Studies On The Photocatalysis And Reaction Mechanism For Reducing CO₂to Methyl Formate

With the development of the world economy the rapid increase in the level ofcarbon dioxide is a matter of great concern. The atmospheric carbon dioxideconcentration will continue to increase due to the fossil fuel consumption. How toreduce CO2in the atmosphere to ease the “greenhouse effect” has become the focus ofthe world attention. A possible avenue for the sustainable development is to use thephotocatalysts for conversing CO2into hydrocarbons with the help of solar energy.Photocatalytic reduction of CO2is under mild reaction conditions, environmentalfriendly and can use solar energy. The use of CO2to synthesis high-value-addedchemicals or fuel can achieve the CO2-chemicals, fuel-CO2circle. In this paper,photocatalytic reduction of CO2in methanol to prepare methyl formate and theirmechanistic studies by on-line ATR-FTIR spectroscopy was investigated. The maincontents were as follows:Firstly, SrTiO3were synthesized by solvothermal method using oleic acid assurfactant, and Ag nanoparticles were loaded on the SrTiO3surface through thephotoreduction method. SrTiO3prepared by using oleic acid as surfactant showed thehigher activity owing to the smaller size and well dispersion. The Ag nanoparticlescan extend the spectral response from UV to visible area and increase light absorption.Ag nanoparticles can be used as electron traps for electron-hole separation, therebyenhancing the photocatalytic activity; however too much Ag loading could cause theformation of large nanoparticles and agglomeration, which was detrimental to thephotocatalysis.Secondly, silver halides were used for the CO2photoreduction, and on lineATR-FTIR spectroscopy was used to study the reaction process. The photocatalysts ofsliver halides were stable under repeated application since no siginificant decrease inthe photocatalytic reaction rate. The XRD patterns of the as-prepared silver halidesand silver halides used after5consecutive photocatalytic reactions showed thecatalysts were relatively stable. After6h of the reaction the CO2peak intensity wasreduced by2.2%, which indicated that CO2was consumed in the photoreaction. Theintermediates such as formic acid and formaldehyde were monitored in the solution.Formic acid was the2-electron reduction product of CO2, and formaldhyde was theprimary oxidation product of methanol. The methyl formate could be producedthrough the esterification of formic acid and methanol, dimerization of formaldehydevia Tishchenko reaction, but mainly through the esterification reaction.Thirdly, ZrO2nanosheets were synthesized by a two-step method and used forthe CO2photoreduction. Layered ZrO2was prepared by the solvothermal method indiethylenetriamine. After calcination DETA was removed and the crystallinity of ZrO2 was improved. The morphology of layered structure was remained and the gapbetween the layers was enlarged. Layered ZrO2was exfoliated to ZrO2nanosheets inisopropanol under ultrasonic treatment. The high degree of crystallinity helped toincrease the photocatalytic activity; ZrO2nanosheets favored for transfering electronsand holes to the surface, which enhanced the photocatalytic activity.Fourthly, the composite photocatalysts of AgCl-ZrO2nanosheets weresynthesized by anchoring AgCl nanocrystals on the surface of ZrO2nanosheets andused for the CO2photoreduction. The heterogeneous precipitation process ofAg+-ZrO2nanosheets suspension with chloride added dropwise was used tosynthesize AgCl-ZrO2nanosheets composites. The optimal composite with40wt%AgCl showed the higher photocatalytic activity. The enhanced photocatalytic activityinduced by the AgCl loading was attributed to the interfacial transfer ofphotogenerated electrons and holes between AgCl and ZrO2nanosheets, which leadsto effective charge separation. High AgCl loading amounts caused agglomeration,which decreased the photocatalytic activity.