Catalysis of the electrochemical reduction of carbon dioxide by iron(0) porphyrins. Synergistic effect of Lewis acid cations

Addition of Lewis acid cations such as Mg2+, Ca2+, Ba2+, Li+ and Na+ considerably improves the catalysis of CO2 reduction by iron(0) tetraphenylporphyrins, in terms of both catalytic efficiency and lifetime of the catalyst. Carbon monoxide is the main product, while formate is formed to a lesser extent (30% with Mg2+ and only 10% with Li+). Cyclic voltammetry indicates that the order of reactivity of these Lewis acid synergists is Mg2+ congruent to Ca2+ > Ba2+ > Li+ > Na+. Systematic analysis of the kinetics allows the proposal of a mechanism for the effect of both the divalent and monovalent cations. In many instances, the assisted catalysis is so strong that the current is affected by product inhibition as observed previously with Bronsted acid synergists. Self-inhibition is more irreversible in the present case and required a different approach to be analyzed. The importance of push-pull mechanisms in the catalysis of CO2 reduction is thus confirmed; electrons are pushed into the CO2 molecule by the electron-rich catalyst and the cleavage of one of the C-O bonds is helped by the presence of an electron deficient synergist.