Chemistry:Photochemical carbon dioxide reduction

Photochemical carbon dioxide reduction harnesses solar energy to convert CO
2 into higher-energy products. The chemical conversion of CO₂ already occurs on an industrial scale in the manufacture of solvents such as formic acid, but photochemical reduction differs in that it relies on a renewable energy source, the sun. Because CO₂ is a greenhouse gas, there is environmental interest in producing artificial systems that are efficient photocatalysts, but the low conversion rates of current methods have prohibited wide-scale industrial application.

Overview

Photochemical reduction follows from chemical reduction (redox). But, it differs in that the electrons used for reduction are generated from the photoexcitation of another molecule, called a photosensitizer. To harness the sun's energy, the photosensitizer must be able to absorb light within the visible and ultraviolet spectrum. Molecular sensitizers that meet this criterion often include a metal center, as the d-orbital splitting in organometallic species often falls within the energy range of far-UV and visible light. The reduction process begins with excitation of the photosensitizer, as mentioned. This causes the movement of an electron from the metal center into the functional ligands. This movement is termed a metal-to-ligand charge transfer (MLCT). Back-electron transfer from the ligands to the metal after the charge transfer, which yields no net result, is prevented by including an electron-donating species in solution. Successful photosensitizers have a long-lived excited state, usually due to the interconversion from singlet to triplet states, that allow time for electron donors to interact with the metal center. Common donors in photochemical reduction include triethylamine (TEA), triethanolamine (TEOA), and 1-benzyl-1,4-dihydronicotinamide (BNAH).

An example of photoexcitation using Ru(bpy)₃ and triethylamine. The net result is a lone electron, originating from the metal, residing in the aromatic bipyridine moiety of Ru(bpy)₃.

After excitation, CO₂ coordinates or otherwise interacts with the inner coordination sphere of the reduced metal. The mechanistic details for this reduction process have not been fully determined, but commonly observed products include formate, formic acid, carbon monoxide, and methanol. Note that light absorption and catalytic reduction may occur at the same metal center or on different metal centers. That is, a photosensitizer and catalyst may be tethered through an organic linkage that provides for electronic communication between the species. In this case, the two metal centers form a bimetallic supramolecular complex. And, the excited electron that had resided on the functional ligands of photosensitizer passes through the ancillary ligands to the catalytic center, which becomes a one-electron reduced (OER) species. The advantage of dividing the two processes among different centers is in the ability to tune each center for a particular task, whether through selecting different metals or ligands.

An example of a supramolecular complex capable of photochemical reduction. Notice the photosensitizer on left tethered to the catalytic complex on the right.

History

Initial work by Lehn and Ziessel in the 1980s led the development of catalytic CO₂ reduction using visible light. In prior work developing photocatalysts for water splitting, Lehn observed that Co(I) species were produced in solutions containing CoCl₂, 2,2'-bipyridine (bpy), a tertiary amine, and a Ru(bpy)₃Cl₂ photosensitizer. The high affinity of CO₂ to cobalt centers led both him and Ziessel to study cobalt centers as electrocatalysts for reduction. In 1982, they reported CO and H₂ as products from the irradiation of a solution containing 700ml of CO₂, Ru(bpy)₃ and Co(bpy).

Current work

Since the work of Lehn and Ziessel, several catalysts have been paired with the Ru(bpy)₃ photosensitizer. When paired with methylviologen, cobalt, and nickel-based catalysts, carbon monoxide and hydrogen gas are observed as products. Paired with rhenium catalysts, carbon monoxide is observed as the major product, and with ruthenium catalysts formic acid is observed. It should be noted, however, that some product selection is attainable through tuning of the reaction environment. Other photosensitizers have also been employed as catalysts. They include FeTPP (TPP=5,10,15,20-tetraphenyl-21H,23H-porphine) and CoTPP, both of which produce CO while the latter produces formate also. Non-metal photocatalysts include pyridine and N-heterocyclic carbenes.

A reaction scheme for the catalytic reduction of CO₂ by Re(bpy)CO₃Cl. CT is an abbreviation for Charge-Transfer.