SURFACE AND INTERFACE ANALYSIS Surf. Interface Anal. 2004; 36: 737–740 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.1751 Effect of the support on the surface composition of vanadium phosphate catalysts in the oxidative dehydrogenation of ethane M. P. Casaletto, 1* L. Lisi, 2 G. Mattogno, 3 P. Patrono 4 and G. Ruoppolo 2 1 Institute of Nanostructured Materials, CNR, Via U. La Malfa 153, I-90146 Palermo, Italy 2 Institute for Research on Combustion, CNR, Napoli, Italy 3 Institute of Nanostructured Materials, CNR, Monterotondo (RM), Italy 4 Institute of Inorganic Methodologies, CNR, Monterotondo (RM), Italy Received 6 August 2003; Revised 24 October 2003; Accepted 16 November 2003 Vanadyl phosphate catalysts supported on different oxides (g-Al 2 O 3 , TiO 2 , SiO 2 ) have been investigated by x-ray photoelectron spectroscopy (XPS). The surface chemical composition has been studied as a function of the thermal treatment under oxidizing (calcination) or reaction conditions in the oxidative dehydrogenation of ethane. Dispersion of vanadyl phosphates on g-Al 2 O 3 and TiO 2 results in the formation of vanadium species in different oxidation states, i.e. V 5+ phosphate, V 5+ and V 4+ oxide species, whose relative fraction depends both on the support and on the reaction temperature. A progressive reduction of vanadium species occurs in the samples as an effect of the temperature. Vanadyl phosphates exhibit a stronger interaction with the titania support, which undergoes surface modifications after impregnation, thermal treatments and catalysis runs. A noticeable interaction of the alumina support and a poor dispersion of the active phase on silica are also revealed by XPS. Copyright  2004 John Wiley & Sons, Ltd. KEYWORDS: XPS; vanadium phosphate catalyst; vanadium oxidation state; surface characterization; oxidative dehydrogenation of ethane INTRODUCTION The aim of this paper is to compare the surface chemical composition of vanadyl phosphate catalysts, supported on different oxides, and to investigate the role of the support. As dispersion depends on the nature of the oxide support, the vanadium content in the catalysts has been optimized to obtain highly dispersed vanadium species. Different vanadium-based catalysts have been extensively investigated for the oxidative dehydrogenation of C 2 –C 4 alkanes. 1,2 The presence of both V 4C and V 5C ions is relevant to the catalytic activity, since it promotes the selective oxidation to olefin or to the oxygenated compound. 2,3 TiO 2 -supported vanadyl phosphate catalysts exhibit the best activity in the oxidative dehydrogenation (ODH) of ethane, 4 which depends also on the preparation method. 5  -Al 2 O 3 and, above all, SiO 2 -supported catalysts show poorer catalytic performances, due to the worse dispersion of the active phase. 6,7 Differences in the catalytic behaviour are generally explained on the basis of the nature and distribution of vanadium species, which are influenced by the vanadium L Correspondence to: M. P. Casaletto, Institute of Nanostructured Materials, CNR, Via U. La Malfa 153, I-90146 Palermo, Italy. E-mail: casaletto@pa.ismn.cnr.it Contract/grant sponsor: National Research Council (CNR), Italy. Contract/grant sponsor: Ministry of University and Scientific Research (MIUR), Italy. loading and the acid–base character of the support. Upon adsorption on TiO 2 and Al 2 O 3 , a homogeneous distribution of highly dispersed species is obtained, as shown by XRD detecting only the signals of the supports. 8 A good dispersion of the active phase is inhibited on SiO 2 by the strong surface acidity of silica. Segregation of bulk-like VOPO 4 aggregates occurred on silica-based samples, probably by polymerization on the acidic surface sites of the support, as shown by XRD. 8 EXPERIMENTAL Vanadyl phosphate dihydrate: VOPO 4 Ð2H 2 O (hereafter indi- cated as VOP) was prepared by refluxing V 2 O 5 in an aqueous solution of H 3 PO 4 (85 wt.%) for 16 h, according to Ladwig’s method. 9 Pure  -Al 2 O 3 (CK-300 Akzo), anatase TiO 2 (Euroti- tania) and amorphous SiO 2 (Sigma) were used as support for the active phase (specific surface area D 190, 125 and 200 m 2 g 1 , respectively). VOP catalysts were prepared by wet impregnation of the support with different amounts of the active phase, corresponding to the theoretical mono- layer coverage (14, 9.6 and 14 wt%, respectively), in aqueous solution. After drying at T D 80 ° C, samples were calcined in flow- ing air at different temperatures (T D 450, 550 and 650 ° C) for 3 h. Catalytic tests in the oxidative dehydrogenation of ethane were carried out in a continuous flow laboratory plant, Copyright  2004 John Wiley & Sons, Ltd.