(348l) On the Coalescence Behavior of Supported and Unsupported Binary Alloy Nanoclusters: A Molecular Dynamics Simulation Study

Ramachandran, S., Argonne National Laboratory
Sankaranarayanan, S., Argonne National Laboratory

Bimetallic nanoclusters of type Pt3M (M=Ni,Co,Cr etc?) are of particular interest because of their enhanced catalytic activity toward oxygen reduction reaction compared to Pt, as well as the ability to reduce the overall levels of platinum loading in the PEM fuel cells. Pt3M alloys are also known to exhibit a unique structure (Monte carlo simulations) wherein an enriched layer of platinum (often referred to as Pt-skin) is found to at the surface of the nanocluster followed by a alloying element enriched layer followed by subsequent layers resembling a bulk mixed configuration. Such surface enrichment of Pt in the alloyed cluster is considered to be a key contributing factor for improved catalytic activity. Supported catalysts, prepared by suspending these alloy nanoclusters on carbon (amorphous) or graphitic carbon, are the common form in which these catalysts are employed in PEM fuel cells. The supports' role on the stability and structure of the nanoparticles is important in order to design an effective catalyst. Stability of the nanoclusters at higher temperatures (experienced either during synthesis or during operation) is an important aspect determining the durability of these catalysts. Coalescence behavior of alloy nanoparticles offers a unique insight into the role of alloying element in the coalescence mechanism as well as a way to estimate the kinetic and transport parameters for such processes. Monitoring the behavior of coalescence on inert support (such as graphite), as well as metallic support (either Pt or Ni) helps determine the role played by the substrate on the stability of nanocluster. This atomistic simulation study investigates the coalescence behavior of two binary alloy nanoparticles for a host of starting temperatures in vacuum, as well as in the presence of an inert and a metal substrate. The coalescence characteristics of bimetallic Pt-Ni nanoclusters of different sizes and compositions were investigated through molecular dynamics simulations using the Quantum Sutton-Chen (QSC) many-body potentials. Monte-Carlo simulations employing the bond order simulation model were used to generate minimum energy configurations, which were utilized as the starting point for molecular dynamics simulations. The coalescence behavior of alloy nanostructures is also characterized by a growing neck phase. Structural changes accompanying the thermal evolution were studied by the bond order parameter method (BOP). The simulated MD trajectories were used to calculate the deformation parameters which are used to characterize the structural evolution resulting from diffusion of Pt and Ni atoms during the coalescence process. The coalescence characteristics were found to depend on the composition, shape and size of the nanocluster. These studies are currently being extended to evaluate the effect of an inert substrate such as graphite as well as metal substrate such as Pt and Ni. Cu-Ni alloy systems were also studied. Such alloys exhibit a well segregated system (with an enriched Ni core and surface segregated Cu). Also, the properties of Cu and Ni are similar unlike the case of Pt and Ni. Thus, this type of system provides us an opportunity to compare and contrast the nature of alloying element (noble Pt vs. Cu), as well as the nanocluster structure (moderately segregated vs. well segregated) and it's impact on coalescence behavior. Nanoparticles having an initial icosahedral as well as cuboctahedron structure were simulated in this study. The interaction of two binary nanoclusters in vacuum was found to strongly impact the alloy segregation, with the extent of mixing dependent on the initial nanocluster shape and size.