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

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 November 8, 2002

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. 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 expressionfor each reaction is given by: Ref. Fishtik & Datta

4. Developing the Model Identify (q) surface intermediates: H2OS, COS, CO2S, H2S, HS, OHS, OS, HCOOS Pre-exponential factors fromtransition state theory • 101 Pa-1s-1 – adsorption/desorption reactions • 1013 s-1 – surface reactions UBI-QEP methodused 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

5. Adsorption and Desorption Reactions 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

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

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: COinlet = 0.15 H2Oinlet = 0.20 CO2 inlet = 0.05 H2 inlet = 0.05 Ref. Xue et al. Catal. Today, 30, 107 (1996).

8. 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

9. 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

10. Reaction Route Analysis • A Reaction Route is the result of a linear combination of q+1 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

11. 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

12. Energy Diagram Analysis

13. RR Contributions on Cu(111) Equilibrium RR1 & RR3 RR2 Total Mechanism

14. RR Contributions on Fe(111) Equilibrium RR1, RR3, RR18 & RR19 Total Mechanism

15. Reaction Route Combination • The ERs of each dominant RR are combined to generate a “net” RR • Simplified Model involving only 13 ERs

16. Quasi-Equilibrium Reactions • Identified by affinity calculations • s1,s2,s3,s4,s5,s7,s11 • All intermediates represented except OHS Reducing the Model Rate Determining Steps • Copper: s6,s8,s10,s15 • Iron: s6,s8,s10,s12,s15 Quasi-Steady State Species • OHS

17. 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 s3+1/2s4+1/2s5 + s11: HCOOS  CO2 + 1/2H2 + S EQ

18. Rate Expressions RR1 RR3 RR19 RR18

19. WGSR Mechanism r6 A6 r8 r10 r12 r15 A8 = A9 = A10 = A12 = A15 r

20. 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

21. Simplified Model

22. 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.

23. Questions…