Dynamic Monitoring of the Structural Evolution of Au@Pd under Electrochemistry

Core–shell nanocatalysts have shown superior catalytic activity than monometallic catalysts. However, these metastable materials are susceptible to structural changes during catalysis. Comprehending the evolution of surface sites and their stability under different reaction conditions is crucial for designing durable and highly active core–shell nanocatalysts. Herein, structural transformation of the atomic layer thickness of Pd shells on Au nanocubes in different electrolytes at various electrochemical windows was investigated by a combination of cyclic voltammetry (CV), surface-enhanced Raman spectroscopy (SERS) of adsorbed probe molecules, and elemental analysis. Pd sites are stable under basic and neutral conditions but experience severe structure evolution under acidic conditions. Pd atoms that are directly coordinated by Au atoms, upon oxidation at evaluated potential, transform into Pd ions via the reaction with H+ which would also be adsorbed on the Au sites. These Pd ions are easily coreduced with the formed Au ions into surface alloys in the backward CV scan. In contrast, Pd atoms in the thick Pd overlayer (>1 monolayer) are likely to dissolute into the electrolyte solution and leach. SERS revealed that the change of Pd sites primarily occurred at contiguous Pd sites and isolated Pd sites were relatively stable. This evolution mechanism provides new insight into the rational design of efficient and stable catalysts and is expected to promote further application of core–shell nanocatalysts.