Monitoring of visible light driven CO2 reduction, a key reaction for directly converting photons to energy dense molecules, by time-resolved ATR FT-IR spectroscopy has recently led to the discovery of hitherto unknown chemical intermediates for heterogeneous and molecular catalytic systems alike. For CO2 reduction on the surface of aqueous Cu nanoparticles, the temporal resolution enabled by the light triggered reaction, combined with isotopic labeling, allowed disentangling of spectrally overlapping carbon oxide species and revealed carbon dioxide radical anion, (C(=O)OCO2-), as the one-electron surface intermediate. The formation of this species provides an alternate, low energy path for the conversion to CO and carbonate compared to CO2 reduction to CO2-. The result points to the exploration of nanostructured Cu catalyst supports with high adsorption capacity for CO2 to maximize the density of surface carbon dioxide pairs for capturing electrons from the Cu catalyst. The technique, particularly if combined with the elucidation of accompanying electronic structure dynamics of active metal centers by X-ray methods plays a key role for exploring multi-metal nanoparticle catalysts for CO2 conversion to more energy dense hydrocarbons currently in progress. Using similar visible light triggered catalysis monitored by rapid-scan FT-IR spectroscopy, the elusive two-electron intermediate of CO2 reduction at a molecular catalyst and spontaneous release of CO product were observed for the first time. The observation was enabled by the delivery of electrons by visible light sensitizers with different reducing potential to the Co tetracycle which, combined with temporal resolution, allowed monitoring of selected steps of the two-electron catalytic cycle. The electronic nature of the two-electron intermediate derived from the vibrational data, which is in excellent agreement with DFT calculations reported in the literature, points to structural properties for enhancing the selectivity of CO2 activation over proton reduction, a longstanding challenge of the field.

Understanding mechanisms of CO2 reduction catalysis by capturing infrared intermediates

ACS Fall 2020 Virtual Meeting

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