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Experimentation and Application of Reaction Route Graph Theory for Mechanistic and Kinetic Analysis of Fuel Reforming Reactions Fuel Cell Center Chemical.

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Presentation on theme: "Experimentation and Application of Reaction Route Graph Theory for Mechanistic and Kinetic Analysis of Fuel Reforming Reactions Fuel Cell Center Chemical."— Presentation transcript:

1 Experimentation and Application of Reaction Route Graph Theory for Mechanistic and Kinetic Analysis of Fuel Reforming Reactions Fuel Cell Center Chemical Engineering Department Worcester Polytechnic Institute Worcester, MA Caitlin A. Callaghan, Ilie Fishtik, and Ravindra Datta Alan Burke, Maria Medeiros, and Louis Carreiro 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 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.

3 Reaction Route Graph Theory 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: 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:

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

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 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 a b c d e fg ih

7 DESORPTION ADSORPTION 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 -1 s -1 for adsorption/desorption reactions and s -1 for surface reactions. On Cu(111) water gas shift reaction

8 Constructing the RR Graph 1.Select the shortest MINIMAL FR s1s1 s2s2 s 14 s 10 s3s3 s5s5 s5s5 s3s3 s 14 s2s2 s1s1 water gas shift reaction 1

9 Constructing the RR Graph 2.Add the shortest MINIMAL ER to include all elementary reaction steps s1s1 s2s2 s 14 s 10 s3s3 s5s5 s5s5 s3s3 s 14 s2s2 s1s1 s 4 + s 6 – s 14 = 0 s 17 s 12 s 17 s 15 s6s6 s6s6 s4s4 s4s4 s9s9 s9s9 s7s7 s8s8 s7s7 s8s8 s 11 s 7 + s 9 – s 10 = 0s 4 + s 11 – s 17 = 0s 4 + s 9 – s 15 = 0s 12 + s 15 – s 17 = 0s 7 + s 8 – s 12 = 0 Only s 13 and s 16 are left to be included water gas shift reaction 2

10 Constructing the RR Graph 3.Add remaining steps to fused RR graph s1s1 s2s2 s 14 s 10 s3s3 s5s5 s5s5 s3s3 s 14 s2s2 s1s1 s 17 s 12 s 17 s 15 s6s6 s6s6 s4s4 s4s4 s9s9 s9s9 s7s7 s8s8 s7s7 s8s8 s 11 s 12 + s 13 – s 16 = 0 s 13 – s 14 + s 15 = 0 s 13 s 16 water gas shift reaction 3

11 Constructing the RR Graph 4.Balance the terminal nodes with the OR s1s1 s2s2 s 14 s 10 s3s3 s5s5 s5s5 s3s3 s 14 s2s2 s1s1 s 17 s 12 s 17 s 15 s6s6 s6s6 s4s4 s4s4 s9s9 s9s9 s7s7 s8s8 s7s7 s 11 s8s8 s 13 s 16 OR water gas shift reaction 4

12 Microkinetics We may eliminate s 13 and s 16 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:CO inlet = 0.10 H 2 O inlet = 0.10 CO 2 inlet = 0.00 H 2 inlet = 0.00 water gas shift reaction

14 Network Reduction

15 Reduced Rate Expression where Assume that OHS is the QSS species. A overall R 10 R8R8 R 11 R6R6 R7R7 n2n2 n3n3 n5n5 n6n6 n7n7 R 15 water gas shift reaction

16 Model vs. Experiment for WGS Reaction Experimental Conditions Space time = 1.80 s FEED:CO inlet = 0.10 H 2 O inlet = 0.10 CO 2 inlet = 0.00 H 2 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. CH 4, C 2 H 6 …, JP-8) In first 1-2 years, utilize theoretical and experimental research to methodically investigate reforming of methane on various catalysts CH 4 + H 2 O CO + 3H 2 (MSR) CH 4 + ½ O 2 CO + 2 H 2 (CPOX) CO + H 2 O CO 2 + H 2 (WGS)

19 Experimental Approach Catalysts of interest: Ni, Cu, Ru, Pt, CeO 2, and commercially available catalysts for steam and autothermal reformation Both integral and differential experiments used to study kinetics (T max 800 o C) 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


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