Presentation on theme: "Experimentation and Application of Reaction Route Graph Theory for Mechanistic and Kinetic Analysis of Fuel Reforming Reactions Caitlin A. Callaghan,"— Presentation transcript:
1 Experimentation and Application of Reaction Route Graph Theory for Mechanistic and Kinetic Analysis of Fuel Reforming ReactionsCaitlin A. Callaghan, Ilie Fishtik, and Ravindra DattaFuel Cell CenterChemical Engineering DepartmentWorcester Polytechnic InstituteWorcester, MAAlan Burke, Maria Medeiros, and Louis CarreiroNaval Undersea Warfare CenterDivision NewportNewport, RI
2 IntroductionPredicted 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 LawsExample: 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. 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:Powerful new tool in graphical and mathematical depiction of reaction mechanismsNew method for mechanistic and kinetic interpretation“RR graph” differs from “Reaction Graphs”Branches elementary reaction stepsNodes multiple species, connectivity of elementary reaction stepsReaction Route Analysis, Reduction and SimplificationEnumeration of direct reaction routesDominant reaction routes via network analysisRDS, QSSA, MARI assumptions based on a rigorous De Donder affinity analysisDerivation of explicit and accurate rate expressions for dominant reaction routesRecently, we have developed a powerful new tool combining both graph theory and electrical network theory for mechanistic and kinetic interpretation.Our RR graphs differ from the more familiar “Reaction Graphs” in that our branches represent the elementary reaction steps and the nodes represent the connectivity of those steps and comprise several species while the Reaction Graph represents each species as a node and their participation in reactions by the branches. Furthermore, our RRGT may be expanded to the case of more complex reaction mechanisms while the Reaction Graph is typically applied to mono-molecular reaction mechanisms.This tool provides a methodology for analysis, reduction and simplification of reaction mechanisms. The enumeration of reaction routes is systematic; the dominant reaction routes are easily identified via the network analysis. The RDS, QSSA and MARI assumptions are no longer arbitrary, but based on rigorous De Donder affinity analyses and result in the derivation of explicit and accurate rate expressions based on those reaction routes which are shown to dominate the kinetics of the mechanism.
4 RR GraphsStopStartA RR graph may be viewed as several hikes through a mountain range:Valleys are the energy levels of reactants and productsElementary reaction is a hike from one valley to adjacent valleyTrek over a mountain pass represents overcoming the energy barrierA RR graph may be viewed as hikes through a mountain range. In this analogy, the valleys represent the energy levels of the reactants and products and a hike from one valley to an adjacent valley is representative of an elementary reaction step. Finally, we view the trek over a mountain pass as the energy barrier an elementary reaction step must overcome to proceed from reactants to products.
5 RR Graph Topology Full Routes (FRs): Empty Routes (ERs): a RR in which the desired OR is producedEmpty Routes (ERs):a RR in which a zero OR is produced (a cycle)Intermediate Nodes (INs):a node including ONLY the elementary reaction stepsTerminal Nodes (TNs):a node including the OR in addition to the elementary reaction stepsThe RR graph is described in terms of four major characteristics: the Overall Reaction Routes, RRs in which the desired OR is produced; the Empty Reaction Routes, RRs in which a zero OR is produced (a cycle); the Intermediate Nodes, nodes in which only the elementary reaction steps are participants; and, finally, the Terminal Nodes, nodes in which, not only do the elementary reaction steps participate, but also the OR.
6 Electrical Analogy Kirchhoff’s Current Law Kirchhoff’s Voltage Law Analogous to conservation of massKirchhoff’s Voltage LawAnalogous to thermodynamic consistencyOhm’s LawViewed in terms of the De Donder RelationabcdefgihThe reduction of a mechanism is the result of the application of electrical network theory to the RR graph. In this analogy, we find that Kirchhoff’s Current Law corresponds to conservation of mass. That is, at any given node, the rates (which we associate with current) must sum to zero. Further, Kirchhoff’s Voltage Law is analogous to thermodynamic consistency (where the affinity, here in terms of dimensionless affinity, corresponds to voltage). In other words, in a cycle, the affinities must sum to zero. Finally, we view each elementary reaction step as a resistor and associate with it a resistance determined by Ohm’s Law as it corresponds to the De Donder relation.
7 The WGSR Mechanism On Cu(111) ADSORPTION DESORPTION 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 MINIMAL FR1s1s2s14s10s3s5s5s3s10s14s2s1water gas shift reaction
9 Constructing the RR Graph 2Add the shortest MINIMAL ER to include all elementary reaction stepss12 + s15 – s17 = 0s4 + s11 – s17 = 0s7 + s8 – s12 = 0s7 + s9 – s10 = 0s4 + s6 – s14 = 0s4 + s9 – s15 = 0s11s17s8s12s1s2s14s10s3s5s6s7s4s9Only s13 and s16 are left to be includeds15s15s7s9s4s6s5s3s10s14s2s1s8s12s17s11water gas shift reaction
10 Constructing the RR Graph Add remaining steps to fused RR graph3s12 + s13 – s16 = 0s13 – s14 + s15 = 0s11s17s8s12s1s2s14s10s3s5s6s7s4s9s15s16s13s13s16s15s7s9s4s6s5s3s10s14s2s1s8s12s17s11water gas shift reaction
11 Constructing the RR Graph 4Balance the terminal nodes with the ORORs1s2s14s10s3s5s13s15s11s8s6s7s16s17s9s12s12s4s4s17s9s16s7s6s8s11s15s13s5s3s10s14s2s1ORwater gas shift reaction
12 MicrokineticsWe may eliminate s13 and s16 from the RR graph; they are not kinetically significant stepsThis results in TWO symmetric sub-graphs; we only need onewater gas shift reaction
18 ULI ObjectivesElucidate 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 catalystsCH4 + 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 reformationBoth integral and differential experiments used to study kinetics (Tmax ≈ 800 oC)WPI: (External reforming)Test in-house fabricated catalystsMethane steam and autothermal reformation reactionsNUWC: (Internal & External reforming)Apparatus available at NUWC for internal reforming with SOFC button cell testsCommercial catalyst testing – external steam and autothermal reforming of methane
23 Benefits to the NavyExtend fundamental understanding of reaction mechanisms involved in logistics fuel reforming reactionsGather data on air-independent autothermal fuel reformation with commercially available catalystsDevelop new catalytic solutions for undersea fuel processingDevelop relationship between ONR and WPI