Bridging Spectroscopy and Reactivity: Identifying the True Reactive CO Species in Electrocatalytic CO Oxidation

In situ electrochemical vibrational spectroscopy has revolutionized our understanding of electrocatalysis. However, establishing a rigorous correlation between spectroscopic signatures and reaction kinetics is still very challenging because high-coverage surface species may dominate the signal, whereas the truly reactive participants may be short-lived or spectroscopically silent. To disentangle spectators from true reaction participants, we developed a high-sensitivity spectroelectrochemical platform that integrates the rotating disk electrode with surface-enhanced Raman spectroscopy (RDE-SERS). This approach enables the simultaneous acquisition of reaction kinetics and interfacial molecular information under well-defined mass transport conditions. Using this versatile approach, we investigated the benchmark carbon monoxide oxidation reaction (COOR) on the Pt surface, which enabled decoupling between the spectroscopically observable species and the catalytically active species. Despite persistent and intense Raman signatures of adsorbed CO (COad) in the diffusion-limited region, COad predominantly acts as a spectator that blocks surface sites without effectively participating in the reaction, as confirmed by the isotope substitution experiments. Conversely, the oxidation current is sustained by the continuous flux of solution-phase CO (COsol), which is identified as the kinetically relevant reactant. These findings offer a new perspective on the roles of adsorbed species versus solution-phase species during dynamic electrochemical reactions. Collectively, the RDE-SERS methodology is established as a reliable strategy for identifying true reaction pathways, bridging molecular-level spectroscopic identification with macroscopic reactivity in complex electrocatalytic systems.