Self-assembled mononuclear complexes: open metal sites and inverse dimension-dependent catalytic activity for the Knoevenagel condensation and CO2 cycloaddition.

To lessen the greenhouse effect, measures such as improving the recovery of crude oil and converting carbon dioxide (CO2) into valuable chemicals are necessary to create a sustainable low-carbon future. To this end, the development of efficient new oil-displacing agents and CO2 conversion has aroused great interest in both academia and industry. The Knoevenagel condensation and CO2 cycloaddition are the key reactions to solve the above problems. Four Cu- or Zn-based molecular complexes built from different ligands possessing hydrophilic-hydrophobic layers and different dimensionalities were chosen as solid catalysts for this study. Structural analysis revealed the presence of hydrophilic-hydrophobic layers and open metal sites in the low-dimensional complexes. To obtain deep insight into the reaction mechanism, first-principles density functional theory (DFT) calculations were carried out. These calculations confirmed that in the Knoevenagel condensation reaction, the final formation of benzylidenemalononitrile is the rate-determining step (an energy barrier (ΔE) value of 73.2 kJ mol-1). The zero-dimensional (0D) Cu molecular complex with unsaturated metal centers, hydrophilic and hydrophobic layers, exhibited higher catalytic activity (yield: 100%, temperature: room temperature, and time: 2 h) compared with one- and two-dimensional Cu complexes. In the presence of a 0D Zn complex co-catalyzed with Br- in the CO2 cycloaddition reaction, the ΔE value reduces to 35.5 kJ mol-1 for the ring opening of styrene oxide (SO), which is significantly lower than Br- catalyzed (80.9 kJ mol-1) reactions. The roles of unsaturated metal centers, hydrophilic-hydrophobic layers and dimensionality in the Knoevenagel condensation and CO2 cycloaddition were explained in the results of structure-activity relationships.

Strong Electronic Metal-Support Interaction between Iridium Single Atoms and a WO3 Support Promotes Highly Efficient and Robust CO2 Cycloaddition.

The carbon dioxide (CO2 ) cycloaddition of epoxides to cyclic carbonates is of great industrial importance owing to the high economical values of its products. Single-atom catalysts (SACs) have great potential in CO2 cycloaddition by virtue of their high atom utilization efficiency and desired activity, but they generally suffer from poor reaction stability and catalytic activity arising from the weak interaction between the active centers and the supports. In this work, Ir single atoms stably anchored on the WO3 support (Ir1 -WO3 ) are developed with a strong electronic metal-support interaction (EMSI). Superior CO2 cycloaddition is realized in the Ir1 -WO3 catalyst via the EMSI effect: 100% conversion efficiency for the CO2 cycloaddition of styrene oxide to styrene carbonate after 15 h at 40 °C and excellent stability with no degradation even after ten reaction cycles for a total of more than 150 h. Density functional theory calculations reveal that the EMSI effect results in significant charge redistribution between the Ir single atoms and the WO3 support, and consequently lowers the energy barrier associated with epoxide ring opening. This work furnishes new insights into the catalytic mechanism of CO2 cycloaddition and would guide the design of stable SACs for efficient CO2 cycloaddition reactions.

Single-ion chelation strategy for synthesis of monodisperse Pd nanoparticles anchored in MOF-808 for highly efficient hydrogenation and cascade reactions.

Ultrafine Pd nanoparticles are prepared using a single-ion precursor on a MOF-808 carrier. The ligand 2,3-pyrazinedicarboxylic acid (Pza) is dispersed in porous MOF-808 via grafting on formic acid sites, and thus Pd2+ ions are chelated by Pza to form a new single-ion precursor Pd@MOF-808-Pza. Then a Pd-nano@MOF-808-Pza catalyst is prepared by direct reduction of this precursor using NaBH4. Material characterization reveals the homogeneous dispersion of 3-6 nm Pd nanoparticles within the MOF-808 matrix. Pd-nano@MOF-808-Pza exhibits excellent catalytic activity in the hydrogenation of unsaturated nitrogen-containing compounds, and other typical reactions, such as the Knoevenagel condensation, Suzuki/Heck cross-coupling, and hydrogen tandem reactions. In addition, density functional theory (DFT) calculations are carried out to elucidate the chelation of Pd2+ ions by Pza on MOF-808 and propose mechanisms of hydrogenation reactions. This work provides an effective reduction catalyst, and more importantly, a single-ion chelation strategy for design and synthesis of metal supported catalysts.

Torsion bidirectional sensor based on tilted-arc long-period fiber grating.

Fiber torsion sensor has been researched for many years due to its various structure and sensitive response. In order to distinguish the torsion direction, fiber sensor still faces some difficult problems, including complex fabricating condition, special fiber structure and limited sensitivity. In this paper, a novel long-period fiber grating (LPFG) formed by tilted-arc grids is designed and fabricated in the normal simple-mode fiber, showing small size and high sensitivity. The asymmetrical tilted-arc grid structure can induce considerable chirality into the tilted-arc LPFG to enable it to distinguish torsion direction, which doesn't need any equipment to rotate or twist the fiber in the fabrication process. Theoretical analysis indicates that the structure can respond opposite wavelength shifting to the opposite torsion directions, and the torsion sensitivity is related to both the radius and tilted angle of grid. A series of tilted-arc LPFGs are fabricated with CO2-laser scanning and tested in torsion experiment, all of whom can distinguish bidirectional torsion. The maximum sensitivity value can reach 0.514 nm/(rad·m-1), which is higher than many normal tilted LPFGs and twisted fiber structures. The novel LPFG has the potential to be applied in directional torsion field due to its direction-distinguishing ability, high sensitivity and simple fabrication.

Crystallization of Gd2O3 nanoparticles: evolution of the microstructure via electron-beam manipulation.

NaGdF4 is one of the most commonly employed phosphor host matrices for lanthanide doping and is one of the most efficient infrared-to-visible up-conversion fluorescent host materials. Although the structure, morphology and luminescence properties of NaREF4 have been sufficiently investigated, there are very few reported instances of introducing localized order/crystallinity by electron-beam (e-beam) irradiation. In this work, we studied the phase transformation of Gd2O3 from an amorphous to crystalline form via manipulation by e-beam irradiation. The amorphous Gd2O3 occurs as an impurity in the cubic-NaGdF4 nanoparticles (NPs). The structural evolutions, including the transformation from amorphous to crystalline, the recrystallization process and the formation of the graphene@NP core-shell structure, are discussed in detail. We also propose an evolution scheme, in which the e-beam manipulation of the organic-containing NPs induces a subtle structural transformation, depending in principle on the microenvironment of the NPs.

Nitrogen-Deficient Graphitic Carbon Nitride with Enhanced Performance for Lithium Ion Battery Anodes.

Graphitic carbon nitride (g-C3N4) behaving as a layered feature with graphite was indexed as a high-content nitrogen-doping carbon material, attracting increasing attention for application in energy storage devices. However, poor conductivity and resulting serious irreversible capacity loss were pronounced for g-C3N4 material due to its high nitrogen content. In this work, magnesiothermic denitriding technology is demonstrated to reduce the nitrogen content of g-C3N4 (especially graphitic nitrogen) for enhanced lithium storage properties as lithium ion battery anodes. The obtained nitrogen-deficient g-C3N4 (ND-g-C3N4) exhibits a thinner and more porous structure composed of an abundance of relatively low nitrogen doping wrinkled graphene nanosheets. A highly reversible lithium storage capacity of 2753 mAh/g was obtained after the 300th cycle with an enhanced cycling stability and rate capability. The presented nitrogen-deficient g-C3N4 with outstanding electrochemical performances may unambiguously promote the application of g-C3N4 materials in energy-storage devices.

Novel g-C3N4/CoO Nanocomposites with Significantly Enhanced Visible-Light Photocatalytic Activity for H2 Evolution.

Novel g-C3N4/CoO nanocomposite application for photocatalytic H2 evolution were designed and fabricated for the first time in this work. The structure and morphology of g-C3N4/CoO were investigated by a wide range of characterization methods. The obtained g-C3N4/CoO composites exhibited more-efficient utilization of solar energy than pure g-C3N4 did, resulting in higher photocatalytic activity for H2 evolution. The optimum photoactivity in H2 evolution under visible-light irradiation for g-C3N4/CoO composites with a CoO mass content of 0.5 wt % (651.3 µmol h-1 g-1) was up to 3 times as high as that of pure g-C3N4 (220.16 µmol h-1 g-1). The remarkably increased photocatalytic performance of g-C3N4/CoO composites was mainly attributed to the synergistic effect of the junction or interface formed between g-C3N4 and CoO.

A simple approach for large-area fabrication of Ag nanorings.

A simple and low-cost method based on a two-step heat treatment of AgNO(3)/SiO(2) film has been developed for fabricating metal Ag nanoring arrays. The as-prepared nanorings have an inner diameter of 70-250 nm and an average wall thickness (namely wire diameter) of approximately 30 nm with a number density of approximately 10(9) cm(-2) on the surface of the SiO(2) matrix. X-ray diffraction (XRD) results reveal that these nanorings exhibit a face-centered cubic crystal structure. Furthermore, a new growth mechanism, namely a molten metal bubble as a self-template, is tentatively proposed for Ag nanorings.