Ion pair formation between the electrogenerated 2,3-dichloro-5,6-dicyano-1,4-benzoquinone dianion and the sodium ion at platinum surfaces

The electrochemical reduction at platinum electrodes of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in the presence of Na⁺ in acetonitrile solution was studied using various electrochemical measurements under conditions with significant mass transport, namely channel flow- and sono-voltammetries. In channel electrode voltammetry, the presence of Na⁺ was observed to change the voltammetric response from that of pure DDQ alone. In particular, at potentials corresponding, in the absence of Na⁺, to the two electron reduction step of DDQ, a spike-like increase and decrease of reductive current was observed on negative going potential scans after which the current returned to the level expected for a one-electron reduction. This behavior can be explained by the following reaction scheme with the formation of a passivated surface due to the adsorption of Na₂⁺ DDQ^2-. DDQ^2-+Na⁺→Na⁺DDQ^2- Na⁺DDQ^2-→Na⁺DDQ_ads^2- Na⁺DDQ_ads^2-+Na⁺DDQ _ads^2-→Na₂⁺ DDQ_ads^2-+DDQ_2- This scheme was found to be consistent with independent sono-voltammetric data and with in situ electrochemical ESR measurements.