Microkinetic investigationsof heterogeneouslycatalyzedreactions (under industrially relevant conditions)

Transcription

1 Microkinetic investigationsof heterogeneouslycatalyzedreactions (under industrially relevant conditions) Jennifer Strunk Junior Research Group Leader Ruhr-University Bochum Laboratory of Industrial Chemistry Lecture series, Fritz-Haber-Institut, Berlin, Oct. 19, 2012.

2 Scope of this lecture Microkinetic investigations what for? Elementary step kinetics Microcalorimetry Temperature-programmed desorption Examples: CO on Cu/ZnO/Al 2 O 3 Methanol decomposition on ZnO This lecture is based on the lecture Modern Microkinetics, Prof. Dr. M. Muhler, Industrial Chemistry, Ruhr-University Bochum.

3 The seven steps in heterogeneous gas-solid solid catalysis 7 1 6b 2a 6a 2b outer surface inner surface 3-5 1) Transport of the reactants from the fluid bulk to the boundary layer 2) Transport of the reactants to the surface a) through the boundary layer to the outer surface b) from the outer surface to the inner surface 3) Adsorption (chemisorption) of the reactant 4) Chemical reaction 5) Desorption of the product boundary layer Very frequently: inner surface >> outer surface 6) Transport of the products to the fluid bulk a) from the inner surface to the outer surface b) from the outer surface through the boundary layer 7) Transport of the products from the boundary layer to the fluid bulk

4 Complexity of Heterogeneously Catalyzed Reactions Mean field approach (mesoscopic) k 7 k 1 k 2 k 3 k 6 k 4 k 5 Global kinetics (macroscopic) 1 Elementary step kinetics (microscopic) 1 k (T, c, p, p...) O. Hinrichsen, in: Catalysis from A to Z: A Concise Encyclopedia (ed. Herrmann, Cornils, Wong, Schlögl), 2 nd edition, Wiley-VCH, Weinheim 2003.

5 Knowledge-base approach: : Microkinetic analysis Single crystal surfaces (UHV) Steady-state experiments Transient experiments Material Gap Microkinetic analysis Characterization Spektroscopic studies Pressure Gap Kinetic theories, thermodynamics Real catalysts (high pressure) O. Hinrichsen, Catal.Today 53 (1999)

6 Why are elementary step kinetics useful? Elementary step kinetics, X Global kinetics Local approach Mechanistically proven model Physical interpretation of kinetic parameters Coverages of intermediates Changes of the morphology Process engineering/ Design, dimensioning Optimized Strategy for catalyst preparation Optimizationof Reaction parameters (Choice of reactor)

7 Example 1: Water gas shift reaction Microkinetic Model * 1) H2O(g) + * H2O* (1) H2O* + * OH* + H* (2) 2 OH* H2O* + O* (3) OH* + * O* + H* (4) 2 H* H2(g) + 2* (5) CO(g) + * CO* (6) CO* +O* CO2* + * (7) Ea r = A exp R T 1 β = K α i γ K g g P P CO CO 2 Power-law Model * 1) P P H H O 2 2 P α CO CO P αh2 H O 2 O P α CO CO2 2 P αh H 2 2 P γ tot apparent reaction order of component i fudge factor correcting the total pressure dependence equilibrium constant for the WGS reaction ( 1 β ) CO2* CO2 (g) + * (8) CO2* + H* HCOO* + * (9) HCOO* + H* H2COO* + * (10) H2COO* + 4 H* CH3OH(g) + H2O(g) + 5* (11) C.V. Ovesen, B.S. Clausen, B.S. Hammershøi, G. Steffensen, T. Askgaard, I. Chorkendorff, J.K. Nørskov, P.B. Rasmussen, P. Stoltze, P. Taylor, J. Catal. 158 (1996)

8 Example 2: Bridging pressure/material gaps for methanol synthesis CO + 2 H 2 = CH 3 OH R H = - 92 kj/mol CO H 2 = CH 3 OH + H 2 O R H = - 50 kj/mol CO + H 2 O = CO 2 + H 2 (WGSR) R H = - 42 kj/mol Cu/ZnO/Al 2 O MPa, 500 K WGSR: water gas shift reaction H O C O Cu formate H H O C O Cu dioxomethylene CH 3 O Cu methoxy WGSR Steps Surface reactions 1 H2O(g) + * 2 H2O* + * 3 2OH* 4 OH* + * 5 2H* 6 CO(g) + * 7 CO* + O* 8 CO 2* 9 CO 2* + H* 10 HCOO* + H* 11 H2COO* + H* 12 H3CO* + H* 13 CH3OH* 14 H2COO* + * 15 HCHO* 16 H COO* + H* * : free surface site X* : adsorbed molecule or atom X 2 H2O* OH* + H* H2O* + O* O* + H* H 2 + 2* CO* CO 2* + * CO 2(g) + * HCOO* + * H2COO* + * H3CO* + O* CH3OH* + * CH3OH(g) + * HCHO* + O* HCHO(g) + * HCHO* + OH* T.S. Askgaard, J.K. Nørskov, C.V. Ovesen, P. Stoltze, J. Catal. 156 (1995)

9 Mechanistic steps of methanol synthesis on copper adsorption and desorption

10 Mechanistic steps of methanol synthesis on copper adsorption and desorption H 2, CO, CO 2

11 Mechanistic steps of methanol synthesis on copper Dissociation and surface reaction formatehco 2

12 Mechanistic steps of methanol synthesis on copper surface reaction: hydrogenation dioxomethyleneh 2 CO 2

13 Mechanistic steps of methanol synthesis on copper surface reaction methoxych 3 O

14 Mechanistic steps of methanol synthesis on copper desorption methanol CH 3 OH

15 Mechanistic steps of methanol synthesis on copper surface reaction, desorption hydroxyl OH, H 2 O

16 Atomic resolution in situ Transmission Electron Microscopy In situ TEM images (A, C, and E) of a Cu/ZnO catalyst in various gas environments together with the corresponding Wulff constructions of the Cu nanocrystals (B, D, and F). (A) The image was recorded at a pressure of 1.5 mbar of H 2 at 220 o C. The electron beam is parallel to the [011] zone axis of copper. (C) Obtained in a gas mixture of H 2 and H 2 O, H 2 : H 2 O = 3:1 at a total pressure of 1.5 mbar at 220 o C. (E) Obtained in a gas mixture of H 2 (95%) and CO (5%) at at total pressure of 5 mbar at 220 o C. P.L. Hansen, J.B. Wagner, S. Helvig, J.R. Rostrup-Nielsen, B.S. Clausen, H. Topsøe, Science 295 (2002)

17 Dynamic Behavior of Cu/ZnO: Alloy Model b) c) a) d) Cu Zn Oxygen vacancies Reduced Zn Oxidized atmosphere oxidizing atmosphere a) Round-shaped particles under oxidizing syngas conditions b) Disc-like particles under more reducing conditions c) Surface Cu-Zn alloying due to stronger reducing conditions d) brass alloy formation due to severe reducing conditions Reduced atmosphere reducing atmosphere J.-D. Grunwaldt, A.M. Molenbroek, N.-Y. Topsøe, H. Topsøe, B.S. Clausen, J. Catal. 194 (2000)

18 Development of a microkinetic model Ab initio calculation Analogies Surface Science Studies TST, collision theory Tentative Surface Reaction Mechanism Reactor Model crucial kinetic parameters Transient Experiments Steady-state kinetics Comparison of Experiment and Simulation optimization of the not-constrained kinetic parameters, sensitivity analysis Microkinetic Model

19 Microkinetic Modeling Microkinetic Modeling Arrheniusform forelementarysteps k = A exp (-E act /(R T)) 4 kinetic parameters (2 preexponential factors, 2 activation energies) per reversible elementary step!

20 Kinetic Parameters based on TST J.A. Dumesic et al., The Microkinetics of Heterogeneous Catalysis, ACS Professional Ref. Book, Washington, DC 1993.

21 Kinetic Parameters based on TST

22 Heat of Adsorption

23 Routes to the Heat of Adsorption Thermal Desorption Spectroscopy yieldse a,des Isosteric Heat of Adsorption using the Clausius-Clapeyron Equation Calorimetry direct measurement of Q

24 Thermal Desorption Spectroscopy Thermal desorption spectra can be easily analysed by application of the Redhead formula. Only valid if no readsorption occurs. QMS turbomolecular pump E = RT m [ ln( νt /β) 3.64] E: Activation energy of the desorption T m : Peak maximum β: Heatingrate ν: Preexponential factor(arrhenius) m

25 Isosteric Heats of Adsorption Clausius-Clapeyron equation lnp 1 T θ = H R ads

26 Microcalorimetry: Experimental set-up The adsorption microcalorimetry set-up is based on the work of B. E. Spiewak and J. A. Dumesic. B. E. Spiewak, J. A. Dumesic, Thermochimica Acta 290 (1996) turbopump calorimeter PI M adsorptive gas hotbox He Small doses of adsorptive gas are expanded into the calorimeter. The Tian-Calvet calorimeter measures the resulting heatflow (isothermal mode, 300 K). The amount of adsorbed molecules is measured volumetrically. The complete set-up is metal-tightened and thermostated. System originally set up at LTC by Dr. Raoul Naumann d Alnoncourt

27 Microcalorimetry: Necessary experimental steps Pretreatment of the sample (up to 3 days) Sealing of the sample in a pyrex capsule filled with He Transfer of the capsule into the calorimeter Degassing of the complete set-up at 418 K (2 days) Reaching thermal equilibrium at RT (over night) Crushing the capsule and reducing the He pressure Waiting for a stable baseline (2 hours) Starting the automatic dosing sequence (50 cycles, 2 days)

28 Microcalorimetry: Measurement procedure Dosing cycle: 1. Evacuating the dosing section 2. Filling the dosing section with adsorptive gas (100 Pa = 1 µmol) 3. Opening the measuring cell 4. Heat flow measurement (60 min)

29 Microcalorimetry: Data processing Calorimetric data and pressure data are collected simultaneously during the measurement. Processing of the calorimetric data (integration of the heat flow for each single pulse) yields the evolved heat. dq/dt / mw t / s

30 Differential heat of adsorption and adsorption isotherm: : CO on Cu Adsorption of CO on a copper catalyst Q / kj/mol Coverage / µmol/g cat Coverage / µmol/g cat Pressure / Pa

31 Temperature-programmed programmed desorption Why measure TPD? Adsorption kinetics of single molecules are probed: Important for improving catalysts Faster adsorption may increase overall rate Heterogeneous surfaces: Are there multiple adsorption sites for my reactant? 1 st vs. 2 nd order desorption: Does my reactant molecule dissociate on my catalyst?

32 TPD: Experimental procedure Four general steps Catalyst pretreatment e. g. oxidation, reduction, reaction conditions Adsorption often small amount of probe molecule in inert gas Purge in inert gas at a fixed temperature Temperature-programmed desorption in inert gas flow, increase temperature (linearly) with time

33 TPD: Experimental procedure advanced Variation of: Heating rate - heating-rate-variation method, e. g. 5 K/min, 2 K/min, 1 K/min - At constant initial coverage: usually θ 0 = 1 Initial coverage - with constant heating rate - different coverages can be obtained by a variation of dosing time & temperature - non-activated adsorption (CO on Cu): initial coverage variation by partial desorption

34 TPD: Experimental set-up

35 First-order desorption kinetics First order: The TPD peaks do not shift as a function of coverage; asymmetric peak shape

36 Second-order order desorption kinetics Second order: The TPD peaks do shift as a function of coverage; more symmetric peak shape

37 Example: Hydrogen desorption from Cu(111) single crystal (TDS) Please note: Due to surface reconstruction, much more complicated desorption traces are obtained in case of Cu(100) and Cu(110). G. Anger et al. SurfaceScience 220 (1989) 1-17.

38 H 2 TPD from Cu/ZnO/Al 2 O 3 : Variation of the Heating Rate Effluent mole fraction H 2 / % K/min, T max = 306 K 10 K/min, T max = 303 K 6 K/min, T max = 297 K 2 K/min, T max = 288 K w cat = 200 mg Q He = 100 Nml/min Temperature / K Pretreatment Methanol Synthesis Flushingin He at 493 K for0.5 h Coolingin He to 240 K Dosing 100% H 2 for 0.5 h at 240 K and at p = 15 bar Coolingin H 2 to 78 K Flushingin He at 78 K second-order desorption from Cu surface sites no readsorption T. Genger, O. Hinrichsen, M. Muhler, Proc. Europacat IV, Rimini 1999, p. 175.

39 2nd Order Plot According to Polanyi-Wigner Equation Cu/ZnO/Al 2 O 3 H 2 TPD, β variation A des = s -1 E des = 78 kjmol -1 2lnT max -lnβ no readsorption, no diffusion limitation 1000 K / T max T. Genger, O. Hinrichsen, M. Muhler, Europacat IV, Rimini T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) β R A n E ln T E RT n 1 des m n 1 des = ln n Θ 2 m m des m

40 H 2 TPD from Cu/ZnO/Al 2 O 3 : Variation of the Heating Rate Effluent mole fraction H 2 / % Cu/ZnO/Al 2 O Effluent mole fraction H 2 / % Temperature / K T. Genger, O. Hinrichsen, M. Muhler, Proc. Europacat IV, Rimini T. Genger, O. Hinrichsen, M. Muhler, Catal. Lett. 59 (1999) A des = s -1 E des = 78 kjmol -1

41 Temperature-programmed programmed adsorption of H 2 on Cu/ZnO/Al 2 O 3 Effluent mole fraction H 2 / % % H 2 in He Q = 20 Nml/min 2 K/min 6 K/min 10 K/min 15 K/min T min = 270 K T min = 299 K Temperature / K T min = 283 K T min = 291 K dissociative adsorption on Cu surface sites high activation barrier Pretreatment Methanol Synthesis Flushingin H 2 and He at 493 K for0.5 h Coolingin He to 78 K H. Wilmer, T. Genger, O. Hinrichsen, J. Catal. 215 (2003)

42 Temperature-programmed programmed adsorption of H 2 on Cu/ZnO/Al 2 O 3 Effluent mole fraction H 2 / % % H 2 in He Q = 20 Nml/min Effluent mole fraction H 2 / % A ads = (Pa s) -1 E ads = 51 kjmol Temperature / K H. Wilmer, T. Genger, O. Hinrichsen, J. Catal. 215 (2003) T. Genger, O. Hinrichsen, M. Muhler, Stud. Surf. Sci. Catal.130 (2000)

43 Combining TPD and Microcalorimetry: CO on Cu/ZnO/Al 2 O 3 TPD transient method temperature dependence processing of data yields information about adsorption kinetics and thermodynamics Microcalorimetry thermodynamic equilibrium isothermal conditions processing of data yields adsorption heats and isotherms (equilibrium data) Combination by microkinetic modelling TPD under the influence of readsorption differential heats from microcalorimetry

44 Microkinetic Modeling of TPD of CO on Cu/ZnO/Al 2 O 3 CO + * CO* adsorption of a single component, readsorption occurring freely (non-activated) rate expression for the forward reaction rate expression for the reverse reaction r r ads = = = k k ads ads des k des p p θ CO CO CO θ frei ( 1 θ ) CO with: rate constants expressed by the Arrhenius-equation k i = A i E exp RT A, i with E A,ads = 0 E A,des = H ads

45 Microkinetic Modeling of TPD of CO on Cu/ZnO/Al 2 O 3 TPD-experiment: reflected adsorption enthalpy is the mean value of enthalpies of all molecules desorbing in a certain coverage range Mean values can be calculated from microcalorimetry data in the same coverage intervals Coverage interval [-] 0,000-0,054 0,000-0,102 0,000-0,167 H ads [kj mol -1 ] 62,1 58,5 55,5 Calculation of A ads from adsorption entropy (Method by Scholten & Konvalinka)

46 Microkinetic modeling of TPD of CO on Cu/ZnO/Al 2 O Effluent mole fraction of CO / % dashed lines: exp. bold lines: theor. 300 K 325 K 350 K Temperature / K Fractional coverage experimental Temkin model Pressure / Pa CO TPD spectra and adsorption isotherms can be modeled in good agreement using calorimetric data. J. Strunk et al., Phys. Chem. Chem. Phys. 8 (2006)

47 Methanol decomposition on ZnO NanoTek effluent mole fraction / % K 517 K 567 K CO 2 H 2 H 2 O CO CH 3 OH -H 2 lattice O temperature / K Dissociative adsorption of methanol as methoxy species (and OH or bulk H) on ZnO (at least 58 % of the Zn 2+ adsorption sites occupied). For pure ZnO the mass balance is closed. At 491 and 517 K two different methoxy species are decomposed to form formate and H 2. Kähler et al., ChemPhysChem. 11 (2010)

48 Methanol decomposition on ZnO NanoTek effluent mole fraction / % Variation of heating rate 540 K 525 K 517 K 515 K 499 K 491 K ln(β/t max 2 ) Arrhenius plot E A =127 kj/mole 1. decomp. peak 2. decomp. peak E A =109 kj/mole temperature / K 1/T max E A for the conversion of methoxy to formate species are 109 kj mol -1 and 127 kj mol -1. Desorption at higher temperatures is ascribed to the decomposition of formates. Kähler et al., ChemPhysChem. 11 (2010)

49 Summary Knowledge of the elementary steps from model experiments under controlled conditions can be used to understand catalytic reaction under industrially relevant conditions. - Example: Cu catalyst in methanol synthesis Cu(111) in UHV Microcalorimetry can be used to obtain heats of adsorption of reactants on industrial catalysts. TPD is a viable method to probe adsorption/desorption kinetics or the decomposition of intermediates. For the modeling, single crystal data (e.g. Cu(111)) or heats of adsorption from microcalorimetry can be used (e.g. CO on Cu/ZnO/Al 2 O 3 ) Microkinetic modeling can bridge pressure and material gaps in studies of heterogeneously catalyzed reactions.