Cobalt-doped molybdenum disulfide for efficient sulfite activation to remove As(III): Preparation, efficacy, and mechanisms

Synergy of single atoms and sulfur vacancies for advanced polysulfide-iodide redox flow battery

Aqueous redox flow batteries (RFBs) incorporating polysulfide/iodide chemistries have received considerable attention due to their safety, high scalability, and cost-effectiveness. However, the sluggish redox kinetics restricted their output energy efficiency and power density. Here we designed a defective MoS2 nanosheets supported Co single-atom catalyst that accelerated the transformation of S2-/Sx2- and I-/I3- redox couples, hence endow the derived polysulfide-iodide RFB with an initial energy efficiency (EE) of 87.9% and an overpotential of 113 mV with an average EE 80.4% at 20 mA cm-2 and 50% state-of-charge for 50 cycles, and a maximal power density of 95.7 mW cm-2 for an extended cycling life exceeding 850 cycles at 10 mA cm-2 and 10% state-of-charge. In situ experimental and theoretical analyses elucidate that Co single atoms induce the generation of abundant sulfur vacancies in MoS2 via a phase transition process, which synergistically contributed to the enhanced adsorption of reactants and key reaction intermediates and improved charge transfer, resulting in the enhanced RFB performance.

New cobalt-based metal-organic frameworks for catalysing sulfite in the degradation of reactive brilliant red

Advanced oxidation processes utilizing metal-organic frameworks (MOFs) to enhance the catalytic degradation efficiency of synthetic dyes in wastewater require a thorough understanding of the activation mechanism. Herein, four new MOFs, named [Co(Hchtc)(bpe)] (1), [Co(Hchtc)(bpy)]·H2O (2), [Co1.5(Hcptc)(bpy)0.5(H2O)2]·2H2O (3) and [Co2(cptc)(bpe)2(H2O)]·H2O (4) (H3chtc = cyclohexane-tricarboxylic acid, H4cptc = cyclopentane-tetracarboxylic acid, bpe = trans-1,2-(bis(4-pyridyl)ethene), bpy = 4,4'-bipyridine), were exploited as catalysts in the degradation of X3B by the activation of sulfites. Complexes 1 and 2, the first MOFs constructed with H3chtc, are similar two-dimensional layered structures composed of double organic linkers and dinuclear units in which the coordination numbers of cobalt ions are five and six, respectively; complex 3 is also a layered framework assembled with six-coordinated trinuclear cobalt units and bpy, whereas complex 4 is a three-dimensional framework including both 5- and 6-coordinated cobalt ions. Interestingly, catalysts 1/SO32- and 4/SO32- demonstrate a superior degradation efficiency on X3B, while 2/SO32- and 3/SO32- exhibit moderate degradation efficacy on X3B. Further investigations suggest that both the adsorption capacity of the dye and the coordination vacancy on the metal ions contribute to the degradation process, with the latter exerting an activation effect of sulfite ions.

Transforming crystal structures of cobalt molybdate to generate electron-rich sites for electrochemical detection of Pb(II)

BACKGROUND

Most of the investigations on distinct crystal structures of catalysts are individually focused on the difference of surface functional groups or adsorption properties, but rarely explore the changes of active sites to affect the electrocatalytic performance. Catalysts with diverse crystal structures had been applied to modified electrodes in different electrocatalytic reactions. However, there is currently a lack of an essential understanding for the role of real active sites in catalysts with crystalline structures in electroanalysis, which is crucial for designing highly sensitive sensing interfaces.

RESULTS

Herein, cobalt molybdate with divergent crystal structures (α-CoMoO4 and β-CoMoO4) were synthesized by adjusting the calcination temperature, indicating that α-CoMoO4 (800 °C) (60.00 μA μM-1) had the highest catalytic ability than β-CoMoO4 (700 °C) (38.68 μA μM-1) and α-CoMoO4 (900 °C) (29.55 μA μM-1) for the catalysis of Pb(II). It was proved that the proportion of Co(II) and Mo(IV) as electron-rich sites in α-CoMoO4 (800 °C) were higher than β-CoMoO4 (700 °C) and α-CoMoO4 (900 °C), possessing more electrons to participate in the valence cycles of Co(II)/Co(III) and Mo(IV)/Mo(VI) to boost the catalytic reduction of Pb(II). Specifically, Co(II) transferred a part of electrons to Mo(VI), promoting the formation of Mo(IV). Co(II) and Mo(IV), as the electron-rich sites, providing electrons to Pb(II), further accelerating the conversion of Pb(II) into Pb(0).

SIGNIFICANCE

In the process of detecting Pb(II), the CoMoO4 structures under different temperatures have distinct content of electron-rich sites Co(II) and Mo(IV). α-CoMoO4 (800 °C), with the highest content are benefited to detect Pb(II). This work is conducive to understanding the effect of the changes of active sites resulting from crystal transformation on the electrocatalytic performance, and provides a way to construct sensitive electrochemical interfaces of distinct active sites.

Sulfite induced degradation of sulfamethoxazole by a silica stabilized ZIF-67(Co) catalyst via non-radical pathways: Formation and role of high-valent Co(IV) and singlet oxygen

The sulfite (S(IV))-based advanced oxidation process (AOP) has emerged as an appealing alternative to the traditional persulfate-based AOP for the elimination of organic contaminants from diverse water matrices. In this work, a silica reinforced ZIF-67(Co) catalyst (CZS) is fabricated, characterized and tested in the activation of S(IV) for the sulfamethoxazole (SMX) degradation. The prepared CZS demonstrates superior stability and catalytic ability for the degradation of SMX compared to ZIF-67(Co) across a broad pH range. Unlike the conventional radical-dominated oxidation systems, the CZS/S(IV) system for SMX degradation operates through a non-radical mechanism, featuring high-valent Co(IV) and singlet oxygen (1O2) as the predominated reactive species. The hydroxylated Co species exposed on the CZS surface is identified as the pivotal active site, realizing the S(IV) activation through a complexation-electron transfer process, resulting in the production of various reactive intermediates. Co(II) undergoes the conversion to Co(IV) by generated HSO5-, and 1O2 predominantly originates from the intermediate SO4•-. Profiting from the highly selective oxidation capacities of Co(IV) and 1O2, the established oxidative system demonstrates a remarkable interference resistance and exhibits an exceptional decontamination performance under real-world water conditions. In short, this work provides a sustainable S(IV)-based oxidation strategy for environmental remediation via non-radical mechanism.

Cobalt(II) mediated calcium sulfite activation for efficient oxidative decontamination in waters: Performance, kinetics and mechanism

To overcome the drawback that excess SO32- from soluble Na2SO3 captures the generated reactive intermediates in sulfite (S(IV))-based advanced oxidation processes (AOP), CaSO3 of the ability to slowly release SO32- is selected as an alternative S(IV) source to establish an enduring S(IV)-based AOP with Co(II). Herein, the Co(II)/CaSO3 process triggers a much better ofloxacin (OFL) degradation than the Co(II)/Na2SO3 process (degradation rate constant: 12.1 > 3.18 mM-1 min-1). The mechanism investigation corroborates that the Co(II) mediated CaSO3 activation follows a Fenton-like process (complexation followed by intramolecular electron transfer). Apart from the conventional sulfate radical (SO4•-), Co(IV) species and singlet oxygen (1O2) are also certifiably involved in Co(II)/CaSO3 process, and their role and formation mechanisms are elucidated comprehensively. Further, the proposed Co(II)/CaSO3 process exhibits an excellent tolerance to complex water matrices (e.g., background ions and humic acid), suggesting its practical application potential for various contaminants abatement in actual wastewater.

Effect and mechanism of phosphate enhanced sulfite activation with cobalt ion for effective iohexol abatement

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) from sulfite activation have recently received attention for abatement of microorganic pollutants in the aquatic environments. Trace-level Co(II) has been demonstrated to be effective for promoting sulfite activation (simplified as the Co(II)/sulfite system) and the corresponding radical formation, yet this process is challenged by the limited valence inter-transformation of Co(II)/Co(III). In order to enhance this valence inter-transformation, a novel Co(II)/HPO42-/sulfite system is developed in this work, because HPO42-, as a typical radical scavenging agent, has the advantage of complexing with Co(II) without quenching effect. In this work, complexation of Co(II) with HPO42- can regulate the electronic structure of Co(II), accelerate electron transfer, and promote valence inter-transformation of Co(II)/Co(III) during the sulfite activation process. The Co(II)/HPO42-/sulfite system exhibits superior iohexol abatement performance under circumneutral conditions. For pH 8.0 and Co(II) dose of 1 μM, the iohexol abatement efficiency is as high as 98%, which is considerably higher than that of the Co(II)/sulfite system (50%). SO4•- is identified as the predominant reactive radical contributing to iohexol abatement. The presence of HPO42- broadens the pH adaptability of the Co(II)/sulfite system for iohexol abatement. In addition, the coexisting Cl- exerts an inhibitory effect on iohexol abatement while the other cations and anions show negligible effect. The Co(II)/HPO42-/sulfite system displays good reusability and adaptability towards various organic pollutants. This study highlights the important role of complexation of Co(II) with HPO42- in sulfite activation and provides a feasible idea for abatement of the microorganic pollutants.

Modulating the >Co(II)/Co(III) redox cycling via confinement of cobalt with WS2 for the ultrafast sulfite activation

Sulfite autoxidation in combination with the cobalt-based heterogeneous activators, has recently emerged as the efficient sulfate radical (SO4•-) generation process for organic micropollutant abatement in the water and wastewater treatment, yet the sluggish >Co(II)/Co(III) redox cycling currently compromises the efficacy of radical generation and the potential applications. Herein, regarding that the reductive W(IV) species in WS2 can modulate the >Co(II)/Co(III) redox cycling in the advanced oxidation processes, confinement of cobalt with WS2 (Co-WS2) is designed and characterized. The Co-WS2/sulfite process achieves an ultrafast tetracycline (TC) abatement (~100 % abatement of TC within 1 min) under circumneutral conditions with lower dosage of sulfite and activator, outperforming the current cobalt-based heterogeneous counterparts. The dominant reactive radicals are identified as SO4•- and hydroxyl radical (HO•), which are quantified to be 9.7 μM and 4.5 μM, respectively. The superior radical generation efficiency and the concomitant TC abatement rely on the excellent redox properties and electron transfer capability of Co-WS2. The inter-transformation of >Co(II)/>Co(III) can be accelerated via the involvement of the reductive W(IV) species with the redox-reversibility of the W(IV)/W(VI) couple in the presence of sulfite. The TC degradation intermediates and the corresponding pathways are also proposed according to the ultra-performance liquid chromatography and quadrupole-time of flight mass spectrometry (UPLC-QTOF-MS) analysis. In addition, the influences of the reactant dosage, coexisting anions (HCO3-, HPO42-, Cl- and NO3-), humic acid and the various real water matrices on TC abatement are thoroughly explored. Especially, the Co-WS2/sulfite process is advantageous owing to the negligible effect of the coexisting anions on the TC abatement. This study provides a novel heterogeneous activator for significantly improving sulfite activation efficacy to achieve the efficient organic micropollutant abatement.