Thermal Treatment of Catalysts

Modern Methods in Heterogeneous Catalysis Research Friederike C. Jentoft, October 31, 2003 Outline

1. Terminology (calcination) 2. Sample vs. oven set temperature 3. Self-generated atmosphere & self-steaming of zeolites 4. Combustion 5. Glow phenomenon & zirconia catalysts 6. Crystallization 7. Loss of surface area 8. Effect of additives 9. Solid-solid wetting 10.Reductive treatments & SMSI Steps of Catalyst Preparation

v IUPAC defines 3 steps of catalyst preparation

1. Preparation of primary solid, associating all the useful compounds 2. Processing of that primary solid to obtain the catalyst precursor for example by heat treatment 3. Activation of the precursor to give the active catalyst (reduction to metal, formation of sulfides, deammoniation of zeolites) Heat Treatment of Intermediate Solids or Precursors

v drying v thermal decomposition of salts (nitrates, ammonium salts) v calcination

v product is a "reasonably inert solid" which can be stored easily Annealing

v in a general meaning: a heating of a material over a long time span; strain and cracks in a crystalline solid can be removed Origin of the Term "Calcination" v latin "calx" = playstone limestone, (greek chálix) v burning of calcium carbonate (limestone) to calcium oxide (quicklime)

-1 CaCO3 → CaO + CO2 ΔH(900°C)=3010 kJ mol v used to construct Giza pyramids (ca. 2800 A.C.), burning of limestone ("Kalkbrennen") mentioned by Cato 184 A.C. v performed in kilns (ovens) at 900°C v addition of air to sustain combustion + cool product Examples for Kilns for Calcination

v Schematic of a vertical shaft kiln. v Schematic of a rotary kiln a) Preheating zone; b) Calcining zone; a) Burner; b) Combustion air; c) Cooling zone c) Pre-heater; d) Kiln; e) Cooler General Definition of "Calcination" v decomposition of a substance through heating, transformation in lime-like substance – Duden v to heat (as inorganic materials) to a high temperature but without fusing in order to drive off volatile matter or to effect changes (as oxidation or pulverization) – Websters v heating (burning) of solids to a certain degree of decomposition, whereby with e.g. soda, gypsum the crystal water is completely or partially removed – Römpp's Chemielexikon v the heating of a solid to a high temperature, below its melting point, to create a condition of thermal decomposition or phase transition other than melting or fusing – Hüsing, Synthesis of Inorganic Materials Definition of "Calcination" in Catalysis Research v thermal treatment (of a catalyst) in oxidizing atmosphere. The calcination temperature is usually slightly higher than that of the catalyst operating temperature – Ullmann's Encyclopedia of Industrial Chemistry v a heat treatment of catalyst precursor in an oxidizing atmosphere for a couple of hours - Catalysis from A to Z, Eds. Cornil et al. v heating in air or oxygen; the term is most likely to be applied to a v often,step in the with preparation respect of to a catalycatalyst sts,- IUPAC an Compendiumoxidizing of Chemical Terminology treatment is meant v however, you will find statements such as "calcined in inert atmosphere" v sample-generated atmosphere may be oxidative (nitrate decomposition) Example from Patent Literature

v extremely vague! unimportant? v no, there is a secret to it! v often only temperature + holding time given

E.J. Hollstein, J.T. Wei, C.-Y. Hsu, US Patent 4,918,041 Calcination Procedure: Temperature Program

900

800

700

600

500

Temperature / K 400

300 0 100 200 300 400 500 Time / min v heating rate, holding time, cooling rate v cooling usually uncontrolled below certain T, slower Actual Temperature Program

v oven may not be able to perform selected program (lack in power): temperature lag of actual temperature behind set temperature v poorly tuned controller may give temperature oscillations v heat needs to be transferred from oven to sample, needs a gradient: temperature lag of sample temperature vs. oven temperature Role of Sample v strongly endo- or exothermic events may interfere with the heating program v endothermic events: solvent evaporation v exothermic events: combustion of organics, crystallization Evaporation of Water: Thermal Effects

v example: a 10 g sample containing 18 % water (0.1 mol) -1 v ΔHevap(H2O, 373 K) = 41 kJ mol v to evaporate in 1 minute: ≈ 70 Watt Example: Calcination of Zirconium Hydroxide

700 10 K min-1 -1 600 3 K min

500

400 1 K min-1

300 Sample bed temperature / K

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time / h v with a 10 g sample, deviations can occur already at moderate heating rates Evaporation of Water: Gas Formation

v example: a 10 g sample containing 18 % water (0.1 mol) v at 400 K: corresponds to ≈ 3.3 l of water vapor v depending on the - form of the bed - the type of furnace (tubular / muffle) - static / dynamic atmosphere (no flow / flow) the sample will be exposed to vapor for minutes! Zeolite Y as a Cracking Catalyst v zeolite Y is used as a cracking catalyst (FCC)

Faujasite structure

+ + v first synthesize NaY, then exchange Na by NH4 (liquid phase) v obtain active HY through thermal decomposition of NH4Y v regular HY not very stable Ultrastable Y zeolite

v McDaniel and Maher 1967 report „new ultra-stable form of faujasite“ v worked with 100 g of zeolite, first exchange then heat

treatment

900

800

700

600

500

Temperature / K 400

300 0 100 200 300 400 500 Time / min v „keeping the elapsed time between the exchange step and the heating step at 815°C to a minimum is quite critical“

C.V. McDaniel, P.K. Maher in Molecular Sieves, Soc. Chem. Ind. London, 1968, p. 186 Ultrastable Y Zeolite

v Kerr 1967: treatment of HY at 700-800°C in inert static atmosphere v „any technique keeping this water in the system during the heating process will result in a stable product“ v published comparison of heating in „deep bed“ or „shallow bed“: deep bed produces stable product v ascribes success of McDaniel & Maher to the large amount that they used

G.T. Kerr, J. Phys. Chem. 1967, 71, 4155 and J. Catal. 1969, 15, 200. Influence of Packing

MS analysis v Packing of a solid m/e = 18 (H2O) influences evolution of gas (water vapor) Evaporation – Autogeneous Pressure

v boiling point of 5.8 mg H2O, sealed water is explosion determined 2.09 mg H O, 50 µm hole in lid 2 properly only in onset crucible with lid + hole in lid! 6.2 mg H2O no lid Stabilization Through Dealumination

v water vapor removes aluminum from zeolite framework (extra-framework aluminum) v leads to stabilization v today, ultrastable Y or USY is obtained through

„steaming“, treatment of NH4-Y 600-800°C in rotary kilns v USY is used in „fluid catalytic cracking“ and „hydrocracking“ and „hydroprocessing“ v „steaming“ is a general method for dealumination of zeolites Exothermic Reactions: Combustion v organic matter may be present, e.g. from sol-gel process, surfactant-assisted synthesis v will combust upon thermal treatment in oxygen- containing environment v look at thermochemical data CRC Handbook of Thermophysical and Thermochemical Data Eds. David R. Lide, Henry V. Kehiaian, CRC Press Boca Raton New York 1994 FHI library 50 E 55 D'Ans Lax, Taschenbuch für Chemiker und Physiker Ed. C. Synowietz, Springer Verlag 1983, FHI library 50 E 54 Example: Pentane Combustion

C5H12 (g) + 8 O2 (g) → 5 CO2 (g) + 6 H2O (g)

L Q D r H = å n i D f H i ° i=1 D H Q = 5 D H° + 6 D H° -1 D H° r f CO2 f H 2O f C5 H12

Q -1 -1 -1 D r H = 5 (-393.51 kJmol ) + 6 (-241.82 kJmol ) -1 (-146.44 kJmol )

Q -1 D r H = 3272.0 kJmol v combustion is strongly exothermic! v oxygenates have higher enthalpies of formation, i.e. enthalpy of combustion becomes smaller Example: Surfactant-Assisted Synthesis Mesoporous Zirconia

v surfactants (hexadecyl-trimethyl-ammonium bromide) form micelles v inorganic matter forms around micelles Example: TG/DTA Analysis of ZrO2-Precursor

80 H2O loss 60 exo 70 50

40

60 combustion endo 30 of organics 20 TG (mg) 50 DTA (mV) 2- 10 SO4 decomposition 40 0

-10 200 400 600 800 1000 1200 Temperature (K) v ZrO2/CTAB composite synthesized with Zr(O-nPr)4 in the presence of sulfate ions at Zr:S:CTAB = 2:2:1, measured with 10 K/min in an air stream Other Exothermic Reactions

v Example: calcination of X-ray amorphous zirconium hydroxide v "ZrO2 * 2.5 H2O" Heating of Zirconium Hydroxide

Heating time / min 115 120 125 130 135 140 145 150 155 950 17.1 ml boat 900 8.4 ml boat 2.2 ml 850

800 2.2 ml boat 8.4 ml

750 700 17.1 ml 650 600 Sample bed temperature / K 550 640 660 680 700 720 740 760 Oven temperature / K

v strong influence of batch size / heat transfer v rapid overheating (up to 40-50 K/s) v overshoot of up to 300 K History of Glow vOverheating is so violent, it is accompanied by emission of visible light ("glow") Berzelius 1812 (antimonates, antimonites) The Glow Phenomenon Oxides Showing a Glow Origin of Glow

v combustion of organic contaminants v heat of crystallization v loss of surface energy through sintering Effect of Combustion?

Heating time /min 165 170 175 180 900 Oxygen 880 Argon Air 860 840

820 800 780

Sample bed temperature / K 760 780 790 800 810 820 830 840 Oven temperature / K v atmosphere little influence on overheating effect v heat not caused by combustion of organic contaminants Origin of Glow

v combustion of organic contaminants v heat of crystallization v loss of surface energy through formation of larger particles -1 Heat of Crystallization of ZrO2 (kJ mol )

?-ZrO2 –28.7 Keshavaraja any ZrO2 –23.2 Srinivasan t- or m-ZrO2 –4.3 to –22.5 Chuah -1 t- or m-ZrO2 –12.9 Tatsumi t-ZrO2 –29.3 to –33.4 Livage t-ZrO –19.6 -53.6Mercera kJ mol 2 : t-ZrO –30.1 2± 0.8 Haberko 2 -ZrO -4.3 and t-ZrO2 –53 Molodetsky t-ZrO2 –13 Xie m-ZrO2 –58.6 ± 3.3 Molodetsky

Changeheat ofin surfacecrystallization energy upon of at -ZrO2 → t-ZrO2 2 -1 (assumedaccording ABET = to 100 literature m ·g ): between+14.6 Molodetsky

Transition t-ZrO2 to m-ZrO2: –5 Molodetsky –6 Xie –6 Coughlin Estimation of Temperature Rise through Crystallization v assume a medium heat of 25 kJ mol-1 v process assumed quasi-adiabatic (δQ = 0) v molar heat of precursor / intermediate assumed similar -1 -1 to that of ZrO2: 82.3 J mol K

DH DT = » 300 K c p v corresponds approximately to observation Does Crystallization Happen During Glow? v use method that allows structure-determination and good time resolution v X-ray absorption spectroscopy at Zr K edge at ESRF (1 spectrum per s; 10 K min-1 heating rate) v allows observation of local environment around Zr4+ ions In situ X-ray Absorption Spectroscopy

v use large pellet to create a sufficiently large glow effect v put small pellet inside that is transparent for X-rays

5mm5 mm XAS Spectra

1.0

0.75Norm. absorption

0.5 200 150 0.25 100 50 Time, seconds

0.0 18 18.25 18.5 18.75 19 Photon energy [keV] Sample Temperature vs. Time

600 Heating rate 550 ≈ 50 K s-1

500

450

Temperature, °C 400

350 110 112 114 116 118 120 122 124 126 128 130 Time, seconds Structural Evolution

6.0 )

2 4.0 (k)*k c FT( 2.0 547 °C 499 °C 447 °C 439 °C 436 °C

0 2 R4 [Å] 6 8 Origin of Glow

v combustion of organic contaminants v heat of crystallization - yes! v loss of surface energy through formation of larger particles Sintering

v a heat treatment at 2/3 to 3/4 of the melting point to solidify shaped bodies from pressed metal powders, occurs in 3 steps v 1. increase of particle contacts through "sinter bridges" 2. formation of a contiguous backbone, original particles lose their identity, shrinkage, formation of new grain boundaries 3. rounding and elimination of pores, further shrinkage, closed pores Tammann and Hüttig Temperature v Tammann temperature temperature necessary for lattice (bulk) recrystallization

for metal oxides TTammann » 0.52 TF v Hüttig temperature temperature necessary for surface recrystallization

for metal oxides THüttig » 0.26 TF

with TF the absolute melting temperature Influence of Particle Size

v decomposition of Al(OH)3, thermogravimetric analysis v smaller particles react at lower temperature

1 µm

50-80 µm 0.2 µm Influence of Particle Size on Melting Point of Au

v Melting point can decrease drastically with decreasing particle size!

Ph. Buffat, J. P. Borel, Phys. Rev. A 13 (1975) 2287-2298 Surface Energy

v loss of surface area through formation of larger crystals

-2 v surface energy of t-ZrO2(101) ≈ 1.1 J m v surface area shrinks from 250 to 150 m2 g-1 during calcination -1 -1 → 110 J g or 13.5 kJ mol (ZrO2) v is not negligible in comparison to crystallization Formation of a Crystalline Solid from Amorphous Precursor

ΔH???

v nature of amorphous precursor usually not well-known v ΔH depends on nature of precursor & product v if, through variation of the treatment conditions, for the same precursor different ΔHs are obtained, the products will be different Effect of Additives

TimeZeit / minmin 120 130 140 150 160 170 180 190 1000 FeSZH MnSZH 950

900 SZH 850 ZH 800

750 700 Probentemperatur / K 650 Rampe 3°/min 17,1 ml-Schiffchen Sample bed temperature / K 600 650 700 750 800 850 OvenOfentemperatur set temperature / K / K v additives shift glow to higher temperatures and reduce overshoot Glow Phenomenon: MnSZ and FeSZ

Heating time / min 165 170 175 180 185 190

1000 1000 25 g 25 g 900 800 950 12 g 700 3 g

900 12 g 3 g 600

850 500 Temperature / K 400

800 2%MnSZH2%MnSZ 300 0 100 200 300 400 500 Sample temperature / K 2%FeSZH 750 Time / min

780 800 820 840 860 Oven temperature / K v max. calcination T may be exceeded v promoters influence calcination chemistry (systemic), Fe and Mn different v strong batch size dependence Influence on Catalytic Activity?!

Zeit / min 165 170 175 180 185 190

1000 25 g 25 g

950 12 g 3 g 900 12 g 3 g

2%FeSZ 850 2%MnSZ

800 Probentemperatur / K 750

780 800 820 840 860 Ofentemperatur / K

16 17.1 ml boat 14 17.1 ml boat 14 8.4 ml boat 12 8.4 ml boat 12 2.2 ml boat 2.2 ml boat 10 10 8 8

-butane (%) i

-butane (%) 6 6 i 4 4 Yield 2 Yield 2 0 0 0 120 240 360 480 0 120 240 360 480 Time on stream / min Time on stream / min

samples calcined in larger batches are more active (1 vol% n-butane at 338 K) characterize catalysts Surface Area & Calcination Batch Size

16 17.1 ml boat 14 17.1 ml boat 14 8.4 ml boat 12 8.4 ml boat 12 2.2 ml boat 2.2 ml boat 10 10 8 8

-butane (%) i

-butane (%) 6 6 i 4 4 Yield 2 Yield 2 0 120 0 0 120 240 360 480 FeSZ 0 120 240 360 480 Time on stream / min 115 Time on stream / min MnSZ -1 g 2%FeSZ 2 110 2%MnSZ 105 100

95 90 Surface area / m 85 80 0 2 4 6 8 10 12 14 16 18 20 Boat size / ml

v surface area increases with calcination batch size v differences in activity exceed differences in surface area Activity and Bulk Structure

16 14 17.1 ml boat 17.1 ml boat 14 12 8.4 ml boat 8.4 ml boat 2.2 ml boat 12 2.2 ml boat 10 10 8 8

-butane (%) i

-butane (%) 6 2%MnSZ i 6 2%FeSZ 4 4 Yield Yield 2 2 0 0 0 120 240 360 480 0 120 240 360 480 Time on stream / min Time on stream / min 1.445 1.445

1.444 1.444

1.443 1.443

1.442 1.442

c/a 1.441 c/a 1.441

1.440 1.440

1.439 1.439

1.438 1.438 0 5 10 15 20 0 5 10 15 20 Boat size / ml Boat size / ml v lattice parameters of tetragonal ZrO2 change Porosity of Iron-Promoted Sulfated Zirconia

70 2.2 ml boat -1 8.4 ml boat *g 3 60 17.1 ml boat

50

40

30

20 Adsorbed volume / cm 10 0.0 0.2 0.4 0.6 0.8 1.0

p/p0 v large batch calcination: formation of mesopores, 1-4 nm Effect of Calcination Batch Size

v samples from the same raw material (precursor) are converted into different products by variation of the batch size during calcination Why Does Overheating Occur?

v heat is generated faster than transferred away v look at heat transfer Heat Transfer Modes

v all of them play role during calcination v estimations can be made! Heat Transfer by Convection

v free vs. forced convection makes a considerable difference Thermal Conductivity

v data are for solids, conduction worse in loose powders v exact material during calcination unknown

ZrO2

Kingery 1955 Radiation

v conduction and convection are proportional to difference between the temperatures (T gradient , ΔT) of the body of interest and the surrounding v radiation is proportional to the difference between the temperatures to the forth power IUPAC Recommendations on Calcination

v all particles of catalyst should be subjected (..) to exactly the same (..) conditions only possible in moving beds (fluid beds, rotating furnaces, spray drying) v supply a sufficient quantity of gas or liquid to the reactor to ensure complete reaction (..); special consideration should be given to mass and heat transfer

J. Haber, J.H. Block, B. Delmon, Pure & Appl. Chem. 67 (1995) 1257-1306 Preparation of Supported Catalysts

v goal: disperse an active phase on an inexpensive and inert (?) support

increase surface area

support Interaction Between Active Phase and Support Spreading Mechanisms

v transport via gas phase also possible Preparation of SCR Catalyst by Solid-Solid-Wetting

v MoO3/TiO2: typical catalyst for selective catalytic reduction

(reaction of NOx with NH3 to give N2 and H2O)

720 K heating in O2, saturated with H2O

v start out with physical mixture: small particles, intimate mixture v evoke spreading through thermal treatment v analysis of surface composition! Ion Scattering Spectroscopy v topmost layer of surface is probed with ion beam (He+)

He+ He+ Ekin (~kV)

E'kin

v also: LEIS = low energy ion scattering v kinetic energy after interaction depends on mass of scattering atom v highly surface sensitive, destructive Spreading

v Spreading of MoO3

on TiO2 can be achieved by thermal treatment

in O2/H2O Supported Metal Catalysts: Metal-Support Interaction Depth Profiling of Supported Noble Metal Catalyst

v Rh/TiO2: a system with strong metal support interaction (SMSI) Conclusion

vMany things can happen during a thermal treatment!