Microkinetic Modeling of the Water Gas Shift Reaction on Copper and Iron Catalysts
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Transcript of Microkinetic Modeling of the Water Gas Shift Reaction on Copper and Iron Catalysts
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
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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:
n
iiji
q
kkjkjojs
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0TIS
Ref. Fishtik & Datta
jijkjo
jijkjo
i
n
ik
q
kj
i
n
ik
q
kjj
PRT
EexpA
PRT
EexpAr
���
��
110
110
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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
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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
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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).
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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
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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
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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
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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
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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
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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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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
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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
s3+1/2s4+1/2s5 + s11: HCOOS CO2 + 1/2H2 + S EQ
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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
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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 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
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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)
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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.
Questions…