Theoretical Investigations On Multi-atom Metal Catalysts

Since the concept of‘single-atom catalysts’（SACs）was proposed in 2011,SACs have become a research hotspot in the field of photocatalysis and electrocatalysis,and have achieved industrialization in 2020.In addition to the advantages of SACs,supported diatomic catalysts（DACs）have higher metal atom loading,more flexible active sites and adsorption sites,making them exhibit better catalytic performance.On the basis of SACs,single-atom nanozymes（SANs）with similar activities to biological natural enzymes have emerged.SANs have well-defined geometric structures and uniform active centers,opening up new avenues for the development of artificial enzymes with catalytic properties of natural enzymes.Combining the advantages of supported DACs and SANs,multi-atom metal catalysts supported by two-dimensional materials was designed,and theirs catalytic performance were studied based on density functional theory,the conversion of methane,the oxygen evolution reaction（ORR）,and the oxidation of carbon monoxide（CORR）were studied.This thesis mainly includes the following research contents:（1）A Fe₂ dimer catalyst supported on two-dimensional C₂N monolayer（Fe₂@C₂N）was constructed to explore its catalytic activity for the partial oxidation of methane,as well as the source of activity and the catalytic mechanism.The results show that the H₂O₂dissociates spontaneously on Fe₂@C₂N and generate a Fe-O-Fe intermediate,which is comparable to the bond length of Fe-O in the methane monooxygenase,and has similar characteristics to the Cu-O-Cu active centers in the current high-efficiency catalyst zeolites for methane conversion.The Fe-O-Fe active center exhibits high activity for C-H cleavage of methane,promoting the formation of methyl radicals,which is beneficial to the conversion of methane.This work enriches the application of two-dimensional material supported dimer catalysts for C-H bond activation,and provides ideas for the development of efficient non-precious metal catalysts for direct oxidation of methane at low temperatures.（2）Frist-principles computations were performed to investigate the detailed reaction mechanism of methane conversion over 26 transition metal（from IB to group VIIIB）and 4 main group metal（M=Al,Ga,Sn,Bi）dimers embedded in phthalocyanine monolayers（M₂-Pc）.We clarified the pathways of methane C-H bond activation assisted by surface oxygen and hydroxyl species（*O and*OH）,obtained by the decomposition of hydrogen peroxide.We also investigated the direct non-oxidative conversion of methane on these M₂-Pc.The results show that methane conversion proceeds via an*OH-assisted mechanism over the Ti₂-Pc and Zr₂-Pc,with energy barriers of 0.85 and 0.86 e V,respectively.A combination of*O-and*OH-assisted mechanism on the surface of Sc₂-Pc,with an energy barrier of 0.63 e V for C-H bond cleavage.The C-H dissociates spontaneously on the surface of the Ta₂-Pc,which is expected to realize the non-oxidative conversion of methane.Our work can not only enrich the catalytic variety of supported dimer catalysts,but also provide strategies for the design of effective supported dimer catalysts for methane conversion of different selectivity.（3）To simplify the complicated structures of the samples obtained in the experiments,combined with SACs,the effects of Mo doping and S defects on the catalytic performance of FeS ultrathin nanosheets for oxygen evolution reaction,and the relationship between the composition-structure-activity of the catalysts were examined.The results show that the introduction of S vacancies in the Mo-doped system will reduce the positive charge of its neighboring Fe sites,which will affect the charge transfer with the reaction intermediates,and regulate the adsorption energy of the intermediates,thereby enhancing the activity of the Fe sites.This work is expected to provide an avenue to explore the application of anionic defects and heteroatom doping in catalysis.（4）Unlike metal atoms were introduced into Ni₁/FeO_xSACs to construct DACs M₁Ni₁/FeO_x（M=Mn,Co,Cu,Zn）,and the synergistic effect of nearby metal atoms on CO oxidation was discussed.Different from the traditional dimer catalysts,the constructed structure contains a dual-metal hetero-bi-atoms.The results show that the introduced metal can tune the adsorption and desorption of reaction intermediates by regulating the electronic structure,thereby changing the catalytic activity of CO oxidation.In particular,the activity of Mn₁Ni₁/FeO_xis even higher than that of the SAC Pt₁/FeO_x,and the energy barrier of the rate-limiting step is only 0.65 e V.This work reduces the use of expensive metals through synergy between cheap metals,opening a new avenue for designing novel nanocatalysts for low-temperature CO oxidation and beyond.