In situ Raman spectroscopy during the partial oxidation of methane to formaldehyde over supported vanadium oxide catalysts

Vanadia was found to be well dispersed and present as a two-dimensional overlayer when supported on SiO2, TiO2, SnO2, 3 wt% TiO2/SiO2, 3 wt% MoO3/SiO2, and 3 wt% SnO2/SiO2. Partial oxidation of methane by oxygen formed formaldehyde most selectively over the V2O5/SiO2 catalyst, but catalytic performance was strongly dependent on vanadia coverage and autocatalytic behavior was observed. At very low conversions, the formaldehyde activity increased linearly with vanadia coverage, indicating that isolated V5+ species were responsible for the active sites. No significant structural changes were revealed by in situ Raman spectroscopy for the V2O5/SiO2 catalyst, which indicated that the fully oxidized surface sites were related to the high formaldehyde selectivity. This selectivity exhibited a maximum at 1 wt% V2O5 content, and the lower selectivities at higher loadings appeared to be due to the increasing Lewis acidity of the catalysts. Space-time yields of 0.1-1.4 kg CH2O/kg cat/hr and selectivities of 2-78% are reported herein for the V2O5/SiO2 catalysts. Deep oxidation products, CO and CO2, were principally produced over the V2O5/TiO2 and V2O5/SnO2 catalysts. For the first time, in situ Raman analysis clearly showed;that for these latter catalysts the surface vanadium(V) oxide species were partially reduced under the steady-state reaction conditions: The performance of the V2O5/TiO2/SiO2 catalyst was similar to that of the V2O5/TiO2 catalyst, consistent with the earlier observation that vanadia was largely bound to the titania overlayer. It appears that formaldehyde selectivity decreased with increasing catalyst reducibility, but no direct correlation of catalyst activity with reducibility was observed. (C) 1997 Academic Press,Inc.