EFFECT OF SURFACE SPECIES ON ACTIVITY AND SELECTIVITY OF MOO3 SIO2 CATALYSTS IN PARTIAL OXIDATION OF METHANE TO FORMALDEHYDE/

The effect of the nature of surface species on the activity and select ivity Of MoO3/SiO2 catalysts has been investigated for the partial oxi dation of methane to formaldehyde. Characterization techniques includi ng BET surface area, ambient and in situ Raman spectroscopy, X-ray pho toelectron spectroscopy, and temperature-programmed reduction were use d in conjunction with steady-state reaction studies to relate the pres ence of different surface species to the activity and selectivity of t he catalyst. Results of these experiments indicate the presence of a h ighly dispersed silicomolybdic species with terminal Mo=O sites appear ing at lower MoO3 loadings. As the weight loading increases, these sit es are transformed into polymolybdate species, forming more Mo-O-Mo br idging sites at the expense of Mo=O sites. At high weight loadings, cr ystalline MoO3 begins to form. The abundance of the Mo=O sites is beli eved to affect activity and selectivity in the partial oxidation of me thane to formaldehyde.