首页|Part I.The morphology,composition and structure of vanadia-supported and -promoted Rh particles upon oxidation and reduction:Structure-activity correlations in thin film model catalysts:CO hydrogenation on Rh/VO_x
Part I.The morphology,composition and structure of vanadia-supported and -promoted Rh particles upon oxidation and reduction:Structure-activity correlations in thin film model catalysts:CO hydrogenation on Rh/VO_x
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NSTL
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 (0.07-25 nm),prepared by reactive deposition of V metal in 10~(-2) Pa O_2.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 O_2 for 1 h and to subsequent reduction in the temperature range 373-873 K.While higher VO_x exposures (mean VO_x coverage >= 3 nm) favour the formation of crystalline V_2O_3 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 O_2 at 673 K the oxidation states of vanadium vary between V~(5+) and V~(2+),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 (V_3Rh_5,Rh_3V,V_3Rh and VRh,respectively) was observed.On a "VO_x/Rh/Al_2O_3" 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.
Metal-support interactionRhodiumVanadiumAlloy formationElectron microscopySelected area electron diffraction
Simon Penner、Bernd Jenewein、Di Wang、Robert Schlogl、Konrad Hayek
NSTL
