Quantitative Determination of the Number of Surface Active Sites and the Turnover Frequencies for Methanol Oxidation over Metal Oxide Catalysts: Application to Bulk Metal Molybdates and Pure Metal Oxide Catalysts

Abstract

The present work investigates the number and nature of the surface active sites and the catalytic activity of bulk metal molybdates and bulk metal oxides on methanol selective oxidation. Bulk metal molybdates were synthesized by coprecipitation and characterized by laser Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and specific surface area analysis. The number of surface active sites (Ns) was determined by measuring the amount of methoxy species produced by methanol chemisorption on the catalysts at 100°C. The specific activity values (TOFs) were calculated by normalizing the reaction rate by the number of surface active sites. The significant differences in catalytic behavior of bulk metal molybdates and bulk metal oxides, and the surface molybdenum enrichment observed on metal molybdates, gave evidences that the surface of bulk metal molybdates may be composed only by molybdenum oxide species in a two-dimensional overlayer. It was not possible to establish the structure of surface molybdenum oxide species; however, it can be concluded that the structure of surface active sites species of bulk metal molybdates and monolayer supported oxide catalysts are different according to the dispersion of the Ns values of these systems. This work shows, for the first time in the literature, that bulk metal molybdates and monolayer supported molybdenum oxide catalysts possess similar activity and TOF in methanol selective oxidation. The number of active surface sites and the specific activity toward selective oxidation products (TOF redox), CO2 (TOF basic), and dimethyl ether (TOF acid) of bulk metal oxides were also determined. In contrast to bulk metal molybdates, most bulk metal oxides catalyze the total combustion of methanol even at low methanol conversion.