Preparation And Characterization Of Cu/Al₂O₃/ZnO?0001?-Zn And Cr₂O₃/ZnO?0001?-Zn Model Catalysts

The surface composition and structure of the catalyst?geometric structure and electronic structure?has a significant influence on the catalytic performance.However,the heterogeneous catalytic system is complex,making it difficult to be characreized.Therefore,using well-defined model catalyst can establish a more reliable structure-catalytic performance relationship and offer a better way to understand the reaction mechanism and active sites.Copper based catalyst?Cu/ZnO/Al₂O₃?has been widely employed in the industrial catalyst for converting syngas?H2,CO,CO2?into methanol.Although a topic of intense research,the nature of the active site as well as the reaction mechanism is still under debate.The copper-based catalyst has also been investigated using model surfaces,but most of them were focused on Cu/ZnO binary ones,The absence of Al₂O₃ is partly because of its electrical insulation,restricting the use of surface electron spectroscopies.In this study,the Cu/Al₂O₃/ZnO?0001?-Zn ternary model catalysts were prepared and characterized,as well as the growth models of the binary model ones:Al₂O₃/Zn0?0001?-Zn,Cu/ZnO?0001?-Zn and Cu/Al₂O₃.It was found that Al₂O₃ grew on the Zn0?0001?-Zn surface by a layer-by-layer model,while Cu grew on the Zn0?0001?-Zn surface and Al₂O₃ surfaces by the form of high dispersion at the beginning and then three dimensional clusters.For ternary model catalyst of Cu/Al₂O₃/Zn0?0001?-Zn,Cu was preferred on the interface of Al₂O₃/Zn0?0001?-Zn.The more Al₂O₃ existed,the more hardly Cu2+ was reduced to Cu0,Cu could be covered by a thin ZnOx film during the reduction processes.Therefore,the active sites may consist of Cu+ ions.Cr₂O₃-ZnO catalyst show very high activity and selectivity to light olefins in Fischer Tropsch synthesis.The understanding of the surface composition and the the interaction of Cr₂O₃/ZnO are not clear enough by now.Thus Cr₂O₃ thin films were prepared on the Zn0?0001?-Zn surface,Its redox properties were also investigated.It was found that Cr₂O₃ grows in two dimensional up to 0.4 ML then three dimensional clusters.Cr³⁺is stable and can be partially oxidated to Cr⁶⁺.