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

1. 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 Alan Burke, Maria Medeiros, and Louis Carreiro Fuel Cell Center Chemical Engineering Department Worcester Polytechnic Institute Worcester, MA Naval Undersea Warfare Center Division Newport Newport, RI

2. 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 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.

3. Reaction Route Graph Theory Ref. Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B108: 5671-5682. Fishtik, I., C. A. Callaghan, et al. (2004). J. Phys. Chem. B108: 5683-5697. Fishtik, I., C. A. Callaghan, et al. (2005). J. Phys. Chem. B109: 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

4. RR Graphs 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 Stop Start

5. 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

6. a b e c d f g i h Electrical 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

7. ADSORPTION DESORPTION The WGSR Mechanism On Cu(111) 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. water gas shift reaction

8. Constructing the RR Graph • Select the shortest MINIMALFR 1 s1 s2 s14 s10 s3 s5 s5 s3 s10 s14 s2 s1 water gas shift reaction

9. Constructing the RR Graph • Add the shortest MINIMAL ER to include all elementary reaction steps 2 s4 + s11 – s17 = 0 s12 + s15 – s17 = 0 s7 + s8 – s12 = 0 s4 + s9 – s15 = 0 s7 + s9 – s10 = 0 s4 + s6 – s14 = 0 s11 s17 s8 s12 s1 s2 s14 s10 s3 s5 s6 s7 s9 s4 Only s13 and s16are left to be included s15 s15 s7 s9 s4 s6 s5 s3 s10 s14 s2 s1 s12 s8 s17 s11 water gas shift reaction

10. 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 s7 s9 s4 s6 s5 s3 s10 s14 s2 s1 s12 s8 s17 s11 water gas shift reaction

11. Constructing the RR Graph • Balance the terminal nodes with the OR 4 OR s1 s2 s14 s10 s3 s5 s15 s13 s11 s8 s6 s7 s16 s17 s9 s12 s12 s4 s4 s17 s9 s16 s7 s6 s8 s11 s15 s13 s5 s3 s10 s14 s2 s1 OR water gas shift reaction

12. Microkinetics • We may eliminate s13and 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

13. 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

14. Network Reduction

15. Reduced Rate Expression R15 R7 n6 R6 R11 R8 n2 n3 n5 n7 R10 Aoverall Assume that OHS is the QSS species. where water gas shift reaction

16. 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

17. Energy Diagram

18. 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)

19. 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

20. MSR/WGSR Apparatus

21. Objective Tasks • Theoretical Work

22. Objective Tasks • Experimental Work

23. 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