Facet-Dependent Restructuring and Catalytic Activity of Cu Single Crystals during CO Electro-Oxidation

Understanding the relationship between the surface structure and catalytic activity is central to the rational design of efficient electrocatalysts. While Cu is well-known for its tendency to restructure under reaction conditions, it remains poorly understood how exactly such dynamic structural changes influence the catalytic activity. Here, we combine electrochemistry with in situ electrochemical scanning tunneling microscopy (EC-STM) to study and directly compare CO electro-oxidation on Cu(111) and Cu(100) single crystals and how it relates to the structural changes on these two faces. We find that both surfaces undergo nanometer-scale restructuring during the reaction, leading to the formation of undercoordinated Cu adatoms, which act as the catalytically active sites for both surfaces. However, their morphological evolution differs markedly: Cu(111) exhibits dynamic and reversible restructuring, maintaining a high density of adatom nanoclusters across successive potential steps, whereas Cu(100) forms clusters that evolve less reversibly with a gradual decrease in cluster density over time and upon repeated potential steps. Notably, the evolution of clusters and their density do not directly correlate with the observed catalytic activity for either facet. Instead, we propose that the facet-dependent differences in activity stem primarily from variations in the effective density of the catalytically active Cu adatoms and their distinct interaction with reactants rather than from different structural motifs. These findings highlight the crucial role of dynamic surface restructuring in governing catalytic performance and emphasize the need to account for facet-specific morphological and structural changes in the rational design of efficient Cu-based electrocatalysts.