The group led by Associate Professor Li Miao from the School of Environment has made new progress in the efficient reduction, removal, and energy utilization of aqueous NO3? pollutants. They have developed a phosphorus-modified cobalt single-atom catalyst (Co-SAC) that achieves highly efficient reduction and removal of aqueous NO3? pollutants and improves the conversion yield of NO3? to NH3.
The imbalance in the nitrogen cycle caused by the global increase in reactive nitrogen has made NO3? one of the most common pollutants in water. Nitrate pollution threatens ecological security and human health. Synthesizing ammonia through nitrate reduction not only helps remove nitrate pollutants from water but also alleviates society’s demand for ammonia energy. With moderate requirements, the electrochemical reaction process is easy to operate, and highly efficient, enabling the direct conversion of nitrates to ammonia. However, during the electrochemical reduction process of nitrates, the N–N coupling reactions that produce nitrogen gas tend to occur on the active sites of nano-sized or larger electrodes, limiting the efficient production of ammonia. Therefore, developing electrode materials with high activity, low cost, and high selectivity is one of the core research areas in this field.
To address the bottlenecks of poor activity and passivation of cobalt (Co) metal electrodes, Li Miao’s group developed a phosphorus-modified Co-SAC material based on the anchoring effect of a defect-rich carbon basal plane. This catalyst effectively avoids the N–N coupling reaction, resulting in enhanced ammonia selectivity and reduction activity. The phosphorus-modified Co-SAC achieves a higher NO3? conversion, with a high Faradic efficiency of 92.0% and a maximum ammonia yield rate of 433.3 μgNH4+·丑?1·肠尘?2.
Figure 1: Structural and morphological analysis results of the single-atom catalyst
The research group used the rare 15NO3? in nature as the nitrogen source and employed the isotope labeling method to further prove that the sole nitrogen source for ammonia production was NO3?. The 1H NMR spectra with 14NO3? and 15NO3? as reactants showed typical triple and double peaks corresponding to the 14NH4+ and 15NH4+ products, respectively. The group analyzed the carrier structure using various experimental techniques. The results showed that the doping of phosphorus further increased the defect level of the carbon-nitrogen carrier, provided more fixed sites for loading SAC, and the defect sites affected the electronic structure and performance of adjacent Co active sites, improving the electrode conductivity.
Figure 2: Electrode performance results
Based on density functional theory calculations, the research group innovatively strengthened the pollution purification theory and methods at the single-atom scale structure regulation level. They explored the conversion reaction mechanism of NO3? on the model SAC catalyst active sites from a molecular level, analyzing reaction pathways and energy changes of NO3?. The results revealed that NO3? undergo stepwise deoxygenation and hydrogenation elementary reactions on single-atom sites. N* can further couple to form nitrogen gas when energy is provided externally or can react spontaneously and gradually with hydrogen to form ammonium products. The defect sites formed after phosphorus doping promote the catalytic conversion of NO3?at the adjacent CoP1N3 sites, and the nitrate reduction process involves an 8-electron transfer to produce ammonium products. Additionally, the research found that the defect structures near the metal active sites help to further enhance the activity of single-atom catalysts, providing theoretical guidance for designing catalysts with high activity sites and revealing the conversion and product distribution rules of nitrate reactions.
Figure 3: Schematic diagram of the reaction mechanism
The research results were published online on July 12th under the title “Boosted ammonium production by single cobalt atom catalysts with high Faradic efficiencies” in the Proceedings of the National Academy of Sciences of the United States of America. The first author of the paper is Dr. Li Jiacheng, a postdoctoral researcher at Tsinghua University’s School of Environment. The corresponding author is Associate Professor Li Miao of Tsinghua University’s School of Environment. Professor Liu Xiang and others from the School of Environment provided important guidance and assistance for the experiments. The research project was funded by the National Natural Science Foundation’s general project and the key research and development plan.
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