Kinetic Map for Destabilization of Pt-Skin Au Nanoparticles via Atomic Scale Rearrangements

A commonly used strategy to enhance the mass activity of Pt-based catalysts involves the synthesis of Au nanoparticles (NPs) with a monolayer-thick Pt-skin layer. The synergistic effect of Au and Pt results in a higher catalytic activity and better Pt utilization. However, the stability of the Pt-skin layer is questionable as our recent equilibrium Monte Carlo simulations predict that eventually the surface Pt is replaced by Au. The role of Au during destabilization of Pt-skin in vacuum and solution is investigated with the help of molecular dynamics. Different starting Au-Pt arrangements are studied mimicking various NP synthesis approaches. Beyond a critical number of atoms in a Pt cluster, the ideal Pt monolayer rapidly transforms to a three-dimensional (3D) Pt cluster. This is supported by our model predicting transition from the Pt monolayer to Volmer-Weber growth in the Au-Pt system. At room temperature, Pt atoms move into the subsurface layer at second timescales mainly via the exchange mechanism involving Au atoms or Au climbing on top of Pt. For all practical purposes, the experimental "Pt-skin" Au NPs may actually correspond to a single layer of surface Au over subsurface Pt layers. Presence of large 3D Pt clusters may slowdown the climbing of Au atoms on Pt, thereby delaying the formation of Au-skin.