CO₂ Capture and Electrochemical Reduction of Low-Concentration CO₂ Using a Re(I)-Complex Catalyst in Ethanol

Direct reduction of low-concentration CO2 from exhaust gases (3–13%) is important for CO2 utilization technologies because CO2 condensation processes require high energy consumption and cost. The Re(I) ethoxide complex fac-[Re(bpy-CH2OH)(CO)3(OEt)] (bpy-CH2OH = 4,4′-bis(hydroxymethyl)-2,2′-bipyridine), which is formed in an EtOH solution containing a base, efficiently captured CO2 to form the carbonate-ester complex fac-[Re(bpy-CH2OH)(CO)3(OCOOEt)] (Re(OCOOEt)) under both 10% and 100% CO2 atmospheres. In an EtOH solution containing 1,1,3,3-tetramethylguanidine (TMG) as the base, the electrocatalytic CO2 reduction reaction proceeded by Re(OCOOEt) with high CO selectivity, Faradaic efficiency, and durability even under a 10% CO2 atmosphere. This high electrocatalysis was retained in the presence of water in the solution up to 2.8 M (5 vol %). On the other hand, the electrocatalytic CO2 reduction reaction did not proceed efficiently in the absence of TMG under 10% CO2. The mechanistic studies and investigation suggest that the formation of the carbonate-ester complex in advance is necessary for the highly efficient electrocatalytic reduction of low-concentration CO2 in EtOH.