Photoreduction Of CO₂ Using TiO₂-Based Nano-Particles

CO₂ emission has become a global environmental problem. The conversion of CO₂ into hydrocarbons has both theoretical and practical significances. Cu and RE （La, Nd, Y） modified TiO₂ nano-particles were separately synthesized by sol-gel technique. The activity of catalysts was evaluated with the yield of formaldehyde, which was the main product of CO₂ photoreduction in our experiment. Furthermore, the mechanism of photocatalysis was also investigated in this research. The crystal structure, crystal morphology and chemical component were characterized using X-ray diffraction （XRD）, transmission electron microscope （TEM） and X-ray photoelectron spectroscopy （XPS）.A series of copper modified titanium dioxide （CuT） nanoparticles were synthesized via sol-gel method. The main phase of all nanoparticles was anatase. After 6 h of UV illumination, the yield of formaldehyde achieved maximum. With the increasing of reaction time, in addition to formaldehyde, acetone was also detected in liquid products. When the copper modifying ratio less than 7 wt.%, copper mainly played the role of doping, and 0.6CuT had the highest yield of formaldehyde. When the copper modifying ratio above 7 wt.%, CuT nanoparticles were not just TiO₂, but the compound semiconductor of TiO₂ and CuO, and 20CuT showed a high photocatalytic performance.RE （La, Nd, Y） doped TiO₂ nanoparticles were prepared by sol-gel method. Doping with rare earth elements, the photoreduction capacities of TiO₂ could be significantly increased. The results shown that 0.1LaT performed the highest photocatalytic activity.Various RE and Cu co-modified TiO₂ nanoparticles were also synthesized by sol-gel method. The XRD analysis shown that the synergy effect between RE and Cu reduced the crystallization of anatase and retarded the transformation of amorphous titanium dioxide to anatase. 0.1La^-20CuT had the highest yield of formaldehyde, which could achieve 953.53μmol/g·cat. under UV illumination for 6 h.The optimum conditions of CO₂ photoreduction were also discussed in this research, including concentration of catalyst, CO₂ flow rate, initial pH value of reaction solution and the addition of hole scavenger. The results shown that the optimum conditions of CO₂ photoreduction were as follows: illumination time was 6 h, initial pH value of reaction solution was 10, CO₂ flow rate was 30 mL/min, and the addition concentration of methanol was 1.0 mol/L.