Metal-Support Interactions in Co/Al_2O_3 Catalysts: A Comparative Study on Reactivity of Support

摘要

Co/Al_2O_3 catalyst have been prepared with coventioanl impregnation and sol-gen methods to very the chemical reactivity of the alumina support. The material system has been investigated with X-ray diffraction (XRD), Fourier transfrom infrared spectroscopy(FTIR), diffuse reflectance Fourier transfrom spectroscopy (DRIFT), Brunauer-Emmett-Teller(BET) method, X-ray photoelectron spectroscopy(XPS), temperature-programmed reduction (TPR)(with thermogravimetric analysis (TGA) and differential thermal analysis (DTA)), and gas chromotography (GC) for the catalytic oxidation of CO. It had been found that the reactivity of the support changes the surface structure and chemical composition of catalysts significantly. When the metal-support interaction is weak, Co_3O_4 is a predominant surface phase (which is interfaced by a “cobalt surface phase”). With an increase in support reactivity, CoO and CoAl_2O_4 are found to be present on the surface. The cobalt content on the surface decreases upon the increase in support reactivity and surface area. At the high atomic ratio of Al/Co, there are more=Al-O-H bonds that show higher basicity. Reducibility of the phases observed decreases on the order of Co_3O_4, CoO, and CoAl_2O_4. In is found that the removal of CO on metallic sites is much easier in the catalyst with lower metal-support interactions. At room temperature, reactions between the adsobed CO and lattice oxygen have been observed in all samples. The catalytic activity for the CO combustion reaction increases with the decreases in metal-support interactions of the catalysts. Possible causes for the above observations have been addressed.