Abstract:
The structure and chemical nature of Pt in combustion-synthesized Pt/CeO(2) catalysts have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and temperature-programmed reaction (TPR). Catalytic oxidation of CO over Pt/CeO(2) is correlated with its structure. High-resolution XRD studies show that the structure could be refined for the composition of Ce(1-x)Pt(x)O(2-delta) in the fluorite structure with 6 percent oxide ion vacancy. TEM images show very few Pt particles on the CeO(2) crystallite surface in as-prepared samples and a decrease in the density of Pt metal particles is observed on heating. XPS studies demonstrate that Pt is dispersed mostly in +2 (72 percent) and +4 (21 percent) oxidation states on CeO(2), whereas only 7 percent is present as Pt metal particles. On heat treatment, Pt2+ species increase at the cost of Pt4+ ions. EXAFS studies show the average coordination number of 1.3 around the platinum ion in the first shell of 1 percent Pt/CeO(2) at a distance of 1.98 Angstrom, indicating oxide ion vacancy around the platinum ion. On heating, the average oxygen coordination of Pt and oxygen increases to 2.3. The second shell at 2.97 Angstrom is due to Pt-Pt coordination, which is absent in PtO(2) and PtO. The third shell at 3.28 Angstrom is not observed either in Pt metal or any of the platinum oxides, which could be attributed to Pt2+-Ce4+ correlation. Thus, Pt/CeO(2) forms a Ce(1-x)Pt(x)O(2-delta) type of solid solution having -0-Pt2+-O-Ce4+- kinds of linkages.
