Study On The Synthesis Of Single-Atom Arrays And Their Catalytic Properties

Since its introduction in 2011,single-atom catalysts have sparked a wave of research in the field of energy catalysis.These catalysts consist of a central metal atom and surrounding ligand atoms embedded in a heterogeneous substrate,allowing for high metal dispersion and selectivity.Furthermore,the heterogeneous substrates within the single-atom catalysts can be easily separated from the reaction system,facilitating the recovery and reuse of efficient catalysts.However,current single-atom synthesis strategies tend to focus on the structural modulation of single-atom sites,resulting in mostly randomly distributed single-atoms in the substrate.This neglects the effect of site distribution on catalytic activity and selectivity.Developing a method to synthesize ordered single atoms will pave the way for researching the relationship between the ordered distribution of single atoms and catalytic properties.In this paper,using metal phthalocyanine precursors,we developed a film surface shrinkage-induced self-assembly strategy to synthesize periodic single-atom arrays with one-dimensional ordered distribution of metal single atoms in the arrays.Information on the distribution and local structure of the single atoms in the arrays was analyzed with the aid of various characterization techniques,the effects of the singleatom distribution on activity and selectivity were analyzed by oxygen reduction reaction.In addition,binary single-atom arrays were also prepared using this strategy with a large number of axial diatomic sites distributed in the arrays,which exhibit higher CO selectivity in the electrocatalytic CO2 reaction.Theoretical calculations further reveal the intrinsic mechanism between site structure and activity.The paper is divided into four chapters,as shown below.In Chapter 1,we provide a concise overview of the development of single-atom catalysts,including the synthesis methods and characterization techniques used to achieve structural modulation and accurate analysis of single-atom sites.Our work demonstrates that a variety of site structures enable efficient conversion of small molecule compounds through electrocatalytic means.In Chapter 2,we developed a synthetic method for surface shrinkage-induced selfassembly and prepared one-dimensional single-atom arrays.By controlling the experimental conditions,we analyzed the specific processes and influential factors of the array formation.Using advanced electron microscopy,we observed that the arrays consist of periodic parallel stripes on the surface of the nanosheets.Each stripe is composed of a number of graphene layers vertically distributed on the surface of the nanosheet.Most of the metal atoms are anchored in the graphene,resulting in a onedimensional ordered distribution of the metal atoms at the microscale.By analyzing the X-ray absorption spectra,we conclude that the metal atoms are atomically dispersed in the array and have a local coordination structure of MN4(M=Fe,Co,Cu)configuration.In Chapter 3,we analyzed the performance of Fe-based single-atom arrays in the oxygen reduction reaction.The half-wave potential of the array(0.892 V)was significantly higher than that of the disordered distribution of the Fe single-atom sample(0.824 V).Additionally,the arrays exhibited low H2O2 yields and high reaction stability.Theoretical calculations indicate that the anchored FeN4 sites in the inner layer of the streaked multilayer graphene structure significantly adjust the electron distribution of FeN4 sites in the outer layer.This adjustment forms lower d-band centers,weakens the free energy for adsorption of oxygen-containing intermediates(*OOH,*O and*OH),alters the reaction decisive step,and ultimately reduces the overpotential of the oxygen reduction reaction.In Chapter 4,we synthesized binary single-atom arrays using two precursors,iron phthalocyanine and nickel phthalocyanine.Within the internal multilayer graphene structure,the adjacent FeN4 and NiN4 sites form axial dual atomic sites.This unique axial arrangement exhibits a three-dimensional confinement effect,effectively suppressing the out-of-plane motility of the central metal atom and ultimately generating dual atomic sites with high yields and stability in the array.In CO2 electroreduction tests,the heteronuclear sites exhibit higher CO selectivity than homonuclear sites at low potentials.The tunable ratio of syngas obtained at high potentials can be used for later production of chemicals and fuels such as saturated aldehydes,methanol,and substitute natural gas.Theoretical analysis shows that the NiN4 site in the inner layer causes significant charge polarization of the FeN4 site in the outer layer.This not only produces a lower d-band center and reduces the energy barrier for the transition from intermediate*CO to gaseous CO but also effectively inhibits the formation of H2,ultimately improving the CO selectivity of the FeN4 site in the CO2 electroreduction reaction.