V-51 and H-1 solid-state MAS NMR studies of vanadia catalysts supported onAl2O3-TiO2 sol-gel mixed oxide

A series of vanadia catalysts supported on sol-gel derived mixed oxide Al2O 3-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 catal ysts V-51 NMR studies indicated the peaks corresponding isolated and distor ted tetrahedral vanadia species at low V2O5 contents and octahedral vanadia species at high vanadia loadings. Upon outgassing the catalysts, a decreas e in the intensity of the peak corresponding to distorted tetrahedral speci es was observed with a consequent increase in the intensity of the peak cor responding to isolated tetrahedral vanadate species due to the removal of w ater molecules or hydroxyl groups from the coordination sphere of vanadium. The H-1 MAS NMR studies showed the presence of terminal and bridged hydrox yl 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 con tents in the DRIFT spectra of the catalysts. (C) 1999 Elsevier Science B.V. All rights reserved.