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Abstract

The combination of (high-resolution) electron microscopy and electron diffraction was applied to study the structural and morphological alterations of a number of Rh/VO_x-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 VO_x 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 VO_x 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 VO_x 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 “VO_x/Rh/Al₂O₃” catalyst, prepared by depositing 1ML VO_x 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.