Turning carbon dioxide (CO2) into useful chemicals is a compelling avenue towards minimizing our CO2 emissions and curbing climate change. Photocatalysts, which can be designed to leverage energy from light to drive CO2 conversion, are a major goal in functional materials science.
Though various types of photocatalysts exist, coordination polymers (CPs) are a particularly attractive option. These heterogeneous materials can integrate the functions of light absorption and CO2 reduction catalysts simultaneously. Moreover, CPs can be synthesized from earth-abundant metals and organic molecules, which makes them widely usable at an industrial level.
In August 2022, Professor Kazuhiko Maeda and colleagues from Institute of Science Tokyo, Japan, reported a precious-metal-free CP called KGF-9 that served as a standalone photocatalyst for converting CO2 to formate. Although KGF-9 was highly selective, its photocatalytic activity was rather low, as evidenced by its low apparent quantum yield (AQY). Now, in a more recent study published in Advanced Functional Materials on November 13, 2024, Maeda and his team managed to greatly improve KGF-9's performance through an alternative synthesis approach, highlighting its previously untapped potential.
The approach in question was a microwave-assisted solvothermal method, which involves heating a solution in a sealed vessel using microwaves. "Volumetric heating with microwaves can directly and uniformly heat the entire reaction mixture, causing molecular rotations that result in a drastic increase in the reaction rate," notes Maeda. On top of speeding up the production of KGF-9 from two full days to as little as an hour, this synthesis route had other important effects.
After thorough testing of the samples produced, the researchers noted that the microwave-assisted method produced thin KGF-9 fibrils with a much greater specific surface area and crystallinity than the previous synthesis route. Subsequent photocatalytic experiments revealed that these improvements led to a massive boost in AQY for CO2-to-formate conversion. To put this into numbers, the AQY of the newly synthesized KGF-9 was 25%, which represents a near ten-fold increase compared to the 2.6% value previously reported. "This AQY represents a record-high value among reported heterogeneous photocatalysts for CO2-to-formate conversion and is even on par with the AQY reported for homogeneous photocatalysts," remarks Maeda.
The research team then conducted mechanistic studies to understand the origin of the observed improvements in CO2-to-formate conversion. Through careful analysis, they concluded that producing well-crystallized KGF-9 with few surface defects was a decisive factor. Interestingly, by combining KGF-9 with a carbonaceous conductor, they found that this compound also proves suitable for the electrocatalytic reduction of CO2, which uses electricity in an aqueous medium instead of light to drive the conversion of CO2 to formate.
Taken together, the findings of this study paint a bright future for KGF-9 and similar photocatalysts, which could soon become key in our efforts towards sustainable societies. With any luck, these affordable and versatile materials will help us achieve carbon neutrality to prevent further damage to our ecosystems.
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About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to create value for and with society."
