The Preparation And Photocatalytic Reduction Of CO₂ With H₂O Of CdS/TiO 2 Photocatalyst

The atmospheric concentration of carbon dioxide has monotonically increased owing to an enormous volume of fossil fuels consumption since the industrial revolution,which is the major cause of the global warming that has been identified as one of the most serious environmental problems. Nowdays, many efforts is being made to reduce carbon dioxide emission through pre-, post- or oxy-fuel combustion processes. The CO₂ capture and geological sequestration (CCS) processes, however, are energy-intensive and high cost. The photocatalytic reduction of CO₂ is a promising technical solution since it can convert CO₂ into hydrocarbons at room temperature and atmospheric pressure. Meanwhile it would consume less energy in comparison to conventional methods by harvesting solar energy, which is abundant, cheap and ecologically clean and safe. As UV and visible light response semiconductor materials, TiO₂ and CdS are two of the most representative materials, they have become the preferred basematerial to study photocatalytic ractions.In this thesis, Pt(Cu)-P25, CdS/P25, Pt(Cu)-CdS/P25, TiNT, Pt(Cu)-TiNT, CdS/TiNT and Pt(Cu)-CdS/TiNT catalysts were synthesized by a series of chemical methods using commerical P25(Degussa) as precursor.Their physical and chemical properties were characterized by XRD, DRS, SEM, BET etc.All the above catalysts's activity of photocatalytic reduction CO₂-H₂O was examined on a self-designed test bench. The influence of different content of Pt(Cu) and CdS to P25 on photocatalytic reduction CO₂ was systematically investigated. The best conditions to prepare titanate nanotubes (TiNT) through hydrothermal-ultrasonic method using P25 as precursor was explored. Studied the effect of synthesized method and load Pt(Cu) to the properties of CdS/TiNT photocatalytic reduction CO₂ and H₂O.Furthermore, the reaction mechanism of the relative photocatalytic CO₂ reduction was also discussed.The experiment results showed that P25 after treated at 450 oC loaded the mass fraction of 0.5%, 1% and 2% Pt, 0.5%Pt-P25 present the highest photocatalytic activity, while loaded the mass fraction of 0.25%, 0.5% and 1% Cu, the photocatalytic activity of 0.5% Cu-P25 is the best.The combination of CdS and P25 an promotethe photo-electrons and holes separation, improve the photocatalytic efficiency. Among the mass fraction of 5%, 10%and 20%CdS, 10%CdS/P25 showed the highest photocatalytic activity, the methane production rate is as high as nearly 10 times of P25. The photocatalyticactivity of 10%CdS/P25 was significantly improved after loaded with Pt, 1% Pt was best among 0.5%, 1%, 1.5% and 2% wt.It was found 150 oC, 24 h hydrothermal treatment was the best condition to prepare TiNT through control the temperature and time of P25 hydrothermally reacted in 10 mol/L NaOH. Expermental results showed that TiNT has photocatalytic reduction CO₂-H₂O activity, but is not as good as P25. When TiNT composited with CdS, the photocatalytic activity was significantly increased, indicating that there existed synergy effect between CdS and TiNT. Activity tests showed that CdS/TiNTprepared via simple ion exchange method had higher photocatalytic activity than by chemical precipitation method. When loaded with 0.1%Pt, CH4 yield reached 0.039μmol·h-1·g-1, but no CO. when loaded with 0.5%Cu, CO and CH4 yields both increased, CH4 yield reached 0.034μmol·h-1·g-1, just slightly lower than that of Pt loading. It is suggested that Cu is probably a good alternative candidate of Pt as co-catalyst in this catalyst system.Based on the experiments in this paper, it is proposed that the trace amount of residual carbon, which comes from surface adsorption of catalyst sample or organic precursor during preparation processes, plays an important role in the initial catalytic reaction, and affect the reaction products as well. It should be considered in the characterization of photocatalytic hydrogenation of CO₂.