The palladium catalysed oxidation of methane: reaction kinetics and the effect of diffusion barriers

The combustion of methane on a palladium catalyst was examined in a monolit h reactor. The rate equation was determined and showed an approximately fir st order dependence in methane concentration and zero order dependence on o xygen concentration. Significant inhibition by water was observed, and inhi bition by carbon dioxide was negligible. At high water concentrations the o rder with respect to water is approximately minus one. A significant reduct ion in both activity and activation energy was observed above temperatures of approximately 820 K with a dry feed. Significant diffusion limitation in the washcoat was observed. The intrinsic volumetric rate constant was foun d to be directly proportional to the palladium loading of the washcoat. The effect on the reaction rate of layers of inert washcoat placed on top of t he active catalyst was investigated. These diffusion barriers reduced the r eaction rate. The reactor performance was modelled using a two-dimensional finite element single channel model that included washcoat diffusion. The e ffect of diffusion barriers was compared to the effect of using a less acti ve catalyst for steady state and transient modes of operation at values of the Lewis number. At low Lewis number the diffusion barrier was effective a t reducing the temperature rise at the entrance to the reactor for large in let reactant concentration. (C) 2001 Elsevier Science Ltd. All rights reser ved.