Abstract: The oxidation of methanol to methyl formate at low temperature (373 K) over the ZrO2-supported RuOx catalyst was studied. The methanol oxidation rate and product selectivity strongly depend on the RuOx structure, which is changed in the Ru surface density range of 0.2[KG-45x]-[KG-20x]3.8 Ru/nm2. The RuOx structure evolves from isolated RuO2-4 species below 0.4 Ru/nm2 to RuO2 clusters above 1.9 Ru/nm2. Such structural evolution with increasing Ru surface density leads to a decrease in the methanol oxidation rate on per Ru atom and on per exposed Ru atom (i.e. turnover rate). This reveals that RuO2-4 species are more reactive than RuO2 species, which is consistent with the higher reducibility of the RuO2-4 structure. These RuO2-4 species are also more selective than the RuO2 species for the methyl formate formation at similar methanol conversion; a ～96% methyl formate selectivity (at ～15% methanol conversion) was attained at 0.2 Ru/nm2 with predominant RuO2-4 structure. These findings suggest that the highly active and selective RuO2-4 structure is required for achieving high methyl formate productivity.

Key words: methanol, selective oxidation, methyl formate, zirconia, ruthenium oxide, supported catalyst, structural effect