TEM study of the microstructural modifications of an alumina-supported palladium combustion catalyst

An alpha-alumina-supported Pd catalyst was studied for the catalytic combus tion of methane. Changes in catalyst activity due to variations in the oxyg en partial pressure were correlated with corresponding changes in the catal yst oxidation state, particle size, and morphology, At a temperature of 107 0 K, the apparent catalyst activity increased with time under the reaction conditions where decomposition of the PdO phase occurs, The formation of la rge, faceted crystallites of metallic Pd, 200-300 nn in size, was observed by TEM during this process. The increase in activity with increasing partic le size is counterintuitive and suggests that metallic Pd surfaces formed u nder reaction conditions are highly reactive. Since the activation energy f or methane oxidation over Pd metal is greater than that over PdO, our resul ts can only be explained by an increase in the pre-exponential factor for t his reaction on metal surfaces. To gain insight into the changes in morphol ogy, a model Pd/alpha-alumina catalyst was subjected to similar thermal tre atments. It was found that decomposition of PdO leads to the formation of l arge Pd metal particles. Upon cooling, these Pd particles re-oxidize by the formation of rough polycrystalline PdO particles rather than by the format ion of uniform layers of Pd oxide on the Pd metal. Our observations suggest a mechanism to explain the phenomenon of Pd catalyst activation observed i n our previous research and by other research groups. (C) 1999 Academic Pre ss.