Direct visualization of inner-sphere electrocatalytic reactions as they occur at detachable electrochemical...

Molecular-level observation of inner-sphere electrocatalytic reactions has remained a longstanding challenge. Although recent advances in in situ and operando mass spectrometry enable probing of complex electrochemical processes, intrinsic limitations of mass spectrometry hinder characterization of adsorbed intermediates at inner-sphere interfaces. Herein, we report a detachable electrochemical interface strategy that enables direct visualization of inner-sphere electrocatalytic reactions as they occur. A positively charged [Au₉(PPh₃)₈]³⁺ nanocluster acts as a molecular interfacial carrier that selectively assembles at the cathodic interface and mediates electron transfer. Under reaction potentials, changes in the charges of the cluster weaken its interaction with the electrode, allowing reaction information-bearing clusters to desorb into solution for immediate mass spectrometric analysis while retaining their interfacial origin. Based on this strategy, we achieved operando visualization of multiple types of inner-sphere electrocatalytic processes. In the hydrogen evolution reaction, we directly observed short-lived hydrogen intermediates on the [Au₉(PPh₃)₈]³⁺ associated with proton-coupled electron transfer (PCET) pathways, with the interfacial species identified as [Au₉H(PPh₃)₈]²⁺. In the reduction of nitrosobenzene, we captured the formation of nitrosobenzene radical anions at the interface ([Au₉(PPh₃)₇ + PhNO]²⁺) and their evolution pathway in the solution phase. Additionally, in the interface reconstruction reaction, we observed the potential-dependent reconstruction and growth of [Au₉(PPh₃)₈]³⁺ and [Au₁₁(PPh₃)₈]³⁺ in the presence of metal ions, providing new insights into interfacial reconstruction phenomena. By converting previously elusive interfacial reactions into directly observable single chemical entities, this work establishes a general operando platform for probing short-lived inner-sphere electrocatalytic intermediates, bridging a critical gap between interfacial reaction chemistry and molecular-level detection.