In Situ SERS Study of Electrocatalytic CO ₂ RR on Au@Pd Bimetallic Catalysts: From Isolated Single Atoms to Monolayer of Pd

ABSTRACT A fundamental understanding of catalytic activity and mechanism for electrocatalytic CO 2 reduction reaction (CO 2 RR) from a single‐metal atom to its monolayer on another metal substrate is crucial for bimetallic catalysts design. Herein, in situ surface‐enhanced Raman scattering (SERS) spectroscopy systematically probed electrocatalytic CO 2 RR on Au@Pd catalysts with varying Pd coverage, from isolated single‐Pd atoms and Pd nanoislands to Pd monolayers on Au surfaces. In situ SERS sensitively detected the adsorption behavior of *CO intermediate, which, combined with electrocatalytic performance, reveals that isolated Pd single atoms on Au show the best activity for CO 2 ‐to‐CO conversion and can enhance the activity of nearby Au sites. This is attributed to the moderate energy of *COOH formation and *CO desorption on single‐Pd atoms as corroborated by calculations. Moreover, we observed different potential‐dependent behavior of *CO adsorbed on Pd nanoisland and monolayer surfaces. Theoretical results suggest that surface *H on Pd reverses the binding energy of *CO(atop) and *CO(bridge) at Pd step edge sites, making *CO(atop) more favorable, whereas *CO(bridge) is more stable without *H adsorption. This reveals the important role of *H in regulating intermediate adsorption configurations during CO 2 RR. These observations are valuable for fine‐tuning the atomic structure of bimetallic catalysts with suitable *H and *CO absorption properties for high CO 2 RR performance.