Lawrence Berkeley National Laboratory

This review addresses the issue of surface segregation in bimetallic alloy nanoparticles, which are relevant to heterogeneous catalysis, in particular for electro-catalysts of fuel cells. We describe and discuss a theoretical approach to predicting surface segregation in such nanoparticles by using the Modified Embedded Atom Method and Monte Carlo simulations. In this manner it is possible to systematically explore the behavior of such nanoparticles as a function of component metals, composition, and particle size, among other variables. We chose to compare Pt75Ni25, Pt75Re25, and Pt80Mo20 alloys as example systems for this discussion, due to the importance of Pt in catalytic processes and its high-cost. It is assumed that the equilibrium nanoparticles of these alloys have a cubo-octahedral shape, the face-centered cubic lattice, and sizes ranging from 2.5 nm to 5.0 nm. By investigating the segregation of Pt atoms to the surfaces of the nanoparticles, we draw the following conclusions from our simulations at T= 600 K. (1) Pt75Ni25 nanoparticles form a surface-sandwich structure in which the Pt atoms are strongly enriched in the outermost and third layers while the Ni atoms are enriched in the second layer. In particular, a nearly pure Pt outermost surface layer can be achieved in those nanoparticles. (2) Equilibrium Pt75Re25 nanoparticles adopt a core-shell structure: a nearly pure Pt shell surrounding a more uniform Pt-Re core. (3) In Pt80Mo20 nanoparticles, the facets are fully occupied by Pt atoms, the Mo atoms only appear at the edges and vertices, and the Pt and Mo atoms arrange themselves in an alternating sequence along the edges and vertices. Our simulations quantitatively agree with previous experimental and theoretical results for the extended surfaces of Pt-Ni, Pt-Re, and Pt-Mo alloys. We further discuss the reasons for the different types of surface segregation found in the different alloys, and some of their implications.