X-ray photoelectron spectroscopy study of V2O5 dispersion on a nanosized Al2O3-TiO2 mixed oxide

The Al2O3-TiO2 (1:1.3 mole ratio) was obtained from dilute mixture solutions of sodium aluminate and titanium tetrachloride by hydrolysis with in situ generated ammonium hydroxide. The calcined (773 K) mixed oxide powder was constituted from nanosized anatase crystallites and amorphous alumina. A nominal 16 wt % V2O5 was impregnated on the calcined Al2O3-TiO2 support by using an oxalic acid solution of NH4VO3. To investigate thermal stability of Al2O3-TiO2 and the dispersion of vanadia on its surface these samples were subjected to thermal treatments from 773 to 1073 K and were examined by X-ray photoelectron spectroscopy, X-ray diffraction, FT-infrared, and Oz chemisorption techniques. The physicochemical characterization results revealed that the Al2O3-TiO2 mixed oxide is homogeneous and accommodates a monolayer equipment of V2O5 in a highly dispersed state when calcined at 773 K. The Ti/Al atomic ratio as determined by XPS suggests a coverage of Al2O3 by TiO2. However, at higher calcination temperatures surface enrichment of alumina occurs due to a concentration gradient. In the case of the V2O5/Al2O3-TiO2 sample, an increase of calcination temperature also resulted in the decrease of specific surface area and the dispersion of vanadium oxide. The impregnated vanadium oxide also exhibited a noticeable influence on the phase transformation of titania anatase. The V/Ti and V/Al atomic ratios revealed that vanadium oxide is distributed equally on both tiania and alumina surfaces when calcined at 773 K; however, surface segregation of vanadium oxide occurred on the titania surface at higher calcination temperatures.