Rhodium particles supported by thin vanadia films as model systems for catalysis: An electron microscopy study

Abstract

Well-shaped Rh particles grown epitaxially on NaCl surfaces were coated with a 25 nm layer of crystalline vanadium oxide by reactive deposition in 10 −4 mbar oxygen. The Rh/VO x film was subjected to consecutive heat treatments in 1 bar oxygen and in 1 bar hydrogen up to 673 K. The structural and morphological changes were monitored by (high-resolution) transmission electron microscopy, selected area electron diffraction and electron energy-loss spectroscopy and compared to the alterations of a bare vanadium oxide film treated under equal conditions. The stoichiometry and structure of the VO x support depend on the temperature of the NaCl template and on the deposition rate. Low deposition rates and high substrate temperatures favour the generation of a pure V 2O 3 phase, with both V 2O 3 and the Rh particles in epitaxial relation to NaCl (001). A treatment in 1 bar O 2 between 300 and 573 K converts the supporting oxide into mixed V 2O 3, VO 2 and V 2O 5 phases. Oxidation at 673 K induces a complete reconstruction into a single V 2O 5 phase, while an oxygen treatment at 723 K transforms also the Rh particles into (β)Rh 2O 3. Reduction of the bare V 2O 5 film in 1 bar hydrogen yields cubic VO at 673 K, but reduction of the Rh/V 2O 5 film leads to VO formation already at 473 K. Finally, a reduction of vanadia-supported Rh particles at and above 573 K results in the formation of Rh/V alloy structures.