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 (RexO,, CrOx, WOs, Moos, and V206) 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 (ReO, , Cr,Os , WixOg, MosO& , V1,,Oir, H2P0c, and HSO; ) when the surface is hydrated. Upon dehydration, only one dehydrated surface ReO, species is present on niobia. For the CrOB, WOs, Moos, V205, PsOx, 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°C). The 1% V205 and 1% P,Os on NbxO, samples possess a high concentration of surface acid sites as well as surface area, and the strength of surface Lewis acid sites on 1% P,0s/Nb206 is stronger than the other systems. In addition, the surface VzOs, MOO,, CrOs, and Re,O, sites on niobia behave as redox sites and the surface WOa, PzOs, 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.