Methanol Oxidation to Formate on ALD-Prepared VO_x/θ-Al₂O₃ Catalysts: A Mechanistic Study

Well-defined supported VO_x/θ-Al₂O₃ catalysts were prepared by atomic layer deposition (ALD) with vanadium coverages of 0.48, 1.20, and 3.40 wt %. In-situ Raman and UV-vis diffuse reflectance spectroscopy confirm that the monovanadate, VO₄, is the predominant vanadium species at low loadings (0.48 and 1.20 wt %), while polyvanadate VO₄ is the predominant vanadium species for the 3.40 wt % VO_x/θ-Al₂O₃ catalyst. In-situ FTIR spectroscopy of methanol oxidation to formate, in the absence of gas-phase oxygen, on the 0.48 wt % VO_x/θ-Al₂O₃, identified two different formates. A comparison of the frequencies for the formates adsorbed on just V₂O₅ and on just θ-Al₂O₃ demonstrates that one of these formates is located on aluminum sites of VO_x/θ-Al₂O₃ while the other is located on vanadium sites. The oxidation state of vanadium for the VO_x/θ-Al₂O₃ catalyst was determined by XPS after different reaction times. On the basis of the time dependence of the formate absorptions and the change in the oxidation state of vanadium in VO_x/θ-Al₂O₃, a mechanism is proposed for methanol oxidation and we discuss the role of the alumina support in the mechanism.