Abstract
The nature of the vanadia-support interaction for silica and alumina-supported V2O5 catalysts was investigated using temperature programmed reduction (TPR), temperature programmed oxidation, and solid-state 51V NMR. Solid-state 51V NMR for the V2O5/SiO2 catalysts indicated the presence of microcrystalline bulk-like vanadia species even at low vanadia 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. 51V NMR indicated that bulk-like vanadia species are present for V2O5/Al2O3 catalysts only at high vanadia loadings. Vanadia was more highly dispersed on alumina than on silica as evidenced by NMR and TPR. The two lowest temperature TPR peaks appear to be related to the reduction of surface vanadia on V2O5/Al2O3. It was found that for V2O5/Al2O3 the average oxidation state of V after reduction to 900°C is consistent with the stoichiometry V+5 → V+4, whereas the V2O5/SiO2 catalysts exhibited 70% reduction of the V2O5 to V2O3 as did bulk V2O5. The amount of surface vanadium as determined by TPR correlates reasonably to the amount of tetrahedral V found by NMR. It is concluded that TPR provides an excellent means by which vanadia dispersion can be estimated on supported vanadia catalysts.
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