Theoretical Study On Structural,Thermal Stability,and Activity Of Nanoalloys Catalyst

In recent years,mental nanoalloys(MNAs)catalysts have attracted lots of attention due to their high specific surface area and unique nanostructures.As a new type of nanometer material,it has important applications in fuel cell,selective hydrogenation,epoxidation of olefins,and other catalysis fields.It is also known as the "fourth generation catalyst".Due to the limitation of current experimental techniques,the computer simulation methods to study the MNAs catalyst have become an important research method.In this paper,the structural properties of MNAs catalyst with important catalytic applications were firstly studied by MC.Afterwards,the thermal stability of MNAs catalyst was further obtained by MD.Finally,catalytic properties of MNAs catalyst were calculated by DFT method.The main conclusions are given as follows:1.Studying the structure stabilities of MNAs catalysts.The structure properties of MNAs catalysts with different sizes and compositions were studied by the software package developed by our group.Moreover,the surface segregation rule of metal atoms was further summarized.(1)Studying the structure stabilities and mechanism of bimetallic MNAs catalysts.The equilibrium structures of Ni-based,Ag-based and Pt-based bimetallic MNAs catalysts were systematically investigated.Based on the MC study,it is found that Ni-Ir and Ni-Ru MNAs catalysts tend to form Ni-Ir(Ru)-Ni "three-layer onion" structure.However,Ni-Pt,Ag-Co,Ag-Cu and Ag-Au MNAs catalysts tend to form a "core-shell" structure.Pd-Pt bimetallic MNAs with different sizes and shapes tend to form crown-Jewel structure.The reasons structures formation are surface energy and atomic size.(2)Studying the structure stabilities and mechanism of trimetallic MNAs catalyst.The equilibrium structures of Pt-based trimetallic MNAs catalysts were systematically investigated.Pt-Au-Pd catalyst tend to form an onion-like structure.The composition has an important effect on the structural stability of Pt-Au-Pd.Similar to the Pt-Au-Pd structure,the Pt-Ag-Ni MNAs catalysts also forms an onion-like structure in which the Ag atoms are located on the surface of the Pt-Ag-Ni trimetallic MNAs,and the Pt atoms mainly occupy to the subsurface,while the Ni atoms are mainly located on the core.In addition,the existence of onion-like structure can be confirmed by excess energy.The formation of onion-like structure for trimetallic MNAs catalysts can be explained by surface energy and atomic size.2.Studying the thermal stability and effect factors of MNAs catalyst.The structure thermal evolution and melting properties of MNAs catalyst are studied by MD method.It can be found that the "core-shell" structure of MNAs catalyst can remain stable during heating,which shows good thermal stability.Based on the Hamiltonian model and MD simulation,a universal thermal evolution descriptor was proposed.Based on this descriptor,196 bimetallic MNAs catalyst systems were rapidly screened to identify 44 bimetallic MNAs catalysts with structural thermal evolution tendency upon heating.The system was verified by MD simulation and experimental results.For the Pd-Pt bimetallic MNAs catalyst,the melting point of Pd-Pt catalyst is mainly affected by shape and size.For the thermal stability of Ni-based MNAs catalysts,it was found that the composition has a significant effect on thermal stability.In addition,it is found that the thermal stability of Ni-based catalysts is related to the morphology.Finally,the thermal stability of Pt-Au-Pd and Pt-Ag-Ni trimetallic MNAs was studied.It is found that the thermal stability is best when the composition is Pt224Au113Pd224 and Pt337Ag112Ni112,and the melting point of the trimetallic MNAs catalyst satisfies the Pawlow's law.3.Studying the catalytic activity of MNAs catalyst.DFT method is used to study the catalytic activity of Pt-free(Ag-Co)and Pt-based(Pd-Pt)MNAs catalyst.(1)Studying the catalytic activity of Ag-Co MNAs catalyst and mechanism explanation.It is found that the composition has an important effect on the oxygen reduction reaction(ORR)catalytic activity of Ag-Co MNAs catalyst.The composition Ag42Co13 has the best ORR activity according to reaction energy barrier.Moreover,the composition also affects the ORR reaction mechnism of the Ag-Co MNAs catalyst.The activity of Ag-Co and other Co-based MNAs catalysts was compared by DFT calculation and theoretical analysis.It is found that the core-shell Ag42Co13 MNAs catalyst has better ORR activity than other Co-based MNAs catalyst due to the charge transfer effect,which agrees well with the experimental observations.(2)Studying the catalytic activity of Pd-Pt MNAs catalyst and mechanism explanation.It can be found that the activity of Pd-Pt MNAs catalys is mainly affected by the morphology,size and oxygen coverage.The activity of Pd-Pt MNAs catalys with different morphologies follows the order of Dec<Cubo<Ico.Moreover,the Pd-Pt MNAs catalyst with small size has the higher activity than that with large size.With the oxygen coverage increasing,the activity of Pd-Pt MNAs catalyst decreases.The effects of size,morphology and oxygen coverage on the activity of Pd-Pt MNAs catalyst were further explained by projected density of states(PDOS)and charge density difference analysis.The charge redistribution and transfer induced by size,morphology and oxygen coverage can affect the activity of Pd-Pt MNAs catalyst.