(305g) A Graph Theoretic Approach to Methanol Decomposition Reaction Mechanism and Kinetics

Methanol decomposition (CH₃OH → CO + 2H₂) is a part of on-site source of hydrogen generation process for fuel cells. It is therefore useful to develop a fundamental understanding of the reaction mechanism and kinetics on different catalysts. Recently, Dumesic et al. [1] and Mavrikakis et al. [2] have investigated the mechanism of this reaction on Pt(111) via DFT calculations. However, a systematic analysis of the reaction pathways has not been accomplished. Our new graph-theoretic approach [3] is, therefore utilized for mechanistic and kinetic analysis of the reaction network based on reaction route theory and their analogy with electrical circuits. The set of 13 steps as considered by Dumesic et al. [1] and Mavrikakis et al. [2] constitutes the backbone of the approach. The activation energies on Pt(111) are based on DFT calculations [1], supplemented with the Unity Bond Index-Quadratic Exponential Potential Method (UBI-QEP) [4] for the adsorption and desorption steps. The Transition State Theory (TST) is used to predict the pre-exponential factors for each step assuming an immobile transition state; however, the pre-exponential factors are adjusted slightly to ensure thermodynamic consistency with the known thermodynamics of the overall reaction [5]. The approach provides a comprehensive overview of the mechanism and allows the enumeration of all reaction pathways. The dominant reaction pathways are determined based on energetic considerations, e.g., by comparing their energy diagrams. Rigorous reduction of the graph is accomplished through resistance comparisons of parallel pathways to yield a simplified graph with the most dominant reaction pathways.

(1) Gokhale A. A., Kandoi S., Greeley J. P., Mavikakis M., Dumesic J. A., Chem. Engg. Sci., 2004, 59, 4679. (2) Greeley J., Mavrikakis M., J. Am. Chem. Soc., 2002, 124, 7193. (3) Fishtik I., Callaghan C. A., Datta R., J. Phys. Chem.B., 2004,108, 5671. (4) Shustorovich, E., Sellers, H., Surf. Sci., 1998, 31, 1. (5) Lund, C., Ind. Eng. Chem. Res., 1996, 35, 2531.