C-H BOND ACTIVATION IN HYDROCARBON OXIDATION ON SOLID CATALYSTS

The activation of C-H bonds in saturated hydrocarbons is the crucial s tep in several different types of oxidation reaction on a variety of c atalysts and could occur through homolytic or heterolytic mechanisms. From the available evidence it appears that the heterolytic mechanism, with the extraction of a proton, is the most likely process on oxide catalysts and this may also apply to metallic catalysts under typical (oxidising) reaction conditions. However, the state of oxidation of a metallic surface under reaction conditions is complex and the degree o f oxidation will depend on the metal, the temperature, the oxygen part ial pressure, the metal particle size, the support and the choice of h ydrocarbon. The importance of coadsorbates in facilitating the dissoci ative adsorption of saturated hydrocarbons seems to be well establishe d and could explain the unusual enhancement of activity observed for t he oxidation of some saturated hydrocarbons when inorganic gases, such as SO2, are added to the reaction mixture. The inhibition by products of the oxidation reaction (CO2 and H2O) can be quite severe, but H2O has by far the greatest effect. This is interpreted in terms of an equ ilibrium involving the formation of surface hydroxide ions which are c onsidered to be inactive for activation of C-H bonds. As a consequence , it is possible, especially at low temperatures, that the rate determ ining step in C-H bond activation could be the regeneration of the act ive sites, through desorption of H2O, rather than C-H bond activation as is commonly assumed. The activation of C-H bonds by NO2 is addresse d in the context of selective reduction of NOx by hydrocarbons on vari ous types of oxide catalyst and possible similarities with the promoti on of the C-H bond activation process by SO2 are discussed.