Electrocatalytic reduction of carbon dioxide by using macrocycles of nickel and cobalt

Carbon dioxide has been activated towards reduction by electrochemically primed transition metal complexes. This indirect reduction allows more favorable electrode potentials than those required when CO2 is reduced directly. The metal complexes employed were Curtis type tetraazamacrocyclic complexes of nickel and cobalt. The electrode potentials ranged from -1.3v to -1.6v vs SCE. Evidence for CO2 reduction was obtained from controlled potential coulometry experiments performed in a gas tight electrolysis cell followed by g.c. analysis. The solvents used were either CH3CN-H2O or water only. Carbon monoxide was found to comprise at least 50% of the total reduced products in all cases; H2 was also produced in most cases. While a protic source was found to be necessary for CO2 reduction to occur, the results show that CO2 competes effectively with H+ for the electrons available from the electrochemically reduced metal complexes. In addition, the reduction was found to be electrocatalytic, with current efficiencies for total reduced products as high as 98%. (Author)

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