Catalytic Reaction Kinetics Why catalytic reaction kinetics Derivation rate expressions Simplifications Rate determining step Initial reaction rate Limiting cases Temperature dependency Pressure dependency Examples Catalysis and Catalysts - Kinetics

Reactor design equation conversion i stoichiometric coefficient i rate expression ‘space time’ catalyst effectiveness Catalysis and Catalysts - Kinetics

Simple example: reversible reaction A B ‘Elementary processes’ A + * k 1 - B 2 3 1. 2. 3. A B 1 3 2 A* B* ‘Langmuir adsorption’ Catalysis and Catalysts - Kinetics

Elementary processes Eliminate unknown surface occupancies Rate expression follows from rate equation: At steady state: Eliminate unknown surface occupancies Catalysis and Catalysts - Kinetics

Elementary processes contd. Site balance: (7.5) Steady-state assumption: (7.6-7) Rate expression: (7.9) Catalysis and Catalysts - Kinetics

Quasi-equilibrium / rate-determining step r = r+2 - r-2 Catalysis and Catalysts - Kinetics

Rate expression r.d.s. Rate determining step: Eliminate unknown occupancies Quasi-equilibrium: So: Catalysis and Catalysts - Kinetics

Rate expression, contd. Substitution: where: Unknown still q* Catalysis and Catalysts - Kinetics

Rate expression, contd. Site balance: Finally: Catalysis and Catalysts - Kinetics

Other rate-determining steps Adsorption r.d.s Surface reaction r.d.s. Desorption r.d.s. Catalysis and Catalysts - Kinetics

Langmuir adsorption Uniform surface (no heterogeneity) Constant number of identical sites Only one molecule per site No interaction between adsorbed species A + * A* 1 100 0.8 KA (bar-1) 10 0.6 0.4 1 0.2 0.1 0.2 0.4 0.6 0.8 1 pA (bar) Catalysis and Catalysts - Kinetics

Thermodynamics Equilibrium constant Adsorption constant Reaction entropy Reaction enthalpy Adsorption constant Adsorption entropy, <0 (J/mol K) Adsorption enthalpy,<0 (J/mol) atm-1 Catalysis and Catalysts - Kinetics

Multicomponent adsorption / inhibition Langmuir adsorption Inhibitors Catalysis and Catalysts - Kinetics

Dissociative adsorption H2 + 2* 2H* Two adjacent sites needed Catalysis and Catalysts - Kinetics

Langmuir-Hinshelwood/Hougen-Watson models (LHHW) includes NT, k(rds) For: A+B C+D pApB-pCpD/Keq molecular: KApA dissociative: (KApA)0.5 = 0, 1, 2 number of species in r.d.s. Catalysis and Catalysts - Kinetics

Verwerking p. 11 t/m 13 Catalysis and Catalysts - Kinetics

Initial rate expressions Forward rates Product terms negligible Adsorption Surface reaction Desorption (K2 and KApA0 >>1) T1 T1 T1 T2 r0 T2 T2 T3 T3 T3 pA0 Catalysis and Catalysts - Kinetics

Ethanol dehydrogenation Franckaerts &Froment Cu-Co cat. C2H5OH « CH3CHO + H2 Model: 1. A + * « A* 2. A* + * « R* + S* (r.d.s.) 3. R* « R + * 4. S* « S + * = Derive rate expression = Catalysis and Catalysts - Kinetics

Initial rates - linear transformation Ethanol dehydrogenation Full expression Initial rate After rearrangement linear form: linear least squares fit trends, positive parameters Catalysis and Catalysts - Kinetics

Initial rates - CO hydrogenation over Rh Van Santen et al. Kinetic model 1. CO + * « CO* 2. CO* + * ® C* + O* (r.d.s.) 0.2 0.4 0.6 0.8 1.0 Occupancy (-) 400 450 500 550 600 Temperature (K) 200 800 Rate Initial rate Catalysis and Catalysts - Kinetics

Temperature and Pressure Dependence Verwerking p. 18 t/m21 *A *A# *B k- # k+ kbarrier Catalysis and Catalysts - Kinetics

Limiting cases - forward rates Surface reaction r.d.s. 1. Strong adsorption A A* # Ea2 A* B* Catalysis and Catalysts - Kinetics

Limiting cases - forward rates Surface reaction r.d.s. 2. Weak adsorption A* # A(g) + * Ea2 HA A* Catalysis and Catalysts - Kinetics

Limiting cases - forward rates Surface reaction r.d.s. 3. Strong adsorption B A* # Ea2 B + *+ A HA - HB A* B* + A Catalysis and Catalysts - Kinetics

Cracking of n-alkanes over ZSM-5 J. Wei I&EC Res.33(1994)2467 Ea2 Eaobs DHA Carbon number kJ/mol 200 100 -200 -100 Catalysis and Catalysts - Kinetics

Observed temperature behaviour T higher coverage lower Highest Ea most favoured Change in r.d.s. 1/T ln robs desorption r.d.s. adsorption r.d.s. Catalysis and Catalysts - Kinetics

‘Kinetic Coupling’ two kinetically significant steps Pt-catalysed dehydrogenation of methylcyclohexane: M  T + H2 Two kinetic significant steps: * + M  .... T*  T + * mari no inhibition by e.g. benzene T* much higher than equilibrium with gas phase T Catalysis and Catalysts - Kinetics

Sabatier principle - Volcano plot Rate Heat of adsorption Catalysis and Catalysts - Kinetics

Summary mechanism kinetics Langmuir adsorption uniform sites no interaction adsorbed species constant number of sites Rate expression series of elementary steps steady state assumption site balance quasi-equilibrium / rate determining step(s) initial rates simpler mechanism kinetics Catalysis and Catalysts - Kinetics

Catalysed N2O decomposition over oxides Winter, Cimino Rate expressions: 1st order strong O2 inhibition moderate inhibition Also: orders 0.5-1 water inhibition = Explain / derive = Catalysis and Catalysts - Kinetics

N2O decomposition over Mn2O3 Vannice et al. 1995 2 N2O 2N2 + O2 Kinetic model 1. N2O + * « N2O* 2. N2O* ® N2 + O* 3. 2 O* « 2* + O2 Rate expression Catalysis and Catalysts - Kinetics

N2O decomposition over Mn2O3 Vannice et al. 1995 order N2O ~0.78 Eaobs= 96 kJ/mol Oxygen inhibition 0.0 2.0 4.0 6.0 8.0 10.0 p O2 / kPa 0.1 0.2 0.3 0.4 r / 10 -6 mol.s -1 .g 648 K 638 K 623 K 608 K 598 K pN2O = 10 kPa = Explain = Catalysis and Catalysts - Kinetics

N2O decomposition over Mn2O3 Vannice et al. 1995 Kinetic model 1. N2O + * « N2O* 2. N2O* ® N2 + O* 3. 2 O* « 2* + O2 Values Rate expression = Thermodynamically consistent = Catalysis and Catalysts - Kinetics

N2O decomposition over ZSM-5 (Co,Cu,Fe) Kapteijn et al. 11th ICC,1996 2 N2O 2N2 + O2 Kinetic model 1. N2O + * ® N2 + O* 2. N2O + O*® N2 + O2 + * Rate expression no oxygen inhibition Catalysis and Catalysts - Kinetics

N2O decomposition over ZSM-5 (Co,Cu,Fe) Kapteijn et al. 11th ICC,1996 2 4 6 8 10 p(O ) / kPa 0.0 0.2 0.4 0.6 0.8 1.0 X(N O) Fe-ZSM-5 Co-ZSM-5 Cu-ZSM-5 743 K 833 K 793 K 733 K 773 K 688K Oxygen inhibition model 1. N2O + * ® N2 + O* 2. N2O + O*® N2 + O2 + * 3. O2 + * « *O2 Rate expression Catalysis and Catalysts - Kinetics

Effect of CO on N2O decomposition CO + O*® CO2 + * CO + * « CO* (Cu+) CO removes oxygen from surface so ‘enhances’ step 2, oxygen removal now observed: rate of step 1 r1 = k1 NT pN2O increase: ~2, >3, >100 Catalysis and Catalysts - Kinetics

Effect of CO on N2O decomposition rate without CO rate with CO ratio = 1 + k1/k2 and: So k1/k2 = : 1 Co >2 Cu >100 Fe 0.7 >0.9 >0.99 Catalysis and Catalysts - Kinetics

Apparent activation energies N2O decomposition CO/ N2O = 2 Fe Cu Catalysis and Catalysts - Kinetics

Apparent activation energies N2O decomposition CO/ N2O = 0 Cu Fe Catalysis and Catalysts - Kinetics

Complex kinetics HDN of Quinone over NiMo/Al2O3 (Prins & Jian, Zurich) Kinetic scheme Purpose: Kinetics of reaction Effects functions Ni and Mo Addition role of P Catalysis and Catalysts - Kinetics

Complex kinetics Subscheme research: HDN of OPA Not observed intermediate, not significant Catalysis and Catalysts - Kinetics

Complex kinetics HDN of OPA Derived global scheme: How can this ‘direct’ step be rationalised? Catalysis and Catalysts - Kinetics

Complex kinetics HDN of OPA (Jiang & Prins) Reaction modelling space time (cs) 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 1 2 3 4 5 OPA NiMo one site model 370C Partial pressure (kPa) OPA PB PCHE PCH strong adsorption N-containg species plug flow reactor excellent fit Catalysis and Catalysts - Kinetics

Complex kinetics HDN of OPA Competitive parallel steps Direct global routes OPA + * OPA* PB + * PCHA* PCHA + * PCHE* PCHE + * PCH + * HCs not adsorbed (weakly compared to N-s) kb ka slow Fast reaction steps Only traces found kc kd ke The direct route to PCH Other hydrogenation functional sites ? Catalysis and Catalysts - Kinetics

Rate expressions Steady state assumption Site balance (one site) Strong adsorption N-species parallel reactions Q: explain zero order OPA direct route from PCHE Catalysis and Catalysts - Kinetics

Catalysis and Catalysts - Kinetics

‘Kinetic coupling’ two steps kinetically significant Decomposition of ammonia over Mo (low p, high T) 2NH3 -> N2 + 3H2 Steps: 2NH3 + * -> 2N* + 3H2 2N* -> N2 surface concentration N much higher than equilibrium with N2 pressure ‘fugacity of N* corresponds with virtual fugacity N2 Catalysis and Catalysts - Kinetics

Virtual fugacity, kinetic coupling Aromatization light alkanes over zeolite Alkanes -> Aromatics + Hydrogen Cracking yields high H*, so high fugacity H* H* not in equilibrium with H2 -> low aromatics selectivity Addition of Ga provides escape route for H* zeolite: alkane -> 2H* + ..... Ga: 2H* -> H2 Kinetic coupling used to increase reaction selectivity for aromatics Catalysis and Catalysts - Kinetics

Kinetic coupling between catalytic cycles effect on selectivity Hydrogenation: butyne -> butene -> butane A1 A2 A3 butyne and butene compete for the same sites but: K1 >> K2 resulting high selectivity for butene (desired) possible even when k2 > k1 since: Meyer and Burwell (JACS 85(1963)2877) mol%: 2-butyne 22.0 cis-2-butene 77.2 trans-2-butene 0.7 1-butene 0.0 butane 0.1 Catalysis and Catalysts - Kinetics

Kinetic coupling between catalytic cycles effect on selectivity Bifunctional catalysis: Reforming Isomerization n-pentane: n-C5 -> i-C5 low concentration close proximity Pt-function: n-C5 -> n-C5= surface diffusion Acid function: n-C5= -> i-C5= Pt-function: i-C5= -> i-C5 Catalytic cycles on different catalysts Affect selectivity: modify surface (change adsorption properties) modify fluid phase (change adsorption properties) benzene hydrogenation M. Soede Catalysis and Catalysts - Kinetics

Competitive adsorption Selective hydrogenation aromatics S.Toppinen,Thesis 1996 Ni-alumina trilobe catalyst 3 mm particles 40 bar H2 125oC semi-batch reactor Consecutive conversion behaviour rate constants ~ similar adsorption constants decrease Propose a rate expression to account for this effect Catalysis and Catalysts - Kinetics

Partial benzene hydrogenation Ru-catalyst - clusters of crystallites Slurry reaction, elevated pressures Water-salt addition increases selectivity + 2 H + H 2 2 Ru Salt-water Adsorption / Desorption properties affected Catalysis and Catalysts - Kinetics

Dual site models: A + B C (r.d.s.) Catalysis and Catalysts - Kinetics

Surface occupancies Empty sites: Occupied by A: Occupied by B: Catalysis and Catalysts - Kinetics

Dual site models, contd. Number of neighbouring sites (here: 6) Catalysis and Catalysts - Kinetics