Mediated Peroxymonosulfate Activation at the Single Atom Fe-N3O1 Sites: Synergistic Degradation of Antibiotics by Two Non-Radical Pathways.

The activation of persulfates to degrade refractory organic pollutants is a hot issue in advanced oxidation right now. Here, it is reported that single-atom Fe-incorporated carbon nitride (Fe-CN-650) can effectively activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) removal. Through some characterization techniques and DFT calculation, it is proved that Fe single atoms in Fe-CN-650 exist mainly in the form of Fe-N3O1 coordination, and Fe-N3O1 exhibited better affinity for PMS than the traditional Fe-N4 structure. The degradation rate constant of SMX in the Fe-CN-650/PMS system reached 0.472 min-1, and 90.80% of SMX can still be effectively degraded within 10 min after five consecutive recovery cycles. The radical quenching experiment and electrochemical analysis confirm that the pollutants are mainly degraded by two non-radical pathways through 1O2 and Fe(IV)═O induced at the Fe-N3O1 sites. In addition, the intermediate products of SMX degradation in the Fe-CN-650/PMS system show toxicity attenuation or non-toxicity. This study offers valuable insights into the design of carbon-based single-atom catalysts and provides a potential remediation technology for the optimum activation of PMS to disintegrate organic pollutants.

Microwave-Assisted Synthesis of Highly Active Single-Atom Fe/N/C Catalysts for High-Performance Aqueous and Flexible All-Solid-State Zn-Air Batteries.

The development of low-cost single-atom electrocatalysts for oxygen reduction reaction (ORR) is highly desired but remains a grand challenge. Superior to the conventional techniques, a microwave-assisted strategy is reported for rapid production of high-quality Fe/N/C single-atom catalysts (SACs) with profoundly enhanced reaction rate and remarkably reduced energy consumption. The as-synthesized catalysts exhibit an excellent ORR performance with a positive half-wave potential up to 0.90 V, a high turnover frequency of 0.76 s-1 , as well as a satisfied stability with a lost half-wave potential of just 27 mV over 9000 cycles (much better than that of Pt/C with 107 mV lost) and good methanol resistance. The open-circuit voltages of as-constructed aqueous and flexible all-solid-state Zn-air batteries (ZABs) are 1.56 and 1.52 V, respectively, higher than those of 20% Pt/C-based ones (i.e., 1.43 and 1.38 V, respectively). Impressively, they afford a peak power density of 235 mW cm-2 , which exceeds that of Pt/C (i.e., 186 mW cm-2 ), and is comparable to the best ones of Fe/N/C-based ZABs ever reported.

Solvent environment engineering to synthesize FeNC nanocubes with densely Fe-Nx sites as oxygen reduction catalysts for Zn-air battery.

Zeolitic imidazole framework (ZIF)-derived iron-nitrogen-carbon (FeNC) materials are expected to be high-efficiency catalysts for oxygen reduction reaction (ORR). However, increasing the density of active sites while avoiding metal accumulation still faces significant challenges. Herein, solvent environment engineering is used to synthesize the FeNC containing dense Fe-Nx moieties by adjusting the solvent during the ZIF precursor synthesis process. Compared with methanol and water/methanol, the aqueous media can provide a more moderate Fe content for the ZIF precursor, which facilitates the construction of high-density Fe-Nx sites and prevent the appearance of iron-based nanoparticles during pyrolysis. Therefore, the FeNC(C) nanocubes synthesized in an aqueous media have the highest single atom Fe loading (0.6 at%) among the prepared samples, which presents excellent oxygen reduction properties and durability under alkaline and acidic conditions. The advantage of FeNC(C) is proven in Zn-air batteries, with outstanding performance and long-term stability.

Defect-Rich Single Atom Catalyst Enhanced Polysulfide Conversion Kinetics to Upgrade Performance of Li-S Batteries.

Lithium-sulfur (Li-S) batteries have attracted considerable attention owing to their extremely high energy densities. However, the application of Li-S batteries has been limited by low sulfur utilization, poor cycle stability, and low rate capability. Accelerating the rapid transformation of polysulfides is an effective approach for addressing these obstacles. In this study, a defect-rich single-atom catalytic material (Fe-N4/DCS) is designed. The abundantly defective environment is favorable for the uniform dispersion and stable existence of single-atom Fe, which not only improves the utilization of single-atom Fe but also efficiently adsorbs polysulfides and catalyzes the rapid transformation of polysulfides. To fully exploit the catalytic activity, catalytic materials are used to modify the routine separator (Fe-N4 /DCS/PP). Density functional theory and in situ Raman spectroscopy are used to demonstrate that Fe-N4 /DCS can effectively inhibit the shuttling of polysulfides and accelerate the redox reaction. Consequently, the Li-S battery with the modified separator achieves an ultralong cycle life (a capacity decay rate of only 0.03% per cycle at a current of 2 C after 800 cycles), and an excellent rate capability (894 mAh g-1 at 3 C). Even at a high sulfur loading of 5.51 mg cm-2 at 0.2 C, the reversible areal capacity still reaches 5.4 mAh cm-2 .

A novel epoxy coating with nanocatalytic anticorrosion performance achieved by single-atom Fe-N-C catalyst.

In view of the critical importance of oxygen to corrosion evolution, to starve corrosion via depleting oxygen in coatings is a promising strategy. In this work, a novel nanocatalytic anticorrosion concept is proposed to design new coating with outstanding corrosion resistance. Different from the passive barrier of traditional coatings and self-repair after corrosion of current stimuli-feedback coatings, such coating could spontaneously eliminate internal diffused oxygen and greatly suppress the corrosion process. As a proof of concept, single-atom Fe-N-C electrocatalyst with isolated FeN4 active sites is synthesized by a simple confined carbonization method, exhibiting excellent oxygen reduction performance (E1/2 = 0.902 V). In composite coating, the evenly dispersed Fe-N-C compensates for the coating defects and serves as oxygen scavengers, which could actively adsorb and consume ambient oxygen, thereby preventing oxygen penetration to the metal substrate surface, eliminating the oxygen contribution to corrosion and significantly boosting the anticorrosion performance of epoxy coating. This in-situ mediation for oxygen in coating prevents metal substrate from receiving new supply of oxygen, while imparting active anticorrosion property to the coating.

Enhanced Dual-Directional Sulfur Redox via a Biotemplated Single-Atomic Fe-N2 Mediator Promises Durable Li-S Batteries.

The lithium-sulfur (Li-S) battery is considered as an appealing candidate for next-generation electrochemical energy storage systems because of high energy and low cost. Nonetheless, its development is plagued by the severe polysulfide shuttling and sluggish reaction kinetics. Although single-atom catalysts (SACs) have emerged as a promising remedy to expedite sulfur redox chemistry, the mediocre single-atom loading, inferior atomic utilization, and elusive catalytic pathway handicap their practical application. To tackle these concerns, in this work, unsaturated Fe single atoms with high loading capacity (≈6.32 wt%) are crafted on a 3D hierarchical C3 N4 architecture (3DFeSA-CN) by means of biotemplated synthesis. By electrokinetic analysis and theoretical calculations, it is uncovered that the 3DFeSA-CN harnesses robust electrocatalytic activity to boost dual-directional sulfur redox. As a result, S@3DFeSA-CN can maintain a durable cyclic performance with a negligible capacity decay rate of 0.031% per cycle over 2000 cycles at 1.0 C. More encouragingly, an assembled Li-S battery with a sulfur loading of 5.75 mg cm-2 can harvest a high areal capacity of 6.18 mAh cm-2 . This work offers a promising solution to optimize the carbonaceous support and coordination environment of SACs, thereby ultimately elevating dual-directional sulfur redox in pragmatic Li-S batteries.

A hierarchically ordered porous Fe, N, S tri-doped carbon electrocatalyst with densely accessible Fe-Nx active sites and uniform sulfur-doping for efficient oxygen reduction reaction.

Doping FeNC catalysts with heteroatoms has been widely recognized as a most promising way to improve the electrocatalytic activity to achieve the replacement of Pt-based electrocatalysts for efficient oxygen reduction reaction (ORR). However, it is still a challenge to combine delicate control of the catalyst structure with heteroatom-doping to achieve optimal electrocatalytic efficiency. Herein, a strategy of combining chemical vapor deposition (CVD) with dual-template cooperative pyrolysis was rationally designed to synthesize the novel electrocatalyst with densely isolated single Fe atoms scattered on hierarchically ordered porous N and S codoped carbon (ISA-Fe/HOPNSC). The three-dimensional hierarchically ordered interconnected macro/mesoporous structure and uniform sulfur-doping make ISA-Fe/HOPNSC exhibit superb ORR performance in both alkaline and acidic electrolytes with a positive half-wave potential (0.920 V in 0.1 M KOH and 0.795 V in 0.5 M H2SO4) as well as outstanding methanol tolerance and long-time stability. As cathode catalyst in Zn-air battery, ISA-Fe/HOPNSC also provides much better maximum power density (210.7 mW cm-2) and specific capacity (796.7 mAh gZn-1) than those of Pt/C (112.6 mW cm-2 and 712.6 mAh gZn-1). Remarkably, the synthesis strategy in this work proposes myriad opportunities for combining delicate control of the catalyst structure with heteroatom doping to synthesize high-performance ORR catalysts.

Iron-Based Dual Active Site-Mediated Peroxymonosulfate Activation for the Degradation of Emerging Organic Pollutants.

It is still a challenge to synthesize highly efficient and stable catalysts for the Fenton-like reaction. In this study, we constructed an integrated catalyst with highly dispersed iron-based dual active sites, in which Fe2N and single-atom Fe (SA-Fe) were embedded into nitrogen- and oxygen-co-doped graphitic carbon (Fe-N-O-GC-350). Extended X-ray absorption fine structure (EXAFS) confirmed the coordination structure of iron, and line combination fitting (LCF) demonstrated the coexistence of Fe2N and SA-Fe with percentages of 75 and 25%, respectively. Iron-based dual active sites endowed Fe-N-O-GC-350 with superior catalytic activity to activate peroxymonosulfate (PMS) as evidenced by the fast degradation rate of sulfamethoxazole (SMX) (0.24 min-1) in the presence of 0.4 mM PMS and 0.1 g/L Fe-N-O-GC-350. Unlike the reported singlet oxygen and high-valent iron oxo-mediated degradation induced by the SA-Fe catalyst, both surface-bound reactive species and singlet oxygen contributed to SMX degradation, while surface-bound reactive species dominated. Density functional theory (DFT) simulation indicated that Fe2N and SA-Fe enhanced the adsorption of PMS, which played a key role in PMS activation. The Fe-N-O-GC-350/PMS system had resistance to the interference of common inorganic anions and high oxidation capacity to recalcitrant organic contaminants. This study elucidated the important role of Fe2N in PMS activation and provide a clue to design rationally catalysts with iron-based dual active sites to activate PMS for the degradation of emerging organic pollutants.

Simultaneously Engineering the Coordination Environment and Pore Architecture of Metal-Organic Framework-Derived Single-Atomic Iron Catalysts for Ultraefficient Oxygen Reduction.

Designing highly efficient and durable electrocatalysts that accelerate sluggish oxygen reduction reaction kinetics for fuel cells and metal-air batteries are highly desirable but challenging. Herein, a facile yet robust strategy is reported to rationally design single iron active centers synergized with local S atoms in metal-organic frameworks derived from hierarchically porous carbon nanorods (Fe/N,S-HC). The cooperative trithiocyanuric acid-based coating not only introduces S atoms that regulate the coordination environment of the active centers, but also facilitates the formation of a hierarchically porous structure. Benefiting from electronic modulation and architectural functionality, Fe/N,S-HC catalyst shows markedly enhanced ORR performance with a half-wave potential (E1/2 ) of 0.912 V and satisfactory long-term durability in alkaline medium, outperforming those of commercial Pt/C. Impressively, Fe/N,S-HC-based Zn-air battery also presents outstanding battery performance and long-term stability. Both electrochemical experimental and density functional theoretical (DFT) calculated results suggest that the FeN4 sites tailored with local S atoms are favorable for the adsorption/desorption of oxygen intermediate, resulting in lower activation energy barrier and ultraefficient oxygen reduction catalytic activity. This work provides an atomic-level combined with porous morphological-level insights into oxygen reduction catalytic property, promoting rational design and development of novel highly efficient single-atom catalysts for the renewable energy applications.

Engineering Fe-N Coordination Structures for Fast Redox Conversion in Lithium-Sulfur Batteries.

Critical drawbacks, including sluggish redox kinetics and undesirable shuttling of polysulfides (Li2 Sn , n = 4-8), seriously deteriorate the electrochemical performance of high-energy-density lithium-sulfur (Li-S) batteries. Herein, these challenges are addressed by constructing an integrated catalyst with dual active sites, where single-atom (SA)-Fe and polar Fe2 N are co-embedded in nitrogen-doped graphene (SA-Fe/Fe2 N@NG). The SA-Fe, with plane-symmetric Fe-4N coordination, and Fe2 N, with triangular pyramidal Fe-3N coordination, in this well-designed configuration exhibit synergistic adsorption of polysulfides and catalytic selectivity for Li2 Sn lithiation and Li2 S delithiation, respectively. These characteristics endow the SA-Fe/Fe2 N@NG-modified separator with an optimal polysulfides confinement-catalysis ability, thus accelerating the bidirectional liquid-solid conversion (Li2 Sn ↔Li2 S) and suppressing the shuttle effect. Consequently, a Li-S battery based on the SA-Fe/Fe2 N@NG separator achieves a high capacity retention of 84.1% over 500 cycles at 1 C (pure S cathode, S content: 70 wt%) and a high areal capacity of 5.02 mAh cm-2 at 0.1 C (SA-Fe/Fe2 N@NG-supported S cathode, S loading = 5 mg cm-2 ). It is expected that the outcomes of the present study will facilitate the design of high-efficiency catalysts for long-lasting Li-S batteries.

Atomically Dispersed Iron Metal Site in a Porphyrin-Based Metal-Organic Framework for Photocatalytic Nitrogen Fixation.

The rational design of photocatalysts for efficient nitrogen (N2) fixation at ambient conditions is important for revolutionizing ammonia production and quite challenging because the great difficulty lies in the adsorption and activation of the inert N2. Inspired by a biological molecule, chlorophyll, featuring a porphyrin structure as the photosensitizer and enzyme nitrogenase featuring an iron (Fe) atom as a favorable binding site for N2via π-backbonding, here we developed a porphyrin-based metal-organic framework (PMOF) with Fe as the active center as an artificial photocatalyst for N2 reduction reaction (NRR) under ambient conditions. The PMOF features aluminum (Al) as metal node imparting high stability and Fe incorporated and atomically dispersed by residing at each porphyrin ring promoting the adsorption and the activation of N2, termed Al-PMOF(Fe). Compared with the pristine Al-PMOF, Al-PMOF(Fe) exhibits a substantial enhancement in NH3 yield (635 µg g-1cat.) and production rate (127 µg h-1 g-1cat.) of 82% and 50%, respectively, on par with the best-performing MOF-based NRR catalysts. Three cycles of photocatalytic NRR experimental results corroborate a stable photocatalytic activity of Al-PMOF(Fe). The combined experimental and theoretical results reveal that the Fe-N site in Al-PMOF(Fe) is the active photocatalytic center that can mitigate the difficulty of the rate-determining step in photocatalytic NRR. The possible reaction pathways of NRR on Al-PMOF(Fe) were established. Our study of porphyrin-based MOF for the photocatalytic NRR will provide insight into the rational design of catalysts for artificial photosynthesis.

Single-atom Fe catalytic amplification-gold nanosol SERS/RRS aptamer as platform for the quantification of trace pollutants.

Bisphenol A (BPA), as a typical endocrine disruptor, poses a serious threat to human health. Therefore, it is urgent to establish a rapid, sensitive, and simple method for the determination of BPA. In this paper, based on the aptamer-mediated single-atom Fe carbon dot catalyst (SAFe) catalyzing the HAuCl4-ethylene glycol (EG) nanoreaction, a new SERS/RRS di-mode detection method for BPA was established. The results show that SAFe exhibits a strong catalytic effect on the HAuCl4-EG nanoreaction, which could generate purple gold nanoparticles (AuNPs) with resonance Rayleigh scattering (RRS) signals and surface-enhanced Raman scattering (SERS) effects. After the addition of BPA aptamer (Apt), it could encapsulate SAFe through intermolecular interaction, thus inhibiting its catalytic action, resulting in the reduction of AuNPs generated and the decrease of RRS and SERS signals of the system. With the addition of BPA, Apt was specifically combined with BPA, and SAFe was re-released to restore the catalytic ability; the generated AuNPs increased. As a result of this RRS and SERS signals of the system recovered, and their increment was linear with the concentration of BPA. Thus, the quantification of 0.1-4.0 nM (RRS) and 0.1-12.0 nM (SERS) BPA was realized, and the detection limits were 0.08 nM and 0.03 nM, respectively. At the same time, we used molecular spectroscopy and electron microscopy to study the SAFe-HAuCl4-ethylene glycol indicator reaction, and proposed a reasonable SAFe catalytic reaction mechanism. Based on Apt-mediated SAFe catalysis gold nanoreaction amplification, a SERS/RRS di-mode analytical platform was established for targets such as BPA.

Surface-confinement assisted synthesis of nitrogen-rich single atom Fe-N/C electrocatalyst with dual nitrogen sources for enhanced oxygen reduction reaction.

The utilization of earth abundant iron and nitrogen doped carbon as a precious-metal-free electrocatalyst for oxygen reduction reaction (ORR) significantly depends on the rational design and construction of desired Fe-Nxmoieties on carbon substrates, which however remains an enormous challenge. Herein a typical nanoporous nitrogen-rich single atom Fe-N/C electrocatalyst on carbon nanotube (NR-CNT@FeN-PC) was successfully prepared by using CNT as carbon substrate, polyaniline (PANI) and dicyandiamine (DCD) as binary nitrogen sources and silica-confinement-assisted pyrolysis, which not only facilitate rich N-doping for the inhibition of the Fe agglomeration and the formation of single atom Fe-Nxsites in carbon matrix, but also generate more micropores for enlarging BET specific surface area (up to 1500 m2·g-1). Benefiting from the advanced composition, nanoporous structure and surface hydrophilicity to guarantee the sufficient accessible active sites for ORR, the NR-CNT@FeN-PC catalyst under optimized conditions delivers prominent ORR performance with a half-wave potential (0.88 V versus RHE) surpass commercial Pt/C catalyst by 20 mV in alkaline electrolyte. When assembled in a home-made Zn-air battery device as cathodic catalyst, it achieved a maximum output power density of 246 mW·cm-2and a specific capacity of 719 mA·h·g-1Znoutperformed commercial Pt/C catalyst, holding encouraging promise for the application in metal-air batteries.

Single-Atom Fe-Anchored Nano-Diamond With Enhanced Dual-Enzyme Mimicking Performance for H2O2 and Glutathione Detection.

Glutathione (GSH) is an important antioxidant and free radical scavenger that converts harmful toxins into harmless substances and excretes them out of the body. In the present study, we successfully prepared single-atom iron oxide-nanoparticle (Fe-NP)-modified nanodiamonds (NDs) named Fe-NDs via a one-pot in situ reduction method. This nanozyme functionally mimics two major enzymes, namely, peroxidase and oxidase. Accordingly, a colorimetric sensing platform was designed to detect hydrogen peroxide (H2O2) and GSH. Owing to their peroxidase-like activity, Fe-NDs can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue with sufficient linearity at H2O2 concentrations of 1-60 µM and with a detection limit of 0.3 µM. Furthermore, using different concentrations of GSH, oxidized TMB can be reduced to TMB, and the color change from blue to nearly colorless can be observed by the naked eye (linear range, 1-25 µM; detection limit, 0.072 µM). The established colorimetric method based on oxidase-like activity can be successfully used to detect reduced GSH in tablets and injections with good selectivity and high sensitivity. The results of this study exhibited reliable consistency with the detection results obtained using high-performance liquid chromatography (HPLC). Therefore, the Fe-NDs colorimetric sensor designed in this study offers adequate accuracy and sensitivity.

Activation of peroxymonosulfate by iron-biochar composites: Comparison of nanoscale Fe with single-atom Fe.

A convenient and efficient method to fabricate isolated Fe single-atom catalysts deposited on Myriophyllum aquaticum-based biochar (ISA-Fe/MC) is reported for peroxymonosulfate-based organics degradation. Firstly, the Fe nanoparticles anchored on the hierarchical porous biochar (nano-Fe/MC) can be obtained by utilizing K2FeO4 as a synchronous activation and graphitization agent. Subsequently, ISA-Fe/MC was achieved by HCl etching of nano-Fe/MC to remove the excess Fe nanoparticles. Compared with nano-Fe/MC, ISA-Fe/MC demonstrated outperformed catalytic capacity towards PMS activation for phenol degradation. The combination of super high surface area, hierarchical porous structure, graphitization structure and atomically dispersed Fe species should be responsible for prominent catalytic oxidation ability and outstanding resistance to common anions and humic acid. Based on the chemical scavengers, EPR experiments and electrochemistry tests, the SO4•- dominated radical degradation pathway for nano-Fe/MC and electron transfer reigned non-radical degradation pathway for ISA-Fe/MC was revealed. In contrast to nano-Fe/MC, density functional theory calculations demonstrated the enhanced density of states around Fermi level in ISA-Fe/MC meaning the increased catalytic performance and more electron transfer between single-atom Fe to adjacent graphitic C and N which could serve as electron transfer channel for PMS activation.