Structural and redox properties of VOx and Pd/VOx thin film model catalysts studied by TEM and SAED

Abstract

The oxidation of pure V₂O₃ and Pd/V₂O₃ films was studied by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) in the temperature range 673–773 K. Thin films of V₂O₃ were prepared by reactive deposition of V metal in 10⁻² Pa O₂ on NaCl(001) cleavage faces. Pd particles were epitaxially grown on NaCl(001) and subsequently embedded in V₂O₃. Oxidation of both pure V₂O₃ and Pd/V₂O₃ at 673 K transforms V₂O₃ into a platelet-like V₂O₅ structure. At temperatures T ≥ 773 K, a reconstruction of the platelet-like V₂O₅ structure into an array of oblong and needle-type V₂O₅ nanocrystals of different size occurs. Subsequent reduction of the so-prepared structures in 1 bar H₂ at 573–673 K results in the formation of the cubic VO phase, whereby the external shape of the original crystals is partially maintained. Upon oxidation at 723 K, Pd is transformed into PdO, but its formation is suppressed in comparison with Pd supported on Al₂O₃ and occurs only at an about 100 K higher temperature than on Pd/Al₂O₃. The Pd particles are stabilized against oxidation up to 673 K, PdO decomposes upon reduction in hydrogen between 573 and 673 K.