SURFACE COORDINATION-NUMBER AND SURFACE REDOX COUPLES ON CATALYST OXIDES, A NEW APPROACH OF THE INTERPRETATION OF ACTIVITY AND SELECTIVITY .2. EXPERIMENTAL AND THEORETICAL BASES

Thermal-desorption and adsorption microcalorimetry data concerning the oxygen adsorption desorption Process on catalyst oxides reveal a "quantification" of oxygen adsorption heats. As discussed previously, this "quantification" implies the surface coordination number n, taking into account the formation enthalpy of the oxide. For symmetrical structures such as NiO, TiO2, Cr2O3, the adsorption heats Q depend mainly on the three parameters, DELTA(f)H-degrees, c the bulk and n the surface coordination numbers of oxygen, so that: Q(n) = DELTA(f)H-degrees X n/c. This simple relationship, which is verified in many cases, is used firstly to define the surface states and the properties of surface sites. Its quantitative exploitation allows the calculation, knowing the enthalpy of formation and the structure of the oxide, of the free enthalpy of adsorption DELTAG of oxygen, i.e. the chemical potential of the surface in each of the discrete states defined by n. The activity depends on the lowest value of n present on the surface. This theoretical approach constitutes a generalization of the classical theory of oxidation-reduction processes. It can be applied to all heterogeneous reactions, and, particularly, it allows one to explain oxygen (or other reactants) adsorption-desorption features. In the case of catalyst oxides, it leads to a better understanding of their activity and selectivity, and to a rationalization of the interpretation of kinetical data.