Molecular Structures and Reactivity of Supported Molybdenum Oxide Catalysts

Abstract

Supported molybdenum oxide catalysts were prepared by an equilibrium adsorption method. The molecular structures of the molybdenum oxide overlayers on different oxide supports (Al 2O 3, TiO 2, ZrO 2, SiO 2, and MgO), under in situ conditions, were investigated by Raman spectroscopy. The molybdenum oxide species on TiO 2, ZrO 2, and Al 2O 3 possess a highly distorted, octahedrally coordinated surface molybdenum oxide species with one short MoO bond regardless of the molybdenum oxide content. The MoO 3/SiO 2, catalysts primarily contain crystalline MoO 3 because of the lower density and reactivity of the silica surface OH groups. The MoO 3/MgO catalysts possess MgMoO 4 and CaMoO 4 compounds due to the high aqueous solubility of MgO and CaO (an impurity in the MgO support) and the strong acid-base interaction between molybdenum oxide and MgO/CaO. The methanol oxidation reaction studies revealed that the MoO 3/TiO 2 (anatase and rutile) and MoO 3/ZrO 2 catalysts are the most active catalysts and that their activities (TOFs) are at least 1-2 orders of magnitude higher than those of the MoO 3/Al 2O 3, MoO 3SiO 2, and MoO 3/MgO catalysts. It was also found that the catalytic activities correlate with the reducibility of the surface molybdenum oxide species on the oxide supports as well as their surface morphology (e.g., molybdenum oxide dispersion and molybdate compound formation). These studies demonstrate that the specific oxide support controls the reactivity of the supported molybdenum oxide phases.