Kinetics of simple inner- and outer-sphere electrochemical reactions at rotating silver electrodes as examined using surface-enhanced raman spectroscopy

An application of surface-enhanced Raman spectroscopy (SERS) to evaluating the kinetics of electrochemical reactions is outlined. This involves monitoring the potential-dependent SERS intensities of the adsorbed reactant at a rotating disk electrode in a potential region where the kinetics are under mixed mass transfer-heterogeneous electron transfer control. Comparisons are made between the kinetic behavior extracted from the SERS intensity-potential dependence and that obtained from the faradaic current flow. For Co(NH₃)₆³⁺ reduction at silver, where the reactant is electrostatically attracted but not bound to the surface, the SERS and electrochemical rate behavior is closely compatible. However, for Cr(NH₃)₅Br²⁺ and Cr(NH₃)₅NCS²⁺ reduction, where the reactants bind directly to the surface prior to electron transfer, the reactivities at the SERS-active sites are substantially higher than for the prevalent adsorbate as obtained electrochemically. These findings suggest that surface sites displaying efficient Raman scattering may also provide centers of catalytic activity.