Surface segregation in bimetallic nanoparticles (NPs) can be critically important for their catalytic activity because the activity is largely determined by the surface composition. Little, however, is known about the atomic scale mechanisms and kinetics of surface segregation. One reason is that it is hard to resolve atomic rearrangements experimentally. It is also difficult to model surface segregation at the atomic scale because the atomic rearrangements can take place on time scales of seconds or minutes – much longer than that can be modeled with molecular dynamics. Here, we use the adaptive kinetic Monte Carlo (AKMC) method to model the segregation dynamics in PdAu NPs over experimentally relevant time scales. Our focus on PdAu NPs is motivated by experimental work showing that both core@shell and random alloy PdAu NPs with diameters of less than 2 nm are stable, even under reaction conditions. This means that one of these structures must be metastable and kinetically trapped. Our simulations show that NPs with the Au@Pd structure are thermodynamically stable and NPs with the PdAu random alloy structure are metastable and kinetically trapped below 400 K over time scales of hours. These AKMC simulations provide insight into the energy landscape of the two NP structures, and the key mechanisms that lead to segregation. In the core-shell NP, surface segregation occurs primarily on the (100) facet through both a vacancy-induced and a vacancy-free mechanism. The system becomes kinetically trapped when all corner sites in the core are occupied by Pd atoms. Higher energy barriers are required for further segregation, so that the metastable NP has a partially alloyed shell. In contrast, surface segregation in the PdAu alloy NP is suppressed because the alloy NP has reduced strain as compared to the Au@Pd NP, and the segregation mechanisms in the alloy require more elastic energy for the exchange of Pd and Au and between the surface and subsurface.

Adaptive kinetic Monte Carlo simulations of surface segregation in PdAu nanoparticles

2019 Spring National Meeting

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