Development of a Nanostructured α‑MnO₂/Carbon Paper Composite for Removal of Ni²⁺/Mn²⁺ Ions by Electrosorption

Toxic metal ions, such as Ni²⁺ and Mn²⁺, in industrial waste streams are nonbiodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than do chemical/biological approaches without the high pressures needed for membranes. Here, nanoneedle-structured α-MnO₂/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electroadsorption and capacitive deionization removal of nickel and manganese ions from pseudoindustrial waste streams. The specific performance of α-MnO₂/CFP (16.4 mg Ni²⁺ per g of active material) not only shows a great improvement in comparison with its original CFP substrate (0.034 Ni²⁺ mg per g), but also is over 6 times that of activated carbon (2.5 mg Ni²⁺ per g). The high performance of α-MnO₂/CFP composites is attributed to their high surface area, desirable mesoporosity, pore-size distribution that permits the further access of ions, and their property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn²⁺ ions are released due to the partial reduction of MnO₂ when operated as a negative electrode. For the removal of Mn²⁺ ions, an asymmetric arrangement, consisting of a MnO₂/CFP positive electrode and an activated carbon negative electrode, was employed. This arrangement reduced the Mn²⁺ concentration from 100 ppm to less than 2 ppm, a vast improvement over the systematical two-activated carbon electrode system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO₂/CFP composite was maintained as a positive electrode, it was completely stable. The technique was able to reduce both Ni²⁺ and Mn²⁺ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore

Hierarchical Porous N‑Doped Carbon Nanofibers with Encapsulated Li₃VO₄ Nanoparticles for Lithium-Ion Storage

Lithium vanadium oxide (Li3VO4) is a highly promising anode material for lithium-ion batteries due to its high theoretical capacity and moderate operation voltage. However, its low intrinsic electronic conductivity leads to an undesirable rate capability and restricts its practical applications. To address this issue, we designed a structure of hierarchical porous carbon network-wrapped Li3VO4 nanoparticles to enhance the overall electrochemical performance, especially at high rates. Polyacrylonitrile (PAN) and poly­(methyl methacrylate) (PMMA) were employed as carbon sources and porous templates during the electrospinning and subsequent calcination processes. This approach can enhance the electronic conductivity, improve the contact area between Li3VO4 and the electrolyte, and decrease the ion/electron diffusion path. As a result, the constructed hierarchical porous N-doped C nanofiber-encapsulated Li3VO4 nanoparticles (P-LVO/NC NFs) exhibited an ultrahigh discharge capacity of 1039.5 mA h g–1 and a stable capacity of 736.8 mA h g–1 after 500 cycles at 0.5 A g–1. Furthermore, they demonstrated an outstanding rate capability of 397.7 mA h g–1 and at 5.0 A g–1. The unique hierarchical porous structure provided excellent reaction kinetics, resulting in exceptional Li-ion storage performance. Therefore, the fabricated P-LVO/NC NFs hold great potential as high-performance anode materials

Chloride Oxidation by One- or Two-Photon Excitation of <i>N</i>‑Phenylphenothiazine

Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C–H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, E° (Cl•/–), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH•+ by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH•+*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M–1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C­(sp3)–H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant

Visible-Light-Driven Chlorine-Atom-Mediated Efficient and Selective Cleavage of C–C Bond in Lignin Models

Lignin valorization to aromatic fine chemicals requires selective cleavage of a high-energy C–C bond under mild conditions. Herein, we report the efficient and selective C–C bond cleavage of lignin model substrates through a chlorine (Cl) atom as the hydrogen atom transfer agent generated by visible-light excitation of the metal-free photocatalyst perylene diimide. With the assistance of the Cl atom, the overall yield of C–C bond cleavage products in model substrate β–O–4 linkages reached 82.5% under visible-light excitation. This work makes use of metal-free photocatalysts and ubiquitous chloride as an alternative sustainable method for lignin depolymerization

Visible Light Generation of a Microsecond Long-Lived Potent Reducing Agent

Photoexcitation of molecular radicals can produce strong reducing agents; however, the limited lifetimes of the doublet excited states preclude many applications. Herein, we propose and demonstrate a general strategy to translate a highly energetic electron from a doublet excited state to a ZrO2 insulator, thereby increasing the lifetime by about 6 orders of magnitude while maintaining a reducing potential less than −2.4 V vs SCE. Specifically, red light excitation of a salicylic acid modified perylene diimide radical anion PDI•– anchored to a ZrO2 insulator yields a ZrO2(e–)|PDI charge separated state with an ∼10 μs lifetime in 23% yield. The ZrO2(e–)­s were shown to drive CO2 → CO reduction with a Re catalyst present in micromolar concentrations. More broadly, this strategy provides new opportunities to reduce important reagents and catalysts at low concentrations through diffusional electron transfer

Ordovician intrusive rocks from the eastern Central Asian Orogenic Belt in Northeast China: chronology and implications for bidirectional subduction of the early Palaeozoic Palaeo-Asian Ocean

<p>The eastern segment of the Central Asian Orogenic Belt is traditionally called the Xing’an Mongolia Orogenic Belt (XMOB). Ordovician intrusive rocks exposed in the XMOB, from north to south, are the Abaga-East Ujimqin Qi-Duobaoshan belt, the Sonid Zuoqi-West Ujimqin Qi belt, and the Damaoqi-Baimaimiao-Tulinkai belt, respectively. Zircon U–Pb dating and geochemical data are presented for the intrusive rocks in East Ujimqin Qi and West Ujimqin Qi, Inner Mongolia. The intrusive rocks from East Ujimqin Qi consist of gabbro, diorite, and granodiorite. LA-MC-ICP-MS zircon U–Pb ages range 446 to 461 Ma. Geochemical data suggest that the gabbros and diorites from East Ujimqin are a tholeiitic series, both of arc-related and N-MORB (mid-ocean ridge basalt) signature, indicating a back-arc basin setting. The granodiorites have a shoshonitic series and arc-related signature. Rare earth element (REE) patterns and trace element characteristics suggest gabbros, diorites, and granodiorites are petrogenetically correlated. These intrusive rocks from East Ujimqin Qi have high light REE, Th, and U concentrations, suggesting the effect of middle–upper continental crustal contamination. Major oxides display positive or negative correlations, with increasing MgO or SiO_2, indicating that fractional crystallization occurred during magma evolution. Geochemical data of diorite from West Ujimqin Qi indicate a tholeiitic series, arc-related signature. Zircon U–Pb dating yielded an age of 441.8 ± 1.5 Ma. Integrated with the regionally exposed Mid–Late Ordovician plutons and metasedimentary strata, we concluded that the northward subduction of the Palaeo-Asian Ocean (PAO) that occurred beneath the southern margin of the South Mongolian Micro-continent along the Sonid Zuoqi-Xilinhot gave rise to early Palaeozoic igneous rocks from the Abaga–East Ujimqin Qi–Duobaoshan and the Sonid Zuoqi–West Ujimqin Qi belts. Southward subduction beneath the North China Craton generated the Damaoqi–Baimaimiao–Tulinkai belt. The results support the bidirectional subduction model of the PAO in the early Palaeozoic.</p

Unassisted Uranyl Photoreduction and Separation in a Donor–Acceptor Covalent Organic Framework

The donor–acceptor covalent organic framework (COF) TTT–DTDA (TTT = thieno­[3,2-b]­thiophene-2,5-dicarbaldehyde and DTDA = 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)­trianiline) was prepared and found to have long-lived excited states (>100 ms) characterized by transient absorption spectroscopy. These excited-state lifetimes were sufficient to perform the direct photoreduction of uranium at ppm concentration levels. The photoreduction of soluble uranyl species to insoluble reduced uranium products is an attractive separation for uranium, typically accomplished with sacrificial reagents and protective gases. In the case of TTT–DTDA, illumination in aqueous solutions containing only uranyl ions produced crystalline uranyl peroxide species ([UO2(O2)]) at the COF that were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, and infrared spectroscopy. The maximum absorption capacity of TTT–DTDA was found to be 123 mg U/g COF at pH 5 after 10 h of illumination in solutions devoid of sacrificial reagents or protective gases. The TTT–DTDA COF was recyclable and maintained high selectivity for uranium in competing ion experiments, which are necessary requirements for a practical uranium extraction strategy based on photochemical uranium reduction

Reversible Amine-to-Imine Chemistry at a Covalent Organic Framework for Sustainable Uranium Redox Separation

The interconversion chemistry of amine-to-imine sites in a covalent organic framework (COF) was developed for the redox-based separation of uranium. Compared to traditional approaches using sacrificial reagents or material decomposition for the reduction and separation of uranium, amine-COF served as the electron donor and was regenerated repeatedly following the oxidation and uranium reduction/separation. The amine-COF, PI-3-AR, was formed from the sodium borohydride (NaBH4) reduction of the imine-linked COF, PI-3, prepared from the solvothermal synthesis of 1,3,5-triformyl benzene (TFB) and 4,4′,4″-(1,3,5-triazine-2,4,6-triyl)trianiline (TTA). PI-3-AR could be converted back to PI-3 via oxidative amination using an excess of the oxidant iodine, I2, or in the photochemical reduction of uranyl ions (UO22+). In consecutive photochemical uranium reduction and separation cycling experiments, the reduced amine COF, PI-3-AR, underwent: (i) oxidation alongside uranium photoreduction and deposition; (ii) acid treatment and uranium extraction; and (iii) NaBH4 reduction and material recovery. The COF, PI-3-AR, and novel separation process involving amine-to-imine interconversion effectively removed uranium (maximum adsorption = 278 mg U/g COF) and maintained >98% uranium recovery over five recycling steps at pH 4.0