Mechanism of the electrochemical reduction of carbon dioxide at inert electrodes in media of low proton availability

Direct electrolysis of CO2 in DMF at an inert electrode, such as mercury, produces mixtures of CO and oxalate, whereas electrolysis catalysed by radical anions of aromatic esters and nitriles produces exclusively oxalate in the same medium. Examination of previous results concerning the direct electrochemical reduction and the reduction by photoinjected electrons reveals that there are no significant specific interactions between reactant, intermediates and products on the one hand, and the electrode material on the other, when this is Hg or Pb. These observations and a systematic study of the variations of the oxalate and CO yields with temperature and CO2 concentration, allow the derivation of a consistent mechanistic model of the direct electrochemical reduction. It involves the formation of oxalate from the coupling of two CO2 radical anions in solution. CO (and an equimolar amount of carbonate) is produced by reduction at the electrode of a CO2-CO2= adduct, the formation of which, at the electrode surface, is rendered exothermic by non-specific electrostatic interactions.