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Experimentation and Application of Reaction Route

Graph Theory for Mechanistic and Kinetic Analysis

of Fuel Reforming Reactions

Caitlin A. Callaghan, Ilie Fishtik, and Ravindra

Datta

- Fuel Cell Center
- Chemical Engineering Department
- Worcester Polytechnic Institute
- Worcester, MA

Alan Burke, Maria Medeiros, and Louis Carreiro

Naval Undersea Warfare Center Division

Newport Newport, RI

Introduction

- Predicted elementary kinetics can provide

reliable microkinetic models. - Reaction network analysis, developed by us, is a

useful tool for reduction, simplification and

rationalization of the microkinetic model. - Analogy between a reaction network and electrical

network exists and provides a useful

interpretation of kinetics and mechanism via

Kirchhoffs Laws - Example the analysis of the WGS reaction

mechanism

Callaghan, C. A., I. Fishtik, et al. (2003).

"An improved microkinetic model for the water gas

shift reaction on copper." Surf. Sci. 541 21.

Reaction Route Graph Theory

Ref. Fishtik, I., C. A. Callaghan, et al.

(2004). J. Phys. Chem. B 108 5671-5682.

Fishtik, I., C. A. Callaghan, et al. (2004). J.

Phys. Chem. B 108 5683-5697. Fishtik, I., C.

A. Callaghan, et al. (2005). J. Phys. Chem. B

109 2710-2722.

- Powerful new tool in graphical and mathematical

depiction of reaction mechanisms - New method for mechanistic and kinetic

interpretation - RR graph differs from Reaction Graphs
- Branches ? elementary reaction steps
- Nodes ? multiple species, connectivity of

elementary reaction steps - Reaction Route Analysis, Reduction and

Simplification - Enumeration of direct reaction routes
- Dominant reaction routes via network analysis
- RDS, QSSA, MARI assumptions based on a rigorous

De Donder affinity analysis - Derivation of explicit and accurate rate

expressions for dominant reaction routes

RR Graphs

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

RR Graph 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

Electrical Analogy

- Kirchhoffs Current Law
- Analogous to conservation of mass
- Kirchhoffs Voltage Law
- Analogous to thermodynamic consistency
- Ohms Law
- Viewed in terms of the De Donder Relation

The WGSR Mechanism

On Cu(111)

a - activation energies in kcal/mol (? ? 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.

water gas shift reaction

Constructing the RR Graph

- Select the shortest MINIMAL FR

1

s1

s2

s14

s10

s3

s5

s5

s3

s10

s14

s2

s1

water gas shift reaction

Constructing the RR Graph

- Add the shortest MINIMAL ER to include all

elementary reaction steps

2

s4 s6 s14 0

s7 s9 s10 0

s4 s11 s17 0

s4 s9 s15 0

s12 s15 s17 0

s7 s8 s12 0

s11

s17

s8

s12

s1

s2

s14

s10

s3

s5

s6

s7

s9

s4

Only s13 and s16 are left to be included

s15

s15

s6

s4

s9

s7

s5

s3

s10

s14

s2

s1

s12

s8

s17

s11

water gas shift reaction

Constructing the RR Graph

- Add remaining steps to fused RR graph

3

s12 s13 s16 0 s13 s14 s15 0

?

s11

?

s17

s8

s12

s1

s2

s14

s10

s3

s5

s6

s7

s9

s4

s15

s16

s13

s13

s16

s15

s6

s4

s9

s7

s5

s3

s10

s14

s2

s1

s12

s8

s17

s11

water gas shift reaction

Constructing the RR Graph

- Balance the terminal nodes with the OR

4

OR

s1

s2

s14

s10

s3

s5

s15

s11

s13

s8

s6

s7

s17

s9

s16

s12

s12

s4

s4

s17

s9

s16

s7

s6

s11

s8

s15

s13

s5

s3

s10

s14

s2

s1

OR

water gas shift reaction

Microkinetics

- 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

water gas shift reaction

Resistance Comparisons

Experimental Conditions Space time 1.80

s Feed COinlet 0.10 H2Oinlet 0.10 CO2

inlet 0.00 H2 inlet 0.00

water gas shift reaction

Network Reduction

Reduced Rate Expression

R7

R15

n6

R8

R11

R6

n2

n3

n5

n7

R10

Aoverall

Assume that OHS is the QSS species.

where

water gas shift reaction

Model vs. Experiment for WGS Reaction

Experimental Conditions Space time 1.80

s FEED COinlet 0.10 H2Oinlet 0.10 CO2

inlet 0.00 H2 inlet 0.00

water gas shift reaction

Energy Diagram

ULI Objectives

- Elucidate the mechanism and kinetics of logistics

fuel processing using a building block approach

(i.e. CH4, C2H6 , JP-8) - In first 1-2 years, utilize theoretical and

experimental research to methodically investigate

reforming of methane on various catalysts - CH4 H2O ? CO 3H2 (MSR)
- CH4 ½ O2 ? CO 2 H2 (CPOX)
- CO H2O ? CO2 H2 (WGS)

Experimental Approach

- Catalysts of interest Ni, Cu, Ru, Pt, CeO2, and

commercially available catalysts for steam and

autothermal reformation - Both integral and differential experiments used

to study kinetics (Tmax 800 oC) - WPI (External reforming)
- Test in-house fabricated catalysts
- Methane steam and autothermal reformation

reactions - NUWC (Internal External reforming)
- Apparatus available at NUWC for internal

reforming with SOFC button cell tests - Commercial catalyst testing external steam and

autothermal reforming of methane

MSR/WGSR Apparatus

Objective Tasks

- Theoretical Work

Objective Tasks

- Experimental Work

Benefits to the Navy

- Extend fundamental understanding of reaction

mechanisms involved in logistics fuel reforming

reactions - Gather data on air-independent autothermal fuel

reformation with commercially available catalysts - Develop new catalytic solutions for undersea fuel

processing - Develop relationship between ONR and WPI