Methane Catalytic Combustion over Hierarchical Pd@CeO2/Si-Al2O3: Effect of the Presence of Water

The influence of water vapor on methane catalytic combustion was studied over a Pd@CeO2/Si-Al2O3 catalyst, carefully designed to maximize Pd-CeO2 interaction and prevent metal sintering and compared to a conventional impregnated catalyst with identical chemical composition. Although the nanostructured Pd@CeO2/Si-Al2O3 catalyst is thermally stable, the addition of water to the reaction feed leads to a transient deactivation at low temperatures, consistent with the well documented competitive adsorption. In addition to this, the hierarchically structured catalyst exhibits an additional severe deactivation after methane oxidation in the presence of water vapor at 600°C that can be reversed only by heating the catalyst above 700°C. The presence of water in the reaction feed deactivates the conventional impregnated catalyst less severely and the activity largely returns upon water removal. Catalytic FTIR and CO-chemisorption data indicate that this severe deactivation process in the hierarchical catalyst is due to the formation of stable OH groups on the surface of the ceria nanoparticles. These hydroxyl groups are suggested to significantly inhibit the oxygen spillover from the CeO2 nanoparticles to Pd, preventing its efficient re-oxidation, as observed by operando X-ray absorption near edge spectroscopy (XANES) experiments. At the same time, their presence can contribute to limit the gas phase accessibility of Pd, as indicated by the decrease of CO chemisorption capability. The presence of hydroxyls plays a minor role on the deactivation of the conventional catalyst at 600°C.