Surface Aspects of Bismuth–Metal Oxide Catalysts

Abstract

A series of conventional and model bismuth–metal oxidecatalysts (vanadates, molybdates, tungstates, and niobates) werephysically and chemically characterzied (Raman spectroscopy,BET, XPS, and methanol oxidation) to obtain additional insightsinto the structure-reactivity relationships of such catalyticmaterials. The reactivity for methanol oxidation over theconventional bismuth–metal oxide catalysts was found to beprimarily related to the surface area of the oxide catalysts andwas essentially independent of the near surface composition andthe bulk structure. The selectivity for methanol oxidation overthe conventional bismuth–metal oxide catalysts was essentiallyfound not to be a function of the surface area, the near surfacecomposition, and the bulk structure. A series of modelbismuth–metal oxide catalysts was synthesized by depositingmetal oxides on the surface of a bismuth oxide support. Themodel studies demonstrated that two-dimensional metal oxideoverlayers are not stable on the bismuth oxide support andreadily react to form bulk bismuth–metal oxide compounds uponheating. Furthermore, the model studies revealed that these bulkbismuth–metal oxide compounds are related to the active sitesfor the partial oxidation reaction.In situRamanspectroscopy in methanol/oxygen, methanol, and oxygen reactionenvironments with helium as the diluent revealed no additionalinformation regarding the nature of the active site. It wasfound that only highly crystalline bismuth–metal oxide phasesare selective for the partial oxidation of methanol toformaldehyde. Thus, selective bismuth–metal oxide catalystswill always possess highly crystalline metal oxide phasescontaining extremely low surface areas which make it difficultto obtain fundamental surface information about the outermost layers.