Microwave-assisted synthesis route leads to record-breaking performance for CO₂ conversion

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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.

More information: Chomponoot Suppaso et al, Fibrous Pb(II)‐Based Coordination Polymer Operable as a Photocatalyst and Electrocatalyst for High‐Rate, Selective CO2‐to‐Formate Conversion, Advanced Functional Materials (2024). DOI: 10.1002/adfm.202417223

Journal information: Advanced Functional Materials

Provided by Institute of Science Tokyo