<<

Periodic Variations in the Catalytic Properties of THE INFLUENCE OF STATE PARAMETERS ON AND CATALYSIS

By G. C. Bond, D.s~.,F.R.I.C. Research Laboratories, & Co Limited

(2) The first row VIII metals ( Of the one hundred or so elements in the metals): Fe, Co, Ni. no less than spventy-jive (3) The second and third row Group VIII are metals, but of these only the twelve in metals ( group metals) : Ru, Groups VIIl and IB have important Rh, Pd, Os, Ir, Pt. catalytic properties. The marked changes in these properties on proceed- Comparison of Groups VIII and IB ing either certically or horizontally There are dramatic differences between the through the groups is discussed in terns catalytic properties of the Group IB metals of the periodic variation of the solid- and those metals lying adjacent in Group state properties of the mijtals. VIII : these differences are particularly marked with and similar It is now well established that the occur- reactions. and are totally unable rence of significant and useful catalytic to activate and hence to catalyse properties is largely confined to the elements reactions involving hydrogen, while their lying in Groups VIII and IB of the Periodic immediate neighbours to the left, Table. Other elements of the transition series and platinum, are among the best known are able to act catalytically, but because of the hydrogenation catalysts. This simple ob- great thermochemical difficulty of reducing servation constitutes the most vivid possible them to the metallic state, and of keeping demonstration of the existence of an electronic them reduced, they do not find any practical factor in catalysis, since geometrically speak- applications. Metals which have either no ing there is very little difference between d-electrons or which have only filled d- palladium and silver, or between platinum electron shells, while sometimes thermo- and gold (see Table I). Their electronic pro- chemically suitable, are generally catalytically perties (magnetic susceptibility, electrical inactive, and catalysis (particularly of hydro- conductivity, etc.) are of course substantially genation and related reactions) has therefore different; the Group VIII metals having come to be particularly associated with the incomplete d-electron bands are paramagnetic, presence of unpaired d-electrons in thc . while the Group IB metals having no un- There are, however, many large and im- paired d-electrons are diamagnetic (see Table portant differences between the twelve metals TI). The presence of unfilled d-electrons is constituting Groups VIII and IB : to facilitate essential to hydrogen , as they discussion we will subdivide the twelve stabilise an intermediate weakly-held form metals as follows: without which cannot occur. (I) The Group IB metals (coinage metals) : A similar but less marked difference in Cu, Ag, Au. catalytic properties exists between

Platinum Metals Rev., 1968, 12, (3), 100-105 100 and . Although nrckel is generally much Table I superior, copper is not without some modest Crystal Structure, Metallic Radius and properties as a hydrogenation catalyst, being Density of the Elements of Groups VIII particularly attractive for the selective hydro- and IB genation of and diolefins (I). The b.c.c. =body-centred cubic reason for the slight activity of copper has c.p.h. =close packed hexagonal been a matter of debate for many years and f.c.c. =facecentred cubic no firm conclusion has been arrived at. It is Fe co Ni cu possible that the d-electrons in copper are h.c.c. f.c.c. f.c.c. f.c.c. 1.248 1.258 1.248 1.278 more readily thermally excited to sp-states 7.87 g/cc 8.90 g/cc 8.90 g/cc 8.96 g/cc than those of silver and gold, and that as Ru Rh Pd Ag before the resulting unpaired d-electrons are c.p.h. f.c.c. f.c.c. f.c.c. responsible for catalytic activity. I'.32lf r.348 1.37A 1.44A 1.90 g/CC 12.44 g/CC 12.02 g/CC 10.49 g/CC The Group IB metals are more active than their Group VIII neighbours in the rare 0s Ir Pt Au c.p.h. f.c.c. f.c.c. f.c.c. cases where the metal is required to act as an 1.338 1.358 1.388 1.448 electron-donor, as for example in the case of 2.48 g/Cc 22.5 g/CC 21.45 g/CC 19.3 g/CC the decomposition of (2). Silver has an important role to play as a There is also a related sharp break between controlled oxidation catalyst, and is used on the first and second row elements with regard a large scale for the oxidation of to electronic properties. The base metals are to ethylene and of to ferromagnetic while the . Copper is too easily oxidised metals are paramagnetic (see Table 11): the to be a possible catalyst, and gold is so noble base metals are readily oxidised and corroded that it is unable even to chemisorb . (and their compounds are correspondingly The Group VIII metals usually give only more difficult to reduce to the metallic state), complete oxidation. and have lower potentials, than the platinum group metals. This has important Comparison of the Base Metals practical consequences in catalyst preparation, with the Platinum Group Metals since for example nickel oxide is not easily It is now desirable to compare the differ- reduced to the metal while the of ences in physical and catalytic properties palladium and platinum can be reduced to observed on moving vertically through each the corresponding metals by hydrogen under sub-group of Group VIII. The base metals ambient conditions. The significant differ- differ from the platinum group metals in ences in the of the elements of many of their physical and chemical pro- Group VIII also have repercussions on pre- perties. The contraction is re- parative procedures for catalysts. Thus there sponsible for the close similarity in the atomic are no palladium or platinum analogues of the and metallic radii of vertically adjacent pairs simple nickel salts such as Ni(NO,), and of the platinum group metals, and this is NiSO,, while hydrated nickel chloride, substantially responsible for other similarities, freely soluble, contrasts with palladium for example, in latent heats of sublimation chloride, which is anhydrous, polymeric in and in melting points (see Table 111) and in structure and virtually insoluble in water. mechanical properties. All the metals of the The differences between the catalytic second and third sub-groups are face-centred properties of the base metals and the platinum cubic in structure, but those in the first sub- group metals are too numerous and well- group are either body-centred cubic or close- known to warrant a detailed exposition. The packed hexagonal (see Table I). latter are generally superior to the former as

Platinum Metals Rev., 1968, 12, (3) 101 Table II methylene groups in certain steroids is more Magnetic Character and Mass Magnetic stereoselective when is the cataIyst Susceptibility (c.g.s. units x 106) of the than when platinum is used (4). As a final Elements of Groups VIII and 1B example, palladium is more active than rho- Fe CO Ni cu dium for the of substituents ferro-. ferro- ferro- dia- (for example, alkoxy groups and magnetic magnetic magnetic magnetic ) from aromatic rings (5). Attempts to - - - -0.09 rationalise effects of this kind have only been made in very limited contexts, the difficulty para- para-, para- dia- being the lack of adequate knowledge concern- magnetic magnetic magnetic magnetic ing metals other than palladium and platinum. 9 +0.50 -+1.11 7 5.4 -0.20 It is instructive to seek the origin of these Ir Pt All para- para- para- dia- differences in catalytic behaviour. The differ- magnetic magnetic magnetic magnetic ent crystal habits (Table I) may have some effect, but it is probably not large. There has been much discussion over a protracted catalysts for hydrogenation and related re- on the relation between the catalytic actions, although the cost differential (about properties and the bulk properties of metals 20 between nickel and palladium) sometimes (6, 7, 8). Although catalytic effects must counteracts the activity difference. For this result from the physico-chemical properties reason also is widely used in of surface atoms, there are grounds for hoping synthesis and decomposition, although the that these, themselves not easily measurable, more costly is in fact more efficient. will reflect the more easily measurable bulk The superiority of platinum as a hydrogena- properties. tion-dehydrogenation catalyst, however, deter- An important effect in determining catalytic mines its selection for reforming activity is undoubtedly the strength of ad- operations, notwithstanding the cost factor. of intermediate species. For optimum The platinum group metals are the undoubted catalytic performancc there is required choice for oxidation processes such as the maximum coverage by species of minimum hydrogen-oxygen reaction, complete hydro- adsorption energy. With metals to the left and ammonia oxidation, of Group VIII activity is lower because the where base metals would he rapidly oxidised adsorption energy is too high; to the right it and inactivated. is lower because the adsorption energy is too small to maintain adequate surface coverage. Catalytic Properties of the The precise position of the activity maximum Platinum Group Metals depends on the nature of the reaction, as is Although the metals of the platinum group demonstrated in Figs I and 2. The view has distinguish themselves from all others in often been expressed that the strength of terms of their catalytic abilities (6, 7) there chemisorption bonds parallels the strength are nevertheless important differences be- of the metal-metal bond in the solid, and so tween the various members of the group. correlations have been sought between cataly- Specific points of difference have been tic activity and latent heat of sublimation (9) described previously (3) and it is only or other physical properties (6) which should necessary to mention a few examples here. reflect the metal-metal bond strength and Ruthenium is very much less active than which vary periodically with increasing palladium for the reduction of aromatic atomic number. Unfortunately the physical nitro-groups, while the reverse is true for properties in question divide themselves into carbonyl group reduction. The reduction of two groups, one of which indicates maximum

Platinum Metals Rev., 1968, 12, (3) 102 cohesive strength in Groups V or VI of the Periodic Table, and the other of which indicates maximum cohesive strength in Group VIII (6). The former group of properties involves changes (sublima- tion, melting, etc., see Table 111) while the latter does not (density, metallic radius, etc., see Table I) and is therefore thought to be more reliable as a guide to cohesive strength. Although these properties vary rationally and periodically with increasing atomic number, because of the difficulties outlined above it may be preferable for the present to seek only to correlate catalytic properties with position in the Periodic Table. In a sense this is avoiding the main issue, but if- some trends can be discerned as the electron/ ratio changes then some rationalisation of catalytic phenomena dent of the physical form of the catalyst then may be hoped for. some measure of order is apparent. Detailed The limited success that quantitative cor- study of the hydrogenation of olefins, dioleiins relations with physical properties enjoyed is and alkynes catalysed by all the Group VIII due in part to the different physical forms in metals has revealed a consistent pattern of which the metals have been used. If, however, behaviour which correlates qualitatively with attention is concentrated upon those catalytic the known stabilities of organometallic olefin properties which are substantially indepen- complexes; this work has been reviewed in detail elsewhere (10). A larger challenge is, however, presented when the hydrogenation of other functional groups is attempted on a Table Ill comparative basis, because the important and Latent Heat of Sublimation and Melting Point of the Elements of Groups VIII useful selectivity consideration is frequently and JB I absent and reliance has to be placed on qualitative rate measurements. Nevertheless Fe co Ni cu gg kcal/g IOZ kcalig IOI kcallg 81 kcalig certain regularities of behaviour are emerg- atom atom atom atom ing, and these are worth sketching in outline. 1540°C 1493°C I455’c 1083°C Ru Rh Pd Ag Hydrogenation of the 155 kcal/g 133 kcal/g 89 kcal/g 68 kcal/g atom atom atom atom Aromatic Ring 2400°C 1966°C 1554°C 960°C Although all the platinum group metals 0s Ir Pt Au are in some measure active for the -phase 187 kcal/g 158 kcal/g 135 kcal/g 84 kcal/g reduction of aromatic , signi- atom atom atom atom ficant differences appear when they are used 3045’c 2454°C 1773°C 1063°C under mild conditions in -phase reduc-

Platinum Metals Rev., 1968, 12, (3) 103 tion. Palladium is inactive, whereas both and ruthenium are active: unfortu- nately it is not possible to quantify this state- ment because of the different kinds of catalyst that have been used. The aromatic ring is expected to adsorb parallel to the surface, and the bond probably involves partial dona- tion of the delocalised 7;-electrons of' the ring to the metal (I I). Hence nai'vely the lower the electron/atom ratio, the stronger the adsorp- tion is expected to be. We therefore conclude that palladium is inactive because it does not adsorb the aromatic ring sufficiently strongly. It is significant that compounds such as phenol and , having substituents which donate electrons to the ring and which hence would be expected to adsorb more strongly, are more reactive than when palladium is used as catalyst. Hydrogenation of the Carbonyl Group Aromatic and are readily hydrogenated in the presence of palladium catalysts while the corresponding aliphatic compounds are unaffected under mild con- ditions. Now because of electromeric effects, the carbonyl group in aromatic compounds will have a greater electron density than in from the metal to the nitro group, the inactivity aliphatic compounds. Hence adsorption of of ruthenium being due to too weak adsorp- the carbonyl group on palladium would seem tion of the reactant. We cannot distinguish to involve electron donation from this group these possibilities without further experimen- zo the metal, and the low of aliphatic tal work. carbonyl compounds is probably due to in- sufficiently strong adsorption. If this is so, we Conclusions may predict that on moving to rhodium and On passing along any row of the Group thence to ruthenium, that is, with decreasing VIII metals from left to right, the increasing electron/atom ratio, the adsorption and hence electron/atom ratio has the effect of de- the reactivity of aliphatic carbonyl com- creasing the strength of metallic bonding since pounds should increase, and qualitatively the number of unpaired electrons is falling. this is what is observed. Hence also the concentration of valency The activity of the second row metals for electrons at the surface is falling, and this hydrogenation of aromatic nitro-compound means decreasing strength of adsorption for conversely increases with increasing electron/ those species where the metal donates elec- atom ratio. We therefore infer that adsorption trons to the adsorbate. Catalytic activity of the nitro-group is either too strong on across the row may either increase or decrease ruthenium (which is inactive) or alternatively depending on where the optimum adsorption that adsorption involves electron donation strength occurs. A knowledge of the sense of

Platinum Metals Rev., 1968, 12, (3) 104 the activity change with increasing electron/ 4 G. I. Gregory, J. S. Hunt, P. J. May, F. A. Nice and G. H. Phillips, 3. Chem. Soc., (C), atom ratio can give valuable clues to the 1666,2201 and can assist the 5 H. A. Smith and R. G. Thompson, Adv. selection of catalysts of optimum activity for Catalysis, 1967, 9, 727 6 G. C. Bond, ‘Catalysis by Metals’, Academic particular reactions. For example, binary Press, London, 1962, (see especially Chapters alloys of metals having activities possibly 2 and 21) superior to those of pure metals may be 7 A. M. Sokol’skaya, S. M. Reshetnikov, A. B. Fasman and D. V. Sokol’skii, Internat. Chem. selected on this basis. Engning, 1967, 7, 525 8 G. C. A. Schuit, L. L. van Reijen and W. M. H. Sachtler, in ‘Actes 2me Congres Internat. Catalyse’, Editions Technip, Paris, References 1961, P. 893 I R. S. Mann and K. C. Khulbe, Indian J. g S. M. Reshetnikov and A. M. Sokol’skaya, Technol’., 1967, 5, 65; A. de Jonge, J. W. E. Zhur. $2 Kham., 1966, 40, 2412 Coenen and C. Okkerse, Nature, 1965, 206, 10 G. C. Bond and P. B. Wells, A‘dv. Catalysis, 573 1964, 15, 92; G. C. Bond, in Kinetics and 2 See for example D. A. Dowden and P. W. Catalysis’, ed. P. B. Weisz and W. K. Hall, Reynolds, Discuss. Faraday SOC.,1950, 8, 194 Amer. Inst. Chem. Eng. Symposium series, 3 See for example G. C. Bond and E. J. 1967,631 3 Sercombe, Platinum Metals Rev., 1965, 9, 74; 11 See for example J. L. Garnett and W. A. ‘Platinum Metal Catalysts’, published by Sollich-Baumgartner, Adv. Catalysis, 1966, Johnson Matthey & Co Ltd 16, 95

High Purity Palladium Brazing Alloys MULTI-STAGE JOINING IN THE MANUFACl’URE OF THERMIONIC VALVES

The manufacture of special purpose While the well known silver-copper eutectic thermionic valves operating at relatively high melting at 778°C can satisfy a good many temperatures and under conditions of high of these requirements when produced under vacuum necessitates the use of brazing alloys careful conditions, alloys containing palladium specifically suited to these requirements. show distinct advantages in terms of both Such alloys must be free from impurities vapour pressure requirements and mechanical that might hinder wetting and flow of the properties, while also giving improved wetting molten brazing material, must have low characteristics on , and vapour pressures at high service temperatures, nickel alloys. In addition they offer reduced high melting points and, where needed, good risk of failure of the joints due to intergranular mechanical properties at high temperatures. penetration. The Pallabraze range of high

~~~~~ purity palladium brazing alloys Pallabraze High Purity Brazing Alloys developed by Johnson Matthey was described in this journal Alloy Composition Melting range “C some years ago by M. H. Sloboda (Platinum Metals Rev., Pallabraze 810 5 Pd-Ag CU 807- 810 1963, 7, S), but a revised Data Sheet just published by the Pallabraze 840 10 Pd-Ag CU 830 - 840 company brings up to date the Pallabraze 850 10 Pd-Ag CU 824 850 - detailed properties and uses of Pallabraze 880 15 Pd-Ag CU 856 - 880 these alloys. As will be seen Pallabraze 900 20 Pd-Ag CU 876 - 900 from the table, a range of Pallabraze 950 25 Pd-Ag CU 901 - 950 brazing materials is available Pallabraze 1010 5 Pd-Ag 970-1010 to provide melting points Pallabraze 1090 18 Pd-Cu 1080-1090 spaced at convenient intervals Pallabraze 1225 30 Pd-Ag 1150-1225 so that complex assemblies can Pallabraze 1237 60 Pd-Ni 1237 be fabricated by the step-by- step technique.

Platinum Metals Rev., 1968, 12, (3) 105