Effect of surface species on activity and selectivity of MoO3/SiO2 catalysts in partial oxidation of methane to formaldehyde

Abstract

The effect of the nature of surface species on the activity and selectivity of MoO3/SiO2 catalysts has been investigated for the partial oxidation of methane to formaldehyde. Characterization techniques including BET surface area, ambient and in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction were used in conjunction with steady-state reaction studies to relate the presence of different surface species to the activity and selectivity of the catalyst. Results of these experiments indicate the presence of a highly dispersed silicomolybdic species with terminal Mo=O sites appearing at lower MoO3 loadings. As the weight loading increases, these sites are transformed into polymolybdate species, forming more Mo-O-Mo bridging sites at the expense of Mo=O sites. At high weight loadings, crystalline MoO3 begins to form. The abundance of the Mo=O sites is believed to affect activity and selectivity in the partial oxidation of methane to formaldehyde.