Title: Microkinetic Modeling of the Water Gas Shift Reaction on Copper and Iron Catalysts
1Microkinetic Modeling of the Water Gas Shift
Reaction on Copper and Iron Catalysts
- Caitlin Callaghan, Ilie Fishtik Ravindra Datta
- Fuel Cell Center
- Chemical Engineering Department
- Worcester Polytechnic Institute
- Worcester, MA
- August 19, 2002
2Research Objectives
- Develop a predictive microkinetic model for LTS
and HTS water gas shift catalysts - Identify the rate determining steps
- Develop reduced model
- Simulate the reaction for different catalysts
(e.g. Cu, Fe, etc.) - Eventual goal is a priori design of catalysts for
the water-gas-shift-reaction in fuel reformers
for fuel cells
3Model Theory
- Mechanism assumed to proceed via a set of ERs
involving the active sites (S), surface
intermediates (Ii), and terminal species (Ti). - The generic rate expression for each reaction is
given by
Ref. Fishtik Datta
4Developing the Model
Identify (q) surface intermediates H2OS, COS,
CO2S, H2S, HS, OHS, OS, HCOOS
- UBI-QEP method used to generate ERs and calculate
the energetic characteristics (??H, Ea) of each
ER based on three types of reactions - 1. AB(g) S ? ABS
- 2. AB(g) S ? AS BS
- 3. AS BCS ? ABS CS
- Pre-exponential factors from transition state
theory - 101 Pa-1s-1 adsorption/desorption reactions
- 1013 s-1 surface reactions
5Elementary Reactions
s1 H2O S ? H2OS s2 CO S ? COS s3
CO2S ? CO2 S s4 HS HS ? H2S S s5
H2S ? H2 S s6 H2OS S ? OHS HS s7 COS
OS ? CO2S S s8 COS OHS ? HCOOS S s9
OHS S ? OS HS s10 COS OHS ? CO2S
HS s11 HCOOS S ? CO2S HS s12 HCOOS
OS ? CO2S OHS s13 H2OS OS ? 2 OHS s14
H2OS HS ? OHS H2S s15 OHS HS ? OH
H2S
Adsorption and Desorption Reactions
6Reaction Energetics
Cu(111) Cu(111) Fe(111) Fe(111)
s1 101 1014 0 13.6 0 17.2
s2 101 1014 0 12.0 0 32.0
s3 4 1012 101 5.3 0 6.9 0
s4 1013 1013 15.5 13.0 24.5 7.6
s5 6 1012 101 5.5 0 7.1 0
s6 1013 1013 25.4 1.6 19.9 12.0
s7 1013 1013 0 17.3 20.6 4.5
s8 1013 1013 0 20.4 9.0 12.2
s9 1013 1013 15.5 20.7 12.4 29.1
s10 1013 1013 0 22.5 10.3 10.9
s11 1013 1013 1.3 3.5 4.4 1.8
s12 1013 1013 4.0 0.9 19.3 0
s13 1013 1013 29.2 0 24.6 0
s14 1013 1013 26.3 0 24.8 0
s15 1013 1013 1.3 4.0 3.4 3.2
- Pre-exponential factors
- Pa-1s-1
- (adsorption/ desorption steps)
- s-1
- (surface reaction)
- Activation energies (kcal/mol)
7Simulation of Microkinetic Model for Cu(111),
13-step
Ref. Fishtik Datta, Surf. Sci. 512 (2002).
Expt. Conditions Space time 0.09
s FEED COinlet 0.15 H2Oinlet 0.20 CO2
inlet 0.05 H2 inlet 0.05
Ref. Xue et al. Catal. Today, 30, 107 (1996).
8Simulation of Microkinetic Model for Cu(111),
15-step
Expt. Conditions Space time 1.80
s FEED COinlet 0.10 H2Oinlet 0.10 CO2
inlet 0.00 H2 inlet 0.00
9Simulation of Microkinetic Model for Fe(111),
15-step
Expt. Conditions Space time 1.17
s FEED COinlet 0.10 H2Oinlet 0.10 CO2
inlet 0.00 H2 inlet 0.00
10Reaction Route Analysis
- A Reaction Route is the result of a linear
combination of q1 ERs that produces the desired
overall reaction. - 210 Possible Reaction Routes were found including
- Empty Roots
- The net reaction is zero.
- Non-Empty Roots
- The net reaction is the WGSR.
- 31 Unique Reaction Routes remain
- 17 Routes previously examined (Fishtik Datta,
Surf. Sci. 512 (2002).) - 14 New Roots based on s14 s15 contribution
11Unique Reaction Routes
- RR1 formate reaction route
- RR2 redox reaction route
- RR3 associative reaction route
12RR Contributions on Cu(111)
Equilibrium
RR1 RR3
RR2
Total Mechanism
13RR Contributions on Fe(111)
Equilibrium
RR1, RR3, RR18 RR19
Total Mechanism
14Reaction Route Combination
- The ERs of each dominant RR are combined to
generate a net RR - Simplified Model involving only 13 ERs
ER s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12 s13 s14 s15
Cu ? ? ? ? ? ? ? ? ? ? ?
Fe ? ? ? ? ? ? ? ? ? ? ? ?
15Reducing the Model
- Quasi-Equilibrium Reactions
- Identified by affinity calculations
- s1,s2,s3,s4,s5,s7,s11
- All intermediates represented except OHS
- Quasi-Steady State Species
- OHS
- Rate Determining Steps
- Copper s6,s8,s10,s15
- Iron s6,s8,s10,s12,s15
1612-Step, 4-Route, 4-RDS Model
s1 H2O S ? H2OS EQ s2 CO S ? COS
EQ s6 H2OS S ? OHS HS RDS s8 COS
OHS ? HCOOS S RDS s10 COS OHS ? CO2S HS
RDS s12 CO2S OHS ? OS HCOOS RDS s15 OHS
HS ? OS H2S RDS s2 s3 s7 CO OS ?
CO2 S EQ s3 CO2S ? CO2 S EQ 1/2(s4
s5) HS ? 1/2H2 S EQ s31/2s41/2s5
s11 HCOOS ? CO2 1/2H2 S EQ
17Rate Expressions
RR1
RR3
RR19
RR18
18WGSR Mechanism
r6
A6
r8
r10
r12
r15
A8 A9 A10 A12 A15
r
19Overall Rate Expression
- IRRs and ERs combine to indicate the dominant
rates of each RR - Cu(111) r12 neglected
- Fe(111) r12 included
- Overall Rate Expression
- r r8 r9 r10 r12 r15
20Simplified Model
21Conclusions
- A reliable predictive microkinetic model for the
WGS reaction on Cu(111) and Fe(111) is developed. - Only a limited number of RRs dominate the
kinetics of the process (RR1,RR3,RR18,RR19). - Prediction of simplified models compare extremely
well with the complete microkinetic model. - The addition of s14 and s15 dramatically affected
the model for WGS on copper the model for iron
remained unaffected. RR18 requires further
investigation.
22Questions