Graphitic carbon nitride quantum dots and metal porphyrins assemble for CO2 reduction Hanna Larsson, Dr. Liam Mistry, Dr. Long Le-Quang, Dr. Gerard Masdeu, Wilma Björkman, Prof. Hanna Härelind, Prof. Maria Abrahamsson Chalmers University of Technology, Gothenburg, Sweden Artificial photosynthesis has been described as a “chemist’s dream” [1] . Addressing the rising levels of atmospheric carbon dioxide and our dependence on fossil fuels, while also providing stability to the inherent fluctuations of renewable energy sources. Many current solutions rely on non-abundant elements, such as Re [2] , Ru [3, 4] , and Ir [5] , which are suboptimal from both sustainability and economic perspectives. Thus, efforts are made to utilize more abundant materials and chemical designs for highly efficient systems. Despite growing interest in this field over the past decades, a comprehensive understanding of the underlying mechanisms remains elusive. This is partly due to the complexity of the reactions involved, the difficulties generalizing between systems and measuring short-lived intermediates [6, 7] . Advancing this understanding could lead to the design of systems with improved efficiency, selectivity, and stability. Therefore, we are investigating a hybrid assembly composed of graphitic carbon nitride quantum dots (g-CNQDs) and metal porphyrins, which works in aqueous solution [8] . Focus is on closely observing the reaction mechanisms and the interactions between the 0D nanomaterial and molecular photocatalysis mainly by using photophysical techniques. By functionalizing g-CNQDs with ethylenediaminetetraacetic acid (EDTA), carboxylic acid groups are introduced, enhancing water solubility and creating binding sites for porphyrins. Independently, the particles exhibit intriguing photophysical properties, including two-photon absorption in the near-infrared (NIR) region and size-dependent emission—features characteristic of quantum dots. A collection of techniques and methods, including steady-state and transient optical spectroscopy, gas chromatography, electrochemistry, and transmission electron microscopy, are used to characterize the materials and reactions. We have previously demonstrated a turnover number exceeding 10^5 and a selectivity of 95% for CO 2 -to-CO reduction using iron 5,10,15,20-tetra(4-N,N,N-trimethylanilinium)porphyrin (Fe-p-TMA), in combination with g-CNQDs and triethanolamine (TEOA) as a sacrificial electron donor [8] . Spectroscopic and photoelectrochemical measurements indicate that the g-CNQDs act as photosensitizers, absorbing light and donating electrons to the porphyrin-based reaction center, where CO 2 is adsorbed and subsequently reduced. Our data suggest that the iron center cycles between Fe(0) and Fe(II) oxidation states during the reaction. Corresponding experiments using cobalt 5,10,15,20-tetra(4-N,N,N-trimethylanilinium)porphyrin (Co-p-TMA) are currently underway. References 1. Molecular Catalysis of CO2 Reduction. ACS Catalysis, 2017. 7 (1): p. 70-88. 2. Sahara, G. and O. Ishitani, Efficient Photocatalysts for CO2 Reduction. Inorganic Chemistry, 2015. 54 (11): p. 5096-5104. 3. Zhao, Y., et al., Polymer Chromophore–Catalyst Assembly for Photocatalytic CO2 Reduction. ACS Applied Energy Materials, 2021. 4 (7): p. 7030-7039. 4. Lian, S., et al., Powering a CO2 Reduction Catalyst with Visible Light through Multiple Sub-picosecond Electron Transfers from a Quantum Dot. Journal of the American Chemical Society, 2017. 139 (26): p. 8931-8938. 5. Rao, H., et al., Visible-light-driven methane formation from CO2 with a molecular iron catalyst. Nature, 2017. 548 (7665): p. 74-77. 6. Gust, D., T.A. Moore, and A.L. Moore, Solar Fuels via Artificial Photosynthesis. Accounts of Chemical Research, 2009. 42 (12): p. 1890-1898. 7. Khalil, M., et al., Photocatalytic conversion of CO2 using earth-abundant catalysts: A review on mechanism and catalytic performance. Renewable and Sustainable Energy Reviews, 2019. 113 : p. 109246. 8. Mistry, L., et al., Selective Photocatalytic Reduction of CO2-to-CO in Water using a Polymeric Carbon Nitride Quantum Dot/ Fe-Porphyrin Hybrid Assembly. ChemCatChem, 2022. 14 (24): p. e202200897.
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