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Oxygen vacancies (OVs)-rich α-Ni 0.2 Fe 1.8 O 3 with dual metal sites was synthesized by Ni2+ isomorphously substituted α-Fe 2 O 3 to activate peroxymonosulfate (PMS) for aceclofenac (ACF) degradation. A novel insight was proposed for the overlooked role of H 2 O in the surface-complexed process between PMS and unsaturated metal sites induced by OVs. The surface-bond SO 4 ∙- and ∙OH were demonstrated as the dominant reactive oxygen species (ROS). In situ ATR and Raman spectra revealed the reaction of HSO 5 - with the surface metal sites by replacing the surface-bonded -OH. The DFT study revealed the spontaneous formation of hydrolyzed -OH on OVs (∆G=−1.45 eV), which was further replaced by HSO 5 - to complex with metal sites (∆G=−3.58 eV). The O Ⅰ of the H-O Ⅰ -O Ⅱ -SO 3 - was more readily bonded with the Ni or Fe sites around OVs, causing the O-O cleavage for SO 4 ∙- generation. This study proposed a brand-new perspective for the interfacial behavior of PMS activation. [Display omitted] • The α-Ni 0.2 Fe 1.8 O 3 with rich oxygen vacancies can excellently activate PMS. • The surface-bond SO 4 ∙- and ∙OH played a major role in the degradation of ACF. • HSO 5 - was mainly bonded to metal sites by replacing surface -OH groups. • The interfacial mechanism for PMS activation by α-Ni 0.2 Fe 1.8 O 3 was proposed. • The degradation pathway of ACF by α-Ni 0.2 Fe 1.8 O 3 /PMS system was deduced. [ABSTRACT FROM AUTHOR]

Singlet oxygen (1O 2) plays a crucial role in Fenton-like reactions due to its high efficiency and selectivity in removing trace organic pollutants from complex water matrices. Defect engineering, which allows the efficient exposure of active sites and optimization of electronic structures, has rapidly emerged as a fundamental strategy for enhancing 1O 2 yield. Herein, we introduce tunable sulfur vacancy (V S) density into Co 9 S 8 catalysts for peroxymonosulfate (PMS) activation. The modulation of the octahedral Co (CoS 6) and tetrahedral Co (CoS 4) electronic structures by V S triggers the unexpected selective generation of 1O 2. The V S /PMS system exhibits excellent resistance to interference and highly selective degradation of electron-donating organic pollutants. Experimental and theoretical calculations revealed a new evolutionary route for 1O 2 involving two phases (Phase I: HSO 5 − → *O, Phase II: *O + HSO 5 − →*OO → 1O 2). This study provides a molecular-level understanding of V S -mediated catalytic selectivity for high-efficient decontamination applications. [Display omitted] • Co 9 S 8 catalysts with tunable V S was fabricated to PMS activation for contaminants degradation. • The optimized V S /PMS system achieved 1O 2 -dominated degradation mechanism. • Regulation d-band center of Co-3d orbital boosted the process of HSO 5 − adsorption and activation. • Combining experiment and DFT results revealed a new evolutionary route of 1O 2. [ABSTRACT FROM AUTHOR]