Formation of Ce1-xPdxO2-δ Solid Solution in Combustion-Synthesized Pd/CeO2 Catalyst: XRD, XPS, and EXAFS Investigation
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Abstract
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 Ce0.99Pd0.01O1.90 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 Ce1-xPdxO2-δ type of solid solution having −Pd2+−O2-−Ce4+− kinds of linkages.
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