Reducibility of vanadium oxide species in MCM41

Vanadium-containing MCM-41 materials were prepared by (i) in situ incorporation of V during hydrothermal synthesis (denoted as V-MCM-41-syn), (ii) chemical vapour deposition of VOCl3 on siliceous MCM-41 (V-MCM-41-cvd), and (iii) impregnation of siliceous MCM-41 with vanadyl acetylacetonate (V-MCM-41-imp). A well-ordered hexagonal structure of the materials is revealed from XRD patterns exhibiting four resolved reflections. The fraction of amorphous non-porous by-product obtained from nitrogen adsorption data increases in the sequence: V-MCM-41-imp approximate to 5%<V-MCM-41-cvd approximate to 8%<V-MCM-41-syn approximate to 20%. UV/vis diffuse reflectance spectroscopy (DRS) reveals predominantly mononuclear V-V oxide in fourfold tetrahedral coordination owing to missing absorptions below 30000 cm(-1) in the dehydrated state for all materials. The content of polynuclear V-V oxide species increases in the order: V-MCM-41-syn <V-MCM-41-imp <V-MCM-41-cvd. Reduction by hydrogen to V-red was monitored by in situ DRS and the fraction of reducible V-V decreases in the sequence: V-MCM-41-imp approximate to 100% > V-MCM-41-cvd approximate to 67% >V-MCM-41-syn approximate to 14%. The non-reducible species are assumed to be buried either in the amorphous by-product or inside the walls. Photoelectron spectroscopy reveals that the reducible vanadium oxide species are located in the channels of the MCM-41 structure. The analysis of the reduction kinetics points to a small fraction of polynuclear V-V oxide reduced at a tenfold higher rate. (C) 1998 Elsevier Science B.V. All rights reserved.