Construction Of Dual-functional Materials Based On LDHs For CO₂ Capture And In-situ Reduction As Wellas Mechanism Studyn

Carbon capture,utilization and storage（CCUS）technology plays a key role in reducing carbon emissions,the new technology of integrated CO₂capture and conversion（i CCC）has attracted more and more researchers in recent years.The advantage of this technology is that it saves the re-release of CO₂after capture and the subsequent compression and transportation process,thus reducing the energy consumption of the system for comprehensive utilization of carbon resources.In addition,the direct conversion of heat energy to chemical energy can be achieved when the technology is applied to CO₂capture from high temperature flue gas.With the rapid development of photocatalytic technology,the coupling of CO₂capture and in-situ reduction with other photocatalytic oxidation reactions,such as glycerol photocatalytic oxidation,can enhance the reaction rate synergistically and realize process efficiency through the high value of oxidation half-reaction products,which opens up a new road for the innovation and development of i CCC technology.The realization of i CCC technology depends on dual-functional materials（DFMs）with CO₂adsorption sites and in situ catalytic sites.However,the properties of DFMs at the active sites and their role in in-situ reduction are still controversial,resulting in the difficult of the synergistic effect of bifunctional sites to maximize.Therefore,rationally construct DFMs with synergistic effect of adsorption and catalytic sites and to further study the mechanism of its capture and transformation reaction is undoubtedly an ideal choice to achieve efficient CO₂capture and in-situ reduction reaction.Layered double hydroxides（LDHs）are anionic compounds with high affinity to CO₃^2-,and have the characteristics of adjustable element composition,laminate and intralayer space confinement effect,providing favorable conditions for CO₂capture and in situ reduction.Based on this,with the aim of constructing an integrated CO₂capture-in-situ reduction system,the controllable design of LDHs-based dual-functional materials and the mechanism of interlayer CO₃^2-in-situ transformation were studied.The optimal reaction conditions for CO₂capture and transformation,as well as the interaction and synergistic mechanism between adsorption and catalytic sites were mainly explored.The reaction mechanism under the condition of thermal/photo-driven was elucidated.（1）In the thermal-driven CO₂capture and in-situ reduction reaction,Co Mg Al-LDHs was used as a CO₂adsorbent to selectively capture CO₂and form an interlayer CO₃^2-with strong fluidity and easy activation,aiming to improve CO₂capture selectivity and reduce in-situ reduction temperature.The CO₂capture-in situ reduction reaction was realized by using Ru nanoparticles dispersed uniformly on LDHs lamellar as H₂activation sites.Furthermore,the interaction between the metal and the support was regulated,and the performance matching between the CO₂capture and the in-situ reduction reaction was achieved.Interlayer CO₃^2-can be reduced to hydrocarbon products in the range of 140-250 ^oC,and220 ^oC is the optimal reaction temperature,which is～100 ^oC lower than the in-situ conversion temperature in the literature.After 2 h reaction,1046.55 and 509.95μmol/g of CH₄and CO yields can be obtained.More importantly,Ru/Co Mg Al-LDHs can capture CO₂at room temperature and reaction temperature,with the captured capacity of 1543.83 and897.77μmol/g,respectively.Excitingly,Ru/Co Mg Al-LDHs can even maintain more than 97%selectivity in flue gas containing NO_xand SO_x,and the yield of capture and in-situ conversion can basically maintain balance.In situ FTIR characterization of ¹²CO₂and isotopically labeled¹³CO₂confirmed that interlayer CO₃^2-can be in-situ activated into monodentate carbonate species,and further hydrogenated to*CO species with*H species generated by Ru nanoparticles.Subsequently,*CO generates methane through the reaction of*H species assisted to*OCH₂species.After the reaction,the catalyst re-captures CO₂and converts it into interlayer CO₃^2-,thus realizing the recycling of the dual-functional materials.（2）On the basis of the above CO₂capture and in-situ reduction reactions,the external-field assisted method was introduced and glycerol was used as the hydrogen donor to carry out the photo-driven CO₂capture-in-situ reduction-glycerol oxidation reaction.By using the controllability of LDHs lamination elements,Ni and Fe elements with light-absorbing ability are introduced into host layer of LDHs,a series of Ni_xMg_yFe₁-LDHs dual-functional adsorption catalytic materials were constructed.After UV-visible light irradiation 24 h,CO₃^2-was in-situ reduced to CO,and the accumulative yield was up to 1.53 mmol/g,and the 0.50 mmol/g H₂yield was obtained,at the same time the glycerol was efficiently oxidized to lactic acid（LA）and a small amount of dihydroxyacetone（DHA）.The accumulative yield and selectivity of LA was 4.33 mmol/g and 93.9%,respectively.By adjusting the ratio of Ni and Fe in LDHs,the matching degree of the half-reaction rate of oxidation and reduction can be further adjusted,and the flexible adjustment of H₂/CO in the range of 0.47 to 0.94 can be achieved.Moreover,1.34 mmol/g H₂generation was also detected in the CO₂capture reaction.This was because the photocatalytic reaction induced the Ni and Fe metal sites on the catalyst surface to pre-store photogenerated electrons,and drove the subsequent CO₂capture conversion of water as an oxygen supply molecule into CO₃^2-,and the electrons reduced H in water to hydrogen.A series of characterization confirmed that Fe³⁺promoted CO₃^2-activation,and metal ions and surface hydroxyl groups on LDHs captured photogenerated electrons and holes respectively,thus providing active sites for reduction and oxidation half-reactions.The remaining electrons in photocatalytic reduction promoted the capture of CO₂.Thus the multi-process coupling of CO₂capture-in situ reduction-glycerol oxidation was realized.