Theory of surface chemistry and reactivity of reducible oxides

The increasing availability of computational data from quantum-chemical calculations on the reactivity and electronic structure of catalytically active oxidic systems make a revisitation of the classical questions on chemical bonding aspects of catalytically reactive systems useful. This Perspective paper starts with a short review of the different available systems. The elementary reactions that are part of the selective oxidation catalysis, such as substrate bond activation or oxygen insertion, are introduced. This is followed by a discussion of basic electronic features of the valence electron structure of oxide materials. We will initially compare the chemical bonding and reactivity of compounds with dominantly ionic versus covalent bonding. Surface reactivity descriptors for the two types of chemical bonds are indicated. It will appear that, except for longer range ionic interactions, binding to the metal cations is dominated by local covalent interactions and that ligand field theory type atomic orbital splitting as well as rehybridisation are important. Trends in bonding as a function of position of reducible metal in the periodic system are emphasized throughout the paper. At the surface, in addition to the changes in electronic structure, atom position relaxation and surface reconstruction effects may also have a large influence on the differences in reactivity. The reactivity of the oxygen atoms varies. They can be proton acceptors and generate Brønsted acidity, can assist heterolytic or radical type CH bond activation or when electrophilic will insert into hydrocarbon double bonds. At the end of the paper trends in catalytic reactivity are analyzed. The double volcano type curve found for the dependence of catalytic reactivity on the position of cation in the third row of the periodic system is shown to relate to the distribution of electrons with high spin over the ligand field split atomic orbitals of the respective cations. This will be discussed in the context of classical catalytic results that are based on Balandin type volcano plots of reactivity versus the catalyst material reactivity parameters.