Predictions of Relationships between Catalytic and Solid Phase Properties by Kinetic Models and Their Validation

The paper illustrates that realistic predictions about steady-state coverages and stability of surface species can be obtained by kinetic analysis. As an example, oxidative coupling of methane to ethane over CaO/CeO2 catalysts is considered. For this reaction, it was earlier predicted by a kinetic model that the steady-state surface coverage with OH groups, oxygen, and vacant 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 which the incorporation of Ca into the CeO2 host lattice can be demonstrated by pronounced effects on the electrical conductivity, has been characterised by XPS, UPS, ISS, and IR techniques. It was found that bulk and surface properties are interrelated but do not change proportionally. The oxygen anion conductivity increases and a change from n- to p-type conductivity occurs 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 predicted by former kinetic analysis in the oxidation of methane, this sample is capable of forming surface OH groups of exceptional thermal stability.