Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation

The use of IR spectroscopic techniques to provide information on the mechanisms of catalytic oxidation over metal oxide catalysts is briefly discussed. The data published on studies of the catalytic oxidation of methanol, of linear C₄ hydrocarbons and of methylaromatics over different metal oxide surfaces are reviewed and discussed. Lattice oxygen appears to act as the active oxygen species in both selective and total oxidation. Generalized mechanisms of these complex oxidation reactions are proposed and the catalyst features affecting selectivities in these reactions are discussed. The reaction network is apparently essentially governed by the organic chemistry of the reacting molecule (thus being substantially the same over the different oxide catalysts). However, the catalyst surface governs the rate of the different steps, favoring some paths over others. Thus, selectivity is determined by the catalyst chemical behavior and by the reaction variables (contact time, temperature, gas-phase composition, presence of steam, etc.). IR studies, if performed under conditions where some intermediates are actually detectable and jointly with other techniques, can give valuable information on the catalysis mechanisms. On the other hand, it has been concluded that in situ studies frequently do not give reliable information on reaction mechanisms, because under reaction conditions spectators rather than intermediates are detected.