Electrocatalytic transformation of oxygen to hydroxyl radicals via three-electron pathway using nitrogen-doped carbon nanotube-encapsulated nickel nanocatalysts for effective organic decontamination.

The selective electrochemical reduction of oxygen (O2) via 3e- pathway for the production of hydroxyl radicals (HO) is a promising alternative to conventional electro-Fenton process. Here, we developed a nitrogen-doped CNT-encapsulated Ni nanoparticle electrocatalyst (Ni@N-CNT) with high O2 reduction selectivity for the generation of HO•via 3e- pathway. Exposed graphitized N on the CNT shell, and Ni nanoparticles encapsulated within the tip of the N-CNT, played a key role in the generation of H2O2 intermediate (*HOOH) via a 2e- oxygen reduction reaction. Meanwhile, those encapsulated Ni nanoparticles at the tip of the N-CNT facilitated the sequential HO• generation by directly decomposing the electrogenerated *H2O2 in a 1e- reduction reaction on the N-CNT shell without inducing Fenton reaction. Improved bisphenol A (BPA) degradation efficiency were observed when compared with conventional batch system (97.5% vs 66.4%). Trials using Ni@N-CNT in a flow-through configuration demonstrated a complete removal of BPA within 30 min (k = 0.12 min-1) with a limited energy consumption of 0.068 kW·h·g-1 TOC.

Particle size optimization of metal-organic frameworks for superior capacitive deionization in oxygenated saline water.

Pyrolysis-free metal-organic frameworks (MOFs) with optimized particle sizes were used as capacitive deionization (CDI) materials in oxygenated saline water. Upon decreasing the particle size of the MOFs, excellent cycling stability and higher CDI performance were achieved. This was possibly due to the improvement in charge transfer and electrolyte permeation, uncovering the significance of particle size control in improving CDI performance.

Electrified nanohybrid filter for enhanced phosphorus removal from water.

Phosphorus (P) has been identified as a major cause of eutrophication. One feasible way to deal with P-containing wastewater is to employ advanced adsorbents with high P affinity. Towards this end, the loading of these sorbents onto a conductive scaffold would facilitate the introduction of an electric field into the reaction system thereby permitting a continuous-flow operation and improved P sorption kinetics. Here, the preparation and evaluation of an electroactive carbon nanotube (CNT) filter functionalized with cerium-based metal organic frameworks (Ce-MOF) is reported. Various advanced characterization techniques confirmed the successful fabrication of the Ce-MOF/CNT nanohybrid filter. The results suggested that the nanohybrid filter had a maximum P adsorption capacity of 22.41 mg g-1, which compared favorably with other state-of-the-art P sorbents. Ce-MOF loading, applied voltage and flow rate each increased the rate constants for phosphate removal by factors of 1.6, 2.1 and 5.8 times relative to the absent states. The underlying P sorption mechanisms involved outer-sphere surface complexation (electrostatic attraction), inner-sphere surface complexation (Ce-O-P) and diffusion. The performance was tolerant of a wide operational pH range and different water matrices. The Ce-MOF/CNT electrochemical filter described in this study provides a viable strategy to address the challenging issues associated with aqueous P pollution.

Carbon nanotube filter functionalized with MIL-101(Fe) for enhanced flow-through electro-Fenton.

Herein, an electrochemical carbon nanotubes (CNT) filter modified with MIL-101(Fe) has been designed for the electro-Fenton applications by serving as a functional flow-through electrode. Under an electric field, the hybrid filter enabled the in situ generation of H2O2via the two-electron oxygen reduction reaction, which promoted the production of HO by the accelerated Fe2+/Fe3+ cycling of MIL-101(Fe). It was observed that 93.2 ± 1.2% tetracycline and 69.0 ± 0.8% total organic carbon (TOC) were removed in 2 h under the optimized conditions. The electron paramagnetic resonance (EPR) analysis and radical scavenging experiments revealed that HO predominated the tetracycline degradation. As compared to the batch reactor, the performance of the proposed system was improved by 5.6 times owing to the convection-enhanced mass transport. The plausible working mechanism and degradation pathway were also subsequently proposed. The findings reported in this study provide a promising insight for the environmental remediation by integrating nanotechnology and Fenton chemistry.

Defect-Rich Hierarchical Porous UiO-66(Zr) for Tunable Phosphate Removal.

The introduction of defects into hierarchical porous metal-organic frameworks (HP-MOFs) is of vital significance to boost their adsorption performance. Herein, an advanced template-assisted strategy has been developed to fine-tune the phosphate adsorption performance of HP-MOFs by dictating the type and number of defects in HP-UiO-66(Zr). To achieve this, monocarboxylic acids of varying chain lengths have been employed as template molecules to fabricate an array of defect-rich HP-UiO-66(Zr) derivatives following removal of the template. The as-prepared HP-UiO-66(Zr) exhibits a higher sorption capacity and faster sorption rate compared to the pristine UiO-66(Zr). Particularly, the octanoic acid-modulated UiO-66(Zr) exhibits a high adsorption capacity of 186.6 mg P/g and an intraparticle diffusion rate of 6.19 mg/g·min0.5, which are 4.8 times and 1.9 times higher than those of pristine UiO-66(Zr), respectively. The results reveal that defect sites play a critical role in boosting the phosphate uptake performance, which is further confirmed by various advanced characterizations. Density functional theory (DFT) calculations reveal the important role of defects in not only providing additional sorption sites but also reducing the sorption energy between HP-UiO-66(Zr) and phosphate. In addition, the hierarchical pores in HP-UiO-66(Zr) can accelerate the phosphate diffusion toward the active sorption sites. This work presents a promising route to tailor the adsorption performance of MOF-based adsorbents via defect engineering.

One-step Sb(III) decontamination using a bifunctional photoelectrochemical filter.

Developing advanced technologies to achieve decontamination of emerging contaminants such as antimony (Sb) is highly demanded. Herein, we successfully designed a dual-functional photoelectrochemical filter system for "one-step" detoxification and sequestration of highly toxic Sb(III). The key to this technology is a photoelectrical-responsive CNT filter functionalized with nanoscale MIL-88B(Fe) photocatalysts. At 2.5 V and under illumination, a 97.7 ±â€¯1.5 % Sb(III) transformation and a 92.9 ±â€¯2.3 % Sbtotal removal efficiency can be obtained using an optimal hybrid filter (e.g. CM(50:3)) over 2 h continuous filtration. This can be explained by the synergistic effects of the filter's flow-through design, photoelectrochemical reactivity, fine pore size, and plentiful exposed sorption sites. Various advanced characterization techniques validated the system efficacy. Improved Sb(III) removal kinetics were observed when compared with conventional batch system (97.5 % vs 75.8 %). A synergistic effect between photocatalytic (PC) and electrochemical (EC) process were identified (kPEC =0.99 h-1 >kPC=0.21 h-1 + kEC =0.30 h-1). EPR and photochemical characterizations suggested that hydroxyl radicals dominated the Sb(III) conversion. The proposed technology works effectively across a wide range of pH values and water matrixes. The outcomes of this study can facilitate mechanistic insights into photoelectrocatalysis and provide a promising nanotechnology for efficient Sb(III) decontamination.

Recent advances on photocatalytic fuel cell for environmental applications-The marriage of photocatalysis and fuel cells.

Environmental pollution and energy crisis have become recent worldwide concerns. Huge amounts of organic wastes are discharged into water bodies, causing serious environmental pollution. Meanwhile, these organic compounds are important carbon and energy sources that could be utilized instead of being discarded. A smart design of a photocatalytic fuel cell (PFC) can achieve double benefits: it can degrade organic pollutants and at the same time generate energy. In this review article, we discuss recent progress in the development of PFC systems, and summarize the principles for constructing advanced PFC systems. We particularly focus on the rational design of electrode materials in terms of surface, morphology, facet, and interfacial reaction engineering. The impact of important operational parameters on PFC performance is further discussed in detail. We then discuss the major limitations and opportunities for future PFCs research. The development of smart and advanced PFC systems depends on highly interdisciplinary collaborations, which require concerted efforts from the communities of materials science, chemistry, engineering, and environmental science.

Heterogeneous Semiconductor Shells Sequentially Coated on Upconversion Nanoplates for NIR-Light Enhanced Photocatalysis.

Combination of upconversion nanocrystals (UCNs) with CeO2 is a decent choice to construct NIR-activated photocatalysts for utilizing the NIR light in the solar spectrum. Herein we present a facile approach to deposit a CeO2 layer with controllable thickness on the plate-shaped NaYF4:Yb,Tm UCNs. The developed core-shell nanocomposites display obvious photocatalytic activity under the NIR light and exhibit enhanced activity under the full solar spectrum. For enhancing the separation of photogenerated electrons and holes on the CeO2 surface, we sequentially coat a ZnO shell on the nanocomposites so as to form a heterojunction structure for achieving a better activity. The developed hybrid photocatalysts have been characterized with TEM, SEM, PL, etc., and the working mechanism of such UCN-semiconductor heterojunction photocatalysts has been proposed.

Controlled Growth of Metal-Organic Framework on Upconversion Nanocrystals for NIR-Enhanced Photocatalysis.

Development of MOF-based photocatalysts is intriguing research due to their structural flexibility and tremendous catalytic sites, whereas most MOFs only can take use of UV/visible light and lack of response to NIR light. Herein, we present a facile approach to integrate upconversion nanoparticles (UCNPs) with MOF to build a NIR-responsive composite photocatalyst. The MOF shell with controllable thickness can be grown on the UCNPs, thus exhibiting tunable photocatalytic activities under NIR irradiation. Furthermore, we extend visible absorption of the MOF shell by adding -NH2 groups so that the composite photocatalysts have a better utilization of UC emissions and sunlight to improve their activities. The developed composite photocatalysts have been characterized by XRD, TEM, PL, etc., and their photocatalytic performances were systematically explored. The formation and working mechanism of the composite photocatalysts were also elucidated.