The relationship between the chemical state of Pd species and the catalytic activity for methane combustion on Pd/CeO2

Three Pd/CeO2 catalysts are synthesized by reduction-deposition and an impregnation method (IMP) to clarify how the chemical state of Pd influences the catalytic performance for CH4 combustion. The results show that Pd/CeO2 (HHA), using hydrazine hydrate as a reductant, shows much higher catalytic activity and stability for CH4 combustion than Pd/CeO2 (FA), using formaldehyde as a reductant, and Pd/CeO2 (IMP). The results of our comprehensive characterization reveal that there are two different chemical states of Pd species on Pd/CeO2 : Pd2+ in the form of PdO particles and Pd delta+ (2 < delta <= 4) in the form of PdxCe1-xO2, and the latter is mainly induced by the Pd species entering the lattice of CeO2, which acts as a transfer channel of active oxygen species from the CeO2 lattice to active Pd species. The activity of Pd/CeO2 directly correlates to the surface concentrations of PdO and adsorbed oxygen. An increase in the amount of PdxCe1-xO2 would thicken the transition layer and decrease the replenishment rate of oxygen vacancies through diffusion of lattice oxygen. Meanwhile, the existence of more PdxCe1-xO2 could promote the capture of produced CO2 on the CeO2 around Pd species and decrease the catalytic activity.