The Synergetic Construction Of Intrinsic Defects Controlled Oxidation As Well As Reduction Active Sites And Mechanism Study Of Photocatalytic CO₂ Reduction

Since the industrial age,the development and utilization of fossil energy has promoted the development and progress of human society and civilization.However,the human society is suffering from environmental pollution and other urgent problems dut to the rampant employment of fossil energy.Under this background,governments agreed to keep global temperature change below1.5°C by 2050 and to develop CO₂ capture,use and storage technology（CCUS）.Inspired by photosynthesis in nature,researchers have developed an artificial photosynthesis system that couples photocatalytic CO₂ reduction with water or biomass oxidation to reduce CO₂ to CO,methane,methanol,ethane and ethanol.Although this process employs green and clean light energy as driving energy and is considered as a potential green way for CO₂ emission reduction as well as comprehensive utilization,there are problems of low catalytic efficiency and difficult control of product selectivity.As this process involves both reduction and oxidation half-reaction processes,reasonable design and construction of the dominant active sites of H₂O/biomass oxidation and CO₂ reduction and the synergy effect of the active sites hold the key to enhance the performance of artificial photosynthesis system,which can also provide guidance and reference for the innovation and breakthrough of efficient photocatalyst.This thesis aims at enhance the performance of photocatalytic CO₂reduction with H₂O oxidation and photocatalytic CO₂reduction with glycerol oxidation.By selecting Ti O₂ and layered double hydroxide（LDHs）novel two-dimensional photocatalytic materials as semiconductors,it focuses on design and construction of metal dispersion and semiconductor defect structure at nano and atomic scale in the metal/semiconductor photocatalyst,to improve the CO₂reduction and water or glycerol oxidation reaction rate as well as induce the synergetic effect between them.Finally,the catalytic and synergetic mechanism of half-reaction dominant sites are also revealed at molecular and atomic scales.（1）In the photocatalytic reaction of CO₂reduction and H₂O oxidation,to improve the efficiency of CO₂ reduction reaction,the Au@Pd nanoparticles loaded on oxygen vacancy-rich Ti O₂ catalyst is prepared.Through the synergy of the two active sites,the catalytic performance is enhanced.The results of HAADF-STEM,EDS and CO-FTIR confirm that the bimetallic particles show typical core-shell structure with ca.1 nm thickness of Pd as shell and Au particles as the core.EPR and XPS characterization show that the concentration of oxygen vacancy defect on the surface of Ti O₂ support is significantly increased after hydrogen reduction treatment.Due to the enhanced activation ability of H₂O by oxygen vacancy defects,the efficiency of water oxidation half-reaction is improved,thus forming a large number of active hydrogen protons.Subsequently,in coordination with Au@Pd nanoparticles,active hydrogen was effectively used for CO₂ reduction,improving the efficiency of the reduction half reaction.By adjusting the ratio of Au@Pd to oxygen vacancy sites,Au₁Pd₂-15%-Ti O₂catalyst can obtain CH₄ with 26.32μmol/（g·h）yield with electron selectivity of 96%.CO₂-TPD,CO₂-FTIR and a series of probes assisted in-situ FTIR characterization identify the sites and adsorped models for CO₂ and active hydrogen,and confirm that Au@Pd-CO₂^·-and Au@Pd-H adsorbed species on the surface of Au@Pd nanoparticles play a key role in synergistic matching between oxidation and reduction half-reactions,resulting in highly efficient and selective reduction of CO₂ to CH₄.（2）In the photocatalytic reaction of CO₂ reduction and H₂O oxidation,the metal vacancy anchored Pt single atom catalyst is constructed with the goal of controlling the reduction selectivity of CO₂.The electronic structure of Pt single atom is adjusted by the in-situ regulation of photogenerated electrons to optimize the activation of the reaction intermediates,thus realizing the selectivity controlment of CO₂ reduction to hydrogencarbon.By introducing the photoactive ions of Zn²⁺,Ni²⁺and Ti⁴⁺into the layers,two-dimensional LDHs photocatalytic materials are prepared.Followed by liqud etching,the Zn²⁺ions are selectively dissolved to form metal vacancy defects.Then,the Pt single atoms are anchored by metal vacancy defects to obtain Pt/Zn Ni Ti-LDHs catalyst.EXAFS characterization results show that the metal-metal coordination number of LDHs decreases after etching,which confirms the formation of metal vacancy defects on the support surface.XPS,XANES and EXAFS further confirm the metal vacancy and Pt exist the electron interaction and clarify that Pt single atoms are linked to metal vacancy defects thtrough Pt-O and Pt-Ti coordination.In combination with in situ CO-FTIR and XANES characterization,the induction effect of photogenerated electrons on the electron structure of Pt single atoms is revealed under dark and light conditions.By controlling the concentration of photogenerated electrons,Pt single atoms close to+2 valence and Pt^δ+single atoms close to 0 valence are selectively generated.Combined with the catalytic performance and a series of in-situ spectroscopic characterization,it is found that the electronic structure of Pt single atoms has a significant effect on the selectivity of hydrocarbons in the photocatalytic CO₂ reduction reaction.The low-valence Pt^δ+single atoms can directly hydrogenate the activated intermediate CO to CH₄,while the high-valence Pt single atom can cooperate with the adjacent LDHs surface metal ion site to open the carbon-carbon coupling path to reduce CO to C₂H₆.（3）In the photocatalytic coupling of CO₂ reduction with glycerol oxidation,the CO₃^2--LDHs materials is designed and constructed to achieve the above reaction on the basis of following characteristics:Firstly,LDHs layer metal ions and hydroxyl groups could capture photogenerated electrons and holes respectively,thus providing active sites for the reduction and oxidation half-reactions of coupling reactions;Secondly,LDHs has a high affinity for CO₃^2-between layers,which could capture CO₂ from air and store it in the form of CO₃^2-in aqueous solution to solve the difficulty of CO₂ enrichment on the catalyst surface in gas-liquid-solid three-phase reaction.Specifically,the Pt/Mg²⁺Fe³⁺-LDHs nanoreactor is constructed with the capability of CO₂ capture and photocatalytic coupling reaction.Furthermore,by reducing the thickness of LDHs layers,the utilization rate of CO₃^2-preenriched in LDHs interlayer is improved,and abundant oxygen vacancy defects are induced to form Fe²⁺by reducing part of Fe³⁺,thus enhancing the performance of the coupled reaction of light-driven CO₂ reduction and glycerol oxidation.AFM,XPS,EPR,EXAFS and TG-MS are used to reveal the effect of Mg Fe-LDHs thickness on oxygen vacancy concentration and interlayer CO₃^2-content.When the thickness of LDHs decreases from 5 nm to 2 nm,the metal-oxygen coordination number of layer ions decreases significantly,the concentration of oxygen vacancy defects increases significantly,and some Fe³⁺are induced to Fe²⁺.Although the content of interlayer anion storage in ultrathin LDHs decreases,the utilization rate of interlaminar CO₃^2-increases due to the increased openness of interlaminar space.In addition to the observation of Pt single atoms by AC-HAADF-STEM,CO-FTIR and XAFS characterization confirm that the oxygen vacancy on the surface of ultrathin LDHs anchored Pt through Pt-O and Pt-Fe coordination,and induced the formation of electron-rich Pt single atoms.After the structure of the nanoreactor is clarified,the possible mechanism of the coupling reaction is proposed by combining DFT calculation with in-situ spectroscopy characterization.Under light,the secondary hydroxyl group of glycerol molecule is activated by the hydroxyl group of LDHs layer,then the secondary hydroxyl group and adjacent C-H bond are dehydrogenated with the help of photogenerated holes to form dihydroxyacetone,lactic acid and active hydrogen.Subsequently,the interlamellar carbonate activated by Fe²⁺in the layer is reduced to CO with the participation of active hydrogen and photogenerated electrons,and CH₄ is formed at the single atom of Pt^δ+to complete the photocatalytic coupling reaction.More importantly,the in-situ FTIR and catalytic performance confirm that the nanoreactor could continue to capture CO₂ in the air after the reaction to supplement the interlayer CO₃^2-consumed by the reaction and show good reusability.The integration of CO₂ capture with photocatalytic coupling reactions also enables innovation in CCUS process.