Utilization of Low-Concentration CO 2 with Molecular Catalysts Assisted by CO 2 -Capturing Ability of Catalysts, Additives, or Reaction Media.

Increasing concentration of atmospheric CO 2 is a worldwide concern and continues to trigger various environmental problems. Photo- or electrocatalytic CO 2 reduction (CO 2 -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 CO 2 in exhaust gases from industries are several % to 20%, and that in the atmosphere is about 400 ppm. Although condensation processes of CO 2 require high energy consumption and cost, pure CO 2 has been used in most of the reported studies for photo- and electrocatalytic CO 2 -Red because the reaction between CO 2 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 CO 2 , which utilize a combination of catalytic CO 2 -Red and CO 2 -capturing reactions (or CO 2 adsorption). First, we describe CO 2 insertions into M-X bonds of the catalysts, which increase the rate constants and/or equilibrium constants for CO 2 binding on the catalysts, and modifications of the second coordination sphere to stabilize the CO 2 -bound catalysts. Furthermore, we discuss the reaction media used for catalytic CO 2 -Red that have the unique effect of increasing CO 2 concentrations around the catalysts. These reaction media include typical CO 2 -capturing additives, ionic liquids, and metal-organic frameworks.