Efficient Photocatalytic Conversion Of Low-Concentration Of Co₂ Based On Metal Catalytic Sites Regulation

Since the industrial revolution,human activities such as the utilization of fossil fuels have emitted a large amount of carbon dioxide（CO₂）,which disrupts the balance of the natural carbon cycle and causes a set of serious environmental problems,such as global climate change.Photocatalytic converting CO₂ into high value-added industrial raw materials such as carbon monoxide（CO）is considered to be an effective approach to alleviate environmental pressure from CO₂ emissions.However,due to the high stability of CO₂ molecular,the large potential barrier for CO₂ reduction and the complexity of the catalytic reaction process,CO₂ photoreduction still suffers from some bottlenecks such as low efficiency and poor selectivity.Due to the greater barrier for CO₂ absorption and activation at low concentration,most of the current researches are conducted in the pure CO₂ atmosphere in order to improve the efficiency and selectivity for CO₂ photoreduction.Nevertheless,the concentration of CO₂ emitted by human beings is often low（ca.3%-15%）.The capture,condensation and purification process of CO₂ not only requires special equipment but also is accompanied by huge energy consumption and new carbon emissions.Therefore,efficient photoconversion of low-concentration CO₂ directly from industrial waste streams into solar fuel is of great significance to the high value-added utilization of anthropogenic CO₂.The metal catalytic sites often play the role of active centers in CO₂photoconversion.Therefore,regulating the metal catalytic sites of catalyst is expected to provide efficient catalysts for the photocatalytic conversion of low concentration CO₂and good research models for investigating the catalytic mechanism.This thesis mainly contains the following three parts:（1）Ni-based metal organic frameworks monolayers（Ni MOLs）with fully exposed metal sites has been constructed by a freezing process-assisted ultrasonication method.In the low concentration of CO₂（10%）,the as-prepared Ni MOLs shows high catalytic performance for CO₂-to-CO conversion with an apparent quantum yield（A.Q.Y.）of1.96%and a selectivity of 96.8%.Such a catalytic performance is not only higher than that of CO₂ photoreduction systems conducted at the low-concentration CO₂ but also higher than that of most pure CO₂ photoreduction systems.As compared with Ni MOLs,its isostructural Co MOLs counterpart is almost inactive at a low concentration of CO₂.Based on the results of systematic experiments and density functional theory（DFT）calculations,it is confirmed for the first time that the strong adsorption energy for CO₂on metal sites is the key factors for determining the catalytic efficiency and product selectivity in photocatalytic reduction of low concentration of CO₂,demonstrating a kind of metal-node-dependent activity and selectivity.（2）Based on the above results,the relationship between the CO₂ adsorption properties and catalytic performance for low-concentration CO₂ photoreduction has been further investigated by studying the catalytic performance of isostructural hierarchical Ni Co₂O₄ and Mg Co₂O₄ hollow nanocages（Ni Co₂O₄ HCs and Mg Co₂O₄HCs）with fully exposed metal sites.The resulting Ni Co₂O₄ HCs shows a high apparent quantum yield（A.Q.Y.）of 1.56%with a CO selectivity of 89%at the low concentration of CO₂,which exceeds the catalytic performance of most inorganic catalysts in pure CO₂.Its isomorphic Mg Co₂O₄ HCs counterpart shows much lower catalytic performance although it features higher CO₂ adsorption capacity.The results of experiments and DFT calculations show that the CO₂ molecules adsorbed on the Ni site can be easily reduced to CO,while the CO₂ adsorbed on the Mg site cannot participate in this reaction.Based on the above results,the concept of active adsorption of CO₂ on metal sites has been proposed for the first time.Only active CO₂ adsorption can accelerate the whole catalytic reaction,thereby revealing the relationship between CO₂adsorption and photoreduction at the atomic level.（3）In view of the inevitable production of H₂ in the photoreduction of low concentration of CO₂ and the consequent low selectivity,this work first proposes and realize the idea of converting diluted CO₂ and H₂O into tunable CO/H₂ mixtures（syngas）,which circumvents the rigorous requirement of high selectivity.Through the organic ligands-related complexation selectivity,Fe metal-organic frameworks（MOFs）and Ni MOFs with different ligands were constructed from electroplating sludge via a gradient recovery approach.The as-prepared Ni MOFs exhibits record-high performance for CO₂-to-CO photoconversion in low-concentration CO₂（10%）,while the Fe MOFs trends to H₂ production.According to a modular catalyst design strategy based on metal sites,syngas with a widely tunable CO/H₂ ratio（1:15-14:1）was produced for the first time from low-concentration CO₂ photoreduction by adjusting the ratio of Fe MOFs and Ni MOFs.The diverse catalytic performance on Fe/Ni MOFs originates from the different affinities towards CO₂/H₂O,which relates to the electron numbers in the 3d orbitals of metal sites.In summary,the relationship between metal catalytic sites and photocatalytic conversion of low-concentration CO₂ has been revealed.It is believed that this thesis not only provides a detailed theoretical basis for designing and constructing catalysts towards CO₂ photoreduction or other applications,but also presents a feasible application outlet for the high-value utilization of low-concentration CO₂.