The Effect On Photo/electrocatalytic CO₂ Reduction By Fine Regulation Of Layered Double Hydroxides And Their Derivatives

The efficient conversion of CO₂is a potential strategy to mitigate the greenhouse effect and prepare high-value-added chemicals.Photo/electrocatalytic reduction of CO₂has attracted wide attention because of its wide energy sources and cleanness.However,the activation of linear CO₂molecules is difficult,and the side hydrogen evolution reaction（HER）is easy to take place,which reduces the selectivity of CO₂reduction reaction.Therefore,it is an urgent problem to design and develop efficient,highly selective,and stable photo/electrocatalytic CO₂reduction catalysts.As a class of supramolecular assembled two-dimensional nanomaterials,layered double hydroxides（LDHs）materials are a kind of potential photo/electrocatalysts for CO₂reduction,due to their adjustability of composition,size,thickness,morphology,defect structure,electronic structure,and band structure.Based on the controllability of the fine structure for LDHs materials,this dissertation realized the fine control of the electronic structure,defect structure,and coordination structure of LDHs by adjusting the composition,thickness,synthesis method,and derivative structure of LDHs at the micro atomic level.It showed excellent activity and selectivity in photocatalytic/electrocatalytic CO₂reduction.Furthermore,the fine microstructure of the catalyst,the mechanism of CO₂reaction,and the structure-activity relationship were further studied by using advanced spectroscopy,in-situ characterization,and DFT theoretical calculation.Therefore,this dissertation provided a new idea and method for realizing efficient photocatalytic/electrocatalytic CO₂reduction through fine regulation of the structure of LDHs and their derivatives.The main contents are shown as follows:1.Based on the adjustability of the composition on the laminate of LDHs,a series of u-MAl-LDHs（M=Mg,Co,Ni,Zn）with different d electron numbers in e_gorbitals in the divalent metal of LDHs were successfully prepared by adjusting the electronic structure of divalent metal elements in u-MAl-LDHs.Interestingly,we found that u-Co Al-LDH with 1 electron in the e_gorbital exhibited the highest photocatalytic activity of CO₂reduction to CO underλ>400 nm,and the CO production rate was 218.13 mmol g^-1h^-1.To our surprise,CO production rate was still as high as 43.73 mmol g^-1h^-1underλ=600 nm irradiation.The photoelectrochemical tests showed that u-Co Al-LDH exhibited the narrowest band gap,the smallest charge transfer impedance,and the higher separation efficiency of carriers among u-MAl-LDHs.Furthermore,DFT calculation found that u-Co Al-LDH displayed the strongest adsorption for reaction substrate H₂O and CO₂compared with other u-MAl-LDHs,which was conducive to promoting the activity of CO₂reaction.2.To utilize the adjustability of the thickness of LDHs,a series of monolayer LDHs（m-Mg Al-LDH,m-Co Al-LDH,m-Ni Co-LDH and m-Ni Fe-LDH）were prepared via the separate nucleation and aging steps（SNAS）method by adjusting and optimizing the synthesis method.XRD,TEM and AFM showed that the 00l characteristic diffraction peak disappeared in the monolayer LDHs,the size of the monolayer LDHs is uniform,and the thickness was about 1 nm.XAFS results displayed that there were abundant defects in monolayer LDHs.The monolayer m-Ni Fe-LDH achieved 81.75%selectivity of CH₄,and the by-product H₂selectivity was less than 3%,which was far better than multilayer LDHs.DFT calculations exhibited that the rich defects in the monolayer structure were conducive to generating more defect states and promoting charge transfer.Meanwhile,the defects as the new active sites improved the reaction performance.3.Ascribed to the adjustability of coordination structure of LDHs,VO₄-modified Ni Mg V-LDH（denoted as V/Ni Mg V-LDH）and traditional ternary Ni Mg V-LDH were successfully fabricated by controlling the synthesis method of LDHs.The V/Ni Mg V-LDH exhibited excellent performance in photocatalytic CO₂reduction（CO₂PR）with 99%selectivity to C1（CO+CH₄）products.The production rate of CH₄on V/Ni Mg V-LDH was 30 times that of Ni Mg V-DH.And the selectivity of H₂decreased from 28.11%of Ni Mg V-LDH to 0.93%of V/Ni Mg V-LDH.More important,the by-product H₂can be completely suppressed atλ=405 nm and 550 nm by using V/Ni Mg V-LDH.In-situ DRIFTS and DFT calculations showed that this was due to the synergy between VO₄and Ni-O in V/Ni Mg V-LDH,which was beneficial to promote CO₂activation.At the same time,V/Ni Mg V-LDH displayed stronger*CO and*H adsorption,which was conducive to the further hydrogenation of*CO to form the intermediate of CHO*and CH₃O*,and then to form CH₄.4.By using LDHs as the precursors,and then pyrolyzed with melamine,a series of Ni single atom and Ni nanoparticle catalysts with different contents were successfully synthesized(denoted as Ni _NPx@Ni _SAy-NG（x,y=1,2,3;NG=N-doped graphene）.The content of Ni nanoparticles is 21.57%for Ni _NP1@Ni _SA1-NG,38.46%for Ni _NP2@Ni _SA2-NG,and 74.07%for Ni _NP3@Ni _SA3-NG,respectively.The HAADF-STEM and XAFS showed that there were Ni single atom sites and Ni nanoparticle sites coexisted in Ni _NPx@Ni _SAy-NG.The obtained samples exhibited a volcano-type trend for the maximum CO Faradaic efficiency with the highest point at Ni _NP2@Ni _SA2-NG in electrochemical reduction of CO₂（CO₂RR）.And Ni _NP2@Ni _SA2-NG achieved～98%CO Faraday efficiency at-0.58 V（vs.RHE）with the current density of-119.03 m A·cm^-2.Furthermore,the current density reached up to-264 m A·cm^-2at-0.98 V（vs.RHE）in the flow cell.In-situ EIS,in-situ ATR-FTIR,and in-situ XAFS results showed that Ni nanoparticles were beneficial to promote the splitting of H₂O to generate H*,and then promote the hydrogenation of CO₂to form*COOH intermediates.The Ni single atom site was conducive to the desorption of CO,thereby promoting the formation of CO products.The content of the two sites played an important role in balancing the formation of H*and the hydrogenation of CO₂.The Ni _NP2@Ni _SA2-NG with the content of Ni single atom sites of 61.54%and Ni nanoparticles of 38.46%can balance the two,thus promoting the efficient electrocatalysis of CO₂to CO.