Structure–activity correlations in thin film model catalysts: CO hydrogenation on Rh/VOx Part I. The morphology, composition and structure of vanadia-supported and -promoted Rh particles upon oxidation and reduction

The combination of (high-resolution) electron microscopy and electron diffraction was applied to study the structural and morphological alterations of a number of Rh/VOx-model systems upon oxidation and reduction, and to discriminate between different phenomena of metal–support interaction. Well-defined Rh particles (mean size 10–15 nm) were grown epitaxially on NaCl(001) surfaces and subsequently covered by layers of VOx of varying thickness (0.07–2 nm), prepared by reactive deposition of V metal in 10−2 Pa O2. Most films were covered with a stabilizing layer of amorphous alumina. The resulting model catalysts were subjected to an oxidative treatment at 673 K in O2 for 1 h and to subsequent reduction in the temperature range 373–873 K. While higher VOx exposures (mean VOx coverage ≥ 3 nm) favour the formation of crystalline V2O3 phases in partial epitaxial orientation to the Rh particles in the as-deposited state, lower exposures result in less ordered layers of cubic VO. Similarly, after a treatment in 1 bar O2 at 673 K the oxidation states of vanadium vary between V5+ and V2+, depending on the film thickness. Decoration of Rh by reduced VOx species was found to be the dominant feature of metal–support interaction upon reduction at low temperatures (T < 573 K), whereas at increasing reduction temperature the formation of distinct Rh–V alloys (V3Rh5, Rh3V, V3Rh and VRh, respectively) was observed. On a “VOx/Rh/Al2O3” catalyst, prepared by depositing 1ML VOx prior to Rh deposition alloy formation was not detected, and decoration of the metal particles was the dominant effect of reduction at 673 K. A counterpart to Rh/V “subsurface” or “surface” alloys, known to be formed on bulk Rh surfaces under similar conditions, could not be observed