CATALYTIC PROPERTIES OF SUPPORTED MOLYBDENUM OXIDE CATALYSTS - IN-SITU RAMAN AND METHANOL OXIDATION STUDIES

The oxidation of methanol was studied over supported molybdenum oxide catalysts as a function of the specific oxide support (TiO2, ZrO2, Nb2 O5, and Al2O3) and molybdenum oxide loading (surface coverage). The su rface molybdenum oxide species were selective for the production of fo rmaldehyde, and the oxide support sites yielded dimethyl ether (alumin a and niobia) and methyl formate (zirconia) or were relatively inactiv e (titania). The turnover frequency (TOF) for the selective oxidation of methanol to formaldehyde varied by a factor of 2-4 with surface mol ybdenum oxide coverage and a factor of approximately 10 with the speci fic oxide support at monolayer coverage. The molecular structures of t he surface molybdenum oxide species (isolated, tetrahedral at low cove rages and polymerized, octahedral/tetrahedral at high coverages) did n ot affect the reaction selectivity but did appear to influence the sli ght increase in TOF with surface coverage. The order of magnitude vari ation in TOF with the specific oxide support correlated with the reduc ibility bf the support and suggests that the Mo-O-support bond is crit ical in controlling the TOF. In situ Raman studies during methanol oxi dation revealed that the supported molybdenum oxide species were 100% dispersed up to monolayer coverage. The percent reduction of the surfa ce molybdenum oxide species, reflected by the decrease in the Raman in tensity of the Mo=O bond, during methanol oxidation was not a strong f unction of surface coverage and the specific oxide support. This sugge sts that the order of magnitude variation in the TOF with the specific oxide support is primarily related to the activity per site of the su rface molybdenum oxide species rather than variation in the number of participating sites.