A reduced mechanism for methane and one-step rate expressions for fuel-lean catalytic combustion of small alkanes on noble metals

A reduced mechanism and a one-step rate expression for fuel-lean methane/air catalytic combustion on an Rh catalyst are proposed. These are developed from a detailed microkinetic model using a computer-aided model reduction strategy that employs reaction path analysis, sensitivity analysis, partial equilibrium analysis, and simple algebra to deduce the most abundant reaction intermediate and the rate-determining step. The mechanism and the one-step rate expression are then tested on Pt catalyst. It is found that the reaction proceeds effectively via the same mechanistic pathway on both noble metals, but the effective reaction orders differ due to the difference in the adsorption strength of oxygen. Based on the homologous series idea, the rate expression is extended to small alkanes (ethane and propane; butane is also briefly discussed) and is found to reasonably describe experimental data. Estimation of the relevant parameters in the rate expression for various fuels and catalysts using the semiempirical bond-order conservation theory, quantum mechanical density functional theory, and/or simple experiments is discussed. Finally, it is proposed that detailed microkinetic models with coverage-dependent parameters can assist in rationalizing the apparent discrepancies between experimental data from various research groups.