Visualizing the Site-Specific Kinetics and Terrace-to-Edge Migration of CO Electrooxidation on a Single Pd Nanocatalyst.

The rational design of electrocatalysts requires a molecular-level understanding of how active sites and surface migration jointly influence catalytic behavior, particularly for reactions involving interactions between surface-bound adsorbates. Directly resolving this interplay under reaction conditions remains challenging because it requires simultaneous chemical sensitivity and nanoscale spatial resolution. Here, we employ in situ electrochemical tip-enhanced Raman spectroscopy (EC-TERS) to track CO electrooxidation on individual Pd nanocatalysts with sub-30 nm dimensions at a spatial resolution of 3 nm. Hyperspectral imaging reveals that oxidation is initiated at low-coordinated edge sites, manifested as preferential CO depletion at the particle periphery. At the same time, a continuous red shift of the C-O stretching mode on terrace sites indicates a decrease in the local CO coverage. Together with the preferential CO depletion at edge sites, this spatial and spectral evolution reveals terrace-to-edge CO migration during electrooxidation. These observations suggest that surface migration contributes to sustaining the reaction and may play a rate-limiting role during the early stages of electrooxidation. Across Pd nanocatalysts with different local structures, the migration behavior evolves differently, linking the nanoscale structure to electrocatalytic performance. More broadly, this work demonstrates how spatially resolved chemical imaging can disentangle site-specific reactivity and molecular migration, providing a framework for understanding migration-mediated electrocatalysis at complex electrochemical interfaces.