Structural characterization of vanadium oxide catalysts supported on nanostructured silica SBA-15 using X-ray absorption spectroscopy

Mixed transitions metal oxides (e.g. MoVNbTe oxides) are active in selective oxidation of propane to acrylic acid. In contrast to various binary oxides (e.g. MoO3 or V2O5), these mixed oxides exhibit a much higher selectivity. However, the origin of the promoting effect of, for instance, vanadium in mixed oxides is largely unknown. Hence, model systems are sought which enable conclusions on structure activity relationships of individual metal centers in active catalysts. For that, supported metal oxides possess two major advantages over bulk oxides. First, particular metal oxide structures which are not readily available for investigations under reaction conditions can be stabilized and studied on suitable support materials [1]. Second, dispersed supported metal oxides simplify correlating the local structure around the metal centers with their catalytic performance. Distinguishing active metal centers at the surface from metal centers in the bulk of conventional oxide catalysts is no longer required.VxOy supported on SBA-15 (nanostructured SiO2) [2] constitutes a suitable model system to investigate the role of vanadium during selective oxidation catalysis [3-6]. Structural characterization of VxOy supported on SiO2 has been subject of many spectroscopic studies including IR [[7-9], XPS [6,10,11], Raman [10-15], UV-VIS [11,13-16] and EXAFS [13,17-22]. A recent review of spectroscopic investigations and structural characteristics of various supported vanadium oxides has been presented by Weckhuysen and Keller [23]. It is assumed, that the structure of supported vanadium oxide depends on both amount of vanadium and degree of hydration [14]. Hence, most studies were performed on VxOy/SiO2 samples exhibiting low vanadium loading (< 10 wt %). At these loadings a monolayer of supported VxOy species is assumed and crystalline V2O5 is not detectable. Under ambient conditions the structure of hydrated vanadium oxide supported on SiO2 resembles that of V2O5 [10,13,18]. Therm