The relationship between the chemical state of Pd species and the catalytic activity for methane combustion on Pd/CeO2Electronic supplementary information (ESI) available: Pd 3d XPS spectra of Pd/CeO2catalysts after the stability test (Fig. S1) and of Pd/CeO2catalysts with low Pd loading (Fig. S2); data for H2-TPR (Fig. S3) and the peak area for each peak from the H2-TPR results (Table S1). See DOI: 10.1039/c8cy00208h

Three Pd/CeO2catalysts are synthesized by reduction-deposition and an impregnation method (IMP) to clarify how the chemical state of Pd influences the catalytic performance for CH4combustion. The results show that Pd/CeO2(HHA), using hydrazine hydrate as a reductant, shows much higher catalytic activity and stability for CH4combustion 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 Pdδ+(2 < δ≤ 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 CeO2lattice to active Pd species. The activity of Pd/CeO2directly correlates to the surface concentrations of PdO and adsorbed oxygen. An increase in the amount of PdxCe1−xO2would thicken the transition layer and decrease the replenishment rate of oxygen vacancies through diffusion of lattice oxygen. Meanwhile, the existence of more PdxCe1−xO2could promote the capture of produced CO2on the CeO2around Pd species and decrease the catalytic activity.