Long-Time Dynamics Simulations of Surface Segregation in Pd-Au Nanoparticles

Zhiyao Duan
University of Texas at Austin

Bimetallic nanoparticles are widely used as catalysts in chemical production, environmental cleaning, and renewable energy generation and utilization. A primary factor that influences the activity of bimetallic catalysts is the elemental distribution in the surface and subsurface region. The elemental distribution is strongly influenced by the chemical environment the catalysts are operating in. The surface structure of the catalyst may differ significantly under reaction conditions from as-synthesized due to differences in environmental conditions. To understand the structure-function relationships of bimetallic catalyst, Monte Carlo simulations have been employed to predict the equilibrium elemental distribution under experimentally relevant conditions. However, kinetics of the structural evolution, which provides important mechanistic insights and time scale information, is rarely simulated due to the timescale gap between what can be simulated with molecular dynamics (MD) and experimentally relevant time scales.. Adaptive kinetic Monte Carlo (aKMC) is a promising algorithm, based upon harmonic transition-sate theory, designed to overcome this time scale gap.

In the study, we applied the aKMC method to understand the structural evolution of Au@Pd core-shell nanoparticles with 201 atoms and a truncated octahedral shape. Thermodynamically, Au surface segregation is favorable in Au@Pd nanoparticles, with all of the Au atoms on the surface. Using aKMC, we have simulated ~0.02 s of dynamics, a significantly longer timescale than the 10-6 s achievable by MD. Au surface segregation was observed in the aKMC simulation. The mechanism of Au surface segregation involves surface defects formation, concerted multiple atom rearrangements, and long-distance diffusion events. These remarkably dynamic surface rearrangements facilitate initial Au surface segregation. After the initial Au surface segregation, when the Au atoms occupy corner and edge sites of the nanoparticle, further Au surface segregation is inhibited. For a large portion of the simulated time, the nanoparticle remains in a partially Au surface-segregated state with Au atoms decorating the corners and edges. These simulations reveal differences between the thermodynamically favorable structure and more realistic structures which are kinetically accessible. In summary, our study, employing the aKMC method, effectively bridges the timescale gap and gives insights into the kinetics of surface segregation in Au@Pd bimetallic nanoparticles.

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