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.

Markedly boosted peroxymonosulfate- and periodate-based Fenton-like activities of iron clusters on sulfur/nitrogen codoped carbon: Key roles of a sulfur dopant and compared activation mechanisms.

Highly dispersed iron nanoclusters on carbon (FeNC@C) hold great promise for wastewater purification in Fenton-like reactions. The microenvironment engineering of central Fe atom is promising to boost the activation capacity of FeNC@C, which is however remains a challenge. This study developed a self-sacrificed templating strategy to S, N-codoped carbon supported Fe nanoclusters (FeNC@SNC) activator and find the key role of sulfur heteroatoms in regulating the electron structure of Fe sites and final activation property. Investigations revealed that the FeNC@SNC composite exhibited unusual bifunctional activity in both peroxymonosulfate (PMS)- and periodate (PI)-based Fenton-like reactions. We also offered insights into the differences between the degradation of organics by the FeNC@SNC/PMS and FeNC@SNC/PI systems. Specifically, under identical conditions, the FeNC@SNC/PMS system delivered a higher oxidation capability and stronger resistance to nontarget matrix constituents, but showed more severe Fe leaching than the FeNC@SNC/PI system. Furthermore, while mediated electron-transfer process was identified as the major route for pollutant decomposition in both systems, the high-valent Fe-oxo species [Fe (IV)] was the auxiliary reactive species found only in the FeNC@SNC/PMS system. Based on these findings, our results provide profound insights into the design of active and durable Fe-based activators toward highly efficient Fenton-like reactions.

Harvesting mechanical energy for hydrogen generation by piezoelectric metal-organic frameworks.

Piezocatalysis, the process of directly converting mechanical energy into chemical energy, has emerged as a promising alternative strategy for green H2 production. Nevertheless, conventional inorganic piezoelectric materials suffer from limited structural tailorability and small surface area, which greatly impedes their mechanically driven catalytic efficiency. Herein, we design and fabricate a novel UiO-66(Zr)-F4 metal-organic framework (MOF) nanosheet for piezocatalytic water splitting, with the highest H2 evolution rate reaching 178.5 µmol g-1 within 5 h under ultrasonic vibration excitation (110 W, 40 kHz), far exceeding that of the original UiO-66 host. A reduced bandgap from 2.78 to 2.43 eV is achieved after introducing a fluorinated ligand. Piezoresponse force microscopy measurements demonstrate a much stronger piezoelectric response for UiO-66(Zr)-F4, which may result from the polarity of the introduced fluorinated ligand. This work highlights the potential of MOF-based porous piezoelectric nanomaterials in harvesting mechanical energy to drive chemical reactions such as water splitting.

Metal-organic framework-derived magnetic carbon for efficient decontamination of organic pollutants via periodate activation: Surface atomic structure and mechanistic considerations.

Practical implementation of periodate-based advanced oxidation processes for environmental remediation largely relies on the development of cost-effective and high-performance activators. Surface atomic engineering toward these activators is desirable but it remains challenging to realize improved activation properties. Here, a surface atomic engineering strategy used to obtain a novel hybrid activator, namely cobalt-coordinated nitrogen-doped graphitic carbon nanosheet-enwrapped cobalt nanoparticles (denoted as Co@NC-rGO), from a sandwich-architectured metal-organic framework/graphene oxide composite is reported. This activator exhibits prominent periodate activation properties toward pollutant degradation, surpassing previously reported transition-metal-based activators. Importantly, the activator shows good stability, magnetic reusability, and the potential for application in a complex water matrix. Density functional theory modeling implies that the strong activation capability of Co@NC-rGO is related to its surface atomic structure for which the embedded cobalt nanoparticles with abundant interfacial Co-N coordinations display modified electronic configurations on the active centers and benefit periodate adsorption. Quenching experiments and electrochemical measurements showed that the system could oxidize organics through a dominant nonradical pathway. Additionally, a lower concentration of cobalt leaching was observed for the Co@NC-rGO/periodate system than for its Co@NC-rGO/persulfate counterpart. Our work provides a pathway toward engineering surface atomic structures in hybrid activators for efficient periodate activation.

Regulation of electronic structures of MOF-derived carbon via ligand adjustment for enhanced Fenton-like reactions.

Peroxymonosulfate (PMS)-based Fenton-like reactions are widely used for wastewater remediation. Metal-free carbonaceous activators can avoid the secondary pollution caused by metal leaching but often suffer from insufficient activity due to limited active centers and mass transfer barriers. Here, we prepared a series of heteroatom (N, S, F)-doped, highly porous carbonaceous materials (UC-X, X = N, S, F) by pyrolyzing UiO-66 precursors assembled by various organic ligands. Density functional theory calculations showed that the heteroatoms modulated the electronic structures of the carbon plane. UC-X exhibited significantly enhanced PMS activation capability compared with the undoped counterpart, in the efficiency order of UC-N > UC-S > UC-F > UC. UC-N (calcined at 1000°C) showed the best PMS activation, exceeding that of commonly used carbocatalysts. The prominent performance of UC-N originated from its unique porous structure and homogeneously dispersed graphitic N moieties. Trapping experiments and electron spin resonance showed a nonradical degradation pathway in the UC-N/PMS system, through which organics were oxidized by donating electrons to UC-N/PMS* metastable complexes. This work not only reports a universal way to access high-performance, metal-free PMS activators but also provides insight into the underlying mechanism of the carbon-activated PMS process.

Cobalt-Enhanced Mass Transfer and Catalytic Production of Sulfate Radicals in MOF-Derived CeO2 • Co3 O4 Nanoflowers for Efficient Degradation of Antibiotics.

Antibiotics discharge has been a critical issue as the abuse in clinical disease treatment and aquaculture industry. Advanced oxidation process (AOPs) is regarded as a promising approach to degrade organic pollutants from wastewater, however, the catalysts for AOPs always present low activities, and uncontrollable porosities, thus hindering their further wider applications. In this work, an aliovalent-substitution strategy is employed in metal-organic framework (MOF) precursors assembly, aiming to introduce Co(II/III) into Ce-O clusters which could modify the structure of the clusters, then change the crystallization, enlarge the surface area, and regulate the morphology. The introduction of Co(II/III) also enlarges the pore size for mass transfer and enriches the active sites for the production of sulfate radicals (SO4• - ) in MOF-derived catalysts, leading to excellent performance in antibiotics removal. Significantly, the CeO2 •Co3 O4 nanoflowers could efficiently enhance the generation of sulfate radical SO4• - and promote the norfloxacin removal efficiency to 99% within 20 min. The CeO2 •Co3 O4 nanoflowers also present remarkable universality toward various antibiotics and organic pollutants. The aliovalent-substitution strategy is anticipated to find wide use in the exploration of high-performance MOF-derived catalysts for various applications.

Ce-based heterogeneous catalysts by partial thermal decomposition of Ce-MOFs in activation of peroxymonosulfate for the removal of organic pollutants under visible light.

Metal-organic framework (MOF) derivatives have drawn considerable attention for applications in various fields. In this work, spindle-shaped Ce-TCPPs were assembled by a rapid microwave-assisted hydrothermal method. After thermal treatment at low temperature under a N2 atmosphere, the Ce-TCPPs were partially pyrolyzed and converted to a novel CeO2/N-doped carbon/Ce-TCPP nanocomposite. Compared to completely decomposed materials, these partially decomposed heterogeneous catalysts exhibited significantly higher photocatalytic activation ability toward PMS for the removal of organic pollutants (e.g., rhodamine B, methylene blue, methyl orange, tetracycline and oxytetracycline). For the optimized sample thermal treated at 450 °C, a 100 mL RhB solution (10 mg/L) can be removed within 20 min with the assistance of PMS under visible light. The significantly enhanced activity can be attributed to the effective spatial separation of photogenerated electrons and holes in the formed Z-scheme CeO2/N-doped carbon/Ce-TCPP system. This work may provide useful guidance for the design and fabrication of MOF-derived photocatalytic systems for environmental remediation.

Atomically Dispersed Cobalt Sites on Graphene as Efficient Periodate Activators for Selective Organic Pollutant Degradation.

Pollutant degradation via periodate (IO4-)-based advanced oxidation processes (AOPs) provides an economical, energy-efficient way for sustainable pollution control. Although single-atomic metal activation (SMA) can be exploited to optimize the pollution degradation process and understand the associated mechanisms governing IO4--based AOPs, studies on this topic are rare. Herein, we demonstrated the first instance of using SMA for IO4- analysis by employing atomically dispersed Co active sites supported by N-doped graphene (N-rGO-CoSA) activators. N-rGO-CoSA efficiently activated IO4- for organic pollutant degradation over a wide pH range without producing radical species. The IO4- species underwent stoichiometric decomposition to generate the iodate (IO3-) species. Whereas Co2+ and Co3O4 could not drive IO4- activation; the Co-N coordination sites exhibited high activation efficiency. The conductive graphene matrix reduced the contaminants/electron transport distance/resistance for these oxidation reactions and boosted the activation capacity by working in conjunction with metal centers. The N-rGO-CoSA/IO4- system exhibited a substrate-dependent reactivity that was not caused by iodyl (IO3·) radicals. Electrochemical experiments demonstrated that the N-rGO-CoSA/IO4- system decomposed organic pollutants via electron-transfer-mediated nonradical processes, where N-rGO-CoSA/periodate* metastable complexes were the predominant oxidants, thereby opening a new avenue for designing efficient IO4- activators for the selective oxidation of organic pollutants.

Enhanced Piezocatalytic Activity of Sr0.5Ba0.5Nb2O6 Nanostructures by Engineering Surface Oxygen Vacancies and Self-Generated Heterojunctions.

Piezocatalysis provides a promising strategy for directly converting weak mechanical energy into chemical energy. In this work, we report a simple one-step hydrogen reduction route for the simultaneous generation of surface defects and heterojunctions in Sr0.5Ba0.5Nb2O6 nanorods fabricated by a molten salt synthesis method. The as-fabricated Sr0.5Ba0.5Nb2O6/Sr2Nb2O7 nanocomposites with controllable oxygen vacancies exhibited excellent piezocatalytic activity under ultrasonic vibration, with an about 7 times enhancement of the rate constant (k = 0.0395 min-1) for rhodamine B degradation and an about 10 times enhancement of the water-splitting efficiency for hydrogen generation (109.4 µmol g-1 h-1) for the optimized sample (H2 annealed at 500 °C) compared to pristine Sr0.5Ba0.5Nb2O6 nanorods. This work demonstrates the essential role of a well-modulated oxygen vacancy concentration in the piezocatalytic activity and provides an inspiring guide for designing self-generated heterojunction piezocatalysts.

Microwave-assisted hydrothermal assembly of 2D copper-porphyrin metal-organic frameworks for the removal of dyes and antibiotics from water.

Adsorption and photocatalysis are promising strategies to remove pollutants of dyes and antibiotics from wastewater. In this study, we demonstrate a rapid microwave-assisted hydrothermal route for the assembly of 2D copper-porphyrin Metal-Organic Frameworks (Cu-TCPP MOFs) within 1 h. The resulting 2D Cu-TCPP nanosheets with excellent crystallinity and a large surface area (342.72 m2/g) exhibited outstanding adsorption performance for typical dyes with adsorption capacities of about 185 mg/g for rhodamine B, 625 mg/g for methylene blue, and 290 mg/g for Congo red, respectively, as well as for representative antibiotics with adsorption capacities of about 130 mg/g for oxytocin, 150 mg/g for tetracycline, and 50 mg/g for norfloxacin, respectively. Meanwhile, the as-prepared 2D Cu-TCPP showed good photocatalytic degradation activity of pollutants after adsorption under irradiation by visible light, reaching removal efficiencies of 81.2 and 86.3% toward rhodamine B and norfloxacin, respectively. These results demonstrate the promising potential of 2D Cu-TCPP for use in the removal of contaminants from wastewater.

N-doped hierarchically porous carbon for highly efficient metal-free catalytic activation of peroxymonosulfate in water: A non-radical mechanism.

Metal-free carbo-catalyst has recently emerged as a promising candidate as a substituent for tradition-metal based heterogeneous catalyst for catalytic activation of peroxymonosulfate (PMS). However, most reported carbo-catalysts suffer from low catalytic efficiency and poor stability, thus a high-performance catalyst is urgently desired. In this study, a novel carbo-catalyst (NHPC-800), prepared by using tannic acid and dicyandiamide as renewable carbon/nitrogen feedstocks via a simple pyrolysis route, is reported as an activator of PMS with highly efficient catalytic ability and stability. The as-prepared NHPC-800 possesses as high as 22.4 atom% of nitrogen dopants and a hierarchically porous structure with abundant meso/macropores, accompanied by the abundant edges and wrinkles, which supply sufficient exposed catalytically active centers and fast electrons/mass transportations. Using rhodamine B as a model pollutant, the NHPC-800 shows a highly efficient catalytic ability which is superior to most reported carbo-catalysts and even some state-of-the-art metal catalysts. Based on competitive quenching experiments and electron paramagnetic resonance (EPR) results, a non-radical pathway involving the generation of 1O2 is responsible for the degradation of pollutants. Given that the NHPC-800 shows good recycling performance and strong resistance to adventitious interference such as anions and natural organic matters, we believe NHPC-800 can be a promising candidate for practical applications, and this study can provide inspirations for the further development of highly efficient carbo-catalysts.

Core-shell Prussian blue analogues@ poly(m-phenylenediamine) as efficient peroxymonosulfate activators for degradation of Rhodamine B with reduced metal leaching.

Herein, Prussian blue analogues@poly(m-phenylenediamine) nanoparticles (PBA@PmPDs) with well-defined core-shell structure were synthesized. The successful coating of poly(m-phenylenediamine) (PmPD) on the surface of FeyCo3-y [Co(CN)6]2 (Fe-Co-Co PBA) was confirmed by SEM, TEM, XRD, TGA, FT-IR and XPS. The catalytic performance of Fe-Co-Co PBA@PmPDs was evaluated by activation of peroxymonosulfate (PMS) for degradation of Rhodamine B (RhB), the effects of different influence factors on the RhB degradation efficiency were investigated, including PMS concentration, temperature, initial solution pH, and co-existing inorganic salts. Cobalt ions leaching and stability of the catalysts were studied, Co ions concentration dissolved into the solution from the solid catalysts at 60 min is reduced by half after the Fe-Co-Co PBA is coated with PmPDs. The RhB removal efficiency is higher than 90% even after four cycles and the nanoparticles still maintain the core-shell structure, indicating that Fe-Co-Co PBA@PmPDs is stabile and reusable. Radical quenching experiments and electron paramagnetic resonance spectra indicate that both SO4- and OH are generated during the PMS activation process and SO4- is the dominant reactive species. In virtue of its superior catalytic activity, excellent reusability and stability, low metallic ion leaching, Fe-Co-Co PBA@PmPDs could be a promising catalyst for the remediation of contaminated water.

Trichloroacetic acid-modulated synthesis of polyoxometalate@UiO-66 for selective adsorption of cationic dyes.

In this study, POM@UiO-66 nanoparticles were fabricated by encapsulation of POM (K6P2W18O62 polyoxometalate) into mesoporous UiO-66 metal organic framework through a solvothermal method with trichloroacetic acid as a modulator to promote the defects formation of UiO-66. The as-prepared samples were characterized by TEM, SEM, PXRD, TG, XPS, and EDX elemental mapping, and the successful combination of POM and UiO-66 was confirmed. Two cationic dyes, rhodamine B and malachite green, and one anionic dye orange G were employed to investigate the adsorption performance of POM@UiO-66. The adsorption data showed that the removal process of cationic dyes by POM@UiO-66 matched well with the pseudo-second-order model and Langmuir isothermal model. The resulting POM@UiO-66 nanoparticles exhibited high adsorption to cationic dyes but low adsorption to anionic dyes, and the adsorption capacities of rhodamine B, malachite green, and anionic dye orange G were 222.6, 190.6 and 40 mg g-1, respectively. Furthermore, the cationic dyes could be selectively removed from a cationic-anionic dye binary system. These results suggested that the novel polyoxometalate-based UiO-66 material is a promising candidate for the adsorption of cationic dyes from effluent.

Efficient incorporation of diverse components into metal organic frameworks via metal phenolic networks.

Metal phenolic networks (MPNs) were used for the first time for the construction of core@shell MOF materials. Diverse functional nanocores with different shapes and sizes could be coated with ZIF-8 shells mediated by MPNs. The obtained Fe3O4@TA-Fe3+@ZIF-8 powder showed a superhydrophobic nature which could efficiently absorb n-hexane from water.