SURFACE MODIFIED NIOBIUM OXIDE CATALYST - SYNTHESIS, CHARACTERIZATION, AND CATALYSIS

The molecular structures and reactivity of surface modified niobium oxide catalysts were determined by Raman spectroscopy, pyridine adsorption as well as the methanol oxidation reaction. Metal oxides (Re2O7, CrO3, WO3, MoO3, and V2O5) and acids (phosphate and sulfate) form a surface overlayer on the niobia support, and possess a structure similar to the metal oxide ionic species present in acidic solutions (ReO4-, Cr3O102-, W12O408-, Mo8O264-, V10O286-, H2PO4-, and HSO4-) when the surface is hydrated. Upon dehydration, only one dehydrated surface ReO4 species is present on niobia. For the CrO3, WO3, MoO3, V2O5, P2O5, and sulfate supported on niobia two dehydrated surface metal oxide species, highly distorted and slightly distorted structures, are present. Acidity and BET studies reveal that the presence of the surface metal oxide species stabilizes the surface acidic properties and surface area of niobia during high calcination temperatures (500-degrees-C). The 1% V2O, and 1% P105 on Nb2O5 samples possess a high concentration of surface acid sites as well as surface area, and the strength of surface Lewis acid sites on 1% P2O5/Nb2O5 is stronger than the other systems. In addition, the surface V2O5, MoO3, CrO3, and Re2O7 sites on niobia behave as redox sites and the surface WO3, P2O5, and sulfate sites on niobia behave as acid sites during methanol oxidation. The weak interaction between the surface rhenia species and the niobia support results in volatilization of surface rhenia during methanol oxidation and a corresponding low activity. Thus, the surface properties of niobia catalysts could be altered by the addition of surface metal oxide overlayer.