Surface segregation and phase separation in bismuth–tin pyrochlores

Abstract

The pyrochlore Bi ₂ Sn _2-x Bi _x O _7-x/2 , which is a catalyst for the oxidative coupling of methane, has been studied in the range of composition -0.2≤x≤1.0. When heated in air in the range 200–1000 °C, the surface cation concentration is enriched in bismuth. Combining X-ray photoelectron spectroscopy (XPS) surface analysis and X-ray diffraction (XRD) phase analysis has allowed a distinction to be drawn between mechanisms contributing to the segregation process. Dependant upon the composition and temperature, an underlying thermodynamically driven surface enrichment in bismuth may be supplemented by the nucleation, separation and growth of a discrete bismuth oxide phase.