MOLECULAR-STRUCTURES OF SUPPORTED NIOBIUM OXIDE CATALYSTS UNDER AMBIENT CONDITIONS

The supported niobium oxide catalysts were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy in order to determine the molecular structure and monolayer coverage of the surface niobium oxide phase on oxide supports (MgO, Al2O3, TiO2, ZrO2, and SiO2). The molecular structures of the surface niobium oxide phases present in the supported niobium oxide catalysts under ambient conditions, where adsorbed moisture is present, are controlled by the surface pH of the system. Basic surfaces result in the formation of highly distorted NbO6 groups and acidic surfaces result in the formation of slightly distorted NbO6, NbO7 and NbO8 groups. The surface niobium oxide overlayer is stable to high calcination temperatures due to the strong surface niobium oxide-support interaction. The monolayer coverage of supported niobium oxide catalysts is reached at approximately 19 wt.% Nb2O5/Al2O3, approximately 7 wt.% Nb2O5/TiO2, approximately 5 wt.% Nb2O5/ZrO2 and approximately 2 wt.% Nb2O5/SiO2, but not for the Nb2O5/MgO system due to the incorporation of Nb+5 into the MgO support.