experimentation and application of reaction route graph theory for mechanistic and kinetic analysis...

Experimentation and Experimentation and Application of Reaction Route Application of Reaction Route Graph Theory for Mechanistic Graph Theory for Mechanistic

and Kinetic Analysis of and Kinetic Analysis of Fuel Reforming ReactionsFuel Reforming Reactions

Fuel Cell CenterChemical Engineering Department

Worcester Polytechnic InstituteWorcester, MA

Caitlin A. Callaghan, Ilie Fishtik, and Ravindra Datta

Alan Burke, Maria Medeiros, and Louis Carreiro

Naval Undersea Warfare CenterDivision Newport

Newport, RI

IntroductionIntroduction 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 Kirchhoff’s 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 Reaction Route Graph TheoryTheory

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

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.

RR RR GraphsGraphs

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

RRRR Graph Topology 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 AnalogyElectrical Analogy Kirchhoff’s Current Law

– Analogous to conservation of mass

Kirchhoff’s Voltage Law– Analogous to thermodynamic consistency

Ohm’s Law– Viewed in terms of the De Donder Relation

ab

c

d

ea b c d e 0r r r r r

f g h i 0 A + A A Af g

i h

Rr

A=

DESORPTION

ADSORPTION

The WGSR MechanismThe WGSR Mechanism

E

A

Elementary Reactions EK

AK

ΔH

s1: 0 106 CO + S COS 12.0 1014 -12.0 a,b s2: 0 106 H2O + S H2OS 13.6 1014 -13.6 a,b s3: 5.3 4 1012 CO2S CO2 + S 0 106 5.3 a,b s4: 15.3 1013 HS + HS H2S + S 12.8 1013 2.5 a s5: 5.5 6 1012 H2S H2 + S 0 106 5.5 a,b s6: 25.4 1013 H2OS + S OHS + HS 1.6 1013 23.8 a s7: 10.7 1013 COS + OS CO2S + S 28.0 1013 -17.3 a s8: 0 1013 COS + OHS HCOOS + S 20.4 1013 -20.4 a s9: 15.5 1013 OHS + S OS + HS 20.7 1013 -5.2 a s10: 0 1013 COS + OHS CO2S + HS 22.5 1013 -22.5 a s11: 1.4 1013 HCOOS + S CO2S + HS 3.5 1013 -2.1 a

s12: 4.0 1013 HCOOS + OS CO2S + OHS 0.9 1013 3.1 a

s13: 29.0 1013 H2OS + OS 2OHS 0 1013 29.0 a s14 : 26.3 1013 H2OS + HS OHS + H2S 0 1013 26.3 a s15 : 1.3 1013 OHS + HS OS + H2S 4.0 1013 -2.7 a s16: 0.9 1013 HCOOS + OHS CO2S + H2OS 26.8 1013 -25.9 a

s17: 14.6 1013 HCOOS + HS CO2S + H2S 14.2 1013 0.4 a

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.

On Cu(111)

water gas shift reaction water gas shift reaction

Constructing the Constructing the RRRR GraphGraph

1. Select the shortest MINIMAL FR

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

s1 s2 s14 s10 s3 s5

s5 s3 s10 s14 s2 s1


1


2. Add the shortest MINIMAL ER to include all elementary reaction steps

s1 s2 s14 s10 s3 s5

s5 s3 s10 s14 s2 s1

s4 + s6 – s14 = 0

s17 s12

s12 s17

s15

s15

s6

s6

s4

s4

s9

s9

s7

s8

s7

s8s11

s11

s7 + s9 – s10 = 0s4 + s11 – s17 = 0s4 + s9 – s15 = 0s12 + s15 – s17 = 0s7 + s8 – s12 = 0

Only s13 and s16

are left to be included


2


3. Add remaining steps to fused RR graph

s1 s2 s14 s10 s3 s5

s5 s3 s10 s14 s2 s1

s17 s12

s12 s17

s15

s15

s6

s6

s4

s4

s9

s9

s7

s8

s7

s8s11

s11

s12 + s13 – s16 = 0s13 – s14 + s15 = 0

s13s13s16

s16


3


4. Balance the terminal nodes with the ORs1 s2 s14 s10 s3 s5

s5 s3 s10 s14 s2 s1

s17

s12s12

s17

s15

s15

s6

s6

s4 s4

s9

s9s7

s8

s7

s11s8

s11

s13

s13

s16

s16

OR

OR


4

MicrokineticsMicrokinetics

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

Aoverall

R1 R2

R14 R10

R5R3R8

R11

R6 R17

R12R7

R9n2

n4

n3 n5

n6

n7

R15

R4

n1 n8n0 n9

Aoverall

R1 R2

R14 R10

R5R3R8

R11

R6 R17

R12R7

R9n2

n4

n3 n5

n6

n7

R15

R4

n1 n8n0 n9


1.E-01

1.E+02

1.E+05

1.E+08

1.E+11

1.E+14

1.E+17

0 100 200 300 400 500 600

Temperature (oC)

Re

sis

tan

ce

-1

1ra

te(s

)

R 14

R 4 + R 6

1.E-04

1.E+00

1.E+04

1.E+08

1.E+12

1.E+16

1.E+20

1.E+24

1.E+28

0 100 200 300 400 500 600

Temperature (oC)

Re

sis

tan

ce

-1

1ra

te(s

)

R 11

R 9 + R 12

1.E-03

1.E+00

1.E+03

1.E+06

1.E+09

1.E+12

1.E+15

1.E+18

1.E+21

1.E+24

1.E+27

0 100 200 300 400 500 600

Temperature (oC)

Re

sis

tan

ce

-1

1ra

te(s

)

R 17

R 4 + R 11

Resistance ComparisonsResistance Comparisons

Experimental Conditions

Space time = 1.80 s

Feed: COinlet = 0.10

H2Oinlet = 0.10

CO2 inlet = 0.00

H2 inlet = 0.00


Network Reductiona b

c d

Aoverall

R1 R2

R14 R10

R5R3R8

R11

R6 R17

R12R7

R9

n2

n4

n3 n5

n6

n7

R15

R4

n1 n8

Aoverall

R1 R2

R10

R5R3R8

R11

R6 R17

R12R7

R9

n2

n4

n3 n5 n7

R15

R4

n1 n8

n6

Aoverall

R1 R2

R10

R5R3R8

R11

R6 R17

R7

n2

n4

n3 n5 n7

R15

R4

n1 n8

Aoverall

R1 R2

R10

R5R3R8R11R6

R7

n2

n4

n3 n5

n6

n7

R15

R4

n1 n8

n0 n9 n0 n9

n0 n9n0 n9

Reduced Rate ExpressionReduced Rate Expression

2 22 2

22

1/ 22 1/ 26 1 H O 0 8 10 2 15 H 4 5 CO H

OR 1/ 2H O CO6 6 15 H

8 10 2 CO1/ 24 5

1COk K P θ k k K P k P K K P P

rKP Pk K k P

k k K PK K

where

2

2

0 1/ 2H

1 H O 2 1/ 24 5

1

1 CO

PK P K P

K K

Assume that OHS is the

QSS species.

Aoverall

R10

R8R11R6

R7

n2 n3 n5

n6

n7

R15


Model vs. Experiment for WGS Model vs. Experiment for WGS ReactionReaction

Experimental Conditions

Space time = 1.80 s

FEED: COinlet = 0.10

H2Oinlet = 0.10

CO2 inlet = 0.00

H2 inlet = 0.00

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


Energy DiagramEnergy Diagram

n1

Pot

enti

al E

ner

gy (

kca

l/mol

)

0

10

20

30

40

50

-10

-20

-30

-40

-50

Reaction Coordinate

s5s3

s15

s4

s7

s6s1

s2

s8

s11

s10

n2

n3

n4 n7

n5 n6

n8

n9

n10

ULI ObjectivesULI 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 ApproachExperimental 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 ApparatusMSR/WGSR Apparatus

Data Acquisition

Vent to Hood

ArAr

digital signalmaterial flowmaterial flow

CO

MFC

CO

MFC

H2

MFC

H2

MFC

N2

MFC

N2

MFC

Furnace

Packed Bed Reactor Condenser

Bypass

Data Acquisition

Gas Chromatograph

DI H2O

MFC

CO2

MFC

MFC

CO2

Syringe Pump

Vaporizing Section

CH4

MFC

CH4

MFC

MFC Readout

Objective TasksObjective Tasks

Theoretical Work 2005 Timeline 2006

Tasks A M J J A S O N D J F M A M J Theory & algorithm of nominimal graphs Development of MATLAB program Ab Initio calculations for energetics of WGS Completion of WGS pathway structure Kinetics and mechanism of MSR CPOX mechanism structure and kinetics Integration of MSR/CPOX mechanisms for ATR Comparison with experiment

Objective TasksObjective Tasks

Experimental Work 2005 Timeline 2006

Tasks A M J J A S O N D J F M A M J Construction of equipment, testing and calibration Catalyst testing – NUWC Catalyst testing – WPI Comparison with theory

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

experimentation and application of reaction route graph theory for mechanistic and kinetic analysis...

Documents

s feed

s2s2 s1s1 s

rr graph s1s1 s2s2 s

s3s3 s5s5 s5s5 s3s3

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overall reaction

reaction network analysis

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