Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe2O3–CeO2 composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe2O3–CeO2 composites with an Fe molar percentage below 0.3 form solid solutions with the CeO2 cubic fluorite structure, in which the doped Fe3+ initially substitutes Ce4+ in fluorite cubic CeO2, but then mostly locate in the interstitial sites after a critical concentration of doped Fe3+. With an Fe molar percentage between 0.3 and 0.95, the Fe2O3–CeO2 composites are mixed oxides of the cubic fluorite CeO2 solid solution and the hematite Fe2O3. XPS results indicate that CeO2 is enriched in the surface region of Fe2O3–CeO2 composites. The Fe2O3–CeO2 composites have much higher catalytic activities in CO oxidation than the individual pure CeO2 and Fe2O3, and the Fe0.1Ce0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe2O3–CeO2 composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO2-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.