Study On The Photocatalytic Reduction Of CO₂ By Ti-based MOFs With High-efficiency Heterojunction

Photocatalysis using solar energy as the driving force can reduce CO₂ into valuable hydrocarbon fuel,which is expected to alleviate the current energy crisis and global warming problems.In the context of the"carbon neutral"strategy,developing efficient and stable photocatalytic material systems is vital to promote the conversion of CO₂into high-value-added products for energy recovery.Z-scheme heterojunction can effectively promote electron-hole separation and retain strong redox ability,which has a promising application in the catalytic conversion of CO₂ to high-value chemicals.NH₂-MIL-125（Ti）has a large specific surface area and high stability.It is one of the most effective catalysts for the photocatalytic reduction of CO₂.However,the photocatalytic activity of NH₂-MIL-125（Ti）is still unsatisfactory,and the photocatalytic reduction of CO₂ is not widely used,mainly because of the following shortcomings:1)the reduction performance of NH₂-MIL-125（Ti）itself is weak,and it still needs to be compounded with other semiconductor materials;2)the traditional type-II heterojunction charge transfer when connected with other reduced semiconductors weakens the performance of the catalyst 3)the visible light absorption range of NH₂-MIL-125（Ti）is narrow;4)the influence of liquid and gas phase conditions on the photocatalytic reduction of CO₂ is poorly studied,resulting in a single application at present.To this end,the following research has been carried out in this paper:to introduce semiconductors and electron transfer media matching the energy level of NH₂-MIL-125（Ti）to construct Z-schemer heterojunction catalysts with strong redox ability.The charge transfer pathway and the mechanism of photocatalytic CO₂reduction were revealed.In addition,from the perspective of biogas purification and utilization,the liquid and gas phase environments during the photoreaction were changed to explore the influence of complex conditions on the reaction process and the reaction mechanism.To address the problems of unsatisfactory reduction performance,weak driving force of Z-scheme heterojunction and weak visible light absorption ability of NH₂-MIL-125（Ti）,Cu₂O/Pt/NH₂-MIL-125（Ti）and Zn S/CQDs/NH₂-MIL-125（Ti）photocatalysts were constructed in this study.The energy level matching of Cu₂O and NH₂-MIL-125（Ti）formed Z-scheme heterojunction exhibited excellent charge separation efficiency and redox ability;further introduction of Pt NPs as a mediator between Cu₂O and NH₂-MIL-125（Ti）could suppress the conventional type-II charge transfer;the light absorption properties of Cu₂O and Pt NPs were utilized to expand the NH₂ MIL-125（Ti）light absorption range;the yield of CH₃OH and CH₃CH₂OH can reach 434.46 and718.47μmol/g after 3 h photoreaction of the optimal composite ratio of Cu₂O/Pt/NH₂-MIL-125（Ti）.In addition,CQDs were extracted from waste sludge using hydrothermal method to replace precious metals as electron transfer medium.It was demonstrated that the Z-scheme charge transfer mechanism between Zn S and NH₂-MIL-125（Ti）was consistent;Zn S and CQDs synergistically enhanced the visible light absorption of NH₂-MIL-125（Ti）;the optimal composite ratio of Zn S/CQDs/NH₂-MIL-125（Ti）after 3 h of visible light irradiation,the yields of CH₃OH and CH₃CH₂OH were 501.57 and 564.70μmol/g,respectively.There are few studies on the effects of liquid and gas phase conditions on photocatalytic CO₂ reduction technology,leading to a single application.Therefore,in this study,the photocatalytic performance was investigated under the liquid phase system of Na OH,Na₂CO₃ and Na HCO₃ solutions and various p H aqueous solutions using 2%Zn S/CQDs/NH₂-MIL-125（Ti）as the catalyst;meanwhile,the photocatalytic performance was investigated under different CO₂:CH₄ ratio conditions from the perspective of biogas purification and utilization.The results showed that the actual reactant of the photocatalytic CO₂ reduction process was CO₂（aq）;the CO₂（aq）/TC ratio affected the yield of photocatalytic CO₂ reduction;among all liquid-phase systems,the best catalyst performance was in 0.1 M Na OH solution,and the yields of CH₃OH and CH₃CH₂OH after 3 h of photoreaction were 600.03 and 583.43μmol/g,respectively;When CO₂:CH₄=6:4 in the gas mixture,the catalyst saturated the adsorption of CO₂（aq）with a saturation of 43.24 mmol/L,at which the yields of CH₃OH and CH₃CH₂OH were479.35 and 556.14μmol/g,respectively;the catalytic system used in this study had no catalytic effect on CH₄ and achieved the selective catalysis of the mixed gas of CO₂.The development of this study provides a new idea for constructing efficient Z-scheme heterojunction Ti-based MOFs catalysts for visible-light-driven catalytic reduction of CO₂ to produce hydrocarbon fuels.It provides a new reference for the future use of photocatalytic CO₂ reduction technology to deal with complex practical situations（biogas purification）.