Monte Carlo simulations of segregation in Pt-Re catalyst nanoparticles

Abstract

. We predict that due to the segregation process the equilibrium Pt-Re nanoparticles would achieve a core-shell structure, with a P t-enriched shell surrounding a Pt-deficient core. For fcc cubo-octahedral Pt 75 Re 25 nanoparticles, the shells consist of almost 100 at.% of Pt atoms. Even in the shells of hcp truncated hexagonal bipyramidal Pt 50 Re 50 nanoparticles, the concentrations of Pt atoms exceed 85 at. % (35 at.% higher than the overall concentration of Pt atoms in these nanoparticles).Most prominently, all Pt atoms will segregate to the surfaces in the hcp truncated hexagonal bipyramidal Pt 25 Re 75 nanoparticles containing less than 1 000 atoms. We also find that the Pt atoms segregate preferentially to the vertex sites, less to edge sites, and least to facet sites on the shell of Pt-Re

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].…”

“…We found from our previous studies [22,23] assumed that their bimetallic nanoparticles also have an fcc crystal structure as well as a cubo-octahedral shape (shown in Fig. 1), which is bounded by {111} and {100} facets.…”

“…Our MEAM potential for Re predicts that hcp Re is more stable by 0.084 eV/atom than fcc Re, whose equilibrium lattice constant is 3.89 Å [22]. We also calculated the surface energies for the relaxed extended surfaces of fcc Re using the MEAM potential.…”

“…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.…”