Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles

Abstract

We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using Modified Embedded Atom Model potentials and the Monte Carlo method. The nanoparticles are assumed to have disordered fcc configurations at two fixed overall concentrations (50 at.% Pt and 75 at.% Pt). We use four kinds of nanoparticle shapes [cube, tetrahedron, octahedron, and cubo-octahedron] terminated by {111} and {100} facets to examine the extent of the Pt segregation to the nanoparticle surfaces and determine the equilibrium structures of Pt-Ni nanoparticles at T=600 K. The model particles contain between 560 and 4631 atoms (particle size ranging from 2.5 to 5 nm).Our results imply that a complete {100}-facet reconstruction could make the cubooctahendral Pt-Ni nanoparticles most energetically favorable, consistent with experimental observations. We predict that at 600 K due to segregation the equilibrium Pt 50 Ni 50 nanoparticles with fewer than 2000 atoms and Pt 75 Ni 25 nanoparticles with fewer than 4000 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that due to an order-disorder transition the Pt 50 Ni 50 cubo-octahedral nanoparticles containing more than 2000 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogenous core. *Corresponding

Quantitative prediction of surface segregation in bimetallic Pt–M alloy nanoparticles (M=Ni,Re,Mo)

“…Therefore, one aim of our investigations for Pt-Ni (fcc-fcc), Pt-Re (fcc-hcp), and Pt-Mo (fcc-bcc) nanoparticles is to establish that the MEAM approach is capable of accurately simulating surface phenomena for a broad spectrum of Pt bimetallic alloy nanoparticles, even when one component might prefer a different lattice. Using MEAM potentials, we have indeed achieved reasonably good simulation results for Pt-rich Pt-Ni [21], Pt-Re [22], and Pt-Mo [23] nanoparticles with the fcc lattices and Re-rich Pt-Re [22] nanoparticles with the hcp lattice.…”

“…The MEAM potentials for the pure metals (Pt, Ni, Re, and Mo) are developed by fitting parameters to reproduce empirical data for cohesive energies, lattice constants, elastic constants, and vacancy formation energies of fcc Pt, fcc Ni, hcp Re, and bcc Mo [21][22][23]. Moreover, as shown in Table 1, the MEAM potentials for pure metals lead to surface energies of relaxed extended low-index pure surfaces that are in good agreement with first-principles calculations [44] and experimental measurements [45,46].…”

“…To lower the cost of fuel cells, it is desirable to design new electrode catalysts containing a low Pt content overall, but high Pt concentration in the surface. To this end, we chose to theoretically investigate the segregation tendency of Pt atoms to the surfaces of Pt-Ni [21], Pt-Re [22], and Pt-Mo [23] nanoparticles.…”

A DFT Study on the O2 Adsorption Properties of Supported PtNi Clusters

“…Previous computational results based on Monte Carlo (MC) simulations have addressed the structural stability of cuboctahedral PtNi NPs, ranging from 2.5 to 5 nm [22]. Using a many-body potential to describe the interactions between metal atoms, MC simulations at 600 K showed that PtNi tends to form surface-sandwich structures, with a segregation pattern in which Pt atoms are enriched at the outermost and third shells, while the Ni atoms are enriched in the second shell.…”

Shape-selected bimetallic nanoparticle electrocatalysts: evolution of their atomic-scale structure, chemical composition, and electrochemical reactivity under various chemical environments

“…Several previous experimental and theoretical studies have investigated temperature- [35][36][37][38][39][40][41][42][43][44] and adsorbate-driven [45][46][47][48] segregation process in Pt-Ni systems, including the effect of such segregation in their catalytic reactivity. 35,37,[49][50][51][52] In general, segregation phenomena in bimetallic systems can be understood based on the surface energies and atomic radius of the two metals, with the element having a larger atomic radius and a lower surface energy segregating towards the free surface.…”