Segregation and chemical state of vanadium and molybdenum in vanadium—molybdenum oxide catalyst studied by x-ray photoelectron spectroscopy

Abstract

The vanadium—molybdenum oxide (VMoO) catalyst has been studied by X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis. (TGA). The results show that the surface composition of VMoO catalyst strongly depends on temperature. At room temperature, the catalyst surface is usually enriched with molybdenum. When the sample is heated to above 350–400°C, the vanadium species diffuse from bulk to surface; with a rise in temperature, the ratio of vanadium-to-molybdenum gradually increases and the surface even becomes enriched with vanadium. Besides, when the VMoO catalyst is heated to a certain temperature, the sample can absorb or evolve oxygen according to the ambient oxygen partial pressure. During the redox process, the vanadium species in VMoO catalyst are liable to change their chemical state, the V 4+ and V 5+ sites seem to act as the inlet and outlet sites, respectively of oxygen. The function of molybdenum species in VMoO catalyst seems to be to promote the evolution of lattice oxygen and tend to retain the ratio of V 4+-to-V 5+ at a certain value, which may be important for high selectivity and activity of the VMoO catalyst.