Isopropanol oxidation by pure metal oxide catalysts: number of active surface sites and turnover frequencies

Abstract

The objective of the present study was to determine the number of active surface sites and their nature, redox or acidic, for bulk metal oxide catalysts using isopropanol as a chemical probe molecule. Isopropanol oxidation activity on the following metal oxides was investigated: MgO, CaO, SrO, BaO, Y 2O 3, La 2O 3, CeO 2, TiO 2, ZrO 2, HfO 2, V 2O 5, Nb 2O 5, Ta 2O 5, Cr 2O 3, MoO 3, WO 3, Mn 2O 3, Fe 2O 3, Co 3O 4, Rh 2O 3, NiO, PdO, PtO, CuO, Ag 2O, Au 2O 3, ZnO, Al 2O 3, Ga 2O 3, In 2O 3, SiO 2, SnO 2 and Bi 2O 3. On average, the number of active surface sites for isopropanol dissociative adsorption on these catalysts was ∼2–4 μmol/m 2. The number of active surface sites enabled quantification of the turnover frequency (TOF) for these catalysts. The TOF values for the various pure metal oxides were normalized at 200 °C. The TOFs of catalysts showing redox activity vary by six-orders of magnitude (10 2 to 10 −4 s −1). For catalyst showing acidic activity, the TOFs varied by over eight-orders of magnitude (10 1 to 10 −7 s −1). The reaction products from isopropanol oxidation at low conversions reflected the nature of the active surface sites, redox or acidic, on these catalysts. Redox surface sites yield acetone and acidic surface sites yield propylene. Small amounts of isopropyl ether formation are sometimes also observed via bimolecular recombination of surface isopropoxide species on acidic surface sites. All catalysts with the exception of Fe 2O 3 and TiO 2, exhibited extremely high selectivity to either redox or acidic products. Except for the sharp decrease in TOFs towards redox products with increasing bulk M–O heats of formation at low −Δ H f, no correlations were found between the TOFs and bulk metal oxide properties (TPR–H 2 and −Δ H f). However, an inverse relation was found between the TOFs (redox) and the surface isopropoxide intermediate decomposition temperature at low decomposition temperatures. At moderate and high decomposition temperatures, the TOFs (redox) were almost independent of the surface isopropoxide decomposition temperature. The selectivity of the metal oxide catalysts was found to be independent of the TOFs.