Vanadia and tungsta grafted TiO2: Influence of the grafting sequence on structural and chemical properties

Ternary V2O5–WO3/TiO2 catalysts were prepared by sequential or simultaneous grafting steps of vanadia and tungsta onto titania and were compared with similarly prepared binary catalysts. Different grafting sequences including alternating grafting of vanadia and tungsta were compared. During the grafting process the metal oxide precursor reacted with hydroxyl groups of the other grafted metal oxide. The interaction of vanadia with tungsta species resulted in a different reduction behavior of both metal oxides compared to the binary catalysts as indicated by temperature-programmed reduction with hydrogen. The strength of interaction of the grafted species depended on preparation sequence and metal loading. At low coverage (‹monolayer) catalyst properties were found to depend strongly on loading, but relatively little on the grafting mode, as indicated by vibrational spectroscopy. Laser Raman experiments at different laser power revealed reversible effects due to temperature induced structural changes of surface vanadia species. For all catalysts, even at a loading of more than one and a half monolayers, no evidence of crystalline vanadia or tungsta could be found. After calcination of WO3/TiO2 catalyst at 1023 K instead of 573 K and subsequent grafting with vanadia, new species with hydroxyl groups showing a vibrational frequency below 3600 cm−1 were formed. The increase of the calcination temperature had no significant influence on the reduction of vanadia by hydrogen