Microkinetic Modeling of the Water Gas Shift Reaction on Copper and Iron Catalysts

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Microkinetic 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
• November 8, 2002

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Research 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

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Model 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
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Developing 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

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Elementary 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 s16 HCOOS OHS
? CO2S H2OS s17 HCOOS HS ? CO2S H2S
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Reaction 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
s16
s17

• Pre-exponential factors
• Pa-1s-1
• (adsorption/ desorption steps)
• s-1
• (surface reaction)
• Activation energies (kcal/mol)

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Simulation 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).
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Simulation 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
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Simulation 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
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Reaction Route Analysis

• A Reaction Route is the result of a linear
  combination of q1 ERs that produces the desired
  overall reaction.
• 450 Possible Reaction Routes were found including
• Empty Roots
• The net reaction is zero.
• Non-Empty Roots
• The net reaction is the WGSR.
• 70 Unique Reaction Routes remain
• 17 Routes previously examined (Fishtik Datta,
  Surf. Sci. 512 (2002).)
• 53 New Roots based on s14,s15,s16 s17
  contribution

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Unique Reaction Routes
formate reaction route RR1 s1 s2 s3 s4
s5 s6 s8 s11 redox reaction route RR2 s1
s2 s3 s4 s5 s6 s7 s9 associative
reaction route RR3 s1 s2 s3 s4 s5 s6
s10 modified redox reaction route RR18 s1
s2 s3 s5 s6 s7 s15
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Energy Diagram Analysis
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RR Contributions on Cu(111)
Equilibrium
RR1 RR3
RR2
Total Mechanism
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RR Contributions on Fe(111)
Equilibrium
RR1, RR3, RR18 RR19
Total Mechanism
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Reaction 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 s16 s17
Cu ? ? ? ? ? ? ? ? ? ? ?
Fe ? ? ? ? ? ? ? ? ? ? ? ?
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Reducing the Model

• Quasi-Steady State Species
• OHS

• Rate Determining Steps
• Copper s6,s8,s10,s15
• Iron s6,s8,s10,s12,s15

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12-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
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Rate Expressions
RR1
RR3
RR19
RR18
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WGSR Mechanism
r6
A6
r8
r10
r12
r15
A8 A9 A10 A12 A15
r
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Overall 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

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Simplified Model
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Conclusions

• 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.

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Questions