Theoretical Studies On Electronic Structures And Catalytic Performances Of Single-atom M-N/C Catalysts

Heterogeneous catalysts are widely used in chemical industry due to the stability and the feature of readily separation from reactants and products.However,the intrinsic complexities of the traditional heterogeneous catalysts active species have hindered the comprehension on level of molecules and atoms,thus make it difficult to design innovative atom effictive catalysts and develop catalytic principle.Nowadays,the explosive extension in research on catalysis by supported single-atom catalyst（SAC）proves the scientific interest in this new frontier of heterogeneous catalysis.SACs with high atom efficiency,stability,tuneable activity and selectivity offer a new avenue for sicentisits to diminish the gap between heterogeneous,homogeneous and enzyme catalysts.The isolated cativity center and distinct coordinating or binding environment of SACs could improve the development of catalytic theory.However,it’s difficult to ues“grand unified theory”to explain all the performance and characteristics of SACs.Single-atom M-N/C（with the active centre being the metal M bonded to the first coordination layer N or C atoms,M-N/C）catalysts are a significant branch of SACs and play an important role in the study of SACs.The combination of theory and experiment also provides a reference for the design and development of such single-atom catalysts.However,there are still many debates electronic structure of such catalysts.Therefore,studying the structure-activity relationship between the electronic structure and catalytic performance of such catalysts,exploring the possible catalytic reaction mechanism,and achieving the atomic-scale quantitative design of specific reaction catalyst systems are potential strategies for improving atomic economy and realizing green chemistry.In this thesis,the geometrical structure,electronic structure and catalytic properties of the single-atom M-N/C catalytic systems are studied by density functional theory.The aim is to provide a theoretical basis for understanding the catalytic nature of the single-atom catalytic reaction and the design of new high-efficiency catalysts.The quantitative constitutive relationships between the electronic properties of the active centres of SACs（Ni-N-C,Au-N/C）and their catalytic activity were first investigated;then the role of the metal-centred coordination environment of SACs（Fe-N/C,Pd-N/C）in regulating the adsorption properties of intermediate species in their catalytic reactions and the mechanism of catalytic oxidation were investigated;finally,the role of the carrier in optimising the catalytic activity of phthalocyanine-based SACs MPc（M=Fe,Co,Ni and Cu）was clarified.The main contents are as follows:Based on the active central structure of hemoglobin,a single-atom Ni-N-C catalyst with an active center of（Ni-N4）---N was constructed by using N-containing phenanthroline and pyridine as ligands.DFT theory revealed that the Ni---N site has excellent catalytic activation of hydrogen molecules,and H₂ undergoes heterocyclysis at this active site to produce Ni-H^δ-and N-H^δ+,with an activation enthalpy change energy barrier of 0.58 e V.Then the heteroclastic H can directly catalyse the ethanol aldehyde hydrogenation reaction with a reaction energy barrier of 0.55 e V.Molecular orbital energy level analysis,local molecular orbital LMOs,electron density difference（EDD）,electron localisation function（ELF）and localisation function of orbitals（LOL）reveal the FLP（frustrated Lewis pair）feature of the active centre.To further investigate the influence of the electronic properties of the active centre on the catalytic activity of the M-N/C SACs,the Au-N/C SACs geometrical configuration were investigated using DFT theory with the coordinate carrier of the semiconducting g-C₃N₄nanosheets,and the Au-N-C structure with Au^Ⅰactive centre was preferably selected as the catalyst for the acetylene hydrochlorination reaction.The Au SAC was determined to have an active Au atom of+1 valence,i.e.the active centre of the monoatomic Au-N-C catalyst is an N-coordinated Au^Ⅰ-N-C.The mechanism of the acetylene hydrochlorination reaction was investigated,and it was found that HCl was directly dissociative adsorbed at the Au-N site,effectively avoiding the activity loss and agglomeration due to the high-valent Au atom reduction by acetylene.The semiconducting nature of g-C₃N₄ ensures a flexible electron transfer mechanism between Au and the carrier,and ensures the structural stability of the active centre.The coordination sensitivity of Fe-N/C SACs were revealed by a comparative study of the effect of different coordination environments of Fe-N/C on their catalytic activity for the oxidation of benzene.The study of the equatorial plane coordination environment of the Fe-N/C active centre shows that the ratio of the N/C atoms in the first coordination layer of its equatorial plane plays a decisive role in the stability of its catalytic structure and the catalytic activity of the O=Fe=O intermediates,the more N atoms,the more stable of structure and the higher catalytic activity of its corresponding O=Fe=O intermediates.Comparing the catalytic activity of the Fe=O intermediate species with that of the axially coordinated O=Fe=O intermediate species,reveals that the axially coordinated second O can directly change the reaction mechanism of benzene oxidation on Fe-N₂C₂,while further reducing the reaction free energy barrier for C-O bonding on Fe-N₄.In order to more thoroughly investigate the mechanism of the metal centre coordination environment of M-N/C SACs on their catalytic oxidation activity,Pd-N/C SACs with trzn-COF as the coordination stable carrier were constructed.And,it was found that the Pd-C active centre coordinated with sp² C and the Pd-N active centre coordinated with the N atom of Schiff C=N bond both had good stability.A study of their CO oxidation reaction mechanism revealed that the adsorption characteristics and catalytic activity of the reaction intermediates of the Pd-C active centre and the Pd-N active centre are regulated by the anion-πinteraction of the specific-δgroup with the+δbenzene ring centre and the electron-withdrawing feature of the imine group,respectively.In order to investigate the mechanism of support influence on the activity of M-N/C SACs,the catalytic activity of phthalocyanine metal MPc（M=Fe,Co,Ni and Cu）as the active centre was investigated to catalyse the CO₂ reduction reaction（CO₂RR）to generate CO,and the effect of support on the activity of MPc was investigated by exploring the modelling of N-doped hollow porous carbon spheres（NHPCSs）.The interaction of MPc with intermediates COOH and CO,electronic structure analysis,and computational hydrogen electrode（CHE）model calculations showed that Co Pc with moderate charge spin had the optimal catalytic activity for the CO₂RR,and the free energy changesΔG₁ andΔG₂ for the COOH and CO adsorption steps are 0.02 e V and 0.33 e V,respectively.The addition of the support brings the overall reaction path closer to the ideal 0 e V and therefore shows better catalytic activity.