Embedding Carbon Quantum Dots into Crystalline Polyimide Covalent Organic Frameworks to Enhance Water Oxidation for Achieving Dual-Channel Photocatalytic H2O2 Generation in a Wide pH Range

Photocatalytic technique is regarded as the cleanest approach for producing H2O2. Herein, two kinds of novel polyimide COFs decorated with CQDs (namely, MPa-COFs/CQDs and MNd-COFs/CQDs) were constructed by using the one-pot hydrothermal method. Due to the electron donor role of CQDs, the recombination of photoinduced electrons and holes was suppressed after the combination of polyimide COFs with CQDs. Importantly, the introduction of CQDs not only boosted the absorbing ability of polyimide COFs toward visible light but also reduced the impedance and improved the charge transfer efficiency. After CQDs were embedded into polyimide COFs, the surface hydrophilicity of catalysts was significantly improved, which provided convenience for the water oxidation reaction. Benefiting from the electron donor-acceptor interaction between polyimide COFs and CQDs, a step-by-step two-electron oxygen reduction reaction over polyimide COFs was enhanced. More interestingly, the embedding of CQDs can create a direct two-electron water oxidation reaction pathway, which played an important role in photocatalytic H2O2 generation. Meanwhile, H+ generated from water oxidation can also be used for the reduction of oxygen to form H2O2. Under the synergistic effects of water oxidation and oxygen reduction, as-prepared MPa-COFs/CQDs-2 displayed excellent performance in photocatalytic H2O2 generation, and its yield was as high as 540 μmol/g within 60 min. In short, the current work shared an effective strategy to improve the performance of polyimide COFs in photocatalytic H2O2 production.

Critical parameters influencing the continuous performance of upflow microbubble airlift photocatalytic process treating pharmaceutical pollutants

The advances in heterogeneous photocatalysts are still confined to evaluating the functional photocatalytic activity of catalysts in simple batch-mode operation. Nevertheless, the long-term stability, recovery-reusability, and cost-effectiveness of photocatalysts are critical issues in practical applications for pollution control. This study examined the critical parameters to improve the photocatalytic degradation activity of the antibiotic tetracycline and strategized successful continuous performance in a two-stage photocatalytic process adopting sequencing batch-mode microbubble upflow airlift reactor (UALR) followed by the centrifugal separation of CdS nanoparticles (NPs). The most effective strategy for NPs separation was a sequential combination of gravity separation (10 min settling) in the settling phase and subsequent high-speed centrifugation (5 min at 25,000×g) of the settled NPs sediments, providing an economic benefit by reducing the centrifugation capacity. During steady state operation under the optimal conditions, the UALR showed reliable performance, resulting in 97-91% and 85-81% degradation efficiency at 60- and 30-min reaction time per cycle, respectively. A weak basic condition (pH 8) and dissolved oxygen (DO) supplementation increased the photocatalytic activity by 12% (0.0292 min^-1) and 30% (0.0363 min^-1) compared to the control. Trapping studies confirmed the enhanced performance using various reactive oxygen species scavengers, revealing an increase in ^•OH generation (6.5%).

Thiadiazole-Based Covalent Organic Frameworks with a Donor-Acceptor Structure: Modulating Intermolecular Charge Transfer for Efficient Photocatalytic Degradation of Typical Emerging Contaminants

As novel metal-free photocatalysts, covalent organic frameworks (COFs) have great potential to decontaminate pollutants in water. Fast charge recombination in COFs yet inhibits their photocatalytic performance. We found that the intramolecular charge transfer within COFs could be modulated via constructing a donor-acceptor (D-A) structure, leading to the improved photocatalytic performance of COFs toward pollutant degradation. By integrating electron donor units (1,3,4-thiadiazole or 1,2,4-thiadiazole ring) and electron acceptor units (quinone), two COFs (COF-TD1 and COF-TD2) with robust D-A characteristics were fabricated as visible-light-driven photocatalysts to decontaminate paracetamol. With the readily excited electrons in 1,3,4-thiadiazole rings, COF-TD1 exhibited efficient electron-hole separation through a push-pull electronic effect, resulting in superior paracetamol photodegradation performance (>98% degradation in 60 min) than COF-TD2 (∼60% degradation within 120 min). COF-TD1 could efficiently photodegrade paracetamol in complicated water matrices even in river water, lake water, and sewage wastewater. Diclofenac, bisphenol A, naproxen, and tetracycline hydrochloride were also effectively degraded by COF-TD1. Efficient photodegradation of paracetamol in a scaled-up reactor could be achieved either by COF-TD1 in a powder form or that immobilized onto a glass slide (to further ease recovery and reuse) under natural sunlight irradiation. Overall, this study provided an effective strategy for designing excellent COF-based photocatalysts to degrade emerging contaminants.

A newly-designed free-standing NiCo2O4 nanosheet array as effective mediator to activate peroxymonosulfate for rapid degradation of emerging organic pollutant with high concentration

Nowadays effective treatment of high concentration organic wastewater is still a formidable task facing human beings. Herein, for the first time, a well-defined ZIF-67-derived NiCo₂O₄ nanosheet array was successfully prepared by a feasible method. In comparison with ordinary NiCo₂O₄ nanosphere, the formation of nanosheet structure could offer more opportunities to exposure internal active sites of NiCo₂O₄, thereby resulting in smaller interface resistance and higher charge transfer efficiency. As expected, ZIF-67-derived NiCo₂O₄ nanosheet array displayed great performance in peroxymonosulfate (PMS) activation. More importantly, recyclable redox couples of Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ endowed the stable catalytic activity of NiCo₂O₄ nanosheet. Interestingly, developed NiCo₂O₄-1/PMS oxidation system could achieve the effective degradation of antibiotics with high concentration in a short time. Both radical and nonradical pathways were involved into PMS activation, wherein SO₄^-, OH, O₂^- and ¹O₂ were major reactive oxygen species. The formation paths of reactive oxygen species and effects of inorganic anions were also investigated. Electrochemical analyses revealed that NiCo₂O₄-1 with nanosheet structure mediated the electron transfer between PMS and tetracycline (TC), which played a vital role in TC degradation. Furthermore, developed NiCo₂O₄-1/PMS oxidation system displayed great removal ability towards TC in actual water samples, and degradation products were low toxicity or no toxicity. In short, current work not only developed an effective oxidation system for completing the rapid degradation of antibiotic with high concentration, but also shared some novel insights into the activation mechanism of SR-AOPs.

Progress in Hybridization of Covalent Organic Frameworks and Metal-Organic Frameworks

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) hybrid materials are a class of porous crystalline materials that integrate MOFs and COFs with hierarchical pore structures. As an emerging porous frame material platform, MOF/COF hybrid materials have attracted tremendous attention, and the field is advancing rapidly and extending into more diverse fields. Extensive studies have shown that a broad variety of MOF/COF hybrid materials with different structures and specific properties can be synthesized from diverse building blocks via different chemical reactions, driving the rapid growth of the field. The allowed complementary utilization of π-conjugated skeletons and nanopores for functional exploration has endowed these hybrid materials with great potential in challenging energy and environmental issues. It is necessary to prepare a "family tree" to accurately trace the developments in the study of MOF/COF hybrid materials. This review comprehensively summarizes the latest achievements and advancements in the design and synthesis of MOF/COF hybrid materials, including COFs covalently bonded to the surface functional groups of MOFs (MOF@COF), MOFs grown on the surface of COFs (COF@MOF), bridge reaction between COF and MOF (MOF+COF), and their various applications in catalysis, energy storage, pollutant adsorption, gas separation, chemical sensing, and biomedicine. It concludes with remarks concerning the trend from the structural design to functional exploration and potential applications of MOF/COF hybrid materials.

Architecting an indirect Z-scheme NiCo2O4@CdS-Ag photocatalytic system with enhanced charge transfer for high-efficiency degradation of emerging pollutants

Bimetallic oxides with spinel structure show great prospects in the photocatalysis owing to many active sites. Herein, a novel 500NiCo₂O₄@CdS-5%Ag composite was fabricated via a feasible strategy. Interestingly, the combination with NiCo₂O₄ could significantly enhance the absorption ability of CdS for visible light. Benefiting from the formation of heterojunction structure between NiCo₂O₄ and CdS, the recombination of photogenerated electrons and holes was remarkably restrained. As an effective mediator, deposition of Ag could further promote the transfer of photogenerated charge carriers, thereby accelerating the reaction rate. Meanwhile, light absorption capacity of composite was also improved, owing to the surface plasmon resonance effect of metallic Ag. More importantly, 500NiCo₂O₄@CdS-5%Ag composite with great stability displayed an excellent performance in the photocatalytic degradation of OFX, and its highest removal efficiency was as high as 99.14%. Possible degradation pathways of OFX were given, and most of OFX could be degraded into CO₂, H₂O and other by-products with no toxicity. Significantly, the separation and transfer of photogenerated charge carriers followed indirect Z-scheme heterojunction mechanism. The O₂^-, OH and ¹O₂ were main active species in photocatalytic reaction system. All in all, current work inspired some new ideas for designing novel photocatalytic system in wastewater treatment.

Insights into the degradation of emerging organic pollutants by peroxydisulfate activated with Co3O4@NiO: role of each component and catalytic mechanism

A novel Co3O4@NiO as a high-efficiency catalyst was employed to activate peroxydisulfate for degrading antibiotics, and the catalytic mechanism was investigated.