Controlling Product Selectivity in Photochemical CO₂ Reduction with the Redox Potential of the Photosensitizer

The ability to selectively reduce CO2 to a particular product or mixture of products is expected to play a key role in mitigation strategies aiming to alleviate the devastating impact of this greenhouse gas in our climate and oceans. Among those, the production of liquid solar fuels from CO2 and H2O will likely need cascade strategies involving multiple catalysts carrying out different functions. This will require that the catalysts doing the initial CO2 reduction steps deliver the right product or products to downstream catalysts. CO, H2 and formate are the most common products in CO2 reduction by molecular catalysts. In this work, we demonstrate control over the selectivity of C1 products in photochemical CO2 reduction with the same catalyst, simply by changing the redox potential of the photosensitizer and/or the water concentration. Turnover numbers for CO generation with one of the photosensitizers under anhydrous conditions reached 85,000, one of the largest values reported to date. A combination of experimental results and DFT calculations show that control of the selectivity is achieved, in part, due to the interplay between regimes under kinetic or thermodynamic control. These regimes are largely dictated by the proton sources and the CO2 reduction byproducts generated.