Selective redox catalysis

This article concerns so-called redox reactions in heterogeneous catalysis [J 1 200][J 1 250] , i.e. in the presence of solid catalysts. These are essentially controlled oxidation reactions of organic compounds (hydrocarbons, alcohols, volatile organic compounds, etc.), or inorganic compounds such as SO ₂ , NO, etc. The desired objective of selective controlled oxidation is to obtain oxygenated compounds such as alcohols, aldehydes, anhydrides/acids, etc., while minimizing the formation of by-products including carbon dioxide. Most of these reactions of interest in basic chemistry take place in the presence of oxide catalysts containing transition metals, whose oxidation state changes during the reaction. The usual oxidants are preferably air or, more recently, oxygen, but also hydrogen peroxide or peroxides in the liquid phase. In the case of total oxidation to CO ₂ [J 1 216] , these reactions are used for the depollution or purification of air, exhaust gases from fixed (thermal power plants) or mobile (transport) installations, or aqueous effluents.

Catalysts for the controlled or total oxidation of hydrocarbons or oxygenated compounds, e.g. methanol, are for the most part composed of Me _x O _y metal oxides or oxysalts (molybdates, sulfates, phosphates, etc.), and much more rarely of metals (Ag, Au). Depending on the electronic configuration of Me in Me _x O _y the nature of Me—O bonds is more or less iono-covalent. The degree of ionicity is strongly dependent on the Me ⁿ⁺ cation, which is more or less Lewis acidic compared to the Lewis base O ^2– . It can be seen that oxides with a more ionic bond (perovskites, spinels,...