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The photo-Fenton performance of Fe-based metal organic frameworks (Fe-MOFs) largely depends on the amount and the local electron density of metal coordinately unsaturated sites (M CUSs). However, a majority of Fe active sites are fully bound by organic ligands leading to decreased Fe CUSs. Additionally, the symmetrical electronic distribution of iron-oxo (Fe-O) clusters and the fast electron-hole recombination are unbeneficial for the directional electron transfer and the following electron accumulation on Fe CUSs. Herein, the structure of Fe-O clusters onto the framework of MIL-88B was controllably regulated via change of Ce doping amount, among which Fe 0.8 Ce 0.2 -MIL-88B exhibited highest removal efficiency of tetracycline (TC). That was mainly ascribed to the following two points: for one, the induced ligand missing defects ameliorated the pore structures and generated more M CUSs; for another, the lower electronegativity of Ce than Fe and the role of ligand missing defects as electron trap state collectively increased the local electron density at Fe CUSs. As a result, the increased M CUSs provided more active sites for H 2 O 2 coordination and the highly concentrated electrons density at Fe CUSs afforded the substantial electron donation towards robust H 2 O 2 dissociation into ∙OH. Furthermore, the increased mesoporous size favored highly-efficient utilization of ∙OH. This work provides a facile strategy to improve photo-Fenton performance of Fe-MOFs., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Bimetallic metal-organic frameworks (MOFs) with iron and another transition metal in the inorganic nodes, as newly developed photo-Fenton catalysts, have gained an ample attention because of the synergistic effect and enhanced properties compared to the monometallic counterparts. However, the photo-Fenton application of bimetallic MOFs still faces the obstacle of unsatisfactory photogenerated electrons transfer due to the fast recombination of photogenerated electron-hole pairs. In this work, perylene diimide (PDI) was modified upon bimetallic NH 2 -MIL-88B(Fe, Mn) (FM88B) via the ammonolysis process to construct PDI/FM88B heterojunction structure. The constructed heterostructure mediated the directional transfer of photogenerated electrons from PDI (electron donor) to FM88 (electron acceptor). The built-in electric field (BIEF) between PDI and FM88B promoted the separation and directional transfer of photogenerated electron-hole pairs. The interfacial amide bond as the high-speed electronic transmission channel reduced the transfer distance and energy barrier of photogenerated electrons from PDI to FM88B, accelerating the photogenerated electrons delivery. As a result, both Fe(III)/Fe(II) and Mn(III)/Mn(II) redox couples were continuously and rapidly cycled, ultimately boosting the performance of FM88B for H 2 O 2 activation. Compared with FM88B, the optimal 6%PDI/FM88B exhibited excellent degradation performance against tetracycline (TC) (degradation efficiency and apparent rate constant, as 89% and 0.067 min−1) and notable synergy between photocatalysis and Fenton-like process (synergistic factor of 77.61%). DFT calculation and HPLC-MS analysis were used to verify the possible degradation pathways of TC. Meanwhile, the corresponding toxicity changes during TC degradation were studied in detail. This work paves the way for improving the heterogeneous photo-Fenton performance of the bimetallic MOFs in environmental remediation. [Display omitted] • Photogenerated electrons in PDI/FM88B could directionally transfer from PDI to FM88B. • Built-in electric field promoted photogenerated electron-hole pairs separation and transfer. • Interfacial amide bond reduced photogenerated electrons transfer distance and energy barrier. • The circulation of both Fe(III)/Fe(II) and Mn(III)/Mn(II) redox couples was accelerated. • Achievement of intensified synergism between photocatalysis and Fenton. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesized FeCo 2/1 -MOFs catalyst optimized for coordination, d-electron count, and energy band structure to boost catalytic performance. • Isomorphous substitution with Co(II) effectively raised d-band center and valence band of FeCo 2/1 -MOFs, • Theoretical calculations elucidated mechanisms enhancing PMS adsorption and activation. Modulations in interatomic localized electron density and occupation states are vital for facilitating efficient Photo-Fenton-like reactions in iron-based metal–organic frameworks (Fe-MOFs). Here, Co-doped Fe-MOFs (FeCo x -MOFs) were synthesized through a one-step hydrothermal method, introducing Co(II) with a distinct Jahn-Teller effect and electronegativity. This led to local ligand deficiency, forming exposed Fe-O-Co clusters. This integration facilitated Co coordinatively unsaturated sites (CUSs) to partially acquire electrons from adjacent Fe CUSs, further optimizing the d-band center. As a result, the d-band centers of both Fe (-1.32 eV) and Co (-1.52 eV) CUSs in FeCo 2/1 -MOFs were higher than that of Fe sites (-1.87 eV) in Fe-MOFs, resulting in enhanced adsorption capability of persulfate (PMS). Notably, Co(II) underwent ligand-driven electron redistribution, populating antibonding orbitals (e g *) and increasing their energy, thereby elevating the conduction band. Concurrently, Co 3d orbitals served as donor energy levels, narrowing the band gap. With the tuned band structure, the e g * of Fe(III) sites experienced rapid electron population upon photoexcitation, expediting Co(III) reduction and enhancing PMS activation. This study provides a unique perspective on the catalytic application of FeCo x -MOFs in photo-Fenton-like reactions. [ABSTRACT FROM AUTHOR]

Fe-based metal organic frameworks (Fe-MOFs) being a newly developed photo-Fenton catalyst, have gained ever-increasing attention for environmental remediation, but have been limited by high synthesis cost (high energy-consumption and low yield). In this work, economical and scalable synthesis of Fe-BDC 1 (BDC denotes as terephthalic acid) was achieved via facile stirring under ambient condition, and its adsorptive, photocatalytic, Fenton-like and photo-Fenton performances were comprehensively explored against contaminants removal. In comparison, synthesized Fe-BDC 2 with the common solvothermal method, the yield of Fe-BDC 1 increased 3.4 times. Also, its adsorption capacity towards Rhodamine B (Rh. B) increased about 5.85 times. Moreover, the significantly improved degradation of Rh. B was observed in the Fenton-like and photo-Fenton systems with the increased reaction rate constants up to 4.90 and 2.33 times, respectively. This enhanced catalytic performance of Fe-BDC 1 was mainly attributed to the enlarged specific surface area (SSA) and favorable electron separation and migration, resulting into the accelerated ≡Fe(II)/≡Fe(III) cycle and increased yield of reactive oxidative species (ROSs). Additionally, the unique light-induced adsorption enhancement further strengthened the contaminants removal. Fe-BDC 1 as photo-Fenton catalyst was found superior over conventional catalysts in terms of advantages including: low dose requirement of catalysts (decrease in an order of magnitude) and H 2 O 2 , wide pH working range (3.0–9.0), excellent reusability and stability, universal removal behavior against all organic pollutants, and remarkable synergism between photocatalysis and Fenton-like process with synergistic factor of 77.23%. Moreover, the Fe coordinative unsaturated sites (Fe CUSs) and ·OH were determined as the active sites and the main ROSs, respectively, responsible for highly efficient pollutant degradation. This work gives a feasible solution for the preparation of Fe-MOFs with low synthesis cost and high yield, paving ways for their practical mass application in heterogeneous photo-Fenton oxidation. [Display omitted] • Successfully synthesis of Fe-MOFs with low cost and high yield. • Significantly enhanced adsorption, Fenton-like and photo-Fenton performances were acquired. • Visible light induced adsorption enhancement further strengthen contaminates removal. • The synergism between photocatalytic and Fenton-like process was demonstrated. • Favorable electron separation and migration accelerated ≡Fe(II)/≡Fe(III) cycle. [ABSTRACT FROM AUTHOR]

• Successful development of novel low-crystalline bimetallic MOFs as excellent photo-Fenton catalysts. • Achievement of intensified synergism between hetero-metal nodes in the low-crystalline bimetallic MOFs. • Low-crystalline state, further strengthened the role of doped metal. • The electron-rich Fe sites were formed in the low-crystalline MOFs. • Low-crystalline MOFs enable exposure of more metal CUSs, beneficial for H 2 O 2 coordination. In this study, the low-crystalline bimetallic metal-organic frameworks (MOFs) of L-MIL-53(Fe, Mn) and L-MIL-53(Fe, Cu) (MIL: Material of Institute Lavoisier) were successfully developed as photo-Fenton catalysts, exhibiting significantly enhanced degradation performance (15.2 and 2.49 times increase in reaction rate constant, respectively) for ciprofloxacin (CIP) in contrast to their crystalline counterparts. That were mainly attributed to the intensified synergism between hetero-metal nodes. Specifically, taking L-MIL-53(Fe, Mn) as an example, the formed electron-rich Fe sites along with the strengthened roles of Mn for enlarged specific surface area, enhanced visible-light absorption and separation and transportation of photo-induced carriers in the low-crystalline state, are responsible for acceleration of Fe(II)/Fe(III) and Mn(II)/Mn(III) cycles, beneficial for H 2 O 2 activation. Moreover, increased metal coordinately unsaturated sites (CUSs) are favorable for more H 2 O 2 coordination. This work gives a deep insight into the heterogeneous photo-Fenton mechanism of the bimetallic MOFs in their low-crystalline state. [ABSTRACT FROM AUTHOR]