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

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