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 $Pt/CeO_2$ 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_-_xPt_xO_2_-_\delta $ in the fluorite structure with 6% 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%) and +4(21%) oxidation states on $CeO_2$, whereas only 7% is present as Pt metal particles. On heat treatment, $Pt^2^+$ species increase at the cost of $Pt^4^+$ ions. EXAFS studies show the average coordination number of 1.3 around the platinum ion in the first shell of 1% $Pt/CeO_2$ at a distance of 1.98 A, 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 A is due to Pt-Pt coordination, which is absent in $PtO_2$ and PtO. The third shell at 3.28 A is not observed either in Pt metal or any of the platinum oxides, which could be attributed to $Pt^2^+-Ce^4^+$ correlation. Thus, $Pt/CeO_2$ forms a $Ce_1_-_xPt_xO_2_-_\delta $ type of solid solution having $-0-Pt^2^+-O-Ce^4^+-$ kinds of linkages.