Scientists have succeeded in creating one of the most efficient systems for solar conversion of carbon dioxide into carbon monoxide, the key component in the synthetic fuel production. The new technology performs up to 165 catalytic cycles per second and facilitates approximately 180,000 CO₂ conversions per catalyst molecule, approaching the efficiency of natural enzymes. This breakthrough opens up new possibilities for the development of technologies capable of converting greenhouse gases into chemical feedstocks and synthetic fuels, using solar energy.
This is stated in a study by researchers at Nanjing University in China, who developed an artificial photocatalytic system inspired by the principles of natural enzymes. The researchers proposed a new way to organize the reaction, allowing them to overcome one of the main limitations of photocatalysis, which is an extremely low concentration of active, light generated particles triggering the chemical transformations.
Today, carbon dioxide conversion into useful chemical products is considered one of the most promising areas for the development of low-carbon energy and the chemical industry. However, most photocatalytic systems are rather inefficient because the active intermediate compounds forming under the impact of light exist for only a fraction of a second and quickly disappear before being able to react with CO₂.
To solve this problem, the scientists turned to a mechanism that has been used by living nature for billions of years. In cells, enzymes pre-concentrate the necessary molecules near the active site, allowing chemical reactions to proceed much faster. The researchers created an artificial analogue of this system – a molecular capsule based on porphyrin with iron atoms. Porphyrin is an organic molecule structurally similar to the ones found in hemoglobin in blood and chlorophyll in plants.
The resulting capsule was able to collect and retain active particles in close proximity to the catalytic centers.
The scientists mixed a photosensitizer (a light-absorbing substance) with a special compound serving as an electron source for a subsequent reaction, and then illuminated the mixture. As a result, a large number of active negatively charged particles formed in the solution. A positively charged capsule added to the system acted like a magnet, attracting and trapping them inside itself, creating a saturated environment ready for catalysis.
This approach has radically changed the course of the reaction. Thanks to a preliminary accumulation of active particles and their concentration inside the capsule, the catalyst began to operate practically in a state of complete saturation, similar to a natural enzyme. This transition from chaotic molecular interactions to a process organized on the principle of enzyme operation allowed for a significant increase in reaction efficiency. The system consistently produced carbon monoxide at a rate of up to 165 catalytic cycles per second, which is comparable to performance of some natural enzymes and the best modern photocatalytic systems.
During operation of the system, a single catalyst molecule facilitated approximately 180,000 cycles of carbon dioxide conversion into carbon monoxide, which is one of the best results ever achieved for the CO₂ photocatalytic reduction. The reaction selectivity exceeded 99.9%, meaning that virtually all the carbon dioxide was converted specifically into the target product. Using a carbon-13 isotope label, the scientists confirmed that the carbon monoxide produced was indeed derived from CO₂. In addition, in a specially designed flow-through microreactor, the researchers managed to raise the number of catalytic cycles to 362,000 over two hours of a continuous operation.
Beyond CO₂ reduction, the scientists demonstrated that the system they had developed can serve as a universal platform for carrying out other chemical transformations. Using the same capsule and the same principle of pre-incubation, the researchers obtained a number of valuable heterocyclic compounds, benzimidazoles and benzothiazoles which are widely used in the pharmaceutical and chemical industries. The reaction efficiency turned out to depend on the chemical nature of the starting compounds: molecules containing nitrogen and sulfur reacted significantly more actively than similar compounds containing oxygen.
