C-H bond activation in hydrocarbon oxidation on solid catalysts

The activation of C-H bonds in saturated hydrocarbons is the crucial step in several different types of oxidation reaction on a variety of catalysts 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 of oxidation will depend on the metal, the temperature, the oxygen partial pressure, the metal particle size, the support and the choice of hydrocarbon. The importance of coadsorbates in facilitating the dissociative adsorption of saturated hydrocarbons seems to be well established and could explain the unusual enhancement of activity observed for the 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 equilibrium involving the formation of surface hydroxide ions which are considered to be inactive for activation of C-H bonds. As a consequence, it is possible, especially at low temperatures, that the rate determining step in C-H bond activation could be the regeneration of the active sites, through desorption of H2O, rather than C-H bond activation as is commonly assumed. The activation of C-H bonds by NO2 is addressed in the context of selective reduction of NOx by hydrocarbons on various types of oxide catalyst and possible similarities with the promotion of the C-H bond activation process by SO2 are discussed.