E. Finocchio et al., THE ACTIVATION OF HYDROCARBON C-H BONDS OVER TRANSITION-METAL OXIDE CATALYSTS - A FTIR STUDY OF HYDROCARBON CATALYTIC COMBUSTION OVER MGCR2O4, Journal of catalysis, 151(1), 1995, pp. 204-215
The interaction of light hydrocarbons (methane, ethane, propane, prope
ne, n-butane, isobutane, 1-butene, benzene, and toluene) with the oxid
ized surface of the spinel MgCr2O4 (an active hydrocarbon combustion c
atalyst) has been investigated by FTIR spectroscopy in the temperature
range 300-773 K. This interaction results in the reduction of the cat
alyst 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, ca
rbon oxides) directly adsorbed on the surface. It has been concluded t
hat 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 carb
onyl compounds and/or carboxylate anions and, finally, carbon oxides.
The earlier intermediates (alkoxides, carbonyl compounds) are detectab
le only with the most reactive hydrocarbons (propane, n-butane, isobut
ane, propene, 1-butene, toluene) while with the least reactive hydroca
rbons (methane and benzene), because of their too high activation temp
erature, only the final intermediates are detectable (carbonates, carb
oxylates). Molecular-level mechanisms for C-H cleavage and hydrocarbon
catalytic combustion are proposed. (C) Academic Press, Inc.