The Fine Tuning Of Magnesium-based Catalysts For Photo/Photothermal Catalytic C-H Bond Activation

The selective activation of C-H bond is long been the goal in the field of chemistry.However,the high binding energy of C-H bond makes its activation highly challenging.Moreover,the activatied C-H bonds are very reactive and easy to be over-oxidated into CO₂,complicating the control of selectivity.Meanwhile,the photo-/photothermal catalysis can utilize the photogenerated carriers to accomplish up-hill redox reactions,offerring opportunities for activating C-H bonds under mild conditions.Hence,the fabrication of a photo/photothermal catalyst with both high activity,selectivity and durability for C-H bond activation is not only a research focous in academic community but also provides possibility for in-depth understanding for the activation mechanism of C-H bonds under mild conditions.This dissertation presents a series of studies on the fine-tuning of magnesium-based catalysts for photo/photothermal catalytic C-H bond activation.By means of non-noble metal doping,noble single-atom loading,and facet regulation,the the coordination,electron,and band structures of magnesium-based catalysts were successfully tuned,realizing the construction of efficient,selective,and durable photo/photothermal catalytic systems.Furthermore,with the help of combined experimental（in-situ XAFS,in-situ DRIFT）and theoritical（DFT calculations）characterizations,we successfully revealed the structural-activity relationship between the catalyst structure and the adsorption/desorption energy of key reaction imtermediates.This dissertation provides a new strategy and approach for the rational design of magnesium-based catalysts for photo/thermal reaction systems.The main research contents of this dissertation are as follows:1.Based on the unique electronic structure of the MgAl-LDH laminate with the uniform divalent and trivalent metal dispersion,we successfully synthesized MgAl-LDH loaded with single Au atoms（Au₁-MgAl-LDH）and MgAl-LDH loaded with Au nanoparticles（Au-NP-MgAl-LDH）.Subsequently,we proposed 6 possible models for the fine structure of Au₁-MgAl-LDH and discussed them in detail.Finally,by combining EXAFS fitting with DFT calculation results,we found that single Au atoms were loaded on MgAl-LDH in the form of Au-O₄ coordination above the Al³⁺of the MgAl-LDH laminate.In the photocatalytic conversion of benzene to phenol,Au₁-MgAl-LDH presented a phenol selectivity of 99%,while Au-NP-MgAl-LDH produced only aliphatic acids,and no production of phenol was detected.By combining in-situ DRIFT,in-situ XAFS,and DFT calculations,we found that the difference in selectivity between Au₁-MgAl-LDH and Au-NP-MgAl-LDH arise from their different adsorption behaviors of substrate benzene:for Au₁-MgAl-LDH,benzene is activated by a single Au-C bond,maintaining the symmetry of the benzene ring and leading to the formation of phenol;while for Au-NP-MgAl-LDH,multiple Au-C bonds are generated during activation of benzene,leading to the destruction of benzene symmetry and the formation of aliphatic acids.2.Based on the topological transformation of NiMgAl-layered double hydroxide（LDH）,a series of Ni²⁺-doped MgO/Al₂O₃ catalysts（NiMgAl-x）were successfully constructed,in which Ni²⁺was doped into MgO crystals and occupied the position of Mg.During the topological transformation process,the electrons cam be transferred from Ni,Mg,and Al to O reached the maximum level in NiMgAl-800,further constructing highly active electron-rich oxygen species on the surface of NiMgAl-800.When applied in the photothermal catalytic coupling of methane,NiMgAl-800 exhibited the highest ethane selectivity（97.8%）and ethane productivity(453.20μmol g^-1 h^-1),which was the highest value among the reported non-noble metal-based methane coupling catalysts,and its catalytic stability can be maintained over 12 h.It was also found that the introduction of a trace amount of water（300μL）in the reaction system can greatly enhance the selectivity and productivity of ethane,and the introduced water are capable of repairing the oxygen defects generated on the surface of the catalyst during the reaction.Such mechanism can be classfied as a Mars-van Krevelen-like mechanism.Meanwhile,the participation of water can induce the reaction pathway shift to favorable for methane coupling,and inhibit the over-oxidation of methane,thereby increasing the yield and the selectivity of ethane.3.Based on the crystal facet engineering of MgO,we successfully synthesized three MgO materials with different exposed facets using various methods,and loaded Au nanoclusters on their surfaces,respectively.Through XRD,HRTEM,and HAADF-STEM characterizations,we successfully demonstrated the morphological and surface atomic arrangement differences among MgO with different exposed facets.Furthermore,we used CO₂ as a probe molecule to further characterize the ratio of each exposed facet for Au/MgO（100）,Au/MgO（110）,and Au/MgO（111）.In the photothermal oxidative coupling of methane,Au/MgO（111）exhibited extremely high ethane production rates(12733.4μmol g^-1 h^-1),high ethane selectivity（90.6%）,and catalytic stability（100 h）.Its C₂₊production rate and catalytic stability surpassed those of most advanced photocatalytic OCM and NOCM systems reported in the literature.By combining in-situ Au L₃-edge XANES with DFT theoretical calculations,we found that during the photothermal processes,Au acts as a hole acceptor,exhibiting strong d-σhybridization with*CH₃/Au,thus serving as a strong and effective*CH₃ coupling site to direct the catalytic reaction path towards C-C coupling.In contrast,under thermal catalytic conditions,Au cannot accept holes,leading to the predominant formation of the over-oxidative product CO₂.