Study On The Photocatalytic Reduction Of CO₂ By Ti-based MOFs With Surface Interface Regulation

The excessive consumption of fossil fuels by the rapidly growing economy makes the content of carbon dioxide in the atmosphere keep rising,which leads to a series of problems such as global warming and energy shortage,which has attracted wide attention from all countries in the world.In addition to saving energy and reducing emissions of CO₂,the capture and effective utilization of CO₂ is also the key.Using photocatalytic technology to convert CO₂ into valuable fuel is an important way to solve global warming and energy crisis.Although considerable progress has been made in the existing research,the synthesis of stable and efficient photocatalysts still faces great challenges,and the current research is still in the preliminary stage.In recent years,metal-organic frameworks（MOFs）have attracted more and more attention due to their large specific surface area,high porosity and strong CO₂ adsorption capacity.In this study,a Ti-based MOFs was used as a precursor template to prepare LiFePO₄/CuO@MIL-125（Ti）composite materials to improve the photocatalytic performance of semiconductor materials in the CO₂ reduction process.Based on material characterization and analysis,the photocatalytic CO₂ reduction performance of the composite photocatalyst under different conditions was investigated,and the mechanism of photocatalytic CO₂ reduction was discussed.The specific research content and results are as follows:（1）LiFePO₄/CuO@MIL-125（Ti）ternary composite was prepared by solvothermal reaction using MIL-125（Ti）as precursor material,and a series of morphology,structure and photoelectrochemical characterization were carried out.The results show that the preparation of LiFePO₄/CuO@MIL-125（Ti）material can significantly improve the surface charge transfer efficiency of the catalyst,effectively inhibit the composite efficiency of photogenerated electrons and holes,and enhance the light absorption capacity.The results showed that under the simulated sunlight irradiation for 3 h,the 2.5%LiFePO₄/1.0%CuO@MIL-125（Ti）composite photocatalyst showed a high photocatalytic activity under the condition of water as the reducing agent,CO₂ was reduced to methanol,ethanol and acetic acid and other products.The yields were 455.38,966.36 and 844.63μmol/g,and the selectivity was 13.0%,55.0%and 32.0%,respectively.After five cycle experiments,the synthetic yield of the photocatalyst was only reduced by 3.18%,which indicated that the photocatalyst had high stability and reusability.After five cycles of experiments,its comprehensive output decreased by only 3.18%,indicating that the photocatalyst has high stability and reusability.The charge transfer mechanism on the surface of the catalyst and the possible formation pathway of CO₂ reduction products were proposed.（2）The 2.5% LiFePO₄/1.0%CuO@MIL-125（Ti）composite material was used as the photocatalyst,four alkaline solutions of different concentrations of sodium hydroxide,sodium carbonate,sodium bicarbonate and ammonium bicarbonate were selected for photocatalytic CO₂ reduction experiment.The experimental results show that HCO₃^-is the main form of existence after the alkaline solution is permeated with CO₂ until saturation.The results showed that the concentration of CO₂（aq）and the ratio of CO₂（aq）/TC were the influencing factors of photocatalytic reduction efficiency,and it was confirmed that CO₂（aq）was the actual reactant of photocatalytic reduction.（3）Different proportions of CH₄ and CO₂ mixtures were prepared to simulate biogas for photocatalytic CO₂ reduction experiments.Experimental results show that pure CH₄under the condition of photocatalysis,no product formation was detected,indicating that the catalyst has no catalytic effect on CH₄ in the mixed gas experiment,which provides the possibility for the purification of biogas.When the mixing ratio of CO₂ and CH₄ was 4:6,which was similar to the content of CO₂ and CH₄ in biogas,the yield of CH₃OH,CH₃CH₂OH and CH₃COOH were 466.62,549.92 and 430.64μmol/g,respectively,and the selectivity were 21.80%,51.38%and 26.82%,respectively.With the change of CO₂ partial pressure,the adsorption saturation of CO₂（aq）on LiFePO₄/CuO@MIL-125（Ti）composite surface was 28.50 mmol/L.It is suggested that HCO₃^-is conducive to the conversion of CH₃OH,while CO₂（aq）is conducive to the conversion of CH₃CH₂OH and CH₃COOH according to the trend of product yield.