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The scientific community now agrees that the rise in atmospheric CO(2), the most abundant green house gas, comes from anthropogenic sources such as the burning of fossil fuels. This atmospheric rise in CO(2) results in global climate change. Therefore methods for photochemically transforming CO(2) 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 (CH(3)OH((l))) with electrons and protons derived from water. Existing infrastructure already supports the delivery of natural gas and liquid fuels, which makes these possible CO(2) reduction products particularly appealing. This Account focuses on molecular approaches to photochemical CO(2) reduction in homogeneous solution. The reduction of CO(2) by one electron to form CO(2)(*-) 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 CO(2) and bent CO(2)(*-). An alternative and more favorable pathway is to reduce CO(2) 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 CO(2) reduction products, and we also propose bicarbonate/carbonate production. For comprehensiveness, we briefly discuss hydrogen formation, a common side reaction that occurs concurrently with CO(2) 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.

Researchers in this field have investigated photochemical and electrochemical CO 2 reduction to CO or formate, even to methanol, using transition metal electrodes, metal complexes, semiconductors, and also organic molecules, and the details of these achievements can be found in many reviews recently published. 14−31 While recent progress in this field is quite remarkable in photochemical CO 2 reduction using semiconductors or heterogeneous systems, most experiments have not been confirmed using labeled CO 2 (i.e., 13 CO 2 ) and H 2 O (i.e., H 2 18Figure 1. Z-Scheme for photocatalytic CO 2 reduction coupled to methanol oxidation. Reproduced with permission from ref 46.

The production of hydrogen by the electrolysis of water, a sustainable and greenhouse-gas-free source, requires an efficient and abundant electrocatalyst that minimizes energy consumption. Interest in transition metal carbides and nitrides has been aroused by their promising properties that make them potential substitutes for Pt-group metals as catalysts for the hydrogen evolution reaction. In this review, we discuss systematically the recent progress in the development of group IV-VI metal carbides and nitrides toward the hydrogen evolution reaction. Some strategies for designing such catalysts and improving their efficiency and reliability, including nanostructuring, optimizing hydrogen binding energy, interaction with the supporting material, and exploiting hybrid structures, are highlighted. We conclude with an outlook on the challenges in designing future HER electrocatalysts.

Green plants convert CO(2) to sugar for energy storage via photosynthesis. We report a novel catalyst that uses CO(2) and hydrogen to store energy in formic acid. Using a homogeneous iridium catalyst with a proton-responsive ligand, we show the first reversible and recyclable hydrogen storage system that operates under mild conditions using CO(2), formate and formic acid. This system is energy-efficient and green because it operates near ambient conditions, uses water as a solvent, produces high-pressure CO-free hydrogen, and uses pH to control hydrogen production or consumption. The extraordinary and switchable catalytic activity is attributed to the multifunctional ligand, which acts as a proton-relay and strong π-donor, and is rationalized by theoretical and experimental studies.

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In our developing world, carbon dioxide has become one of the most abundant greenhouse gases in the atmosphere. It is a stable, inert, small molecule that continues to present significant challenges toward its chemical activation as a useful carbon end product. This tutorial review describes one approach to the reduction of carbon dioxide to carbon fuels, using cobalt and nickel molecular catalysts, with particular focus on studying the thermodynamics and kinetics of CO(2) binding to metal catalytic sites.

Excited-state properties of fac-[Re(dmb)(CO)3(CH3CN)]PF6, [Re(dmb)(CO)3]2 (where dmb ) 4,4′dimethyl-2,2′-bipyridine), and other tricarbonyl rhenium(I) complexes were investigated by transient FTIR and UV-vis spectroscopy in CH3CN or THF. The one-electron reduced monomer, Re(dmb)(CO)3S (S ) CH3CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)-(CO)3(CH3CN)]PF6 or by homolysis of [Re(dmb)(CO)3]2. In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)3S dimerizes slowly in THF, kd ) 40 ( 5 M -1 s -1 . This rate constant is much smaller than those of other organometallic radicals which are typically 10 9 M -1 s -1 . The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K ≈ 10 -4 ). The reaction of Re(dmb)(CO)3S with CO2 is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)3]2 in the presence of CO2 produces CO with a 25-50% yield based on [Re]. A CO2 bridged dimer, (CO)3(dmb)Re-CO(O)-Re(dmb)(CO)3 is identified as an intermediate. Both [Re(dmb)(CO)3]2(OCO2) and Re(dmb)(CO)3(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.