Selective activation of peroxymonosulfate to singlet oxygen by engineering oxygen vacancy defects in Ti3CNTx MXene for effective removal of micropollutants in water

Nitrogen-doped 2D MXene-based catalysts: Synthesis, properties and applications for electrochemical hydrogen production

Designing advanced materials with a trade-off between overall electrocatalytic efficiency and economic cost for electrochemical hydrogen production is crucial to overcoming the current energy crisis and environmental issues. On the more 10-year journey since the discovery, transition-metal carbides/nitrides nanosheets (MXenes) have increasingly attracted attention as potential materials toward hydrogen/oxygen evolution reactions (HER/OER) because of their unique physical and chemical characteristics, but the layered restacking and low intrinsic electrochemical activity are dragging them out water-splitting technology. Doping MXenes with nitrogen atoms has recently been introduced as a facile but efficient strategy to accelerate the HER/OER efficiency by the optimization of electronic structure, surface terminations, and adsorption/desorption energies of intermediates on pristine MXenes. However, a comprehensive evaluation of the doping mechanism and content-structure-performance relationship of N-doped 2D MXene-related catalysts is still lacking. Thus, we herein systematically summarize synthetic strategies, theoretical calculations, properties, and applications of nitrogen-doped 2D MXenes for the HER and OER to give more fundamental insights into physicochemical characteristics of nitrogen-doped 2D MXenes to further design next-generation catalysts for the electrochemical hydrogen production and other applications.

Efficient activation of peroxymonosulfate by Mo2TiC2Tx@Co for sustained emerging micropollutant removal: Mo vacancy-mediated activation in Fenton-like reactions

Developing advanced heterogeneous catalysts through structural modifications effectively enhances the catalytic activity of non-homogeneous catalysts for removing emerging micropollutants (EMPs). In this study, Mo2TiC2Tx@Co with Mo vacancies was synthesized using the Lewis molten salt method, which efficiently activates peroxymonosulfate (PMS) and continuously degrades EMPs in water. The abundant Mo vacancy structure in the material acts as an anchoring site for Co nanoparticles and a co-catalytic site for Fenton-like reactions, enabling PMS adsorption and activation. Furthermore, Mo facilitates the redox cycling of Co3+/Co2+ through electron transfer. Mo vacancy-mediated activation in Fenton-like reactions enabled the Mo2TiC2Tx@Co/PMS system to achieve superior degradation efficiency for sulfamethoxazole (SMX) and several other EMPs, with the SMX degradation rate being 52.7 times higher than that of the Mo2TiAlC2/PMS system. The system exhibited robust resistance to various anionic species and maintained high activity over a wide pH range. The Mo2TiC2Tx@Co /PMS system degrades EMPs in water through both free radical (SO4•- and •OH) and non-radical (1O2) mechanisms, enhancing EMPs removal from complex water environments. This study aims to develop an efficient and sustainable heterogeneous catalyst, offering a viable solution for the long-term and effective degradation of EMPs in water.

A review on reactive oxygen species-induced mechanism pathways of pharmaceutical waste degradation: Acetaminophen as a drug waste model

Innately designed to induce physiological changes, pharmaceuticals are foreknowingly hazardous to the ecosystem. Advanced oxidation processes (AOPs) are recognized as a set of contemporary and highly efficient methods being used as a contrivance for the removal of pharmaceutical residues. Since reactive oxygen species (ROS) are formed in these processes to interact and contribute directly toward the oxidation of target contaminant(s), a profound insight regarding the mechanisms of ROS leading to the degradation of pharmaceuticals is fundamentally significant. The conceptualization of some specific reaction mechanisms allows the design of an effective and safe degradation process that can empirically reduce the environmental impact of the micropollutants. This review mainly deliberates the mechanistic reaction pathways for ROS-mediated degradation of pharmaceuticals often leading to complete mineralization, with a focus on acetaminophen as a drug waste model.

Atomically dispersed cobalt activator with nitrogen and sulfur co-coordination for high-efficiency Fenton-like catalysis: Insights into density-dependent activity and mechanisms

Atomically dispersed metal activators (ADMAs) have demonstrated unique advantages in environmental remediation, but how to controllably regulate the active site density and electronic structure of ADMAs to further enhance activation efficiency remains challenging. Here, we introduce a sulfur-atom-doping approach that allows the fine-tuning of atomic Co site content and electronic structure, enabling exploration of density-dependent activation performance of ADMAs for peroxymonosulfate (PMS)-based Fenton-like catalysis. Our investigation reveals a direct correlation between activation capacity and single-Co-site density. The optimal SNC@CoSA-0.05 activator with densely populated Co-N3S1 sites (10.1 wt%) displays exceptional efficacy in eliminating Rhodamine B, with specific activity of 31.0 min-1 g-1 L, outperforming most previously published activators. Moreover, SNC@CoSA-0.05 showed a remarkedly reduced metal leaching (47.4 μg L-1) than its nanocluster counterpart (194 μg L-1) at pH 3.2. Experimental and theoretical analyses unveiled that coordinated sulfur actively modulates the electronic structure of the central Co atom, enhancing the adsorption and activation of PMS, thereby improving decontamination efficiency. Mechanistic studies further elucidate the predominant electron-transfer regime involved in oxidizing micropollutants by SNC@CoSA-0.05/PMS, with Co(IV)=O, •OH, and SO4•- being the auxiliary oxidizing species. This study not only offers a method for concurrent adjustment of active site density and electronic structure in ADMAs but also sheds light on the activation mechanisms of atomic metal sites.

Low-temperature molten-salt synthesis of Co3O4 nanoparticles grown on MXene can rapidly remove ornidazole via peroxymonosulfate activation

We further developed previous work on MXene materials prepared using molten salt methodology. We substituted single, with mixed salts, and reduced the melting point from >724 °C to <360 °C. Cobalt (Co) compounds were simultaneously etched and doped while the MXene material was created using various techniques in which Co compounds occur as Co3O4. The synthesized Co3O4/MXene compound was used as a peroxymonosulfate (PMS) activator that would generate free radicals to degrade antibiotic ornidazole (ONZ). Under optimal conditions, almost 100% of ONZ (30 mg/L) was degraded within 10 min. The Co3O4/MXene + PMS system efficiently degraded ONZ in natural water bodies, and had a broad pH adaptation range (4-11), and strong anion anti-interference. We investigated how the four active substances were generated using radical quenching and electron paramagnetic resonance (EPR) spectroscopy. We identified 12 ONZ intermediates by liquid chromatography-mass spectrometry and propose a plausible degradative mechanism.

Mo Vacancy-Mediated Activation of Peroxymonosulfate for Ultrafast Micropollutant Removal Using an Electrified MXene Filter Functionalized with Fe Single Atoms

Developing advanced heterogeneous catalysts with atomically dispersed active sites is an efficient strategy to boost the kinetics of peroxymonosulfate (PMS) activation for micropollutant removal. Here, we report a binary Mo₂TiC₂T_x MXene-based electroactive filter system with abundant surface Mo vacancies for effective activation of PMS. The Mo vacancies assumed two essential roles: (i) as anchoring sites for Fe single atoms (Fe-SA) and (ii) as cocatalytic sites for the Fenton-like reaction. Fe-SA formed strong metal-oxygen bonds with the Mo₂TiC₂T_x support, stabilizing at the sites previously occupied by Mo. The resulting Fe-SA/Mo₂TiC₂T_x nanohybrid filter achieved 100% degradation of sulfamethoxazole (SMX) in the single-pass mode (hydraulic retention time <2 s) when assisted by an electric field (2.0 V). The rate constant (k = 2.89 min^-1) for SMX removal was 24 and 67 times greater than that of Fe nanoparticles immobilized on Mo₂TiC₂T_x and the pristine Mo₂TiC₂T_x filter, respectively. Operation in the flow-through configuration outperformed the conventional batch reactor model (k = 0.17 min^-1) due to convection-enhanced mass transport. The results obtained from experimental investigations and theoretical calculations suggested that atomically dispersed Fe-SA, anchored on Mo vacancies, was responsible for the adsorption and activation of PMS to produce sulfate radicals (SO₄^•-) in the presence of an electric field. This study provides a proof-of-concept demonstration of an electroactive Fe-SA/Mo₂TiC₂T_x filter for broader application in the treatment of water contaminated by emerging micropollutants.