Methane Oxidation on Pd@ZrO2/Si–Al2O3Is Enhanced by Surface Reduction of ZrO2

The catalytic properties of Pd@ZrO2 core–shell catalysts supported on Si-modified alumina were studied for application to methane oxidation and compared to the analogous Pd@CeO2 catalysts. In the absence of water (dry conditions), both Pd@ZrO2 and Pd@CeO2 were highly active and showed nearly identical reaction rates and thermal stabilities. However, unlike catalysts based on Pd@CeO2, the Pd@ZrO2 catalysts were also very stable in the presence of high concentrations of water vapor. By means of Coulometric titration and pulse-reactor studies, we demonstrate that ZrO2 in contact with Pd can be reduced. Additionally, Coulometric titration showed that the Pd-PdO equilibrium at 600 °C is shifted to much lower P(O2) in the Pd@ZrO2 catalyst compared to conventional Pd/ZrO2 or Pd/Al2O3 catalysts. Because PdO is more active for methane oxidation, this observation provides a possible explanation for the superior performance of the Pd@ZrO2 catalyst.