Studies About Monolayer Dispersion And Structure Of Supported Oxide

Supported oxides have been widely used as catalysts and other materials in modern industries. Oxides can spontaneously disperse on surface of the support, which forms monolayer or sub-monolayer dispersion structure, according to Prof. Xie's and Tang's model. The maximal dispersion capacity is called dispersion threshold. Properties of a dispersed oxide are different from those of their crystal forms, determined by the dispersion structure. In developing novel catalysts or other materials, it's important to research into the dispersion structures. The purpose of this dissertation is to get more knowledge about the essence of monolayer dispersion by researching into the two-component monolayer dispersion systems.Several supported structures, including CuO/anatase, cobalt oxide/ anatase, cobalt oxide/γ-alumina, MoO₃/anatase and MoO₃/α-alumina have been characterized mainly by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and extended x-ray absorption fine structure (EXAFS). Experimental results showed the three oxides all formed monolayer dispersion states on their corresponding supports. Thedispersion thresholds were quantified by analyzing the experimental results. The coordination structures of metal centers in the monolayer dispersion states have been obtained by fitting the EXAFS data using corresponding pure crystal oxide as reference. The EXAFS analyzing tool was UWXAFS software kit.CuO/anatase samples were prepared by impregnation method and calcined at 400°C. Results showed that CuO's dispersion threshold is 7.2mgCuO/gTiO2 or 5.6Cu atoms/nm2 T1O2. When CuO amount is very lower than the threshold, Cu2+ existed as octahedral [CuO6]. The tetrahedrally coordinated Cu2+ ions appeared and amount of the [CuO4] unit increased with CuO loading until dispersion threshold. After achieving the dispersion threshold, the ratio between [CuO6] and [CUO4] kept constant and the extra CuO existed in crystalline.Cobalt oxide/anatase and cobalt oxide/y-alumina samples were also prepared by impregnation method. Calcination temperatures were 400°C for anatase-supported samples and 300°C for y-alumina supported ones. Cobalt oxide could form monolayer dispersion structure on corresponding supports. The dispersion thresholds were 20.8mg CoO/gTiO2 or 1.4 Co atoms/nm2 TiO2 for anatase-supported samples and 35.2mg CoO/gy-Al2O3 or 2.03 Co atoms/nm2 Y-AI2O3 for y-alumina-supported samples, respectively. For both anatase-supported and y-alumina-supported samples, Below the dispersion threshold, Co atoms existed only in octahedral coordination, [CoO6] on both anatase and y-alumina surface. Beyond the dispersion threshold, Co3O4 crystals could be found in samples. The anatase and y-alumina support could stabilize monolayer dispersed bivalence Co2+.MoOs/anatase and MoO3/a-alumina samples were both prepared by dry-mixing method and calcined at 450 °C, MOO3 could spontaneously...