Interface Stabilization of Undercoordinated Iron Centers on Manganese Oxides for Nature-Inspired Peroxide Activation

Triggering of Low-Valence Molybdenum in Multiphasic MoS2 for Effective Reactive Oxygen Species Output in Catalytic Fenton-like Reactions

Utilization of photocatalytic reactions to trigger persistent large-scale reactions could be an alternative path for practical solar energy conversion to relieve environmental pressure nowadays. We took the view that the photoinduction of transition states was critical for improving the activity of catalytic reactions. On the basis of theoretical predictions, the reaction Gibbs free energy of permonosulfate (PMS) activation can be rapidly reduced by molybdenum with low valence. We therefore constructed a multiphasic molybdenum dichalcogenide (MoS₂) heterostructure-based photosystem that enabled generation of Mo transition states by visible light excitation. According to combination results of electron paramagnetic resonance, photoelectrochemical analysis, and X-ray photoelectron spectroscopy, we confirmed that the optimized 2H/1T heterojunction permitted the transport of excited interfacial electrons from the semiconductive 2H phase to the metallic 1T phase, and synchronously partially reduced Mo(IV) to Mo(III) at the interface. This intensified the charge transfer between the MoS₂ and PMS-containing solution, thereby efficiently splitting the PMS molecules into ^•OH and SO₄^•- radicals. In this system, a type of refractory herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), can be degraded within 60 min at a rate constant of 6.20 × 10^-2 min^-1 using multiphasic MoS₂ with a 1T/2H ratio of 1:1.

Confining Free Radicals in Close Vicinity to Contaminants Enables Ultrafast Fenton-like Processes in the Interspacing of MoS2 Membranes

Heterogenous Fenton-like reactions are frequently proposed for treating persistent pollutants through the generation of reactive radicals. Despite great efforts to optimize catalyst activity, their broad application in practical settings has been restricted by the low efficiency of hydrogen peroxide or persulfate decomposition as well as ultrafast self-quenching of the activated radicals. Theoretical calculations predicted that two-dimensional (2D) metallic 1T phase MoS₂ materials with exposed (001) surfaces and (100) edges should have remarkable affinity towards crucial intermediates in the peroxymonosulfate (PMS) activation process. X-ray photoelectron spectroscopy and in situ Raman spectroscopy were used to show that the exposed metallic Mo sites accelerate the rate-limiting step of electron transfer. A lamellar membrane made from a stack of 2D MoS₂ with tunable interspacing was then designed as the catalyst. The non-linear transport between the MoS₂ nanolayers leads to high water diffusivity so that the short-lived reactive radicals efficiently oxidize contaminants.

Intercalation of Nanosized Fe3C in Iron/Carbon To Construct Multifunctional Interface with Reduction, Catalysis, Corrosion Resistance, and Immobilization Capabilities

As a robust reducing system in industrial wastewater treatment, iron/carbon (Fe/C) microelectrolysis suffers from surface passivation and low utilization efficiency. Herein, we introduced Fe₃C into the Fe/C system to develop a core-shell Fe⁰/Fe₃C/C nanorod with a multifunctional interface (Fe₃C/C) providing reduction, catalysis, adsorption, and corrosion resistance. The results proved that the fabricated Fe⁰/Fe₃C/C possesses 5.6 times higher reduction capacity (220 mg/g) for Cr(VI) reduction but a relatively lower Fe leakage (2.7 mg/L) than Fe/C. On the basis of the results of electrochemical characterization (Tafel polarization curves and electrochemical impedance spectroscopy), the corrosion-resistant Fe₃C/C shell can significantly prevent surface passivation of the Fe⁰ core, whereas Fe₃C efficiently catalyzes electron transfer from the inner Fe⁰ to the external carbon surface. Moreover, the reductive species involved in Cr(VI) removal were identified as hydrogen atoms, adsorbed Fe(II) ions, and electrons tunneling from Fe⁰. STEM, XPS, and Mössbauer spectroscopies were further adopted to characterize the interface reaction of Fe⁰/Fe₃C/C during the Cr(VI) removal process. Finally, the reaction mechanism for Cr(VI) reduction over Fe⁰/Fe₃C/C was proposed, and the distribution of active sites was inferred.

The Regular/Persistent Free Radicals and Associated Reaction Mechanism for the Degradation of 1,2,4-Trichlorobenzene over Different MnO2 Polymorphs

The role of regular/persistent free radicals on the catalytic activity of K⁺-tuned MnO₂ tunnel structures is poorly understood to date. Herein, three MnO₂ polymorphs (α-, β-, and δ-MnO₂) were synthesized and examined toward the degradation of 1,2,4-trichlorobenzene (1,2,4-TrCBz) at 300 °C. δ-MnO₂, with a two-dimensional-layered tunnel structure tuned by K⁺, exhibited the highest activity among the three MnO₂ polymorphs. The electron spin resonance spectroscopy results confirmed that δ-MnO₂ featured the most abundant reactive oxygen species (ROS: O₂^-•, •OH, and ¹O₂), followed by α-MnO₂ (O₂^-• and ¹O₂), and β-MnO₂ (O₂^-•), being supported by the calculated energy barrier. It was, intriguingly, noted that persistent organic free radicals, newly recognized as emerging surface-stabilized compound, were remarkably detected in α- and β-MnO₂/1,2,4-TrCBz systems but not in more reactive δ-MnO₂/1,2,4-TrCBz system. These might contribute to discrepant oxidative degradation process. During the oxidative process, intermediates, including benzoic acid and glycerol, formed via attack by ROS. Upon further attack, these intermediates fragmented into smaller molecules such as formic, acetic, propionic, and butyric acids. The present findings give deeper insights into the role of free radicals on the catalytic degradation of chlorinated aromatics.

Harnessing the Wisdom in Colloidal Chemistry to Make Stable Single-Atom Catalysts

Research on single-atom catalysts (SACs), or atomically dispersed catalysts, has been quickly gaining momentum over the past few years. Although the unique electronic structure of singly dispersed atoms enables uncommon-sometimes exceptional-activities and selectivities for various catalytic applications, developing reliable and general procedures for preparing stable, active SACs in particular for applications under reductive conditions remains a major issue. Herein, the challenges associated with the synthesis of SACs are highlighted semiquantitatively and three stabilization techniques inspired by colloidal science including steric, ligand, and electrostatic stabilization are proposed. Some recent examples are discussed in detail to showcase the power of these strategies in the synthesis of stable SACs without compromising catalytic activity. The substantial further potential of steric, ligand, and electrostatic effects for developing SACs is emphasized. A perspective is given to point out opportunities and remaining obstacles, with special attention given to electrostatic stabilization where little is done so far. The stabilization strategies presented herein have a wide applicability in the synthesis of a series of new SACs with improved performances.

Site-specific surface tailoring for metal ion selectivity via under-coordinated structure engineering

Coordinatively unsaturated atoms play an important role in structural and electronic tuning, while their effects on surface site tailoring for selective adsorption have not been well explored. We demonstrate a new concept based on elaborate tuning of the chemical states of lattice atoms to control the accessibility of different surface sites. The under-coordinated manganese structure developed herein not only benefits the formation of hydroxylated lateral edge sites for preferential complexation of lead (Pb) ions, but also favors a decrease in the proportions of metal-ion-nonspecific octahedral vacancy sites. On the basis of this strategy, a common core-shell structure was devised to further assemble highly exposed edge sites, achieving high selectivity coefficients (31.2-172.0) for Pb(ii) against various metal cations. Even when these metal cations coexist at higher concentrations, the general specificity is still maintained, with further pH-enabled switchable sorption-desorption for adsorbent recyclability. Different from frequently reported specific ligand-induced selective systems, intrinsic structure modification as described herein will lead to a new paradigm for surface site tailoring that enables a versatile and tunable platform in environmental remediation, resource recovery and analyte sensing.

Biocompatible FeOOH-Carbon quantum dots nanocomposites for gaseous NOx removal under visible light: Improved charge separation and High selectivity

Development of biocompatible photocatalysts with improved charge separation and high selectivity is essential for effective removal of air pollutants. Iron-containing catalysts have attracted extensive attention due to their low-toxicity and high natural abundance. Here, carbon quantum dots (CQDs) modified FeOOH nanocomposites fabricated using a facile hydrothermal route showed enhanced NO removal efficiency (22%) compared to pure FeOOH. Moreover, generation of toxic NO₂ intermediates was significantly inhibited using the nanocomposites, demonstrating high selectivity for final nitrate formation. Photo-electrochemical results showed that both charge separation and transfer efficiency were significantly improved by CQDs addition, and the lifetime of photo-generated carriers was increased eventually. Density functional theory calculations further elucidated that the suppressed recombination of photo-induced electron-hole pairs was due to enhanced electron migration from the FeOOH to CQDs. A NO degradation mechanism was proposed based on detection of the reactive oxygen species using electron paramagnetic spectroscopy. In addition, the nanocomposite showed good biocompatibility and low cytotoxity, ensuring minimal environmental impact for potential application in large-scale.

Controlled growth of γ-MnO2 nanoflakes on OMS-2 for efficient decomposition of organic dyes in aqueous solution via peroxymonosulfate activation

The development of green and efficient catalysts for peroxymonosulfate (PMS) activation and abatement of organic pollutants in wastewater is of significant practical interest. In this paper, the three-dimensional mixed manganese oxides of OMS-2 and γ-MnO₂ were fabricated through a simple refluxing method from KMnO₄ and MnSO₄. It was found that growth of γ-MnO₂ nanoflakes on OMS-2 can be controlled by the concentration of MnSO₄. The catalysts not only have many excellent structural properties such as interconnected network and highly exposed active sites, but also show the high ratio of low valent manganese species. In particular, the catalysts exhibited much higher efficiency for Acid Orange 7 degradation in the presence of PMS than pure OMS-2 or γ-MnO₂. The oxidation of Mn(III) species by PMS occurs in the system with the formation of sulfate and hydroxyl radicals contributed to the dye degradation. Moreover, the catalysts showed good stability and reusability during four consecutive cycles. Thus, the environmental friendly mixed manganese oxides of γ-MnO₂ and OMS-2 with low cost, facile synthesis process and high efficiency are very promising catalysts for PMS activation and pollutants degradation.