ABSTRACT
Metal oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or as supports. Metal oxides are used for both their acid-base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis [1-6]. The three key features of metal oxides, which are essential for their application in catalysis, are (i) coordination environment of the surface atoms, (ii) redox properties of the oxide, and (iii) oxidation state of the surface. Surface coordination environment can be controlled by the choice of crystal plane exposed and by the preparation procedures employed; however, specification of redox properties is largely a matter of choice of the oxide. The majority of oxide catalysts correspond to more or less complex transition metal oxides containing cations of variable oxidation state. These cations introduce redox properties and, in addition, acid-base properties. The acid-base properties of the oxides are usually interrelated to their redox behavior. Many attempts were made
“DK3029_C008” — #2
in the literature to find correlations between acid-base and redox characteristics, and the catalytic properties. It is obvious that both characteristics are not independent since cations are Lewis acids while lattice oxygen anions are basic and hydroxyl groups could be either acidic (Brønsted site) or basic. The acidic character of the cations depends on their positive charge and size while the basic character of the lattice oxygen anions depends on the ionic character of the metal-oxygen bonds [1,5,6].
