Homogeneous Electrocatalytic CO2 Reduction Using a Porphyrin Complex with Flexible Triazole Units in the Second Coordination Sphere

The electrochemical reduction of carbon dioxide (CO2) to produce value-added chemicals is of great significance in mitigating environmental and energy concerns. In this work, an iron porphyrin catalyst, FePEG8T, with multiple triazole units tethered to a porphyrin ligand via flexible oxymethylene linkers, is reported for efficient electrocatalytic reduction of CO2 to afford carbon monoxide (CO). The electrocatalyst exhibits an excellent catalytic activity with a current density of -17.5 mA/cm(2) and CO Faradaic efficiency of 95% at -2.5 V vs Fc/Fc(+) in acetonitrile using water as the proton source. The maximum turnover frequency (TOFmax) was calculated to be 5.5 x 10(4) s(-1) using foot-of-the-wave analysis, which is thirty times higher than the result from our previous zinc complex with the same triazole-porphyrin ligand. Control experiments on an iron porphyrin complex without triazole units confirm the contribution of triazole units on high catalytic activity. Long-term electrolysis of 40 h was also performed and demonstrated high catalyst stability. A normal KIE of 6.92 was obtained with H2O/D2O as the proton source at varying concentration ranges (0.5-5 M), suggesting that protonation of the catalyst-substrate intermediate is a rate-limiting step. Furthermore, the Tafel plot was generated for the catalyst FePEG8T for comparison with previously reported iron porphyrin catalysts. This work demonstrates an efficient CO2 reduction catalyst containing an iron metal center and a flexible triazole in the second coordination sphere toward CO formation with high stability, activity, and selectivity.