Abstract:

MoOx supported on pure monoclinic zirconia (m-ZrO2) and tetragonal zirconia (t-ZrO2) with different Mo surface densities were prepared and their structures were characterized by X-ray diffraction, Raman spectroscopy and temperature-programmed reduction in H2. At Mo surface densities lower than the theoretical monolayer coverage (~5 nm-2), isolated MoOx and two-dimensional polymolybdates were dominant on the m-ZrO2 surface while MoO3 crystallites were still present on the t-ZrO2 surface indicating a favorable dispersion of MoOx on m-ZrO2. As the Mo surface densities increased to more than monolayer coverage, MoOx tended to form ZrMo2O8 on the ZrO2 surface by a solid reaction at 600 ℃, which was more prevalent on m-ZrO2 than on t-ZrO2. The acidity of the MoOx/ZrO2 catalysts increased with an increase in the Mo surface density indicating that ZrMo2O8 is more acidic than dispersed MoOx. Catalysts prepared using t-ZrO2 resulted in more acidic MoOx/ZrO2 and consequently higher selectivity for dimethyl ether during the selective oxidation of methanol was achieved. The MoOx structures were more reducible on the m-ZrO2 surface leading to improved activity for the oxidation of methanol to formaldehyde, dimethoxymethane and methylformate. An understanding of the effect of ZrO2 crystallite phase on the structures of the MoOx species and their catalytic properties for the selective oxidation of methanol provides useful information for the study of other metal oxides such as VOx, and the design of catalysts that are bifunctional in terms of acidity and redox activity.

Key words: Selective oxidation, Monoclinic zirconia, Tetragonal zirconia, Crystallite phase effect, Molybdena, Methanol