SURFACE COORDINATION-NUMBER AND SURFACE REDOX COUPLES ON CATALYST OXIDES, A NEW APPROACH OF THE INTERPRETATION OF ACTIVITY AND SELECTIVITY .1. INTERPRETATION OF OXYGEN THERMAL-DESORPTION SPECTRA

Desorption activation energies, E(d), of the different oxygen species detected in temperature-programmed desorption (TPD) spectra of about twenty catalyst oxides, are explained using the effective coordination numbers n of oxygen atoms whose coordinations are unsaturated at the surface. The E(d) are calculated from n and the bulk formation enthalpies, DELTA(f)H-degrees, taking into account the bulk coordination numbers, c. They can be considered as stabilization energies, E(d) = E(n/c) = DELTA(f)H-degrees X n/c, characteristic of each crystallographic state of surface oxygen ions "O2-". The number of TPD peaks is explained, taking into account that the desorbable species correspond to the lowest n values (n less-than-or-equal-to c/2). In spite of several approximations, a very good agreement is observed between the calculated energies E(n/c), and the experimental E(d) ones, and this approach appears to be highly predictive and efficient in explaining reactivity and selectivity.