Construction And Performance Of A Nanoreactor For Light-driven CO₂ Reductionbiomass Oxidation Coupled Reaction

In the context of global carbon neutrality,it is of great significance to carry out research on photocatalytic CO₂ reduction driven by renewable energy.However,due to the high C=O bond energy of CO₂ and the slow kinetic rate of the water oxidation half-reaction,the process is inefficient,limiting its practical application.In recent years,researchers have used a hole sacrificial agent represented by triethanolamine to increase the reduction efficiency of CO₂ on the premise of accelerating the oxidation kinetic rate by consuming more holes.However,the added value of the oxidation product of the hole sacrificial agent is low,resulting in a waste of hole oxidation energy.5-Hydroxymethylfurfural（5-HMF）,as a class of alcohol biomass platform compounds,the oxidation of its hydroxyl group can not only improve the photocatalytic reaction efficiency,but also the obtained oxidation products have high added value.Therefore,coupling CO₂ reduction with 5-HMF oxidation process can not only achieve CO₂ emission reduction and comprehensive utilization,but also obtain biomass-based chemicals with higher added value.Layered double hydroxides（LDHs）are a class of anionic layered compounds with controllable and compatible compositions,and the confinement of their intralayer space can effectively enrich reactants and improve their catalytic performance.Based on the above characteristics,LDHs exhibit typical nanoreactor characteristics.In addition,the interlayer of LDHs has a very high affinity for CO₃^2-,and metal ions and hydroxyl groups on the laminate can capture photogenerated electrons and holes,respectively,and have the ability of photocatalytic conversion,and through the design and construction of the dominant sites for reduction and oxidation half-reactions,the coupling of CO₂ in-situ reduction and 5-HMF oxidation can be realized.（1）Based on the strong affinity of LDHs for CO₃^2-,a ZnNiFe-CO₃^2--LDHs nanoreactor was constructed by using CO₂ in the air as the carbon source by co-precipitation method.The FTIR results showed that ZnNiFe-LDHs realized the pre-concentration of CO₂ in the form of CO₃^2-,which laid the foundation for the subsequent reactions.The selective etching method was further used to introduce metal vacancy defects.The electronic structure of the laminate metal and hydroxyl ions is regulated.Specifically,using XRD and SEM characterization is proved that the nano-reaction still maintains a relatively good crystal structure and sheet-like morphology after etching;ICP-AES characterization results show that with the increase of etching time,the Zn²⁺content on the laminate gradually decreased;the TG-MS characterization proved that the CO₃^2-content between the nanoreactor layers remained basically unchanged after etching.EPR and XPS results show that the concentration of metal vacancy defects in the nanoreactor is proportional to the etching time,and with the introduction of metal vacancy defects,electron-rich Ni²⁺and electron-deficient hydroxyl ions that are more favorable for CO₂ and 5-HMF activation.（2）Furthermore,the properties of ZnNiFe-LDHs before and after etching were evaluated on photocatalytic coupling reaction in Ar atmosphere.The study found that ZnNiFe-LDHs can meet the realization of photocatalytic coupling reaction,and the performance of the obtained nanoreactors after etching is enhanced,and the ZnNiFe-LDHs-E3h nanoreactors etched for 3 h showed the best catalytic performance in the reaction time of 4 h,the cumulative yield of CO,the main product at the reducing end,was 76.32μmol/g,and the main product,2,5-furandicarboxaldehyde（DFF）at the oxidation end,was 55.12μmol/g,and the nanoreactor can still maintain 90%of the initial performance after five cycles of reaction.In order to explore its performance enhancement mechanism,PL spectroscopy and photocurrent response characterization proved that ZnNiFe-LDHs-E3h has the strongest photogenerated carrier separation ability.Using in-situ FTIR characterization,it was proved that the existence of metal vacancy defects and the changes of the electronic structures of the metal and hydroxyl groups after etching were helpful for the activation of the reaction substrates CO₃^2-and 5-HMF.Further characterized by series ¹³C-labeled in situ FTIR,it is demonstrated that the nanoreactor can supplement the interlayer CO₃^2-converted in situ during the reaction by capturing CO₂ from air.Based on the above characterization results,the following reaction mechanism is proposed:due to the induction of metal vacancy defects,the activation of CO₃^2-is promoted by the electron-rich Ni²⁺sites on the LDHs layer;The electron-deficient hydroxyl sites promote the adsorption and activation of 5-HMF,and the performance of the ZnNiFe-LDHs nanoreactor in the photocatalytic coupling reaction is improved under the synergistic effect of the two types of sites.（3）In order to further enhance the efficiency of MgAl-LDH in the coupled reaction of photocatalytic CO₂ in-situ reduction and 5-HMF oxidation,a MgAl-LDH/TiO₂ nanoreactor with type Ⅱ heterostructure was constructed based on the excellent photogenerated electron capture ability of TiO₂.The existence of type Ⅱ heterostructures promotes the separation efficiency of photogenerated carriers in the nanoreactor and realizes the enhancement of coupled reaction performance.Specifically,XRD,SEM and XPS were used to characterize the successful composite of MgAl-LDH and TiO₂.Using UV-Vis and energy band spectrum characterization,it is proved that the photoresponse ability of the composite nanoreactor is significantly improved compared with MgAl-LDH,and combined with the analysis of the characteristics of type Ⅱ heterojunction materials,the MgAl-LDH in the composite nanoreactor is not only responsible for the carbon source required for the reaction,but also responsible for the 5-HMF oxidation in the coupling reaction;TiO₂ is responsible for the CO₃^2-reduction in the coupled reaction.The results of PL spectrum and photocurrent response spectrum show that the composite nanoreactor has a stronger ability to separate photogenerated carriers.Based on the above characterization results,the following performance enhancement mechanism is proposed.The construction of the composite nanoreactor can enhance the separation ability of photogenerated carriers,thereby realizing the enhancement of the photocatalytic coupling reaction performance.