Formation of Ce_1-xPd_xO_2-δ solid solution in combustion-synthesized Pd/CeO₂ catalyst: XRD, XPS, and EXAFS investigation

Pd/CeO₂ (1 at. %) prepared by the solution-combustion method shows a higher catalytic activity for CO oxidation and NO reduction than Pd metal, PdO, and Pd dispersed over CeO₂ by the conventional method. To understand the higher catalytic properties, the structure of 1 at. % Pd/CeO₂ catalyst material has been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The diffraction lines corresponding to Pd or PdO are not observed in the high-resolution XRD pattern of 1 at. % Pd/CeO₂. The structure of 1 at. % Pd/CeO₂ could be refined for the composition of Ce_0.99Pd_0.01O_1.90 in the fluorite structure with 5% oxide ion vacancy. Pd(3d) peaks in the XPS in 1 at. % Pd/CeO₂ are shifted by 3 eV indicating that Pd is in a highly ionic +2 state. EXAFS studies show the average coordination number of 3 around Pd²⁺ ion in the first shell of 1 at. % Pd/CeO₂ at a distance of 2.02 Å, instead of 4 as in PdO. The second shell at 2.72 Å is due to Pd-Pd correlation which is larger than 2.69 Å in PdO. The third shell at 3.31 Å having 7 coordination is absent either in Pd metal or PdO, which can be attributed to -Pd²⁺-Ce⁴⁺-correlation. Thus, 1 at. % Pd/CeO₂ forms the Ce_1-xPd_xO_2-δ type of solid solution having -Pd²⁺-O^2--Ce⁴⁺- kinds of linkages.