Evolution of metastable structures on bimetallic surfaces from microscopy and machine-learning molecular dynamics*

Restructuring of interfaces plays a crucial role in materials science and heterogeneous catalysis. In particular, bimetallic surfaces often adopt very different composition and morphology compared to the bulk. For the first time, we reveal a detailed long-timescale restructuring of Pd deposited on Ag by combining microscopy, spectroscopy, and novel simulation methods. We develop and perform accelerated machine-learning molecular dynamics, followed by automated mechanistic characterization, to discover surface restructuring events in an unbiased fashion, including Pd-Ag place exchange and Ag pop-out. We find that encapsulation of Pd islands by Ag always precedes layer-by-layer dissolution of Pd into Ag, resulting in significant migration of Ag out of the surface and leaving behind extensive vacancy pits within microseconds. Because Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd, these metastable structures are of vital catalytic importance. Systematic investigation of other bimetallic systems (e.g. Pd/Au, Pt/Ag, Co/Pt) and adsorbate-induced restructuring (e.g. CO, H2) is underway.