Cobalt and nickel complexes of 2,2′ : 6′,2″ : 6″,2‴-quaterpyridine as catalysts for electrochemical reduction of carbon dioxide
Abstract
The complexes [Co(qtpy)(OH2)2][ClO4]2 and [Ni(qtpy)(MeCN)2][ClO4]2(qtpy = 2,2′ : 6′,2″ : 6′,2‴-quaterpyridine) were prepared by treating qtpy with Co(ClO4)2·6H2O and Ni(ClO4)·6H2O respectively in acetonitrile. In acetonitrile both complexes show one reversible couple assignable as the [M(qtpy)L2]2+/+ couple {E1/2 for [Co(qtpy)(OH2)2]2+/+=–0.67 V; [Ni(qtpy)(MeCN)2]2+/+=–0.79 V vs. saturated calomel electrode, SCE}, one quasi-reversible couple assignable as [M(qtpy)]+/0(E1/2=–0.96 for Co, –1.15 V for Ni) and one reduction wave at about –1.98 V vs. SCE assignable as reduction of [M(qtpy)] to [M(qtpy)]–. Constant-potential electrolysis of a 0.2–0.4 mmol dm–3 solution of [Co(qtpy)(OH2)2]2+ in the presence of CO2 at –1.7 V resulted in the production of CO with a current efficiency of about 80%. At bulk concentration of [Co(qtpy)(OH2)2]2+ 0.2 mmol dm–3, electrodeposition of the cobalt complex occurs at –1.65 V vs. SCE to give a dark brown electroactive film on the electrode surface. The cobalt-modified electrode is catalytically active towards the electrochemical reduction of carbon dioxide both in acetonitrile and in water. Constant-potential electrolysis with the cobalt-modified electrode at –1.7 V in acetonitrile and at –1.3 V in water resulted in the production of CO with current efficiencies of about 35%. No film formation on the electrode was observed for [Ni(qtpy)(MeCN)2]2+ and its catalytic activity towards reduction of CO2 is much less than that of the cobalt complex.