In the artificial photosynthesis of solar fuels from CO2 there is presently little agreement about what carbon-containing molecule should be made from CO2 in the first place – carbon monoxide, formic acid, methanol, methane, or higher hydrocarbons. The reduction of CO2 must also be selective since production of solar fuels generally requires protons, and proton reduction to hydrogen is favored thermodynamically compared CO2 reduction. Several classes of catalysts, old and new, will be discussed in terms of their activities, selectivities, and general promise in artificial photosynthetic devices. New strategies for the lowering of overpotentials of electrocatalysts for the reduction of CO2, especially through the use of supramolecular interactions will be discussed. In particular, the addition of methyl acetamidomethyl groups at the 4,4'-positions of a 2,2'-bipyridyl ligand is found to enhance the rate of a bimolecular reduction mechanism of CO2 by ReI fac-tricarbonyl chloride complexes. Electrochemical studies, spectroelectrochemical measurements, and molecular dynamics simulations indicate that these methyl acetamidomethyl groups promote the formation of a hydrogen-bonded dimer. This supramolecular complex catalyzes the reductive disproportionation of CO2 to CO and CO32– at a lower overpotential (ca. 250 mV) than the corresponding single-site 2 e– reduction of CO2 to CO and H2O catalyzed by the corresponding model complex with a 4,4'-dimethyl-2,2'-bipyridyl ligand. These findings demonstrate that non-covalent self-assembly can modulate the catalytic properties of metal complexes by favoring alternate catalytic pathways. The latest results in extending the use of supramolecular interactions in guiding catalytic mechanisms for CO2 reduction will be presented.

Carbon dioxide reduction by earth abundant catalysts in artificial photosynthesis: Mechanistic insights and challenges

249th National Meeting (2015)

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