Molecular Dynamics of Free and Graphite-Supported Pt-Pd Nanoparticles

Abstract

The thermal characteristics of bimetallic Pt-Pd nanoparticles, both free and graphite-supported, were investigated through molecular dynamics simulations using quantum Sutton-Chen many-body potentials for the metal-metal interactions. The graphite substrate was represented as layers of fixed carbons sites and modeled with the Lennard-Jones potential model. The melting temperatures for bimetallic nanoparticles were estimated based on variations in thermodynamic properties such as potential energy and heat capacity. Melting temperatures of the nanoparticles were found to be considerably lower than those of bulk Pt and Pd. The Pt-Pd clusters exhibited a two-stage melting, where surface melting of the external atoms is followed by homogeneous melting of the internal atoms. The melting transition temperature was found to increase when the particle is on the graphite support, with an increase at least 180 K higher than that of the same-sized free nanoparticle. The results of the density distributions perpendicular to the surface indicate that the Pd atoms have a tendency to remain at the surface, and the Pd atoms wet the graphite surface more than the Pt atoms, while root mean squares suggest that surface melting starts from the cluster surface, and surface melting was seen in both free and graphite-supported nanoparticles. Structural changes accompanying the thermal evolution were studied by the bond-orientational order parameter method.

Atomic Surface Segregation and Structural Characterization of PdPt Bimetallic Nanoparticles

“…In addition, both Pt and Pd are totally miscible and their bulk melting temperature is 2045 K and 1827 K, respectively, so according to the segregation rules reported previously by G. Guisbiers et al, the segregated element is the one with the lowest surface energy, therefore Pd goes to the surface due to Pd having a lower surface energy compared to Pt [ 40 ]. This evidence shows a trend of Pd atoms to segregate on the surface of nanoparticles and to remain there even after the completion of the heating process in correspondence with the results published in previous works [ 19 , 41 ], i.e., C. Fernández-Navarro et al [ 42 ] studied the dynamics of Pd-Pt nanoparticles in a process of heating until reaching the melting condition, and compared the melting process in an unsupported particle against that of a particle supported on a graphite substrate. Their results exhibited a local rearrangement of metal atoms near the carbon substrate in such a way that the metal lattice had a better match with the carbon substrate, and that the Pd atoms migrated to preferentially occupy the region in direct contact with the carbon substrate.…”

Island Formation on Pt/Pd(557) Surface Alloys in the Presence of Adsorbed CO: A Molecular Dynamics Study

“…The bare systems were run in the canonical (NVT) ensemble at 850 K for 200 ps and the microcanonical (NVE) ensemble for 1 ns and displayed minimal changes in the (557) structure during this period. This temperature is well below previously simulated melting temperatures for Pt/Pd alloyed nanoparticles in both free and graphite-supported configurations. − …”

The study of dynamics and phase transitions of small Ag/Pd motifs using molecular dynamics and histogram methods

“…Several computational methods such as electronic structure calculations [12][13][14][15], generic algorithms [16][17][18], Monte Carlo [19,20] and molecular dynamics [21,22,4,23] methods have been used to investigate the most stable structures of bimetallic clusters. In comparison on these methods, electronic structure calculation is a challenging task for transition metal system due to the complexity of their potential energy landscape and the complexity increases further in bimetallic alloys [24].…”