1. A Systematic And Mechanistic Analysis Of The Water-Gas Shift Reaction Kinetics On Low And High Temperature Shift Catalysts
CA Callaghan, I Fishtik, and R Datta
Fuel Cell Center
Department of Chemical Engineering
Worcester Polytechnic Institute
Worcester, MA , USA

2. introduction and motivation
Fuel Cells
H2 Infrastructure
Fuel Processing
Water-Gas Shift
Better understanding of mechanism & kinetics of reforming reactions (e.g. WGSR)
More systematic design of reformers and their catalysts

3. the WGSR mechanism
a – activation energies in kcal/mol (θ  0 limit) estimated according to Shustorovich & Sellers (1998) and coinciding with the estimations made in Ovesen, et al. (1996); pre-exponential factors from Dumesic, et al. (1993).
b – pre-exponential factors adjusted so as to fit the thermodynamics of the overall reaction; The units of the pre-exponential factors are Pa-1s-1 for adsorption/desorption reactions and s-1 for surface reactions.

4. reaction route graphs and topology
Full Routes (FRs):
a RR in which the desired OR is produced
Empty Routes (ERs):
a RR in which a zero OR is produced (a cycle)
Intermediate Nodes (INs):
a node including ONLY the elementary reaction steps
Terminal Nodes (TNs):
a node including the OR in addition to the elementary reaction steps
Stop
Start
A RR graph may be viewed as several hikes through a mountain range:
Valleys are the energy levels of reactants and products
Elementary reaction is a hike from one valley to adjacent valley
Trek over a mountain pass represents overcoming the energy barrier
Ref. Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B 108:
Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B 108:
Fishtik, I., C. A. Callaghan, et al. (2005). J. Phys. Chem. B 109:

5. the RR graph of the WGSR OR s1 s2 s14 s10 s3 s5 s13 s15 s11 s8 s6 s7

6. the electrical analogy
Kirchhoff’s Current Law
Analogous to
conservation of mass
Kirchhoff’s Voltage Law
thermodynamic
consistency
Ohm’s Law
Viewed in terms of the
De Donder Relation a b c d e f g i h

7. simplified RR network
We may eliminate s13 and s16 from the RR graph; they are not kinetically significant steps
This results in TWO symmetric sub-graphs; we only need one

8. overview of analysis, reduction and simplification procedure
R4 + R6 vs. R14
Effect of R14 on Conversion
Experimental Conditions: Space time = 1.80 s
FEED: COinlet = 0.10; H2Oinlet = 0.10
CO2 inlet = 0.00; H2 inlet = 0.00
On Cu(111)

9. the reduced network
Cu(111)
Fe(110)

10. the mechanism CO H2O s1 s2 s6 s8 s4 CO2 s3 H2 s5 s10 s15 s11 s7 s9
Formate RR
CO2 s3 H2 s5
s10
s15
Modified
Redox RR
Associative RR
s11 s7 C O H s9

11. the reduced rate expression
Cu(111)
Fe(110)
Experimental Conditions: Space time = 1.80 s
FEED: COinlet = 0.10; H2Oinlet = 0.10
CO2 inlet = 0.00; H2 inlet = 0.00

12. Energy diagram
Cu(111)

13. conclusions and acknowledgements
Reaction network analysis is a useful tool for reduction, simplification and rationalization of the microkinetic model.
Reaction stoichiometry translates into the network connectivity (i.e. IN, TN)
Allows for a more systematic approach for the analysis of microkinetic mechanisms.
Analogy between a reaction network and electrical network exists:
rate = current
affinity = voltage
resistance = affinity/rate
Application of RR graph theory to the analysis of the WGS reaction mechanism validated the reduced model and confirmed earlier results* based solely on a conventional microkinetic analysis.
A reliable predictive microkinetic model for the WGS reaction on Cu(111) and Fe(110) is developed
Only a limited number of RRs dominate the kinetics of the process
Prediction of simplified models compare extremely well with the complete microkinetic model.
Funding: Office of Naval Research
* Callaghan, C. A., I. Fishtik, et al. (2003). Surf. Sci. 541: 21.

14. topological characteristics
Full Reaction Routes
FR1: OR = s1 + s2 + s3 + s4 + s5 + s6 + s10
FR2: OR = s1 + s2 + s3 + s4 + s5 + s6 + s7 + s9
FR3: OR = s1 + s2 + s3 + s4 + s5 + s6 + s8 + s11
FR4: OR = s1 + s2 + s3 + s5 + s6 + s7 + s15
FR5: OR = s1 + s2 + s3 + s5 + s6 + s7 + s9 - s11 + s17
Empty Reaction Routes
ER1: 0 = -s4 - s6 + s14
ER2: 0 = -s4 - s9 + s15
ER3: 0 = -s8 + s10 - s11
ER4: 0 = -s4 - s11 + s12 + s15
ER5: 0 = -s4 + s8 - s10 + s17
Intermediate Nodes
IN1: r2 - r6 - r13 - r14 + r16
IN2: r1 - r7 - r8 - r10
IN3: -r3 + r7 + r10 + r11 + r12 + r16 +r17
IN4: r4 - r5 + r14 + r15 + r17
IN5: r6 - r8 - r9 - r10 + r12 + 2r13 + r14 - r15 - r16
Terminal Nodes
TN1: -s9 - s10 - s11 + s13 - s15 - s16 - s17 + OR
TN2: s8 - s11 - s12 - s16 - s17 + OR
TN3: -s7 - s10 - s11 - s12 - s16 - s17 + OR
TN4: s6 + s13 + s14 - s16 + OR
TN5: -s5 + OR
Example: the water gas shift reaction