Surface Characterization of La₂O₃−TiO₂ and V₂O₅/La₂O₃−TiO₂ Catalysts

Abstract

The techniques of X-ray diffraction, O2 chemisorption, FT-Raman, and X-ray photoelectron spectroscopy were utilized to characterize La2O3−TiO2 composite oxide and V2O5/La2O3−TiO2 catalyst calcined at 773 and 1073 K temperatures. The investigated La2O3−TiO2 (1:5 mole ratio) mixed oxide was obtained by a homogeneous coprecipitation method with in situ generated ammonium hydroxide. A nominal 12 wt % V2O5 was impregnated over the calcined (773 K) composite oxide by adopting a wet impregnation method from ammonium metavanadate dissolved in aqueous oxalic acid solution. The characterization results suggest that the calcined La2O3−TiO2 mixed oxide primarily consists of a mixture of TiO2 anatase and La−Ti oxides. The La2O3−TiO2 also accommodates a monolayer equivalent of vanadia in a highly dispersed state. The O 1s, Ti 2p, La 3d, and V 2p photoelectron peaks of the V2O5/La2O3−TiO2 sample are highly sensitive to the calcination temperature. The XPS line shapes and the corresponding binding energies indicate that the dispersed vanadium oxide interacts selectively with the lanthana portion of the La2O3−TiO2 mixed oxide and readily forms a LaVO4 compound. The XRD and FT-Raman techniques, in particular, provide direct evidence for the formation of LaVO4 compound. Interestingly, the presence of lanthana in V2O5/La2O3−TiO2 catalysts retards the phase transformation of anatase into rutile under the influence of vanadia.