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

The combustion of methane on a palladium catalyst was examd. in a monolith reactor. The rate equation was detd. and showed an approx. first order dependence in methane concn. and zero order dependence on oxygen concn. Significant inhibition by water was obsd., and inhibition by carbon dioxide was negligible. At high water concns. the order with respect to water is approx. minus one. A significant redn. in both activity and activation energy was obsd. above temps. of approx. 820 K with a dry feed. Significant diffusion limitation in the washcoat was obsd. The intrinsic volumetric rate const. was directly proportional to the palladium loading of the washcoat. The effect on the reaction rate of layers of inert washcoat placed on top of the active catalyst was investigated. These diffusion barriers reduced the reaction rate. The reactor performance was modeled using a two-dimensional finite element single channel model that included washcoat diffusion. The effect of diffusion barriers was compared to the effect of using a less active catalyst for steady state and transient modes of operation at values of the Lewis no. At low Lewis no. the diffusion barrier was effective at reducing the temp. rise at the entrance to the reactor for large inlet reactant concn.