Methanol: a "smart" chemical probe molecule

A novel chemisorption method was employed for the dissociative adsorption o f methanol to surface methoxy intermediates in order to quantitatively dete rmine the number of surface active sites on one-component metal oxide catal ysts (MgO, CaO, SrO, BaO, Y2O3, La2O3, CeO2, TiO2, ZrO2, HfO2, V2O5, Nb2O5, Ta2O5, Cr2O3, MoO3, WO3. Mn2O3, Fe2O3. Co3O4, Rh2O3, NiO, PdO, PtO, CuO, A g2O, Au2O3, ZnO. Al2O3, Ga2O3, In2O3, SiO2, GeO2, SnO2, P2O5, Sb2O3, Bi2O3, SeO2 and TeO2). The number of surface active sites for methanol dissociati ve adsorption corresponds to similar to3 mu mol/m(2) on average for many of the metal oxide catalysts. Furthermore, the methanol oxidation product dis tribution at low conversions reflects the nature of the surface active site s on metal oxides since redox sites yield H2CO, acidic sites yield CH3OCH3 and basic sites yield CO2. The distribution of the different types of surfa ce active sites was found to vary widely for the different metal oxide cata lysts, In addition, the commonality of the surface methoxy intermediate dur ing dissociative chemisorption of methanol and methanol oxidation on oxide catalysts also allows for the quantitative determination of the turnover fr equency (TOF) values. The TOF values for the various metal oxide catalysts were found to vary over seven orders of magnitude (10(-3) to 10(4) s(-1)). An inverse relationship (for metal oxide catalysts displaying high (>85%) s electivity to either redox or acidic products) was found between the methan ol 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 me tal oxide catalysts.