The continuous discharge of recalcitrant organic pollutants such as antibiotics into water bodies poses a significant challenge to aquatic ecological security. Advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) can generate highly reactive oxygen species, offering an effective route for the deep removal of recalcitrant organic pollutants. Single-atom catalysts (SACs) have attracted extensive attention in the field of PMS activation owing to their theoretically 100% metal utilization efficiency and well-defined coordination structures. However, conventional symmetric metal–heteroatom coordination structures have limited ability to regulate the electronic state of the central metal; meanwhile, powder-type SACs face engineering obstacles in packed beds, such as high flow resistance, easy loss, and difficult recovery. Continuous-flow catalysis offers advantages including high mass transfer efficiency, controllable residence time, easy scale-up, and stable continuous operation, which better meet the requirements of practical water treatment scenarios for high throughput and long-term stability. Therefore, effectively integrating atomic-scale active-site regulation with macroscopic structured reactor design is a key issue in advancing single-atom catalysis from the laboratory to continuous water treatment applications.
Recently, the team of Prof. Jiangbo Xi at Wuhan Institute of Technology, in collaboration with the team of Prof. Zhongxin Chen at The Chinese University of Hong Kong (Shenzhen), constructed a structured cobalt single-atom catalyst with asymmetric Co–N 3 O 1 coordination sites and directly integrated it into a continuous-flow fixed-bed Fenton-like reactor (Figure 1). Using graphene oxide as the support precursor, cobalt nitrate as the metal source, and polyurethane foam as both the three-dimensional framework and nitrogen source, the study achieved synergistic assembly of nitrogen-doped graphene and the polyurethane framework through a one-step low-temperature hydrothermal process at 120 °C. The resulting material retained a three-dimensional interconnected porous structure, which helps reduce flow resistance and enhance mass transfer. Aberration-corrected transmission electron microscopy confirmed that Co was uniformly dispersed as single atoms; X-ray absorption spectroscopy revealed that the Co centers were coordinated with approximately three N atoms and one O atom, forming an asymmetric Co-N 3 O 1 configuration. In contrast, the control sample prepared at 180 °C mainly formed a symmetric Co-N 4 structure, indicating that low-temperature structured assembly plays a key role in stabilizing the asymmetric coordination environment.
This synergistic strategy significantly improved the continuous-flow catalytic performance, achieving a maximum flow rate of up to 206 mL min -1 in the fixed-bed reactor, approximately twice that of the powder system. Under continuous-flow conditions, 92% of tetracycline (TC) was removed within 2 min, with a turnover frequency (TOF) as high as 1.23 min -1 , far exceeding that of the sample with symmetric Co-N 4 sites; the same catalyst removed only 58% in 14 min in a conventional stirred batch system, whereas the continuous-flow system ultimately achieved 98% removal. This gap indicates that continuous-flow operation, by enhancing interfacial mass transfer, fully unleashes the catalytic potential of the asymmetric sites, with the structural effect and site effect being multiplicatively coupled rather than simply additive. The catalyst also exhibited good environmental adaptability and engineering stability, maintaining a removal efficiency of over 95% after 15 days of continuous operation in a scaled-up continuous-flow device. After operation, the framework and single-atom dispersion state remained essentially stable, indicating that the structured support effectively protects the single-atom sites and provides a feasible solution to the problems of loss and recovery in the engineering application of powder catalysts.
To reveal the intrinsic mechanism by which asymmetric coordination promotes PMS activation, the research team combined radical quenching, competitive kinetics, electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations. Competitive kinetics results showed that the relative contributions of •OH, SO 4 • - , and 1 O 2 to the oxidation reaction were approximately 20%, 33%, and 47%, respectively. DFT calculations revealed that the Co metal d-band center of the asymmetric sites shifted significantly upward after the introduction of the oxygen ligand, enhancing the electronic coupling between the Co center and the terminal O of PMS and lowering the reaction barrier associated with ¹O₂ generation by about 0.25 eV compared with Co-N 4 sites. The adsorption energy of PMS on Co-N 3 O 1 sites was -2.43 eV, also stronger than the -2.28 eV on Co-N 4 sites, while Co-N 3 O 1 transferred about 0.78 e⁻ to PMS, promoting O-O bond elongation and activation. Thus, asymmetric coordination, by reconstructing the local electronic structure of the Co center, achieves synergistic regulation of PMS adsorption, electron transfer, and reactive oxygen species generation pathways, and provides a clear explanation at the electronic structure level.
In summary, this study, through the synergistic construction of asymmetric single-atom sites and a three-dimensional structured support, achieved cross-scale integration from “atomic-scale electronic structure regulation” to a “macroscopic continuous-flow reactor.” Asymmetric coordination significantly promotes PMS adsorption and electron transfer and regulates the generation of major active species such as 1 O 2 and SO 4 • - . In addition, the three-dimensional interconnected structure effectively reduces flow resistance, enhances mass transfer, and endows the catalyst with good mechanical stability. The efficient long-term continuous operation further demonstrates that this work provides a technical reference for overcoming the limitations of traditional powder-type single-atom catalysts in recovery, mass transfer, and continuous operation, while also offering a reliable design strategy for efficient, stable, and scalable advanced oxidation water treatment systems.
Science China Chemistry
Experimental study