Size-matched hierarchical porous carbon materials anchoring single-atom Fe-N4 sites for PMS activation: An in-depth study of key active species and catalytic mechanisms

State-Of-The-Art Structural Regulation Methods and Quantum Chemistry for Carbon-Based Single-Atom Catalysts in Advanced Oxidation Process: Critical Perspectives into Molecular Level

Advanced oxidation processes (AOPs) by carbon-based single-atom catalysts (SACs) are recognized as an attractive scientific frontier for water treatment, with the outstanding benefits of ultra-effective and anti-interference capability. However, most of the research has paid more attention to the performance of SACs, while the in-depth understanding of catalytic regulation by molecular interaction is relatively deficient. This critical review delves into deciphering the catalytic mechanism through a micro-level, which makes it more convenient to interpret apparent catalytic phenomena. It first summarizes basic theories of quantum chemistry, which provide mechanism interpretation and prediction for molecular-oxidation systems. Additionally, corresponding oxidation pathways of common oxidants are underscored. Following the oxidants, state-of-the-art regulation methods are discussed with special attention to involved molecular interactions and pollutants. Particularly, the preliminary insights into the "oxidant-catalyst-pollutants" internal relationships are provided to help construct the SAC-AOP system from a molecular standpoint. Meanwhile, some cutting-edge laboratory devices and pilot-scale engineering are presented to illustrate the ultimate purpose of scientific molecular exploration. Eventually, relative challenges of SACs-AOPs upon the design of catalytic systems and investigation methods are provided. This review aims to promote the large-scale potential of SACs-based AOPs in practical water treatment by emphasizing the pivotal role of micro-insights.

Highly Efficient Degradation of Bisphenol A by Peroxymonosulfate Activation Using Bamboo Kraft Lignin Single-Atom Catalyst

A nitrogen-coordinated Fe single-atom catalyst (SA Fe-N/C) is synthesized using a homogeneous ethanol-based dissolution system with bamboo kraft lignin serving as the carbon source. Uniformly dispersed Fe atoms with an interatomic distance of less than 2 Å throughout the SA Fe-N/C structure are revealed through X-ray absorption spectral analysis and HAADF-STEM images, which possessed a high Fe loading of 2.69%. The degradation rate of bisphenol A (BPA) approached 99% within 5 min, with the observed rate constant (kobs) of the catalysts markedly increasing from 0.070 to 0.615 min-1. The catalyst-mediated electron transfer pathway is identified as the predominant mechanism for BPA degradation. Both experimental data and DFT analysis of the nitrogen ligands demonstrated that pyridinic N-coordinated Fe single atoms are the principal active sites, attributed to the enhanced electron density and delocalization concentrated around the Fe sites. These findings significantly elucidate the role of nitrogen ligands in designing efficient lignin-derived carbon single-atom catalysts for environmental applications.

Bridge-oxygen bonding modulates Ru single atoms for peroxymonosulfate activation: Importance of high-valent Ru species and 1O2

The application of single-atom catalysts (SACs) to advanced oxidation processes (AOPs) based on peroxymonosulfate (PMS) has attracted considerable attention. However, the catalytic pathways and mechanisms underlying these processes remain unclear. In this study, NiFe-LDH was synthesized and single Ru atoms were stably loaded onto it by forming Ru-O-M (M=Ni or Fe) bonds (Ru@NiFe-LDH). This was demonstrated using high-angle annular dark-field scanning TEM (HAADF-STEM) and X-ray absorption fine structure spectra (XANES). The Ru@NiFe-LDH/PMS system showed a high catalytic reactivity (100 % sulfamethoxazole degradation in only 30 min), high stability (97 % reactivity was maintained after continuous operation for 400 min), and wide pH suitability (working pH range 3-11) for AOPs. The crucial roles of the high-valent species (Ru(V) = O) and 1O2 in this reaction were verified. Density functional theory (DFT) calculations revealed that electron transfer produced a positively charged Ru. This enhances the adsorption of negatively charged PMS anions onto the Ru monoatomic sites, thereby, causing the formation of Ru-PMS* complexes. This study implies that the structure-function relationship between organic compounds and SACs plays a significant role in PMS-based AOPs, and provides a comprehensive mechanism for the role of high-valent species in heterogeneous Fenton-like systems.

Efficient degradation of organic pollutants without any external assistance over a wide pH range using carbon vacancy-modified Fe-N-C catalysts

Currently, water treatment usually requires additional light illumination or oxidants for the degradation of organic pollutants, which increases the costs and is not conducive to practical application. In this study, carbon vacancy-modified Fe-N-C single-atom catalysts (Cv-Fe-N-C SACs) were prepared through one simple acid-etching and pyrolysis process. Interestingly, we found that Cv-Fe-N-C SACs could degrade organic pollutants without any external assistance (such as oxidants or light illumination). The Cv-Fe-N-C SACs could remove over 99% of Rhodamine B (RhB) within 10 min at room temperature. The degradation of organic pollutants with the Cv-Fe-N-C SACs was attributed to their ability to activate dissolved oxygen for producing superoxide (O2˙-). In addition, the catalysts showed high activity over a broad pH range (3-11) and held rather good stability after 5 recycles. This study proved that the Cv-Fe-N-C SACs are highly efficient catalysts for degrading organic pollutants. These catalysts have the potential to make sewage treatment more efficient and less expensive.

Non-radical oxidation driven by iron-based materials without energy assistance in wastewater treatment

Chemical oxidation is extensively utilized to mitigate the impact of organic pollutants in wastewater. The non-radical oxidation driven by iron-based materials is noted for its environmental friendliness and resistance to wastewater matrix, and it is a promising approach for practical wastewater treatment. However, the complexity of heterogeneous systems and the diversity of evolutionary pathways make the mechanisms of non-radical oxidation driven by iron-based materials elusive. This work provides a systematic review of various non-radical oxidation systems driven by iron-based materials, including singlet oxygen (1O2), reactive iron species (RFeS), and interfacial electron transfer. The unique mechanisms by which iron-based materials activate different oxidants (ozone, hydrogen peroxide, persulfate, periodate, and peracetic acid) to produce non-radical oxidation are described. The roles of active sites and the unique structures of iron-based materials in facilitating non-radical oxidation are discussed. Commonly employed identification methods in wastewater treatment are compared, such as quenching, chemical probes, spectroscopy, mass spectrometry, and electrochemical testing. According to the process of iron-based materials driving non-radical oxidation to remove organic pollutants, the driving factors at different stages are summarized. Finally, challenges and countermeasures are proposed in terms of mechanism exploration, detection methods and practical applications of non-radical oxidation driven by iron-based materials. This work provides valuable insights for understanding and developing non-radical oxidation systems.

S-Doping Regulated Iron Spin States in Fe-N-C Single-Atom Material for Enhanced Peroxidase-Mimicking Activity at Neutral pH

Designing biomimetic nanomaterials with peroxidase (POD)-like activity at neutral pH remains a significant challenge. An S-doping strategy is developed to afford an iron single-atom nanomaterial (Fe1@CN-S) with high POD-like activity under neutral conditions. To the best of knowledge, there is the first example on the achievement of excellent POD-like activity under neutral conditions by regulating the active site structure. S-doping not only promotes the dissociation of the N─H bond in 3,3″,5,5″-tetramethylbenzidine (TMB), but also facilitates the desorption of OH* by the transformation of iron species' spin states from middle-spin (MS FeII) to low-spin (LS FeII). Meanwhile, LS FeII sites typically have more unfilled d orbitals, thereby exhibiting stronger interactions with H2O2 than MS FeII, which can enhance POD-like activity. Finally, a one-pot visual detection of glucose at pH 7 is performed, demonstrating the best selectivity and sensitivity than previous reports.

Single-atom catalysts activate persulfate to degrade emerging organic contaminants in aqueous environments

Single-atom catalysts (SACs) exhibit outstanding catalytic activity due to their highly dispersed metal centers. Activating persulfates (PS) with SACs can generate various reactive oxygen species (ROS) to efficiently degrade emerging organic contaminants (EOCs) in aqueous environments, offering unique advantages such as high reaction rates and excellent stability. This technique has been extensively researched and holds enormous potential applications. In this paper, we comprehensively elaborated on the synthesis methods of SACs and their limitations, and factors influencing the catalytic performance of SACs, including metal center characteristics, coordination environment, and types of substrates. We also analyzed practical considerations for application. Subsequently, we discussed the mechanism of SACs activating PS for EOCs degradation, encompassing adsorption processes, radical pathways, and non-radical pathways. Finally, we provide prospects and outline our vision for future research, aiming to guide advancements in applying this technique.

Peroxymonosulfate activation by superparamagnetic mixed-valent Cu/N-(L-cysteine)-O-(carboxymethyl)chitosan/cobalt ferrate-rice hull hybrid nanocomposite for efficient degradation of naproxen: Synergetic adsorption-catalysis, kinetics, pathway, and relevant mechanism

Naproxen (NPX) as an emerging anthropogenic contaminant was detected in many water sources, which can pose a serious threat to the environment and human health. Peroxymonosulfate (PMS) decomposed by Cu(I) has been considered an effective activation method to produce reactive species. However, this decontamination process is restricted by the slow transformation of Cu(II)/Cu(I) by PMS. Herein, new N-(L-cysteine/triazine)-O-(carboxymethyl)-chitosan/cobalt ferrate-rice hull hybrid biocomposite was constructed to anchor the mixed-valent Cu(I)-Cu (II) (CuI, II-CCCF) for removing pharmaceutical pollutants (i.e., naproxen, ciprofloxacin, tetracycline, levofloxacin, and paracetamol). The structural, morphological, and catalytic properties of the CuI,II-CCCF have been fully identified by a series of physicochemical characterizations. Results demonstrated that the multifunctional, hydrophilic character, and negative ζ-potential of the activator, accelerating the redox cycle of Cu(II)/Cu(I) with hydroxyl amine (HA). The negligible metal leaching, well-balanced thermodynamic-kinetic properties, and efficient adsorption-catalysis synergy are the main reasons for the significantly enhanced catalytic performance of CuI,II-CCCF in the removal of NPX (98.6 % at 7.0 min). The main active species in the catalytic degradation of NPX were identified (●OH > SO4●- > 1O2 > > O2●-) and consequently suggested a degradation path. It can be noted that these types of carbohydrate-based nanocomposite offer numerous advantages, encompassing simple preparation, excellent decontamination capabilities, and long-term stability.