Electrocatalytic reduction of CO2 to CO in the presence of a mononuclear polypyridyl ruthenium(II) complex

Abstract

A new polypyridyl Ru(II) complex, [Ru(tptz)(CH3CN)Cl2] (tptz=2,4,6-tri(2-pyridyl)-1,3,5-triazine) has been prepared and characterized. It crystallizes with one molecule of acetonitrile and a partly occupied water solvent molecule. X-ray analysis of the complex revealed that the central Ru(II) atom has a distorted octahedral Cl2N3N' coordination environment. The complex was used as an electrocatalyst for the reduction of CO2 to CO in an acetonitrile solution as monitored by cyclic voltammetry. The electrocatalytic activity of the complex was investigated for CO2 reduction under different reaction conditions. The results show that the CO2 reduction peak current increases with increasing concentration of the electrocatalyst from 0.5 to 2mM, increasing purging time of CO2 from 5 to 30min, and increasing scan rate from 0.1 to 0.5Vs−1. The catalytic activity of the Ru(II) complex is decreased at lower temperatures because of the relationship between the activation energy and temperature. The suggested electrocatalytic mechanism was confirmed by computational studies. The calculations indicate that the electrocatalytic mechanism includes six steps. The electrocatalyst undergoes two sequential one-electron tptz-based reductions followed by substitution of the CH3CN by CO2 to yield a metallocarboxylate intermediate. This intermediate (RuII(η1-CO22–)) is involved in further tptz-based reductions and an oxide (O2–) transfer to the free CO2 to yield a monocarbonyl Ru(II) complex ([RuII(tptz2–)(CO)Cl2]2–) and the carbonate anion. The monocarbonyl intermediate undergoes a rapid loss of the coordinating CO molecule at room temperature. Finally, the dianionic intermediate reacts with a new molecule CO2 restarting the electrocatalytic cycle.