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

Caitlin Callaghan, Ilie Fishtik & Ravindra DattaCaitlin Callaghan, Ilie Fishtik & Ravindra Datta

Fuel Cell CenterFuel Cell CenterChemical Engineering DepartmentChemical Engineering DepartmentWorcester Polytechnic InstituteWorcester Polytechnic InstituteWorcester, MAWorcester, MA

August 19, 2002August 19, 2002

22

Research ObjectivesResearch Objectives

Develop a predictive microkinetic model for Develop a predictive microkinetic model for LTS and HTS water gas shift catalystsLTS and HTS water gas shift catalysts Identify the rate determining stepsIdentify the rate determining steps Develop reduced modelDevelop reduced model

Simulate the reaction for different catalysts Simulate the reaction for different catalysts (e.g. Cu, Fe, etc.)(e.g. Cu, Fe, etc.)

Eventual goal is Eventual goal is a prioria priori design of catalysts for design of catalysts for the water-gas-shift-reaction in fuel reformers the water-gas-shift-reaction in fuel reformers for fuel cellsfor fuel cells

33

Model TheoryModel Theory Mechanism assumed to proceed via a set of Mechanism assumed to proceed via a set of

ERs involving the ERs involving the active sitesactive sites ( (SS), ), surface surface intermediates intermediates ((IIii), and ), and terminal speciesterminal species ( (TTii).).

The generic The generic rate expression rate expression for each reaction is for each reaction is given by: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

44

Developing the ModelDeveloping 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 from transition state theorytransition state theory

101 Pa-1s-1 – adsorption/desorption reactions

1013 s-1 – surface reactions

55

Elementary ReactionsElementary Reactionsss11 : : HH22O + S O + S HH22OS 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

Adsorption and DesorptionReactions

66

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

Reaction EnergeticsReaction Energetics

Pre-exponential Pre-exponential factors factors PaPa-1-1ss-1-1

(adsorption/ (adsorption/ desorption desorption steps) steps)

ss-1-1 (surface (surface reaction)reaction)

Activation Activation energies energies (kcal/mol)(kcal/mol)

77

0

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1

0 100 200 300 400 500 600Temperature (oC)

Co

nve

rsio

n o

f C

O

Xue

Cu13model

Equilibrium

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

88

Simulation of Microkinetic Simulation of Microkinetic Model for Cu(111), 15-stepModel 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

99

Simulation of Microkinetic Simulation of Microkinetic Model for Fe(111), 15-stepModel for Fe(111), 15-step

0

0.2

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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

1010

Reaction Route AnalysisReaction Route Analysis A A Reaction RouteReaction Route is the result of a linear combination is the result of a linear combination

of of qq+1 ERs that produces the desired overall reaction.+1 ERs that produces the desired overall reaction.

210 Possible Reaction Routes were found including210 Possible Reaction Routes were found including Empty Roots Empty Roots

The net reaction is zero.The net reaction is zero. Non-Empty RootsNon-Empty Roots

The net reaction is the WGSR.The net reaction is the WGSR.

31 Unique Reaction Routes remain31 Unique Reaction Routes remain 17 Routes previously examined (17 Routes previously examined (Fishtik & Datta, Surf. Sci. Fishtik & Datta, Surf. Sci.

512 (2002).)512 (2002).)

14 14 New RootsNew Roots based on based on ss1414 & & ss1515 contribution contribution

1111

Unique Reaction RoutesUnique 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 + s11 - 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 - s12 + 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

RRRR11 – formate reaction route – formate reaction route

RRRR22 – redox reaction route – redox reaction route

RRRR33 – associative reaction route – associative reaction route

1212

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Co

nve

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f C

ORR Contributions on Cu(111)RR Contributions on Cu(111)

RR2

RR1 & RR3

TotalMechanism

Equilibrium

1313

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Co

nv

ers

ion

of

CO

RR Contributions on Fe(111)RR Contributions on Fe(111)

RR1, RR3,RR18 & RR19

TotalMechanism

Equilibrium

1414

Reaction Route CombinationReaction Route Combination

The ERs of each dominant RR are The ERs of each dominant RR are combined to generate a “net” RRcombined to generate a “net” RR

Simplified Model involving only 13 ERsSimplified Model involving only 13 ERs

ERER ss11 ss22 ss33 ss44 ss55 ss66 ss77 ss88 ss99 ss1010 ss1111 ss1212 ss1313 ss1414 ss1515

CuCu

FeFe

1515

Quasi-Equilibrium ReactionsQuasi-Equilibrium Reactions Identified by affinity calculationsIdentified by affinity calculations ss11,,ss22,,ss33,,ss44,,ss55,,ss77,,ss1111

All intermediates represented except All intermediates represented except OHSOHS

n

iiji

q

kkjkojojj PKA

RT 11

lnlnlnln1

Reducing the ModelReducing the Model

Quasi-Steady State SpeciesQuasi-Steady State Species OHS

Rate Determining StepsRate Determining Steps Copper: Copper: ss66,,ss88,,ss1010,,ss1515

Iron: Iron: ss66,,ss88,,ss1010,,ss1212,,ss1515

1616

12-Step, 4-Route, 4-RDS Model12-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

1717

Rate ExpressionsRate Expressions

RR1

RR3

RR19

RR18

1818

WGSR MechanismWGSR 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

1919

Overall Rate ExpressionOverall Rate Expression IRRs and ERs combine to indicate the IRRs and ERs combine to indicate the

dominant rates of each RRdominant rates of each RR Cu(111):Cu(111): rr1212 neglected neglected

Fe(111):Fe(111): rr1212 included included

Overall Rate ExpressionOverall Rate Expressionrr = = rr88 + + rr99 + + rr1010 + + rr1212 + + rr1515

2020

Simplified ModelSimplified Model

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Co

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13-step mechanism Cu(111)

simplified model Cu(111)

equilibrium

15-step mechanism Fe(111)

simplified model Fe(111)

2121

ConclusionsConclusions

A reliable predictive microkinetic model for the WGS A reliable predictive microkinetic model for the WGS reaction on Cu(111) and Fe(111) is developed.reaction on Cu(111) and Fe(111) is developed.

Only a limited number of RRs dominate the kinetics of Only a limited number of RRs dominate the kinetics of the process (RRthe process (RR11,RR,RR33,RR,RR1818,RR,RR1919).).

Prediction of simplified models compare extremely well Prediction of simplified models compare extremely well with the complete microkinetic model.with the complete microkinetic model.

The addition of The addition of ss1414 and and ss1515 dramatically affected the dramatically affected the model for WGS on copper; the model for iron model for WGS on copper; the model for iron remained unaffected. RRremained unaffected. RR1818 requires further requires further investigation.investigation.

Questions…Questions…