Ionic dispersion of Pt over CeO₂ by the combustion method: structural investigation by XRD, TEM, XPS, and EXAFS

Abstract

The structure and chemical nature of Pt in combustion-synthesized Pt/CeO₂ 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/CeO₂ is correlated with its structure. High-resolution XRD studies show that the structure could be refined for the composition of Ce_1-xPt_xO_2-δ in the fluorite structure with 6% oxide ion vacancy. TEM images show very few Pt particles on the CeO₂ 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%) and +4 (21%) oxidation states on CeO₂, whereas only 7% 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% Pt/CeO₂ at a distance of 1.98 Å, 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 Å is due to Pt-Pt coordination, which is absent in PtO₂ and PtO. The third shell at 3.28 Å is not observed either in Pt metal or any of the platinum oxides, which could be attributed to Pt²⁺-Ce⁴⁺ correlation. Thus, Pt/CeO₂ forms a Ce_1-xPt_xO_2-δ type of solid solution having -Pt²⁺-O-Ce⁴⁺- kinds of linkages.