Molecular Approaches to the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels

Abstract

The scientific community now agrees that the rise in atmospheric CO₂, the most abundant green house gas, comes from anthropogenic sources such as the burning of fossil fuels. This atmospheric rise in CO₂ results in global climate change. Therefore methods for photochemically transforming CO₂ into a source of fuel could offer an attractive way to decrease atmospheric concentrations. One way to accomplish this conversion is through the light-driven reduction of carbon dioxide to methane (CH_4(g)) or methanol (CH₃OH_(l)) with electrons and protons derived from water. Existing infrastructure already supports the delivery of natural gas and liquid fuels, which makes these possible CO₂ reduction products particularly appealing. This Account focuses on molecular approaches to photochemical CO₂ reduction in homogeneous solution. The reduction of CO₂ by one electron to form CO₂^•− is highly unfavorable, having a formal reduction potential of −2.14 V vs SCE. Rapid reduction requires an overpotential of up to 0.6 V, due at least in part to the kinetic restrictions imposed by the structural difference between linear CO₂ and bent CO₂^•−. An alternative and more favorable pathway is to reduce CO₂ though proton-assisted multiple-electron transfer. The development of catalysts, redox mediators, or both that efficiently drive these reactions remains an important and active area of research. We divide these reactions into two class types. In Type I photocatalysis, a molecular light absorber and a transition metal catalyst work in concert. We also consider a special case of Type 1 photocatalysis, where a saturated hydrocarbon links the catalyst and the light absorber in a supramolecular compound. In Type II photocatalysis, the light absorber and the catalyst are the same molecule. In these reactions, transition-metal coordination compounds often serve as catalysts because they can absorb a significant portion of the solar spectrum and can promote activation of small molecules. This Account discusses four classes of transition-metal catalysts: (A) metal tetraaza-macrocyclic compounds; (B) supramolecular complexes; (C) metalloporphyrins and related metallomacrocycles; (D) Re(CO)₃(bpy)X-based compounds where bpy = 2,2′-bipyridine. Carbon monoxide and formate are the primary CO₂ reduction products, and we also propose bicarbonate/carbonate production. For comprehensiveness, we briefly discuss hydrogen formation, a common side reaction that occurs concurrently with CO₂ reduction, though the details of that process are beyond the scope of this Account. It is our hope that drawing attention both to current mechanistic hypotheses and to the areas that are poorly understood will stimulate research that could one day provide an efficient solution to this global problem.