In photosynthetic organisms, CO2 reduction is achieved via hydride transfer from NADH cofactors, while the catalytic cycle is closed using photochemical reduction of NAD+ by water. We mimic this chemistry using artificial NADH analogs adsorbed on the photocathode surface for photochemical regeneration (see figure). Our biomimetic approach promises to direct the CO2 reduction selectively to methanol, using inexpensive metal-free photocatalysts. Here, we report our findings on the dark and photochemical half reactions of the catalytic cycle. The dark reaction involves a hydride transfer from NADH analogs to CO2 and was studied by determining, experimentally and computationally, the thermodynamic hydricities of a series of NADH analogs. This study provided the structure-property relationship in model compounds and the results have pinpointed two molecular motifs that show promise as potential catalysts for CO2 reduction. One of those NADH analogs, a benzimidazole-based hydride donor, has been further probed as the reductant for CO2. We find that the reduction to formate occurred efficiently, providing the first experimental evidence of CO2 reduction by organic hydride donors. The photochemical regeneration of NADH analogs was studied using a dye-sensitized two-electron reduction process involving p-type electrodes (GaP and NiO). Ultrafast laser spectroscopy was utilized to evaluate factors that control the rates of interfacial charge transfer and recombination. We find that the charge-separated states exhibit short lifetimes and we propose future directions that can be utilized to reduce the rates of unwanted charge recombination processes. This study provides important mechanistic insights into the CO2 reduction by metal-free catalysts.

Photocatalytic CO2 reduction by biomimetic NADH analogs

2019 Spring National Meeting

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