Systematic Assessment of Adsorption-Coupled Electron Transfer Toward Voltammetric Discrimination between Concerted and Non-Concerted Mechanisms

The electron transfer and specific adsorption of a redox-active molecule are coupled in many important electrode reactions. Herein, we report a theoretical framework for the voltammetric discrimination of the concerted and non-concerted mechanisms of adsorption-coupled electron-transfer (ACET) reactions. In the concerted mechanism, an oxidant in the solution is simultaneously reduced and adsorbed to deposit a reductant on the electrode surface. Alternatively, electron-transfer and adsorption steps are mediated separately in the non-concerted mechanism. Our model involves the common adsorption step for both mechanisms to ensure consistent adsorption properties of the redox couple. For simplicity, we assumed a weak adsorption step that does not contribute to the current response. We predicted that not only a kinetically controlled adsorption step but also a chemically reversible electron-transfer step is required for the voltammetric identification of the reaction mechanism. High scan rates were required during cyclic voltammetry (CV) for the kinetic control of the adsorption step. Unique CV shapes, or characteristic changes therein, were expected for each mechanism during the reversible adsorption of oxidants or reductants. We modelled the reversible adsorption of both the oxidant and reductant for the reduction of benzyl chloride at a Ag electrode. The experimental CV of this chemically irreversible ACET reaction kinetically controlled the adsorption step but was consistent with either mechanism to quantitatively validate our model. A voltammetric discrimination of the concerted and non-concerted mechanisms has not been demonstrated, but it will be possible if both requirements are satisfied.