Mechanistic study of photocatalytic CO2 reduction using a Ru(II)-Re(I) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)(3)](2+) photosensitiser and fac-[Re(diimine)(CO)(3){OC(O)OC2H4NR2}] catalyst units can be used to reduce CO2 to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO2 to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO2, e.g., 10% CO2, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO2 was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)(-), which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)(-) is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)(-) produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)(-) proceeds with an observed first-order rate constant of approximately 1.8 s(-1) at 298 K and is a unimolecular reaction, independent of the concentrations of both CO2 and RuC2(Re)(-).

Automated summary

The study investigates the photocatalytic reduction mechanism of CO2 using a supramolecular photocatalyst RuC2Re with a carbonate ester ligand. The experiments reveal the stable and efficient nature of RuC2Re, as well as provide detailed insights into the kinetics of the photochemical formation processes of the key intermediate RuC2(Re)(-).