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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
water gas shift reaction water gas shift reaction
1
Constructing the Constructing the RRRR GraphGraph
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
water gas shift reaction water gas shift reaction
2
Constructing the Constructing the RRRR GraphGraph
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
water gas shift reaction water gas shift reaction
3
Constructing the Constructing the RRRR GraphGraph
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
water gas shift reaction water gas shift reaction
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
water gas shift reaction water gas shift reaction
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
water gas shift reaction water gas shift reaction
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
water gas shift reaction water gas shift reaction
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
water gas shift reaction water gas shift reaction
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