Predictions of relationships between catalytic and solid phase properties by kinetic models and their validation

The paper illustrates that realistic predictions about steady-state coverag es and stability of surface species can be obtained by kinetic analysis. As an example, oxidative coupling of methane to ethane over CaO/CeO2 catalyst s is considered. For this reaction, it was earlier predicted by a kinetic m odel that the steady-state surface coverage with OH groups, oxygen, and vac ant sites depends on the CaO content of the mixed oxide catalysts. Now, the surface of solid solutions of CaO in CeO2 (5, 10, and 20 mole % CaO), in w hich the incorporation of Ca into the CeO2 host lattice can be demonstrated by pronounced effects on the electrical conductivity, has been characteris ed by XPS, UPS, ISS, and IR techniques. It was found that bulk and surface properties are interrelated but do not change proportionally. The oxygen an ion conductivity increases and a change from n- to p-type conductivity occu rs with increasing CaO-content. The external surface of the mixed oxides is strongly enriched in CaO, which forms CaCO3 under ambient conditions, but the degree of surface enrichment exhibits a marked minimum at 10% CaO. As p redicted by former kinetic analysis in the oxidation of methane, this sampl e is capable of forming surface OH groups of exceptional thermal stability. (C) 2001 Academic Press.