Preparation And Photocatalysis Behavior Of Mesoporous Coupled-Semiconductor NiO-TiO₂

Direct synthesizing dimethyl carbonate from CO₂ and methanol has great theoretical and practical significance both in synthesis of hydrocarbon oxygenates and utilization of carbon recourse. In this thesis, the study was extended to the gas-solid reaction system by using photocatalytic reaction technology. The mesoporous n-p coupled semiconductors were also designed and prepared, whose surface structure, the structure of energy band, light absorption ability and photocatalytic activity were systematically investigated.1. NiO-TiO₂ and NiO-V₂O₅ coupled semiconductors were successfully synthesized with using organic templates, whose specific surface area is obviously higher than that of the materials prepared via hydroxide co-precipitation.2. XRD, Raman, TPR and TEM results show that NiO-TiO₂ exists with the shape of nanotubes while NiO-V₂O₅ with small particles of diameter below 30nm, and NiO was well-dispersed on the surface of the solid materials. The extremely small particles of NiO and V₂O₅, NiO and TiO₂ crystallites exist on the surface of NiO-TiO₂ and NiO-V₂O₅ coupled-semiconductors respectively. Another distinct characterization of these materials on their surface structures is that the semiconductors V₂O₅ and TiO₂ are coupled with NiO. The interaction between NiO and TiO₂ is so strong which results in the formation of NiTiO3, however, the interaction between NiO and V₂O₅ is weak which lead to the existence of extremely small particles in the surface of V₂O₅. In addition, there exist abundant active sites on the surfaces of the two coupled semiconductors such as Lewis acid sites Ti4+, V5+ and Lewis base sites V=O or (V, Ti)-O-Ni.3. The results of Eg evaluated by Kubella-munk function and UV-vis DRS indicate that broaden by the n-p semiconductor couple effect, light absorption field and the efficient of photo-generated carriers separation improved. The increasing amount of NiO led to the red shift of the absorption edge and distinct absorption enhancement to the visible light of the NiO-TiO₂ coupled semiconductor, however, the increase of calcination temperature decreased the light absorption in the wavelength region from 400nm-550nm. The addition of NiO did not broaden the absorption field in NiO-V₂O₅ semiconductor, which means no strong n-p coupled effect took place. 4. The photocatalysis reaction evaluating results of the coupled semiconductors indicate that DMC can be successfully synthesized from CO₂ and CH3OH under the condition of UV irradiation (365nm, 0.65mw/cm²). The conversion of the reactants and the selectivity of the products are related with the preparation method of the semiconductor, surface chemistry structure, energy band edge position, light absorption ability, reaction temperature. The photocatalysis activities of the coupled semiconductor prepared by organic templates are superior to that of coupled semiconductor prepared via hydroxide co-precipitation, and the photocatalysis activities of NiO-TiO₂ coupled semiconductors are superior to that of NiO-V₂O₅ coupled semiconductor. Under the radiation of UV light (intensity 0.65 mw/cm2, wavelength 365nm), temperature 120℃, space velocity 270h-1, the conversion of CH3OH 4.4%, the selectivity of DMC 70.4% can be obtained over mesoporous 4% NiO-TiO₂ coupled-semiconductor.