51V and 1H solid-state MAS NMR studies of vanadia catalysts supported on Al2O3-TiO2 sol-gel mixed oxide

A series of vanadia catalysts supported on sol-gel derived mixed oxide Al2O3-TiO2 were synthesized with a V2O5 content varying from 1 to 25 wt.%. The catalysts were characterized by employing V-51, Al-27, H-1 MAS NMR, diffuse reflectance FT-IR and BET surface area measurements. In the calcined catalysts V-51 NMR studies indicated the peaks corresponding isolated and distorted tetrahedral vanadia species at low V2O5 contents and octahedral vanadia species at high vanadia loadings. Upon outgassing the catalysts, a decrease in the intensity of the peak corresponding to distorted tetrahedral species was observed with a consequent increase in the intensity of the peak corresponding to isolated tetrahedral vanadate species due to the removal of water molecules or hydroxyl groups from the coordination sphere of vanadium. The H-1 MAS NMR studies showed the presence of terminal and bridged hydroxyl groups of alumina and titania in the Al2O3-TiO2 mixed oxide support. The overtone bands corresponding to V = O were seen only at higher vanadia contents in the DRIFT spectra of the catalysts. (C) 1999 Elsevier Science B.V. All rights reserved.