Modeling of equilibrium conformation of Pt 2 Ru 3 nanoparticles using the density functional theory and Monte Carlo simu
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In this study, the density functional theory (DFT) and Monte Carlo (MC) simulations were conducted to determine the equilibrium conformation of Pt2Ru3 nanoparticles with diameters 1.0–3.5 nm at finite temperature. DFT calculations were carried out to estimate the binding energy using slab configurations and energy could be correlated with some structural descriptors and multilinear regression equations to calculate the binding energy from descriptors related to the number of a specific bond to neighboring atoms. MC simulations were carried out to obtain the equilibrium conformation of atoms in Pt2Ru3 at 150–363 K. MC simulations’ result shows that atoms of the same element tend to segregate each other, and Pt/Ru ratio on the surface increases with increasing particle size; also, most of the Pt are located on the surface whereas most of the Ru are located on the subsurface or at the core sites. It is qualitatively exhibited that the Pt/Ru ratio on the surface decreases with increasing temperature. I. INTRODUCTION
In recent years, polymer electrolyte fuel cells (PEFCs) have attracted increasing interest because they can provide clean energy with high efficiency. Pt has long been considered to be the most effective electroanode catalyst for PEFCs. However, Pt is a precious metal and can be easily poisoned by carbon monoxide (CO), which exists as an impurity in the fuel used in PEFCs.1–3 The catalytic properties of a metal can be markedly changed by alloying with a second metal.4 Alloys provide a means by which the strength of bonds between metal and adsorbates can be modified, thus opening a mechanism by which catalytic chemistry can be controlled. Pt–Ru alloys are known to substantially improve the catalytic performance in the electrochemical oxidation of CO from CO contaminated hydrogen fuels or to suppress CO adsorption on the catalyst. The reason for these improvements has been a subject of many studies but still remains an unresolved issue. Within the ligand effect approximation, the addition of Ru increases the CO tolerance via electronic interactions that may reduce the Pt–CO bond strength, which may facilitate CO oxidation5–7 or decrease CO adsorption. In the popular bi-functional mechanism,8,9 it is assumed that Ru provides an oxygenated surface species by dissociating water at the Ru sites at lower potentials against the pure Pt catalyst, leading to accelerated CO2 formation and thus improving the tolerance of CO. The addition of Ru8,10 lowers the overpotential Contributing Editor: Susan B. Sinnott a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.57
depending on the composition and atomic structure of the alloy. Han et al. reported a density functional theory (DFT) that showed that alloying Pt with Ru on the surface only slightly enhances the adsorption energy of CO relative to that of pure Pt; after O or OH oxidized Ru, the adsorption energy of CO on neighboring Pt is dramatically reduced.11 Babu et al. have reported a series of electrochemical 195Pt and 13C
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