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Development of monatomic iron centers for customized water treatment

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In a recently published study in Chinese Journal of CatalysisProfessor Shaobin Wang from the University of Adelaide and Hui Zhang from Wuhan University elucidated the mechanisms of PMS activation by monatomic iron catalysts. They also identified the relationships between the geometric and electronic structures of monatomic Fe centers and the selective production of different reactive species/pathways.

Single-atom catalysts revolutionize water treatment: Efficient and selective generation of reactive oxygen species for global water challenges. Researchers describe in detail how shrinking active sites to the atomic level in single-atom catalysts improves the efficiency and selectivity of reactive oxygen species generation by peroxymonosulfate-based advanced oxidation. Their review explains the underlying mechanism and identifies critical structure-activity/selectivity relationships, providing valuable insights into the rational design of catalysts for environmental remediation.
Single-atom catalysts revolutionize water purification: Efficient and selective generation of reactive oxygen species for global water challenges. Researchers describe in detail how shrinking active sites to the atomic level in single-atom catalysts improves the efficiency and selectivity of reactive oxygen species generation by peroxymonosulfate-based advanced oxidation. Their review article explains the underlying mechanism and identifies critical structure-activity/selectivity relationships, providing valuable insights into the rational design of catalysts for environmental remediation. Image credit: Chinese Journal of Catalysis

Due to centuries of rapid growth in global industrialization, too many hazardous chemicals are entering the environment, posing a significant threat to aquatic ecology and human health.

Advanced peroxymonosulfate-based oxidation processes (PMS-AOPs) represent attractive methods for treating these harmful pollutants. Aggressive organic pollutants (AOPs) are designed to oxidize or even mineralize by utilizing a variety of reactive oxygen species (ROS).

With their homogeneous and well-defined active sites and optimal atom utilization, single atom catalysts (SACs) show considerable potential for efficient and targeted PMS activation. However, due to the numerous ROS production pathways and their complex interactions, the relationships between structure, activity and selectivity are not yet fully understood.

The activation of PMS by single atom sites typically occurs in three steps. The adsorption of HSO5on the monatomic Fe nucleus with different molecular orientations is the first stage.

The second step is the charge transfer between the adsorbed HSO5 and the active Fe center, which causes a shift in the charge density and even the splitting of the HSO5–O bond. To restore the active site, the third phase involves the removal of the activated HSO5 either by a spontaneous release of reactive species (such as free radicals and 1Ö2) or the degradation of surface-bound species with additional substrates (such as catalyst PMS* and FeIV=O).

Even at a single Fe atom site, different ROS are generated because PMS activation involves multiple steps and ROS formation involves multiple chemical intermediates. Although the energetically ideal PMS adsorption structure can be found theoretically, each reaction step must have an appropriate reaction free energy to generate specific ROS.

Therefore, controlling the PMS adsorption energy and the reaction free energy of essential basic processes is required to control the ROS selectivity. In this case, controlling the electronic structure of the active site is useful to control the PMS activation activity and ROS selectivity to assemble the compounds.

The creation of Fe–N2–O2 and Fe–N5 sites to enhance 1O2 and FeIV =O formation as well as a Fe–N3 site to form catalyst-PMS complexes requires a moderate increase in the charge density of the Fe sites. Radical production is favored by an additional increase in the charge density over Fe–N.3-P1 and Fe–N4 (pyrrole-N) sites.

The Australian Research Council and the National Natural Science Foundation of China funded the research. The China Scholarship Council also provided a one-year research fellowship at the University of Adelaide.

Journal reference:

‌Cheng, C., et al. (2024) Single-atom iron catalysts for peroxymonosulfate-based advanced oxidation processes: coordination structure versus reactive species. Chinese Journal of Catalysis. doi.org/10.1016/s1872-2067(23)64611-x.

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