Description: This review addresses the issue of surface segregation inbimetallic alloy nanoparticles, which are relevant to heterogeneouscatalysis, in particular for electro-catalysts of fuel cells. We describeand discuss a theoretical approach to predicting surface segregation insuch nanoparticles by using the Modified Embedded Atom Method and MonteCarlo simulations. In this manner it is possible to systematicallyexplore the behavior of such nanoparticles as a function of componentmetals, composition, and particle size, among other variables. We choseto compare Pt75Ni25, Pt75Re25, and Pt80Mo20 alloys as example systems forthis discussion, due to the importance of Pt in catalytic processes andits high-cost. It is assumed that the equilibrium nanoparticles of thesealloys have a cubo-octahedral shape, the face-centered cubic lattice, andsizes ranging from 2.5 nm to 5.0 nm. By investigating the segregation ofPt atoms to the surfaces of the nanoparticles, we draw the followingconclusions from our simulations at T= 600 K. (1) Pt75Ni25 nanoparticlesform a surface-sandwich structure in which the Pt atoms are stronglyenriched in the outermost and third layers while the Ni atoms areenriched in the second layer. In particular, a nearly pure Pt outermostsurface layer can be achieved in those nanoparticles. (2) EquilibriumPt75Re25 nanoparticles adopt a core-shell structure: a nearly pure Ptshell surrounding a more uniform Pt-Re core. (3) In Pt80Mo20nanoparticles, the facets are fully occupied by Pt atoms, the Mo atomsonly appear at the edges and vertices, and the Pt and Mo atoms arrangethemselves in an alternating sequence along the edges and vertices. Oursimulations quantitatively agree with previous experimental andtheoretical results for the extended surfaces of Pt-Ni, Pt-Re, and Pt-Moalloys. We further discuss the reasons for the different types of surfacesegregation found in the different alloys, and some of theirimplications.
Date: June 20, 2005
Creator: Wang, Guofeng; Van Hove, Michel A.; Ross, Phil N. & Baskes,Michael I.
Item Type: Refine your search to only Article
Partner: UNT Libraries Government Documents Department