Structure And Thermal Stability Of Platinum-based Metal Alloy Nanocatalysts

Metal clusters at the nanoscale have better catalytic activity than bulk materials due to their large specific surface area,which make it important applications in the chemical industry.Among them,nano-alloy catalysts synthesized from two or more metals exhibit superior catalytic activity than its single metals.In addition,the morphology,size and atomic arrangement of the catalyst are key factors in determining its performance.In this paper,molecular simulation and theoretical phase diagram methods are used to study the structure and properties of platinum-based nano-alloy catalysts from these three factors.Firstly,the structure properties of Pt-based bimetallic nanoclusters with three morphologies(icosahedron,decahedron and cuboctahedron)and Pt-Co-Ni Trimetallic clusters with different compositoins were studied by Monte Carlo method.It is found that the Core-Shell bimetallic nanoclusters consisting of M(Cu,Co,Ni)core/Pt shell have better stability than that of Pt core/M(Cu,Co,Ni)shell.In the Crown-Jewel structure system,the cluster is more stable when Pt atoms are distributed at the inflection point of clusters surface.Moreover,the size and morphology of the clusters have no effect on the surface aggregation of Pt.For Pt-M(Cu,Co,Ni)bimetallic nanoclusters with different configurations,the structure stability of Pt-Ni system is stronger than that of Pt-Cu and Pt-Co system.For Pt-Co-Ni nanoclusters,Pt,Co,Ni atoms are mainly distributed in the surface,core and sub-surface layers of clusters.When the atoms of Pt:Co:Ni=2:1:2,the system show greatest structure stability,and when Pt:Co:Ni=1:3:1,the structure stability is the worst.Secondly,the melting properties of Pt-based bimetallic nanoclusters with three morphologies(icosahedron,decahedron and cuboctahedron)and Pt-Co-Ni trimetallic nanoclusters with different compositions have been studied by molecular dynamics method.It is shown that the thermal stability order of Pt-M(Cu,Co,Ni)bimetallic under three different morphologies was:Pt-Ni>Pt-Co>Pt-Cu.As to the same bimetallic system,the thermal stability of Core-Shell structure is better than that of Crown-Jewel structure.Interestingly,for Pt-Co-Ni trimetallic nanoclusters,the thermal stability is superior to others when Pt:Co:Ni=2:1:2 and the thermal stability is the worst when Pt:Co:Ni=1:3:1,which is consistent with the structure stability.In addition,the thermal stability of bimetallic and trimetallic nanoclusters increases with the increasing size of clusters,and the relationship between melting point and N-1/3 conforms to Pawlow's law.Finally,the thermodynamic properties of bimetallic nanoalloys are studied by phase diagram model.Phase diagrams of nanoalloys(4-100 nm)with different morphologies were studied by nano-thermodynamic model.It was found that the morphology of dodecahedron,truncated octahedron and icosahedron exhibite better phase stability,and the order of phase stability of bimetallic nano-alloys is Pt-Ni>Pt-Co>Pt-Cu,which is consistent with the results of MD simulation.Quasichemical model was used to study the phase separation properties of Pt-Cu nanoalloys.The results show that Pt atoms were more likely to be separated from the surface of the alloys,which explained that Core-Shell structure was more stable than others.Importantly,we propose a modified model for the first time,combining molecular simulation with nano-thermodynamic model,to predict the melting point of small sized(1-100 nm)nanoalloys.