Structural, acidic and redox properties of V2O5-TiO2-SO42− catalysts

Abstract

Abstract A series of V2O5-TiO2 (VT) catalysts with V2O5 contents from 15 to 75 wt% were prepared by the co-precipitation method and doped with SO42− ions. The structural properties were characterized by X-ray diffraction (XRD), Raman spectroscopy (LRS) and X-ray photoelectron spectroscopy (XPS). The surface acidity was determined by the techniques of NH3 adsorption microcalorimetry and pyridine adsorption infrared spectroscopy (FT-IR). Isopropanol (IPA) and methanol probe reactions in the presence of O2 were employed to provide the information about surface acidity and redox properties simultaneously. The results from XRD and LRS showed that V2O5 was well dispersed on the surface of TiO2 when the vanadia content was below 25 wt%. XPS showed that both vanadium and titanium were present in their fully oxidized states in all the samples. The SO42− doped VT samples had the sulfur oxidation state of +6. The results of NH3 adsorption microcalorimetry and pyridine adsorption FT-IR indicated that the VT catalysts possessed identical surface acid densities independently of the V2O5 content, and that both Bronsted and Lewis acid sites were present on their surfaces. The results of isopropanol probe and methanol oxidation reactions suggested that the surface acidity of VT catalysts was improved upon SO42− doping, as evidenced by the simultaneous decrease in the amounts of oxidation products and increase in the amounts of dehydration products. A high selectivity of 90% to dimethoxymethane (DMM) was measured on the SO42− modified 25%V2O5-75%TiO2 catalyst, with 54% conversion of methanol at 413 K.