Iron Oxo Research Articles

High-Valent Iron–Oxo species in heterogeneous Catalysis: Formation, Mechanism, and reactivity for the degradation of various emerging organic contaminants

High-valent iron–oxo species (FeⅣ/Ⅴ=O), generated via peroxide activation with Fe-based catalysts, have shown potential in the selective and efficient degradation of organic contaminants. However, a comprehensive understanding of the oxidative degradation mechanism induced by FeⅣ/Ⅴ=O is still pending. In this research, to unravel this oxidation mechanism, an Fe-single atom catalyst (FeN4-C) was synthesized, and FeⅤ=O was formed as the predominant reactive species upon peroxymonosulfate (PMS) activation. The FeN4-C/PMS system was experimentally studied on its catalytic oxidation behavior for the degradation of 34 typical emerging organic contaminants (EOCs) in water, exhibiting a significant variation in the pseudo first-order kinetic rate constants (kobs) among the EOCs. A quantitative structure–activity relationship (QSAR) model was established based on the molecular structure feature and measured kobs of the EOCs. The model infers that EOCs with higher electron donating ability (EHOMO) are more susceptible towards FeN4-C/PMS oxidation, implying an electrophilic reaction following the single electron transfer (SET) mechanism. Further exploration of the degradation pathways for seven representative EOCs revealed that the oxidation occurs through the abstraction of either a hydrogen atom or a nonbonded (or π) electron by FeⅤ=O, enabling processes such as C-, N-, and S-oxidation, along with N- and O-dealkylation and deamination, to effectively degrade EOCs. These research findings provide a robust theoretical framework for understanding the reaction mechanisms and predicting the degradability and removal of various EOCs by FeⅣ/Ⅴ=O for water purification.

Enhanced Alkene Oxidative Cleavage in Water via Photoexcited FeIV Species within Covalent Organic Framework Thin Films.

The oxidative cleavage of alkenes is a crucial step in synthesizing key organic molecules featuring carbonyl functional groups prevalent in natural products and pharmaceuticals. We introduce a photochemical method for heterogeneous C=C bond cleavage, employing photo-catalytically generated [(bTAML)FeIV-O-FeIV(bTAML)]- species (where bTAML stands for biuret-modified tetraamido macrocyclic ligand) in aqueous environments under gentle conditions. Leveraging the photosensitizing properties of Covalent Organic Frameworks (COFs) and their advantageous morphological traits as films, we enhance the reaction by closely associating the substrate with the catalyst. This study marks the inaugural demonstration of Fe2 IV-μ-oxo radical cation and FeIV=O species facilitating alkene cleavage in water against a backdrop of a hydrophobic COF. Through comprehensive mechanistic studies, including control experiments, we confirm that these two high-valent iron oxo species collaborate to cleave alkenes, forming an intermediate epoxide. Our approach yields moderate to high success across various alkenes, displaying diverse functional groups (achieving up to 75 % yield) with notable efficiency and selectivity towards aldehyde/ketone products. Moreover, the heterogeneous COF film, immobilizing (Et4N)2[FeIII(Cl)bTAML], exhibits exceptional recyclability, enduring up to four cycles.

Enhanced Alkene Oxidative Cleavage in Water via Photoexcited FeIV Species within Covalent Organic Framework Thin Films

AbstractThe oxidative cleavage of alkenes is a crucial step in synthesizing key organic molecules featuring carbonyl functional groups prevalent in natural products and pharmaceuticals. We introduce a photochemical method for heterogeneous C=C bond cleavage, employing photo‐catalytically generated [(bTAML)FeIV−O−FeIV(bTAML)]− species (where bTAML stands for biuret‐modified tetraamido macrocyclic ligand) in aqueous environments under gentle conditions. Leveraging the photosensitizing properties of Covalent Organic Frameworks (COFs) and their advantageous morphological traits as films, we enhance the reaction by closely associating the substrate with the catalyst. This study marks the inaugural demonstration of Fe2IV‐μ‐oxo radical cation and FeIV=O species facilitating alkene cleavage in water against a backdrop of a hydrophobic COF. Through comprehensive mechanistic studies, including control experiments, we confirm that these two high‐valent iron oxo species collaborate to cleave alkenes, forming an intermediate epoxide. Our approach yields moderate to high success across various alkenes, displaying diverse functional groups (achieving up to 75 % yield) with notable efficiency and selectivity towards aldehyde/ketone products. Moreover, the heterogeneous COF film, immobilizing (Et4N)2[FeIII(Cl)bTAML], exhibits exceptional recyclability, enduring up to four cycles.

Electrochemically producing high-valent iron–oxo species for phenolics-laden high chloride wastewater pretreatment

Electrochemical advanced oxidation processes (EAOPs) have shown great promise for treating industrial wastewater contaminated with phenolic compounds. However, the presence of chloride in the wastewater leads to the production of undesirable chlorinated organic and inorganic byproducts, limiting the application of EAOPs. To address this challenge, we investigated the potential of incorporating Fe(II) and Fe(III) into the EAOPs with a boron-doped diamond (BDD) anode under near-neutral conditions. Our findings revealed that both Fe(II) and Fe(III) facilitated the generation of high-valent iron-oxo species (Fe(IV) and Fe(V)) in the anodic compartment, thereby reducing the oxidation contribution of reactive chlorine species. Remarkably, the addition of 1000 μM Fe(II) under high chloride conditions resulted in over a 2.8-fold increase in the oxidation rate of 50 μM phenolic contaminants at pH 6.5. Furthermore, 1000 μM Fe(II) contributed to a reduction of more than 66% in the formation of chlorinated byproducts, consequently enhancing the biodegradability of the treated water. Additionally, transitioning from batch mode to continuous flow mode further amplified the positive effects of Fe(II) on the EAOPs. Overall, this study presents a modified electrochemical approach that simultaneously enhanced the degradation of phenolic contaminants and improved the biodegradability of wastewater with high chloride concentrations.

Visible light-enhanced interface interaction for PMS activation towards the removal of emerging organic pollutants: performance, mechanism and toxicity

The peroxymonosulfate (PMS)-based advanced oxidation process is considered as an effective way to remove emerging organic pollutants in the wastewater. However, the heterogeneous catalysts used for PMS activation suffers from low catalytic stability and PMS utilization efficiency. Herein, sulfur-doped cobalt ferrite loaded graphitic carbon nitride (S-CoFe2O4/CN) was firstly synthesized for PMS activation toward the sulfamethoxazole (SMX) removal. The S-CoFe2O4/CN possessed excellent PMS catalytic activity and photocatalytic activity. The first-order kinetic constant (k) of SMX degradation in the presence of visible light reached 0.3564 min−1 with high PMS utilization efficiency (78.3 %). Mechanism analysis elucidated that compared to the original CoFe2O4, sulfur doping created more oxygen vacancies, while visible light further promoted the regeneration of Co(II), Fe(II) and oxygen vacancy, the formation of high-valent iron oxo as well as the reduction of S2- oxidation. In addition, the visible light/S-CoFe2O4/CN/PMS system showed high resistance to inorganic ions, and superior catalytic stability. Both acute and chronic toxicity of treated SMX solution decreased. This study provides an effective system to remove emerging organic pollutants in the wastewater.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C-H Oxidation.

Precise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)-H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site-selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C-H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis-Menten kinetics and increased reaction rates. In addition, C-H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Proximity Effects on the Reactivity of a Nonheme Iron (IV) Oxo Complex in C−H Oxidation

AbstractPrecise control of substrate positioning and orientation (its proximity to the reactive unit) is often invoked to rationalize the superior enzymatic reaction rates and selectivities when compared to synthetic models. Artificial nonheme iron (IV) oxo (Fe(IV)=O) complexes react with C(sp3)−H bonds via a biomimetic Hydrogen Atom Transfer/Hydroxyl Rebound mechanism, but rates, site‐selectivity and even hydroxyl rebound efficiency (ligand rebound versus substrate radical diffusion) are smaller than in oxygenases. Herein, we quantitatively analyze how substrate binding modulates nonheme Fe(IV)=O reactivity by comparing rates and outcomes of C−H oxidation by a pair of Fe(IV)=O complexes that share the same first coordination sphere but only one contains a crown ether receptor that recognizes the substrate. Substrate binding makes the reaction intramolecular, exhibiting Michaelis–Menten kinetics and increased reaction rates. In addition, C−H oxidation occurs with high site selectivity for remote sites. Analysis of Effective Molarity reveals that the system operates at its maximal theoretical capability for the oxidation of these remote sites. Remarkably, substrate positioning also affects Hydroxyl Rebound, whose efficiency only increases on the sites placed in proximity by recognition. Overall, these observations provide evidence that supramolecular control of substrate positioning can effectively modulate the reactivity of oxygenases and its models.

Coupling iron-based zeolitic imidazolate Framework-8 and cellulose nanofiber for efficient peroxymonosulfate oxidation

The development of biocompatible Fenton-like catalysts with outstanding activities and stabilities, using low-cost sustainable bio-sourced materials, has garnered significant attention, yet remains a substantial challenge. In this study, we successfully constructed monodispersed iron anchored on a 3D porous N-doped carbon substrate (CNF@Fe-NC) via one-step annealing of Fe-based zeolitic imidazolate frameworks (ZIFs) and bacterial cellulose nanofibers (CNFs) as highly efficient peroxide-activation catalysts. Due to abundant micropores and mesopores in the interconnected CNF support and ZIFs, even after annealing, the obtained catalysts have a specific surface area of up to 739.4 m2/g. Owing to monodispersed Fe-Nx species and abundant nanofibrous structure, CNF@Fe-NC exhibited outstanding catalytic stability and superior activity for peroxymonosulfate (PMS) activation toward organic contaminant oxidation, outperforming the benchmark Fenton system. Surface Fe-Nx sites and CNF defects could adsorb negatively charged PMS to produce nonradical CNF@Fe-NC-PMS* complexes and high-valent iron oxo species (FeV = O), rather than radicals and 1O2, for eliminating pollutants. The strong iron − support electronic interaction and abundant π-conjugation in the CNF support facilitated the efficient adsorption and activation of PMS and the uniformly dispersed Fe species in CNF@Fe-NC improved atom-utilization efficiency. This study provides a clue to design biocompatible cellulosic-based Fenton-like catalysts for the removal of organic contaminants.

Impact of carboxylate ligation on the C-H activation reactivity of a non-heme Fe(IV)O complex: a computational investigation.

A comprehensive DFT investigation has been presented to predict how a carboxylate-rich macrocycle would affect the reactivity of a non-heme Fe(IV)O complex towards C-H activation. The popular non-heme iron oxo complex [FeIV(O)(N4Py)]2+, (N4Py = N,N-(bis(2-pyridyl)methyl)N-bis(2-pyridylmethyl)amine) (1), has been selected here as the primary compound. It is transformed to the compound [FeIV(O)(nBu-P2DA)], where nBu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)iminodiacetate (2) after the replacement of two pyridine donors of N4Py with carboxylate groups. Two other complexes, namely 3 and 4, have been predicted sequentially substituting nitrogen with the carboxylate groups. Ethylbenzene and dihydrotoluene were chosen as substrates. In terms of C-H activation reactivity, an interesting pattern emerges: as the carboxylate group becomes more equatorially enriched, the reactivity increases, following the trend 1 < 2 < 3 < 4. This also aligns with available experimental reports related to complexes 1 and 2. Fe(IV)O complexes exhibit two-state reactivity (triplet and quintet), whereas the quintet state is more favourable due to the stabilization of the transition states through exchange interactions involving a greater number of unpaired electrons. A detailed analysis of the factors influencing reactivity has been performed, including distortion energy (which decreases for the transition state with the addition of carboxylate groups), the triplet-quintet oxidant energy gap (which consistently decreases as carboxylate group enrichment increases), steric factors, and quantum mechanical tunneling. This investigation provides a detailed explanation of the observed outcomes and predicts the higher reactivity of carboxylate-enriched Fe(IV)O complexes. After potential experimental verification, this could lead to the development of new, optimal catalysts for C-H activation.

Synchronous and basic asynchronous hydrogen-atom abstraction of benzylic substrates by high-valent iron–oxo porphyrin species

The unsubstituted iron–oxo porphyrin reacts with unsubstituted benzylic C–H bond substrates via a synchronous HAT mechanism. In contrast, when an electron-rich iron–oxo porphyrin reacts with electron-poor substrates, it follows a basic asynchronous HAT mechanism.

Dearomative syn-Dihydroxylation of Naphthalenes with a Biomimetic Iron Catalyst.

Arenes are interesting feedstocks for organic synthesis because of their natural abundance. However, the stability conferred by aromaticity severely limits their reactivity, mostly to reactions where aromaticity is retained. Methods for oxidative dearomatization of unactivated arenes are exceedingly rare but particularly valuable because the introduction of Csp3-O bonds transforms the flat aromatic ring in 3D skeletons and confers the oxygenated molecules with a very rich chemistry suitable for diversification. Mimicking the activity of naphthalene dioxygenase (NDO), a non-heme iron-dependent bacterial enzyme, herein we describe the catalytic syn-dihydroxylation of naphthalenes with hydrogen peroxide, employing a sterically encumbered and exceedingly reactive yet chemoselective iron catalyst. The high electrophilicity of hypervalent iron oxo species is devised as a key to enabling overcoming the aromatically promoted kinetic stability. Interestingly, the first dihydroxylation of the arene renders a reactive olefinic site ready for further dihydroxylation. Sequential bis-dihydroxylation of a broad range of naphthalenes provides valuable tetrahydroxylated products in preparative yields, amenable for rapid diversification.

Ionizing radiation-assisted in-situ synthesis of nitrogen-doped graphene oxide-supported nano Fe3O4 for PMS activation to degrade emerging pollutants

This study firstly reported the in-situ preparation of nitrogen-doped graphene oxide-supported nano Fe3O4 (Fe3O4@NRGO) by ionizing radiation. The conversion of Fe(II) to iron oxide was modulated by absorbed dose in solution, which regulated the catalytic activity of Fe3O4/NRGO. When the initial concentration of Fe(II) was 0.5 mM and the absorbed dose was 10 kGy, the as-prepared catalyst (Fe3O4@NRGO-10) showed superior catalytic activity for activating peroxymonosulfate (PMS) to degrade phenol. In the presence of 0.4 mM PMS and 0.5 g/L Fe3O4@NRGO-10, the pseudo first-order kinetic rate of phenol degradation reached 0.0286 min−1. Sulfate radicals, hydroxyl radicals and high-valent iron oxo made major contribution to phenol degradation. The superior catalytic activity of Fe3O4@NRGO-10 might be attributed to the synergistic effect of NRGO and Fe3O4, in which NRGO promoted the conversion of Fe(III) to Fe(II). Fe3O4@NRGO-10 exhibited good stability in the cycling experiments. In addition to phenol, the Fe3O4@NRGO-10/PMS system could degrade sulfamethoxazole, bisphenol A and atrazine, indicating the wide applicability. This study reported a facile way to prepare carbon-supported metal oxide catalysts and convenient way to modulate the catalytic activity by absorbed dose.

Piezoelectricity ameliorates high-valent iron oxo species production in peroxymonosulfate activation for refractory atrazine remediation

Over the past few years, high-valent iron oxo species (Fe(IV)) have shown considerable promise. However, an improved solution is needed for the bottleneck of unsatisfactory electron transfer efficiency in Fe-based catalyst/PMS systems. In this study, Enteromorpha-derived biochar was pyrolyzed with iron and barium titanate (FeBCBa). Under ultrasonic treatment, it removes 94.5% of atrazine (10 mg/L) within 60 min, and is environmentally friendly. BaTiO3′s piezoelectricity enhances Fe(IV) production in FeBCBa, resulting in superior performance. In the ultrasonic condition, the apparent reaction rate was 1.42 times higher than in the non-ultrasonic condition. Using density functional theory calculations, it can be shown that due to the Fe dopant, electrons in ATZ’s LUMO are more easily transferred to the catalyst’s HOMO, which is beneficial for ATZ removal. The results of this study provide new guidance for constructing stable and efficient catalysts for environmental remediation.

Hydrogen atom abstraction by a high-spin [FeIII=S] complex

Iron sulfur clusters are critical to a plethora of biological processes; however, little is known about the elementary unit of these clusters, namely the [Fe=S]n+ fragment. Here, we report the synthesis and characterization of a terminal iron sulfido complex. Despite its high spin (S = 5/2) ground state, structural, spectroscopic, and computational characterization provide evidence for iron sulfur multiple bond character. Intriguingly, the complex reacts with additional sulfur to afford an S = 3/2 iron(III) disulfido (S2 2-) complex. Preliminary studies reveal that the sulfido complex reacts with dihydroanthracene to afford an iron(II) hydrosulfido complex, akin to the reactivity of iron oxo complexes. By contrast, there is no reaction with the disulfido complex. These results provide important insight into the nature of the iron sulfide unit.

Bio-Inspired Iron Pentadentate Complexes as Dioxygen Activators in the Oxidation of Cyclohexene and Limonene.

The use of dioxygen as an oxidant in fine chemicals production is an emerging problem in chemistry for environmental and economical reasons. In acetonitrile, the [(N4Py)FeII]2+ complex, [N4Py-N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine] in the presence of the substrate activates dioxygen for the oxygenation of cyclohexene and limonene. Cyclohexane is oxidized mainly to 2-cyclohexen-1-one, and 2-cyclohexen-1-ol, cyclohexene oxide is formed in much smaller amounts. Limonene gives as the main products limonene oxide, carvone, and carveol. Perillaldehyde and perillyl alcohol are also present in the products but to a lesser extent. The investigated system is twice as efficient as the [(bpy)2FeII]2+/O2/cyclohexene system and comparable to the [(bpy)2MnII]2+/O2/limonene system. Using cyclic voltammetry, it has been shown that, when the catalyst, dioxgen, and substrate are present simultaneously in the reaction mixture, the iron(IV) oxo adduct [(N4Py)FeIV=O]2+ is formed, which is the oxidative species. This observation is supported by DFT calculations.

Slow O–H Dissociation in the First-Order Oxygen Evolution Reaction Kinetics on Polycrystalline γ-FeO(OH)

To unravel the reaction mechanism of the electrochemical oxygen evolution reaction (OER), analysis of the intrinsic activity parameters obtained via electrokinetic investigation remains effective. In this study, polycrystalline γ-FeO(OH), synthesized at room temperature, was used as a stable, albeit reactive, anode for OER, and electrokinetic studies were performed to unravel the oxygen evolution reaction pathway. The cell temperature, hydroxyl ion concentration, and the cation of the supporting electrolyte were varied, and the influence of external bias on the OER activity was recorded. Changes in the cell temperature from 30 to 65 °C lead to an enhanced OER activity due to small overpotential, Tafel slope, and charge-transfer resistance (Rct) values at high temperatures, which unambiguously highlights the influence of the thermodynamic barrier and electron-transfer kinetics. The calculated anodic charge-transfer coefficient (αa) and specific exchange current density (j0,s) values are 0.65 and 2.24 × 10–5 mA cm–2, respectively. The faster OER kinetics on polycrystalline γ-FeO(OH) can also be attributed to an appreciably low activation energy of 8.26 ± 2 kJ mol–1, where variation of the electrolyte concentration indicated a first-order dependence on [OH]−. Experimentally obtained intrinsic kinetic parameters such as the Tafel slope, anodic charge-transfer coefficient, exchange current density, activation energy (Ea), reaction order (m), and deuterium isotope effect implicate the dissociation of hydroxyl ions on the polycrystalline γ-FeO(OH) as the rate-determining step. The direct effect of cations such as Li+, Na+, and K+ of the electrolyte on OER highlights a weak interaction of the cations with the surface-active [FeIII-OH] species. A change in the electrode substance from three-dimensional highly conductive nickel foam (NF) to two-dimensional less conductive carbon cloth (CC) resulted in a change in the slow step, leading to the formation of a high-valency iron oxo intermediate from the iron hydroxy species.

Comparative oxidative ability of mononuclear and dinuclear high-valent iron-oxo species towards the activation of methane: does the axial/bridge atom modulate the reactivity?

Over the years, mononuclear FeIVO species have been extensively studied, but the presence of dinuclear FeIVO species in soluble methane monooxygenase (sMMO) has inspired the development of biomimic models that could activate inert substrates such as methane. There are some successful attempts; particularly the [(Por)(m-CBA) FeIV(μ-N)FeIV(O)(Por˙+)]- species has been reported to activate methane and yield decent catalytic turnover numbers and therefore regarded as the closest to the sMMO enzyme functional model, as no mononuclear FeIVO analogues could achieve this feat. In this work, we have studied a series of mono and dinuclear models using DFT and ab initio DLPNO-CCSD(T) calculations to probe the importance of nuclearity in enhancing the reactivity. We have probed the catalytic activities of four complexes: [(HO)FeIV(O)(Por)]- (1), [(HO)FeIV(O)(Por˙+)] (2), μ-oxo dinuclear iron species [(Por)(m-CBA)FeIV(μ-O)FeIV(O) (Por˙+)]- (3) and N-bridged dinuclear iron species [(Por)(m-CBA)FeIV(μ-N)FeIV(O)(Por˙+)]- (4) towards the activation of methane. Additionally, calculations were performed on the mononuclear models [(X)FeIV(O)(Por˙+)]n {X = N 4a (n = -2), NH 4b (n = -1) and NH24c (n = 0)} to understand the role of nuclearity in the reactivity. DFT calculations performed on species 1-4 suggest an interesting variation among them, with species 1-3 possessing an intermediate spin (S = 1) as a ground state and species 4 possessing a high-spin (S = 2) as a ground state. Furthermore, the two FeIV centres in species 3 and 4 are antiferromagnetically coupled, yielding a singlet state with a distinct difference in their electronic structure. On the other hand, species 2 exhibits a ferromagnetic coupling between the FeIV and the Por˙+ moiety. Our calculations suggest that the higher barriers for the C-H bond activation of methane and the rebound step for species 1 and 3 are very high in energy, rendering them unreactive towards methane, while species 2 and 4 have lower barriers, suggesting their reactivity towards methane. Studies on the system reveal that model 4a has multiple FeN bonds facilitating greater reactivity, whereas the other two models have longer Fe-N bonds and less radical character with steeper barriers. Strong electronic cooperativity is found to be facilitated by the bridging nitride atom, and this cooperativity is suppressed by substituents such as oxygen, rendering them inactive. Thus, our study unravels that apart from enhancing the nuclearity, bridging atoms that facilitate strong cooperation between the metals are required to activate very inert substrates such as methane, and our results are broadly in agreement with earlier experimental findings.

Recent developments of Fe-based metal-organic frameworks and their composites in photocatalytic applications: fundamentals, synthesis and challenges

Nowadays, the use of efficient nanomaterials in the photocatalytic applications are highly demanded to maximize the utilization of solar light energy for sustainable fuel production and environmental remediation. Recently, there has been a growing research on the use of metal-organic framework (MOF) materials as photocatalysts owing to their unique structures and optoelectronic properties. Among these MOF materials, Fe-based MOF photocatalysts have attracted much attention in all fields of photocatalysis due to the presence of extensive iron–oxo (Fe–O) clusters which increase the visible light harvesting. Moreover, iron is considered as one of the low-cost and earth-abundant metals. In this mini-review, the recent developments in Fe-based MOF synthesis techniques with their major photocatalytic applications in oxygen production, hydrogen production, CO&lt;sub&gt;2&lt;/sub&gt; reduction and pollutant photodegradation are summarized and deliberately explained. Finally, the main challenges regarding the Fe-based MOF photocatalysis with the future recommendations are addressed &lt;br&gt; The bibliography includes 175 references.

Catalytic activity enhancement by P and S co-doping of a single-atom Fe catalyst for peroxymonosulfate-based oxidation

High-valent iron–oxo species (FeV=O) generated through peroxide activation by Fe-based catalysts can realize the selective and efficient degradation of organic pollutants in water treatment. However, the rapid and efficient generation of FeV=O for the selective degradation of pollutants over a wide pH range with high stability is challenging. In this study, a N, P, and S co-doped Fe-single atom (SA) catalyst (FeSA-NPS@C) with FeN4 active center was synthesized as a highly efficient catalyst for the peroxymonosulfate (PMS)-based oxidation. The FeSA-NPS@C catalyst exhibited an excellent catalytic performance at low dosages of both the catalyst and PMS for the degradation of pollutants over a wide pH range with high stability. For the degradation of ofloxacin (OFX), the rate constant pertaining to PMS with FeSA-NPS@C was 1.68 min−1, considerably higher than those obtained using the N-doped and N and P co-doped Fe-SA catalysts. The degradation pathway of OFX revealed that the electrostatic effect was of significance in its degradation by FeVN4=O with FeVN4=O being the dominant reactive species. Density functional theory calculations revealed that the heteroatom doping effectively altered the electronic structure of the FeN4 site of FeSA-NPS@C, which strengthen its coordination with PMS, promote the electron transfer to PMS, and facilitate the interaction between FeVN4 = O and OFX, and thus significantly enhanced its catalytic activity. These findings provide new insights into the oxidation mechanism of FeV=O in heterogeneous systems and the catalytic ability enhancement through heteroatom doping.

Engineering electronic and geometric structures of Fe, N-doped carbon polyhedrons toward organic contaminant oxidation

A universal gas diffusion strategy was developed to derive highly efficient PMS-activation catalysts comprising isolated Fe atoms coordinated with N species embedded in carbon polyhedrons (Fe-NC). Their electronic and geometric structures were modulated by altering the precursor ratio and calcination temperature. Benefiting from the maximized atomic utilization, the obtained catalysts exhibited superior catalytic performances, outperforming almost all previously reported metal materials. The enhanced activity is likely due to an optimal particle size and Fe content, favorable Fe-Nx active sites, and conductive carbon materials. Scavenging and probing experiments indicated that surface Fe-NC and the defective edges with oxygen functional groups were the active sites for PMS activation to produce surface-bonding active complexes (Fe-NC-PMS) and high-valent iron oxo species (FeV = O) for organic degradation, unlike the reported radicals and singlet oxygen mediated degradation. Overall, this work provides a universal gas diffusion strategy to design dimension-controlled Fe-NC catalysts for the elimination of organic pollutants.