Structural, microstructural and surface properties of a specific CeO2–Bi2O3 multiphase system obtained at 600 °C

Polycrystalline samples of (1−x) CeO2−x/2 Bi2O3 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 Bi2O3 metastable structural varieties, are observed. However, the crystal cell volumes of these β′ or β Bi2O3 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–CH4 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 CH4 and CO gases into the CO2 gas. The transformations of CH4 and CO molecules as a function of time and temperature are determined through the intensities of FTIR CO2 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–CH4 gas flows, while, for the highest bismuth compositions, a high catalytic reactivity is observed with air–CO gas flows.

Graphical abstractCatalytic efficiencies of CeO2–Bi2O3system: catalytic actions on methane (on the left) or carbon monoxide (on the right) of (1−x)CeO2−x/2 Bi2O3 samples, as a function of the fraction x, and for fixed temperatures: on the vertical axis, the intensities of CO2 FTIR absorption bands are reported. Strong efficiency of bismuth rich samples for CO conversion.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Stabilization of metastable polymorph Bi2O3 phases in the mix system [(1−x)CeO2+(x/2)Bi2O3] at 600 °C. ► Solid gas interactions between this system and air–CO or air–CH4 gas flows at various temperatures and bismuth compositions. ► High efficiency of bismuth rich samples to convert CO into CO2.