X-ray Photoelectron Spectroscopy Study of V₂O₅ Dispersion on a Nanosized Al₂O₃₋TiO₂ Mixed Oxide

The Al 2 O 3 −TiO 2 (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 % V 2 O 5 was impregnated on the calcined Al 2 O 3 −TiO 2 support by using an oxalic acid solution of NH 4 VO 3 . To investigate thermal stability of Al 2 O 3 −TiO 2 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 O 2 chemisorption techniques. The physicochemical characterization results revealed that the Al 2 O 3 −TiO 2 mixed oxide is homogeneous and accommodates a monolayer equipment of V 2 O 5 in a highly dispersed state when calcined at 773 K. The Ti/Al atomic ratio as determined by XPS suggests a coverage of Al 2 O 3 by TiO 2 . However, at higher calcination temperatures surface enrichment of alumina occurs due to a concentration gradient. In the case of the V 2 O 5 /Al 2 O 3 −TiO 2 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.