Utilization of Low-Concentration CO2 with Molecular Catalysts Assisted by CO2-Capturing Ability of Catalysts, Additives, or Reaction Media

Increasing concentration of atmospheric CO2 is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO2 reduction (CO2-Red) using solar energy, i.e., artificial photosynthesis, is a prospective technique owing to its sustainability and the usefulness of the reaction products. Concentrations of CO2 in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO2 require high energy consumption and cost, pure CO2 has been used in most of the reported studies for photo- and electrocatalytic CO2-Red because the reaction between CO2 and the catalyst could be one of the rate-limiting steps. To address these issues and provide a repository of potential techniques for other researchers, this perspective summarizes the catalytic systems reported for the reduction of low-concentration CO2, which utilize a combination of catalytic CO2-Red and CO2-capturing reactions (or CO2 adsorption). First, we describe CO2 insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO2 binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO2-bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO2-Red that have the unique effect of increasing CO2 concentrations around the catalysts. These reaction media include typical CO2-capturing additives, ionic liquids, and metal-organic frameworks.