Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 51 / c / iii / Solution

A useful electrode-surface scheme for the proposed ECEC mechanism isElectron-transfer arrows occur at the electrode, whereas both capture steps occur in the adjacent solution layer. CO${}_2$ is attacked by an oxygen-centered nucleophilic component of the reduced quinone, forming an O-carbonate linkage rather than simply physically adsorbing on neutral AQ.

At potentials more negative than $-2.0\mathrm{V}$, direct second reduction of any unbound semiquinone is also accessible: $\mathrm{A}{\mathrm{Q}}^{\bullet -}\to \mathrm{A}{\mathrm{Q}}^{2-}$, followed by carbonate formation. Thus the fully reduced unbound and mono-adduct species can feed the bis-adduct, and the drawing should include CO${}_2$ transport into the layer and adduct transport away. ECEC is the proposed sequence; competing EECC transfer/capture ordering is possible at such negative potentials.