Methanol: A “Smart” Chemical Probe Molecule

Abstract

A novel chemisorption method was employed for the dissociative adsorption of methanol to surface methoxy intermediates in order to quantitatively determine the number of surface active sites on one-component metal oxide catalysts (MgO, CaO, SrO, BaO, Y₂O₃, La₂O₃, CeO₂, TiO₂, ZrO₂, HfO₂, V₂O₅, Nb₂O₅, Ta₂O₅, Cr₂O₃, MoO₃, WO₃, Mn₂O₃, Fe₂O₃, Co₃O₄, Rh₂O₃, NiO, PdO, PtO, CuO, Ag₂O, Au₂O₃, ZnO, Al₂O₃, Ga₂O₃, In₂O₃, SiO₂, GeO₂, SnO₂, P₂O₅, Sb₂O₃, Bi₂O₃, SeO₂ and TeO₂). The number of surface active sites for methanol dissociative adsorption corresponds to ∼3 μmol/m² on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product distribution at low conversions reflects the nature of the surface active sites on metal oxides since redox sites yield H₂CO, acidic sites yield CH₃OCH₃ and basic sites yield CO₂. The distribution of the different types of surface active sites was found to vary widely for the different metal oxide catalysts. In addition, the commonality of the surface methoxy intermediate during dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover frequency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10⁻³ to 10⁴ s⁻¹). An inverse relationship (for metal oxide catalysts displaying high (>85%) selectivity to either redox or acidic products) was found between the methanol oxidation TOF values and the decomposition temperatures of the surface M–OCH₃ intermediates reflecting that the decomposition of the surface M–OCH₃ species is the rate-determining step during methanol oxidation over the metal oxide catalysts.