Synergistic Effect between Pd and Nonstoichiometric Cerium Oxide for Oxygen Activation in Methane Oxidation

The catalytic behavior of activated oxygen on Pd/CeO2-x/Al2O3 catalyst as well as the synergistic effect between Pd and nonstoichiometric cerium oxide has been revealed through methane oxidation by comparison with Pd catalysts such as Pd/CeO2/Al2O3, Pd/CeO2/SiO2, Pd/Al2O3, and Pd/SiO2 catalysts. The highest activity for methane oxidation was observed on the Pd/CeO2-x/Al2O3 catalyst. This was ascribed to the deactivation of Pd, which was inhibited by the oxygen spillover from Pd to CeO2-x, where the enhanced oxygen storage capacity (OSC) was observed by the nonstoichiometric structure and the oxygen species were reversibly stored/released in the presence of methane. Kinetics studies of methane oxidation resulted in it being first order with respect to methane and a half order with respect to oxygen for Pd/CeO2-x/Al2O3 but nearly zero order with respect to oxygen for other catalysts. From temperature-programmed desorption (TPD), reduction (TPR), oxidation (TPO), and 18O tracer technique analyses, it was found that the oxygen species in Pd/CeO2-x/Al2O3 behave reversibly, going from Pd to the lattice of cerium oxide by the spillover, and then are consumed on Pd in methane oxidation by the inverse spillover from cerium oxide. It is thus concluded that the high activity of oxygen activation on the Pd/CeO2-x/Al2O3 catalyst is induced by both the nonstoichiometric structure of cerium oxide and Pd as the active center.