Strain-Engineered FeN₄ Sites Accelerate Singlet Oxygen Formation in Fenton-Like Reactions

Understanding the structure-performance relationship for the selective generation of reactive oxygen species is critical in Fenton-like catalysis research. Single-atom catalysts (SACs) anchored on nanocarriers can maximize their catalytic capacity, but the inherent high curvature of nanocarriers may induce strain of active sites with unclear impact on their catalytic performance. Here, we uncover the strain-dependent reactivity of single-atom FeN₄ sites anchored on carbon nanotubes (CNTs) for peroxymonosulfate (PMS) activation. Density functional theory calculations show that the curvature-induced strain of CNTs distorted the FeN₄ geometry and redistributed its electronic structure, thereby steering PMS activation toward a singlet oxygen (¹O₂) pathway. Experimental validation confirms that FeN₄-CNTs with an 8 nm diameter achieved the highest degradation rate of sulfamethazine in a PMS activation system with exclusive ¹O₂ selectivity, which was 7.2 and 5.4 times that of counterparts on 2 and 50 nm CNTs, respectively. Spectroscopic and theoretical analyses reveal that rational strain enhanced Fe-3d and O-2p coupling, facilitating electron transfer and elongating O-H bonds in PMS to promote ¹O₂ generation. This work elucidates the mechanistic origin of strain effects in single-atom catalysis and highlights strain engineering as a powerful strategy for selective PMS activation and high-efficiency environmental catalysis.