1. 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

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

3. 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

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

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

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

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

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

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

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

11. 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

12. 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

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

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

15. 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

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

17. 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

18. 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

19. 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

20. 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

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

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

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

24. 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

25. 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

26. 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

27. ‘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

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

29. 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

30. 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

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

32. 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

33. 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

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

35. 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

36. 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 r1 = k1 NT pN2O
increase: ~2, >3, >100
Catalysis and Catalysts - Kinetics

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

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

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

40. 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

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

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

43. 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

44. 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

45. 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

46. Catalysis and Catalysts - Kinetics

47. ‘Kinetic coupling’ two steps kinetically significant
Decomposition of ammonia over Mo (low p, high T)
2NH3 -> N H2
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

48. 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

49. Kinetic coupling between catalytic cycles effect on selectivity
Hydrogenation: butyne -> butene -> butane
A1 A 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
butane
Catalysis and Catalysts - Kinetics

50. 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

51. 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

52. 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

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

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

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