Study On Photo-catalysis Of Synthesizing Acetic Acid From Methane And Carbon Dioxide

Photocatalysis of synthesizing acetic acid from methane and carbon dioxide is of great significance to utilization of alkanes, synthesis of oxygenated hydrocarbons and protection of environment. In this work, a gas-solid photocatalysis reaction technology was applied to this reaction. A series of supported metal modified coupled-semiconductors were designed and prepared. The surface configuration, energy band position, optical absorption property, chemisorption ability to the reactants and photocatalytic performance of the materials were systematically investigated.Supported metal modified coupled-semiconductors Cu/V₂O₅-TiO₂/SiO₂ and Cu/CeO₂-TiO₂/SiO₂ were prepared by the impregnation- chemical modification method successfully. The characterization by BET, XRD, IR, TPR and UV-vis DRS techniques showed that the surface area of these materials were above 200m2/g, the nano-metered crystalline of V₂O₅ or CeO₂ and TiO₂ were dispersed well on the silica support while metal Cu was highly dispersed on these metal oxides. V₂O₅ or CeO₂ coupled with TiO₂ by V-O-Ti or Ce-O-Ti bonds and chemical bonds with silica (V-O-Si or Ce-O-Si) were also formed. Three kinds of active sites, metallic site Cu, Lewis acid sites Tin+, V5+or Ce4+and Lewis basic sites, terminal or bridge O2- existed on the surface of photocatalysts.The optical absorption properties and edge energy of the supported coupled-semiconductors were investigated by the UV-vis DRS experiments and K-M functions. The results demonstrated that the supported coupled-semiconductors had a larger Eg compared to bulk phase, the optical absorption are also enhanced due to the coupled effect. Modifying Cu could extend the optical absorption region of the photocatalysts. According to these results, the energy band structures of supported active species Cu-V₂O₅-TiO₂ and Cu-CeO₂-TiO₂ and the matching principles of the Eg, Ec and Ev in their position were figured out. The different migration patterns and the related redox abilities of the photo-excited electrons and holes were also discussed. The experimental results studied by the chemisorptions-IR and TPD-MS techniques indicated that CO2 could be mainly chemisorbed on the surfaces of Cu/V₂O₅-TiO₂/SiO₂ and Cu/CeO₂-TiO₂/SiO₂ to form the high active horizontal state Cu-(CO)O→Tin+ with the synergistic effects of metallic site Cu and Lewis acid sites Tin+. CH4 molecularly adsorbed on the terminal oxygen of V=O over Cu/V₂O₅-TiO₂/SiO₂ or bridge oxygen of Ce-O-Ti over Cu/CeO₂-TiO₂/SiO₂.