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

Caitlin Callaghan, Ilie Fishtik & Ravindra Datta

Fuel Cell CenterChemical Engineering DepartmentWorcester Polytechnic InstituteWorcester, 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 expression for each reaction is given by:

n

iiji

q

kkjkjojs

11

0TIS

Ref. Fishtik & Datta

jijkjo

jijkjo

i

n

ik

q

kj

i

n

ik

q

kjj

PRT

EexpA

PRT

EexpAr

���

��

110

110

4

Developing the Model

Identify (Identify (qq))surface intermediates: surface intermediates:

H2OS, COS, CO2S, H2S, HS, OHS, OS, HCOOS

UBI-QEP methodUBI-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 ABS2. AB(g) +S AS + BS3. AS + BCS ABS + CS

Pre-exponential factors fromPre-exponential factors from transition state theorytransition state theory 101 Pa-1s-1 – adsorption/desorption

reactions 1013 s-1 – surface reactions

5

Elementary Reactions

ss11 : : HH22O + S O + S H H22OS OS

ss22 : : CO + S CO + S COS COS

ss33 : : COCO22S S COCO22 + S + S

ss44 : : HS + HS HS + HS HH22S + S S + S

ss55 : : HH22S S HH22 + S + S

ss66 : : HH22OS +S OS +S OHS + HSOHS + HS

ss77 : : COS + OSCOS + OS COCO22S + S S + S

ss88 : : COS + OHSCOS + OHS HCOOS + SHCOOS + S

ss99 : : OHS + S OHS + S OS + HS OS + HS

ss1010 : : COS + OHS COS + OHS COCO22S + HSS + HS

ss1111 : : HCOOS + S HCOOS + S COCO22S + HS S + HS

ss1212 : : HCOOS + OS HCOOS + OS COCO22S + OHSS + OHS

ss1313 : : HH22OS + OS OS + OS 2 OHS2 OHS

ss1414 : : HH22OS + HS OS + HS OHS + HOHS + H22SS

ss1515 : : OHS + HS OHS + HS OH + HOH + H22SS

ss16 16 :: HCOOS + OHS HCOOS + OHS CO CO22S + HS + H22OSOS

ss1717:: HCOOS + HS HCOOS + HS CO CO22S + HS + H22SS

Adsorption and DesorptionReactions

6

Cu(111) Fe(111)

s1101 1014 0 13.6 0 17.2

s2101 1014 0 12.0 0 32.0

s34 1012 101 5.3 0 6.9 0

s41013 1013 15.5 13.0 24.5 7.6

s56 1012 101 5.5 0 7.1 0

s61013 1013 25.4 1.6 19.9 12.0

s71013 1013 0 17.3 20.6 4.5

s81013 1013 0 20.4 9.0 12.2

s91013 1013 15.5 20.7 12.4 29.1

s101013 1013 0 22.5 10.3 10.9

s111013 1013 1.3 3.5 4.4 1.8

s121013 1013 4.0 0.9 19.3 0

s131013 1013 29.2 0 24.6 0

s141013 1013 26.3 0 24.8 0

s151013 1013 1.3 4.0 3.4 3.2

s16

s17

Reaction Energetics

ERs jA

jA�

jE

jE�

jE

jE� Pre-exponential

factors Pa-1s-1

(adsorption/ desorption steps)

s-1 (surface reaction)

Activation energies (kcal/mol)

7

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600Temperature (oC)

Co

nve

rsio

n o

f C

O

Xue

Cu13model

Equilibrium

Simulation of Microkinetic Model for Cu(111), 13-stepRef. 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

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Temperature (oC)

Co

nv

ers

ion

of

CO

Experiment

Equilibrium

Simplified Model

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

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600

Temperature (oC)

Co

nve

rsio

n o

f C

O

Experiment

Fe15model

Equilibrium

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 RoutesRR1 = s1 + s2 + s3 + s4 + s5 + s6 + s8 + s11 RR2 = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s9 RR3 = s1 + s2 + s3 + s4 + s5 + s6 + s10 RR4 = s1 + s2 + s3 + s4 + s5 + 2s6 + s7 - s13 RR5 = s1 + s2 + s3 + s4 + s5 + s10 + s11 - s12 + s13 RR6 = s1 + s2 + s3 + s4 + s5 + s9 + s10 + s13 RR7 = s1 + s2 + s3 + s4 + s5 + s8 + 2s11 - s12 + s13 RR8 = s1 + s2 + s3 + s4 + s5 – s8 + 2s10 - s12 + s13 RR9 = s1 + s2 + s3 + s4 + s5 + s8 + 2s9 + s12 + s13 RR10 = s1 + s2 + s3 + s4 + s5 + s8 + s9 + s11 + s13 RR11 = s1 + s2 + s3 + s4 + s5 + s7 + 2s11 - 2s12 + s13 RR12 = s1 + s2 + s3 + s4 + s5 + s7 + 2s9 + s13 RR13 = s1 + s2 + s3 + s4 + s5 – s7 + 2s10 + s13 RR14 = s1 + s2 + s3 + s4 + s5 – s7 + 2s8 + 2s11 + s13 RR15 = s1 + s2 + s3 + s4 + s5 + 2s6 + s8 + s12 - s13 RR16 = s1 + s2 + s3 + s4 + s5 + s6 + s8 + s9 + s12 RR17 = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s11 - s12 RR18 = s1 + s2 + s3 + s5 + s6 + s7 + s15 RR19 = s1 + s2 + s3 + s5 + s6 + s8 + s12 + s15 RR20 = s1 + s2 + s3 + s5 + s7 + s9 + s14

RR21 = s1 + s2 + s3 + s5 + s10 + s14

RR22 = s1 + s2 + s3 + s5 + s8 + s11 + s14

RR23 = s1 + s2 + s3 - s4 + s5 + s7 - s13 + 2s14

RR24 = s1 + s2 + s3 - s4 + s5 + s7 + s13 + 2s15

RR25 = s1 + s2 + s3 - s4 + s5 + s7 + s14 + s15

RR26 = s1 + s2 + s3 + s5 + s7 + s11 - s12 + s14

RR27 = s1 + s2 + s3 + s5 + s8 + s9 + s12 + s14

RR28 = s1 + s2 + s3 + s5 + s10 + s13 + s15

RR29 = s1 + s2 + s3 + s5 + s8 + s11 + s13 + s15

RR30 = s1 + s2 + s3 - s4 + s5 + s8 + s12 - s13 + 2s14

RR31 = s1 + s2 + s3 - s4 + s5 + s8 + s12 + s14 + s15

RR32 = s1 + s2 + s3 - s4 + s5 + s8 + s12 + s13 + 2s15 RR33 = s1 + s2 + s3 + s4 + s5 + 2s6 + s7 - s12 + s16 RR34 = s1 + s2 + s3 + s4 + s5 + 2s6 + s8 + s16

RR35 = s1 + s2 + s3 - s4 + s5 + 2s7 - s8 + 2s15 - s16

RR36 = s1 + s2 + s3 + s4 + s5 + 2s7 - s8 + 2s9 - s16

RR37 = s1 + s2 + s3 + s4 + s5 + s10 + s11 - s16

RR38 = s1 + s2 + s3 + s4 + s5 - s7 + 2s10 + s12 - s16

RR39 = s1 + s2 + s3 + s4 + s5 + s7 + 2s11 – s12 - s16

RR40 = s1 + s2 + s3 + s4 + s5 + s7 + 2s11 – s13 - 2s16

RR41 = s1 + s2 + s3 - s4 + s5 + s7 - 2s12 + s13 + s17

RR42 = s1 + s2 + s3 - s4 + s5 - s7 + 2s8 + s13 + 2s17

RR43 = s1 + s2 + s3 + s4 + s5 + s7 + 2s9 + s12 - s16

RR44 = s1 + s2 + s3 - s4 + s5 + s7 - s12 + 2s14 + s16

RR45 = s1 + s2 + s3 - s4 + s5 + s7 + s12 + 2s15 - s16

RR46 = s1 + s2 + s3 - s4 + s5 + s7 – s12 + s14 + s17

RR47 = s1 + s2 + s3 - s4 + s5 + s7 – s12 – s16 + 2s17

RR48 = s1 + s2 + s3 - s4 + s5 + s7 – s13 - 2s16 + 2s17

RR49 = s1 + s2 + s3 - s4 + s5 + s7 + s15 - s16 + s17

RR50 = s1 + s2 + s3 + s4 + s5 + s7 + s9 + s11 - s16

RR51 = s1 + s2 + s3 + s4 + s5 - s8 + 2s10 - s16

RR52 = s1 + s2 + s3 + s4 + s5 + s8 + 2s11 - s16

RR53 = s1 + s2 + s3 - s4 + s5 + s8 + 2s12 + 2s15 - s16

RR54 = s1 + s2 + s3 - s4 + s5 + s8 + 2s13 + 2s15 + s16

RR55 = s1 + s2 + s3 - s4 + s5 + s8 + 2s14 + s16

RR56 = s1 + s2 + s3 + s4 + s5 + s8 + 2s9 + s12 - s16

RR57 = s1 + s2 + s3 + s4 + s5 + s8 + 2s9 + 2s13 + s16

RR58 = s1 + s2 + s3 - s4 + s5 + s8 - s12 + s13 + 2s17

RR59 = s1 + s2 + s3 - s4 + s5 + s8 + s13 + s15 + s17

RR60 = s1 + s2 + s3 - s4 + s5 + s8 + s14 + s17

RR61 = s1 + s2 + s3 - s4 + s5 + s8 – s16 + 2s17

RR62 = s1 + s2 + s3 + s4 + s5 + s9 + s10 + s12 - s16

RR63 = s1 + s2 + s3 + s5 + s10 - s12 + s13 + s17

RR64 = s1 + s2 + s3 + s5 + s10 + s12 + s15 + s16

RR65 = s1 + s2 + s3 + s5 + s10 - s16 + s17

RR66 = s1 + s2 + s3 + s5 + s6 + s7 - s12 + s17

RR67 = s1 + s2 + s3 + s5 + s6 + s8 + s17

RR68 = s1 + s2 + s3 + s5 + s7 + s11 + s15 - s16

RR69 = s1 + s2 + s3 + s5 + s7 + s9 - s16 + s17

RR70 = s1 + s2 + s3 + s5 + s8 + s9 + s13 + s17

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 AnalysisP

oten

t ial

Ene

rgsy

(ca

l/m

ol)

0

1 0

2 0

3 0

4 0

5 0

-1 0

-2 0

-3 0

-4 0

-5 0

R ea c tio n C o o rd in ate

H 2 O , C O

H 2 , C O 2

13

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600Temperature (oC)

Co

nve

rsio

n o

f C

O

RR Contributions on Cu(111)

RR2

RR1 & RR3

TotalMechanism

Equilibrium

14

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600Temperature (oC)

Co

nv

ers

ion

of

CO

RR Contributions on Fe(111)

RR1, RR3,RR18 & RR19

TotalMechanism

Equilibrium

15

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

16

Quasi-Equilibrium Reactions Identified by affinity calculations s1,s2,s3,s4,s5,s7,s11

All intermediates represented except OHS

n

iiji

q

kkjkojojj PKA

RT 11

lnlnlnln1

Reducing the Model

Quasi-Steady State Species OHS

Rate Determining Steps Copper: s6,s8,s10,s15

Iron: s6,s8,s10,s12,s15

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

COOH

HCO

CO1

312CO21082/154

2/1H1566

20COOH2186

8

2

22

2

2

2 1PKP

PP

PKkPKkkKK

PkKk

θPPKKkkr

COOH

HCO

CO1

312CO21082/154

2/1H1566

20COOH21106

10

2

22

2

2

2 1PKP

PP

PKkPKkkKK

PkKk

θPPKKkkr

COOH

HCO

CO1

312CO21082/154

2/1H1566

20COOH

131126

12

2

22

2

2

22 1PKP

PP

PKkPKkkKK

PkKk

θPPKKkkr

COOH

HCO

CO1

312CO21082/154

2/1H1566

20

2/1HOH

2/1541156

15

2

22

2

2

22 1PKP

PP

PKkPKkkKK

PkKk

θPPKKKkkr

RR1

RR3

RR19

RR18

19

WGSR Mechanismr6

r8 r10 r12 r15

r

1

6

r

1

8

r 1

10

r 1

12

r 1

15

r

A6

A8 = A9 = A10 = A12 = A15

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 Expressionr = r8 + r9 + r10 + r12 + r15

COOH

HCO

CO1

312CO21082/154

2/1H1566

2/1H

2/15415

131292108

20OH16

2

22

2

2

222 1PKP

PP

PKkPKkkKK

PkKk

PKKkPKkkPKkkθPKkr COCO

21

Simplified Model

0

0.2

0.4

0.6

0.8

1

0 100 200 300 400 500 600Temperature (oC)

Co

nve

rsio

n o

f C

O

13-step mechanism Cu(111)

simplified model Cu(111)

equilibrium

15-step mechanism Fe(111)

simplified model Fe(111)

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.

Questions…