| Recently,photocatalytic reduction of CO2 into hydrocarbon fuels is regarded as one of the promising approaches to relieve the enhanced concentration of CO2 in atmosphere and the energy shortage.However,it is still limited by the complicated synthetic methods of the photocatalysts,ambiguous reaction mechanisms and low photocatalytic performances.Although some researchers developed a lot of new photocatalytic materials to improve the photocatalytic performance for the reduction of CO2 under visible light irradiation,the activities was far from satisfaction.In this thesis,the nanocomposite materials have been constructed using element semiconductor S8 and metallic oxide as basic building units.The as-prepared composite materials exhibit high photocatalytic activities for the redution of CO2 under visible light irradiation,although the single building unit precursor is inactive.Taken the Ta2O5,Nb2O5 and TiO2 as examples,the affect factors for the high photocatalytic performaces of nanocomposite materials are revealed and the possible photocatalytic mechanisms are proposed.The main results and conclusions obtained from the present researches are as follows:(1)S-Ta2O5 composite materials have been synthesized by a thermal synthesis method using S8 and Ta2O5 as precursor.The as-prepared samples exhibit high photoabsorption performance in the visible light region.In photocatalytic reduction of CO2,the single S8 and Ta2O5 are inactive,while the S-Ta2O5 composite materials display high photocatalytic activities for the transformation of CO2 into CH4.When the mass ratio of S8 and Ta2O5 is 1:1,the composite material shows the highest activities and the productive rate of methane reaches 1,37 ?molg-1h-1.Furthermore,the composite also exists a high stability of photocatalytic activities.The adsorption capacity of CO2 and photoresponse current over S-Ta2O5 is much higher than that of the precursors.Comparing with single Ta2O5,the charge transfer resistance and the overpotential of the CO2 reduction are considerably decreased over S-Ta2O5.All of these factors may contribute to the high photocatalytic properties of the composite.The in-situ FTIR experiments show that CO2-species may be the predominant intermediate on the surface of S-Ta2O5 during the reaction processes.H2O may serve as proton donor and photo-generated holes sacrificial agent.(2)S-Nb205,S-TiO2,S-ZnO,S-CdS,S-WO3 and S-ZrO2 are prepared by a similar method of S-Ta2O5.The as-prepared samples have been characterized in details.The photocatalytic activities of the samples are evaluated by the reduction of CO2.Results shows that the S-Nb2O5 and S-Ti02 composites exhibit excellent photocatalytic activities under visible light irradiation.The 0.3 S-Nb and 1.5 S-Ti samples show the optimal photocatalytic activities.The productive rates of CH4 over these two samples are 1.61 and 1.87 ?mol/g/h,respectively.It should be noted that the S-ZnO,S-CdS,S-WO3 and S-ZrO2 composites do not display the catalytic performances for the reduction of CO2.The results from the materials characterizations and electrochemical experiments demonstrate that if the composites may catalyze the reduction of CO2,they may possess the strong photoabsorption performance in the visible light region,the high photoelectric response,the high adsorption capacity of CO2,the low charge transfer resistance,and the low overpotential of CO2 reduction.Among these affect factors,the decrease of the overpotential value of the CO2 reduction reaction over the composite may be a most important factor.(3)The activities of the thermal and photo-thermal synergistic catalytic reduction of CO2 over the as-prepared S-Ta2O5 and S-Nb2O5 have been evaluated.The as-prepared composites exhibit the high thermally catalytic activities of the reduction of CO2 into CH4 as low as 60?.The significantly synergistic effect of light irradiation and reaction temperature over S-Ta205 and S-Nb2O5 samples on the catalytic activity during the reduction of CO2 has been proved. |
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