1. 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 August 19, 2002

2. 2 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

3. 3 Model Theory  Mechanism assumed to proceed via a set of ERs involving the active sites (S), surface intermediates (I i ), and terminal species (T i ).  The generic rate expression for each reaction is given by: Ref. Fishtik & Datta

4. 4 Developing the Model Identify (q) surface intermediates: surface intermediates: H 2 OS, COS, CO 2 S, H 2 S, HS, OHS, OS, HCOOS UBI-QEP method UBI-QEP method used to generate ERs and calculate the energetic characteristics (  H, E a ) 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 transition state theory Pre-exponential factors from transition state theory   10 1 Pa -1 s -1 – adsorption/desorption reactions   10 13 s -1 – surface reactions

5. 5 Elementary Reactions s 1 : H 2 O + S  H 2 OS s 2 :CO + S  COS s 3 : CO 2 S  CO 2 + S s 4 : HS + HS  H 2 S + S s 5 : H 2 S  H 2 + S s 6 : H 2 OS +S  OHS + HS s 7 : COS + OS  CO 2 S + S s 8 : COS + OHS  HCOOS + S s 9 : OHS + S  OS + HS s 10 : COS + OHS  CO 2 S + HS s 11 : HCOOS + S  CO 2 S + HS s 12 : HCOOS + OS  CO 2 S + OHS s 13 : H 2 OS + OS  2 OHS s 14 : H 2 OS + HS  OHS + H 2 S s 15 : OHS + HS  OH + H 2 S Adsorption and Desorption Reactions

6. 6 Cu(111)Fe(111) s1s1 10 1 10 14 013.6017.2 s2s2 10 1 10 14 012.0032.0 s3s3 4 10 12 10 1 5.306.90 s4s4 10 13 15.513.024.57.6 s5s5 6 10 12 10 1 5.507.10 s6s6 10 13 25.41.619.912.0 s7s7 10 13 017.320.64.5 s8s8 10 13 020.49.012.2 s9s9 10 13 15.520.712.429.1 s 10 10 13 022.510.310.9 s 11 10 13 1.33.54.41.8 s 12 10 13 4.00.919.30 s 13 10 13 29.2024.60 s 14 10 13 26.3024.80 s 15 10 13 1.34.03.43.2 Reaction Energetics  Pre-exponential factors  Pa -1 s -1 (adsorption/ desorption steps)  s -1 (surface reaction)  Activation energies (kcal/mol)

7. 7 Simulation of Microkinetic Model for Cu(111), 13-step Ref. Fishtik & Datta, Surf. Sci. 512 (2002). Expt. Conditions Space time = 0.09 s FEED:CO inlet = 0.15 H 2 O inlet = 0.20 CO 2 inlet = 0.05 H 2 inlet = 0.05 Ref. Xue et al. Catal. Today, 30, 107 (1996).

8. 8 Simulation of Microkinetic Model for Cu(111), 15-step Expt. Conditions Space time = 1.80 s FEED:CO inlet = 0.10 H 2 O inlet = 0.10 CO 2 inlet = 0.00 H 2 inlet = 0.00

9. 9 Simulation of Microkinetic Model for Fe(111), 15-step Expt. Conditions Space time = 1.17 s FEED:CO inlet = 0.10 H 2 O inlet = 0.10 CO 2 inlet = 0.00 H 2 inlet = 0.00

10. 10 Reaction Route Analysis  A Reaction Route is the result of a linear combination of q+1 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 s 14 & s 15 contribution

11. 11 Unique Reaction Routes  RR 1 – formate reaction route  RR 2 – redox reaction route  RR 3 – associative reaction route

12. 12 RR Contributions on Cu(111) RR 2 RR 1 & RR 3 Total Mechanism Equilibrium

13. 13 RR Contributions on Fe(111) RR 1, RR 3, RR 18 & RR 19 Total Mechanism Equilibrium

14. 14 Reaction Route Combination  The ERs of each dominant RR are combined to generate a “net” RR  Simplified Model involving only 13 ERs ER s1s1s1s1 s2s2s2s2 s3s3s3s3 s4s4s4s4 s5s5s5s5 s6s6s6s6 s7s7s7s7 s8s8s8s8 s9s9s9s9 s 10 s 11 s 12 s 13 s 14 s 15 Cu Fe

15. 15 Quasi-Equilibrium Reactions  Identified by affinity calculations  s 1,s 2,s 3,s 4,s 5,s 7,s 11  All intermediates represented except OHS Reducing the Model Quasi-Steady State Species  OHS Rate Determining Steps  Copper: s 6,s 8,s 10,s 15  Iron: s 6,s 8,s 10,s 12,s 15

16. 16 12-Step, 4-Route, 4-RDS Model s 1 : H 2 O + S  H 2 OS EQ s 2 : CO + S  COS EQ s 6 : H 2 OS + S  OHS + HS RDS s 8 : COS + OHS  HCOOS + S RDS s 10 : COS + OHS  CO 2 S + HS RDS s 12 :CO 2 S + OHS  OS + HCOOSRDS s 15 : OHS + HS  OS + H 2 SRDS s 2 + s 3 + s 7 : CO + OS  CO 2 + S EQ s 3 : CO 2 S  CO 2 + SEQ 1/2(s 4 + s 5 ): HS  1/2H 2 + SEQ s 3 +1/2s 4 +1/2s 5 + s 11 :HCOOS  CO 2 + 1/2H 2 + S EQ

17. 17 Rate Expressions RR 1 RR 3 RR 19 RR 18

18. 18 WGSR Mechanism r6r6 r8r8 r 10 r 12 r 15 r A6A6 A 8 = A 9 = A 10 = A 12 = A 15

19. 19 Overall Rate Expression  IRRs and ERs combine to indicate the dominant rates of each RR  Cu(111): r 12 neglected  Fe(111): r 12 included  Overall Rate Expression r = r 8 + r 9 + r 10 + r 12 + r 15

20. 20 Simplified Model

21. 21 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 (RR 1,RR 3,RR 18,RR 19 ).  Prediction of simplified models compare extremely well with the complete microkinetic model.  The addition of s 14 and s 15 dramatically affected the model for WGS on copper; the model for iron remained unaffected. RR 18 requires further investigation.

22. Questions…