CHARACTERIZATION OF SILICA-SUPPORTED AND ALUMINA-SUPPORTED VANADIA CATALYSTS USING TEMPERATURE-PROGRAMMED REDUCTION

The nature of the vanadia-support interaction for silica and alumina-s upported V2O5 catalysts was investigated using temperature programmed reduction (TPR), temperature programmed oxidation, and solid-state V-5 1 NMR. Solid-state V-51 NMR for the V2O5/SiO2 catalysts indicated the presence of microcrystalline bulk-like vanadia species even at low van adia loadings. Temperature programmed reduction of V2O5/SiO2 exhibited multiple peaks. It is suggested that the low temperature peak is due to reduction of surface vanadia. This appears to be the case also for bulk V2O5. V-51 NMR indicated that bulk-like vanadia species are prese nt for V2O5/Al2O3 catalysts only at high vanadia loadings. Vanadia was more highly dispersed on alumina than on silica as evidenced by NMR a nd TPR. The two lowest temperature TPR peaks appear to be related to t he reduction of surface vanadia on V2O5/Al2O3. It was found that for V 2O5/Al2O3 the average oxidation state of V after reduction to 900-degr ees-C is consistent with the stoichiometry V+5 --> V+4, whereas the V2 O5/SiO2 catalysts exhibited 70% reduction of the V2O5 to V2O3 as did b ulk V2O5. The amount of surface vanadium as determined by TPR correlat es reasonably to the amount of tetrahedral V found by NMR. It is concl uded that TPR provides an excellent means by which vanadia dispersion can be estimated on supported vanadia catalysts. (C) 1994 Academic Pre ss, Inc.