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JOHNSON MATTHEY TECHNOLOGY REVIEW

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Johnson Matthey’s international journal of research exploring science and technology in industrial applications

Contents Volume 59, Issue 3, July 2015

174 Selected Electrical Resistivity Values for the Platinum Group of Metals Part I: Palladium and Platinum By John W. Arblaster 182 12th Greenhouse Gas Control Technologies Conference A conference review by Christopher Starkie 188 Platinum Group Metal and Washcoat Chemistry Effects on Coated Gasoline Particulate Filter Design By Chris Morgan 193 “Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing”, 2nd Edition A book review by Jonathan Edgar and Saxon Tint 199 Temperature Dependent Heat Transfer Performance of Multi-walled Carbon Nanotube- based Aqueous Nanoﬂ uids at Very Low Particle Loadings By Meher Wan, Raja Ram Yadav, Giridhar Mishra, Devraj Singh and Bipin Joshi 207 The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and Microstructures By Teresa Fryé, Joseph Tunick Strauss, Jörg Fischer-Bühner and Ulrich E. Klotz 218 “Exploring Materials through Patent Information” A book review by Julia O’Farrelly 221 “Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts” An essay book review by Martyn V. Twigg 233 Sintering and Additive Manufacturing: The New Paradigm for the Jewellery Manufacturer By Frank Cooper 243 Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain By Jason Dawes, Robert Bowerman and Ross Trepleton 257 Atomic-Scale Modelling and its Application to Catalytic Materials Science By Misbah Sarwar, Crispin Cooper, Ludovic Briquet, Aniekan Ukpong, Christopher Perry and Glenn Jones 284 In the Lab: Combining Catalyst and Reagent Design for Electrophilic Alkynylation Featuring Professor Jérôme Waser 287 Johnson Matthey Highlights http://dx.doi.org/10.1595/205651315X688091 Johnson Matthey Technol. Rev., 2015, 59, (3), 174–181 JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Selected Electrical Resistivity Values for the Platinum Group of Metals Part I: Palladium and Platinum

Improved values obtained for liquid phases of palladium and platinum

John W. Arblaster The measured electrical resistivity (ρ) usually consists

Wombourne, West Midlands, UK of a temperature dependent intrinsic resistivity, ρi, which is due to the pure metal and is caused by the Email: [email protected] scattering of the charge carriers (electrons or holes) by phonons (quantised vibrations of the lattice) and by their collisions with each other, and a residual resistivity

Electrical resistivity values for both the solid and liquid (ρ0) due to impurities which also scatter the carriers and phases of the platinum group metals (pgms) palladium increase the resistivity. The quantity ρ0 is considered to and platinum are evaluated. In particular improved be a summation of the effects of different impurities and values are obtained for the liquid phases of these is also considered to be temperature independent. The metals. Previous reviews on electrical resistivity which two contributions to the total resistivity are combined included evaluations for the pgms included those of according to Matthiessen’s Rule: ρ = ρ0 + ρi and because Meaden (1), Bass (2), Savitskii et al. (3) and Binkele ρ0 may vary from sample to sample then attempts are and Brunen (4) as well as individual reviews by Matula made to evaluate values of ρi which should be universal (5) on palladium and White (6) on platinum. for a speciﬁ c metal.

1.1 Correction for Thermal Expansion Effects 1. Introduction In order to obtain a reference value to which all other Electrical resistivity (ρ) is deﬁ ned in terms of the measurements are adjusted the electrical resistivity is International System of Units (SI units) as: evaluated at 273.15 K (0ºC). In the low temperature region below about 30 K the ρ = R A / l (i) 2 5 resistivity can be represented by ρ = ρ0 + A T + B T where where the temperature dependent terms represent R is the electrical resistance of a uniform specimen of the intrinsic resistivity, whilst up to room temperature material in ohms (Ω) the experimental values are generally given in such A is the cross-sectional area of the specimen in square a form that interpolation can be achieved by using metres (m2) simple polynomials rather than using the complicated l is the length of the specimen in metres (m) Bloch-Grüneisen formula (7–9). In the deﬁ nition The units of ρ are therefore Ω m although practically of resistivity as ρ = R A / l then A and l are usually the most useful units are μΩ cm. measured at room temperature and therefore at different

174 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688091 Johnson Matthey Technol. Rev., 2015, 59, (3) temperatures both A and l have to be corrected for have been superseded by the later high precision thermal expansion effects. It is found below room measurements of Williams and Weaver (15) (0 K–300 K) temperature that for the level of accuracy given for ρ, and Khellar and Vuillemin (16) (17 K–300 K), with the thermal expansion corrections are generally negligible latter given only in the form of an equation which was but at higher temperature the measurements have to evaluated at 17 K and then at 10 K intervals from 20 K be corrected, especially if they are based entirely on to 270 K. The measurements of Williams and Weaver the room temperature values for A and l which are were interpolated above 100 K so as to also obtain a usually measured at 293.15 K, the accepted reference full evaluation at 10 K intervals from 20 K to 270 K. temperature for length change measurements: The measurements of Schriempf and of Williams and Weaver agree satisfactorily and were averaged to 10 K ρ (corrected) = ρ (uncorrected) [(A / A ) × (l / l )] (ii) T 293.15 293.15 T with the measurements of Williams and Weaver being extended to 16 K. The measurements of the latter = ρ (uncorrected) [1 + (lT – l293.15) / l293.15] (iii) and of Khellar and Vuillemin do not agree below 35 K. where Equation (iii) can be considered to be a close However the equation of Khellar and Vuillemin showed approximation of Equation (ii). However since 273.15 K peculiar behaviour below this temperature with derived is the actual reference temperature then corrected values being 6% higher than those of Williams and values of ρ(T) should be further corrected for thermal Weaver at 17 K but 31% lower at 20 K. Therefore the expansion from 293.15 K to 273.15 K. Since this latter measurements were given preference up to 35 K. correction is usually negligible at the level of accuracy At this temperature and above values from the two sets given then it is not applied. of measurements were averaged. Overall agreement In the case of rapid pulse heating to high temperatures, is to within 0.5% between 60 K and 180 K and to within because of inertia l generally is unaltered and it is A 0.1% above 180 K. The selected values of Matula that changes. If D is the diameter of the wire then: below 273.15 K are based on a combination of the measurements of White and Woods and of Laubitz and ρ (T) = ρ (measured) (D 2 / D 2) = ρ (measured) (V / V ) T 293.15 T 293.15 Matsumura and on average the intrinsic values show a (iv) bias of 0.02 μΩ cm above the more recently selected where VT is the volume of the sample at temperature values. Other measurements in the low temperature T and V293.15 is the volume at 293.15 K. These are region were discussed by Matula. 2 2 essentially DT and D293.15 respectively since l is In the high temperature region Matula (5) selected assumed to be unaltered. only the measurements of Laubitz and Matsumura (14)

(90 K–1300 K). After correction for ρ0 = 0.020 μΩ cm 2. Palladium the values were calculated at 50 K intervals from 350 to 1300 K. In the present evaluation these measurements Palladium has a face-centred cubic structure and were combined with the more recent measurements of the melting point is a secondary ﬁ xed point on the Khellaf et al. (18) (295 K–1700 K) which were given in International Temperature Scale of 1990 (ITS-90) at the form of an equation which was also evaluated at 1828.0 ± 0.1 K (10). 50 K intervals but over the range 350 K to 1750 K. After correction of both sets of measurements for thermal 2.1 Solid expansion using the values selected by the present Electrical resistivity values for solid palladium at author (19) they were ﬁ tted to Equation (v) which 273.15 K are given in Table I. The selected value is an has an overall accuracy as a standard deviation of average of the last three determinations. The ρ0 correction ± 0.13 μΩ cm. The two sets of measurements show to the measurement of Laubitz and Matsumura (14) a maximum disagreement of 1.0% at 1300 K. The was suggested by Matula (5) who also appears to have equation was extrapolated to the melting point and selected this value as the reference value. selected values are given in Table II. From 71 data sets for solid palladium Matula (5) Measurements of Milošević and Babić (20) selected only the measurements of Schriempf (17) (250 K–1800 K) were independently corrected for (1.6 K–10.6 K), White and Woods (13) (10 K–295 K) and thermal expansion. Their equation differs from the Laubitz and Matsumura (14) (90 K–1300 K). However selected equation sinusoidally by trending from initially it is considered that the values of White and Woods 0.3% high to 1.7% high at 400 K to 0.9% low at 1400 K

Table I Electrical Resistivity of Palladium at 273.15 K

ρ , Authors Ref. i Temperature of data μΩ cm

Powell et al. 11 9.79 At 273.15 K. Corrected for ρ0 0.144 μΩ m

Powell et al. 12 9.75 Interpolated 200 – 400 K. Corrected for ρ0 0.143 μΩ m White and Woods 13 9.70 At 273.15 K. Average of three samples

Laubitz and Matsumura 14 9.760 Interpolated 250–300 K. Corrected for ρ0 0.020 μΩ m

Williams and Weaver 15 9.751 At 273.15 K. Corrected for ρ0 0.007 μΩ m Khellar and Vuillemin 16 9.765 Calculated. Fit 17–300 K Selected 9.76 ± 0.01 At 273.15 K

Table II Intrinsic Electrical Resistivity of Palladium

Temperature, ρi, Temperature, ρi, Temperature, ρi, K μΩ cm K μΩ cm K μΩ cm Solid 5 0.0008 140 4.36 400 14.47 10 0.0038 150 4.79 500 17.92 15 0.011 160 5.21 600 21.14 20 0.028 170 5.63 700 24.15 25 0.061 180 6.04 800 26.96 30 0.113 190 6.45 900 29.59 35 0.189 200 6.86 1000 32.03 40 0.294 210 7.26 1100 34.30 45 0.420 220 7.66 1200 36.42 50 0.566 230 8.06 1300 38.39 60 0.908 240 8.46 1400 40.23 70 1.29 250 8.85 1500 41.95 80 1.71 260 9.25 1600 43.55 90 2.14 270 9.64 1700 45.05 100 2.59 273.15 9.76 1800 46.46 110 3.04 280 10.02 1828 46.84 120 3.48 290 10.41 130 3.92 300 10.79 Liquid 1828 81.4 2200 82.2 2700 83.3 1850 81.5 2300 82.4 2800 83.5 1900 81.6 2400 82.6 2900 83.7 2000 81.8 2500 82.8 2100 82.0 2600 83.1

176 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688091 Johnson Matthey Technol. Rev., 2015, 59, (3) to 0.4% high at 1800 K. Figure 1 shows the deviations Löffl er (23) (295 K–1100 K) were corrected from of the selected values of Matula (which are considered RT/R295 to RT/R273.15 and were also corrected for thermal as incorporating the measurements of Laubitz and expansion. On this basis the differences reached Matsumura) and the experimental values of Khellaf a maximum of 4.1% high at 450 K but then showed et al. and Milošević and Babić from the fi tted curve. some scatter varying between 1.0% low at 800 K and Measurements of Binkele and Brunen (4) (273–1423 K) 1.6% high at 1100 K. Figure 2 shows the deviations of which were also independently corrected for thermal these three sets of measurements from the fi tted curve expansion, showed systematic biases of 1.3% high for where the resistivity ratios of García and Löffl er were runs 1 and 2 and 1.7% high for run 3. converted to electrical resistivity values for comparison purposes.

2 2.2 Liquid Ref. (5) 1.5 Electrical resistivity values for palladium at the melting Ref. (18) point are given in Table III. In the liquid state neither Ref. (20) 1 Dupree et al. (24) (1832 K–1924 K) nor Güntherodt et al. (25) (1864 K–2019 K) obtained evidence for 0.5 any variation of resistivity with temperature. Although 0 Seydel and Fischer (26) (1825 K–3000 K) did obtain 200 400 600 800 1000 1200 1400 1600 1800 Deviation, % evidence of such a variation, the values of Pottlacher Temperature, K –0.5 (22) (1828 K–2900 K) were selected and ﬁ tted to –1 7 –1.5 6 Ref. (21) Fig. 1. Solid palladium – percentage deviations from Ref. (23) selected curve 5 Ref. (22) 4 3 Also in the high temperature region there are a number of other measurements which were published Deviation, % 2 after the review of Matula. After correction for thermal 1 expansion (19) the electrical resistivity measurements 0 of Miiller and Cezairliyan (21) (1400 K–1800 K) trend 300 500 700 900 1100 1300 1500 1700 1900 Temperature, K from 4.0% to 6.9% high whilst the measurement of Pottlacher (22) at the melting point is 5.9% high. Fig. 2. Solid palladium – percentage deviations from selected curve Resistivity ratio measurements of García and

Table III Differences Between the Solid and Liquid Electrical Resistivity of Palladium at the Melting Point

ρ , ρ , Authors Reference S L ρ /ρ Notes μΩ cm μΩ cm L S Dupree et al. 24 (48.8) 83.0 1.700 (a) Güntherodt et al. 25 47.3 78.8 1.666 Seydel and Fischer 26 50.2 79.1 1.576 Khellaf et al. 18 (45.2) 77.3 1.710 (b) Pottlacher 22 49.6 81.4 1.641 Present assessment – 46.84 81.4 1.738 Notes to Table III (a) Solid value based on (ρL – ρS)/ ρS = 0.70 ± 0.05 (b) Solid value based on ρL /ρS = 1.71

Equation (vi) with selected values for the electrical Rosso (27) (1000 K–2000 K), Laubitz and van der resistivity of the liquid and are also given in Table II. Meer (28) (300 K–1500 K), and Flynn and O’Hagan (29) (273 K–1373 K) and the resistance ratios of Roeser (30) 3. Platinum (73 K–1773 K) and Kraftmakher (31) (1000 K–2000 K) together with resistivity measurements given by Martin Platinum has a face-centred cubic structure and the et al. (32) (300 K–1200 K). White ﬁ tted all selected melting point is a secondary ﬁ xed point on ITS-90 at values from 100 K to 2000 K to Equation (vii) which 2041.3 ± 0.4 K (10). was extrapolated to the melting point. Differences between values derived from this equation and the 3.1 Solid tabulated values of White as given in Table IV do not

The resistance ratio of platinum, WT = RT/R273.15, forms exceed 0.01 μΩ cm. An abridged version of the values the basis of the International Temperature Scale which for the solid phase as given in Table IV was originally White (6) extended to 1300 K and calculated values given in Platinum Metals Review by Corti (33). of intrinsic resistivity using the ﬁ xed reference value of For comparison between these measurements 9.82 ± 0.01 μΩ cm at 273.15 K. Above 1300 K White and the selected values as given in Figure 3, the combined the selected values to this temperature with resistivity ratios of Roeser (30) and Kraftmakher (31) the electrical resistivity measurements of Righini and were converted to electrical resistivity values and all

Table IV Intrinsic Electrical Resistivity of Platinum

Temperature, ρi, Temperature, ρi, Temperature, ρi, K μΩ cm K μΩ cm K μΩ cm

Solid

10 0.0026 150 4.89 500 18.45 15 0.0119 160 5.30 600 22.07 20 0.0367 170 5.70 700 25.59 25 0.0855 180 6.11 800 29.00 30 0.163 190 6.52 900 32.29 35 0.270 200 6.92 1000 35.47 40 0.403 210 7.32 1100 38.54 45 0.560 220 7.72 1200 41.50 50 0.734 230 8.12 1300 44.35 60 1.12 240 8.51 1400 47.09 70 1.53 250 8.91 1500 49.74 80 1.95 260 9.30 1600 52.34 90 2.38 270 9.70 1700 54.93 100 2.80 273.15 9.82 1800 57.51 110 3.23 280 10.09 1900 60.11 120 3.65 290 10.48 2000 62.76 130 4.06 300 10.87 2041.3 63.87 140 4.48 400 14.71

Liquid

2041.3 102.8 2300 105.3 2700 109.1 2050 102.9 2400 106.2 2800 110.1 2100 103.4 2500 107.2 2900 111.1 2200 104.3 2600 108.2

In the case of additional electrical resistivity 1 Ref. (4), runs 1 and 5 Ref. (4), runs 2, 3 and 4 measurements of Birkele and Brunen (4) (273–1497 K), 0.8 Ref. (28) combined runs 1 and 5 trend from initially 0.8% high to 0.1% high at 1200 K to 0.4% high at 1373 K whilst 0.6 Ref. (29) combined runs 2, 3 and 4 trend to an average of 0.5% 0.4 low above 1000 K. These trends are also shown in Figure 3. 0.2 Electrical resistivity measurements of Pottlacher (22) 0 (473 K–1573 K and 1740 K–2042 K in the solid range) 300 500 700 900 1100 1300 1500 Temperature, K are initially 1% higher then trend to an average of 3% –0.2

Deviation, % higher between 900 and 1573 K before trending to –0.4 1.2% higher and then to 0.5% higher between 1740 K and the melting point. These differences are also –0.6 shown in Figure 5. –0.8

Fig. 3. Solid platinum – percentage deviations from selected 3.5 curve 3

2.5 Ref. (27) measurements except those of Flynn and O’Hagan Ref. (22), run 1 2 (29) were corrected for thermal expansion using values Ref. (22), run 2 1.5 selected by the present author (34). In addition the measurements of Martin et al. (32) were corrected to 1 correspond to the selected electrical resistivity value at Deviation, % 0.5 273.15 K. Because of their larger deviations values of 0 Righini and Rosso (27) are compared with the selected 300 500 700 900 1100 1300 1500 1700 1900 2100 values in Figure 4. Temperature, K Fig. 5. Solid platinum – percentage deviations from selected curve

0.6 3.2 Liquid Ref. (30) 0.4 Ref. (31) Electrical resistivity values of platinum at the Ref. (32) melting point are given in Table V. In the liquid state 0.2 electrical resistivity measurements of Pottlacher (22) (2042 K–2900 K) were selected as Equation (viii) since in the overlap region they are closely 0 200 400 600 800 1000 1200 1400 1600 1800 2000 conﬁ rmed by measurements of Gathers et al. (36) Temperature, K

Deviation, % (2100 K–7300 K) obtained at a pressure of 0.3 GPa –0.2 which trend from 0.5% low at 2100 K to 1.0% high at 2900 K. Measurements of Hixson and Winkler (37) –0.4 (2042 K–5100 K) are initially 7% low at the melting point and trend 1% low to 1% high between 2100 K and 2900 K but above 3000 K, in direct comparison –0.6 with the measurements of Gathers et al., the trend is to an average of 2% low. Selected values for the Fig. 4. Solid platinum – percentage deviations from selected curve electrical resistivity of liquid platinum from the melting point to 2900 K are also given in Table IV.

Table V Differences Between the Solid and Liquid Electrical Resistivity of Platinum at the Melting Point

ρ , ρ , Authors Reference S L ρ /ρ μΩ cm μΩ cm L S

Martynyuk and Tsapkov 35 62.1 92.6 1.491

Pottlacher 22 64.2 102.8 1.601

Present assessment – 63.87 102.8 1.610

High Temperature Intrinsic Resistivity of Solid Palladium (273.15 to 1828 K) –2 –5 2 –9 3 ρi (μΩ cm) = 4.58639 × 10 T – 1.39098 × 10 T + 1.84118 × 10 T – 1.76742 (v) Intrinsic Resistivity of Liquid Palladium (1828 to 2900 K) –3 ρi (μΩ cm) = 2.058 × 10 T + 77.7 (vi) Intrinsic Resistivity of Solid Platinum (100 to 2041.3 K) –2 –5 2 –8 3 ρi (μΩ cm) = 4.681197 × 10 T – 3.258075 × 10 T + 8.554023 × 10 T – 1.594242 × 10–10 T 4 + 1.837342 × 10–13 T 5 – 1.316886 × 10–16 T 6 + 5.678222 × 10–20 T 7 – 1.340980 × 10–23 T 8 + 1.329896 × 10–27 T 9 – 1.621733 (vii) Intrinsic Resistivity of Liquid Platinum (2041.3 to 2900 K) –3 ρi (μΩ cm) = 9.604 × 10 T + 83.2 (viii)

References 4. L. Binkele and M. Brunen, “Thermal Conductivity, Electrical Resistivity and Lorentz Function Data 1. G. T. Meadon, “Electrical Resistance of Metals”, for Metallic Elements in the Range 273 to 1500 K”, Plenum Press, New York, USA, 1965 Forschungszentrum Jülich, Institut für Werkstoffe der 2. J. Bass, ‘Electrical Resistivity of Pure Metals and Energietechnik, Zentralbibliothek, Germany, 1994 Dilute Alloys’, in “Electrical Resistivity, Kondo 5. R. A. Matula, J. Phys. Chem. Ref. Data, 1979, 8, and Spin Fluctuation Systems, Spin Glasses and (4), 1147 Thermopower”, eds. K.-H. Hellwege and J. L. Olsen, Landolt-Börnstein Numerical Data and Functional 6. G. K. White, ‘Recommended Values of Electrical Relationships in Science and Technology, New Resistivity and Thermal Conductivity of Platinum’, in Series, Group III: Crystal and Solid State Physics, “Thermal Conductivity 17”, Gaithersburg, Maryland, Vol. 15a, Springer-Verlag, Berlin, Heidelberg, New USA, 15th–19th June, 1981, Proceedings of the York, 1982, p. 1 Seventeenth International Thermal Conductivity Conference, ed. J. G. Hust, Purdue Research 3. A. Andryushchenko, Yu. D. Chistyakov, A. P. Dostanko, Foundation, Plenum Press, New York, USA, 1983, T. L. Evstigneeva, E. V. Galoshina, A. D. Genkin, p. 95 N. B. Gorina, V. M. Gryaznov, G. S. Khayak, M. M. Kirillova, E. I. Klabunovskii, A. A. Kuranov, V. L. Lisin, 7. F. Bloch, Z. Physik, 1929, 52, (7–8), 555 V. M. Malyshev, V. A. Matveev, V. A. Mityushov, L. V. 8. F. Bloch, Z. Physik, 1930, 59, (3–4), 208 Nomerovannaya, V. P. Polyakova, M. V. Raevskaya, 9. E. Grüneisen, Ann. Phys., 1933, 408, (5), 530 D. V. Rumyantsev, E. I. Rytvin, N. M. Sinitsyn, A. M. Skundin, E. M. Sokolovskaya, I. P. Starchenko, 10. R. E. Bedford, G. Bonnier, H. Maas and F. Pavese, N. I. Timofeyev, N. A. Vatolin, L. I. Voronova and V. Metrologia, 1996, 33, (2), 133 E. Zinov’yev, “Blagorodnye Metally, Spravochnik” 11. R. W. Powell, R. P. Tye and M. J. Woodman, Platinum (“Handbook of Precious Metals”), ed. E. M. Savitskii, Metals Rev., 1962, 6, (4), 138 Metallurgiya Publishers, Moscow, Russia, 1984 (in 12. R. W. Powell, R. P. Tye and M. J. Woodman, J. Less Russian); English translation by S. N. Gorin, P. P. Common Met., 1967, 12, (1), 1 Pozdeev, B. A. Nikolaev and Yu. P. Liverov, English Edition, ed. A. Prince, Hemisphere Publishing Corp, 13. G. K. White and S. B. Woods, Phil. Trans. R. Soc. New York, USA, 1989 Lond. A, 1959, 251, (995), 273

14. M. J. Laubitz and T. Matsumura, Can. J. Phys., 1972, 26. U. Seydel and U. Fischer, J. Phys. F: Met. Phys., 50, (3), 196 1978, 8, (7), 1397

15. R. K. Williams and F. J. Weaver, Phys. Rev. B, 1982, 27. F. Righini and A. Rosso, High Temp.-High Pressures, 25, (6), 3663 1980, 12, (3), 335 28. M. J. Laubitz and M. P. Van Der Meer, Can. J. Phys., 16. A. Khellar and J. J. Vuillemin, J. Phys.: Condens. 1966, 44, (12), 66 Matter, 1992, 4, (7), 1757 29. D. R. Flynn and M. E. O’Hagan, J. Res. Natl. Bur. 17. J. T. Schriempf, Phys. Rev. Lett., 1968, 20, (19), 1034 Stand., 1967, C71, (4), 255 18. A. Khellaf, R. M. Emrick and J. J. Vuillemin, J. Phys. F: 30. W. Roeser, “Temperature: Its Measurement and Met. Phys., 1987, 17, (10), 2081 Control in Science and Industry”, ed. M. S. van Dusen, Vol. I, Reinhold Publishing Corp, New York, USA, 19. J. W. Arblaster, Platinum Metals Rev., 2012, 56, 1941, p. 1312 (3), 181 31. Ya. A. Kraftmakher, High Temp.-High Pressures, 20. N. Milošević and M. Babić, Int. J. Mater. Res., 2013, 1973, 5, (4), 433 104, (5), 462 32. J. J. Martin, P. H. Sidles and G. C. Danielson, J. Appl. 21. A. P. Miiller and A. Cezairliyan, Int. J. Thermophys., Phys., 1967, 38, (8), 3075 1980, 1, (2), 217 33. C. W. Corti, Platinum Metals Rev., 1984, 28, (4), 164 22. G. Pottlacher, “High Temperature Thermophysical Properties of 22 Pure Metals”, Edition Keiper, Graz, 34. J. W. Arblaster, Platinum Metals Rev., 1997, 41, (1), 12 Austria, 2010, p. 76 35. M. M. Martynyuk and V. I. Tsapkov, Fiz. Metal. 23. E. Y. García and D. G. Löfﬂ er, J. Chem. Eng. Data, Metalloved., 1974, 37, (1), 49; translated into English 1985, 30, (3), 304 in Phys. Met. Metallogr., 1974, 37, (1), 40 24. B. C. Dupree, J. B. Van Zytveld and J. E. Enderby, J. 36. G. R. Gathers, J. W. Shaner and W. M. Hodgson, High Phys. F: Met. Phys., 1975, 5, (11), L200 Temp.-High Pressures, 1979, 11, (5), 529 25. H.-J. Güntherodt, E. Hauser, H. U. Künzi and R. 37. R. S. Hixson and M. A. Winkler, Int. J. Thermophys., Müller, Phys. Lett. A., 1975, 54, (4), 291 1993, 14, (3), 409

The Author

John W. Arblaster is interested in the history of science and the evaluation of the thermodynamic and crystallographic properties of the elements. Now retired, he previously worked as a metallurgical chemist in a number of commercial laboratories and was involved in the analysis of a wide range of ferrous and non-ferrous alloys.

12th Greenhouse Gas Control Technologies Conference

Advances in carbon capture and storage research

Reviewed by Christopher Starkie Johnson Matthey Technology Centre, Plenary Speakers Sonning Common, Reading RG4 9NH, UK Julio Friedmann (US From Vision to Government, USA) Inheritance – The Email: [email protected] Technical Foundation for the Next Decade of CCS Projects

David G. Victor Global Climate Policy and Introduction (University of California, the Future of CCS San Diego, USA) The International Conference on Greenhouse Gas Control Technology is a biennial meeting now in its Juho Lipponen Gas-Fired Power (International Energy Generation With CCS – A twelfth incarnation and is a highlight for carbon dioxide Agency, France) Competitive Option sequestration researchers around the globe. The conference was held between 4th–9th October 2014 Michael J. Monea Boundary Dam – The at the Austin Convention Center, Texas, USA. Over (SaskPower, Canada) Future is Here four days the conference encompassed all aspects of the carbon capture value chain. Approximately 30% Suk Yee Lam The UK’s CCS (Department for Energy Programme: Policy and of the sessions focused on CO capture technology 2 and Climate Change, UK) Delivery and 30% on CO2 storage, with the remaining sessions covering case studies, CO2 utilisation, commercial Gary T. Rochelle (The From Lubbock TX issues, CO2 transport, policy and social science. University of Texas at to Thompsons, TX: The conference was attended by 1166 delegates Austin, USA) Amine Scrubbing for comprising of an almost even distribution of students, Commercial CO2 Capture academics, industrial representatives, research From Power Plants institutes and government agencies. There was Xu Shishen (Huaneng Greengen and CO2 a high level of participation with 874 contributions Clean Energy Research Capture Projects in China presented throughout the seven parallel sessions and Institute, China) well-planned poster sessions. Furthermore a small selection of exhibitors complimented the technical Emma ter Mors (Leiden The Value of Social programme providing details of commercial ventures University, Netherlands) Science Research for CCS Deployment and institutional programmes. This selective review highlights interesting advances Greg Schnacke (Denbury CO2 EOR: U.S. presented at the conference. For a more comprehensive Resources Inc, USA) Opportunities and overview of the area the reader is directed to a number Challenges of encompassing reviews on the topic (1–3).

Status of Carbon Capture and Storage

Carbon capture and storage is at a pivotal stage. + SaskPower’s Boundary Dam power station has started H to operate the fi rst large scale carbon capture unit on H H H – O O a commercial power plant. The next two years will see Zn I Zn the commissioning of two larger power generation N N N N N N projects: Kemper County in 2016 and Petra Nova, Texas in 2016. There are over twenty large scale – HCO3 projects in operation or under construction across II IV CO the globe (4). These developments are built upon the 2 lessons of hundreds of pilot plants around the globe H2O O O and thousands of research hours. In the past ten C H H – years the energy consumption of amine scrubbing has OO III O O Zn Zn been reduced from 300–350 kWh per tonne CO2 to N N N N N N 200–250 kWh per tonne CO2. Although promising, this approach to capture is limited by the large energies required to regenerate the solvent, and by long term Fig. 1. Carbonic anhydrase cycle solvent durability. Cost reduction is the motivation of much of the ongoing research with advances strongly electron donating groups thereby facilitating that increase the rate of adsorption or reduce the bicarbonate dissociation and retarding inhibition. regeneration energy highly sought after. It was a core The work used Zn salen (N,N’-bis(salicylidene) theme of the conference that carbon capture and ethylenediamine) hexafl uorophosphate complexes storage has the potential to mitigate emissions from as catalysts for liquid amine systems (Figure 2). The power generation and industrial processes for which incorporation of such catalysts led to a signifi cant there are no substitutes. increase in the rates of CO2 hydration. The catalytic cycle is thought to mimic that of the enzyme. If scalable New Capture Technologies the addition of these catalysts could vastly improve the CO2 adsorption kinetics leading to more effi cient Owing to differences in their adsorption mechanism, adsorption systems. tertiary amines are promising candidates for CO2 Carbonic anhydrase also provided inspiration for adsorption with the potential to offer higher amine Richard Blom et al. (SINTEF, Norway) who built upon effi ciency and lower desorption requirements compared the work by Murthy et al. presenting the Zn complex to simple primary and secondary amines. The main [Zn{N[CH2(2-py)]3}(μ-OH)]2(NO3)2 (8). As well as limitation of tertiary amines in liquid scrubber systems catalysing the hydration of CO2 such complexes can is the sluggish formation of carbonic acid. Cameron directly react with CO2 to form a metastable complex. Lippert (University of Kentucky, USA) took inspiration The complex was found to absorb CO2 in the presence from the enzyme carbonic anhydrase which readily of water forming a trimeric Zn species bridged by a catalyses the hydration of CO2 in the majority of plants carbonate species (Figure 3) (9). The absorption could and animals. The mechanism of carbonic anhydrase is be completed at 40ºC with purely thermal regeneration well understood, beginning with the deprotonation of the possible at 80ºC; a temperature at which liquid amines ligated water coordinated to the zinc centre (Figure 1) show very little desorption. Such low desorption (5). This is followed by the nucleophilic attack of the enzyme on CO2. Finally ligand exchange occurs with the Zn bound HCO – replaced by water regenerating – – 3 PF6 PF6 the starting species. Previous studies have shown + N N + that this enzyme denatures under conditions akin to N N N N Zn industrial capture units (6, 7). Much prior research has O O focused on mimics, however these are retarded by the coordination of anions blocking the active site. To Fig. 2. Carbonic anhydrase mimic circumvent this issue Lippert investigated ligands with

N N

N Zn N N N N H N O N OO 3/2 Zn Zn (NO3)2 + CO2 N Zn (NO3)3(OH) + H2O N N O N N N H N N O Zn N N 1 N

Fig. 3. Trimeric zinc complex, adapted from (9)

temperatures are promising, offering signifi cant to yield a coaxial fl ow. A poly(vinyl alcohol) (PVA) reductions in regeneration requirements. Furthermore stabiliser is pumped in a counter current manner to this complex was found to readily remove CO2 from achieve hydrodynamic focusing with the three streams air at low temperatures opening up a series of niche passing through a single exit where they form spheres. applications for this technology. The behaviour of the By using a silicone polymer and additives the outer PVA complex is also being investigated under different and silicone shells can be cured under ultraviolet (UV) solvents to avoid the additional heating requirements light to yield aqueous monoethanolamine solutions brought about by the high heat capacity of water and to encapsulated in a porous polymer shell. overcome issues relating to solubility. Microfl uidic devices enable monodisperse spheres

There were a number of talks highlighting new CO2 to be easily produced with particle sizes controlled capture technologies for post combustion capture. by the dimensions of the device. The team began Joshuah Stolaroff (Lawrence Livermore National encapsulating simple amines such as aqueous Laboratory, USA) gave a talk on microencapsulated solutions of MEA and piperazine which yielded rapid sorbents. These double emulsion materials feature an absorption of CO2. Their recent work in this area active liquid phase constrained within a porous polymer highlighted that the rate determining step was not mass shell. They are produced using a microfl uidic device transit through the polymer shell with encapsulated in a linear process at a rate of 50 Hz (Figure 4). In absorbents having a rate twelve times that of their liquid such a process mixtures of monoethanolamine (MEA) counterparts (11). They found that the working fl uid was and water are passed through the inner capillary with stable over numerous absorption-desorption cycles. a silicone polymer pumped through the middle tube Furthermore the capsules have soft gelatinous physical characteristics negating the issue of attrition prevalent Outer fl uid Middle fl uid in solid adsorbents and catalysts. Encapsulated sodium carbonate slurries mitigate the issue of precipitates and Inner fl uid offer signifi cantly better rates of absorption. Currently such double emulsions are made in a serial fashion and Stolaroff and his team are working to improve the Collection tube Injection tube production capacity of such systems by combining parallel arrays of the microfl uidic devices.

Amine Degradation

Liquid amine based CO2 absorption is the most developed capture technology and the most likely Fig. 4. Microﬂ uidic device (Reproduced with permission candidate for initial full scale deployment. A typical from (10)) liquid amine absorber includes a water wash at the

184 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687614 Johnson Matthey Technol. Rev., 2015, 59, (3) end of the absorption process to remove the amine driven by changing legislation and as such systems and other water soluble products from the exhaust to reduce these emissions further are still under gases. With liquid amine columns appearing to be development. the first generation of carbon capture technology The extent of solvent loss, nitrosamine formation the issue of amine slip and degradation products and potential release is highly dependent on the fl ue needs to be thoroughly investigated. There is gas and exact nature of the process used. A number much discussion surrounding the potential release of solutions were presented at the conference. of nitrosamines and nitroamines formed by the Nathan Fine (The University of Texas at Austin) reaction of nitrogen oxides and amines (12–14). investigated the rates of thermal decomposition Various nitrosamine structures are believed to of MEA and piperazine nitrosamines. For fresh be carcinogenic and could present a significantly solutions of MEA they found minimal conversion to increased hazard compared to the parent amine. nitrosamine. In cases where 1% of the MEA had Nitrosamines have been detected in laboratory degraded to N-(2-hydroxyethyl)glycine a build-up of experiments, photochemical experiments, numerous nitroso-2-hydroxyglycine was apparent. MEA is pilot plant studies and are formed by the reaction regenerated at 120ºC to preserve the structure of the between an amine and a suitable nitrosating agent amine and at this temperature the nitrosamines were (15, 16). In the flue gas, nitrogen oxide can react not suffi ciently thermally degraded. It was found that with nitrogen dioxide forming dinitrogen trioxide 15% of the NO2 absorbed by piperazine was converted (Equation (i)). Nitrogen dioxide can also dimerise to nitropiperazine. Piperazine systems could be to form dinitrogen tetraoxide (N2O4) (Equation (ii)). regenerated at 150ºC leading to appreciable thermal These species can then react with various amines decomposition of the nitropiperazine. to yield nitrosamines (Equations (iii) and (iv)) or A forward thinking presentation by Jesse Thompson nitroamines (Equation (v)) depending on the reacting (University of Kentucky) explored the possibility of isomer. Furthermore nitrate can be generated in nitrosamine destruction as an end of pipe treatment. solution by the decomposition of either N2O3 or The benefi t of nitrosamine reduction is twofold; N2O4 which can directly react with amines. mitigating the release of nitrosamine and regenerating degraded solvent. Using nitrosopyrrolidine as a probe NO + NO N O (i) 2 2 3 molecule and a circulating up-fl ow reactor Thompson et

NO2 + NO2 N2O4 (ii) al. demonstrated that a commercial palladium catalyst cleanly regenerated pyrrolidine at typical desorption NO HO NH + N2O3 HO N + HNO2 (iii) temperatures. Given the low fraction of nitrosamine in HO HO the solution, selectivity between the nitrosamine and the parent amine is crucial. Silica based Pd, Ni and Fe NO catalysts were investigated but found to have limited HO NH + ONONO2 HO N + HNO3 (iv) stability over successive cycling. This limited stability was attributed to the highly alkaline environment slowly NO2 HO NH + O2NH2NO2 HO N + HNO2 (v) degrading the silica surface leading to metal leaching. In a move to combat this, 2×2 manganese octahedral HO HO molecular sieves (OMS-2) (Figure 5) co-impregnated These species and their tautomers can react with with Fe, Pd and Ni were prepared. amines to form nitrosamines or nitroamines either in In such structures the guest metal ion resides the absorber column or desorber and recirculating in the centre of the cage with catalytic activity wash water. There is potential for these pollutants to believed to be maintained by electron transfer from volatilise and contaminate local air or water resources. the vacant states of the OMS-2 structure. The Pd Studies suggest that NOx concentrations down to OMS-2 showed enhanced activity compared to Ni and around 25 ppm lead to the formation of nitrosamines Fe–OMS-2 however nitrosamine destruction was limited (17). For reference a typical fl ue gas from a coal fi red to 60%. Although possessing a lower activity than Pd, power station contains 50–100 ppm NOx. After fl ue iron(II) oxide (FeO) was also investigated as a catalyst gas desulfurisation and deNOx this drops to 5 ppm. owing to its ready availability in fl y ash. Fly ash containing Traditionally the performance of deNOx systems is 5.37 wt% FeO yielded fair nitrosamine destruction

nitrosamines. The closing session presented the clear message that as a technology carbon capture and storage is ready, with the greatest challenge being policy support to incentivise deployment. 4.6 Å References 1 M. E. Boot-Handford, J. C. Abanades, E. J. Anthony, M. J. Blunt, S. Brandani, N. M. Dowell, J. R. Fernández, M.-C. Ferrari, R. Gross, J. P. Hallett, R. S. Haszeldine, Fig. 5. Structure of OMS-2 (Reprinted with permission from P. Heptonstall, A. Lyngfelt, Z. Makuch, E. Mangano, (18). Copyright (2012) American Chemical Society) R. T. J. Porter, M. Pourkashanian, G. T. Rochelle, N. Shah, J. G. Yao and P. S. Fennell, Energy Environ. Sci., 2014, 7, (1), 130 behaviour under hydrogenation conditions. Successive 2 T. C. Drage, C. E. Snape, L. A. Stevens, J. Wood, J. cycling of the fl y ash catalyst exhibited a decrease in Wang, A. I. Cooper, R. Dawson, X. Guo, C. Satterley performance attributed to active phase leaching into and R. Irons, J. Mater. Chem., 2012, 22, (7), 2815 the amine solution. Given the conditions, fl ow rates and 3 D. M. D’Alessandro, B. Smit and J. R. Long, Angew. competitor species this is a challenging hydrogenation Chem. Int. Ed., 2010, 49, (35), 6058 however this work shows that hydrogenation has 4 The Global Status of CCS 2014, Global CCS Institute, potential to limit nitrosamine formation and regenerate 2014, Melbourne, Australia the solvent. Thompson and his team demonstrated that 5 M. Bräuer, J. L. Pérez-Lustres, J. Weston and E. supported Pd, Ni and Fe catalysts show promise for the Anders, Inorg. Chem., 2002, 41, (6), 1454 hydrogenation of nitrosamines back into their parent 6 R. Lavecchia and M. Zugaro, FEBS Lett,. 1991, 292, amines in the solvent native to the process (Figure 6). (1–2), 162 7 G. M. Bond, J. Stringer, D. K. Brandvold, F. A. Simsek, M. G. Medina and G. Egeland, Energy Fuels, 2001, 15, (2), 309 H , catalyst 2 8 N. N. Murthy and K. D. Karlin, J. Chem. Soc., Chem. N N Commun., 1993, (15), 1236 NO H 9 R. H. Heyn, U. E. Aronu, S. J. Vevelstad, K. A. Hoff, T. Fig. 6. The nitrosamine reduction concept put forward Didriksen, B. Arstad and R. Blom, Energy Procedia, by Thompson et al. 2014, 63, 1805 10 A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone and D. A. Weitz, Science, 2005, 308, (5721), 537 Conclusion 11 J. J. Vericella, S. E. Baker, J. K. Stolaroff, E. B. Duoss, J. O. Hardin IV, J. Lewicki, E. Glogowski, W. This review presents only a fragment of the C. Floyd, C. A. Valdez, W. L. Smith, J. H. Satcher Jr, presentations and posters disclosed. The popularity W. L. Bourcier, C. M. Spadaccini, J. A. Lewis and R. D. of the conference, combined with the large number of Aines, Nature Commun, 2015, 6, 6124 key stakeholders present highlights the importance of 12 C. J. Nielsen, H. Herrmann and C. Weller, Chem. Soc. carbon capture and storage. A signifi cant number of the Rev., 2012, 41, (19), 6684 talks and posters discussed pilot plant experiences of 13 A. D. Shah, N. Dai, and W. A. Mitch, Environ. Sci. both capture and storage technologies. Carbon capture Technol., 2013, 47, (6), 2799 and storage is at a crucial stage in development with 14 E. D. Wagner, J. Osiol, W. A. Mitch and M. J. Plewa, fi rst generation systems becoming a commercial reality. Environ. Sci. Technol., 2014, 48, (14), 8203 The large variety of novel concepts and technologies 15 B. R. Strazisar, R. R. Anderson and C. M. White, indicate the potential for further advances in second and Energy Fuels, 2003, 17, (4), 1034 third generation systems. These technologies look to 16 B. Fostås, A. Gangstad, B. Nenseter, S. Pedersen, M address the core issues of regeneration requirements, Sjøvoll and A. L. Sørensen, Energy Procedia, 2011, material stability and the possible behaviour of 4, 1566

17 N. Dai, A. D. Shah, L. Hu, M. J. Plewa, B. McKague and W. A. Mitch, Environ. Sci. Technol., 2012, 46, (17), 9793 18 G. D. Yadav, P. A. Chandan and D. P. Tekale, Ind. Eng. Chem Res., 2012, 51, (4), 1549

The Reviewer

Christopher Starkie graduated with a MSci in Chemistry from the University of Nottingham, UK. He is currently undertaking postgraduate study at the Engineering Doctorate Centre in Efﬁ cient Fossil Energy Technologies under the supervision of Professor Ed Lester, Professor Sean Rigby and Professor Trevor Drage. Working in collaboration with Johnson Matthey he is investigating novel

functionalised materials for CO2 separation. His research interests include adsorbents, surface functionalisation of porous materials and novel material synthesis.

Platinum Group Metal and Washcoat Chemistry Effects on Coated Gasoline Particulate Filter Design

Development of gasoline particulate ﬁ lters to meet Euro 6c

By Chris Morgan Effect of Driving Conditions on Particle Number Johnson Matthey Emission Control Technologies, Emissions Orchard Road, Royston, Hertfordshire, SG8 5HE, UK Almost all GDI vehicles can meet the current 6 × 1012 Email: [email protected] km–1 PN target over the NEDC. It has been reported that a limit of 6 × 1011 km–1 can be achieved through engine design measures such as the use of high Gasoline particulate fi lters (GPFs) are being developed pressure fuel injectors, injection and combustion to enable compliance with future particulate number timing and careful design of spray patterns and piston (PN) limits for passenger cars equipped with gasoline heads to improve mixture formation and to avoid wall direct injection (GDI) engines. A PN emissions wetting (1–3). However, the NEDC is characterised limit of 6 × 1011 km–1 over the New European Drive as having moderate acceleration and deceleration Cycle (NEDC) will apply for new GDI vehicles from rates and extended cruises at constant speed, which September 2017. (A three year derogation allowing a are generally unrepresentative of typical real-world higher PN limit of 6 × 1012 km–1 is currently in force.) driving. Harder acceleration rates and more transient Real-world Driving Emissions (RDE) legislation is driving are observed to give higher PN emissions. For being fi nalised by the European Commission, which is example a Euro 5 GDI vehicle equipped with a fl ow expected to impose additional restrictions on particle through three-way catalyst (TWC) system emitted number emissions in typical driving conditions. 2.8 × 1012 particles km–1 over the NEDC. PN emissions Legislation proposed for Beijing, China, (commonly from the same vehicle increased almost fourfold to known as Beijing 6) is also expected to set a limit on 9.6 × 1012 km–1 when a transient drive cycle with harsh PN emissions from GDI vehicles. acceleration rates was used. Similarly, experiments This paper, based on a presentation given to the on multiple GDI vehicles indicated 30–85% higher PN Society of Automotive Engineers (SAE) Light Duty emissions from testing using the more transient US Emissions Symposium in Detroit, USA, in December Federal Test Procedure (FTP)-75 drive cycle compared 2014, discusses the results from Johnson Matthey to NEDC data. Driver to driver variability has also been test programmes to understand the effects of different observed. Therefore, it is likely that real-world PN driving conditions on engine out PN emissions, the emissions from GDI vehicles are signifi cantly higher benefi ts obtained from applying a platinum group than NEDC test data indicate. metal (pgm)-containing coating onto a GPF and Furthermore, ambient temperature has a signifi cant the impact of such a coating on soot combustion effect on PN emissions. NEDC testing is conducted at properties. a temperature of 23ºC, signifi cantly above the average

188 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688109 Johnson Matthey Technol. Rev., 2015, 59, (3) temperature in the UK and many other European gasoline aftertreatment system. Many European countries. A Euro 6 GDI vehicle equipped with a fl ow exhaust systems have their catalyst volume and through TWC system was measured to emit <4 × 1011 precious metal content optimised to meet current particles km–1 over the NEDC when tested at 23ºC NEDC emissions targets. It is well known that (Figure 1). When the test car was cooled to 10ºC conversion of NOx requires more catalyst volume than before the start of the test, PN emissions increased conversion of CO or hydrocarbons. NEDC-optimised threefold to 1.1 × 1012 km–1. Cooling the car to 0ºC catalyst systems may not provide suffi cient NOx before the start of the test doubled PN emissions again conversion activity under more demanding conditions to 2.3 × 1012 km–1. Therefore, even vehicles designed with higher space velocities. Adding effective catalyst to give low PN emissions during the certifi ed test at volume by applying a pgm-containing coating onto 23ºC are likely to emit signifi cantly more in the ambient the GPF can increase the conversion of gaseous conditions many drivers experience daily. pollutants under such conditions. For example, a Euro 5 1.0 l GDI application was tested over the NEDC Benefi ts of a Coated Gasoline Particulate Filter and World-Harmonised Light-Duty Test Cycle (WLTC) with uncoated or coated GPFs fi tted downstream of GPFs have been shown to be very effective at the series TWC. With the uncoated fi lter system NOx attenuating PN emissions (4, 5). Cordierite wall emissions over the NEDC were 56 mg km–1, increasing fl ow fi lters are most commonly used in development to 82 mg km–1 over the more transient WLTC for which programmes and at least one series application the TWC was undersized. With a coated GPF NEDC employing uncoated cordierite GPFs is on sale today. emissions were ~10% lower than with the uncoated Adding an uncoated fi lter downstream of the existing fi lter, at 50 mg km–1. Furthermore, the coated GPF aftertreatment system allows PN control without gave good control over the WLTC with emissions of requiring signifi cant changes to engine calibration or 47 mg km–1. The coated GPF is particularly effective on-board diagnostic (OBD) strategies. However, the in avoiding NOx slip during the higher speed, higher addition of an extra unit adds canning cost and the space velocity conditions experienced in the fi nal 600 s space required can be problematic on smaller vehicles. of the WLTC (Figure 2). Applying a suitable TWC coating onto the GPF allows The benefi ts of a coated GPF can be further enhanced it to be substituted for fl ow through TWC volume in by optimising the precious metal content of the coating. the existing aftertreatment system, resulting in a more For example, Figure 3 shows NOx emissions from compact architecture. Johnson Matthey and other testing of a TWC plus coated GPF system on a Euro 5 coaters (6–8) have demonstrated such technologies 2.0 l GDI vehicle over the transient Artemis test cycle. previously. Thrifting precious metal from the close-coupled TWC However, expected RDE limits on nitrogen oxides resulted in a ca. 20% increase in NOx emissions. (NOx) emissions will add further demands on the However, increasing the rhodium loading on the

23ºC 14ºC 10ºC 0ºC –7ºC –9ºC Scheduled speed, kph 4.0E + 12 160 3.5E + 12 PN dependent on soak temperature 140 3.0E + 12 120 Speed, kph 2.5E + 12 100 2.0E + 12 80 1.5E + 12 60 NO per km 1.0E + 12 40

Cumulative particulate 5.0E + 11 20 0.0E + 00 0 0 200 400 600 800 1000 1200 Time, s Fig. 1. Impact of vehicle soak temperature on PN emissions over the NEDC

Engine out NOx Post TWC NOx Tailpipe NOx Scheduled speed 2.0 300 1.8 NOx conversion by the coated GPF 250 1.6

1.4 Vehicle speed, kph 200 1.2 1.0 150 0.8 100 0.6 0.4

Accumulated NOx emissions, g 50 0.2 0.0 0 0 200 400 600 800 1000 1200 1400 1600 1800 Time, s Fig. 2. Comparison of TWC plus coated or uncoated GPF over the WLTC

TWC 100 // cGPF Rh@2 TWC 80 // cGPF Rh@2 TWC 80 // cGPF Rh@5 Scheduled speed, kph 3.5 140 Increased Rh loading on ﬁ lter helps 3.0 with NOx breakthrough 120 Scheduled speed, kph 2.5 100

2.0 80

1.5 60

1.0 40

Cumulative NOx emissions, g 0.5 20

0.0 0 0 500 1000 1500 2000 2500 3000 Time, s

Fig. 3. Effect of GPF Rh loading in compensating for emissions breakthroughs from pgm thrifting of the upstream TWC

downstream coated GPF from 2 g ft–3 to 5 g ft–3 more Soot Combustion than compensated for this effect, with signifi cantly improved conversion at higher speeds. A concern about the use of uncoated GPFs, particularly Therefore, as well as controlling PN emissions, when located in remote underfl oor locations, is whether coated GPFs offer advantages over uncoated GPFs they will regularly reach suffi cient temperatures to in conversion of gaseous pollutants for RDE and regenerate collected soot without the use of extreme more transient drive cycles. Additional pgm and engine operating conditions to artifi cially increase the oxygen storage capacity (OSC) on the coated fi lter fi lter temperature. It was hypothesised that the presence help to control emissions breakthroughs during harsh of an active coating on the GPF would enhance soot accelerations and high speed driving. combustion. To investigate this soot-loaded coated

190 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688109 Johnson Matthey Technol. Rev., 2015, 59, (3) and uncoated GPFs were fi tted to a Euro 5 1.4 l Conclusions GDI vehicle and tested over a cycle comprising the NEDC with the fi nal, higher speed, Extra-Urban Drive Control of PN emissions from GDI engines is of Cycle (EUDC) repeated ten times. The fi nal EUDC increasing importance to meet Euro 6c and proposed deceleration from 120 kph triggers a fuel cut-off and RDE and Beijing 6 emissions standards and GPFs are the resulting oxygen-rich environment can lead to soot effective in reducing tailpipe PN emissions. However, combustion at suitable fi lter temperatures. Peak fi lter emissions of particulates and gaseous pollutants inlet temperatures during the NEDC were controlled can be signifi cantly higher in real-world conditions, to values between 520ºC and 650ºC by varying the with more transient driving, higher maximum speeds dynamometer gradient from 0 to 3%. Soot combustion and lower ambient temperatures all shown to have a was monitored through measurement of pressure drop detrimental effect. Use of a pgm-containing coated over the aftertreatment system and by weighing the GPF offers benefi ts over an uncoated GPF, including fi lter before and after each test to measure the mass the abilities to control emissions breakthroughs from of soot removed. upstream TWC components, to enable regeneration of In the uncoated fi lter only 50% of the soot was collected soot at lower temperatures and at a faster removed at a peak temperature of 600ºC, increasing to rate and to allow substitution for TWC volume for 90% at 650ºC (Figure 4). In contrast, soot combustion system compactness. These benefi ts can be enhanced in the coated fi lter increased rapidly at temperatures through careful design of the washcoat chemistry and above 550ºC, with complete removal of the soot at ca. precious metal content. 570ºC. Therefore, the presence of a pgm-containing TWC coating reduced soot combustion temperatures Acknowledgements by approximately 100ºC, delivering signifi cant benefi ts for vehicle strategies to control soot build up in a GPF. The author would like to thank the Gasoline Product Detailed analysis of the differential pressure data Development and Catalyst Test Laboratory teams at showed that at 570ºC the backpressure reduced to the Johnson Matthey Emission Control Technologies a stable level after the fi rst EUDC, indicating rapid Centre in Royston for the development work and test promotion of soot combustion. data described in this paper. The chemistry of the TWC coating can also be optimised to enhance soot combustion properties. Powder reactor studies confi rmed that the presence of References pgm and ceria-containing OSC materials signifi cantly 1 G. Fraidl, P. Hollerer, P. Kapus, M. Ogris and K. reduce soot combustion temperatures and that the Vidmar, ‘Particulate Number for EU6+: Challenges choice of OSC material can infl uence peak soot and Solutions’, IQPC Advanced Emission Control combustion temperatures by more than 50ºC. Concepts for Gasoline Engines, Stuttgart, Germany, 21st–23rd May, 2012

Bare GPF Coated GPF 2 M. Winkler, ‘Particle Number Emissions of Direct 100 Injected Gasoline Engines’, IQPC Advanced Emission 90 Control Concepts for Gasoline Engines, Stuttgart, 80 Germany, 21st–23rd May, 2012 70 3 O. Berkemeier, K. Grieser, K. Hohenboeken, E. 60 Kervounis and K. M. Springer, ‘Strategies to Control 50 40 Particulate Emissions of Gasoline Direct Injection 30 Engines’, FISITA 2012 World Automotive Congress, 20 Beijing, China, 27th–30th November, 2012 10 4 T. Shimoda, Y. Ito, C. Saito, T. Nakatani, Y. Shibagaki, K.

% Soot burnt after 10 fuel cuts 0 450 500 550 600 650 700 Yuuki, H. Sakamoto, C. Vogt, T. Matsumoto, Y. Furuta, Filter inlet temperature, ºC W. Heuss, P. Kattouah and M. Makino, ‘Potential of a Low Pressure Drop Filter Concept for Direct Injection Fig. 4. Effect of a pgm-containing GPF coating on soot Gasoline Engines to Reduce Particulate Number combustion under vehicle deceleration fuel cut-off conditions Emission’, SAE Technical Paper 2012-01-1241

5 K. Ogyu, ‘Requirement of Exhaust Emission Control on 7 J. M. Richter, R. Klingmann, S. Spiess and K.-F. Direct Injection Gasoline Engines – a New Approach Wong, ‘Application of Catalyzed Gasoline Particulate on Gasoline Particulate Filters’, IQPC Advanced Filters to GDI Vehicles’, SAE Technical Paper Emission Control Concepts for Gasoline Engines, 2012-01-1244 Stuttgart, Germany, 21st–23rd May, 2012 8 K. Harth, K. Wassermann, M. Arnold, S. Siemund, 6 C. Morgan, ‘Three Way Filters for Particulate Number A. Siani, T. Schmitz and T. Neubauer, “Catalyzed Control’, IQPC Advanced Emission Control Concepts Gasoline Particulate Filters: Integrated Solutions for for Gasoline Engines, Stuttgart, Germany, 21st–23rd Stringent Emission Control”, 34th International Vienna May, 2012 Motor Symposium, Austria, 25th–26th April, 2013

The Author

Chris Morgan is Technology Director for Johnson Matthey’s European Emission Control Technologies business, responsible for the development and scale-up of autocatalyst coatings for light duty gasoline and diesel applications. Chris previously managed the Gasoline Product Development team, developing new families of three-way catalysts and leading Johnson Matthey’s early work on coatings for gasoline particulate ﬁ lters. He joined Johnson Matthey in 1997, after completing a DPhil at the University of Oxford, UK, on high temperature ceramic superconductors.

“Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing”, 2nd Edition

By Ian Gibson (Deakin University, Australia), David Rosen (Georgia Institute of Technology, USA) and Brent Stucker (University of Louisville, USA), Springer Science+Business Media, New York, USA, 2015, 498 pages, ISBN: 978-1-4939-2112-6, £81.00, €96.29, US$119.00

Reviewed by Jonathan Edgar* The book is structured to provide justifi cation and Johnson Matthey Technology Centre, information for the use and development of AM Blount’s Court, Sonning Common, Reading RG4 9NH, UK by using standardised terminology to conform to standards (American Society for Testing and Materials Saxon Tint (ASTM) F42) introduced since the fi rst edition. The Johnson Matthey Noble Metals, basic principles and historical developments for Orchard Road, Royston, Hertfordshire SG8 5HE, UK AM are introduced in summary in the fi rst three chapters of the book and this serves as an excellent *Email: [email protected] introduction for the uninitiated. Chapters 4–11 focus on the core technologies of AM individually and, in most cases, in comprehensive detail which gives those interested in the technical application and Introduction development of the technologies a solid footing. The remaining chapters provide guidelines and examples “Additive Manufacturing Technologies: 3D Printing, for various stages of the process including machine Rapid Prototyping, and Direct Digital Manufacturing” and/or materials selection, design considerations and is authored by Ian Gibson, David Rosen and Brent software limitations, applications and post-processing Stucker, who collectively possess 60 years’ experience considerations. in the fi eld of additive manufacturing (AM). This is the second edition of the book which aims to include Principles and Processes current developments and innovations in a rapidly changing fi eld. Its primary aim is to serve as a teaching The fi rst three chapters provide a basic understanding aid for developing and established curricula, therefore of why someone might want to utilise AM in the context becoming an all-encompassing introductory text for of traditional methods as well as developments in AM this purpose. It is also noted that researchers may fi nd since its inception. In the initial chapters the reason the text useful as a guide to the ‘state-of-the-art’ and to for introducing standardised terminology is justifi ed in identify research opportunities. contrast to the historical terms. For example, the use

193 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688406 Johnson Matthey Technol. Rev., 2015, 59, (3) of ‘rapid prototyping’ to describe the fi eld is no longer be applied generally. Curable materials are discussed, relevant as the technology is now used for functional including an overview of photopolymer chemistry and components as well. their interactions with radiation. Another method, called Brief summaries of the process are also provided in the ‘two-photon’ approach, describes how a dual light these initial chapters in increasing detail as the reader source can increase part resolution – feature sizes of approaches the technical chapters. This provides a 0.2 μm have been achieved. Advantages of VP include realistic view of actions required throughout the process part accuracy, surface fi nish and process fl exibility. The and gives context to references made throughout the main drawback is their usage of photopolymers, which book. In its simplest form the process can be universally generally do not have the impact strength or durability summarised as: of good quality injection moulded thermoplastics. • conceptualisation and computer aided design (CAD) Powder Bed Fusion • conversion to STereoLithography (STL)/additive manufacturing fi le (AMF) PBF methods deposit layers of powder which are • transfer to AM machine and fi le manipulation sequentially fused together by an energy source • machine setup resulting in solid parts residing in a powder bed. The • build prominent methods are selective laser sintering (SLS) • removal and cleanup and electron beam melting (EBM), and these two are well • post-processing compared: EBM processes benefi t from the fl exibility • application. and high energy of their heat source, which can move nearly instantaneously and with split beams; EBM builds Additive Layer Manufacturing Technologies in typically maintain the bed at high temperature and can Detail create parts with a cast, low porosity, low residual stress microstructure. However their requirement for Chapters 4–11 describe seven different printing a vacuum chamber and conductive target material processes in differing degrees of detail. Topics such as restricts material capabilities whereas SLS machines ‘vat photopolymerisation’ (VP) and ‘powder bed fusion’ can process materials such as polymers and ceramics (PBF) are covered in such comprehensive detail that in gaseous atmospheres. This chapter builds on the their chapters are double the length compared with process model from the previous chapter, applying the ‘binder jetting’ (BJ), despite the latter being a very concepts to the fusion process of solids and methods well used industrial technique. Printable materials for reducing internal stresses in solidifi ed parts. PBF and delivery mechanisms are also listed here and the is a well commercialised technology, but due to the chapters include a ‘process benefi ts and drawbacks’ lapsing of major patents there are some open-source section, useful for the beginner. It should be noted machines available which have led inventors to create that some processes (extrusion, sheet lamination non-engineering applications of the technology too. and directed-energy deposition) are omitted from this review. Materials Jetting

Vat Photopolymerisation The ﬁ rst generation of materials jetting (MJ) machines was commercialised in the 1980s. They relied on heated VP processes make use of liquid, light-curable resins waxy thermoplastics deposited by inkjet printheads, as their primary materials. Upon irradiation, these lending themselves to modelling and investment materials undergo photopolymerisation, becoming casting manufacture; however the more recent focus solid. Ultraviolet (UV) light is projected onto the build has been on deposition of acrylate photopolymers, plate, curing a layer before recoating. Methods for wherein droplets of liquid monomer are formed and then illuminating the photopolymers are presented, including exposed to UV light to initiate polymerisation. Machines masked projection, layer-wise processing and vector with build spaces as big as 1000 × 800 × 800 mm are scan point-wise processing. Expressions relating available, as well as multi-material capability which laser power, scan speed, spot size and cure depth are can print 1000+ materials by varying the composition derived, forming the basis of a process model which can of several photopolymers. Research groups around

194 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688406 Johnson Matthey Technol. Rev., 2015, 59, (3) the world are working on other material groups such and ranked based on the relevance of the input as non-photopolymers, ceramic suspensions and parameters (for example geometric considerations low melting point metals (including aluminium). The and mechanical properties). A particular example, challenge of forming molten droplets, then controlling RM Select, is provided (installation fi le and manual deposition and solidifi cation characteristics has kept are available (1, 2)). This information is particularly these material systems in the research arena. A section pertinent to those interested in production applications on the fl uid mechanics of droplet formation and jetting of AM. is included in a process model for the MJ process (a section which is also relevant to Chapter 8). Post-processing

Binder Jetting AM is often viewed as a complete process and therefore, to the uninitiated, it can be a surprise to learn of the BJ methods were primarily developed at Massachusetts requirement for post-processing. The book introduces Institute of Technology (MIT), USA, during the early the various types of support structures required to 1990s and feature a powder bed similar to that of the achieve and maintain all levels of geometric complexity PBF process (Chapter 5). However, instead of using and the subsequent post-processing methods and an energy source to fuse the material together, an design considerations to aid their removal. Depending inkjet head deposits a binder onto each layer to form on the application of the part, surface fi nishing may part cross-sections. The binder forms agglomerates also be required. In this case it may be necessary to with the powder particles and provides bonding design extra material in the areas to be post-processed with the layer below. The chapter describes some to ensure the desired dimensions are maintained. commercial materials available, and notes that many Powder bed methods possess ‘natural’ supports of them require post-processing to increase strength, where the built object is encased in excess material. for example by infi ltration with another material. They are named as such as they are an intrinsic part of Materials include polymers (for example, poly(methyl the process. The main disadvantage of natural supports methacrylate) (PMMA)), ceramics, foundry sands and is that the part must be designed so that the excess metals such as 420 stainless steel or Inconel alloy 625 material can be removed, for example for powder bed infi ltrated with bronze. This technology can be scaled methods parts that are designed to be hollow must up quite easily, as demonstrated by machines with have an escape hole for loose powder. In this case the an incredible 4 × 2 × 1 m build volume. Although the excess material is recycled as much as possible with subject of ink drop formation is covered in the previous some sieving usually required. chapter, a discussion surrounding ink interaction with ‘Synthetic’ supports are required for methods that the powder bed is missing here. Disappointingly for a do not have ‘natural’ supports or for methods that process described as one of the most readily scalable, have stresses as an intrinsic part of the process, for the chapter is one of the shortest – although some example powder bed fusion parts need to be tethered aspects, such as powder handling, are discussed in to the substrate to prevent warping from stresses other chapters. created by the presence of signifi cant temperature gradients. It is noted that these stresses can be Guidelines for Process Selection and Ancillary reduced, and therefore the requirement for synthetic Process Tasks supports is decreased, by raising the temperature of the build environment. For MJ and materials extrusion The remainder of the book provides outlines and methods the support structures can be constructed information on software applications and physical from the build material or a secondary material. In the processes necessary for gaining maximum advantage case of secondary materials a second ‘printhead’ or from AM. a purge cycle is required to prevent contamination. If the ‘synthetic’ supports are constructed of the build Process Selection material then they must be designed such that they can be easily removed in post-processing, it is common There are software applications where the performance for printer software to include automatic support attributes of each AM technology option are weighted generation. Alternatively, the ‘synthetic’ supports can

195 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688406 Johnson Matthey Technol. Rev., 2015, 59, (3) be constructed of a secondary material with differential with materials properties defi ned for each. As yet, solubility (with respect to the build material) and can there is not an agreed upon standard fi le format for simply be dissolved away post-process. multi-material objects. Post-processing is an integral procedure of AM and understanding the many factors to streamline and Design Considerations safeguard the integrity of the printed part is paramount. Particular focus is made on the use of designed Software Issues and Considerations cellular structures and void fi lling. These structures can be symmetrical or conformal (with variable cell The fi le standard for AM technologies is the STL fi le size to appropriately fi ll the void space). The primary format. The origin of this fi le extension hails to the advantage of using cellular structures is the reduction fi rst stereolithography machines commercialised by of the required build material without compromising 3D Systems Inc, USA. This format is expressed as overall strength. Cellular structures are a particular a list, either in binary or American Standard Code for strength of AM processes given the fact that the Information Interchange (ASCII) (text), of the vertices entire structure can be designed, and with the help of triangular facets used to approximate the surface of of software applications, optimised to achieve the the digital part. This approximation is most relevant, most effi cient strength-to-weight properties. The with respect to deviation from the true geometry, when output cannot always be reproduced but a near there are a signifi cant number of curves in the part. The approximation can be achieved with little compromise orientation of the facets is defi ned by the unit normally in performance. Examples of topology optimisation expressed in vector coordinates. software given are Abaqus from Dassault Systèmes Due to the nature of the process, i.e. layer-by-layer, and OptiStruct from Altair. there is also the requirement to ‘slice’ the fi le prior to initiating the printing process. The generation of the Applications support structures may be done before or during the slicing operation. This section offers a fl avour of the multitude of possible One problem with STL, unlike other CAD formats, is applications of AM technologies. ‘Rapid tooling’ refers that there are no units associated with the fi le itself. to the use of AM to create production tools. Typically Therefore the scale/dimensions must be checked to these are reusable moulds, impressions or patterns ensure the correct dimensions are applied. This can be from which the tools can be created. This method may particularly important for international communications, be applied, in particular, if the material required for part for example USA to EU or vice versa. Furthermore, it production is not currently available in an AM technology should be noted that not all graphics software packages or the mould can be improved upon by utilising the can create STL fi les with adequate accuracy. One design freedoms of AM, for example conformal cooling common issue when converting to STL is the creation channels for injection moulds. Medical and aerospace/ of parts with holes (i.e. incomplete surfaces), which automotive applications have received signifi cant are not compatible with AM machines. More modern attention from the implementation of AM technologies. dedicated AM software packages are emerging, Medical procedures can be streamlined by developing making this issue less prevalent. surgical guides and tools. Prostheses and implants are Colour and materials properties are another factor to of particular interest for the application of AM as they include into the defi nition of a part to print. For coloured can be customised on a case-by-case basis. Signifi cant parts this can be done by colouring of individual facets research efforts are currently being employed to print with solid colour (colour STL) or an image (virtual with living cells and biomaterials for direct transplant. reality modelling language (VRML)). The advantage Another method is to print a biocompatible scaffold for of applying an image is that the applied colour is not post-process treatment with living cells to encourage limited by the resolution of the facet. As the STL fi le natural material growth, for example Osteopore format is surface data only there is the assumption produces ‘bioresorbable’ implants to encourage natural that the underlying material is homogeneous. For bone growth over trephination holes from neurosurgery. AM technologies capable of multi-material printing Aerospace applications of AM have been quite the fi le typically has to be defi ned as distinct objects diverse with examples of engine system and

non-structural components. The primary beneﬁ t for the “Additive Manufacturing aerospace industry is improved fuel efﬁ ciency by better Technologies: 3D Printing, performance and lighter weight components. Rapid Prototyping, and Direct Digital Manufacturing”

Business Opportunities and Future Directions

The book is rounded out by highlighting the fact that AM offers genuinely new avenues for production and product development and thus adjusted business models and practices could be required to accommodate the rapidly changing landscape of manufacturing.

Conclusion

Readers desiring a comprehensive introduction to the many technologies of AM should be satisﬁ ed. Although it is aimed primarily at students and educators, the authors do very well to appeal to those in research and References manufacturing positions too. Excellent explanations 1 D. Rosen, ‘RM Selection, Software User Manual’, of basic concepts through to the state-of-the-art make Version 1, Georgia Institute of Technology, Atlanta, this a great starting point for in-depth research, whilst USA, 12th August, 2005 the process selection tools and business opportunities 2 The Georgia Institute of Technology: The Systems chapters will be very useful for manufacturers looking Realization Laboratory: http://www.srl.gatech.edu/ to explore this technology. Members/drosen (Accessed on 28th May 2015)

The Reviewers

Jonathan Edgar received a BSc and PhD in nanotechnology from the University of Technology, Sydney, Australia. Currently he is working for Johnson Matthey on a core science and materials development project using additive manufacturing technologies.

Saxon Tint has an MEng in Materials Science and Engineering from Imperial College London, UK. He joined Johnson Matthey Noble Metals in 2011 as a Materials Scientist and has worked on a range of projects including alloy development for spark plug tips, powder metallurgy and AM.

As a FTSE100 company that develops advanced materials and specialises in precious metals, Johnson Matthey has a strong interest in precious metal and speciality metal powders. If you have an interest in precious metal powders or speciality metal powders for additive layer manufacturing, 3D printing or other applications we’d be keen to hear from you. Please contact Alexandra French, Sales and Marketing Director, Noble Metals on: [email protected] or +44 (0) 1763 253856 / +44 (0) 7968 568532

For more information about Johnson Matthey’s expertise as well as its investment in research and development, please visit the corporate website: http://www.matthey.com/

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Temperature Dependent Heat Transfer Performance of Multi-walled Carbon Nanotube-based Aqueous Nanofluids at Very Low Particle Loadings

Investigating the mechanism of thermal conductivity enhancement

By Meher Wan sodium dodecyl sulfate (SDS) was used as a surfactant Department of Metallurgical and Materials to minimise the agglomeration of the MWCNTs. An Engineering, Indian Institute of Technology, effective enhancement in thermal conductivity was Kharagpur-721302, India observed at different temperatures. The obtained results are explained with percolation theory. Raja Ram Yadav Department of Physics, University of Allahabad, 1. Introduction Allahabad-211002, India Heat transfer management is becoming increasingly Giridhar Mishra and Devraj Singh* critical in the infrastructure, industry, transportation, Department of Applied Physics, Amity School of defence and aerospace sectors. Several cooling Engineering and Technology, methods have been investigated recently to meet An affiliated institute of Guru Gobind Singh the heat transfer requirements of 21st century high- Indraprastha University, technology, high density heat producing industrial Bijwasan, New Delhi-110061, India equipment. Water, ethylene glycol and mineral oil have been used as conventional coolants for different *Email: [email protected] industries. Conventional heat transfer systems used in a wide range of applications, including petrochemicals, Bipin Joshi refining, power generation and microelectronic devices, Department of Science and Technology, Technology are rather large and involve a significant amount of heat Bhavan, New Mehrauli Road, New Delhi-110016, India transfer fluids in comparison to biomedical engineering based applications (1). Managing high density heat generation in microelectronic industries is a good Aqueous suspensions of multi-walled carbon example (2, 3). nanotubes (MWCNTs + deionised water) have been Modern, small scale cooling applications require very synthesised. Carbon nanotubes (CNTs) were derived effective coolants. Existing heat management systems by chemical vapour deposition (CVD). Transmission can be improved by enhancing the performance of heat electron microscopy (TEM) measurements show the transfer fluids through the use of nanofluids, resulting in formation of MWCNTs. Three samples of CNT-based lower heat exchange surface area, lower capital costs aqueous nanofluids having MWCNT concentrations of and higher energy efficiencies (4). Several techniques 0.01 vol%, 0.03 vol% and 0.05 vol% were prepared with have been investigated to enhance the thermal exchange the help of ultrasonic irradiation. A very small amount of performance of the fluids. Out of many tried methods,

199 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688163 Johnson Matthey Technol. Rev., 2015, 59, (3) one is to add a very small percentage of nanoparticles salen was found to produce CNTs via a tip-growth having high thermal conductivity into heat transfer mechanism. Small Ni particles were observed at the fluids to improve their overall thermal conductivity tips of the CNTs which were otherwise free of Ni. Since (5–8). Such additives may include novel metals, metal Ni is used as a catalyst in relatively small amounts and oxides and carbon nanostructures. Nanoparticles is embedded inside the CNT tips, its contribution to the may possess either spherical, cylindrical, fibril or thermal conductivity enhancement can be ignored. The sheet-like structures. Cylindrical carbon nanostructures MWCNTs have an average outside diameter of 30 nm, (4, 9, 10) or CNTs have very high thermal conductivity and a length of several micrometers, as observed by of the order of 3000 W m–1 K–1 (11). MWCNTs have electron microscopy (field emission scanning electron very exotic physical properties and are relatively easy microscopy (FE-SEM) and TEM) (Figures 1(a) and to synthesise. They have been shown to be promising 1(b)). additives in conventional heat transfer fluids for diverse Nanofluids were prepared using a two step method. heat transfer applications (9, 12). Several researchers First the required amounts of MWCNTs and deionised have tested different combinations and permutations water needed for the sample preparation were of CNTs, the effects of their aspect ratios (diameter determined. Then a very small amount of surfactant and length) and base fluids on thermal conductivity (SDS) was dissolved in the liquid matrix, followed in CNT-based nanofluids (13). For a more uniform by appropriate amounts of MWCNTs for synthesis of dispersion of CNTs, the effects of different quantities different concentrations of nanofluids. SDS was used of surfactant materials on thermal conductivity have to minimise the agglomeration of nanotubes in the also been studied (14). However the addition of a large base fluid. The samples having MWCNTs were stirred weight percentage of filler in a fluid affects its viscous using a magnetic stirrer and were ultrasonicated for properties. Higher viscosity does not support better 20 minutes with 100 W intensity at 20 kHz ultrasonic heat conduction due to the need for higher pumping frequency (Sonics VC 505). As the probe sonicates power as well as changes in other fluidic properties. within a limited conic volume, to facilitate uniform The present work is focused on the preparation dispersion, sonication was followed by 10 minutes of nanofluids by dispersing MWCNTs in deionised magnetic stirring. According to the density data water and measuring the temperature dependent provided by Jana et al. (18), 0.026 g, 0.078 g and thermal conductivity of the nanofluids at very small 0.130 g of CNTs were dispersed in 100 ml of deionised particle loadings. The experimental observations water to prepare nanofluids with 0.01 vol%, 0.03 vol% and regarding thermal conductivity enhancements can 0.05 vol% CNTs. 100 ml suspensions of each be explained with the help of appropriate theoretical composition were prepared. models. It has now been established that when CNTs Thermal conductivity of the synthesised nanofluids are suspended in conventional heat transfer fluids, was measured with a Hot Disk TPS 500 Thermal anomalous enhancements in thermal conductivity are Constant Analyser which works on the temperature observed (15, 16). The motivation behind the present coefficient of resistance principle. The transient study is to find the mechanism of thermal conductivity plane source (TPS) method is an updated version enhancement at very small concentrations of MWCNTs of the transient hot wire (THW) technique (19). The in suspension and the effects of MWCNT aqueous uncertainty in the measurements in this method nanofluids on thermal performance at a range of is ±4%. TPS overcomes many of the drawbacks of temperatures. It is assumed that the viscous properties THW due to its sensor structure and shape. TPS of the base fluid do not change with the inclusion of has a computer controlled temperature controller for very low CNT loadings. accurate temperature settings and precise results at typical temperatures. The TPS based instrument 2. Materials and Methods can be used to measure thermal constants such as conductivity and diffusivity via a Ni sensor, using Deionised water and MWCNTs were used to produce the temperature coefficient of resistance. Inside the nanofluids. The MWCNTs were synthesised by CVD using instrument, the TPS element behaves both like a nickel salen [N,N’-ethylene-bis(salicylideneiminato)]- temperature sensor and a heat source. TPS uses nickel(II); Ni(C16H14N2O2) as a catalyst. The details the Fourier law of heat conduction as its fundamental of the CNT growth are described elsewhere (17). Ni principle for measurements.

(a) (b)

200 nm 0.2 µm

Fig. 1. (a) FE-SEM images of the as-grown MWCNTs; (b) TEM of the MWCNTs

3. Results and Discussion be claimed that the percolation threshold exists below 0.03 vol%, leading to the conclusion that the CNTs do Thermal conductivity measurements were taken for all not have continuous chains from one end to the other the nanofluid samples at temperatures from 10ºC to below a certain particle loading, as predicted by Sastry 80ºC. The experimental data for thermal conductivity et al. (20). of the nanofluid samples are presented in Figure 2. Although a number of models based on different For comparison, the temperature dependent thermal mechanisms are available currently in the literature, conductivity data of the base fluid (water) are also given. no theoretical model is able to explain the anomalous It can be seen that there is no significant increase in the thermal conductivity enhancement observed thermal conductivity with the suspension of 0.01 vol% experimentally due to controlled and uncontrolled MWCNTs. On the other hand, there is a significant parameters in the experimental setup (21). Some anomalous increase in the thermal conductivity of models, for example the Sastry model, are in reasonable water with suspension of 0.03 vol% CNTs. Thus, it can agreement with the experimental data under different assumptions and conditions. A critical analysis of the theoretical models has been carried out by Lamas et al. (21) who concluded that the available models 1.5 show a negative effect of temperature on thermal DI-water + SDS 1.4 conductivity enhancement in these systems. However, –1 0.01 vol% K this is contradictory to the available experimental –1 1.3 0.03 vol% 1.2 0.05 vol% results. The agreement of theory and experiment was 1.1 achieved by adjusting parameters such as the CNT 1.0 geometry (22) and the value of interfacial conductance 0.9 (23). These theoretical models had various degrees 0.8 of empiricism and provided a limited physical insight 0.7 into the experimental observations. In past studies,

Thermal conductivity, W m Thermal conductivity, 0.6 experimental data have showed that the dispersion 0.5 technique and interaction between the CNTs and 0 10 20 30 40 50 60 70 80 90 the base liquid also play a strong role, causing the Temperature, ºC enhancement to vary by as much as 7%–40% for

Fig. 2. Thermal conductivity of nanofluids containing CNTs at water-MWCNT nanofluid (24). different temperatures For the present study, very low concentrations were chosen for two reasons. Firstly, to maintain the fluidic

201 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688163 Johnson Matthey Technol. Rev., 2015, 59, (3) or viscous properties of the base fluid; and secondly to L L sincφθos where iP, ; R = iP, iiand investigate the existence of a percolation threshold to RCNTiP, = Fi,P KACNT CS KAliq liq explain anomalous thermal conductivity enhancements 1 RC = 2 as predicted by Sastry and his coworkers. A number of GdN physical mechanisms have been proposed to explain Finally effective thermal conductivity can be written as the thermal conductivity enhancement of a nanofluid. L An early concept was interfacial layering: the formation cell keff = ; where Lcell is the length of the cell and of a solid-like, liquid molecular layer close to the CNT RAnet Cell interface, which has much higher thermal conductivity Acell is the effective area of the cell in which the than the bulk liquid itself (25). Alternatively, due to the percolation exists. The thermal conductivity high aspect ratio of CNTs, interaction between them is enhancement will not be observed below a certain thought to be highly probable resulting in the formation particle loading, where the formation of chains of CNTs of a network. The phenomenon is often termed does not occur. The enhancement in thermal percolation. The formation of a highly conductive conductivity for 0.01 vol% at higher temperatures is heat flow path in the liquid by the percolation network due to Brownian motion, which dominates the can potentially explain the enhancement due to percolation chain mechanism at high temperatures. concentration of CNTs in relatively low temperature The percolation mechanism can be applied for zones where Brownian motion is not dominant. A mechanically stable suspensions only. At high critical volume fraction of MWCNT loading, called temperatures, suspensions become less stable due to the percolation threshold, was thought to exist above Brownian motion. which the electrical conductivity of the MWCNT Figure 3 shows the variation of thermal conductivity nanofluid would abruptly increase multifold. However, enhancement ratio (Keff/Kliq) with temperature at different initial experiments did not show any such sudden rise MWCNT loadings. Thermal conductivity ratio is considered in the effective thermal conductivity of the nanofluid. as a much more suitable parameter to understand the Thus it was concluded that there is no percolation enhancement in thermal conductivity, irrespective of threshold in thermal transport in those experiments thermal conductivity of the base fluid or liquid in which (25, 26), a view that was contradicted recently MWCNTs are dispersed to form nanofluids. (20). Sastry et al. considered percolation without The present experimental study was carried specifically approaching it from a threshold perspective out in the temperature range 10ºC to 80ºC. The (20). Their approach relies on the basic premise of thermal conductivity was observed to increase with three-dimensional MWCNT chain formation in the liquid temperature, contradictory to predictions made by (percolation) and consideration of a thermal resistance network with higher resistance contact points of CNTs and very low resistance CNT channels. Random CNT 1.55 liq orientation and CNT-CNT contacts are introduced /K 1.50

eff 0.01 vol% by probability density function. They considered a K 1.45 0.03 vol% cubical volume of nanofluid formed by dispersion 1.40 0.05 vol% of CNTs. The fundamental basis of the model is an 1.35 Theoretical curve (Prasher et al.) interaction between the CNTs touching each other 1.30 1.25 to form percolating chains in the suspension. In this 1.20 mechanism, resistive points are formed at the contact 1.15 centres of the CNTs. Thus, a series of resistive contact 1.10 points offer resistance in the thermally conductive 1.05 medium of a CNT chain. The net resistance has been 1.00

calculated by Sastry et al. (20) as given in Equation (i): Thermal conductivity enhancement, 0.95 0 10 20 30 40 50 60 70 80 90   RR+ 2   CNTiP, C Temperature, ºC    × RFi,p  N  M  R =   (i) net ∑   Fig. 3. K/K of nanofluids containing CNTs at different i =1  RRCNTiP, + 2 C  f   + RFi,p  temperatures  M  

202 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688163 Johnson Matthey Technol. Rev., 2015, 59, (3) available theoretical models (21). The temperature K ()12++αφ21()− α  (v) effect on thermal conductivity of nanofluids can be =   Km  ()12+−αφ()1− α  explained with the help of Brownian motion. The thermal conductivity of the CNT-water nanofluids was therefore which is based on the traditional Maxwell-Garnett calculated to examine the Brownian motion effect. (MG) model where f is the particle volume fraction, Prasher et al. (27) developed a model for water based α = 2RKbm/ dN , and Rb is the interfacial resistance. spherical nanoparticle nanofluids. Based on Prasher’s The effective K of the nanofluid can be written as approach, the present experimental observations were Equation (vi): modelled for CNT containing water-based nanofluids K Re ×Pr ()12++αφ21()− α  (vi) at high temperatures. Since the CNTs have very small =+(1 )  K 4 ()12+−αφ()1− α diameters when suspended in the liquid, Brownian f   movement of the CNTs is quite possible. The root- mean-square velocity (vN) of Brownian particles can be Equation (vi), together with the definition of Re given defined (18) as (Equation (ii)): in Equation (iii) has all the necessary ingredients for predicting K, because it includes: 3kTB 1 18kTB (ii) vN = = (a) conduction contribution of the CNTs mdNNπρNNd (b) Rb between the nanotubes and the water

(c) convection contribution (effective for smaller dN). where kB is the Boltzmann constant, T the temperature, So far no empiricism has been included in the model. mN the particle mass, ρN the density, and dN the average Taking the clue from correlations for particle-to-fluid diameter of the CNTs. heat transfer a general correlation for h of the form: m 0.333 Now we consider the effect of the convection of the h = (Kf/a) [1+ A Re Pr φ] is proposed for the Brownian liquid near the CNTs due to their Brownian movement. motion induced convection from multiple nanotubes,

The Reynolds number (Re) based on νN given by where A and m are constants. Convective heat transfer Equation (ii) can be written as Equation (iii): relations are regime dependent, and so depending on

Re, these relations can change. Therefore it is most 1 18kT R = B (iii) likely A should be independent of the fluid type whereas e νπρ d NN m depends on the fluid type. This modification leads to Equation (vii): where n is the kinematic viscosity of the liquid. K ()12++αφ21()− α  m 0.333φ (vii) The Re for 30 nm CNTs in water, Re = 0.0132, is very =+()1 ARe Pr   K ()12+−αφ()1− α small and therefore for convection the flow falls in f   Stokes regime. If a particle is embedded in a semi- infinite medium of thermal conductivity Km, then the If Equation (vii) is valid, A and m should be the same for Nusselt number (Nu) based on the radius of the CNTs different experimental data for a particular fluid. can be shown to be 1; i.e., h = (Km/a). In Stokes regime Figure 3 shows the semi-empirical model for K of ‘h’ is given (28) as h = (Kf/a)[1+ (1/4)Re + Pr] where Pr various water based nanofluids (different f), taking Rb is the Prandtl number, ‘a’ is the radius of the particle = 2.5 × 10–8 Km2 W–1, m = 2.4 and A = 40,000 (as and ‘h’ is the heat transfer coefficient. Note these in similar materials) for the best fit. Here in the model relations are derived analytically from first principles. a thermal boundary layer δT = 3 δF/Pr is arbitrarily This means that the effective K of the fluid due to defined, where Pr is the Prandtl number and δF is the convection caused by the movement of a single the diameter of the base fluid molecule. Here Pr = sphere is: 10 has been taken. Nusselt number (Nu) is given as 2 2 α = Nu ~ Re Pr ; 2RKbm/ dN = 0.967 has been Km = Kf[1+ (1/4)Re ×Pr] (iv) computed; Km = thermal conductivity of water. Re has Note that this is based on a single isolated CNT. In a been computed as 0.0132. real system there will be interaction in the convection It is important to note that in the present case currents from different CNTs. The value of Km is an increase in the viscosity at higher temperature substituted from Equation (iv) in Equation (v): increases Brownian motion. Figure 3 reveals

203 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688163 Johnson Matthey Technol. Rev., 2015, 59, (3) significant enhancement in the thermal conductivity of observations, it can be claimed that the percolation water containing a suspension of 0.05 vol% of CNTs threshold is between 0.01 and 0.03 vol% CNT loadings at different temperatures, in good agreement with in base fluid. Thermal conductivity enhancement the theoretically modelled curve based on order of increases with temperature and concentration of magnitude calculations. CNTs in the base fluid. The temperature dependence The temperature variation of the enhancement of the of the thermal conductivity enhancement is explained thermal conductivity (K/Kf) on experimental observation well with Brownian motion theory as chains of CNTs is also in good agreement with the theoretical curve will not form at higher temperatures due to instability within the measurement accuracy of the apparatus. imposed by the temperature increase. These MWCNT Thus, it may be concluded that convection induced due + deionised water nanofluids are very stable with time. to Brownian movement of the CNTs is the main cause They therefore show potential to be used as coolants in for the observed enhancement in K of the nanofluid at different fields of industry. higher temperatures in the present investigation. The interfacial resistance Rb has also been incorporated Acknowledgements as interfacial resistance greatly depends on bonding between the solid and the liquid (27). Meher Wan is thankful to the Council of Scientific and The shelf life of the nanofluids was also investigated. Industrial Research (CSIR) of the Government of India In Figure 4 the temporal stability of nanofluid containing for a senior research fellowship to carry out the work. 0.03 vol% CNTs is shown at room temperature. The nanofluid is stable for a long time. References 1 D. G. Cahill, P. V. Braun, G. Chen, D. R. Clarke, 0.80 S. Fan, K. E. Goodson, P. Keblinski, W. P. King, G. D. Mahan, A. Majumdar, H. J. Maris, S. R. Phillpot, E. Pop and L. Shi, , 2014, 1, –1 Appl. Phys. Rev. K

–1 0.75 (1), 011305 2 Z. Ling, Z. Zhang, G. Shi, X. Fang, L. Wang, X. Gao, Y. Fang, T. Xu, S. Wang and X. Liu, Renew. Sustain. 0.70 Energy Rev., 2014, 31, 427

CNT nanofluid (0.03 vol%) 3 T. Dixit and I. Ghosh, Renew. Sustain. Energy Rev., 0.65 2015, 41, 1298 4 R. Taylor, S. Coulombe, T. Otanicar, P. Phelan, A. Thermal conductivity, W m Thermal conductivity, Gunawan, W. Lv, G. Rosengarten, R. Prasher and H. 0.60 Tyagi, J. Appl. Phys., 2013, 113, (1), 011301 0 5 10 15 20 25 30 Time, days 5 R. Saidur, K.Y. Leong and H. A. Mohammad, Renew. Sustain. Energy Rev., 2011, 15, (3), 1646 Fig. 4. Thermal conductivity of nanofluid containing 0.03 vol% CNTs at different times 6 S. Özerinç, S. Kakaç and A. G. Yazıcıoğlu, Microﬂuid. Nanoﬂuid., 2010, 8, (2), 145 7 S.-H. Lee and S. P. Jang, Int. J. Heat Mass Transfer, 2013, 67, 930 4. Conclusions 8 O. Mahian, A. Kianifar, S. A. Kalogirou, I. Pop and S. Wongwises, Int. J. Heat Mass Transfer, 2013, 57, It can be concluded that the observed thermal (2), 582 conductivity enhancement in prepared nanofluids is 9 Z. Haddad, C. Abid, H. F. Oztop and A. Mataoui, due to the suspension of CNTs in deionised water. Int. J. Therm. Sci., 2014, 76, 168 Sudden enhancement is observed at a concentration of 0.03 vol% CNTs in water at lower temperatures, which 10 C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Millán, V. S. Amaral, F. Palacio and L. D. Carlos, , is attributed to formation of CNT chains with very high Nanoscale 2013, 5, (16), 7572 conductivity in the medium. The abrupt enhancement of thermal conductivity at 0.03 vol% can be explained 11 H. Maruyama, R. Kariya and F. Arai, Appl. Phys. with the help of percolation phenomena. From Lett., 2013, 103, (16),161905

12 S. Halelfadl, T. Maré and P. Estellé, Exp. Therm. Fluid 21 B. Lamas, B. Abreu, A. Fonseca, N. Martins and M. Sci., 2014, 53, 104 Oliveira, Int. J. Therm. Sci., 2014, 78, 65 13 S. M. S. Murshed and C. A. Nieto de Castro, Renew. 22 W. Yu and S. U. S. Choi, J. Nanopart. Res., 2004, 6, Sustain. Energy Rev., 2014, 37, 155 (4), 355 14 P. Estellé, S. Halelfadl and T. Maré, Int. Commun. 23 L. Xue, P. Keblinski, S. R. Phillpot, S. U.-S. Choi and Heat Mass, 2014, 57, 8 J. A. Eastman, Int. J. Heat Mass Transfer, 2004, 47, 15 Y. Ding, H. Alias, D. Wen and R. A. Williams, Int. J. (19–20), 4277 Heat Mass Transfer, 2006, 49, (1–2), 240 24 M. J. Assael, I. N. Metaxa, K. Kakosimos and D. 16 S. U. S. Choi, Z. G. Zhang, W. Yu, F. E. Lockwood and Constantinou, Int. J. Thermophys., 2006, 27, (4), 999 E. A. Grulke, Appl. Phys. Lett., 2001, 79, (14), 2252 25 M. Foygel, R. D. Morris, D. Anez, S. French and V. L. 17 J. Sengupta, A. Jana, N. D. P. Singh, C. Mitra and C. Sobolev, Phys. Rev. B, 2005, 71, (10), 104201 Jacob, Nanotechnology, 2010, 21, (41), 415605 26 M. J. Biercuk, M. C. Llaguno, M. Radosavljevic, J. K. 18 S. Jana, A. Salehi-Khojin and W.-H. Zhong, Hyun, A. T. Johnson and J. E. Fischer, Appl. Phys. Thermochim. Acta, 2007, 462, (1–2), 45 Lett., 2002, 80, (15), 2767 19 S. E. Gustafsson, Rev. Sci. Instrum., 1991, 62, (3), 797 27 R. Prasher, P. Bhattacharya and P. E. Phelan, Phys. , 2005, 94, (2), 025901 20 N. N. V. Sastry, A. Bhunia, T. Sundararajan and S. K. Rev. Lett. Das, Nanotechnology, 2008, 19, (5), 055704 28 T. D. Taylor, Phys. Fluids, 1963, 6, 987

The Authors

Dr Meher Wan obtained his DPhil from the Physics Department, University of Allahabad, India. He is currently working as research fellow in the Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Kharagpur, India. His research interests are thermal conductivity, phonon transport in nanostructured composite materials and non-destructive techniques for characterisation of materials including ultrasonics. He has published several important studies on heat transfer phenomena of nanofluids in internationally reputed journals.

Professor Dr Raja Ram Yadav is presently Professor of Physics at the Department of Physics, University of Allahabad. His research interests are in the non-destructive ultrasonic and thermal characterisation of nanomaterials, lyotropic liquid crystalline materials, intermetallics and semiconductors; the development of nanomaterials for biomedical applications; and theoretical calculations of nonlinear elastic and ultrasonic properties of crystalline materials. He was awarded the prestigious Indian National Science Academy (INSA) Teachers’ Award for the year 2012.

Dr Giridhar Mishra is Assistant Professor at the Department of Applied Physics, Amity School of Engineering and Technology, New Delhi, India. He obtained his DPhil in Physics from the University of Allahabad. He has worked as a Research Fellow in a project sponsored by the Department of Science and Technology, New Delhi, in the field of materials science. His current research interests are focused on the study of ultrasonic and thermal properties of nanofluids, nanomaterials and other materials. He is a life member of the Indian Association of Physics Teachers (IAPT).

Dr Devraj Singh is Assistant Professor and Head of the Department of Applied Physics at Amity School of Engineering and Technology. His research interests are in the ultrasonic non-destructive characterisation of condensed materials. Presently, he is working on ultrasonic studies of rare earth materials for engineering applications.

Dr Bipin Joshi is a Scientist at the Department of Science and Technology, Government of India, New Delhi. His research area is optical characterisation of nanostructured fabric semiconductors. He is a life member of various scientific societies.

The Effects of Hot Isostatic Pressing of Platinum Alloy Castings on Mechanical Properties and Microstructures

Post processing of parts for jewellery and other applications

Teresa Fryé* Introduction TechForm Advanced Casting Technology, 5558-D SE International Way, Portland, Oregon 97222, USA In earlier research a better understanding of the solidifi cation characteristics for a number of platinum- *Email: [email protected] based alloys was established (1). The fi ndings demonstrated a strong tendency toward the formation Joseph Tunick Strauss of shrinkage and gas porosity upon solidifi cation, and HJE Company, 820 Quaker Rd, Queensbury, HIP, a high-pressure thermal treatment developed as New York 12804, USA a densifi cation process, was proven to be an effective method to minimise or eliminate this porosity. Jörg Fischer-Bühner While the previous results made it clear that porosity Indutherm Erwärmungsanlagen GmbH, 75045 had been signifi cantly reduced following the HIP Walzbachtal-Wössingen, Germany; and Legor Group process, the authors had not yet explored the full SpA, Via del Lavoro, 1 36050 Bressanvido (VI), Italy range of HIP’s effects in terms of post-processed microstructure and mechanical properties. The Ulrich E. Klotz goal of this new phase of research is to further our fem Research Institute for Precious Metals & Metals understanding by characterising the post-HIP effects on Chemistry, Katharinenstraße 17, 73525 Schwäbisch platinum based castings with respect to grain size and Gmünd, Germany shape, chemical distribution and mechanical strength.

Overview of Hot Isostatic Pressing The effects of hot isostatic pressing (HIP) on castings produced in a variety of platinum alloys was investigated. Companies that build HIP equipment or perform the HIP A number of beneﬁ ts were observed, including process describe an isostatic press as something that a reduction in porosity and improvements to the forms and densiﬁ es powdered and cast materials using microstructure and mechanical properties. Differences liquid or gas under extremely high pressure. Unlike in the response to HIP of individual alloys is evaluated mechanical force which compresses a workpiece from as well as some inherent limitations of the HIP process. one or two sides, the isostatic pressure is applied

207 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688451 Johnson Matthey Technol. Rev., 2015, 59, (3) uniformly on all sides of an object, eliminating internal and is a powerful representation of the pore collapsing porosity without changing net shape. Typical product that can occur with HIP. improvements cited by the HIP industry are the To better demonstrate how the HIP process works, elimination of internal voids, improvements in product Figure 2 shows the schematic for a typical HIP unit. consistency, and improvements in the soundness and The unit contains a high temperature furnace enclosed mechanical properties of materials. The fundamental in a pressure vessel. Parts are typically placed within material change underlying these improvements is the the chamber in vertical layers with the use of graphite, attainment of a higher density material in comparison steel or ceramic shelving to maximise load capacity. with its pre-HIP condition. During operation the HIP chamber is fi rst placed under The isostatic nature of pressure in the HIP process as vacuum, followed by fl ooding with an inert gas, usually described above is key to maintaining the dimensional argon, which is used to apply the isostatic pressure. The integrity of a casting during HIP; the pressure being temperature and pressure is then ramped up and left equal on all sides lends to a uniform compression of the to dwell for a specifi ed period of time depending upon material with product dimensions typically remaining the material’s properties. Parts become densifi ed when intact. Although, as seen in Figure 1, local deformations the material’s yield strength is surpassed, creating a in the form of ‘dimpling’ can occur when the sizes of plastic fl ow that forces internal voids to collapse under internal pores are extremely large and diffusion bonding differential pressure. The internal surfaces of the voids collapses exterior surfaces inward. This diffi cult to feed diffusion bond together, increasing density and thereby thick-to-thin geometry for the channel band represents improving the material properties. HIP unit sizes span an extreme example of subsurface shrinkage porosity from small laboratory size up to large-scale industrial. The unit shown in Figure 3 is an example of a medium

(a) scale unit. (b) Not all metals will HIP effectively and the extent to which an alloy will respond to HIP is a function of its creep resistance. Creep is a solid material’s tendency to move slowly and deform permanently under stress. In metals, creep increases with temperature and starts at approximately 30% to 50% of an alloy’s melt 2 cm temperature (2).The rate of creep is a function of temperature, the material’s properties, and the amount Fig. 1. HIP dimples in a 95Pt5Ru channel band of pressure that is applied. In order to achieve optimum material properties, the parameters used in a HIP cycle

Inert Power gas Cooling pump controller Cover Compressor Cooling jacket Ceramic Pressure piece vessel Viscous Pressurised coating Vacuum pump gas (optional) Pressure Heater Thermocouple line Electric Vacuum line line Temperature Exhaust controller valve

Fig. 2. Schematic of a typical HIP unit

shape, chemical distribution, and mechanical strength. The following sections report the methods used, results and conclusions.

Test Geometry

A tapered test specimen was chosen to assess microscopic porosity levels, density and hardness before and after HIP. As shown in Figure 4, the test specimen was designed to promote directional solidiﬁ cation with a single heavy sprue attached to the thickest end.

(a) 23.586 Fig. 3. HIP unit, courtesy Avure Technologies, USA 1.686 4.941 must be precisely dialled in according to the needs of the alloy. Typical parameters including both metals and ceramics will generally fall into the ranges shown in Table I. (b)

Table I HIP Parameter Rangesa Parameter Typical lower Typical upper limit limit 1.917 5.682 Temperature 500ºC 1400ºC Pressure 7000 PSI 45,000 PSI Fig. 4. Tapered test specimen (units in mm) Dwell Time 2 hours 4 hours Cooling Rate 1ºC per minute 100ºC per minute Casting Parameters aCourtesy Avure Technologies The casting parameters and conditions for these trials Comparative Study of the Effects of HIP on are shown in Table II. Standard pour temperatures, Platinum Alloy Microstructure and Mechanical fl ask temperatures and fi ring curves were used. The Properties fl asks were cast using a centrifugal casting machine with induction heating and each tree contained two test The goal of this research was to characterise post- geometries for each alloy. One casting was retained for HIP effects on castings with respect to grain size and as-cast sampling and the second casting was HIPed.

Table II Casting Parameters Alloy Pour temp., ºC Flask temp., ºC Casting condition 95Pt5Ru 1870 850 1 As-cast; 1 HIPed 90Pt10Ir 1870 850 1 As-cast; 1 HIPed 90Pt10Rh 1960 850 1 As-cast; 1 HIPed 95Pt5Co 1850 850 1 As-cast; 1 HIPed Notes to Table II (a) All patterns 3D printed for dimensional precision (b) All trees were identically assembled with two samples per tree (c) All ﬂ asks air cooled identically (d) All alloys were HIPed in the same load

Effects of HIP on Microstructure as the HIP processing (without the use of pressure) is neither capable of fully closing the pores from the Given the high levels of porosity seen in 95Pt5Ru as-cast condition, nor maintaining grain size (Figure 7). and the relatively low levels seen in 95Pt5Co, these While the amount and the size of pores are clearly two alloys were chosen for the present report on reduced, grains are growing substantially during heat microstructural changes brought about by the HIP treatment. Thus, any benefi cial effect of porosity process. Figure 5 demonstrates signifi cant porosity reduction is compromised by grain growth. Based on levels in the as-cast state of 95Pt5Ru. The pores in this result, it would appear that the pressure used in this alloy are interdendritic microshrinkage pores; such the HIP process has the added benefi t of retarding pores form during the spontaneous solidifi cation of grain growth. Figure 8 demonstrates the comparative the alloy that occurs so rapidly that continued feeding grain sizes of 95Pt5Ru in the as-cast, HIPed and is not possible. The HIP process has successfully heat-treated conditions. closed these microshrinkage pores, such that the The casting sample of 95Pt5Co shows no visible microstructure is completely dense after HIPing. macroscopic or microscopic porosity in the as-cast Another important fi nding is that grain size is not condition (Figure 9). However, compared to 95Pt5Ru negatively affected by the HIP process (Figure 6). A the grains are extremely large. Their size and shape simple heat treatment to the same thermal parameters indicates a relatively slow solidifi cation process

50 m

Fig. 5. 95Pt5Ru as-cast showing numerous interdentric microshrinkage pores present in the as-cast condition and uneven grain size distribution with coarse columnar grains at the surface

50 m Fig. 6. 95Pt5Ru HIP which has dense and pore-free microstructure and even grain size distribution. HIP pressure appears to retard grain growth

50 m Fig. 7. 95Pt5Ru heat treatment using same thermal curve as HIP, showing reduction of microshrinkage porosity and heavy grain coarsening during thermal processing

(a) (b) (c)

50 m 50 m 50 m

Fig. 8. Comparative microstructures of 95Pt5Ru: (a) as-cast; (b) heat treated; (c) HIPed

50 m

Fig. 9. 95Pt5Co as-cast showing very large columnar grains growing from the surface during solidifi cation and no macroscopic or microscopic porosity in as-cast condition where a few grains were nucleated at the surface say that HIP does not provide any benefi t to 95Pt5Co of the part, which then grew into the centre. As a castings. The previous research on larger samples consequence, there was suffi cient time for feeding demonstrated a tendency of this alloy to form large and the microstructure is free of pores. Therefore, gas pores and centreline shrinkage porosity that during HIPing few changes of the microstructure were either eliminated or reduced in size by the HIP occurred in the 95Pt5Co (Figure 10). This is not to process.

50 m

Fig. 10. 95Pt5Co HIP showing no signiﬁ cant change of microstructure during HIPing

Alloy Density Certain defect types either do not respond to HIP or have a lower densifi cation response. A key limitation of Density of the test samples was measured using the process is that only porosity that is fully subsurface the Archimedes’ Principle (3) to determine levels will collapse; if pores are open to the surface of the of densifi cation achieved through the HIP process. casting in any way, they will not respond to HIP. Although the density of the as-cast samples was already This effect can be seen in the shrinkage porosity very high, HIP increased the levels to near 100%. This (Figure 11(a)). Another limitation of HIP is seen in result is impressive and effectively puts castings on a gas pores. Pores created by gas are less responsive par with wrought material. As can be seen in Table III, to HIP than shrinkage pores due to the pressure they HIP most effectively increased density in 95Pt5Ru and contain. Rather than being eliminated, the pores are 90Pt10 Rh and was slightly less effective for 90Pt10Ir, typically reduced in size by HIP as can be seen in the which we can see already starts out with higher as-cast cross-section (Figure 11(b)). density. This result correlates well with the generally lower levels of visible porosity seen in 90Pt10Ir cross- Alloy Homogeneity and its Effects on sections from the 2011 study (1). Although 95Pt5Co Segregation was not tested for density, one would expect similar fi ndings as in the case of 90Pt10Ir due to the lower Another aspect of the present study was determining levels of porosity in the as-cast state. whether there had been any change in segregation

Table III Alloy Density Results

Alloy Condition Density, g cm–3 Relative density

95Pt5Ru As-cast 20.32 98.4%

95Pt5Ru HIPed 20.62 99.9%

90Pt10Ir As-cast 21.39 99.5%

90Pt10Ir HIPed 21.48 99.9%

90Pt10Rh As-cast 19.58 98.2%

90Pt10Rh HIPed 19.89 99.7%

(a) (b)

1000 m 200 m

Fig. 11. (a) Surface connected shrinkage porosity in 95Pt5Ru; (b) sectioned Pt90Ir10 gas pore after HIP

of the alloying elements during high temperature platinum casting alloys including two that are also heat treatment or HIP. By contrasting EDX mapping covered in the present testing. It is notable that the of 95Pt5Ru in the as-cast, heat treated, and HIPed values they published for the same alloys tested here conditions, we found that Ru segregated to the primary were appreciably lower in the as-cast state. Although it is dendrites during solidiﬁ cation in a similar manner for not known why this was the case, a plausible explanation all three conditions. Thus, neither heat treatment nor might be the difﬁ culty of obtaining high quality cast test HIP changed the segregation of Ru. As can be seen bars with the technologies available in 1978. in the comparative images in Figure 12, after HIP the This relative scarcity of hard data is not so surprising dendrites have coarsened, but the microstructure is not given that sophisticated platinum casting is a relatively negatively affected because the dendrites have arms new development. It was not until the mid-1990s that that can coarsen without changing the overall size of induction machines capable of handling platinum’s high the dendrite (Figure 12(c)). temperature requirements became mainstream. Prior to that, small-scale oxyhydrogen torch melting was the Tensile Testing only method available and inconsistent quality coupled with low pour weight capacity prevented investment A literature search for mechanical properties data casting from becoming a mainstream industrial process on cast platinum alloys showed that there are few for platinum. All of that has of course changed and publications in this area, with the exception of platinum based alloys are now routinely investment microhardness values that are frequently cited by cast with induction melting methods on a global basis. jewellery industry sources. A publication from 1978 by While as-cast tensile properties of platinum alloys Ainsley and Rushforth (4) was likely one of the earliest are of keen interest in their own right, an additional to look at tensile properties from actual castings versus motivation to perform this testing was as a means to the more commonly cited mill product values. These compare the strength characteristics of as-cast versus authors published values on nearly a dozen different HIPed platinum alloys. In theory, the higher density

(a) (b) (c)

90 m 90 m 90 m

Fig. 12. (a) Pt95Ru5 as-cast; (b) Pt95Ru5 heat treated; (c) Pt95Ru5 HIPed

213 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688451 Johnson Matthey Technol. Rev., 2015, 59, (3) of HIPed product would show increased values for a number of tensile properties. Table IV outlines the testing plan that was followed to produce the data, 4.000 followed by Figure 13 depicting the test bar geometry used for the tensile tests. Tensile testing was carried out in accordance with international standards (5). 19.000 The results in Table V report values for yield strength (YS), ultimate tensile strength (UTS), elongation (ε) and reduction of area (ROA). Yield strength describes stress levels above which plastic deformation occurs 59.720 and will generally increase with decreasing grain size. Following yielding, the material work hardens by the generation of dislocations. As a consequence, the required stress for further deformation increases until the ultimate tensile strength is reached. In metals, 19.000 the UTS values will generally correlate with Vickers hardness values. 3.000 At strains above UTS, which marks the maximum of the stress-strain curve (Figure 14), the cross-section is locally reduced through necking of the sample. Further Fig. 13. Test bar geometry: simple design, heavy molten deformation is localised in the necking region and as a feed to gauge area with double end gates, directional solidifi cation from interior of bar towards outer heavy consequence the required stress for further deformation sections optimizes gauge area (units in mm) is continually decreasing until failure occurs. The total elongation (ε) indicates how much plastic deformation the material can withstand. Pores in the material will While UTS or hardness are clearly important signifi cantly reduce the elongation because they act properties to measure, they are not necessarily the as stress concentration sites. The effect of pores is most critical properties to predict failure in a broad even more pronounced on the reduction of area, which number of applications. When it comes to fatigue life, indicates how much necking occurs until the sample elongation and reduction of area are generally viewed fi nally fails. as being more important. Specifi cally, in cases where

Table IV Casting Speciﬁ cations for Tensile Bars

Alloy Pour temp., ºC Flask temp., ºC Number of test bars Casting condition

95Pt5Ru 1870 850 12 6 As-cast; 6 HIPed

90Pt10Ir 1870 850 8 4 As-cast; 4 HIPed

90Pt10Rh 1960 850 8 4 As-cast; 4 HIPed

95Pt5Co 1850 850 8 4 As-cast; 4 HIPed

Notes to Table IV (a) Total bars: 36; minimum tests required: 18 (b) Test bar locations in ‘upper’ and ‘lower’ centrifuge orientation (c) All waxes turned on lathe for dimensional precision (d) All bars identically wax assembled with double end gates (e) All bars cooled identically (f) All bars HIPed to the same parameters

Table V Tensile Properties

Alloy Condition Yield strength, Ultimate Elongation, % Reduction of Change in composition MPa tensile area,% reduction of strength, MPa area,%

95Pt5Ru As-cast 225 412 30 55 –

95Pt5Ru HIPed 236 420 39 87 +32

90Pt10Ir As-cast 219 353 33 90 –

90Pt10Ir HIPed 226 358 36 87 –3

90Pt10Rh As-cast 140 330 37 64 –

90Pt10Rh HIPed 144 333 43 89 +25

95Pt5Co As-cast 220 452 36 76 –

95Pt5Co HIPed 189 449 38 82 +6

(a) 500 UTS 400

300

200 YS Strength, MPa 100 ROA

0 0 10 20  30 40 Strain, % (b) 500

400

300

200

Strength, MPa 100

0 0 10 20 30 40 Strain, %

Fig. 14. UTS scatter of 95Pt5Ru: (a) as-cast; (b) HIPed

subsequent cold working of the material is involved, HIPing. These results correlate well with the porosity an increased ability to bend before cracking is of levels of the different alloys, and are a clear indication paramount importance. that reduction of porosity increases ductility in the The HIP treatment affects the mechanical properties alloys. of the four alloys in a different way. For all alloys the Another interesting observation came from an effect on YS and UTS is rather low. For 90Pt10Ir and analysis of UTS scatter in the sample population. The 95Pt5Co there is little effect on elongation and ROA. graphs in Figure 14 demonstrate the difference in However, for 95Pt5Ru and 90Pt10Rh a signiﬁ cant spread between the as-cast and HIPed groups. The increase in elongation and ROA is found through HIPed bars exhibit a very tight distribution, whereas the

215 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688451 Johnson Matthey Technol. Rev., 2015, 59, (3) as-cast bars are more scattered. This result correlates 95Pt5Ru, all alloys report values that are so close in the well with observations of lower porosity levels together before and after HIP conditions that any difference is with a more homogeneous grain size and structure in seen as essentially inconsequential. Even the 95Pt5Ru the HIPed samples. that shows a 12-point spread is not considered enough As stated above, reduction of area values posted to be characterised as an appreciably harder material the most impressive gains in the HIPed product. by performance. Thus we can conclude that hardness This property is of particular interest in the jewellery is not signifi cantly impacted by HIP on the platinum- industry given the substantial amount of bending and based alloys we tested. forming that is inherent in stone setting, engraving, sizing and myriad bench operations. Reduction of area Conclusions indicates a material’s ductility and is crucial to successful performance in many of these operations. Figures 15(a) The most signifi cant impact of HIP on platinum-based and 15(b) demonstrate a profound visual difference in alloys is a reduction in porosity. Reduced levels of ductility between the test bar fractures in the as-cast porosity have several associated benefi ts, including and HIPed 95Pt5Ru. a marked increase in ductility in the majority of the alloys tested without sacrifi cing strength. Of the tensile properties tested, the most impressive response was (a) found in the values for ROA, a key indicator of an alloy’s ductility. Another meaningful result demonstrated here was HIP’s effect on grain structure and size. Although further testing is needed to fully characterise this aspect, our initial research confi rms a more uniform grain size and structure in the HIPed samples without (b) any increases in grain size, at least for the alloy 95Pt5Ru. Findings also confi rm that the response to HIP is strongly impacted by the alloy’s composition. 95Pt5Ru benefi ts the most due to its higher levels of porosity present in the as-cast condition, and 95Pt5Co having Fig. 15. (a) 95Pt5Ru as-cast 55% ROA; (b) 95Pt5Ru HIPed lower porosity benefi ts the least. 87% ROA Further work is recommended to more closely assess the impact of qualitative changes in the HIPed product on manufacturing operations. Empirical Hardness Testing evidence strongly suggests greater ease in post- cast operations due to the elimination of sub-surface Table VI reports Vickers hardness values (6) for three micro-porosity and a generally more consistent of the tested alloys. With the possible exception of metallurgical condition, including uniformity of grains. In addition, the increased ductility of HIPed platinum Table VI Vickers Microhardness Results alloys should, in theory, result in a lower number of

a failures during metal bending and forming operations. Vickers Hardness HV1 Heat Alloy As-cast HIPed treated Acknowledgements 90Pt10Ir 113 111 123 The authors would like to acknowledge the original 90Pt10Rh 89 89 n/a publishers, Santa Fe Symposium on Jewelry Manufacturing Technology 2014, ed. Eddie Bell 95Pt10Ru 113 125 128 (Albuquerque: Met-Chem Research, 2014) for 95Pt5Co 126 122 n/a permission to re-publish this work. a Error: ± 3 HV1

3. “Standard Test Method for Density, Oil Content, and This research was supported in part by the German Interconnected Porosity of Sintered Metal Structural Federal Ministry for Economic Affairs and Energy (BMWi) Parts and Oil-Impregnated Bearings”, (Withdrawn under the IGF program (Project No. AiF-IGF 16413N). 2009), ASTM Standard B328-96(2003)e1, ASTM International, West Conshohocken, Pennsylvania, References USA, 2003 1. T. Fryé and J. Fischer-Bühner, ‘Platinum Alloys in the 4. G. Ainsley, A. A. Bourne and R. W. E. Rushforth, 21st Century: A Comparative Study’, in "The Santa Platinum Metals Rev.,1978, 22, (3), 78 Fe Symposium on Jewelry Manufacturing Technology 5. “Metallic Materials – Tensile Testing – Part 1: Method 2011", ed. E. Bell, Proceedings of the 25th Symposium of Test at Room Temperature”, DIN EN ISO 6892-1: in Albuquerque, New Mexico, 15th–18th May, 2011, 2009-12, Deutsches Institut für Normung e.V., Berlin, Met-Chem Research Inc, Albuquerque, New Mexico, Germany, 2009 USA, 2011, pp. 201–230 6. “Metallic Materials – Vickers Hardness Test – Part 2. “Elements of Metallurgy and Engineering Alloys”, ed. 1: Test Method”, DIN EN ISO 6507-1:2006-03, F. C. Campbell, ASM International, Ohio, USA, 2008, Deutsches Institut für Normung e.V., Berlin, Germany, p. 265 2006

The Authors

Ms Teresa Fryé has over 25 years’ experience working in the investment casting industry. She started her career at Precision Castparts Corp, USA, one of the world’s largest investment casters, serving the international customer base for high-temperature aerospace castings. In 1994 she co- founded TechForm Advanced Casting Technology, a company that specialises in shell casting of platinum group metals. She holds a BA in International Affairs and graduate studies in Psychology from Lewis & Clark College in Portland, Oregon, USA.

Joe Strauss has a PhD in Materials Engineering from Rensselaer Polytechnic Institute, USA. He has over 30 years of experience in developing atomisation and powder metallurgy technologies for non-traditional materials including precious metals for jewellery, dental, electronic and medical applications. His work includes design and fabrication of atomisation systems, research and development in powder metallurgy and engineering troubleshooting for manufacturing processes.

Dr-Ing Jörg Fischer-Bühner holds a PhD in Physical Metallurgy and Materials Technology from the technical university RWTH Aachen, Germany. He is currently active in Research & Development for Legor Group SpA (Bressanvido, Italy) as well as INDUTHERM Erwärmungsanlagen GmbH (Walzbachtal, Germany). His work has included manufacturing support, failure analysis, training and consultancy to manufacturing companies, and his research has focused on alloy properties and manufacturing technologies, especially precious metal alloys for jewellery, dental and electrical engineering applications.

Ulrich E. Klotz graduated from the University of Stuttgart, Germany, as a Diploma Engineer in Physical Metallurgy and has a PhD in Materials Science from ETH Zürich, Switzerland. He is Head of the Department of Physical Metallurgy at the fem.

“Exploring Materials through Patent Information”

By David Segal (Abingdon, Oxfordshire, UK), Royal Society of Chemistry, Cambridge, UK, 2015, 244 pages, ISBN:978-1-78262-112-6, £24.99, €31.24, US$40.00

Reviewed by Julia O’Farrelly hard drives and magnetic resonance imaging (MRI) Johnson Matthey Technology Centre, Blounts Court, scanners. The success of additive layer manufacturing Sonning Common, Reading RG4 9NH, UK (ALM), also known as additive manufacturing (AM) or ‘three-dimensional (3D) printing’ technology, depends Email: [email protected] on the availability of plastics that can be converted into molten droplets and of ceramic and metal powders that can be formulated for use in a range of different ALM The majority of books and reviews on any area of techniques. technology development tend to focus on information Chapter 1 also provides a basic introduction to published in the journal literature; reviews of the patents, covering ‘what is a patent?’ and patent patent literature are more often confi ned to the prior structure (front page data, background to the invention, art sections of patent documents. However, patents summary of the invention, embodiments, drawings remain one of the best sources of detailed technical and claims). Sections are included on patent fi ling, information, particularly where the invention may have patent infringement and patent searching. The book commercial signifi cance, and it is good to see this stresses the importance of seeking specialist advice reviewed in a book. from qualifi ed practitioners. As with much of the The author, David Segal, is a graduate of Trinity book, however, too many topics are included, making College Cambridge, UK. In his long career he has coverage superfi cial. worked widely in the fi elds of chemicals and materials Chapters 2–13 review a wide range of emerging and has published a number of patents and papers. technologies in the context of patent information: light emitting diodes, quantum dots, organic light emitting Introduction: Exploring the World of Materials diodes (OLEDs), liquid crystals and liquid crystal through Patent Information displays, ALM or ‘3D printing’, healthcare, block copolymers, aerogels, ionic liquids, fl ame retardants, In Chapter 1 Segal provides a general introduction to the graphene, hydrogels and super-hydrophobic materials. topic of materials for the non-specialist. He makes the I have chosen three of these chapters to review below. point that materials underpin many of the technological advances that we know in everyday life; for example, Additive Layer Manufacturing the use of lanthanide elements as phosphors to generate colour in display screens, or the application of Chapter 6 is titled ‘3D Printing’. Segal describes the neodymium iron boron (NdFeB) magnets in computer evolution of this process starting from Swanson and

Kremer’s photopolymerisation by computer controlled Organic Light Emitting Diodes laser beams. In fused deposition modelling, 3D parts are built up layer-by-layer from a thermoplastic Chapter 4 discusses the development of OLEDs. material using a digital representation of the part. In It starts with a review of the early development selective layer sintering a layer of metal, ceramic or of electroluminescence, for example the work by polymer powder is deposited under computer control Gurnee and Fernandez which showed that a doped and sintered by scanning a laser across the surface. In host material with a conjugated structure, such as each case the process is repeated to ultimately form a anthracene, gave off green light when placed between 3D part. The use of the term ‘3D printing’ was coined two electrodes (4). Pioneering work by Friend et al. on later by Sachs et al. and used, for example, in their conjugated polymers such as poly(p-phenylvinylene) 1993 patent (1). (PPV) formed the basis of polymer light emitting The chapter goes on to give a number of examples of diodes (PLEDs) (5). Later it was discovered that patents on applications of ALM (Figure 1). Aerospace organic phosphorescent materials, which can emit components are discussed, as are dental prostheses light in the triplet state, are potentially more effi cient and biomedical implants. Among the aerospace than fl uorescent materials which can emit only in examples, Segal describes a high temperature AM the singlet state. The example is given of the green process to make titanium near-net-shape metal leading phosphorescent emitter tris(2-phenylpyridine)iridium, edge protective strips for aerofoils (2). A key feature Ir(ppy)3. Of course the phosphorescent emitter layer is is heat transfer away from the support mandrel. In the only one of a complex series of material layers and the dental fi eld he describes the use of selective laser development of complex circuitry is an essential part of powder processing to build up a jaw structure from the realisation of a working OLED device. titanium (3). Although Segal reproduces technical details of the laser required to melt the metal particles, Graphene he does not discuss the signifi cance of such a process, which in this case is the ability to fabricate complex Chapter 12 discusses graphene, a material which has shapes without lengthy pre- or post-processing and received much interest since its discovery in 2004. the ability to customise parts to individual patient The development of methods for making it, including requirements in the same production run. wet chemical processes, vapour phase processes, Some examples are also given of components needed the use of ionic liquids and electrochemical methods, for ALM machines such as shutter mechanisms and have been key; details of these processes are best wiper blades. found in the patent literature. The chapter goes on to

500 450 400 350 led 300 250 200 150 Number ﬁ 100 50 0 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 1991 Year Fig. 1. Filing rates in additive layer manufacturing (Copyright Coller IP)

219 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688424 Johnson Matthey Technol. Rev., 2015, 59, (3) discuss potential methods for large scale production have helped to round out a promising but somewhat of graphene, essential for successful exploitation; disappointing volume. chemical vapour deposition is one such method. Incorporation of graphene fl akes into the polymer “Exploring Materials through matrix of a fi bre reinforced composite can increase its Patent Information” compressive strength, potentially useful in racquets and other sports equipment. In lithium-ion batteries, multilayer graphene fi lms have been evaluated for use as an anode and graphene platelets have been used to reinforce the matrix of a cathode material.

Conclusion

Overall, the book does effectively make the point that patents contain a wealth of technical information and should be considered alongside journals and other sources when researching a topic. It successfully gives a broad overview of the historical development of each topic, with references in the form of patent numbers References which serve as a starting point for further reading. 1 E. M. Sachs, J. S. Haggerty, M. J. Cima and P. A. The focus is very much geared to demonstrating the Williams, Massachusetts Institute of Technology, technical detail that can be found, with examples ‘Three-dimensional Printing Techniques’, US Patent of material compositions and experimental data as 5,204,055; 1993 claimed in individual patents. 2 M. W. Peretti and T. Trapp, General Electric However, this reviewer believes that it tries to Company, ‘Methods for Making Near Net Shape cover too much material in too small a volume and Airfoil Leading Edge Protection’, US Patent Appl. as a result does not really do any individual topic full 2011/0,143,042 justice. Too many patents are referenced and too much 3 J.-P. Kruth, I. Naert and B. Vandenbroucke, non-essential experimental detail from the patents is ‘Procedure for Design and Production of Implant- included. At the same time, the signiﬁ cance of each based Frameworks for Complex Dental Prostheses’, invention is often not clear without going back to the US Patent Appl. 2008/0,206,710 original patent source and this detracts from the overall 4 E. F. Gurnee and R. T. Fernandez, The Dow Chemical readability. Company, ‘Organic Electroluminescent Phosphors’, Segal’s book does not touch on how patents have US Patent 3,172,862; 1965 altered the course of development of a technology by 5 R. H. Friend, J. H. Burroughes and D. D. Bradley, impacting different companies’ freedom to exploit it; Cambridge Research and Innovation Ltd, Cambridge nor does it make any reference to how the existence Capital Management Ltd and Lynxvale Ltd, of patents has affected the commercial development of ‘Electroluminescent Devices’, US Patent 5,247, a technology. In my opinion, such a discussion would 190; 1993

The Reviewer

Julia O’Farrelly is a Principal Information Analyst in the Technology Forecasting and Information group at Johnson Matthey Technology Centre, Sonning Common, UK. She is interested in emerging technologies and their development and commercialisation and the use of patents as a tool in understanding technology landscapes.

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

“Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts”

Edited by Isabella Nova and Enrico Tronconi (Politecnico di Milano, Italy), Fundamental and Applied Catalysis, Springer Science+Business Media, New York, USA, 2014, 716 pages, ISBN: 978-1-4899-8071-7, £171.00, €239.99, US$249.00

An essay book review by Martyn V. Twigg Under these conditions TWCs cannot be used and TST Ltd, Caxton, Cambridge CB23 3PQ, UK alternative technologies were developed for the control of HCs and CO by oxidation catalysts. An undesirable *Correspondence may be sent via Johnson Matthey characteristic of older diesel engines was the black Technology Review: [email protected] soot they produced. This was considerably reduced by fuelling and combustion engineering improvements and was effectively eliminated by the use of diesel The introduction and development of catalytic control particulate filters (DPFs) which were introduced a for exhaust gas emissions from vehicles has been one decade ago. The remaining difficult challenge has of the major technical achievements over the last four been the control of NOx emissions from both light and decades. A huge number of cars were manufactured heavy duty diesel vehicles. Two technologies have during this time that provided society with a high been recently introduced to do this, though only one, degree of personal mobility and without the continuous ammonia selective catalytic reduction (SCR), appears development of emissions control technologies the to be able to provide the necessary performance atmospheric pollution derived from them would have for future demands under a wide range of driving been overwhelming. Three-way catalysts (TWC) were conditions. The present book is about diesel engine introduced on traditional gasoline powered cars in the NOx emissions control by ammonia (derived from early 1980s to control the emissions of hydrocarbons urea) SCR, and before detailing the book’s contents (HC), carbon monoxide (CO) and nitrogen oxides some background information is given which provides (NOx) and have since been developed so that today a suitable context. Because of higher exhaust gas tailpipe emissions of these pollutants can be reduced temperatures control of emissions from heavy duty by more than 99.5% and tailpipe emission levels can be diesel vehicles is less demanding than with light duty less than in the surrounding ambient air. During more ones, so the emphasis here is on diesel cars. recent years, and especially in Europe, the proportion of diesel powered cars has increased rapidly so 1. Background now about half of new European cars have a diesel 1.1 Exhaust Gas Temperature engine. Control of their tailpipe emissions has been particularly challenging because of their low exhaust The control of tailpipe emissions from vehicles powered gas temperature and the presence of excess oxygen. by traditional stoichiometric gasoline engines with

TWC is now highly advanced and can achieve almost Once diesel fuel sulfur levels were reduced from the complete removal of the three gaseous pollutants very high levels of two decades ago in Europe control CO, HCs and NOx under normal driving conditions. In of CO and HC emissions from all diesel engines by practice reductions of more than 99.5% are possible, oxidation catalysts became feasible although special and a contributing factor for such a good performance catalysts had to be developed for the low operating is the relatively high temperature of the exhaust temperature cars. gas. In contrast the control of diesel engine exhaust 1.2 Particulate Control with Filters gas emissions under lean conditions has been more problematic for two main reasons. The first problem This was followed by control of particulate matter results from the efficiency of diesel engines that under (PM or soot) by the introduction of filter technologies part load conditions can result in particularly low that enabled engine measures to further reduce exhaust gas temperatures. For example, the exhaust engine-out NOx levels without being overly concerned gas temperature of a small European car with a gasoline about increased PM that was handled by the filter engine may typically be in the region of 350ºC to 475ºC system. Traditionally the main approach for controlling in the urban part of the European test cycle, whereas NOx from small diesel engines has been via engine the same car with a diesel engine may be around measures, including the use of exhaust gas recycle 150ºC as shown in Figure 1, and designing catalysts to (EGR) and improving injection fuelling to produce ever operate efficiently at such low temperatures has been finer spraysvia multiple smaller injector nozzles and via a major challenge, particularly when fuel sulfur levels increasing fuel pressures. EGR works by decreasing were higher than they are today! the amount of oxygen in the combustion charge that Heavy duty diesel engines in trucks generally operate reduces the fuel burn rate and the peak temperatures under higher engine percentage loads and over as well as somewhat increasing heat capacity of the appropriate duty cycles they can have much higher combusting charge. exhaust gas temperatures than their passenger car There is a trade-off between NOx and PM. Reducing counterparts, typically up to around 400ºC. So here engine-out NOx normally results in an increase in PM. there is considerably more scope for catalytic emissions This is because a higher combustion temperature control. reduces PM by increased burning of residual

800

700 1.6 l Gasoline 1.8 l Diesel

600

500

400

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Inlet temperature, ºC/speed, kph 100

0 0 200 400 600 800 1000 1200 Time, s Fig. 1. Exhaust gas temperatures during the European test cycle for the same family size car equipped with a similar displacement gasoline and diesel engine. Throughout the test cycle the exhaust gas temperature is much lower with the diesel engine than that with the gasoline engine

carbonaceous PM but thermodynamically high 2M(NO3)2 → 2MO + 4NO + 3O2 (iii) temperatures favour formation of endothermic NOx (see Figure 2). This trade-off was broken by fitment of 2NO + 2CO → N2 + 2CO2 (iv) DPF that recently enabled achievement of car diesel engines with engine-out NOx levels significantly below This technology works well on smaller diesel cars, 0.1 g km–1 in the combined European test cycle. although it has temperature limitations reflecting the thermodynamic stability of the metal nitrates 1.3 NOx Control Technologies concerned. Notwithstanding the improvements just mentioned, The second NOx control technology, and the more recently NOx control has become increasingly subject of the present book, is ammonia SCR that important, driven by ever more stringent NOx emissions involves reaction of NOx with ammonia to form legislation. This legislation requires additional catalytic nitrogen and water. Ammonia SCR technology was aftertreatment to meet the NOx standards for diesel introduced on power plant applications in Japan in vehicles, especially cars, and two approaches have the early 1970s, and some twenty years later it was become established. adopted for use in heavy duty diesel vehicles that In the first of these catalytic approaches, NOx trapping, have exhaust gas temperatures appropriately high under normal driving lean conditions NO is oxidised to use the traditional vanadium-based catalysts. to NO2 as in Equation (i). This undergoes further Ammonia was derived from urea solution that was oxidation as it is stored as a metal nitrate, Equation (ii), injected into the exhaust gas where it hydrolyses followed at intervals by a reductive regeneration that forming ammonia and carbon dioxide (see converts the stored NOx to nitrogen. In this process Equation (v)). NOx is liberated usually as NO, Equation (iii), that is However, the temperatures on light duty diesel reduced over a rhodium component in much the same vehicles are too low for efficient operation of the older way as a TWC functions on a traditional gasoline car, SCR vanadium-based catalyst formulations and so Equation (iv). after much effort base metal zeolite catalysts were introduced that can operate effectively at remarkably NO + ½O à NO (i) 2 2 low temperatures and already increasingly large numbers of cars on European roads are equipped MCO3 + NO2 à MNO3 + CO2 (ii) with this SCR technology.

5000

4000

3000

2000

1000 Nitric oxide concentration, ppm

1000 1250 1500 1750 2000 2250 2500 Temperature, K Fig. 2. Concentrations of NO at high temperatures in equilibrium with nitrogen (0.8 bar) and various amounts of oxygen; the highest curve corresponds to 0.2 bar, and subsequent lower curves 0.15, 0.05 and 0.02 bar respectively. Derived from measurements made by W. Nernst (1); modern theoretical values are somewhat higher

More than thirty years ago a very important 2. Topics Covered discovery was made about the effect of the ratio of

NO2 to NO on the rate of the ammonia SCR reaction This book has 22 chapters by eminent contributors and over vanadium-based catalysts. The reaction is much is appropriately edited by Professors Isabella Nova and faster when both are present. The reactions involved Enrico Tronconi from the Politecnico di Milano, Italy, when urea ((NH2)2CO) is the source of ammonia are: whose Laboratory of Catalysis and Catalytic Processes urea hydrolysis to give ammonia, shown overall in (LCCP) has a worldwide reputation for research on the Equation (v); a rapid reaction when just NO is present, control of NOx emissions especially by ammonia SCR reactions. This very well produced book includes some Equation (vi); a particularly slow reaction when NO2 is present alone, Equation (vii); and an amazingly colour illustrations and it is divided into eight parts that are detailed in the following sections. fast reaction when the ratio of NO2 to NO is 1:1, that is known as the fast SCR reaction, Equation (viii). 2.1 Part 1. Selective Catalytic Reduction Depending on the actual SCR catalyst used it can Technology therefore be important that an appropriate upstream oxidation catalyst provides the SCR catalyst with The first part of the book has two chapters with the first entitled ‘Review of Selective Catalytic Reduction (SCR) a suitable mixture of NO and NO2, although some and Related Technologies for Mobile Applications’ modern copper zeolite catalysts are less sensitive to by Timothy Johnson (Corning, USA). It provides an the NO/NO2 ratio than other catalysts. overview of relevant legislation and progress in engine

Urea hydrolysis (NH2)2CO + H2O à 2NH3 + CO2 (v) developments to reduce engine-out NOx levels, before detailing mobile SCR systems using urea in solution Standard reaction 4NH3 + 4NO + O2 à 4N2 + 6H2O (vi) as the source of ammonia. This chapter relies heavily on illustrations reproduced from a variety of original Slow reaction 4NH3 + 4NO2 à 4N2 + 6H2O + O2 (vii) publications that appear not to have been redrawn so there is, unfortunately, a lack of style consistency. Fast SCR reaction 4NH3 + 2NO + 2NO2 à 4N2 + 6H2O(viii) Notwithstanding this the chapter collects together much valuable and practical information. There have been extensive studies on the The second chapter called ‘SCR Technology for mechanism of ammonia SCR reactions, and by Off-highway (Large Diesel Engine) Applications’ analogy with known reactions of discrete compounds is by Daniel Chatterjee and Klaus Rusch (MTU it may be suggested that rapid decomposition of Friedrichshafen GmbH, Germany) and is concerned ammonium nitrite (Equation (ix)) is important in the with large diesel engines used in marine applications, SCR surface catalysed process forming nitrogen. mining trucks and trains as well as in electrical power At higher temperatures one route to undesirable generation units. These engines usually operate under nitrous oxide (N2O) emissions might be from high load conditions so have high temperature exhaust decomposition of ammonium nitrate-like surface gas, enabling good SCR performance with conventional species, Equation (x). The former reaction has been vanadium-based catalysts, but their fuel invariably used to prepare chemically pure nitrogen (free of contains high sulfur levels and this can cause a variety atmospheric argon) and the latter to manufacture of problems. For example, newer zeolite-based SCR N2O. catalysts are poisoned and do not work well, and at these quite high operating temperatures some sulfur NH4NO2 à N2 + 2H2O (ix) dioxide (SO2) can be oxidised to sulfur trioxide (SO3), Equation (xi). If any ammonia slip is present this can NH4NO3 à N2O + 2H2O (x) form particulate ammonium sulfate and/or ammonium However, the surface SCR reactions are complex bisulfate according to Equations (xii) and (xiii), as well and recently nitrate species have been shown as sulfuric acid mist that can itself cause difficulties, to have important roles in the fast SCR reaction. Equation (xiv). Chapters in this book go a long way to help the reader to unravel some of the mechanistic details. SO2 + ½O2 à SO3 (xi)

2NH3 + SO3 + H2O à (NH4)2SO4 (xii) absorb more ammonia than do iron ones and under dynamic transient conditions this can provide a

NH3 + SO3 + H2O à NH4HSO4 (xiii) significant performance advantage. Because of their superior low-temperature SO3 + H2O à H2SO4 (xiv) performance copper zeolite catalysts have been adopted for use in car applications and these are This well-illustrated chapter goes on to discuss discussed in the third chapter in this section by Hai-Ying combined SCR systems such as SCR/filter Chen (Johnson Matthey, USA). The emphasis is on combinations, and large scale SCR units, as well the the impact of the nature and physical properties of the automated control strategies that are usually involved. zeolite type on catalytic performance, and in particular the size of the zeolite pores classified as small, medium 2.2 Part 2. Catalysts or large. Small pore zeolites such as chabazites and The second part of the book has four chapters that other small pore molecular sieve materials such as focus on SCR catalysts, and the first of these by Jonas the silicon substituted aluminium phosphate SAPO-34 Jansson (Volvo, Sweden) discusses vanadium-based have outstanding hydrothermal stability, excellent SCR catalysts used in heavy duty mobile SCR applications activity and importantly they form very low amounts and highlights the legislative requirements before of the undesirable byproduct N2O. The introduction considering catalyst properties. Because the vanadium- of these copper molecular sieve SCR catalysts into based catalyst operates in the temperature range of the series production of diesel cars in Europe was an optimum activity (say 300ºC–500ºC) they have been outstanding technical achievement that will provide a widely used. Typical catalyst compositions are given high degree of NOx emissions control into the future. as 1%–3% V2O5 plus about 10% tungsten trioxide Indeed one might expect that new materials will be (WO3) impregnated onto a high surface area titania discovered that provide the necessary acidity and (normally anatase) that is coated onto flow-through environment around the copper atoms to provide good substrates. Related extruded catalyst compositions SCR activity and durability. However, these features have also been widely used. Practical aspects such as are not unique in providing excellent ammonia SCR selecting appropriate catalyst size (dimensioning), the performance. Some simple metal oxide catalysts have effects of space velocity and ageing effects (thermal been shown to perform well and these are the subject and poisoning) are considered, and it is clear vanadium of the last chapter in the part on SCR catalysts. catalysts have had and will continue to have a major The last chapter in this part on catalysts is the result role in this area. However, with reduced sulfur fuel of a collaboration by Gongshin Qi (General Motors, levels the newer, higher activity zeolite-based catalysts USA) and Lifeng Wang and Ralf T. Yang (University discussed in the following chapter will probably become of Michigan, USA) that deals with low-temperature increasingly important. SCR involving both zeolite and metal oxide ammonia Appropriately the next chapter, by Todd J. Toop, John SCR catalysts as well as touching on developments A. Pihl and William P. Partridge (Oak Ridge National with hydrogen SCR. They highlight the importance of Laboratory, USA), is about iron-zeolite SCR catalysts. the method of making iron-ZSM5 catalysts. Aqueous These were amongst the first metal zeolite catalysts impregnation with iron(III) salts does not lead to full used in SCR applications, and because they can have metal incorporation into the pores because, it is good high-temperature performance coupled with suggested, the heavily hydrated metal cations are reasonable stability, they were introduced into gas too large for easy penetration, whereas impregnation turbine applications at an early stage. In contrast copper with iron(II) species makes highly active catalysts. zeolite catalysts usually have better low-temperature The interpretation of the origin of this effect may be activity that falls off at higher temperatures as ammonia more complex because reduction of catalysts derived is oxidised to NOx. Making sweeping generalisations from iron(III) salts gives improved activity. Again the about the relative performance of SCR catalysts can be importance of small pore acidic molecular sieves is problematic because several factors are involved such noted, as is the wide range of activities that can be as: the type of zeolite involved, its silica to alumina obtained with different copper zeolite catalysts and ratio, the metal loading and importantly the preparation their dual role in providing acid sites for formation of method. However, in general copper-based catalysts ammonium cations and metal-based oxidation of NO

to NO2, leading to highly reactive ammonium nitrite- agreement that binary V—O—V moieties including like species that decompose to nitrogen and water. a Brønsted site are the most active structures and Manganese oxides can have excellent low-temperature a well-accepted mechanism is available for this ammonia SCR activity, and clearly their oxidation site. Isolated VO2+ species exchanged into zeolite capability is important. A wide variety of promoted structures are also active, apparently via a different oxides have been investigated, but it appears their mechanism. Tungsten promoted vanadium appears adoption has been restricted by a lack of tolerance to be effective by encouraging formation of isolated to water and in particular sulfur poisoning. Moreover, V—O—V species. The active sites in iron and copper increasing activity by using higher manganese loadings exchanged zeolites are then considered; here a huge appears to result in the formation of more N2O. It is amount of research has been done over several noted perhaps the most successful development in decades trying to identify the intimate mechanistic this area was made by Shell who in the early 1990s details and the nature of the active SCR sites. Much developed a relatively low-temperature ammonia SCR of the earlier work involved exchanged ZSM-5, process using a vanadium on titania catalyst promoted and then more recently beta-zeolite and small pore by transition metal species. molecular sieves were studied. As previously noted Hydrogen can be a reductant in NOx SCR reactions a key feature is the low temperature performance and over platinum group metal (pgm) catalysts. of the copper catalysts and the higher temperature The reactions that can take place are shown in durability of the iron catalysts. The metal centres may Equations (xv)–(xvii). be associated with NO oxidation. An added advantage of the iron catalysts, like the earlier vanadium ones, 2NO + 4H + O à N + 4H O (xv) 2 2 2 2 is tolerance towards sulfur that is in marked contrast to the poison sensitive copper catalysts. Although ½O2 + H2 à H2O (xvi) there has been considerable speculation about the roles of monomeric, dimeric and oligomeric metal 2NO + 3H2 + O2 à N2O + 3H2O (xvii) active centres their general relative importance is High conversions of NO in the presence of oxygen at unclear. Brønsted acidic zeolite sites have been low temperatures are possible, although as might be thought to be a means of concentrating ammonium expected, at higher temperatures the direct reaction ions close to the metal centres, but the importance of of hydrogen with oxygen, Equation (xvi), increasingly this is questioned by more recent work on non-zeolite takes place, and NO conversion decreases because conventional oxide catalysts some of which can have less hydrogen is available for the SCR reaction. As a good performance. result an operational temperature window is formed in The next chapter by Masaoki Iwasaki (Toyota, Japan), which NO conversion is optimised. A serious detraction is about mechanistic aspects of the ammonia/NO from these hydrogen SCR pgm catalyst systems is the reaction in excess oxygen, Equation (vi), that is the high proportion of N2O that can be formed. A better traditional standard or rapid ammonia SCR reaction. catalyst in this respect appears to be a palladium Results are given for reactions involving copper and iron promoted vanadium on titania/alumina that retains exchanged ZSM-5, a tungsten-promoted vanadium on good low-temperature SCR performance and has titania catalyst as well as the acid form of ZSM-5. The reduced N2O formation although this is probably still expected reaction order of copper was greater than too high for practical applications. iron and vanadium catalysts. Kinetic parameters such as apparent activation energies and apparent reaction 2.3 Part 3. Mechanistic Aspects orders were reported for the separate oxidation of This part of the book is concerned with the mechanistic ammonia and NO as well as the ammonia-NO-oxygen aspects of SCR reactions and has three chapters, the reaction. Generalising for the ammonia NO SCR first of which is by Wolfgang Grunert (Ruhr University reaction the order in NO is positive and close to one, Bochum, Germany) on the nature of SCR active sites. the order in oxygen is fractional and that for ammonia is The range of available characterisation techniques are negative, reflecting its strong adsorption that can result first outlined before the surface science techniques in reaction inhibition. There is a strong correlation that have been used are highlighted. Vanadium-based between SCR activity and NO oxidation activity. A catalysts are considered first, and there is general considerable amount of carefully determined transient

response data is reported and several catalytic cycles and NO2 is much faster. The overall fast SCR reaction are presented. The mechanistic conclusions are similar may be considered to go via the disproportionation to those previously noted. of NO2 to nitrite, the nitrate oxidation of NO to nitrite There then follows an important chapter on the role of and the formation of surface ammonium nitrite that

NO2 in ammonia SCR reactions by the editors Isabella spontaneously decomposes to water and nitrogen. Nova and Enrico Tronconi (Politecnico di Milano, Italy). All the steps involved in the fast SCR reaction are

The most obvious role of NO2 is in combination with summarised in Table I. The required oxidant provided NO in the fast SCR reaction. Ammonia and NO2 are by NO2 in the fast SCR reaction can also be supplied strongly adsorbed and interact on the catalyst surface. by addition of ammonium nitrate in what is called

The reaction of NO2 with surface oxide ions affords enhanced SCR. nitrate and nitrite ions, according to Equation (xviii), 2.4 Part 4. Reaction Kinetics with the latter being further oxidised by NO2 to nitrate and NO via Equation (xix) so the overall stoichiometry There are three chapters in the part of the book on is as shown in Equation (xx). These reactions are in the reaction kinetics of ammonia SCR reactions, equilibrium and depend on concentration, temperature and fittingly the first is by Isabella Nova and Enrico and catalyst oxidation state. Tronconi. This is on SCR reactions over vanadium(V) oxide (V O )/WO supported on titania catalyst, and 2NO + O2– NO – + NO – (xviii) 2 5 3 2 3 2 they explain how measured unsteady state kinetic

– ⇌ – parameters for all of the reactions concerned can be NO2 + NO2 NO + NO3 (xix) incorporated into a computer model for the control of 2– ⇌ – heavy duty diesel NOx control systems. At an intimate 3NO2 + O 2NO3 + NO (xx) mechanism level surface sites are indicated that include The intimate mechanism⇌ of the SCR process is based a surface redox site at which oxygen is adsorbed, a on nitrogen redox chemistry. In the standard slow SCR reaction site at which NO is adsorbed and an acidic reaction oxygen is the oxidant taking NO to nitrite, and site to bond to ammonia. Reduced vanadium centres in the fast SCR reaction the more powerful oxidiser NO2 are reoxidised by nitrate. It is concluded the fast SCR is available and so the mixed reaction involving NO reaction proceeds via dimerisation of NO2 followed

Table I Summary of the Individual Steps Involved in the Fast SCR Reaction over Vanadium-based and Zeolite Metal Promoted Catalysts

Involving NO2 only

2NO2 N2O4 NO2 dimerisation

2– – – N2O4 +⇌ O NO2 + NO3 Disproportionation

– – NO2 + NO2 ⇌ NO + NO3 Nitrite oxidation by NO2

In the presence⇌ of NH3

+ 2– 2NH3 + H2O 2NH4 + O NH3 adsorption

+ – NH4 + NO2 ⇌ [NH4NO2] → N2 + 2H2O Nitrite reduction by NH3

+ – NH4 + NO3 ⇌ NH4NO3 Formation/dissociation of NH4NO3

NH4NO3 → N2⇌O + 2H2O Formation of N2O

In the presence of NO

– – NO + NO3 NO2 + NO2 Reduction of nitrate by NO

Fast SCR ⇌

2NH3 + NO + NO2 → 2N2 + 3H2O Overall reaction

227 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688280 Johnson Matthey Technol. Rev., 2015, 59, (3) by its disproportionation to surface nitrite and nitrate. aftertreatment components are discussed because of Ammonium nitrite decomposes to nitrogen while nitrate the consequences it has on their rate of heating after is reduced by NO to reform NO2. the engine starts. A number of other interesting aspects The next chapter by Michael P. Harold (University are discussed including the complex behaviour of SCR of Houston, USA) and Parnit Metkar (DuPont, USA) catalysts incorporated into a particulate filter. In the provides a very good overview of the published absence of soot in the filter the pressure driven flow of mechanistic work on ammonia SCR of NOx. They gas through the filter walls containing catalyst provides consider not only kinetics and mechanisms but also better performance than the same amount of catalyst the role of transport effects, especially in reactions on a flow-through substrate because of the absence over iron exchanged zeolites and layered catalysts of diffusion resistances. However, the presence of a comprising separate copper and iron zeolite layers. A substantial layer of soot can modify the situation: there number of particularly important points are highlighted. is the potential reaction of NO2 with soot that reduces Diffusion limitations can become significant for the fast the NO2/NO ratio which, with some SCR catalysts, SCR reaction at temperatures just above 200ºC, first can reduce its performance. To compensate for this diffusion within the catalyst pores; and increasing the effect more catalyst will be required. This might not amount of ammonia rather than increasing the rate of the be physically possible and if it were possible more standard SCR reaction with NO does not enhance the catalyst would increase backpressure across the filter. reaction rate but rather slows it due to strong ammonia It is therefore important SCR catalyst incorporated into adsorption causing site blocking. The reaction orders filters lack NO2/NO ratio sensitivity. are one in NO, half in oxygen and –0.3 in ammonia and The other chapter in this part includes discussion the corresponding activation energy of around 10 kcal about the understanding and measurements needed for mol–1 could reflect a relatively low barrier for the rate SCR control systems by Ming-Feng Hsieh (Cummins, limiting step since this was estimated under conditions USA) and Junmin Wang (Ohio State University, USA). where diffusion effects were thought to be absent. SCR control systems have to take into account varying Curiously on iron zeolite NO oxidation is inhibited by engine NOx emissions during real world driving and water, but the standard SCR reaction is not. However, adopt the urea solution injections accordingly. Forward the results of isotopic labelling experiments are control strategies have been used which make major consistent with the decomposition of ammonium nitrite assumptions about catalyst ageing and degradation being involved, Equation (ix), and a potentially important of ammonia capacity, but alone they are not adequate route to ammonium nitrite is from NO reduction of the and some degree of feedback control using sensors is nitrate. It is clear the mechanistic situation for the fast necessary. However, the present NOx sensors suffer SCR reaction can be significantly complex, and the interference from ammonia, and this has to be taken chapter concludes with an examination of two layer into consideration via sophisticated algorithms. In fact copper zeolite/iron zeolite catalyst arrangement, and NOx sensors also have a sensitivity to the NO2/NO aspects of reactor modelling. ratio resulting from the extra oxygen present in NO2. The last chapter in this part, by Louise Olsson Ammonia sensors are being experimented with to (Charmers University, Sweden), complements overcome some of the practical difficulties, but there the previous one because it concerns the kinetic remain significant challenges so SCR control system modelling of ammonia SCR reactions over copper development is an area of much activity. zeolite catalysts. An often unappreciated fact, that is 2.6 Part 6. Ammonia Supply highlighted, is under operating conditions the zeolite will adsorb a large amount of water in addition to ammonia The three chapters in this part are about the production with an enthalpy of adsorption of around 100 kJ mol–1 of a spray of urea solution in the exhaust gas flow, its in the absence of competing adsorbates. conversion into ammonia gas, storage of ammonia in SCR catalysts and the modelling of these processes. The 2.5 Part 5. Modelling and Control first chapter by Ryan Floyd (Tenneco, USA) and Levin The first chapter in this part, about reactor models for Michael and Zafar Shaikh (Ford, USA) is about system flow-through and wall-flow converters, is by Dimitrios architecture and includes the design of injectors and Karamitros and Grigorios Koltsakis (Aristotle University mixing devices. The computer-based design of these Thessaloniki, Greece). The arrangement of the different systems has resulted in reliable production of gaseous

ammonia with minimal deposition of troublesome CO(NH2)2 à NH3 + HNCO (xxi) solids. The second chapter is about ammonia storage and release by Daniel Peitz, Andreas Bernhard (Paul HNCO + H2O à NH3 + CO2 (xxii) Scherrer Institute, Switzerland) and Oliver Kröcher (EPFL, Switzerland) that focuses attention on the 2.7 Part 7. Integrated Systems chemical reactions involved in converting urea to For performance, space constraints and cost ammonia, Equations (xxi) and (xxii), before going on considerations it is desirable to integrate emissions to discuss alternative ammonia sources. While some control functionality as much as possible, and the three of these alternatives have some attraction, the use and chapters in this part of the book are about this topic. distribution of urea solution is now so well established The first chapter details an experimental and modelling it seems unlikely it will be displaced. The third and study of dual-layer ammonia slip catalysts (ASCs) final chapter in this part is about gas flow modelling by by Isabella Nova, Massimo Colombo and Enrico Gianluca Montenegro and Angelo Onorati (Politecnico Tronconi (Politecnico di Milano) and Volker Schmeiβer, di Milano). Computational fluid dynamics (CFD) have Brigitte Bandl-Konrad and Lisa Zimmermann (Daimler, been used for many years to optimise distributed Germany). The amount of ammonia fed to a SCR flow through monolithic honeycomb catalysts and the catalyst must be sufficient to reduce the varying amounts exhaust system as a whole, and these techniques have of NOx produced by the engine while maintaining the been successfully applied to systems involving SCR quantity of ammonia stored in the catalyst to ensure NOx reduction (Figure 3). A high degree of mixing optimum NOx reduction performance. As highlighted ammonia with the exhaust gas is essential for high elsewhere in this review, the control systems designed overall performance. to maintain this situation under dynamic transient

Electronic control unit (EDC 17) Tank or dosing control unit (DCU 17) incl. SCR functions

Supply module SM 5.1 (PC) or SM 5.2 (LD) Actuators Sensors (Defined welding interface to the Heater control unit tank. Heater, lifetime filter, level (HPU-PC) and temperature sensor on module (only with EDC 17) for tank integration. Pump module Engine CAN consisting of supply and emptying pump as replacement part) Glow control unit (GCU) with integrated heater control Coolant Differential Particle NOx (only with EDC 17) pressure sensor sensor sensor Dosing module DM 3.4 Lambda sensor 2 temp. sensors

Exhaust Mixer SCR-on-filter

Oxi-cat

Fig. 3. Schematic diagram of a car exhaust gas emissions control system comprising an oxidation catalyst, wall-flow particulate filter, and flow-through SCR catalyst. Key components include a urea solution tank (heated in cold weather), dosing spray module and static mixer, temperature and NOx sensors. (Source: Robert Bosch GmbH)

229 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688280 Johnson Matthey Technol. Rev., 2015, 59, (3) conditions can be effective but occasionally in some has been a major challenge that has been overcome circumstances excess ammonia may slip from the and such filters are now in series production on some SCR catalyst, and oxidation catalysts have been European diesel cars. developed to control the amount of escaping ammonia 2.8 Part 8. Case Histories by converting it to nitrogen. They need to have high selectivity towards the production of nitrogen, and they There are two chapters in the last part of the book about can have SCR activity should any NOx be present. The practical applications of urea SCR systems to series situations examined were the traditional arrangement of production vehicles. The first, entitled ‘Development of a separate special oxidation catalyst after a SCR of the 2010 Ford Diesel Truck Catalyst System’ by catalyst, a layer of oxidation catalyst above which was Christine Lambert and Giovanni Cavataio (Ford, a layer of SCR catalyst as well as a physical mixture of USA), is a well written contribution with well sized clear the two catalysts. An iron zeolite catalyst was used and illustrations. It provides a nice overview of the SCR when this was present as an upper layer on the oxidation work done in Ford since the early 1990s. By 1995 they catalyst there was enhanced selectivity to nitrogen and had demonstrated a SCR NOx-control system on a a small amount of additional NOx reduction. light duty diesel vehicle, and development continued The second chapter in this part is about combining culminating in the USA with the introduction of the 2010 NOx-trapping catalysts with downstream SCR catalysts truck system. The evolution of copper zeolite catalysts on diesel cars and is by Fabien Can, Xavier Courtois is detailed and practical aspects such as the importance and Daniel Duprez (University of Pointiers, France). of durability of the upstream oxidation catalyst to When a NOx-trap is regenerated by periodic enrichment maintain high NOx conversion through the then of the exhaust gas ammonia can be formed, and the necessary appropriate NO2 to NO ratio. Also covered reactions involved in this process are detailed before is the influence of packaging constraints, backpressure giving the fascinating history of the use of this ammonia problems, and the temperature requirements for the with a SCR catalyst. The ability of the SCR catalyst to NOx conversions required. It was clear a rapid heating store significant amounts of ammonia enables it to cold start strategy was needed to enhance the exhaust reduce NOx that is not retained in the NOx-trap during gas temperature so the emissions control system normal lean operation. Although optimised systems would work efficiently at a sufficiently early stage. have been used on series production cars it seems The 2011 model year system comprised two oxidation likely further advances will be made in the future in this catalysts, urea solution injection, two SCR catalysts, important area because it has the practical advantage and a silicon carbide particulate filter. The optimisation of not requiring to store and inject urea solution into the work included substituting a proportion of the platinum exhaust system. for palladium in the oxidation catalyst as a cost save, The final chapter in this part is by Thorsten Boger although this resulted in poor (if any with an aged

(Corning, Germany) about the integration of SCR catalyst) NO oxidation to NO2. This was acceptable catalysts into DPFs. The DPF materials in series because a NO2 insensitive copper/zeolite catalyst had production are cordierite, various forms of aluminium been selected. Platinum/palladium formulations also titanate, and silicon carbide. To reduce component had the advantages of reducing potential volatilisation count, cost and possibly improve performance there of traces of platinum via its oxide that could influence has been a move to incorporate catalytic functionality SCR catalyst selectivity, reduced low level emissions into particulate filters especially those in light duty of N2O and not oxidising traces of SO2 to the more diesel vehicles. This was first done with oxidation potent catalyst poison SO3. Both the oxidation catalyst catalyst that removes CO and HCs during normal and the SCR catalyst had to have high thermal stability driving and periodically provides high temperature to because they experience high temperatures during initiate filter regeneration. This is done by oxidising active filter regeneration. The palladium-containing partial combustion products from late injection of oxidation catalyst had durability, but early copper/zeolite fuel into the engine. Recently SCR catalyst has been SCR catalysts and even those based on beta zeolite incorporated into filters, and a large amount of catalyst did not have sufficient thermal stability. The availability is required so that exceptionally high porosity filters of SCR catalysts based on small pore zeolites in 2007 are needed. Having sufficient strength and filtration provided the required higher thermal stability. The efficiency with the necessary high porosity material ammonia storage capacity of SCR catalyst with a

230 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688280 Johnson Matthey Technol. Rev., 2015, 59, (3) suitable urea solution dosing strategy can significantly huge number of publications cited in this multi-author enhance low temperature NOx conversion, although book. A wide variety of materials are catalytically this has to be balanced with the possibility of ammonia active in ammonia SCR reactions, and several high slip during high temperature filter regeneration. On the performance catalysts have become established vehicle this requirement was obtained by the control commercially. These have the attributes of high system. Exotherm problems on the SCR catalyst activity, the necessary good selectivity with minimal during filter regeneration caused by HC adsorption and undesirable formation of N2O as well as very good carbon formation were all but eliminated with the small longevity associated with high thermal durability. pore zeolite SCR catalyst. This chapter illustrates the The book provides an important up-to-date survey huge amount of fundamental and development work of the state of SCR science and technology that over that goes into the introduction of a successful vehicle recent years has undergone tremendous advances. emissions control system incorporating urea-based Exceptionally high conversions of NOx to nitrogen SCR that, of course, continues to be improved upon. with amazingly high selectivity are now possible at The final chapter in this part, and the last inthe temperatures so low they were thought impossible book, is by Michel Weibel, Volker Schmeiβer and a decade ago. These improvements resulted from Frank Hofmann (Daimler, Germany) and is a short development work targeting low-temperature NOx contribution about computer models for simulation and control of emissions from diesel engine powered development of exhaust gas systems incorporating cars. Development work continues in this area urea-based SCR NOx control. Factors such as and further exciting developments are likely in the maintaining the level of ammonia stored in the catalyst not too distant future that could take the form of are particularly important with copper-based catalysts substituting urea as a source of ammonia for some that operate best with a significant amount of stored other reductant derived from on board sources such ammonia. The urea solution dosing strategy has to as water or diesel fuel. satisfy this requirement under most engine operating The lack of consistent illustration style, equation conditions without there being excess ammonia that numbering that could have been unified during would be wasteful and potentially be an emission copyediting are easily criticised, as could the all too problem. Independently determined kinetics for each brief index that does not for instance have important of the catalytic reactions and catalysts involved are terms such as chabazite and SAPO. However, these parameterised for ease of use in computer modules, failings do not detract from this book being a mine of and in some instances compiled in data maps. The information that will be of value to researchers working resulting simulation models are important during in the SCR area as well as a reference for students the development and optimisation of the individual in chemistry, catalysis and chemical engineering. The components and in identifying practical ways of editors are to be congratulated for bringing together obtaining optimum overall operating performance. so many eminent contributors and completing such This is complex and made more so by a need to take a major endeavour. This book should therefore be into account the engine operation that determines made available in academic and industrial research engine-out NOx emissions. libraries alike.

3. Conclusions

Ammonia SCR has become the technology of choice Reference for control of NOx emissions from all but the smallest diesel vehicles, and its importance is reflected in the 1 W. Nernst, Z. Anorg. Chem., 1906, 49, (1), 213

The Reviewer

Following Fellowships at the Universities of Toronto and Cambridge Martyn Twigg joined a polymer group at ICI’s Corporate Laboratory in the North West of England, and after involvement in projects at Agricultural Division at Billingham moved there in 1977. Martyn worked on catalysts and catalytic processes including synthesis gas production via naphtha and natural gas steam reforming, methanol and ammonia synthesis, and proprietary catalysts and processes for herbicide manufacture and environmental protection applications. He studied catalyst activation and built a much used off-site catalyst reduction unit. After managing an international polymerisation project he was head-hunted to work at Johnson Matthey as Technology Director in the autocatalyst area that he successfully led until being appointed Chief Scientist. This provided an opportunity for research diversity that included carbon nanotube manufacture and catalysts for medical applications. He was associated with four Queen’s Awards, and was awarded the Royal Society of Chemistry Applied Catalysis Prize. He has more than 200 papers, co-authored books on transition metal mediated organic syntheses and catalytic carbonylation. He produced the “Catalyst Handbook”, co-edits the Fundamental and Applied Catalysis series with Michael Spencer, and has 150 published patent families on catalysts and catalytic processes. Martyn has on-going collaborations with universities and holds honorary academic positions, and runs an active consultancy and catalyst development business.

Erratum Computer Simulation of Automotive Emission Control Systems

It has come to our attention that there was a mistake in the published article: M. Ahmadinejad, J. E. Etheridge, T. C. Watling, Å. Johansson and G. John, Johnson Matthey Technol. Rev., 2015, 59, (2), 152 (Equation (xix)):

k A k = Max 1A+ x (xix) The equation should read:

kMax A x k = 1 + A x (xix)

Sintering and Additive Manufacturing: The New Paradigm for the Jewellery Manufacturer

European jewellery industry poised to develop potential of direct metal laser melting in precious metals

By Frank Cooper Rationale Jewellery Industry Innovation Centre, School of Jewellery, Birmingham City University, This paper intends to explore and open up for debate by Birmingham, UK the jewellery industry what actions and understanding might be required in order to facilitate the transfer Email: [email protected] and acceptance of precious metal direct metal laser melting (DMLM) technologies and processes into a manufacturing process specifi cally tailored for the The use of various sintering technologies, allied to jewellery manufacturing industries and their related suitable powder metallurgy, has long been the subject value and supply chains. The goal of the Jewellery of discussion within the global jewellery manufacturing Industry Innovation Centre (JIIC) and its parent community. This exciting, once theoretical and institution, the Birmingham School of Jewellery, UK, is to experimental technology is now undoubtedly a practical encourage its students to develop, design and produce application suitable for the jewellery industry. All parts computer aided design (CAD) examples of jewellery of the jewellery industry supply and value chains, and products to challenge, prove, and democratise the especially design and manufacturing, now need to processes and materials required for the application of become aware very quickly of just how unsettling and precious metal DMLM technology into the production disruptive this technology introduction has the potential facilities of small and medium-sized enterprises to become. This paper will offer various viewpoints that (SMEs) within the jewellery manufacturing sector. consider not only the technology and its application to The paper also assesses and attempts to quantify the jewellery manufacture but will also consider the new current perceived industry needs for an adaptable, design potentials of the technology to the jewellery low-volume and innovative new technology that will industry. It will also briefl y consider how that design facilitate rapid responses by SMEs to the consumer’s potential is being taught to future generations of demands for more custom-made, individually designed jewellery designers at the Birmingham School of and personalised jewellery products. Typical jewellery Jewellery. We shall also discuss in some detail the manufacturing processes like lost-wax investment economics of and potential for new and different casting or stamping do not have either the necessary business models that this technological paradigm quick response times or, more importantly, the design might offer the jewellery industry. and production fl exibility required to address these

233 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688523 Johnson Matthey Technol. Rev., 2015, 59, (3) issues. This paper is intended to help increase industry could well be increasingly driven by a growing demand awareness, knowledge, and especially it is hoped to for custom-made, personalised, individually designed speed up jewellery industry uptake of the new design and innovative designs of high-quality and high-value and production capabilities offered by the DMLM jewellery as high precious metal prices have resulted processes for working with precious metals. in many consumers now considering the design and innovation within a jewellery item as equal to if not The Economic Argument more important than its base intrinsic value (4). Increased affl uence in the newly emerging economies, This section will focus primarily on the European coupled with a rising number of marriages, working jewellery market sector and its fi nancial models. women, increased shopping opportunities and an The European Union (EU) has traditionally been an online interest in fashion, are thought to be the main important supplier of high-quality jewellery to the world’s driving forces behind the latest growth in precious metal markets, and is also considered to be the second or jewellery sales in these areas. However, consumers third largest market for jewellery consumption, after are also more careful with their spending. There is a the USA (China and India vie for the other places growing fatigue towards ‘fast turnover fashions’ and depending on the statistical analysis method used). many consumers now have increasing opportunity Sales of jewellery in most EU countries are thought to favour good and uniquely personalised ‘statement’ to have risen steadily in the decade from 2005 to the designs and regard these as more important than the present; however, this volume market is predominantly intrinsic value of a jewellery piece. supplied with jewellery items manufactured outside the Global economic uncertainty makes it diffi cult to EU. Socio-economic factors mean that it is very diffi cult predict future jewellery industry trends, but the market for the European jewellery manufacturing industry is predicted in some quarters to begin to expand. In to be competitive on price alone. Recent global, and the near future the EU jewellery market is expected particularly EU, economic and fi nancial crises have to grow, especially in the Eastern EU and accession further impacted the EU jewellery manufacturing countries due in part to their newly emerging middle industry and recent massive price rises in all precious classes. It is the contention of the present article that metals have somewhat weakened jewellery sales. the EU market in particular will increasingly demand Consumers seem to have reduced their expenditure on higher quality products, coupled with original designs jewellery and sought personalised pieces with greater and statement jewellery with added perceived value, associated personal value (1). personalisation, or new production technologies (5–8). High-quality jewellery manufacturing has long A key question is therefore: “Is there a viable been an important sector within the EU economy. economic argument for considering adoption of Detailed information on employment statistics for the DMLM technology?”. The present author believes EU jewellery industry is diffi cult to source and defi ne that the jewellery manufacturing sector in the EU accurately but there are thought to be some 30,000 has the potential to grow signifi cantly further if new smaller companies, with less than 250 employees each, high-technology approaches such as DMLM are and around 200 larger companies with an estimated adopted and exploited effectively. While all materials total of 180,000 employees. These are companies used in the production of precious metal jewellery are that specialise either in a style of jewellery design or intrinsically expensive, DMLM offers a technology shift in a stage of production (2). The World Gold Council that is able to potentially reduce material usage while has estimated that in 2014 the global demand for gold offering new market opportunities. jewellery was US $100 billion (3). Within the EU and other developed economies, Technology and Design consumers have been educated by the fast-moving, digital, online revolution to expect a continuous and DMLM was developed during the 1990s in Germany regularly updated choice of new and innovative (9). Beginning with CAD data, several layers of metallic products. This has impacted consumer buying patterns, powder are successively deposited one on top of the resulting in a surfeit of choice and an ever increasing other. Each layer of powder is heated using a focused competition for their disposable incomes. It could safely laser beam corresponding to a selected cross-section of be predicted that the future high-value jewellery market the part to be produced. The powder bed is then dropped

234 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688523 Johnson Matthey Technol. Rev., 2015, 59, (3) incrementally and another layer of powder is applied concepts and ideas into jewellery products is achieved and smoothed by a blade prior to application of the next through a variety of technical processes including pass of the laser beam, simultaneously fusing each new CAD (12), prototyping and light engineering based layer of powder to the layer below it. The method differs processes and technical feasibility is traditionally a from the related technique of direct metal laser sintering vital consideration at each stage of the design process. (DMLS) in that the layer of metallic powder is fully molten As previously discussed, consumer demand for throughout. The method does not require any binders or increasingly novel products has resulted in the need fl uxing agents. Each run of the laser beam partly overlaps for extreme fl exibility in the design and production of the preceding run, and a protective gas atmosphere is jewellery, the ability to respond rapidly to ever changing maintained above the interaction zone of the laser beam demands, and the implementation of a streamlined and the metallic powder. Once fi nished the powder bed product development process by manufacturers of is removed from the machine and excess powder is personalised and custom-made products, not just then removed and can be fully recycled, although some jewellery. The UK jewellery industry has a signifi cant sieving may be required. Figure 1 shows a schematic of global reputation for producing well-designed, well-made, the process (10). high quality jewellery products, manufactured in A key opportunity is presented to the EU jewellery controlled and regulated environments that meet the manufacturing sector through the harnessing of the high expectations of their end consumers. A radically emergent and rapidly maturing DMLM technologies new manufacturing approach could be considered as and processes, which will facilitate the manufacture of being useful to help re-energise the precious metal uniquely designed, high-value-added, often custom- jewellery manufacturing base and to help facilitate new made, personalised, jewellery products that will be opportunities to boost production, increase profi tability inherently resistant to being copied (11). and regain market share. DMLM, which is nowadays The initial concept and design phases defi ne the routinely considered to be part of the rapid or additive innovative nature of a jewellery product during the manufacturing (AM) stable of technologies, has now early stages of its development. The process of become an accepted production solution within a design enables the defi nition and development of range of industrial manufacturing sectors including concepts and ideas and individual personalisation or aerospace, automotive, dental and medical, where AM customisation of an item, facilitating commercially is used to manufacture parts in a range of ceramic, viable new product development. The transfer of these polymeric and base metallic materials.

Tilted mirror with focus Laser

Burning point (partially sintered granules) Object

Powder Blade for spreading powder Platform and retractable table

Powder delivery system Build space

Fig. 1. Schematic of the DMLM process (10) (Reproduced with the permission of the Verein Deutscher Ingenieure eV, Germany)

Early research undertaken in Sweden in 2005 (13) at the Birmingham School of Jewellery. Figure 2 and much more recently in the UK (14) and Europe shows just a few examples of their work. Each of them has demonstrated that DMLM technologies could be explores and takes advantage various aspects of the extended to the manufacture of products in precious geometric design freedoms that the DMLM process metals, including 18 carat golds in various colours, offers. These items were produced for the students in silver alloys and even platinum group metals. This a number of different metals by a UK based supplier discovery led to some early global interest in the use of of a DMLM technology and were built on a Concept DMLM for precious metal parts manufacture. However, Laser Mlab LaserCUSING® by ES Technology Ltd. the principal experience of precious metal DMLM to The JIIC is also currently actively involved with a UK date has been largely limited to the cosmetic dental government funded DMLM of precious metals research industry, which has adopted some digital production scheme called the Precious project, whose mission solutions in precious metals (15). Additionally, this use statement reads: of the technology in dentistry was restricted to a small “To demonstrate the viability of precious metal number of specialist gold alloys used for restorative additive manufacturing within the UK Jewellery dental crowns in non-jewellery specifi c alloys. Many industry from design and manufacturing through potential alternative uses for precious metal DMLM to fi nishing, polishing and retail” (18). have also been identifi ed including electronics, fuel cell, medical, catalytic and satellite applications and in the This is a two-year project aimed at elevating the current manufacture of low-volume, high-value components in state of the art of DMLM AM within the UK precious the prestige automotive, biomedical and marine sectors metal jewellery industry. (16). There has also been much discussion concerning After a piece of jewellery has been designed and the potential for precious metal DMLM and its intrinsic before it can be manufactured using DMLM a small design benefi ts within the jewellery and high-value number of core activities need to take place: goods sectors, but the research, capital investment, • Pre-Processing (Preparing pieces for and metallurgical knowledge base required to set up manufacture). This essentially refers to all front a precious metals DMLM sector have until now been end software-related activities including the design considered largely prohibitive (17). process Currently there are a small number of different DMLM • Processing (Manufacturing jewellery items using technologies at various stages of development and use DMLM). This refers to the actual manufacturing in and around the European jewellery sector. In 2011 process using DMLM technologies the JIIC introduced and continues to deliver a teaching • Post-Processing (Manual and automated module specifi cally about DMLM technologies and fi nishing and polishing processes). This refers to their adaptation for jewellery design and manufacture the post-DMLM manufacture processing stages up to the cohort of the Design for Industry (DFI) students to the point where an article is ready for sale.

Fig. 2. Examples of students’ work from the Birmingham School of Jewellery DFI DMLM module. From left to right: Natalia Antunovity, Suyang Li, Tesni Odonnell, Tomas Binkevic

Each of these steps is interdependent on the others and they must take place in a logical sequence. When combined together effectively this can result in the production of novel and unique quality jewellery items. Understanding these various activity interrelationships has a profound effect on the eventual quality of any DMLM printed jewellery. If a jewellery designer understands, even on a fairly superﬁ cial level, what is involved in each of these core activities then they will be able to better design jewellery that not only takes advantage of the geometric freedoms that DMLM offers but also can be suitably post-processed to an acceptable quality of ﬁ nish. Fig. 4. An example of the CAD created support structures on an Ojo Case Study: The Ojo Project

To illustrate this a pendant piece from the Precious technology one on top of the other. Parts being built project, called ‘the Ojo’, will be used to show the various using DMLM require a support structure (19), this is a stages of designing and manufacturing jewellery for the scaffold-like construction, and supports all overhanging DMLM process. The Ojo is intended to be an iterative parts enabling undercuts, voids and holes to be design series of 100 pendants where each pendant produced. A jewellery designer will not need to be produced is signifi cantly different from the pendant able to create these supports, as they are added by before and the pendant after. This is achieved by the the DMLM machine setter and the machine’s software use of a CAD design algorithm that continually morphs package, but they will need an appreciation of the use the basic pendant design. This pendant was conceived and application of supports as they leave a witness and designed by Lionel T. Dean of Future Factories who mark or scarring when they have been removed, which is a member of the Precious consortium of companies. will require extra cleaning up and fi nishing (similar to The CAD fi le is created, saved as a stereolithography that required when removing a casting sprue). In theory (STL) fi le and is shown in Figure 3. The next step is it is possible to build any shape, however if supported to use a suitable software to generate and place the areas are visible but inaccessible, then the result will support structures (Figure 4) that are a necessary be perceivably poor surface quality. A small change to part of the DMLM process which requires the jewellery a design can eliminate the need for a lot of support design to be ‘sliced’ in the software. The parts are built structures. up of multiples of these slices printed by the DMLM Support structures are required in most, if not all, laser- powered, metal-based DMLM processes, and there are a number of divergent reasons for their presence. To build complex geometries with overhanging and undercut surfaces, support structures are required to assist in controlling any potential defects in or on the part being built. These defects may be caused by the typical thermal stresses of the DMLM process, occasionally by overheating, or most commonly by being dragged over and disturbed by the re-coater blade applying the next layer of powder. Supports are most principally required because the powder bed surrounding the very small melt pool created by the laser is not suffi cient to support the liquid metal phase in place. Other functions of supports are the bonding Fig. 3. CAD images of the Ojo of the part to the build plate and providing a thermally conductive connection between the part and the build

237 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688523 Johnson Matthey Technol. Rev., 2015, 59, (3) plate to rapidly and effectively dissipate heat from the Once the build process has been completed the melt zone. build plate piston is slowly raised, the surplus unused The next step is to fi ll the machine with powder, load powder is carefully swept away and the ‘additively up the STL fi le and set it into motion. A re-coater blade, manufactured’ jewellery item is exposed (Figure 6). or brush, pushes fi ne, powdered, gold build material The part is then removed from the build platform and from a carefully measured powder supply hopper to the support structures are also removed (Figure 7). create a uniform layer over the build platform. The laser The witness marks or scarring left by the supports also scanning system literally draws the two-dimensional have to be removed in much the same way as a casting (2D) cross-section of the CAD fi le slice on the surface sprue has to be removed and cleaned up. Because of the build material, melting it into a solid deposit layer the support structures are much smaller than a typical or slice (Figure 5). After the fi rst layer is produced, the sprue, a small, fi ne, burr on a pendant or Dremmel drill piston beneath the build platform is lowered fractionally will often suffi ce. and another powder layer is pushed into place using Once the supports have been removed the parts the blade. The laser beam melts the second layer are then ready for fi nishing and polishing (Figure 8). and at the same time fuses or bonds it to the layer Mechanical, or mass fi nishing, techniques are often below. This process is repeated layer by layer until the found to be the best for this stage because the DMLM part is completed. It is this layer adding process that of jewellery will probably prove to be most commercially leads to this technology being described as ‘additive effective when used to produce geometrically complex manufacturing’ in many quarters. designs. These designs by their very nature will

Fig. 5. A laser beam scanning over the surface of the Fig. 7. Support removal from the Ojo powder and melting the gold

Fig. 6. The Ojo emerges from the powder bed Fig. 8. Three variants of the Ojo before ﬁ nishing

238 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688523 Johnson Matthey Technol. Rev., 2015, 59, (3) present many unique and product specifi c challenges fi xture similar to an electric drill chuck on an extended when ready to be polished and fi nished. Mass fi nishing shaft (Figure 9). Because the rotating head is fi xed technologies are based on the correct application of but with an adjustable angle of attack when immersed media fl ow pressure and speed to the jewellery item to into the bowl, it can be easily automated and has be polished. Generally, the higher the pressure exerted shown excellent reliability and repeatability. The use by the media on the jewellery, and the faster the media of small, light media can produce an excellent fi nish, fl ows across the jewellery parts, the faster the desired the fi nishing energy coming from the relative speed of fi nishing results can be achieved. But this fl ow has to both the jewellery part and the medium. A fi nal polish be either uniform or directed, depending where the by hand completes the process (Figure 10). polishing is required. Centrifugal disc fi nishing is an industrial mass fi nishing Comparison with Casting process adapted for the surface treatment of jewellery. The process is carried out in a cylindrical container The most widely used technology in jewellery which is open at the top, while the bottom consists of a manufacturing worldwide continues to be the lost-wax turntable-like disc separated from the container wall by investment casting process, accounting for an estimated a microscopically small gap. During operation, the work 80% of all jewellery production. Current investment pieces and the grinding or polishing media in which they casting processes have been highly developed over are immersed rotate at a high speed, creating a toroidal time principally to facilitate traditional historical demands abrasive fl ow; the relative difference in speed of the for high-volume, batch- and mass-produced jewellery components and media produces the polishing effect. products. In contrast, there is an emerging need for a The contact between the jewellery pieces and the low-volume, rapid response to consumer demands for medium generates a very intense fi nishing effect which custom-made, individually designed products, in simple is up to 20 times more effi cient than can be achieved terms producing in volumes of one. DMLM could meet with conventional systems like vibratory fi nishers. this need. Additionally, yet more pragmatically, it also A process refi ned by Precious consortium partner offers the potential for creating items that appear solid Finishing Techniques Ltd is the ‘stream fi nishing’ yet, if sectioned through, would prove to be entirely process (sometimes referred to as immersion hollow or contain a simple honeycomb or scaffolding polishing). This is a fairly new concept to jewellery structure added for strength, a process sometimes polishing and features short processing times because described as ‘volume without mass’. Such forms the medium is compressed against the wall of a large and designs are not currently achievable using the spinning bowl (centrifugal disc fi nishing) and the parts traditional jewellery manufacturing processes. Caution are held and rotated in this fl ow by use of a rotating must be applied, of course, in selecting appropriate

(a) (b) (c)

Fig. 9. Stream ﬁ nishing of an 18 carat gold Ojo: (a) the part held in a rotating ﬁ xture; (b) immersed in the polishing medium; (c) removed from the medium

(b) (d) (a)

(c)

Fig. 10. 18 carat gold Ojo: (a) after 1 hour of stream polishing; (b) after approximately 3 hours of stream polishing; (c) after ﬁ nal hand/mop polishing; (d) the ﬁ nished Ojo (Ojo is Spanish for eye!)

items for this type of manufacture. For instance making savings in the intrinsic cost of expensive raw materials a typical wedding band hollow would result in an item is undoubtedly now approaching the point at which that feels valueless and cheap; whereas a bulky, this novel manufacturing process can become more heavy, watch bezel could be produced using a weight commercially viable and attractive to the jewellery reducing hollow profi le and the resultant watch would manufacturer and the consumer. still feel right, with weight being added by the watch Furthermore, it is entirely possible and correct to movement and wrist band (Figure 11). The potential consider at this point that the widespread business model used in jewellery manufacturing throughout the UK, especially in relation to lost-wax investment casting – namely the use of sub-contract bureau service providers – is equally applicable to smaller (a) jewellery designers and manufacturers accessing the DMLM technological advance. In conventional jewellery manufacturing there is a measurable correlation between part complexity and its manufactured cost. Using DMLM means that not only is complexity independent of tooling costs, but also that virtually any geometry conceived by the designer is theoretically possible to produce. Conventional design methods are based on the ‘design for manufacture’ principle, in which manufacturing constraints are (b) included at the earliest stages of the design process. This often results in modular designs with standardised components, meaning designers inevitably modify their design intent to enable the item to be manufactured using a specifi c manufacturing process. Using DMLM would allow the removal of many of these constraints, although (as in any other manufacturing process) DMLM has its own limitations. It will be necessary to Fig. 11. Volume without mass (right and wrong): (a) watch develop specifi c ‘design rules’ expertise to manage and bezel; (b) CAD image of a hollow wedding band optimise these new and exciting possibilities. Current research at the JIIC in conjunction with the School of

(a) (b)

Fig. 12. (a) An 18 carat gold galleon ‘printed’ on (b) a Cooksongold Precious M 080 machine

Jewellery and its students is based around discovering modifi cations, as well as the availability of tool-less these rules, which will minimise the limitations imposed fabrication, will infl uence what is designed, how it is by machine modifi cations and CAD design adjustments, designed, and the quantity of products offered. to attain the objective of being able to produce the Innovative design could be considered vital for widest possible range of geometries. the survival of the high-value-added industries, The Jewellery Industry Innovation Centre has including jewellery manufacturing, though it recently purchased a Cooksongold Precious M 080 should be remembered that the manufacture of direct metal ‘3D printing’ DMLM machine for teaching well-designed unique products remains an intensive, and research purposes, and Figure 12 shows an expensive, and consumer-centred process. The example of one of the Centre’s early explorations of commercial pressures to reduce costs to remain the machine’s capabilities. The ship is a little over competitive, while retaining design quality, challenge 1.5 centimetres high. jewellery manufacturers to fi nd ever more innovative manufacturing techniques as well as consider The Future alternative routes to their markets and consumers. The jewellery industry and other design-led creative Manufacturers are generally limited in their methods industries are ideally suited for developing the new of fabrication by the cost of tooling, which must interfaces between the customer and designers, be amortised over the number of parts produced and they will also need to consider new production during the life cycle of a tooling product. In existing technology approaches that maintain and exploit this conventional jewellery manufacturing there is a competitive edge. Jewellery is therefore in a unique direct link between the complexity of the part and position to capitalise and further develop the potential its manufactured cost; this can be signifi cantly of DMLM, while using generic fabrication criteria reduced with use of the DMLM processes. In gaining which are relevant to many other high-value-added knowledge and understanding of the potential design industries where custom-made products command and manufacturing advantages of DMLM, jewellery correspondingly higher consumer prices. manufacturers should be provided with the economic impetus to consider adopting DMLM processes, References as appropriate to their company’s needs. Many 1. M. Krijger, ‘CBI Trade Statistics for Jewellery’, Global high-value products are made in small volumes or Intelligence Alliance/Centre for the Promotion of require individual, personalised adaptations for each Imports from developing countries (CBI), Ministry of customer or application. The ability to provide such Foreign Affairs, The Hague, The Netherlands, 2014

2. ‘Growth, Dedicated Call 10/00’, Topic IV 31, The Agreements’), December 2009 (Withdrawn December European Virtual Institute For Jewellery Technology, 2014), Verein Deutscher Ingenieure eV, Düsseldorf, EC Funded Project, Reference G7RT-CT-2001-05065, Germany, 2009 Milan, Italy, 2002 11. A. M. Carey, ‘The Changing Demands on the Creative 3. L. Street, K. Gopaul, M. Kumar, C. Lu and A. Hewitt, Process as a Consequence of New Technologies’, in ‘Gold Demand Trends: First Quarter 2015’, World “The Santa Fe Symposium on Jewelry Manufacturing Gold Council, London, UK, 2015 Technology 2010”, ed. E. Bell, Met-Chem Research, 4. M. Krijger, ‘CBI Trends: Jewellery’, Global Intelligence Albuquerque, New Mexico, USA, 2010, pp. 101–118 Alliance/Centre for the Promotion of Imports from 12. S. Adler and T. Fryé, ‘The Revolution of CAD/CAM Developing Countries (CBI), Ministry of Foreign in the Casting of Fine Jewelry’, in “The Santa Fe Affairs, The Hague, The Netherlands, 2015 Symposium on Jewelry Manufacturing Technology 5. D. G. Penfold, ‘New Product Development in the West 2005”, ed. E. Bell Met-Chem Research, Albuquerque, Midlands Region of the UK Jewelry Manufacturing New Mexico, USA, 2005, pp. 1–24 Industry – Evaluating the Impact of Design Support’, in “The Santa Fe Symposium on Jewelry Manufacturing 13. N. Towe, ‘Laser Sintering Process for Making Hollow Technology 2007”, ed. E. Bell, Met-Chem Research, Jewellery’, Jewellery Technology Forum Proceedings, Albuquerque, New Mexico, USA, 2007, pp. 445–456 June, 2006, Vicenza, Italy, Legor Group, Italy, 2006 6. ‘Jewellery & Watches’, Key Note Market Report, Key 14. M. Khan and P. Dickens, Gold Bull., 2010, 43, (2), 114 Note Ltd, Richmond on Thames, UK, 2014 15. A. L. Hancox and J. A. McDaniel, Int. J. Powder 7. D. G. Penfold, Design J., 2007, 10, (1), 3 Metall., 2009, 45, (5), 43 8. M. Karydes, ‘Bold gold jewelry is back in style’, 16. L. S. Bertol, W. K. Júnior, F. P. da Silva and C. Aumund- Fortune, 9th May, 2015 Kopp, Mater. Design, 2010, 31, (8), 3982 9. W. Meiners, K. Wissenbach and A. Gasser, Fraunhofer 17. A. Simchi, Mater. Sci. Eng.: A, 2006, 428, (1–2), 148 Ges Forschung, Germany, ‘Shaped Body Especially Prototype or Replacement Part Production’, German 18. Precious, Innovate UK, The Technology Strategy Board, Patent 19,649,865; 1998 Swindon, UK: http://www.precious-project.co.uk/ (Accessed on 29th June 2015) 10. ‘VDI-Richtlinie: VDI 3404 Generative Fertigungsverfahren – Rapid-Technologien (Rapid Prototyping) – Grundlagen, 19. F. Cooper, ‘DMLM Supports: are they the Jewellery Begriffe, Qualitätskenngrößen, Liefervereinbarungen’, Industry’s New Sprue, Riser and Gate Feed?’, in (Engl. Transl. ‘VDI Guideline: VDI 3404 Additive “The Santa Fe Symposium on Jewelry Manufacturing fabrication – Rapid Technologies (Rapid Prototyping) Technology 2014”, ed. E. Bell, Met-Chem Research, – Fundamentals, Terms, Quality Parameters, Supply Albuquerque, New Mexico, USA, 2014, pp. 89–109

The Author

Frank Cooper is a lifelong jewellery industry professional and is now a Senior Lecturer in Jewellery Manufacturing Technologies, and Technical Manager of the Jewellery Industry Innovation Centre, at the Birmingham School of Jewellery, UK. He sits on the Goldsmiths’ Craft and Design Council and is a globally recognised expert in the application of various additive manufacturing, prototyping, CAD and ‘3D printing’ technologies used in the jewellery industry. He is an active participant in a number of jewellery industry-related research initiatives and has published and presented many technical papers and articles in the UK and Europe, as well as at the Santa Fe Symposium in the USA.

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain

Exploring the production and supply of metal powders for AM processes

By Jason Dawes*, Robert Bowerman and 1. Introduction Ross Trepleton Component Technologies, Manufacturing Technology A component fabricated using powder bed may consist Centre, Pilot Way, Ansty Business Park, Coventry, of many thousands of finely spread powder layers. The CV7 9JU, UK uniformity of these layers can affect the properties of the final component. The way in which a powder ‘spreads’ *Email: [email protected] during AM depends on the properties of the powder used. As will be discussed in this paper, even when chemically equivalent, the properties of metal powders The supply chain for metal powders used in additive vary widely depending on both the atomisation method manufacturing (AM) is currently experiencing used and the manufacturing process conditions. To exponential growth and with this growth come obtain a greater degree of control over AM processes new powder suppliers, new powder manufacturing service providers must be able to control the quality of methods and increased competition. The high the raw powder feedstock. number of potential supply chain options provides The overall AM market has seen exponential growth AM service providers with a significant challenge over the last five years and during this time the sale when making decisions on powder procurement. of powder bed metal AM equipment, services and This paper provides an overview of the metal powder products has also followed an exponential trend (1) supply chain for the AM market and aims to give AM due to increased adoption from the aerospace, oil service providers the information necessary to make and gas, marine, automobile and medical sectors. As informed decisions when procuring metal powders. the benefits of using AM to manufacture functional The procurement options are categorised into three metallic components start to outweigh the blockers, main groups, namely: procuring powders from AM more component manufacturers are looking towards equipment suppliers, procuring powders from third metal powder bed technologies to allow them to realise party suppliers and procuring powders directly from their next generation of innovatively and functionally powder atomisers. Each of the procurement options designed products. has its own unique advantages and disadvantages. Research has shown that metal powder costs will be The relative importance of these will depend on what the biggest continuous expense through the life of an the AM equipment is being used for, for example AM machine (1). The quality and consistency of the AM research, rapid prototyping or productionisation. components depends, in part, on the characteristics The future of the metal AM powder market is also of the starting powder feedstock. Hence, controlling discussed. and understanding the quality of the powder both in

243 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) its as-supplied and reused condition is essential in Current predictions forecast that this rapid growth order to achieve the desired mechanical properties of will continue and that there will be a five-fold market the laser melted components. Given the significance value increase by 2021 (1). This growth trend in AM of the metal powder feedstock it is important that AM technology is also reflected in global raw materials users make informed decisions when procuring the (powder) sales. Powder sales in the AM sector over the raw metal powder. The current state of the AM metal past decade are shown in Figure 2. After a decline in powder supply chain is that there are multiple possible sales in 2009, due to market reaction at the beginning methods for the manufacture of metal powders and of the fiscal crisis, both the AM sector value and AM many times as many potential suppliers. Furthermore material sales have seen rapid growth since 2010. not all metal powders are equal in terms of their Specific to metal powder AM, the sale of powder for fundamental properties even when manufactured via metal processes is also shown in Figure 2. It can be the same technique (when procured from different seen that metal sales have followed the market trend vendors). This presents quite a challenge to beginners since 2010, with sales more than doubling in a three in AM technology when deciding on a powder supplier. year period. However, the value of the metal powder However, some AM equipment suppliers, such as EOS, AM market is a relatively small proportion of the whole sell ‘validated powder’. ‘Validated powder’ is a powder market. This highlights the opportunity for growth for which has been identified as suitable for use in AM. powder suppliers offering products into the metals AM Whilst validated powder can de-risk procuring powders market. for AM it does limit users to a single source supplier Based on data from 2013 there are 855 powder and inhibits the development of in-house expertise. manufacturers worldwide (425 located in North Given the complexity of the AM metal powder supply America, 205 in Europe and 225 in the Asia-Pacific chain this review article aims to resolve some of the region) capable of producing an estimated 1.12 confusion involved and address some of the frequently million metric tonnes, to a value of approximately asked questions by users. Additionally, the article will US$6.9 billion (2). The top six powder manufacturers highlight key issues that the market needs to address, have a combined market share of 44% and generally and make potential users aware of some of the key serve the press and sinter market. The remaining factors to consider when selecting the most appropriate market share is made up of small businesses, likely powder supplier. producing powder for a specific purpose or application. When the metal powder market is evaluated, only 2. Overview of the Powder Bed AM Market US$32.6 million was sold for AM usage (0.0047%). This shows that despite the enormous anticipation of Over the last 20 year period the AM market has grown the impact of AM, traditional powder processes such rapidly from an industry worth

600

All AM Materials 528.80 500 Metallic AM materials only 3500 417.00 Services 400 3000 Products 327.10

2500 300 265.90 238.00 220.90 217.80 2000 200 189.50 151.00 128.90 93.40 1500 US$million Value, 81.20 100 71.00 32.60 24.90 18.00 13.50

1000 12.00 0 500 Market value, US$million 0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Year 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 Year Fig. 2. History of materials sales for AM systems worldwide: (–) all AM materials, (–) metallic AM materials only (metallic Fig. 1. AM market growth 1991–2013 (1) powder sale data not available prior to 2009) (1)

244 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) as press and sinter and metal injection moulding the unused powder on the actual powder properties (MIM) still dominate the marketplace. However, as AM and hence subsequent component properties has not processes become more established as component been the subject of intensive scientific study. In the few manufacturing routes, rather than rapid prototyping scientific journals published in the field of metal powder technologies, the potential for growth in the metal AM recycling in AM, it has been observed that recycling powder supply is considerable. powders in powder bed AM processes results in an increase in powder particle size distribution (PSD) (2, 3. Selection of AM Powder 3). The thermal effects that result from the process, such as chamber temperature and the radiation energy 3.1 The Importance of Powder in selective laser melting (SLM) of metal powders, may cause physical as well as chemical changes to The AM process uses powder as its raw material the recycled powder. Furthermore contamination, feedstock, as such the consistency of the powders either through impurities, foreign bodies or interstitial used to build AM components will have a critical elements may be introduced to the powder as a result influence on the final component properties. During the of handling during pre-processing or post-processing build sequence of an AM component, the raw powder stages. feedstock is stored in a hopper, the design of the hopper The first step in understanding powder requirements and the method by which powder is introduced into for AM processes is to assess the types of metallic the build chamber depends entirely on the equipment powder that are available. The following sections manufacturer. A discrete amount of powder from the provide an overview of the methods of metal powder hopper is spread (either using a rake or roller system) production routes. across the build chamber to form a thin (no more than one to two particle diameters) continuous layer of 3.2 Routes of Powder Production powder. After spreading it is critical that the layer is The production of AM metal powder generally consists homogenous over the entire area of the build chamber, of three major stages as outlined in the flow diagram any degree of inhomogeneity may result in porosity (in shown in Figure 3. Briefly, the first stage involves the the absence of powder) or incomplete through-thickness mining and extracting of ore to form a pure or alloyed melting (too much powder pooled up in one area). The metal product (ingot, billet and wire) appropriate spread layer is selectively fused using either a laser for powder production; the second stage is powder source or an electron beam based on an input sliced production and the final stage is classification and three-dimensional (3D) computer aided design (CAD) validation. model. Following selective sintering another layer of The supply chain of taking ore and extracting a metal powder is spread over the first. This iterative process of powder spreading followed by selective melting is is well established and supplies a vast range of pure continued until the build is complete. The total number metals and specific alloys to global markets. Once an of powder layers spread will of course depend on the ingot of the metal or alloy has been formed a number size of component being built but the number could be of additional processing steps may be required to in the region of 7000 layers. Furthermore it is common make the feedstock suitable for the chosen atomisation to build multiple components during one build event. process. For example, plasma atomisation requires the The layer spreading, hopper dosing and bulk packing feedstock material to be either in wire form or powder performance of the AM powder will depend entirely form, thus adding additional rolling and drawing work or on the properties of the powder being used. Further a first step powder production route. complicating the use of AM powder is that the volume Once the first processing form has been obtained of the actual component built can be significantly less there are a number of methods available to produce than the total volume of powder that has been spread. metal powders including, but not limited to: solid-state As a consequence there is a large amount of unused reduction, electrolysis, various chemical processes, powder left over in the build chamber, given the high atomisation and milling. Historically, for reasons that cost of metal powders it is essential that the unused will be discussed, atomisation has been identified as powder is effectively recovered and reused in future the best way to form metal powders for AM due to the builds. However, the effect of continued recycling of geometrical properties of the powder it yields.

Stage 1 Ore

Hydrogenation and Extraction dehydrogenation

Forming

Forming Ingot (billet, wire)

Stage 2

Plasma, PREP, REP Water atomisation or EIGA atomisation Gas or centrifugal Atomisation atomisation Atomisation

Drying

Powder

Post processing

Stage 3 Validation

Fig. 3. From ore to validated AM powder – powder production steps flow chart

None of the powder production routes actually powder exits at the bottom of the chamber, where it is produce a 100% powder yield in the required size collected. Additional processing steps are then required fractions. Some post processing is therefore necessary. to dry the powder. Metal powder produced in this As a minimum, the as-produced metal powder must be way is typically highly irregular in morphology which classified into a well-defined particle size distribution reduces both packing properties and flow properties. suitable for the required process: typically 15–45 µm for Water atomisation is the main method of producing iron SLM and 45–106 µm for electron beam melting (EBM). and steel powders and typically feeds into the press- and-sintered industries rather than the specialised AM 3.2.1 Water Atomisation industry. All atomisation processes begin with melting the 3.2.2 Gas Atomisation feedstock alloy. The melting process has a number of variations, but generally when atomising using water, The gas atomisation process mimics water atomisation, the feedstock is first melted in a furnace before being with the differentiator being the use of gas instead transferred to a tundish (a crucible that regulates the of water during processing. Air can be used as the flow rate of the melt into the atomiser). The liquid alloy atomising media, but it’s more likely that an inert gas enters the atomisation chamber from above; here (nitrogen or argon) will be used to reduce the risk of it is free to fall through the chamber. Water jets are oxidation and contamination of the metal. The process symmetrically positioned around the stream of liquid of melting the metal ingots can be the same as described metal, atomising and solidifying the particles. The final for water atomisation, however for powders produced

246 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) for high end applications such as aerospace, the need just before entering the atomisation chamber, as shown to control interstitial elements has led to increased in Figure 5. This application is used when processing use of vacuum induction melting (VIM) furnaces. A reactive alloys, such as Ti-6Al-4V, minimising the risk of VIM furnace is typically installed directly above the contamination from exposure of the molten titanium to atomisation chamber such that the molten stream of the crucible and the atmosphere (5). liquid metal enters the atomisation chamber directly from the furnace rather than through a tundish, similar to the set-up shown in Figure 4. The stream of liquid metal is atomised by high pressure jets of gas. Due to the lower heat capacity of the gas (compared to water) the metal droplets have an increased solidification time which results in comparatively more spherical powder particles (i.e. droplet spheroidisation time is shorter than the solidification time). Whilst it is not possible to have complete control over the particle size of as- atomised powder, the distribution can be influenced by varying the ratio of the gas to melt flow rate. Research in the field of gas atomisation has shown that even finer Fig. 5. Schematic of the EIGA process for production of AM powders (Courtesy of LPW Technology, UK) particle size distributions can be achieved through the use of hot gas atomisation (4). Although interstitial elements can be well controlled 3.2.3 Plasma Atomisation in gas atomised powders, there are still potential contamination risks. Contamination most pertinent to Plasma atomisation is a method of producing highly non-static critical components, such as aero-engine spherical particles. The feedstock used in the process parts, include refractory materials which can originate can either be in wire form such as the method used by from the ceramic crucibles and atomising nozzles AP&C Advanced Powders and Coatings Inc, Canada, used. One solution to this is to use electrode induction or in powder form such as the method used by Tekna melting gas atomisation (EIGA). EIGA is a variation of Plasma Systems Inc, Canada. The spool of wire or gas atomisation where the metal is fed into the atomiser powder feedstock is fed into the atomisation chamber, in the form of a rod that is melted by an induction coil where it is simultaneously melted and atomised by coaxial plasma torches and gas jets, such as that shown in Figure 6. Plasma rotating electrode process (PREP) is a Melt variation of plasma atomisation whereby a bar of rotating feedstock is used instead of a wire feed. As the Gas source rotating bar enters the atomisation chamber plasma and pump torches melt the end of the bar, ejecting material from its surface. The melt solidifies before hitting the walls of the chamber.

Fine powder 3.2.4 Hydride-Dehydride Process The hydride-dehydride (HDH) method (6) of powder Nozzle production differs from the atomisation processes described above due to the fact that it does not involve Collection melting of the metal feedstock. Instead, it involves chamber crushing, milling and screening to resize larger lumps of metal feedstock into finer powder particles. The Fig. 4. Schematic of the gas atomisation process for HDH process relies on the brittle nature of certain production of AM powders (Courtesy of LPW Technology, metals, such as titanium, when exposed to hydrogen. UK) In the case of titanium, titanium hydrides are formed

3.2.6 Summary of Powder Production Routes Each of these processes yields powder with varying characteristics, a summary of which can be seen in Table I. A series of micrographs highlighting the

Titanium various particle morphologies obtained from each Plasma spool torches manufacturing route is shown in Figure 7. 3.3 Powder Key Process Variables The quality of a component built in an SLM process is assessed based on part density, dimensional accuracy, surface finish, build rate and mechanical Vacuum properties. In order to achieve predictable and pump consistent component qualities it is desirable that the characteristics of the powder bed and the Collection parameters of the machine are maintained at a chamber constant level since the powder bed and machine parameters are closely correlated. In order to Fig. 6. Schematic of the plasma atomisation process for maintain a constant powder bed during each production of AM powders (Courtesy of LPW Technology, SLM build process it is important to understand UK) and control characteristics such as powder bed temperature and density. These characteristics are governed by the KPVs of the starting powder. Due to the complex nature of powders, characterising their in a hydride unit by introducing hydrogen and heat. performance is not a trivial task. A list of variables The brittle lumps can then be crushed and screened (and analysis techniques) that may be considered into the required particle size distribution (PSD). The to have an impact on performance is provided in powder is then returned to the hydride unit to remove Table II. the excess hydrogen from the metal powder particles. 3.3.1 Particle Morphology Powder particles produced using HDH are typically highly irregular. HDH powders are typically used either Particle morphology will have a significant impact on in their as-made condition or used as the powder the bulk packing and flow properties of a powder batch. feedstock for plasma atomisation. Spherical or regular, equiaxed particles are likely to arrange and pack more efficiently than irregular particles 3.2.5 TiROTM Process (9). Research into the effect of particle morphology on The TiROTM process (7, 8) is a relatively new the AM process has shown that morphology can have a method for the production of pure titanium powder significant influence on the powder bed packing density developed by CSIRO, Australia. The TiROTM and consequently on the final component density process is a two stage continuous production (10–12), where the more irregular the particle method in which titanium tetrachloride (TiCl4) is morphology the lower the final density. As a first thermally reduced to an MgCl2/Ti composite consequence of this highly spherical particles tend to under the presence of magnesium in a fluidised bed be favoured in the AM process. This limits the use of reactor. The MgCl2/Ti composite is then separated potentially cheaper powder production routes such as using vacuum distillation to produce high purity Ti water atomisation and HDH. Furthermore as can be powder. The as-processed Ti powder is unsuitable observed in Figure 7(b) gas atomised powders are for AM processes due to the particle size range only nominally spherical. In the case of the titanium primarily being 150–600 µm. As such it is necessary alloy Ti-6Al-4V, this has led to widespread adoption of that the powder is modified using a high shear plasma atomised powder. Plasma atomised powder is milling process in a controlled environment to resize typically highly spherical, but is currently produced by a the powder to a range suitable for AM. single source – AP&C, Canada.

Table I Summary of Powder Characteristics by Manufacturing Process

Manufacturing Particle size, µm Advantages Disadvantages Common uses Process Water 0–500 High throughput Post processing Non-reactive atomisation Range of particle required to remove sizes water Only requires Irregular particle feedstock in ingot morphology form Satellites present Wide PSD Low yield of powder between 20–150 μm Gas atomisation 0–500 Wide range of alloys Satellites present Ni, Co, Fe, Ti (EIGA), (inc. EIGA) available Wide PSD Al Suitable for reactive Low yield of powder alloys between 20–150 μm Only requires feedstock in ingot form High throughput Range of particle sizes Use of EIGA allows for reactive powders to be processed Spherical particles Plasma 0–200 Extremely spherical Requires feedstock to Ti (Ti64 most atomisation particles either be in wire form or common) powder form High cost Plasma rotating 0–100 High purity powders Low productivity Ti electrode process Highly spherical High cost Exotics powder Centrifugal 0–600 Wide range of particle Difficult to make Solder pastes, Zinc of atomisation sizes with very narrow extremely fine powder alkaline batteries, Ti PSD unless very high speed and steel shot can be achieved Hydride– 45–500 Low cost option Irregular particle Ti6/4 dehydride morphology Limited to metals process High interstitial content which form a brittle (H, O) hydride

3.3.2 Particle Size Distribution content produce components with a higher fractional density (13, 14). However, the use of fine materials Characterisation of PSD in a batch of powder ensures increases the risk of health and safety issues. This is that the optimum range of particles, by size, are used particularly true when processing reactive materials in each process. In general, EBM uses a nominal such as titanium where finer particulates are likely to PSD between 45–106 µm, whilst SLM uses a finer be more flammable and explosive. PSD between 15–45 µm. PSD will have an obvious impact on both the minimum layer thickness and the resolution of the finest detail in the component. 3.3.3 Bulk Packing and Flow Properties An inappropriate combination of PSD and layer thickness can potentially lead to in situ segregation Powder flowability is one of the most important due to the mechanical re-coater pushing coarser technological requirements for powders used in AM. particles away from the bed (13), segregation in this The density homogeneity of the final part depends on sense could lead to variation in build quality in the the layer-by-layer melting being performed on thin and vertical direction. It is generally well reported that uniform layers that are accurately deposited by the using powders with a wide PSD and a high fine feeding device. Cohesive powders which exhibit poor

(a) (b)

Fig. 7. Example SEM micrographs of typical particle morphologies obtained using: (a) HDH process; (b) gas atomisation; (c) plasma atomisation; and (d) plasma rotating electrode process. In all micrographs the powder shown is Ti-6Al-4V and images were taken using a Hitachi TM3000 SEM

Table II Powder KPVs and Techniques that Could be Used for their Measurement

Particulate properties Bulk properties

Powder property Assessment technique Powder property Assessment technique

Particle shape (morphology) SEM Apparent density Hall flow Optical microscopy Freeman FT4 Tap density Tapped density tester Particle size and particle size Sieve Flowability Hall flow distribution Laser diffraction Dynamic flow testing (e.g. Optical microscopy revolution, Freeman FT4) Shear cell Angle of repose Cohesiveness Freeman FT4

Particle Porosity Particle polishing and optical Surface Area BET surface area analysis microscope Chemical composition ICP-OES XRD Inert gas fusion Combustion infrared detection

250 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) flow properties are likely to be more problematic in terms to recycle the large amount of unused powder. The of obtaining homogenous density layers throughout effect of continuous powder reuse on the KPVs is the build than powders which are comparatively more an area that has up until recently received relatively free flowing. Powder flow is difficult to relate toany little scientific attention. A handful of researchers have one given parameter of a powder but there are some investigated the effect of continuous reuse of powders general rules which can typically be applied (15, 16): (2, 3) for example, however further study is required (a) Spherical particles are generally more free flowing to fully understand the impact of recycling on process than irregular or angular particles performance. (b) Particle size has a significant influence on flow – larger particles are generally more free flowing 4. Procurement Options than smaller particles (c) Moisture content in powders can reduce flow due Once a suitable atomisation route has been selected, to capillary forces acting between particles the AM user then faces more decisions around (d) Flow properties often show a dependency on the powder supply. The market opportunity for metal AM packing density at the time of measurement – powder supply has not gone unnoticed, and three powders with a higher packing density are less free main options for powder supply have emerged. Firstly, flowing than powders with a lower packing density users can choose to procure powder directly from the (e) Short range attractive forces such as van der Waals AM machine provider. Secondly, users could choose forces and electrostatic forces can adversely affect to procure powder from third party companies, who powder flow and may cause particle agglomeration offer AM machine ‘validated’ powders. Finally powder (short range forces have a bigger impact on finer can be sourced direct from an atomisation company. particles). Indeed several powder manufacturers are now offering AM specific powders as part of their product portfolio 3.3.4 Chemical Composition (the largest of these, and the alloys they provide are The laser sintering behaviour of a metal powder will listed in Table III). A summary of the advantages and not only depend on the physical properties, it will of disadvantages of each procurement option is presented course also depend on the chemical properties. in Table IV. Powder chemical composition for AM should ideally At present the majority of powder sales are through be optimised for the machine or application. Validating AM machine manufacturers or third party suppliers. chemical composition helps to ensure that the The powder provided by these suppliers has been manufactured component has homogenous material optimised for each additive process, also known as properties. being ‘validated’. As well as the bulk alloying chemistry, it is important to By validating a powder, the supplier is ensuring that the understand the effect of interstitial elements, such as O powder given to the customer is of suitable quality so and N, since component properties will depend on the that, when being processed, it will behave as intended, amount of interstitial elements present. For example, leading to a successfully built part that will adhere to the it is well known that the tensile strength and ductility chemical composition and the mechanical properties of properties of Ti-6Al-4V are influenced by O content the given metal or alloy. Simply put, the machine will whereby an increase in O results in an increase in tensile successfully build with that material, thus de-risking strength and a subsequent decrease in elongation (17). powder supply for the end user. Research has also shown that interstitial elements Machine suppliers can validate powder as they will can influence the melting kinetics of the powder by develop processing parameters on their machines for interfering with the surface tension of the melt pool each specific material. Once the machine repeatedly resulting in Marangoni flow (18). Marangoni flow can builds reliable, mechanically suitable parts then the have a negative influence on the porosity of the final parameters will be stored and sent out with batches component (11, 19). of that material to users. The powder being used will be characterised using some of the techniques 3.4 Powder Recycling discussed in Section 3.3 and all subsequent batches It has been mentioned previously that for a powder bed will undertake the same testing to ensure that they AM process to be economically viable it is necessary adhere to the same specification. This ensures that the

Table III Supplier List of Powders Specified for AMa Manufacturing Supplier type Material Building process processes

Company EBM SLM Location Manufacturer Third party Fe-Based Al-Based Ti-Based Ni-Based Co-Based Cu-Based Precious metals Water Gas Plasma Centrifugal

Advanced Canada P P P P P Powders and Coatings – AP&C Carpenter US P P P P P P P Technology Corp GKN US P P P P Hoeganaes Corp H.C. Starck EU P P P P P P P P GmbH Höganäs EU P P P P P P P P Sweden AB Sandvik EU P P P P P P P P Materials Technology TLS Technik EU P P P P P P P GmbH & Co. Spezialpulver KG LPW UK P P P P P P P P P P P P Technology Ltd aInformation obtained from the supplier websites (Accessed April 2015)

end user is receiving consistent powder across batches it contributes an almost insignificant proportion of the that has been validated for their specific process. Third income generated by powder producers (i.e. 0.0047% party suppliers offer a similar service, refining powder of the income generated from metal powders was due size and morphology to ensure they work in process. to sales into the AM market). Since the AM powder From this it can be seen that machine manufacturers market is not currently a major source of income for and third party suppliers undertake a lot of work to powder producers it is likely that powders will not be ensure the powder they provide is suitable for their AM produced to the strict requirements of AM. A further process. There are a number of obvious advantages risk with this procurement method is losing the support from procuring powder through these routes. However, of the AM machine manufacturer. This can be slightly this increased level of material supply security does de-risked by purchasing material from one of the small also draw some limitations. number of powder manufacturers who are starting Alternatively, it is possible to procure powder directly validate their own powder for AM processes. from the powder manufacturer. There are a range of The major advantage of procuring powder from this benefits from purchasing powder directly from the route is the increased choice of materials and cheaper manufacturer; however, there are some underlying procurement costs. To ensure that the specification of risks that a customer must also be aware of. The most the powder is met, a customer may need to undertake pertinent of these risks was alluded to in Section 2. The their own characterisation analysis. Further to this a current state of the metallic AM powder market is that manufacturer may only supply the powder in a wider

Table IV Advantages and Disadvantages of Procuring Powder from Machine Manufacturers, Third Party Suppliers and Direct from Powder Manufacturers

Supplier type Advantages Disadvantages

Machine manufacturers Standard machine parameters are Potentially higher cost of materials provided and are ready to use Powder that has been ‘tried and tested’ Material options are limited Support from the supplier should a build Experimenting with powder of a have issues different specification is limited Ease of sourcing Lack of traceability of material source and manufacturing process Already established procurement routes Validated third party suppliers Able to select powder from the entire Lack of traceability of material source powder metallurgy industry and manufacturing process A wide range of batch sizes are offered Lack of support from the machine manufacturer should a build fail Powder that has been ‘tried and tested’ Potentially higher cost of materials Ease of sourcing Direct from powder manufacturers Wide range of material choices Lack of support from the machine manufacturer should a build fail Potentially lower cost of powder (highly No guarantee that the material will dependent of material/process) produce a successful build Choice of manufacturing process, Minimum batch orders may apply, allowing a degree of control over powder due to minimum powder yields from a characteristics manufacturing process Can use local manufacturers Will powders be produced to the exacting standards required for AM? An increased level of material traceability Lack of powder specification with each order

PSD than the customer wants, therefore additional Additionally, the customer should always consider sieving may be required. Both of these add complexity the use of the machine and powder. For instance: is to the powder supply. the machine being used in production or research? What is the use of the end component? A machine 5. Customer Considerations used purely for production purposes will be required to make parts of the highest quality, therefore powder When deciding where to purchase powder from, a from the AM machine supplier or third party supplier customer has a number of considerations to make. is likely to be the best procurement route. The added Most importantly: benefit of this is that the machine manufacturer will • Can they supply me with the material I require? support the customer if there are any build issues. • Is the price of the powder competitive? However, if complete control and traceability of the • Can they provide the batch size I require? powder used for the build is required, then there • Is there traceability of the material source? Do I may be a lack of transparency from the AM machine require traceability? supplier as to the complete history of their powder. • Do I require knowledge over the powder Research-based machines have a different range specification? Do I require control over the powder of considerations to make. If the primary use of the specification? machine is to prototype or develop the technology,

253 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) then the customer will likely want the additional to procure large batches of powder and pass the control over the powder that they are using. The savings onto the end user. customer will also likely want to attempt building (d) Will machine and powder supply lock down or open with new materials or experiment with machine up? This will be an extremely important moment parameters. Therefore sourcing material directly for metal AM. Other industries, namely polymer- from the powder manufacturer may be best suited based AM, have seen companies use bar-coded here. systems on their machines, such that a system will There is no right answer for the procurement route not physically build unless a bar code of a material that customer takes. However, careful consideration that they supply is scanned. If this is the case with needs to be given to the application and desired metal AM machines then it could cause a severe end results of the AM system. tightening on the research capabilities of these machines. However, this is only likely to happen on 6. Future of Powder Supply Chain the highest value, production use systems. Again, as seen with polymer-based machines, competitors There are a number of theories of how this growing with machines of more open architecture are likely material market will develop over the next five to ten to enter the market. It is highly likely that a lot of years. Here, some ideas are explored. machines of this nature will emerge as patents from (a) Increased production and industrialisation of the major machine manufacturers start to expire. AM drive the price of powder down: all market predictions show the continued growth of metal AM 7. Conclusions over the coming years. Naturally, as the technology becomes widely used, the supply chain of The three main options available for the procurement materials will also grow. Factors such as increasing of AM powders include AM equipment providers, third competition and larger production runs should see party suppliers and directly from powder atomisers. the cost of powder per kilogram be driven down. As There is some degree of security in purchasing powder the powder metallurgy supply chain infrastructure is directly from AM equipment suppliers. This is because already well established, the increase in suppliers the powder batch they supply will be at least nominally of specific AM powders is likely to happen rapidly. the same grade as the powder batch used to develop This theory also applies to processes that are melt theme parameters. However, the AM equipment currently expensive to run, for example plasma supplier has ownership over the powder source thus atomisation. If, as the market develops, this is seen limiting the powder supply chain competiveness. This as the best atomisation route then the amount of results in powder costs which remain the highest of all powder produced by it will significantly increase. procurement options. Feedstock for the process will become cheaper Procuring powder directly from the atomisers may and it is likely that the range of materials available be a cheaper alternative. However, despite the recent will increase. exponential growth, the AM market is currently a (b) Game changing powder production techniques relatively small source of revenue for most powder emerge: throughout this article only atomisation atomisers. Furthermore, because of the specific as a way of producing powders for AM has been particle size fractions used in AM, powder atomisers discussed. However, in the near future, there is the may not produce powder specifically for AM. Instead likelihood that an altogether new manufacturing atomisers may obtain the required size fractions from method will eclipse atomisation, providing suitable an atomised powder batch intended for use in other powder for a fraction of the production cost. industries such as powder hot isostatic pressing (PHIP) Companies such as Metalysis, UK, are developing or press and sinter. The originally intended process new ways in which powder can be made at a for these powder batches may not require the same significantly reduced price. high quality as powders used in AM and as such their (c) Introduction of third party suppliers increases performance in an AM process may not be adequate or competition: the emergence of third party suppliers as expected. could see the price of powder driven down further. This risk of going direct to atomisers can be limited Purely through competition, suppliers will be able by using a third party powder supplier. In this case

254 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688686 Johnson Matthey Technol. Rev., 2015, 59, (3) the third party supplier takes on the associated risks Technology Organisation, Brussels, Belgium, 2006 of procuring powder batches in much higher quantities 3. V. Seyda, N. Kaufmann and C. Emmelmann, Phys. than would be needed by any one AM service provider. Proc., 2012, 39, 425 This then allows the AM service provider to procure only 4. J. J. Dunkley, ‘Hot Gas Atomisation – Economic and the amount they need. The higher powder quantities Engineering Aspects’, in Proceedings of the World procured by a third party supplier potentially provides Congress and Exhibition on Powder Metallurgy, them with the necessary influence to demand higher PM2004, 17th–22nd October, 2004, Vienna, Austria, quality powder batches that are atomised specifically The European Powder Metallurgy Association, with AM as the intended end use. The level of pre- Shrewsbury, UK, 2005 sale powder qualification is also much more detailed 5. R. Gerling, H. Clemens and F. P. Schimansky, Adv. from third party suppliers than from the atomisers Eng. Mater., 2004, 6, (1–2), 23 themselves. Even with this option the procurement 6. C. McCracken, Powder Injection Moulding Int., 2008, costs can be higher than going directly to the atomisers 2, (2), 55 and less support will be made available from equipment 7. C. Doblin, D. Freeman and M. Richards, Key Eng. suppliers should the procured powder be considered a Mater., 2013, 551, 37 potential factor for build failures. 8. C. Doblin, A. Chryss and A. Monch, Key Eng. Mater., What increases the complexity, and indeed 2013, 520, 95 uncertainty, of procuring powders for AM is the lack of 9. N. P. Karapatis, G. Egger, P.-E. Gygax and R. AM specific powder specifications. It is commonplace Glardon, ‘Optimization of Powder Layer Density in to make decisions to accept or reject powder batches Selective Laser Sintering’, in Proceedings of the 10th based on specifications used for press and sinter International Solid Freeform Fabrication Symposium, applications. These specifications would at best include The University of Texas at Austin, USA, 9th–11th chemical composition, sizing by sieve analysis and August, 1999, pp. 255–264 flow assessment by Hall flow. Such specifications can 10. I. Gibson, D. W. Rosen and B. Stucker, “Additive be inadequate to use as a benchmark for AM powder Manufacturing Technologies: Rapid Prototyping to quality. As discussed throughout this article predicting Direct Digital Manufacturing”, Springer, New York, powder performance is highly complex and can be USA, 2010 difficult to characterise using simple techniques. Future 11. H. J. Niu and I. T. H. Chang, Scripta Mater., 1999, 41, work needs to be aimed at systematically identifying (1), 25 the properties of metal powders that have the biggest 12. D. Manfredi, F. Calignano, M. Krishnan, R. Canali, E. influence on the powder performance in terms of P. Ambrosio and E. Atzeni, Materials, 2013, 6, (3), 856 hopper discharge and powder spreading and also how 13. A. Simchi, Metall. Mater. Trans. B, 2004, 35, (5), 937 the powder responds to the AM melting process. Work 14. A. B. Spierings and G. Levy, ‘Comparison of Density of in this field will allow the development of specifications Stainless Steel 316L Parts Produced with Selective Laser which adequately define and control the key process Melting using Different Powder Grades’, in Proceedings of variables of powders used in AM. the 20th Annual International Solid Freeform Fabrication Symposium, The University of Texas at Austin, USA, References 3rd–5th August, 2009, pp. 342–353 15. P. C. Angelo and R. Subramanian, “Powder Metallurgy: 1. ‘Wohlers Report 2014: 3D Printing and Additive Science, Technology and Applications”, PHI Learning Manufacturing State of the Industry Annual Worldwide Pvt Ltd, New Delhi, India, 2008, pp. 76–80 Progress Report’, Wohlers Associates, Inc, Fort 16. R. M. German, “Powder Metallurgy Science”, Metal Collins, Colorado, USA, 2014 Powder Industries Federation, Princeton, New Jersey, 2. P. A. Carroll, P. Brown, G. Ng, R. Scudamore, A. J. USA, 1994, pp. 9–58 Pinkerton, W. U. H. Syed, H. K. Sezer, L. Li and J. 17. J.-M. Oh, B.-G. Lee, S.-W. Cho, S.-W. Lee, G.-S. Choi Allen, ‘The Effect of Powder Recycling in Direct Metal and J.-W. Lim, , 2011, 17, (5), 733 Laser Deposition on Powder and Manufactured Metal. Mater. Int. Part Characteristics’, in “Proceedings of AVT-139 18. M. Rombouts, J. P. Kruth, L. Froyen and P. Mercelis, Specialists Meeting on Cost Effective Manufacture CIRP Annals, 2006, 55, (1), 187 via Net Shape Processing”, 15th–19th May, 2006, 19. D. Boisselier and S. Sankaré, Phys. Procedia, 2012, 39, Amsterdam, The Netherlands, NATO Science and 455

The Authors Jason Dawes is a Senior Research Engineer at the Manufacturing Technology Centre (MTC) where he leads the Particulate Engineering research group. His role is in the technical management of highly innovative research projects involving powder based manufacturing technologies such as additive manufacturing, laser cladding and hot isostatic pressing. He was awarded EngD from University of Birmingham, UK, in 2014 in the field of Chemical Engineering.

Robert Bowerman is a Research Engineer at MTC where he leads projects within the field of additive manufacturing. He has experience in the following technologies: electron beam melting (EBM), selective laser melting (SLM), stereolithography (SLA), digital light processing (DLP) and fused deposition melting (FDM). His projects focus on innovative research and development work primarily on the Manufacturing Capability Readiness Levels (MCRLs) 4 to 6. Work carried out covers all areas of additive manufacturing, including development and productionisation of technology and designing for AM. He is presently working towards Chartered Engineer status.

Ross Trepleton is the Component Technologies Group Technology Manager at the MTC. He is responsible for the coordination and management of the MTC Research Programme, in the area of Component Technology to meet the needs of clients and stakeholders. The aim of this area of research is to develop new technologies that enable improved component manufacturing. He was awarded a PhD from University of Birmingham in the field of Metallurgy and Materials.

JOHNSON MATTHEY TECHNOLOGY REVIEW www.technology.matthey.com

Atomic-Scale Modelling and its Application to Catalytic Materials Science

Developing an interdisciplinary approach to modelling

By Misbah Sarwar, Crispin Cooper and becoming increasingly commonplace. This is a field Ludovic Briquet that entails linking fundamental chemical properties, Johnson Matthey Technology Centre, Blounts Court, for instance electronic and geometric structure, to Sonning Common, Reading, RG4 9NH, UK activity, ultimately providing the basis from which enhanced materials can be predicted (3). A particularly Aniekan Ukpong, Christopher Perry and Glenn attractive feature of computational approaches is the Jones* flexibility and applicability to all divisions of Johnson Johnson Matthey Research Centre, CSIR, Meiring Matthey’s business. For instance within New Business Naude Road, Brummeria, Pretoria, 0184, South Africa Development methods to simulate optical properties of materials (for smart glass applications) are being *Email: [email protected] applied; within platinum group metals (pgm) refining we are exploring metal extraction and applying engineering process modelling; and within glass and Computational methods are a burgeoning science colour technologies we are using thermodynamic tools within industry. In particular, recent advances have to help us understand the formation and properties seen first-principles atomic-scale modelling leave the of glass compositions. Of particular importance to realm of the academic theory lab and enter mainstream Johnson Matthey is catalyst technology (Chemical industrial research. Herein we present an overview, Catalysis, Emission Control, Process Technologies and focusing on catalytic applications in fuel cells, emission Chiral Catalysis Technologies) where we are working control and process catalysis and looking at some towards a multiscale modelling capability that links the real industrial examples being undertaken within the macroscopic engineering world to the atomic-scale Johnson Matthey Technology Centre. We proceed to chemistry and physical properties of materials. discuss some underpinning research projects and give A key philosophy within the research group and what a perspective on where developments will come in the makes the field so important for industrially relevant short to mid-term. applications, is the importance of linking theory with experiment whether through measurement and 1. Introduction validation of the computational methods or by actively seeking collaboration with experimentalists who can The use of atomic-scale modelling, whether force-field synthesise, characterise and test interesting materials. methods or electronic structure theory calculations This overview article will firstly discuss some of the (for example density functional theory (DFT) (1, 2)) background to the methods employed, then move on in chemical and material research within industry is to discuss some recent projects that have been carried

257 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) out with a particular focus on catalytic applications. state-of-the-art approaches, with the implicit limitations The aim is to give a feel for the types of application thereof, can be applied to a specific catalytic reaction we have been tackling, and the underpinning research or application of interest (12–15). Critical in this we are conducting, before moving on to conclude by approach is the development of a suitable atomic-scale highlighting areas where continued research effort will model, from which we can extract information about the be required in the future. electronic properties or certain chemical quantities, for Computational catalysis is a field that has been example reaction energies and activation barriers. The flourishing over the past two decades. This is a result idea is to understand the reaction sufficiently, such that of two primary drivers: the development of efficient we can develop predictive models that can influence the generic codes or algorithms and the improvement in materials development process. This can be achieved at available computer power on which to run these codes a number of levels starting from potential energies and (Figure 1) (4, 5). Computational catalysis as a sub- electronic properties (occupancies, d-band centres and discipline has arguably emerged from surface science highest occupied molecular orbital/lowest unoccupied (6, 7). This is unsurprising given that most of molecular orbital (HOMO/LUMO) gaps), before moving heterogeneous catalytic chemistry occurs at the surface on to thermodynamic models and ultimately full kinetic of materials. However, it is also inextricably linked to models (Figure 2) (16). One key concept is the solid-state materials chemistry, for example the bulk identification of a key property or trend that can then be properties of complex oxides determine the surface used as a descriptor to ‘screen’ through other potential facets that are present and the complex reaction candidates (3, 12). This approach has met with some atmospheres present during catalysis lead to various success, and has led to a number of initiatives working oxidation states of the material, which all play a crucial on generating databases and computational tools to role in the activity and stability of a given catalyst (8–11). accelerate or aid the discovery of novel materials using A key interest of computational catalysis to the ab initio approaches (17–22). Close collaboration with industrial arena is that of material prediction. Here experimentalists then completes the material discovery

Modelling methods across the length scales 20 integral in Johnson Matthey’s development

19 JMTC – Pretoria; Computational 18 Catalysis and Materials Science 17 JMTC Computational Chemistry 16 Group Nobel Prize 15 Walter Kohn and John Pople DFT and computational chemistry Multi- scale modelling; large scale DFT 14 Perdew calculations; holistic simulation of systems Wang

(computing power/FLOPPS) 13 DFT for chemists Application of 10 DFT to reactions; predictive modelling log 12 of catalysts and other materials Development 11 of DFT for chemisorption problems 10 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 Year Fig. 1. Log plot showing the most powerful supercomputer, as measured in floating point operations per second (FLOPPS) (5) vs. year. In grey are key developments in algorithms and then in blue, on the development of atomic-scale modelling within Johnson Matthey, as an example of an industrial user

(a) NiAl_1 NiAl_2 (b)

NiAl_3 NiAl_4

1.0 Ni:Sn 2 Ni:C –1 0.5 Ni:Ga Ni:Zn –4 Ni 0.0 –7 Ni:Al

O binding, eV –10 –0.5 Ni:Mo Ni:Cr –13 Ni:Zr –1.0 Ni:Ti –16

–1.5 –19 –1.0 –0.5 0.0 0.5 1.0 1.5 2.0 C binding, eV Fig. 2. The process of developing a surface model, studying the adsorption of intermediates and transition states, culminating in the kinetic model that is used to screen for new catalyst candidates. (Republished with permission of Maney Publishing, from (16); permission conveyed through Copyright Clearance Center Inc)

process, allowing iterative refinements of models and interface of fundamental research we are working ultimately the synthesis of candidate materials. towards the future development of Johnson Matthey Beyond the immediate task of predicting materials technology. for current needs, the second important focus of computational catalysis is to look to the future: asking 2. Homogeneous Catalysis questions such as how to go beyond the current state of the art to describe systems more accurately or how Homogeneous catalysts operate in the same phase to place the atomic scale picture in the context of the as the reactants, usually but not exclusively dissolved complete catalytic solution. Often this involves trying in a solvent. A wide variety of homogeneous catalysts out new methods and establishing collaborations with exist: these include Lewis and Brønsted acids, metal academic researchers. ions, metal complexes, organometallic complexes In order to illustrate some of the above we now proceed and biocatalysts. However, more recently, the term to discuss some laboratory projects from the areas of homogeneous catalyst is often applied to organometallic homogeneous catalysis, heterogeneous catalysis and or coordination complexes. electrocatalysis that are, or have been, conducted Owing to the fact that upon completion of a reaction, within the Johnson Matthey Technology Centre in more the catalyst must be recovered from the products (often detail. After which we shall proceed to discuss some a costly process) and that in general heterogeneous of the future directions of our research and show how catalysts are more stable, homogeneous catalysts by working at the cusp of the industrial and academic enjoy only limited application in industry. There are

259 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) however, a number of important industrial reactions In collaboration with experimentalists from Johnson catalysed by homogeneous catalysts, including Matthey Catalysis and Chiral Technologies, a hydroformylation, methathesis, carbonylation and modelling study was undertaken to explore some polymerisation using Ziegler-Natta catalysts. Perhaps anomalies pertaining to the direct arylation of oxazole it is in the pharmaceuticals and fine chemicals and 4-bromotoluene. Oxazole, 1, with three reactive industries where homogeneous catalysts enjoy their C–H sites, usually displays the reactivity order: greatest success. Coupling reactions such as the C5>C2>C4. However, in certain instances, the reaction Suzuki-Miyaura reaction, Sonogashira coupling, the at the C5 site appears to be disfavoured, leading to a Heck reaction, Stille cross-coupling and the Buchwald- predominance of C2 product. Table I shows how the Hartwig reaction have long been employed to bring product distribution is affected by altering the amount about various organic transformations. of added phosphine ligand. Computational studies with DFT methods on the transmetallation step of the various cross-coupling reactions first appeared just over a decade ago (23–28) 1 O and provided useful insight into the energetics and 5 2 mechanism of the reaction. Within a couple of years, 4 N 3

DFT results for the full catalytic cycles were reported Oxazole, 1 (29–34). DFT theoretical studies have also recently been used to investigate cross-coupling reactions involving metals other than palladium; examples include: Table I Product Distribution for the Reaction rhodium-catalysed coupling via C–H bond activation (35, 36), nickel-catalysed cross-coupling reactions of Oxazole with 4-Bromotoluene Using (37–42), delineating the mechanism of iron-catalysed the Pre-Catalyst Palladium Acetate cross-coupling reaction (43–47), dialkylzinc-mediated (Pd(OAc)2) cross-coupling (48), copper-catalysed C–N cyclisation Catalyst Ligand % % % Di % Di Mono Mono 2,5 4,5 (49) and enantioselective nucleophilic borylation (50). 5 2

2.1 Palladium-Catalysed C–H Bond Activation Pd(OAc)2 – 66 0 28 3 of Heterocycles t Pd(OAc)2 1 × P Bu3 31 4 59 1

t Over the past decade, Pd-catalysed direct C–H arylation Pd(OAc)2 2 × P Bu3 6 35 50 0 has emerged as an important alternative to traditional cross-coupling reactions for the synthesis of a wide range of arylated heterocycles (51–56). Compared In the absence of added phosphine ligand, to traditional Suzuki-type cross-coupling methods, dimethylacetamide (DMA) solvent is believed to direct arylation has the advantage that it does not coordinate to Pd. Figure 3 shows the reaction coordinate require the preparation of stoichiometric quantities of diagram for the C–H activation step (believed to be organometallic reagents (typically alkyl or aryl boronic rate-limiting) of oxazole at positions C5, C4 and C2, acids), thereby eliminating steps in the synthesis and by the well-established CMD mechanism. The results reducing the amount of toxic metallic waste. correlate well with the experimental observation that for

The most widely accepted mechanism for these Pd(OAc)2 in the absence of added phosphine, mono‑5- reactions is the concerted metallation-deprotonation product is formed in preference to both mono‑2- and (CMD) pathway, involving simultaneous Pd–C bond mono-4-substituted product. formation and aromatic C–H bond cleavage to yield Figure 4 shows the reaction coordinate for oxazole a diaryl Pd(II) species (57–59). A second mechanism, arylation, via the CMD mechanism, in the presence t involving a cyclometallated complex, has also recently of P Bu3 ligand. Formation of the initial intermediate been proposed to account for the much lower than appears to be disfavoured for this mechanism, for both expected reactivity observed when certain tri-tert- coordination through N3, as well as through the C4=C5 butylphosphine-containing Pd catalysts are reacted pi bond. The barrier heights for all three intermediates with heteroarenes in isolation to the catalytic cycle are also fairly large. Our calculations thus suggest (60, 61). that C–H activation at any of the reactive sites via the

30.00

25.00 N O O H

O Pd Ar 20.00 DMA O O H N 15.00 O Pd Ar DMA N –1 O 10.00 OH O Pd Ar N DMA 5.00 N O H O

ΔG, Kcal mol O 14.0 O O Pd Ar O + O Pd Ar N 0.00 DMA H DMA –O O O Pd Ar DMA O Pd Ar N –5.00 O DMA + N+ O O Pd Ar H O –O –10.00 O DMA N O Pd Ar O DMA –15.00 N O Pd Ar O DMA Fig. 3. Free energy diagram for direct arylation of oxazole with [(DMA)Pd(Ar)(OAc)] via the CMD mechanism

N O 30.00 O H O Pd Ar t O PBu3 25.00 O H N N O Pd Ar OH O t 20.00 PBu3 O Pd Ar PtBu 3 O N N + 15.00 O N O H O H –O O 11.0 O Pd Ar O Pd Ar O Pd Ar –1 t t PtBu PBu3 10.00 PBu3 3 + N N O O H –O 5.00 O Pd Ar O Pd Ar t ΔG, Kcal mol PBu3 O t PBu3 0.00 O

O N –5.00 O Pd Ar O Pd Ar PtBu O t 3 PBu3 + –10.00 O N

–15.00 t Fig. 4. Free energy diagram for direct arylation of oxazole with [(P Bu3)Pd(Ar)(OAc)] via the CMD mechanism

t CMD mechanism in the presence of P Bu3 ligand is A better understanding of the factors that control the unfavourable. reactivity and regioselectivity of various heteroaromatic Hartwig et al. (60, 61) have shown that direct substrates is important in designing reaction conditions t arylation of pyridine N-oxide with (P Bu3)Pd(Ar) that will favour activating a specific C–H bond in (OAc) involves cooperative catalysis between two a particular substrate. Additionally, it would allow distinct Pd centres. Figure 5 shows the modelling Catalysis and Chiral Technologies to better market results for oxazole C–H activation via this recently their range of Pd-catalysed cross-coupling catalysts proposed ‘cooperative’ mechanism and is in towards specific applications. Future work includes agreement with our experimental findings. The investigating the regioselectivities of the di-substituted t initial complex that forms has oxazole coordinated products, as well as the effect that the amount of P Bu3 to Pd either through N3 or through the C4=C5 has on the product distribution. pi bond. In this case, however, formation of the –1 latter is unfavourable by around 2 kcal mol . C5 3. Heterogeneous Catalysis activation still has a marginally lower barrier height, but initial intermediate formation is unfavourable. Heterogeneous catalytic materials come in several

The energy barrier for C2–H activation is still quite forms. Generically one could say that these are large (around 21 kcal mol–1), but not as large as either unsupported (for example zeolites or PdO) or for the DMA ligand. These findings suggest that supported (for example nickel/alumina). The majority

C5–H activation is not significantly favoured over of our research has focused on the latter type, where C2–H activation as is observed in the absence of a either pure, doped or alloy nanoparticles are deposited phosphine ligand. on a support that may be either inert, acid/base or This work has shown how modelling can be redox active. Historically, extended infinite surfaces instrumental in explaining experimental observations. have been used as models corresponding to the limit

30.00 N O O H

25.00 O Pd P tBu tBu N 20.00 O O H N OH O Pd O O Pd P t P 15.00 tBu Bu tBu tBu O N+ H –O N Pd –1 10.00 N O O O H O t P O Bu 12.8 tBu O Pd O Pd 5.00 P P tBu tBu tBu tBu N+ O ΔG, Kcal mol H N – 0.00 O O O Pd P O O Pd tBu O P tBu O Pd tBu –5.00 tBu tBu P O tBu –10.00 + N O O Pd O P N tBu –15.00 tBu

t Fig. 5. Free energy diagram for direct arylation of oxazole with [(P Bu3)Pd(Ar)(OAc)] via the cooperative mechanism

262 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) of large nanoparticles (>3 nm – this number is slightly on specific reactions, which may be a combination arbitrary due to the computational effort required to of electronic and geometric influences (Figures 6 explicitly define this value). There is a plethora of real and 7), and also the influence of the so-called ‘metal examples where materials design has been influenced support interaction’ in which electronic, geometric by the result of calculations on such systems (62, and ‘spill-over effects’ play a role (64). In fact, these 63) and indeed this is often the first approach taken influences can be reaction specific and sometimes with our in-house modelling. However, the complexity even condition specific. It still remains a challenge to of a catalyst is such that detailed questions remain, theoretically address these questions fully and this surrounding for example the influence of particle size leads to much vital underpinning research.

(a) 0.8 (b)

CN: 6 CN: 7 0.6 CN: 8 or 9 CN: 12

0.4 Terraces Fraction of atoms 0.2 Steps

Corners 0 0 1 2 3 4 5 6 d{[100]–[–100]}, nm Fig. 6. (a) Graph showing the fraction of atoms of a given type (terrace, step or corner group by coordination number (CN)) as a function of particle size. The plot has been generated for Pt particles using a Wulff construction from surface energies; for clarity only CN 6, 7, 8, 9 and 12 are shown. (b) From top to bottom: three representative surface models are shown: {111} for a terrace (CN: 9), {211} for a step (CN: 8), {532} for a corner atom (CN: 6)

(a) (b) 10 0.8 Single atom 0.4 Bulk

Total DOS Total 0.0 –10 –5 0 5 10 8 201 0.8 6 cluster 0.4 165

Total DOS Total 0.0 6 135 –10 –5 0 5 10 0.8 13 cluster 79 0.4 DOS 4 55

Total DOS Total 0.0 –10 –5 0 5 10 43 0.8 79 cluster 0.4 2 19

Total DOS Total 0.0 –10 –5 0 5 10 13 0.8 Bulk 0 1 0.4 –10 –8 –6 –4 –2 0 2 4

Total DOS Total 0.0 E–Ef, eV –10 –5 0 5 10

Fig. 7. Density of states for: (a) small gold clusters as a function of size (increasing from top to bottom); (b) Pt clusters as a function of size (decreasing from top bottom). When viewed in conjunction with Figure 3, it can clearly be seen that overlaid on the geometric effect of coordination is an electronic effect, partly as a result of low coordination, but also as a result of localised electronic states intermediate between atomic and bulk-like

3.1 Methane Activation of sulfur adsorption on the steam reforming reaction over a Ni{211} and the resulting activity trends for a The activation of methane is central to many series of alternative alloys. technologies, from hydrocarbon combustion and During the steam reforming reaction the dissociation methane oxidation in catalytic emission control to the of methane on a nickel catalyst can lead to the synthesis of hydrogen and syngas as a feedstock for formation of polymeric carbon on the catalyst surface Fischer-Tropsch and ammonia syntheses (65). (Figure 10). The carbon filaments so formed are very In the area of steam reforming catalysis, models problematic, ultimately leading to deactivation of the have been developed to go beyond understanding catalyst and shutting down of the reactor. Using DFT the intrinsic activity of steam reforming catalysts modelling, the origin of the deactivation has previously and begin to consider the influence of poisons and been investigated along with possible mitigation unwanted side reactions. For instance, building strategies (71–73). Two fundamental approaches on published models (66), extensions have been have been studied in-house: the first is to examine made to account for: (a) site blocking and (b) the the role of dopants and alloys on the initial formation electronic deactivation caused by the presence of and nucleation of the problematic carbon, the second sulfur (67). Using the modified model the influence to look at burn-off of any formed carbon using oxygen of sulfur can be explicitly simulated (68), enabling based species and how this can be promoted. strategies to be developed to mitigate its influence. The work has shown that in general if one inhibits Figure 8 shows an example of a sulfur overlayer, the formation of carbon there is a negative effect on along with a calculated thermodynamic isostere the activity of the catalyst; there is the unfortunate for sulfur coverage on Ni {111}, as a function of problem that the very surface sites responsible for temperature and partial pressure of hydrogen catalytic activity are also the primary nucleation sites sulfide (H2S). This figure allows direct comparison for carbon formation. This leads one to consider the to published experimental work (69), against which promotion of burn-off as the pragmatic route to deal calculated enthalpies (ΔH) and entropies (ΔS) with it (Figure 10). Through modification of the support can be validated. This exercise showed excellent or surface composition of the catalyst it is hoped that agreement (ΔH (800 K): –145 (Exp. –155.2) kJ mol –1, promotion of this mechanistic route can be achieved ΔS (800 K) = +38 (Exp. +35.9) J K –1 mol–1), indicating and this is the subject of ongoing research. that these more complex reactivity studies result in Ordinarily, the presence of excess carbon is unwanted. good trends. Figure 9 (70) illustrates the influence However, there are also reactions where the presence

(a) (b) –10

–12

–14

–16

–18

–20

–22 1.0 1.2 1.4 1.6 1.8 2.0 1000 k/T Fig. 8. (a) Optimised structure of 0.25 ml S on a Ni surface; (b) isostere of S calculated from first-principles. Good agreement for saturation coverage (just below 0.25 ml) and entropies is found with the work of McCarty and Wise (69)

(a) (b)

0 0 TOF Rh3Ru 1500 Rh3Ni θH –1 θ –1 C Rh θS θ log (TOF/s CH2 –2 θO –2 –1 1000 θOH θ* log (θ) –3 –3 –1 Ru3Rh ) Pt3Ru 500 Ni3Rh

0 50% drop in activity Price, $ kmol –4 –4 Pd3CoPd3Ru Ru3Ru Ru3Pd Ru3Ni Co3Pd Ni3Ru –1 0 Ni3Ni –5 –9 –8 –7 –6 –5 –5 –9 –8 –7 –6 –5 –4 –3 –2 0 0.2 0.4 0.6 log (PH S/Pstan.) 2 Activity Fig. 9. (a) Calculated turn over frequency and coverage of surface species for methane steam reforming in the presence of S over a Ni {211} surface. Kinetic models like this can be used as the basis for theoretical screening studies; (b) Pareto optimal, intrinsic metal cost (2009), vs. activity for methanol steam reforming (MSR) catalyst, oxidised catalysts have been filtered out following the method outlined in (70)

(a) (b) (c)

Fig. 10. Illustration of: (a) nucleation of C at step edge (or defect); (b) growth of graphitic-like carbon; (c) reaction of graphitic carbon with surface OH (burn-off mechanism)

of carbon growth has been found to promote catalytic studies (79) and experimental evidence (80) suggest activity (74). This has inspired a fundamental research that YSZ will favourably adsorb molecular oxygen over project looking at how defective graphene sheets can intrinsic vacancy sites and partially reduce it creating potentially act as catalysts for converting propane to stabilised superoxide-like species. DFT calculations propene (75). Furthermore, carbon is also used as of molecular oxygen adsorbed over a vacancy site on a support in applications such as fuel cells. These the YSZ [111] surface showed that the O2 species was observations highlight that it is not straightforward partially reduced. Charge distribution analysis showing to classify whether a given element is a poison; as it to be in an approximately –0.5 oxidation state, with exemplified by the case of carbon in catalysis, it can be the O–O bond lengthened slightly and large regions both a friend and a foe. of unpaired electron density on each oxygen atom, Partial oxidation of methane over yttrium stabilised all demonstrating transfer of electron density from the zirconia (YSZ) has been experimentally shown to be surface in to the oxygen π* anti-bonding orbital. The catalytically active for the partial oxidation of methane adsorption was found to be energetically favourable, (76, 77). First-principles electronic structure calculations the DFT model giving an adsorption energy of –0.47eV. have been applied to explore the possible mechanism The oxygenated surface model described above was of methane C–H bond activation over the oxygenated used to study the methane activation process. The pure

YSZ [111] surface (Figure 11) (78). Previous theoretical zirconia (ZrO2) and unoxygenated YSZ surfaces were

Alloying Pt with another transition metal is one possible H way of overcoming some of these issues and this is an H H H H area that has been subject to intensive research in the H last few years (84–89). H The ORR at the cathode consists of several reaction ~0.5– – H O O O O pathways, the detailed understanding of which is still O2– –O 4+ the subject of ongoing research (90). Generally, the Zr Zr4+ reaction can follow two pathways: the four electron

Delocalised route to water (Equation (i)) and the two electron route YSZ surface YSZ surface to H2O2 (Equation (ii)):

+ – Fig. 11. Schematic diagram of the proposed methane O2 + 4H + 4e à 2H2O (i) activation mechanism over oxygenated YSZ surface. + – The excess charge balance on the product is delocalised O2 + 2H + 2e à H2O2 (ii) throughout the ZrO2 system (78) On Pt and Pt alloy surfaces it is generally agreed that the ORR follows the four electron pathway to water. This reaction can be broken down into the following found to be relatively inert towards methane. However, elementary steps (Equations (iii)–(v)): on the oxygenated surface, methane was found to ½O à O* (iii) transfer a hydrogen to a surface lattice oxygen ion in 2 a site neighbouring an adsorbed O species to yield a 2 O* + H+ + e-à OH* (iv) surface OH group and a triangular planar CH3 molecular species in the gas phase. Analysis of the charge density + - OH* + H + e àH2O + * (v) distribution confirms that the CH3 entity is a charge neutral methyl radical and that the H3C–H bond has (where * denotes a surface atom or empty surface site). undergone homolytic cleavage. The overall process In order for a surface to be an effective catalyst it is predicted to be energetically favourable (–0.23 should follow Sabatier’s principle (91, 92). This states eV). Transition state calculations using a constrained that a good working catalyst should have the ability algorithm in combination with the nudged elastic band to break bonds and generate intermediates. However, method (81), revealed a relatively low activation barrier it should also have a low enough interaction energy of approximately 0.4 eV, suggesting that the reaction with these intermediates not to stabilise them on should occur readily. Analysis of the changing geometry the surface, so that they can react further and free and electronic structure allowed the prediction of the up adsorption sites. It has been suggested however simplified mechanism given in Figure 11, with the that the reason for the slow ORR may be due to adsorbed oxygen acting as an electron acceptor. It is the O intermediate binding too strongly to the Pt likely that similar mechanisms may operate over other surface, therefore accumulating on the surface and related metal oxide materials. Further information and blocking active sites. The surface d-band centre and analysis of this work is available (78). oxygen binding energies have been widely used as descriptors for the ORR. Both these show a volcano 4. Electrocatalysis type relationship with catalytic activity (85). 4.1 Transition Metal Alloy Nanocatalysts Proton exchange membrane (PEM) fuel cells offer a promising clean energy source for a range of both High throughput screening of materials using stationary and automotive applications. However, there computational approaches such as DFT has become a are a number of issues which must be overcome in powerful and valuable tool in catalyst design. The search order for these to be commercialised. These include for the optimal material, based on predicted activity and the high cost of the platinum electrodes, slow kinetics stability, among a great number of alloy combinations of the oxygen reduction reaction (ORR) and stability is both a materials and a combinatorial problem. A high of the electrodes under operating conditions (82, 83). throughput approach was developed and adopted for

266 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) investigating the impact of varying the ratios of metals in the alloy surface layer to determine which compositions are most stable and electrochemically active. In this work, through collaboration with project partners Accelrys and CMR Fuel Cells, a combined theoretical and experimental approach was taken to investigate trends in the stability of Pt-M-Pt and M-Pt-Pt core-shell type catalysts (M = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Au, Ta, Hf, Cr, Nb, V, Y, Sc, Ti and Zr). The surfaces were modelled using a 2×2 unit cell of the face-centred cubic (fcc) crystal of Pt (lattice constant Fig. 12. Schematic depicting the under/overlayer structure of bulk Pt system: a = 4.010 Å). The Pt-M-Pt configuration core-shell type catalysts: (a) Pt-M-Pt structure; (b) M-Pt-Pt was modelled by substitution of the second atomic layer structure of Pt with M atoms, whereas the M-Pt-Pt configuration consisted of the Pt atoms in the first atomic layer being between the Pt-M-Pt and M-Pt-Pt structures with and substituted with M atoms (Figure 12). without O/OH adsorbed. If the segregation energy is The stability of the electrode under fuel cell operating negative the Pt skin structure is favoured, if it is positive conditions is a significant challenge to understand, as a surface of M atoms is favoured (Equation (vi)): the highly corrosive environment can promote surface segregation of the alloying component, which can Eseg = E (Pt-M-Pt) – E (M-Pt-Pt) (vi) then be leached away removing any benefit obtained from alloying (93–95). The adsorption of O and OH and in the presence of adsorbate (O or OH) (Equation (vii)): on the surfaces of Pt-M-Pt and M-Pt-Pt alloys was E = E – E (vii) investigated, sampling all possible adsorption sites to seg (Pt-M-Pt)(O/OH) (M-Pt-Pt)(O/OH) identify the most stable. The stability of the surface was Figure 13 shows the segregation energies for the assessed by calculating the segregation energy, Eseg, structures with and without O and OH adsorbed. firstly in vacuum (Equation (vi)) and then with theO The value of the segregation energy indicates how or OH atom adsorbed (Equation (vii)). The segregation strongly a particular configuration is favoured. A more energy is defined as the difference in total energy negative segregation energy indicates that a Pt skin is

2.00 PtAg PtAu PtCo PtCu PtFe PtIr PtNi PtOs PtPd PtRh PtRu PtAl PtBe PtCr PtHf PtMn PtNb PtSi PtTa PtTi PtV 1.00 0.00 –1.00

–2.00 Clean , eV

seg –3.00 Oads E OH –4.00 ads –5.00 –6.00 –7.00 Alloy Fig. 13. Calculated segregation energies for clean {111} surfaces of various Pt alloys in vacuum and with ¼ ml of O or OH adsorbed

267 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) strongly favoured. In vacuum it can be seen that the predicted to bind O more weakly than Pt. It is these Pt-M-Pt structure is favoured in all cases except Au alloys which bind O slightly more weakly than Pt that and Ag (purple bars). However when O is adsorbed are predicted to have enhanced ORR activity. However the segregation energy is weakened in comparison to alloys such as Pt-Ti-Pt and Pt-Al-Pt are predicted to the clean surface (blue bars). For the ORR reaction, have poor ORR activity as they bind O too weakly. the materials that retain a Pt skin are most promising. Using this approach over 2000 alloy combinations For Pt-Ni-Pt, Pt-Pd-Pt, Pt-Be-Pt and Pt-Y-Pt surface were screened and several potential candidates segregation of M is predicted in the presence of identified to put forward for further experimental testing. adsorbed O. For the remaining alloys a Pt skin is This theoretical pre-screening approach allowed predicted though this is not favoured as strongly as for the list of combinations for testing to be significantly the clean surface. A reversal in segregation energies narrowed down, saving valuable experimental time is observed for Pt-Ag-Pt and Pt-Au-Pt, which are more and resource. stable with a Pt skin in the presence of O. Adsorption of OH on the surface destabilises the 4.2 Carbide Core Nanocatalysts system resulting in a weaker segregation energy compared to the clean surface. This destabilisation An alternative strategy to using a transition metal is less pronounced than previously seen for O at the alloy as a core particle is to explore the possibility of surface, with more negative segregation energies using transition metal nanoparticles (96). The use of obtained for OH. This indicates that the tendency to carbides as non-transition metal cores supporting retain a Pt skin in the presence of OH is stronger than thin films of Pt has been investigated. Through the in the presence of O. study of various carbides and their interaction with Pt The most promising ORR catalysts are ones in which overlayers the project has been able to elucidate (a) a Pt skin is retained, acting as a protective barrier the geometric and electronic influences on the stability to prevent the leaching of M from the subsurface. of Pt on these carbides; and (b) how the presence Analysis of the segregation energies in the presence of the different carbide cores influences predicted of both O and OH indicate that in terms of stability catalytic activity (Figure 15). Working closely with the all compositions except Pt-Be-Pt, Pt-Ag-Pt, Pt-Au-Pt, synthetic chemists at Johnson Matthey Technology Pt-Ni-Pt and Pt-Pd-Pt are promising, as a Pt skin is Centre, Sonning Common, the research has created retained in the presence of O and OH when adsorption inspiration and directions for research which are being takes place. followed in-house. Figure 14 shows the adsorption energy of ¼ ml of It is understood that synthesising a material where Pt oxygen on the surface of each Pt-M-Pt alloy. Alloys wets a carbide surface or other non-metal cores may to the left of Pt have a lower d-band centre and are be challenging from a thermodynamic perspective.

–3.4 –2.9 –2.4 –1.9 –1.4 –2

–2.2 PtAl PtTi PtSc –2.4 PtNb PtHf/PtZr –2.6 PtV PtTa –2.8

, eV PtCr PtCo

ads –3 PtMn PtRu PtCu

E O –3.2 PtOs PtFe PtRh PtAg Pt –3.4 PtIr PtAu –3.6

–3.8 PtPd –4 Ed–Ef, eV Fig. 14. Correlation of d-band centre and O adsorption energy for Pt-M-Pt alloys

(a) (b)

1.5 Eseg (100)

–1 Eseg (111) 0.5 Eseg (001) ΔEads (100) Ir Pd Ga Sn ΔEads (111) –0.5 E (001)

, eV atom ads seg

E –1.5

–2.5

Fig. 15. (a) Stability of Pt forming an outer shell with several tie-layer candidates. Overlaid is the oxygen adsorption energy relative to pure Pt; (b) schematic illustrating the carbide core material with the tie-layer/Pt overlayers

Therefore the work, in its second phase, investigated zeolites used for SCR of nitrogen oxides (NOx) species strategies to promote the stability of Pt, for instance for emission control – in particular to help elucidate the through the use of so-called tie-layers. Figure 15 location of ions within the pores and to understand illustrates the stability of a test set of tie-layer/Pt how these interact with other species located within combinations, showing the propensity for the Pt to the voids, such as water, nitrogen oxide (NO) and remain as a skin or to be sandwiched between the tie- ammonia (NH3). layer and carbide support. Small pore zeolites such as CHA have recently gained a lot of interest due to their very good low- 5. Emission Control Catalysis temperature activity and enhanced stability compared to medium and large pore zeolites such as MFI and beta The use of catalysts in emission control is wide (97–103). An understanding of the nature and location of and varied. Along with the supported nanoparticle transition metal ions such as Cu and Fe within the pores catalysts discussed to this point, another significant and how these interact with key NH3-SCR adsorbates class of catalyst is the zeolite. Zeolites are crystalline, such as NO and H2O is crucial in understanding the microporous materials in which the atoms are mechanism of NOx reduction in these zeolites and this arranged to form a network of molecular sized pores in turn can help in devising strategies to improve the and channels. This unique porous structure combined performance of these catalysts. A combined simulation with their huge internal surface area gives rise to a and experimental approach was used to investigate vast number of applications. By tailoring the pores and Cu location in CHA using techniques such as energy channels, molecules can be excluded on the basis minimisation based on interatomic potentials and of size and shape and catalytic processes can be quantum mechanics/molecular mechanics (QM/MM) driven to yield only preferred reaction products. Within methods to identify stable cation exchange sites Johnson Matthey’s business areas alone, zeolites are within the pores and study the influence of adsorbate key components of selective catalytic reduction (SCR) interactions (Figure 16). These results were compared catalysts, as additives for fluidised catalytic cracking to high resolution X-ray diffraction (HR-XRD) and (FCC) catalysts, and as catalysts in the petrochemicals infrared (IR) measurements of probe molecules to refining process. elucidate the ion location and how this is modified by Zeolites have been extensively studied over the last interaction with adsorbates. decade using a wide range of modelling techniques. The Diffusion of reactants and products to and from the structure of the framework, effects of doping, location active sites within the zeolite pores is a key part of the of ions and adsorbates within the pores, diffusion of catalytic cycle and can have a significant impact on a reactants and products within the structure and the catalyst’s performance. Therefore an understanding of reaction pathways leading to the catalytic breakdown this process is crucial to optimising the use of zeolites of molecules can be readily computed. Atomistic as catalysts. An ongoing project is using modelling to modelling has been used to aid the characterisation of understand diffusion of various molecules within these

research for the methods used to enable continuous (a) (b) improvement in modelling capability. This research is often carried out in collaboration with academic groups. We now proceed to discuss some of the academic 2.869 2.177 collaboration Johnson Matthey has been involved in 2.148 2.020 2.035 and how the underpinning knowledge is being used to 2.010 2.562 improve understanding of catalytic materials. 6.1 Redox Active Oxides and Metal Support Interactions Certain oxides provide a challenge to standard computational methods. Working with academics at Fig. 16. Two low energy sites for Cu+ identified using: (a) interatomic potentials; and (b) QM/MM methods Cardiff University, UK, and also via a Royal Society Industrial Fellowship at University College London (UCL), UK, we have been running academic projects looking at pragmatic ways to model ceria and other materials, including NO, NH3, H2O, propane, xylene redox active metal oxides for catalytic activities. A and CO2. The atoms in a zeolite can be arranged in a pertinent question in this work was whether trends in variety of ways to form rings, channels and pores with metal screening studies need to go beyond standard different sizes and dimensionalities, leading to a large DFT methods (Figure 18). The inclusion of the so number of possible framework structures. Molecular called Hubbard U parameter has been found not dynamics (MD) simulations are being used to try and only to be vital for obtaining the ‘correct’ electronic understand the influence of ring size, pore volume, structure (105), but also to have a significant influence dimensionality and chemical composition on diffusion on obtained formation energies and, importantly for through the framework and to predict which structures catalysis, shows that reaction energetics can be highly and compositions would allow optimal diffusion. sensitive to the choice of U (Figure 19) (105–107). The self-diffusion coefficients calculated using MD Once a reasonable model for the support phase of simulations can also be measured using experimental the catalyst is obtained, the interaction between the techniques such as pulse field gradient-nuclear nanoparticle and the support must be considered. magnetic resonance (PFG-NMR) and quasi-elastic A crucial question here is: how big are your neutron scattering (QENS) techniques and a project nanoparticulate catalysts? The answer clearly has a is underway in collaboration with University of bearing on how to consider modelling the system. For Cambridge, UK, and the UK Catalysis Hub at Harwell, instance Figure 6 shows three separate regimes for UK, to measure diffusion of selected molecules in catalysts of different sizes: smaller than 2 nm, between selected frameworks to compare with MD simulations. 2 nm and 3 nm and beyond 3 nm where the facets of Some initial MD simulation results investigating xylene diffusion in USY are presented in Figure 17 for ortho-, meta- and para-xylene. Figure 17 shows diffusion 3.5 coefficients at temperatures between 300 K and 700 K 3 for each isomer. The diffusion coefficients increase with –9 2.5 temperature and indicate that diffusion of the para , ×10 2 –1 ortho s

isomer is faster than or isomers. Initial PFG- 2 meta ortho 1.5 meta NMR measurements at University of Cambridge at 1 para

–10 2 –1 Ds, m 318 K gave a diffusion coefficient of 4.6 × 10 m s 0.5 which is in good agreement with MD simulated values 0 of 5.92 × 10–10 m2 s–1 at 323 K. 0 100 200 300 400 500 600 700 800 Temperature, K 6. Underpinning Research Fig. 17. Diffusion coefficient (Ds) as a function of temperature (T) for ortho, meta and para xylene in zeolite In addition to the more applied research reported structure USY above, a key philosophy is to look at underpinning

(a) (b) (c) 5 5 50 0 0 0 2 5 5.5 4 4 40

3 3 30

2 2 20 Density of states Density of states Density of states 1 1 10

0 0 0 –10 –8 –6 –4 –2 0 2 4 –10 –8 –6 –4 –2 0 2 4 –6 –4 –2 0 2 4 6 Energy, eV Energy, eV Energy, eV Fig. 18. Density of states calculated using standard DFT (red) and with the Hubbard U correction (blue): (a) lanthanum oxide (La2O3), wide band gap oxides are reproduced quite well; (b) palladium (II) oxide (PdO), standard DFT predicts PdO to be a metal, the Hubbard U correction is required to obtain the correct band gap (as seen in Figure 13, this also improves the calculated energetics); (c) CeO2, whilst the overall band gap (2p–5d) is reproduced well with standard approaches, the width and location of the Ce-4f is highly sensitive to the choice of Hubbard U parameter, it is this state that is responsible for the facile redox properties of CeO2 (104)

(a) (b) CeO2 + CO CO/CeO2 CeO2–d + CO2 2 0

1 –1 0

–1 –2

Energy, eV Energy, 0.0 eV ΔE (Pd4O4) 2.5 eV –2 ΔE (band gap) 4.5 eV ΔE (vacancy) –3 5.5 eV Adsorption energy, eV Adsorption energy, 6.5 eV 7.5 eV –3 8.5 eV

0 1 2 3 4 5 6 –4 U, eV Fig. 19. (a) dependence of various properties of PdO as a function of Hubbard U parameter, compared to benchmark results. Vacancy formation energy compared to Heyd-Scuseria-Ernzerhof (HSE) calculations of Delley (106). Band gap compared to (107) and Pd4O4 compared to in-house coupled cluster calculations. The region where the difference in properties and energetics approach zero identifies the required value of U; (b) CeO2 reaction profile adapted from (105), here the acute dependence of reaction profile on the choice of U parameter is illustrated, which necessitates a deeper study of choosing U for catalyst problems (105)

the nanoparticle are sufficiently large to begin to think (a) <2 nm: a regime that is modulated by both electronic in bulk terms (it should be noted the 3 nm bound is a and geometric effects and significantly different slightly arbitrary choice of bound for this regime). From reactivity from the bulk terminated surfaces would Figure 7, bulk-like electronic structure can be observed be expected even before the facets grow significantly large. This has (b) 2–3 nm: a regime where the intrinsic electronic now been confirmed by massively parallel calculations nature of the metal is ‘bulk-like’, but influence of of nanoparticles (108–110). This suggests the following transport or limited numbers of a given facet could general regimes: be influencing the observed catalysis (for example

the availability of hydrogen in the methanation for the chemical industry and have been the focus reaction) of numerous computational modelling investigations (c) >3 nm: the geometry and electronic influences are (113–118). The typical size of these metal particles sufficiently bulk-like to use extended surfaces as is of the order of the nanometre. At such scales the ‘good’ models. interaction of the particle with its chemical surrounding In general the size of a nanoparticle has a profound is of great importance as it impacts its stability (with influence on the observed chemistry and subsequent public health and economic impacts) and may impact catalytic behaviour. This has been well documented for its catalytic activity as well. As a consequence, a PhD Au, however limited attention has been given to other project in collaboration with UCL was initiated with the metals (111, 112). In Figure 20, this point is illustrated aim of using DFT computational modelling methods to for the activation of methane over silver. Not only does investigate how transition metals such as Pd, Pt and Ni the small size increase the activity of the particle relative interact with an α-alumina support. to the extended system, it produces a catalyst that is in The initial investigation studied how single metal a different chemical state, thereby opening up chemical atoms adsorb on the (0001) and (1102) α-alumina routes that would not otherwise have been present. surfaces. On both surfaces, the binding strength For small clusters and nanoparticles, the combination follows the trend Pt>Ni>Pd (119). When bonding to the of small size and the presence of surface oxide species surface, all transition metals promote a charge transfer, opens up even more combinations of chemistry. For moving electron density from a neighbouring surface the case of a small nanoparticle which can be modelled oxygen to a surface aluminium atom (Figure 21). explicitly, the presence of the support introduces an In order to take into account alumina’s strong affinity interfacial region that is critical for the chemistry and for water, a thermodynamic model of a different surface for the case of reducible oxides it also changes the environment was developed. By including the chemical availability of oxygen for reaction. potential of water in the gas phase it was possible to compute the Gibbs free energy of several water 6.2 Metal Support Interactions coverages of α-alumina surfaces and to predict the state The chemical interactions between a metal particle of the surface at a given pressure and temperature. and its metal oxide support are of critical importance Figure 22 presents the evolution of the Gibbs free

(a) (b) 3 3

NO* NO* 2.5 CH * CH3* 3 2 CH O* CH3O* CH3O* 2O* 2O* 2 2H* 2H* 1 1.5

1 0 eV eV

0.5 –1 0 –2 –0.5

–1 –3 Reaction coordinate (gas → surface) ReactionReaction coordinate coordinate (gas (gas → → surface) surface)

Fig. 20. Free energy diagrams for surface species present on: (a) a Ag {111} facet and (b) 13 atom cluster: solid = 423 K, dashed = 623 K, dot-dashed = 823 K. It can be seen for the Ag {111} surface only O is present at low temperature and at higher temperatures the surface is clean. However for the small cluster at 423 K, O, H, NO and CH3O are present, while at 623 K O and NO are present and at 823 K, O is still on the surface. The presence of O has significant influence on the calculated activation barriers. Eact[Ag {211}] = 2.15 eV, Eact[Ag13] = 1.57 eV, Eact[Ag13O6] = 0.74 eV; implying that O covered Ag13 is as active as Ni for the dissociation of methane (8)

enables a significant stabilisation of Pt and Ni atoms on the alumina surface (120). In order to gain some insight into the catalytic activity of supported metal nanoparticles, the interactions

of a carbon monoxide probe with a supported Ni6 nanocluster has been studied. CO was chosen for its relative simplicity and its wide use in catalytic processes such as steam reforming, where supported Ni catalysts are also of relevance. The (0001) α-alumina surface has a strong influence on the CO interaction with

Fig. 21. Top view of an adsorbed Pd single atom on (0001) the Ni6 cluster. The favoured adsorption site on the α-Al2O3 Figure adapted from (119) supported Ni6 cluster is on a hollow site on the side of the particle. On that site, CO is also able to interact with a surface aluminium atom (Figure 23) (121). The CO energy of the clean, low (5 OH nm–2) and high hydrated bond is elongated (1.28 Å vs. 1.13 Å in the gas phase (10 OH nm–2) states of the (0001) surface. At 3.2 kPa, and 1.20 Å on a (111) Ni hexagonal close-packed (hcp) the (0001) surface gradually changes from a heavy site) and its stretching frequency is lowered (1397 cm–1 hydration state to a fully clean surface at around 900 vs. 1899 cm–1 on a (111) Ni hcp site). This dramatic K. On the (1102) surface, the transition between the high activation of the CO molecule illustrates how, even OH coverage and the clean surface is much more sudden though considered as inert, the catalyst support may without an intermediate moderate hydration state (120). play an active role in the reaction cycle. Surface hydration has a strong impact on the For small nanoparticles, it is important to provide an interaction of the transition metal with its support as explicit computational model to describe mechanistic the surface aluminium sites are now fully occupied by aspects of sub-nanometre particles supported on OH groups and are not available to receive electron oxides. Work in collaboration with David Willock, density moved from surface oxygen atoms. Instead, Cardiff University, has been bringing together studies upon adsorption, all three investigated transition metal on redox active supports and small metal nanoparticles atoms trigger the rupture of a surface OH group, in a bid to understand the metal support interaction and followed by the migration of the hydrogen atom to the its influence on reactivity (122). For the probe reaction metal atom. This mechanism is called ‘spill-over’ and of CO oxidation, new mechanistic pathways have been opened up due to the metal/oxide interface that radically changes the rate-determining step (Figure 24) (123). The theme of metal support interaction has also 3.0 seen some experimental contribution and recent work with Tsang’s group in Oxford has been published on the 2.5 –2 2.0

1.5

1.0 Surface energy, J m Surface energy, 0.5

0.0 200 400 600 800 1000 1200 1400 1600 Temperature, K Fig. 22. Gibbs surface free energy at 3.2 kPa water partial pressure for the clean (blue), 5 OH nm–2 (green), and 10 OH –2 Fig. 23. Side view of CO on the Ni6 (0001)) α-Al2O3 system nm (red) hydration coverage of (0001)) α-Al2O3 (Adapted with permission from (120). Copyright (2003) American (Adapted with permission from (121). Copyright (2010) Chemical Society) American Chemical Society)

(a) (b) O..CO (ts) 100 catalyst 50 + CO (g) E Grey: [O] = Au-O 0 Black: [O] = Fe-O –50 C, D –100 B –150 A –200 CO (ads) –250 –300 CO (ads) –350 2

Fig. 24. (a) Au_10 nanoparticle adsorbed on iron (III) oxide (Fe2O3) (0001) surface, (blue: Fe, red: surface oxygen, yellow: Au, grey: carbon, green: CO oxygen); (b) reaction profile for the adsorption and reaction of CO with different surface O in the vicinity of the Au cluster. It can be seen there are a several distinct adsorption site Fe2O3, Au and interfacial, that lead to very different adsorption energies, furthermore the different reacting surface oxygens also lead to different activation barriers. (Adapted from Scott Hoh PhD thesis (123))

role of hydrogen spill-over influence on the reducibility methods are currently being explored to screen for of ceria supported pgm catalysts (64). novel catalyst candidates. Obtaining an explicit description of large metal Whilst models can be developed either explicitly nanoparticles interacting with an oxide support is still or indirectly for the metal/support interaction for the a computational challenge beyond current computing two outer-bounds of the nanoparticle regimes under capacity. However, a simple thermodynamic screening investigation, there is still a significant computational model has been developed with the potential to predict challenge in simulating the intermediate regime. The the general behaviour of certain transition metal/oxide following section describes work towards developing support combinations, in the limit of large nanoparticles. a capability that will allow the 2–3 nm regime to be Taking inspiration from the field of electrochemistry tackled explicitly, starting with an electronic structure where a chemical reaction is considered in terms theory of the metal nanoparticle itself. of the half-reaction, we can consider the following 6.3 Large-scale Calculations of Metal approximations: Nanoparticles (a) metal supported oxides can be split into two components (metal and oxide) One of the significant barriers to developing complex (b) redox catalysts can be considered analogous to models of real catalysts is the limitation on system size electrochemical half reactions that can be tackled explicitly. Significant progress can (c) one half reaction occurs on the metal, the other on be made with a judicious choice of model and some the oxide. prior understanding of the system in question. If one is Taking the example of simultaneous CO oxidation simply interested in geometric structure then arguably, and NO reduction it can be assumed that NO reduction embedded atom or other parameterised potential occurs on the metallic component while CO oxidation approaches can be utilised (124, 125). However occurs on the oxide component. Calculating the free in catalysis, especially when trying to understand energy profile for a simplified reaction pathway allows chemical activity, there is often interest in linking firstly existing databases of metal components to be the electronic structure to reactivity. This requires used, secondly new databases to be developed for methods for conducting large scale electronic structure the oxide component and finally metal/oxide pairings theory calculations, for instance computer codes with to be identified using calculations that are currently favourable scaling over many computer cores (for tractable on available resources. Figure 25 illustrates example GPAW, Order-N Electronic Total Energy the approach being used in the example of CO and Package (ONETEP) (126, 127)) or semi-empirical NO. Early results in this area are promising and new approaches (128).

4 Au Ag Ceria surface Cu 2 Pd Pt Ni Rh 0 Co Ru Fe , eV Re –2 Mo

(450 K) W ΔG –4 Metal surface

–6

–8 M * M * M * M * M * 2 + O M * + O + O + O 2 2 2 (g) + 0.5N CO (g) + NO (g)CO (g) + NO 2 Ox * + 0.5N (g) + 0.5NCO CO (g) + N 2 CO (g) + 0.5NCO CO

Fig. 25. Reaction profile for the NO reduction, CO oxidation, occurring on a series of metals left hand side and Ce surface, right hand side. The highlighted dashed line illustrates the interpolation between reactants and products. To a first approximation this interpolation minimises the free energy pathway and shows the region where an optimal catalyst can be found

One part of this research is looking at exploiting and the propensity for the oxidation state of the the linear scaling capabilities of ONETEP through surface to change during reaction. A significant collaboration with Chris-Kriton Skylaris, Southampton challenge to utilising the DFTB approach is to find University, UK. Enhanced functionality of ONETEP has truly transferable parameters to describe all of the been exploited to help in the simulation of metals (129) necessary interaction present in the simulation and work is now underway benchmarking speed and (130). Figure 26 illustrates geometrical structures scaling for the adsorption of molecules such as O2, CO, obtained from ONETEP, GPAW and DFTB for OH and atomic O. The difference in adsorption energies nanoparticles that are tractable with current between constrained but pre-optimised nanoparticles computational resources, the future will see this and ligands and between fully geometry-optimised capability extended. systems of nanoparticles and ligands has been studied There are several fundamental experimental projects by looking at various sizes of cuboctahedral Pt clusters to augment the theoretical work on nanoparticles.

(Pt13, Pt55, Pt147) and comparing to more conventional The first of these involves the Nellist group from slab models. The results show significant deviations Oxford Materials, UK, who have derived experimental in binding strength between the fully relaxed and methods using quantitative annular dark-field scanning constrained systems. transmission electron microscopy (ADF-STEM) Another project with the University of Limpopo, (131). Pt nanoparticle atomic coordinates have

South Africa, is developing parameters to describe been determined for particles up to Pt943 atoms. The Pd nanoparticles and their interaction with oxide systems were geometry optimised using a damped supports via the semi-empirical, density-functional MD and interatomic force-field approach, before being tight binding (DFTB) approach, which potentially electronically minimised in ONETEP and studied allows thousands of atoms to be simulated using quantum mechanically. Atomic oxygen adsorption limited computational resources. A model system calculations were then performed on the most exposed to understand the usefulness of this approach facet, which exhibits fcc-{111} surface symmetry and has been methane oxidation using Pd supported the least coordinated sites were found to be the most on titania (TiO2). This system has a number of strongly binding, agreeing with terrace measurements complexities, including the metal/ceramic interface from experimental literature.

methods are being developed to create well defined (a) (b) nanoparticle samples for use in ambient pressure X-ray photoelectron spectroscopy (XPS) synchrotron beamlines (132). The ultimate aim is to bridge the ubiquitous temperature, pressure and materials gaps between surface science and real catalysts. The ambient pressure XPS allows the oxidation of different sized nanoparticles to be observed as a function of (c) (d) reactive atmosphere, which in turn can be correlated back to theoretical predictions. This project is ongoing and in the future will make use of the forthcoming versatile soft X-ray (VERSOX) beamline at Diamond Light Source, Harwell, UK, to develop even further new understanding of catalysis materials.

6.4 Moving Beyond the Atomic Scale Fig. 26. Representations of geometrically relaxed The emerging field of modelling across length scales nanoparticles, calculated using the different approaches is ideally suited to modelling catalysis since catalyst discussed in the text. (a) Pt309 nanoparticle electrostatic potential and (b) Pt309 electron density, both calculated solutions rely not only on the intrinsic chemical using ONETEP; (c) Pd321 nanoparticle calculated using properties of an active material, but also on the transport the DFTB + approach; (d) 79 atom Pd/Pt core-shell with a properties of the material at different scales. Figure 27 monolayer of O adsorbed calculated using GPAW (133) illustrates the case of an automotive monolith, highlighting the levels that need to be modelled to The second experimental project is looking at develop a holistic description of the catalytic solution. methane oxidation, in order to provide data about A complete description of a technical solution includes nanoparticle surface composition under a reactive gas- describing, for example, the atomic or electronic level phase atmosphere that in turn can be used to validate of matter, the porous structure of the catalyst layer theoretical predictions. The project is being run with (at two distinct levels: small nanopores and large Professor Georg Held, University of Reading, UK, and macropores) and the reactor itself. Furthermore it also

Picosecond Seconds, minutes Minutes, hours Femtosecond Days Microscale (macropores) Nanoscale 1000 (mesopores)

900 macroscale 800 Inlet ck (z), T (z) ~1 mm 700 gas 0 z l ~10 cm 600

500 ~100 nm 400 0.0 0.5 1.0 1.5 ~10 µm Fig. 27. Schematic illustrating some of the regimes encountered when modelling a catalyst solution, in this case a monolith reactor for emission control. Starting at the left hand side we have the intrinsic kinetics resulting from the atomic scale interactions, moving across to the meso and macroporous structure where diffusion of the reactant gases is necessary to find the active catalytic sites, finally on the right hand side the macroscale model of the reactor channel in the monolith. (Adapted from (133))

276 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) requires the study of different time scales ranging from for determining diffusivity that can be utilised in simpler femtoseconds (the timescale on which the chemistry models of an entire catalytic reactor (144). With the happens) to years (corresponding to the expected development of ab initio oxidation models a fully holistic lifetime of the catalyst). A combination of physics, ab initio description of the catalytic system is becoming chemistry and engineering are required to generate closer. statistical methods and engineering models, which along with chemical insight into mechanisms will 7. Research Outlook provide a more complete picture of the catalytic system. This project started through a Royal Society This brief review has described some of the projects Industrial Fellowship held at UCL, Johnson Matthey run within the atomic-scale groups at Johnson Matthey. Technology Centre and the Institute of Chemical Looking to the future, the following areas appear as Technology, Prague. Its aim is to look at applying the natural progressions. Broadly speaking there are three Prague group’s meso-scale methods to simulation of aspects: transport and reaction in porous catalysts (133–142). Firstly, theoretical development and understanding. This project has been developing over the last few Continued work on model systems aims to enhance the years and has an experimental component, which description of nanoparticles and oxides and ultimately is essentially for benchmarking and validating the bring them together. The description is not just about the theoretical methods. Through the careful preparation structure of the material, it is perhaps more importantly of active catalyst it has been possible to construct about using thermodynamic models to understand the layered catalysts consisting of a uniform active layer state of a catalyst under reaction conditions. If one has overlaid by an inert layer with well-defined morphology a good model of the catalyst under reaction conditions, (thickness, porosity, particle size) that serves as a the reaction can be decomposed to its elementary diffusion layer (Figure 28) (143, 144). steps and the key bond making and breaking that Through this approach it has been possible, firstly, to controls the activity and reactivity of a given material validate the computational models by simulating the can be understood. transport and activity of the precise geometry prepared Secondly, further development of the correlation in the lab; and secondly, to develop a novel approach between a theoretical understanding of catalyst to determining the diffusivity of gases through porous structure, activity and stability with experimental media. This second aspect is important because characterisation. There has undoubtedly been huge it provides insight into the structural properties of progress in this respect over the last two decades. the catalyst layer and how they influence its overall However, specific challenges remain. For instance performance, as well as providing a simple relationship the combination of synthesising ideal shapes and

0.5 0.08 –1 0.4 0.06

0.3 0.04 inert

y CO, % 0.2 0.02 r CO, kmol s 0.1 0.00 0.0 active 20 µm

Fig. 28. (a) Scanning electron micrograph depicting the inert alumina layer, active Pt/alumina layer and substrate material; (b) simulated concentration profile of CO in reconstructed porous media; (c) rate of CO consumption as a function of position in the porous media. Note the activity is purely located in the active region of the catalyst. (Adapted from (144))

277 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X687975 Johnson Matthey Technol. Rev., 2015, 59, (3) structured nanoparticles for catalyst testing, with for funding of Collaborative Awards in Science and advanced characterisation techniques (for example the Engineering (CASE) and doctoral training centre upcoming VERSOX beamline at Diamond) along with students. Misbah Sarwar acknowledges J. L. Gavartin the detailed models from ab initio theory, provides an (Accelrys) and the Technology Strategy Board (TSB) exciting prospect for developing a deep fundamental for support in the iCatDesign project (DTI Project No: understanding of catalyst materials. /5/MAT/6/I/H0379C). Glenn Jones acknowledges the Thirdly, the possibility of going beyond catalyst EU for supporting the IMPRESS project (Contract materials problems to include thermodynamic NMP3-CT-2004-500635, co-funded by the European modelling (using the MTData code (145)) and Commission in the sixth Framework Programme and process modelling using tools like gPROMS (146). European Space Agency) and the Royal Society for Furthermore, traditional materials physics problems supporting his industrial fellowship at UCL (IF090100). can also be studied, for instance smart materials Ludovic Briquet acknowledges the EU for supporting where the challenge lies in describing both the his Framework 6 Program Marie-Curie Early Stage thermodynamic and electronic properties adequately Training studentship. Crispin Cooper gratefully to make material predictions. Another burgeoning acknowledges support from the EPSRC Collaborative area is in battery materials technology where Training Account: UCL (GR/T11364/01). Glenn Jones simulation and modelling will play a significant role. and Crispin Cooper also acknowledge the UK’s super computing facility for time on HECToR, which has 8. Conclusions now been superseded by ARCHER, through the UK’s HPC Materials Chemistry Consortium (EPRSC: EP/ To conclude, if the last twenty years have seen growth L000202). in the capability of computational chemistry and widespread acceptance of the field as a sub-discipline References of science, the next two decades hold the prospect of even greater progress. Utilisation of petascale 1 P. Hohenberg and W. Kohn, Phys. Rev., 1964, 136, computing, the accessibility of high performance (3B), B864 computing and open source code development 2 W. Kohn and L. J. Sham, Phys. Rev., 1965, 140, (4A), means that even larger and more complex systems A1133 are accessible and can be tackled by more 3 J. K. Nørskov, F. Abild-Pedersen, F. Studt and T. people. Multiscale models that capture not only the Bligaard, Proc. Nat. Acad. Sci., 2011, 108, (3), 937 fundamental physics and chemistry of the electronic 4 F. Göltl and P. Sautet, ‘Density Functional Theory and molecular level process, but also the macroscopic as a Key Approach in Surface Chemistry and properties of the device in question will also become Heterogeneous Catalysis’, in “Comprehensive routine. Automation of simulations coupled with high Inorganic Chemistry II: From Elements to Applications”, throughput synthesis will revolutionise the way in which 2nd Edn., eds. J. Reedijk and K. Poeppelmeier, we discover new materials. Atomic-scale modelling Volume 7, Chapter 15, Elsevier, The Netherlands, and its subsequent expansion to other scales is 2013, pp. 405–420 helping Johnson Matthey be part of this computational 5 ‘Top 500 Supercomputing Sites’, TOP500.org, New revolution that will continue to provide increasing Orleans, Louisiana, USA (Accessed on 2nd April opportunities and will be at the heart of developing 2015) catalytic and materials science into the 21st century. 6 J. L. Whitten and H. Yang, Surf. Sci. Rep., 1996, 24, 9. Acknowledgements (3–4), 55 The work presented herein is a result of a number 7 B. Hammer and J. K. Nørskov, Adv. Catal., 2000, of internal and external collaborations. It is hard to 45, 71 acknowledge everyone who has been involved in the 8 C. Chizallet, G. Bonnard, E. Krebs, L. Bisson, C. projects over the last seven years or so, however along Thomazeau and P. Raybaud, J. Phys. Chem. C, 2011, with our experimental colleagues within Johnson 115, (24), 12135 Matthey, the Engineering and Physical Sciences 9 J. Rossmeisl, Z.-W. Qu, H. Zhu, G.-J. Kroes and J. K. Research Council (EPSRC) should be acknowledged Nørskov, J. Electroanal. Chem., 2007, 607, (1–2), 83

10 K. Reuter and M. Scheffler, Phys. Rev. B, 2002, 65, 27 S. Kozuch, C. Amatore, A. Jutand and S. Shaik, (3), 035406 Organometallics, 2005, 24, (10), 2319 11 M. Digne, P. Sautet, P. Raybaud, P. Euzen and H. 28 R. Álvarez, O. N. Faza, C. S. López and Á. R. de Lera, Tulhoat, J. Catal., 2002, 211, (1), 1 Org. Lett., 2006, 8, (1), 35 12 J. K. Nørskov, F. Studt, F. Abild-Pedersen and T. 29 T.-X. Zhang and Z. Li, Comput. Theor. Chem., 2013, Bligaard, “Fundamental Concepts in Heterogeneous 1016, 28 Catalysis”, John Wiley and Sons, Hoboken, New 30 A. A. C. Braga, G. Ujaque and F. Maseras, Jersey, USA, 2014 Organometallics, 2006, 25, (15), 3647 13 A. Walsh, A. A. Sokol and C. R. A. Catlow, 31 L. Zhang and D.-C. Fang, J. Org. Chem., 2013, 78, “Computational Approaches to Energy Materials”, (6), 2405 John Wiley and Sons, Chichester, UK, 2013 32 A. Fromm, C. van Wüllen, D. Hackenberger and L. J. 14 R. A. van Santen and M. Neurock “Molecular Gooßen, J. Am. Chem. Soc., 2014, 136, (28), 10007 Heterogeneous Catalysis: A Conceptual and 33 L. Sikk, J. Tammiku-Taul, P. Burk and A. Kotschy, J. Computational Approach”, Wiley-VCH Verlag GmbH Mol. Model., 2012, 18, (7), 3025 & Co KGaA, Weinheim, Germany, 2006 34 M. García-Melchor, M. C. Pacheco, C. Nájera, A. 15 Chorkendorff and J. W. Niemantsverdriet, “Concepts Lledós and G. Ujaque, ACS Catal., 2012, 2, (1), 135 of Modern Catalysis and Kinetics”, Wiley-VCH Verlag 35 D. Zhao, X. Li, K. Han, X. Li and Y. Wang, J. Phys. GmbH & Co KGaA, Weinheim, Germany, 2003 Chem. A, 2015, 119, (12), 2989 16 G. Jones, Catal. Struct. React., 2015, 00, 1 36 S. Yu, S. Liu, Y. Lan, B. Wan and X. Li, J. Am. Chem. Soc., 2015, 137, (4), 1623 17 The White House: About the Materials Genome 37 Y. Wu, L. Liu, K. Yan, P. Xu, Y. Gao and Y. Zhao, Initiative: https://www.whitehouse.gov/mgi (Accessed J. ., 2014, 79, (17), 8118 on 20th April 2015) Org. Chem 38 Q. Ren, F. Jiang and H. Gong, J. Organomet. Chem., 18 A. Jain, S. P. Ong, G. Hautier, W. Chen, W. D. Richards, 2014, 770, 130 S. Dacek, S. Cholia, D. Gunter, D. Skinner, G. Ceder 39 L. Liu, Y. Lv, Y. Wu, Gao, X. Zeng, Z. Gao, Y. Tang and and K. A. Persson, Appl. Phys. Lett.: Mater., 2013, 1, (1), 011002 G. Zhao, RSC Adv., 2014, 4, (5), 2322 19 J. S. Hummelshøj, F. Abild-Pedersen, F. Studt, T. 40 D. Liu, Y. Li, X. Qi, C. Liu, Y. Lan and A. Lei, Org. Lett., 2015, 17, (4), 998 Bligaard and J. K. Nørskov, Angew. Chem. Int. Ed., 2012, 51, (1), 272 41 B. Zheng, F. Tang, J. Luo, J. W. Schultz, N. P. Rath 20 Quantum Materials Informatics Project: Publications and L. M. Mirica, J. Am. Chem. Soc., 2014, 136, (17), http://www.qmip.org/qmip.org/Welcome.html 6499 (Accessed on 20th April 2015) 42 S. K. Sontag, J. A. Bilbrey, N. E. Huddleston, G. R. 21 G. Pizzi, A. Cepellotti, R. Sabatini, N. Marzari, B. Sheppard, W. D. Allen and J. Locklin, J. Org. Chem., Kozinsky, ‘AiiDA: Automated Interactive Infrastructure 2014, 79, (4), 1836 and Database for Computational Science’, 43 A. Hedström, Z. Izakian, I. Vreto, C.-J. Wallentin and arXiv:1504.01163v1 [physics.comp-ph], 5th April, P.-O. Norrby, Chem. Eur. J., 2015, 21, (15), 5946 2015 44 A. Bekhradnia and P.-O. Norrby, Dalton Trans., 2015, 22 Iowa State University, College of Engineering: 44, (9), 3959 Combinatorial Sciences and Materials Informatics 45 R. B. Bedford, P. B. Brenner, E. Carter, J. Clifton, P. M. Collaboratory (CoSMIC): http://cosmic.mse.iastate.edu/ Cogswell, N. J. Gower, M. F. Haddow, J. N. Harvey, J. (Accessed on 20th April 2015) A. Kehl, D. M. Murphy, E. C. Neeve, M. L. Neidig, J. 23 A. A. C. Braga, N. H. Morgon, G. Ujaque and F. Nunn, B. E. R. Snyder and J. Taylor, Organometallics, Maseras, J. Am. Chem. Soc., 2005, 127, (25), 9298 2014, 33, (20), 5767 24 D. García-Cuadrado, A. A. C. Braga, F. Maseras and 46 S. L. Daifuku, M. H. Al-Afyouni, B. E. R. Snyder, J. L. A. M. Echavarren, J. Am. Chem. Soc., 2006, 128, (4), Kneebone and M. L. Neidig, J. Am. Chem. Soc., 2014, 1066 136, (25), 9132 25 L. J. Goossen, D. Koley, H. L. Hermann and W. Thiel, 47 Y. Sun, H. Jiang, H. Tang, H. Xu, H. Liu, K. Sun and X. Organometallics, 2006, 25, (1), 54 Huang, Mol. Phys., 2014, 112, (16), 2107 26 E. Napolitano, V. Farina and M. Persico, 48 H. Kato, K. Hirano, D. Kurauchi, N. Toriumi and M. Organometallics, 2003, 22, (20), 4030 Uchiyama, Chem. Eur. J., 2015, 21, (10), 3895

49 G.-Y. Ruan, Y. Zhang, Z.-H. Qi, D.-X. Ai, W. Liu and Y. 71 S. Saadi, B. Hinnemann, C. C. Appel, S. Helveg, F Wang, Comput. Theor. Chem., 2015, 1054, 16 Abild-Pedersen and J. K. Nørskov, Surf. Sci., 2011, 50 K. Kubota, E. Yamamoto and H. Ito, J. Am. Chem. 605, (5–6), 582 Soc., 2015, 137, (1), 420 72 S. Saadi, F. Abild-Pedersen, S. Helveg, J. Sehested, 51 D. Alberico, M. E. Scott and M. Lautens, Chem. Rev., B. Hinnemann, C. C. Appel and J. K. Nørskov, J. Phys. 2007, 107, (1), 174 Chem. C, 2010, 114, (25), 11221 52 L. Ackermann, R. Vicente and A. R. Kapdi, Angew. 73 S. Helveg, C. López-Cartes, J. Sehested, P. L. Chem. Int. Ed., 2009, 48, (52), 9792 Hansen, B. S. Clausen, J. R. Rostrup-Nielsen, F. Abild-Pedersen and J. K. Nørskov, Nature, 2004, 427, 53 A. Sharma, D. Vacchani and E. Van der Eycken, (6973), 426 Chem. Eur. J., 2013, 19, (4), 1158 74 E. H. Stitt, M. J. Watson, L. Gladden, J. McGregor, 54 J. Roger, A. L. Gottumukkala and H. Doucet, World Patent 2010/140005 ChemCatChem, 2010, 2, (1), 20 75 A. Ukpong and G. Jones, ‘Ab initio Insight into the 55 N. Kuhl, M. N. Hopkinson, J. Wencel-Delord and F. Catalytic Dehydrogenation of Propane to Propene Glorius, Angew. Chem. Int. Ed., 2012, 51, (41), 10236 over Defective Graphene’, submitted 56 R. Rossi, F. Bellina, M. Lessi and C. Manzini, Adv. 76 J. Zhu, J. G. van Ommen and L. Lefferts, J. Catal., ., 2014, 356, (1), 17 Synth. Catal 2004, 225, (4), 388 57 L. Ackermann, ., 2011, 111, (2), 1315 Chem. Rev 77 J. Zhu, J. G. van Ommen, H. J. M. Bouwmeester and 58 S. I. Gorelsky, D. Lapointe and K. Fagnou, J. Am. L. Lefferts, J. Catal., 2005, 233, (2), 434 2008, 130, (33), 10848 Chem. Soc., 78 C. S. Cooper, R. J. Oldman and C. R. A. Catlow, 59 S. I. Gorelsky, D. Lapointe and K. Fagnou, J. Org. Chem. Comm., 2015, 51, 5856 ., 2012, 77, (1), 658 Chem 79 X. Xia, R. J. Oldman and C. R. A. Catlow, J. Mater. 60 Y. Tan, F. Barrios-Landeros and J. F. Hartwig, J. Am. Chem., 2012, 22, (17), 8594 Chem. Soc., 2012, 134, (8), 3683 80 M. Anpo, M. Che, B. Fubini, E. Garrone, E. Giamello 61 Y. Tan and J. F. Hartwig, J. Am. Chem. Soc., 2011, and M. C. Paganini, Top. Catal., 1999, 8, (3–4), 189 133, (10), 3308 81 G. Mills, H. Jónsson and G. K. Schenter, Surf. Sci., 62 J. K. Nørskov, T. Bligaard, J. Rossmeisl and C. H. 1995, 324, (2–3), 305 Christensen, Nature Chem., 2009, 1, (1), 37 82 ‘The Fuel Cell Today Industry Review 2011’, Johnson 63 F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C. F. Matthey Plc, Royston, UK, 2011 Elkjaer, J. S. Hummelshøj, S. Dahl, I. Chorkendorff 83 ‘Technical Plan — Fuel Cells’, Multi-Year Research, and J. K. Nørskov, Nature Chem., 2014, 6, (4), 320 Development and Demonstration Plan, US 64 N. Acerbi, S. C. E. Tsang, G. Jones, S. Golunski and P. Government, Washington, DC, USA: http://energy. Collier, Angew. Chem. Int. Ed, 2013, 52, (30), 7737 gov/sites/prod/files/2014/12/f19/fcto_myrdd_fuel_ 65 O. R. Inderwildi and S. J. Jenkins, Chem. Soc. cells.pdf (Accessed on 2nd April 2015) Rev., 2008, 37, (10), 2274 84 J. Greeley and J. K. Nørskov, J. Phys. Chem. C, 2009, 66 G. Jones, J. G. Jakobsen, S. S. Shim, J. Kleis, M. 113, (12), 4932 P. Andersson, J. Rossmeisl, F. Abild-Pedersen, T. 85 J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, Bligaard, S. Helveg, B. Hinnemann, J. R. Rostrup- J. R. Kitchin, T. Bligaard and H. Jónsson, J. Phys. Nielsen, I. Chorkendorff, J. Sehested and J. K. Chem. B, 2004, 108, (46), 17886 Nørskov, J. Catal., 2008, 259, (1), 147 86 V. Stamenković, T. J. Schmidt, P. N. Ross and N. M. 67 F. Abild-Pedersen, O. Lytken, J. Engbæk, G. Nielsen, Marković, J. Phys. Chem. B, 2002, 106, (46), 11970 Ib Chorkendorff and J. K. Nørskov, Surf. Sci., 2005, 87 I. E. L. Stephens, A. S. Bondarenko, F. J. Perez- 590, (2–3), 127 Alonso, F. Calle-Vallejo, L. Bech, T. P. Johansson, A. 68 G. Jones, ‘Preliminary results for S desorption from K. Jepsen, R. Frydendal, B. P. Knudsen, J. Rossmeisl, Ni{111}’, Internal Report, Johnson Matthey Plc, and Ib Chorkendorff, J. Am. Chem. Soc., 2011, 133, Sonning Common, UK, 2010 (14), 5485 69 G. McCarty and H. Wise, J. Chem. Phys., 1980, 72, 88 V. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. (12), 6332 Ross, N. M. Markovic, J. Rossmeisl, J. Greeley and 70 T. R. Munter, D. D. Landis, F. Abild-Pedersen, G. J. K. Nørskov, Angew. Chem. Int. Ed., 2006, 45, (18), Jones, S. Wang and T. Bligaard, Comput. Sci. Disc., 2897 2009, 2, 015006 89 Y. Xu, A. V. Ruban and M. Mavrikakis, J. Am. Chem.

Soc., 2004, 126, (14), 4717 C. Glinsvad, F. Abild-Pedersen, J. Greeley, K. W. 90 V. Viswanathan, H. A. Hansen, J. Rossmeisl and J. K. Jacobsen and J. K. Nørskov, J. Phys. Chem. Lett., Nørskov, J. Phys. Chem. Lett., 2012, 3, (20), 2948 2013, 4, (1), 222 91 P. Sabatier, Ber. Deut. Chem. Gesell., 1911, 44, (3), 110 A. H. Larsen, ‘Efficient Electronic Structure Methods 1984 Applied to Metal Nanoparticles’, PhD Thesis, 92 T. Bligaard, J. K. Nørskov, S. Dahl, J. Matthiesen, C. Department of Physics, Technical University of Denmark, 2011 H. Christensen and J. Sehested, J. Catal., 2004, 224, (1), 206 111 T. Jiang, D. J. Mowbray, S. Dobrin, H. Falsig, B. 93 Y. Ma and P. B. Balbuena, Surf. Sci., 2008, 602, (1), Hvolbæk, T. Bligaard, and J. K. Nørskov, J. Phys. 107 Chem. C, 2009, 113, (24), 10548 94 G. E. Ramirez-Caballero and P. B. Balbuena, Chem. 112 H. Falsig, J. Shen, T. S. Khan, W. Guo, G. Jones, S. Phys. Lett., 2008, 456, (1–3), 64 Dahl and T. Bligaard, Top. Catal., 2014, 57, (1–4), 80 95 J. S. Hummelshøj, F. Abild-Pedersen, F. Studt, T. 113 C. Verdozzi, D. R. Jennison, P. A. Schultz and M. P. Bligaard and J. K. Nørskov, Angew. Chem. Int. Ed., Sears, Phys. Rev. Lett., 1999, 82, (4), 799 2012, 51, (1), 272 114 Z. Łodziana and J. K. Nørskov, J. Chem. Phys., 2001, 96 J. Yates, G. H. Spikes and G. Jones, Phys. Chem. 115, (24), 11261 Chem. Phys., 2015, 17, (6), 4250 115 J. R. B. Gomes, Z. Lodziana and F. Illas, J. Phys. 97 D. W. Fickel and R. F. Lobo, J. Phys. Chem. C, 2010, Chem. B, 2003, 107, (26), 6411 114, (3), 1633 116 M. C. Valero, P. Raybaud and P. Sautet, J. Phys. 98 A. Godiksen, F. N. Stappen, P. N. R. Vennestrøm, Chem. B, 2006, 110, (4), 1759 F. Giordanino, S. B. Rasmussen, L. F. Lundegaard 117 R. Grau-Crespo, N. C. Hernández, J. F. Sanz and and S. Mossin, J. Phys. Chem. C, 2014, 118, N. H. de Leeuw, J. Phys. Chem. C, 2007, 111, (28), (40), 23126 10448 99 F. Gao, E. D. Walter, M. Kollar, Y. Wang, J. Szanyi and 118 C. Chizallet and P. Raybaud, Catal. Sci. Technol., C. H. F. Peden, J. Catal., 2014, 319, 1 2014, 4, (9), 2797 100 F. Gao, E. D. Walter, E. M. Karp, J. Luo, R. G. Tonkyn, 119 L. G. V. Briquet, C. R. A. Catlow and S. A. French, J. J. H. Kwak, J. Szanyi and C. H. F. Peden, J. Catal., Phys. Chem. C, 2008, 112, (48), 18948 2013, 300, 20 120 L. G. V. Briquet, C. R. A. Catlow and S. A. French, J. 101 E. Borfecchia, K. A. Lomachenko, F. Giordanino, H. Phys. Chem. C, 2009, 113, (38), 16747 Falsig, P. Beato, A. V. Soldatov, S. Bordiga and C. 121 L. G. V. Briquet, C. R. A. Catlow and S. A. French, J. Lamberti, Chem. Sci., 2015, 6, (1), 548 Phys. Chem. C, 2010, 114, (50), 22155 102 U. Deka, I. Lezcano-Gonzalez, B. M. Weckhuysen 122 S. W. Hoh, L. Thomas, G. Jones and D. J. Willock, and A. M. Beale, ACS Catal., 2013, 3, (3), 413 Res. Chem. Intermediat., 2015, doi: 10.1007/s11164- 103 S. T. Korhonen, D. W. Fickel, R. F. Lobo, B. M. 015-1984-7 Weckhuysen and A. M. Beale, Chem. Commun., 2011, 47, (2), 800 123 S. W. Hoh, ‘Oxidation Catalysis Using Transition Metals and Rare Earth Oxides’, PhD Thesis, School 104 Z.-P. Liu, S. J. Jenkins and D. A. King, Phys. Rev. of Chemistry, Cardiff University, UK, 2014 Lett., 2005, 94, (19–20), 196102 124 A. P. Sutton and J. Chen, Phil. Mag. Lett., 1990, 61, 105 L. J. Bennett and G. Jones, Phys. Chem. Chem. (3), 139 Phys., 2014, 16, (39), 21032 125 K. W. Jacobsen, J. K. Norskov and M. J. Puska, 106 M. K. Bruska, I. Czekaj, B. Delley, J. Mantzaras and Phys. Rev. B, 1987, 35, (14), 7423 A. Wokaun, Phys. Chem. Chem. Phys., 2011, 13, (35), 15947 126 J. J. Mortensen, L. B. Hansen and K. W. Jacobsen, Phys. Rev. B, 2005, 71, (3), 035109 107 J. Waser, H. A. Levy and S. W. Peterson, Acta Cryst., 1953, 6, (7), 661 127 C.-K. Skylaris, P. D. Haynes, A. A. Mostofi and M. C. 108 J. Kleis, J. Greeley, N. A. Romero, V. A. Morozov, H. Payne, J. Chem. Phys., 2005, 122, (8), 084119 Falsig, A. H. Larsen, H. Lu, J. J. Mortensen, M. Dułak, 128 P. Koskinen and V. Mäkinen, Comput. Mater. Sci., K. S. Thygesen, J. K. Nørskov and K. W. Jacobsen, 2009, 47, (1), 237 Catal. Lett., 2011, 141, (8), 1067 129 Á. Ruiz-Serrano and C.-K. Skylaris, J. Chem. Phys., 109 L. Li, A. H. Larsen, A. H. Romero, V. A. Morozov, 2013, 139, (5), 054107

130 M. H. Chuma, H. R. Chauke, G. Jones and P. E. 137 V. Novák, F. Štěpánek, P. Kočí, M. Marek and M. Ngoepe, ‘Parameterization and Validation of Pd and Kubíček, Chem. Eng. Sci., 2010, 65, (7), 2352 Pd Nanoclusters Using SCC-DFTB’, in “Proceedings 138 P. Kočí, F. Štěpánek, M. Kubíček and M. Marek, of SAIP2013: the 58th Annual Conference of the Chem. Eng. Sci., 2006, 61, (10), 3240 South African Institute of Physics”, eds. R. Botha 139 M. Marek, Z. Grof, P. Kocí, M. Kohout, J. Kosek and F. and T. Jili, The 58th Annual Conference of the South African Institute of Physics, University of Zululand, Štepánek, J. Chem. Eng. Jpn, 2007, 40, (11), 879 South Africa, 8th–12th July, 2013, pp. 8–12 140 F. Štěpánek, M. Marek, J. Hanika and P. M. Adler, 131 L. Jones, K. E. MacArthur, V. T. Fauske, A. T. J. van Catal. Today, 2001, 66, (2–4), 249 Helvoot and P. D. Nellist, Nano Lett., 2014, 14, (11), 141 P. Kočí, F. Štěpánek, M. Kubíček and M. Marek, 6336 Chem. Eng. Sci., 2007, 62, (18–20), 5380 132 R. Price, T. Erlap-Erden, E. Crumlin, S. Garcia, S. 142 P. Kočí, F. Štěpánek, M. Kubíček and M. Marek, Mol. Rani, R. Smith, G. Jones, L. Deacon, C. Euarusakul Simul., 2007, 33, (4–5), 369 and G. Held, ‘The Partial Oxidation of Methane with 143 V. Novák, P. Kočí, M. Marek, F. Štěpánek, P. Blanco- Catalytic Pd/Al2O3 Nanoparticles, Studied in-situ García and G. Jones, , 2012, 188, (1), 62 by Near Ambient-Pressure X-Ray Photoelectron Catal. Today Spectroscopy’, submitted 144 M. Dudák, V. Novák, P. Kočí, M. Marek, P. Blanco- 133 V. Novák, P. Kočí, F. Štěpánek and M. Marek, Ind. García and G. Jones, Appl. Catal. B: Environ., 2014, Eng. Chem. Res., 2011, 50, (23), 12904 150–151, 446 134 V. Novák, P. Kočí, F. Štěpánek, M. Kubíček and M. 145 R. H. Davies, A. T. Dinsdale, J. A. Gisby, J. A. J. Marek, Comput. Chem. Eng., 2011, 35, (5), 964 Robinson and S. M. Martin, Calphad, 2002, 26, (2), 135 J. Kosek, F. Štěpánek and M. Marek, Adv. Chem. 229 Eng., 2005, 30, 137 146 gPROMS, Process Systems Enterprise Ltd, London, 136 P. Kočí, V. Novák, F. Štěpánek, M. Marek and M. UK, 1997–2014: www.psenterprise.com/gproms Kubíček, Chem. Eng. Sci., 2010, 65, (1), 412 (Accessed on 2nd April 2015)

The Authors

Misbah Sarwar is currently a Principal Scientist at Johnson Matthey Technology Centre (JMTC), Sonning Common, UK. She has an MSci in Chemistry from University College London (UCL), UK, and a PhD from UCL and the Royal Institution (RI). Misbah joined Johnson Matthey in 2007 to work on the Technology Strategy Board (TSB) funded project iCatDesign. In April 2014 Misbah joined the Emission Control Technology research group at JMTC.

Crispin Cooper is a computational chemist working in the JMTC Emission Control research group. He has a BSc (hons) in Chemistry for Drug Discovery from the University of Bath, UK, and an EngD in Molecular Modelling and Materials Simulation from UCL. He has recently joined Johnson Matthey following an industrial postdoctoral project modelling complex metal oxide catalyst materials.

Ludovic Briquet is a Senior Scientist at JMTC, Sonning Common. He gained his PhD in Chemistry at UCL, UK, in 2010 and specialised in molecular modelling applied to various systems. His main interests however lie in surface science and catalysis.

Aniekan Ukpong is a Senior Scientist at JMTC, Pretoria, where he has been working since 2013. He completed his PhD in Physics at the University of Cape Town, South Africa, in 2008. His research focuses on the fundamental study of materials from theory to applications.

Christopher Perry originally trained as a bio-inorganic chemist. He became interested in modelling whilst lecturing at the University of the Witwatersrand, South Africa. Prior to that, he completed two postdoctoral research projects: one in nuclear magnetic resonance (NMR) spectroscopy at Manchester University, UK, and a second in organic synthesis at the University of the Witwatersrand. He has been working at Johnson Matthey since 2013 and is a Senior Scientist at JMTC, Pretoria.

Glenn Jones has extensive experience in the fields of surface science, catalysis and computational materials chemistry. He gained his PhD from the University of Cambridge, UK, before moving to the Technical University of Denmark as a post-doctoral student. He joined JMTC, Sonning Common, UK, in 2008 and was awarded a Royal Society Industrial Fellowship in 2010 which he held jointly between JMTC and UCL. He moved to Pretoria, South Africa, in 2013 to initiate JMTC’s new modelling laboratory in South Africa, where he is currently Research Manager.

In the Lab Combining Catalyst and Reagent Design for Electrophilic Alkynylation

Johnson Matthey Technology Review features new laboratory research

Jérôme Waser is an Associate Professor in the Institute of Chemical Sciences and Engineering at the About the Researcher Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland. His research focuses on the development of new reactions based on catalysis and synthons with non-conventional reactivity.

About the Research

Alkynes are essential building blocks in synthetic and • Name: Jérôme Waser medicinal chemistry, materials science and chemical biology. Due to their linear geometry and electronic • Position: Associate Professor properties, they are important structural elements in • Department: Institute of Chemical Sciences and supramolecular assemblies and organic materials. The Engineering unique reactivity of the triple bond also makes them • University: EPFL ideal precursors of other functional groups, not only in a classical chemistry setting, but also for biological • Street: EPFL SB ISIC LCSO, BCH 4306, Av. Forel 2 applications. The development of new methods to • City: Lausanne make alkynes is consequently an important fi eld of research in fundamental organic chemistry. • Post Code: 1015 The transfer of terminal alkynes is one of the most • Country: Switzerland successful approaches for introducing triple bonds into organic molecules. This fi eld has been largely dominated • Email Address: jerome.waser@epfl .ch by the use of acetylide anions or their equivalents as nucleophiles, due to their ease of formation. Processes Waser’s group have designed new methods for such as the Sonogashira coupling and the addition the introduction of alkynes into organic molecules of alkynes to carbonyls are highly reliable and are using transition-metal catalysis and electrophilic widely used in synthetic chemistry. Nevertheless, alkynylation reagents. The direct alkynylation of the drawback of this approach is that alkynes can electron-rich heterocycles was developed fi rst be introduced only to the electrophilic positions of (Scheme I). The group harnessed the unique properties molecules . If good electrophilic alkyne synthons were of ethynylbenziodoxolone (EBX) reagents for the available, alkyne chemistry would become even more gold-catalysed alkynylation of indoles, pyrroles, versatile for applications in chemistry and in biology. thiophenes and furans. The cyclic hypervalent iodine

C–H Alkynylation: most electron-rich position Domino cyclisation-alkynylation: unreactive position R1 R1 R R R R N N S O H O R2 MeO N R

i AuCl catalyst I SiiPr O I Si Pr3 O 3 III II Room temperature, F3C Au or Pt catalyst O open ﬂ asks F3C TIPS-F -EBX Room temperature, TIPS-EBX 6 open ﬂ asks

i Si Pr3 i SiPr3 1 R 1 R R R R i N S O Pr3Si H i i i Si Pr3 N SiPr3 SiPr3 N R R R2 O C2-pyrroles and C3-indoles C2-thiophenes C2-furans C3-furans C5 and C6-indoles 48–93% 48–83% 45–90% 53–97% 31–84% 21 examples 21 examples 12 examples 12 examples 24 examples

Scheme I. Gold and platinum-catalysed C–H alkynylation vs. domino cyclisation functionalisation for accessing alkynylated heterocycles

reagent 1-[(triisopropylsilyl)ethynyl]-1,2-benziodoxol- To access triple bonds on other positions of aromatic 3(1H)-one (TIPS-EBX) (Figure 1) was initially rings, the group decided to use a new strategy discovered by Zhdankin and although it displays an based on a domino cyclisation-alkynylation process enhanced stability in the presence of transition metals, (Scheme I). This approach was fi rst applied to the it still acts as a strong electrophilic alkyne source. The synthesis of 3-alkynylated furans starting from allene alkynylation of heterocycles with TIPS-EBX is a user- ketones. The key for success was electronic tuning of friendly method, which proceeds in open fl asks at the hypervalent reagent, with TIPS-F6-EBX being most room temperature and can tolerate a broad range of successful. The synthesis of indoles alkynylated on functional groups. the benzene ring is even more challenging, due to the The developed C–H alkynylation is highly selective much higher reactivity of the pyrrole ring. In this case, for the most electron-rich position of heterocycles. no successful C–H functionalisation method has yet been reported. Using the platinum-catalysed domino cyclisation-alkynylation of homopropargylic alkynyl pyrrole ethers, 5- or 6-alkynylated indoles could be synthesised in good yield and selectivity. Overall, the domino strategy is therefore highly complementary to C–H alkynylation. To introduce alkynes onto C–sp3 centres, Waser’s group focused on the metal-catalysed multi- functionalisation of olefi ns. The intramolecular oxy- and amino-alkynylation of olefi ns using TIPS-EBX and a palladium(II) catalyst to give lactones and lactams at room temperature in open fl asks was developed fi rst (Scheme II). To access tetrahydrofurans and pyrrolidines, the combination of a Pd(0) catalyst and alkynyl bromides was more successful. In this case, the reaction was run at 65ºC under inert gas. Currently, Fig. 1. X-ray structure of TIPS-EBX the transformation cannot be made intermolecularly.

i O R O Me Si Pr3 10 examples 22 examples R3 43–87% 55–92% R3 O I SiiPr R1 3 O O Boc 3 i 20 examples N R Br R TIPS-EBX R O Si Pr3 10 examples 51–86% R3 69–83% Pd0 catalyst PdII catalyst 2 R1 R 65ºC, N2 Room temperature, F3C XH O 3 2 air R R i 20 examples 28 examples 4 O 2 R 1 R3X NTs Si Pr 51–98% R N R Y R 3 50–89% 2 R3 R

Scheme II. Palladium-catalysed oleﬁ n oxy- and amino-alkynylation

Nevertheless the group recently reported a fi rst step Y. Li and J. Waser, Angew. Chem. Int. Ed., 2015, 54, in this direction by the use of an in situ tethering (18), 5438 strategy for palladium-catalysed synthesis of vicinal Y. Li, J. P. Brand and J. Waser, Angew. Chem. Int. Ed., aminoalcohols bearing an alkyne group starting directly 2013, 52, (26), 6743 from allyl amines. The use of trifl uoroacetaldehyde in its commercially available hemiacetal form as a tether J. P. Brand and J. Waser, Chem. Soc. Rev., 2012, 41, (11), played an important role in this reaction. 4165 In conclusion, the metal-catalysed electrophilic S. Nicolai, C. Piemontesi and J. Waser, Angew. Chem. Int. alkynylation approach has allowed heterocyclic and Ed., 2011, 50, (20), 4680 aliphatic alkynes to be synthesised with high effi ciency. S. Nicolai and J. Waser, Org. Lett., 2011, 13, (23), 6324 The obtained products are expected to be highly useful as building blocks in synthetic and medicinal chemistry, J. P. Brand and J. Waser, Angew. Chem. Int. Ed., 2010, as well as in materials science. 49, (40), 7304 S. Nicolai, S. Erard, D. Fernández González and J. Waser, Selected Publications Org. Lett., 2010, 12, (2), 384 U. Orcel and J. Waser, Angew. Chem. Int. Ed., 2015, 54, J. P. Brand, J. Charpentier and J. Waser, Angew. Chem. (17), 5250 Int. Ed., 2009, 48, (49), 9346

Johnson Matthey Highlights

A selection of recent publications by Johnson Matthey R&D staff and collaborators

EMISSION CONTROL TECHNOLOGIES The signifi cance of palladium-catalysed ɑ-arylation methods are discussed and a number of case studies Increased NO Concentration in the Diesel Engine 2 have been included. Exhaust for Improved Ag/Al2O3 Catalyst NH3-SCR Activity The Effects of 1-pentyne Hydrogenation on the Atomic W. Wang, J. M. Herreros, A. Tsolakis and A. P. E. York, Structures of Size-selected AuN and PdN (N = 923 and Chem. Eng. J., 2015, 270, 582 2057) Nanoclusters Fast-SCR was investigated for NOx reduction in internal K.-J. Hu, S. R. Plant, P. R. Ellis, C. M. Brown, P. T. Bishop and R. E. Palmer, Phys. Chem. Chem. Phys., combustion engines. H2 addition was found to increase 2014, 16, (48), 26631 NO2 formation over a Ag/Al2O3 catalyst. This led to an improved NH3-SCR activity at low temperature. It The variation in atomic structures of size-selected Au is concluded that NO2 formation before the Ag/Al2O3 and Pd nanoclusters (containing 923 and 2057 atoms) catalyst either in the engine or a Pt/Al2O3 based DOC supported on amorphous carbon fi lms before and after will improve SCR performance. This NO2 promotion being exposed to the vapour-phase hydrogenation effect was less at higher temperatures. of 1-pentyne was studied. The populations of the nanoclusters were studied at atomic resolution before Thermochemical Recovery Technology for Improved and after the reaction using an aberration-corrected Modern Engine Fuel Economy – Part 1: Analysis of a high-angle annular dark fi eld (HAADF) scanning Prototype Exhaust Gas Fuel Reformer transmission electron microscopy (STEM). The D. Fennell, J. Herreros, A. Tsolakis, K. Cockle, J. Pignon atomic structures of the observed nanoclusters were and P. Millington, RSC Adv., 2015, 5, (44), 35252 determined by comparing the multi-slice HAADF-STEM Reformed exhaust gas recirculation (REGR) provides and experimental images for a full range of cluster H2 to the combustion process to recover heat from orientations. The results show that Au nanoclusters exhaust and improve fuel conversion effi ciency. A full consisting of 923 ± 20 and 2057 ± 45 atoms are robust scale prototype reformer for gasoline direct injection and exhibit high structural stability. A big proportion of engines is presented and its performance is assessed. Pd923 ± 26 nanoclusters, on the other hand, appear to be The performance is better at higher temperatures with amorphous before the treatment and after the reaction a decline in performance at lower exhaust temperature. were found to exhibit high symmetry structures which The reformate quality is also dependent on process suggests the reduction of oxidised Pd nanoclusters in temperature and reactant composition. reaction conditions.

FINE CHEMICALS FINE CHEMICALS: CATALYSIS AND CHIRAL TECHNOLOGIES Palladium-Catalyzed ɑ-Arylation Reactions in Total Synthesis Stereoselective Synthesis of the Halaven C14-C26 S. T. Sivanandan, A. Shaji, I. Ibnusaud, C. C. C. Fragment from D-Quinic Acid: Crystallization-Induced Johansson Seechurn and T. J. Colacot, Eur. J. Org. Diastereoselective Transformation of an α-Methyl Nitrile Chem., 2015, (1), 38 F. Belanger, C. E. Chase, A. Endo, F. G. Fang, J. Li, S. R. Mathieu, A. Z. Wilcoxen and H. Zhang, Angew. New methods for synthesising natural products and Chem. Int. Ed., 2015, 54, (17), 5108 active pharmaceutical ingredients have been explored using palladium-catalysed ɑ-arylation of carbonyl A series of substrate controlled stereoselective reactions compounds. The advantages of this particular method with crystalline intermediates was carried out via an are an increase in the overall yield, an improved α-methyl nitrile to produce a C14–C26 fragment of synthesis scope and a reduction in the number of steps. halichondrin B/Halaven. The synthesis does not require

287 © 2015 Johnson Matthey http://dx.doi.org/10.1595/205651315X688596 Johnson Matthey Technol. Rev., 2015, 59, (3) chromatography and relies entirely on crystallisation for quality control. D-quinic acid is the starting material, [010] [110] [1010] providing all four chiral centres, and is readily available. [001] [0110] [001] Raw material cost and waste are both reduced by the present synthesis.

A Halogen- and Hydrogen-Bonding [2]Catenane for Anion Recognition and Sensing J. M. Mercurio, A. Caballero, J. Cookson and P. D. Beer, RSC Adv., 2015, 5, (12), 9298 Halogen bonding has been little explored outside the areas of solid state crystal engineering. A novel halogen [100] bonding rotaxane structure was prepared for use in [111] [0001] anion recognition and exhibits good anion recognition [010] [110] [1010] and sensing properties. An ion templated Grubbs’ II- [001] [001] [0110] catalysed RCM clipping mechanical bond forming Reproduced by permission of the PCCP Owner Societies was used to synthesise the structure which contains from J. L. R. Yates, G. H. Spikes and G. Jones, Phys. both halogen- and hydrogen-bonding macrocyclic Chem. Chem. Phys., 2015, 17, (6), 4250 components. 1H NMR spectroscopy and ﬂ uorescence titration experiments were carried out and showed that the new structure can strongly associate with acetate is needed to rationally design better catalysts by and dihydrogen phosphate. surface tailoring. Electronic and geometric effects of various metals on unsupported Pd nanocrystals were NEW BUSINESSES: FUEL CELLS investigated using the decomposition of HCOOH to H2 and CO2 as a probe reaction. Bi was found to Platinum-carbide Interactions: Core-shells for Catalytic occupy high index sites causing a decrease in HCOOH Use dehydration, Te occupies terrace sites which reduces J. L. R. Yates, G. H. Spikes and G. Jones, Phys. Chem. the dehydrogenation rate while Ag induced strong Chem. Phys., 2015, 17, (6), 4250 electronic effects and increased the activity of the Pd surface sites. Ag and Bi were concluded to be the Five carbides (TiC, NbC, TaC, WC and SiC) were most effective additives for a surface reaction, while Te investigated using density functional theory with the aim should be added at corner sites to promote the desired of determining their suitability as core-shell components reaction route. in fuel cell applications. The fcc forms of the carbides were compared with hexagonal close-packed (hcp) Surfactant Mediated CO2 Adsorption: The Role of the WC and zinc blende SiC and the latter was found to Coimpregnation Species support Pt overlayers on surfaces, therefore, showing C. M. Starkie, A. Amieiro-Fonseca, S. P. Rigby, T. C. potential for full Pt encapsulation. The transition metal Drage and E. H. Lester, Energy Procedia, 2014, 63, surface resonances (TMSRs) play a vital role during the adsorption of Pt on fcc (111) carbide surfaces and fcc 2323 (100) was found to be adverse towards Pt adsorption. Carbon capture and storage requires novel, second Reduced oxygen adsorption energies was displayed by generation adsorbent systems to potentially reduce several Pt-WC surfaces during the oxygen adsorption costs associated with this technology. Solid supported study; the authors conclude that ORR activity should be amines have been investigated. These consist of basic promoted or maintained with respect to nanoparticulate amines either tethered or impregnated on silica or Pt catalysts. alumina and co-impregnated with surfactant additives. The mechanisms of adsorption of these systems were PROCESS TECHNOLOGIES studied and they were shown to have 55% improved Dual Doping Effects (Site Blockage and Electronic working capacity relative to single component systems. Promotion) Imposed by Adatoms on Pd Nanocrystals Triethanolamine and sodium dodecylsulfate produced for Catalytic Hydrogen Production the best adsorbent properties. S. Jones, S. M. Fairclough, M. Gordon-Brown, W. The Synergistic Effect in the Fe-Co Bimetallic Catalyst Zheng, A. Kolpin, B. Pang, W. C. H. Kuo, J. M. Smith System for the Growth of Carbon Nanotube Forests and S. C. E. Tsang, Chem. Commun., 2015, 51, (1), 46 D. Hardeman, S. Esconjauregui, R. Cartwright, S. Additives based on polymer or metal adatoms can Bhardwaj, L. D’Arsié, D. Oakes, J. Clark, C. Cepek, C. modify the electronic structure of metal nanoparticles Ducati and J. Robertson, J. Appl. Phys., 2015, 117, (4), but greater understanding of atomic level effects 044308

The growth of multi-walled carbon nanotube forests using steel. The ODS steel was irradiated simultaneously with an active bimetallic Fe-Co catalyst was investigated. Fe8+, He+ and H+ at room temperature to a damage When this bimetallic catalyst system was compared of 4.4 dpa at the Joint Accelerators for Nanosciences to pure Fe or Co a synergistic effect is observed. and NUclear Simulation (JANNUS) Saclay facility. The The height of the forests was considerably increased authors concluded that ODS nanoparticles are very and an improvement of the homogeneity in the stable under these irradiation conditions. as-grown nanotubes was found. The catalyst system was characterised using energy dispersive spectroscopy An Experimental Investigation of Biodiesel Steam and in situ X-ray photoelectron spectroscopy. The Reforming authors conclude that the growth rate of the nanotubes is greatly improved in the presence of Fe and Co. S. Martin, G. Kraaij, T. Ascher, D. Wails and A. Wörner, Intl. J. Hydrogen Energy, 2015, 40, (1), 95 TEM Characterization of Simultaneous Triple Ion Implanted ODS Fe12Cr The optimum operating conditions of a proprietary precious metal based catalyst for biodiesel steam V. de Castro, M. Briceno, S. Lozano-Perez, P. Trocellier, S. G. Roberts and R. Pareja, J. Nucl. Mater., 2014, 455, reforming was investigated with the aim of preventing (1–3), 157 catalyst deactivation. Different operating conditions include varying the temperature from 600ºC to 800ºC, The performance of oxide dispersion strengthened applying different pressure from 1 bar to 5 bar and (ODS) ferritic/martensitic steels under irradiation is altering the molar steam-to-carbon ratio from 3 to 5. studied. This is essential in the design of advanced fusion reactors. Transmission electron microscopy was used Coke formation and sintering have been determined to characterise a simultaneous triple ion implanted ODS as the main deactivation mechanisms. The authors Fe12Cr steel with the aim of investigating the impact of conclude that coking can be reduced by using low feed irradiation on the grain and dislocation structures, oxide ﬂ ow rates (31 g h–1 cm–2) and a relatively high catalyst nanoparticles and other secondary phases present in inlet temperature (>750ºC).

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