Structural, microstructural and surface properties of a specific CeO 2–Bi 2O 3 multiphase system obtained at 600 °C

Abstract

Polycrystalline samples of (1− x) CeO 2− x/2 Bi 2O 3 phases, where x is the atom fraction of bismuth have been synthesized by the precipitation process and after the thermal treatment at 600 °C, under air. Samples are first characterized by the X-ray diffraction and scanning electron microscopy. To determine the samples specific surface areas, Brunauer–Emmett–Teller (BET) analyses have been performed. In the composition range 0≤ x≤0.20, a cubic solid solution with fluorite structure is obtained. For compositions x comprised between 0.30 and 0.90, two types of T′ (or β′) and T (or β) tetragonal phases, similar to the well-known β′ or β Bi 2O 3 metastable structural varieties, are observed. However, the crystal cell volumes of these β′ or β Bi 2O 3 phases increase with the composition x in bismuth: this might be due to the presence of defects or substitution by cerium atoms, in the tetragonal lattices. Using X-ray diffraction profile analyses, correlations between bismuth composition x and crystal sizes or lattice distortions have been established. The solid–gas interactions between these polycrystalline materials and air–CH 4 and air–CO flows have been studied as a function of temperature and composition x, using Fourier transform infrared (FTIR) analyses of the conversions of CH 4 and CO gases into the CO 2 gas. The transformations of CH 4 and CO molecules as a function of time and temperature are determined through the intensities of FTIR CO 2 absorption bands. Using the specific surface areas determined from BET analyses, these FTIR intensities have been normalized and compared. For all bismuth compositions, a low catalytic reactivity is observed with air–CH 4 gas flows, while, for the highest bismuth compositions, a high catalytic reactivity is observed with air–CO gas flows.