THE ACTIVATION OF HYDROCARBON C-H BONDS OVER TRANSITION-METAL OXIDE CATALYSTS - A FTIR STUDY OF HYDROCARBON CATALYTIC COMBUSTION OVER MGCR2O4

The interaction of light hydrocarbons (methane, ethane, propane, propene, n-butane, isobutane, 1-butene, benzene, and toluene) with the oxidized surface of the spinel MgCr2O4 (an active hydrocarbon combustion catalyst) has been investigated by FTIR spectroscopy in the temperature range 300-773 K. This interaction results in the reduction of the catalyst and the production of oxygen-containing adsorbed species. These species have been identified by comparison with the spectra of oxygen-containing species (alcohols, aldehydes, ketones, carboxylic acids, carbon oxides) directly adsorbed on the surface. It has been concluded that every hydrocarbon reacts at its weakest C-H bond on Cr-n+=O (n = 5 or 6) surface sites giving rise by hydrogen abstraction and C-O bond formation to alkoxy groups. These species are further oxidized to carbonyl compounds and/or carboxylate anions and, finally, carbon oxides. The earlier intermediates (alkoxides, carbonyl compounds) are detectable only with the most reactive hydrocarbons (propane, n-butane, isobutane, propene, 1-butene, toluene) while with the least reactive hydrocarbons (methane and benzene), because of their too high activation temperature, only the final intermediates are detectable (carbonates, carboxylates). Molecular-level mechanisms for C-H cleavage and hydrocarbon catalytic combustion are proposed. (C) Academic Press, Inc.