Nature of catalytic active sites for Sb-V-O mixed metal oxides

A series of bulk oxides (Sb₂O₃, Sb₂O ₄, VSbO₄, and V₂O₅) and alumina-supported oxides (Sb-O, V-O, and Sb-V-O) were synthesized by conventional methods. The resulting catalysts were characterized with XRD, BET, Raman, and CH₃OH-temperature programmed surface reaction (TPSR) spectroscopy. XRD and Raman spectroscopy confirmed that the bulk oxides are present as crystalline phases and the alumina-supported oxides consisted of two-dimensional surface metal oxide species on the alumina support. In the case of aluminasupported Sb-V-O, VSbO₄ nanoparticles were formed from the solid-state reaction between the two oxides. CH₃OH-TPSR spectroscopy was employed to determine the number (Ns) and chemical nature of active surface sites (redox and acidic) present in the formed catalysts. The bulk and alumina-supported antimonates were found to contain very few active surface redox and acid sites. In contrast, the bulk and alumina-supported vanadium oxides possess a significant number of surface redox sites with a minor amount of surface acid sites. Catalysts that possess both antimonates and vanadium oxides, however, were found to exhibit enhanced redox activity from promotion of the redox surface V_x sites by the antimonates. The series of bulk and aluminasupported catalysts was examined for their performance for the selective ammoxidation of propane to acrylonitrile. Only the catalysts containing both vanadium oxide and antimonates were found active and selective for acrylonitrile formation. The enhanced performance of these catalysts is related to formation of crystalline VSbO₄ phases that promote the surface vanadia redox sites.