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1.
ACS Nano ; 2020 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-32003975

RESUMO

Herein, we report hierarchical 3D NiMn-layered double hydroxide (NiMn-LDHs) shells grown on conductive silver nanowire (Ag NWs) cores as efficient, low-cost, and durable oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional electrocatalysts for metal-air batteries. The hierarchical 3D architectured Ag NW@NiMn-LDH catalysts exhibit superb OER/ORR activities in alkaline conditions. The outstanding bifunctional activities of Ag NW@NiMn-LDHs are essentially attributed to increasing both site activity and site populations. The synergistic contributions from the hierarchical 3D open-pore structure of the LDH shells, improved electrical conductivity, and small thickness of the LDHs shells are associated with more accessible site populations. Moreover, the charge transfer between Ag cores and metals of LDH shells and the formation of defective and distorted sites (less coordinated Ni and Mn sites) strongly enhance the site activity. Thus, Ag NW@NiMn-LDH hybrids exhibit a 0.75 V overvoltage difference between ORR and OER with excellent durability for 30 h, demonstrating the distinguished bifunctional electrocatalyst reported to date. Interestingly, the homemade rechargeable Zn-air battery using the hybrid Ag NW@NiMn-LDHs (1:2) catalyst as the air electrode exhibits a charge-discharge voltage gap of ∼0.77 V at 10 mA cm-2 and shows excellent cycling stability. Thus, the concept of the hierarchical 3D architecture of Ag NW@NiMn-LDHs considerably advances the practice of LDHs toward metal-air batteries and oxygen electrocatalysts.

2.
J Am Chem Soc ; 142(6): 2857-2867, 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-31955572

RESUMO

Understanding the role of the oxidation state of the Cu surface and surface-adsorbed intermediate species in electrochemical CO2 reduction is crucial for the development of selective CO2-to-fuel electrocatalysts. In this study, the electrochemical CO2 reduction mechanism over the Cu catalysts with various oxidation states was studied by using in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), in situ soft X-ray absorption spectroscopy (Cu L-edge), and online gas chromatography measurements. The atop-adsorbed CO (COatop) intermediate is obtained on the electrodeposited Cu surface which primarily has the oxidation state of Cu(I). COatop is further reduced, followed by the formation of C1 product such as CH4. The residual bridge-adsorbed CO (CObridge) is formed on the as-prepared Cu surface with Cu(0) which inhibits hydrocarbon formation. In contrast, the CV-treated Cu electrode prepared by oxidizing the as-prepared Cu surface contains different amounts of Cu(I) and Cu(0) states. The major theme of this work is that in situ SEIRAS results show the coexistence of COatop and CObridge as the reaction intermediates during CO2 reduction and that the selectivity of CO2-to-ethylene conversion is further enhanced in the CV-treated Cu electrode. The Cu catalysts modulated by the electrochemical method exhibit different oxidation states and reaction intermediates as well as electrocatalytic properties.

3.
Faraday Discuss ; 220(0): 105-112, 2019 12 02.
Artigo em Inglês | MEDLINE | ID: mdl-31532426

RESUMO

The mechanistic investigation of copper-catalysed transformations has been an important and fundamental task. Herein, we report via XAS and EPR spectroscopy that the sodium bis(trimethylsilyl)amide could reduce Cu(ii) to a Cu(i) species serving as an electron donor. XAS spectroscopy demonstrates that the newly formed Cu(i) species is the Cu[N(TMS)2]2Na ate complex, in which the nitrogen atoms coordinate with copper linearly.

4.
Chemistry ; 25(43): 10204-10213, 2019 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-31144786

RESUMO

We present herein the synthesis, crystal structure, and electric and magnetic properties of the spin-crossover salt [Mn(5-Cl-sal-N-1,5,8,12)]TCNQ1.5 ⋅2 CH3 CN (I), where 5-Cl-sal-N-1,5,8,12=N,N'-bis(3-(2-oxy-5-chlorobenzylideneamino)propyl)-ethylenediamine, containing distinct conductive and magnetic blocks along with acetonitrile solvent molecules. The MnIII complex with a Schiff-base ligand, [Mn(5-Cl-sal-N-1,5,8,12)]+ , acts as the magnetic unit, and the π-electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ- ) is the conducting unit. The title compound (I) exhibits semiconducting behavior with room temperature conductivity σRT ≈1×10-4  ohm-1 cm-1 and activation energy Δ ≈0.20 eV. In the temperature range 73-123 K, it experiences a hysteretic phase transition accompanied by a crossover between the low-spin S=1 and high-spin S=2 states of MnIII and changes in bond lengths within the MnN4 O2 octahedra. The pronounced shrinkage of the basal Mn-N bonds in I at the spin crossover suggests that the d x 2 - y 2 orbital is occupied/deoccupied in this transition. Interestingly, the bromo isomorphic counterpart [Mn(5-Br-sal-N-1,5,8,12)]TCNQ1.5 ⋅2 CH3 CN (II) of the title compound evidences no spin-crossover phenomena and remains in the high-spin state in the temperature range 2-300 K. Comparison of the chloro and bromo compounds allows the thermal and spin-crossover contributions to the overall variation in bond lengths to be distinguished. The difference in magnetic behavior of these two salts has been ascribed to intermolecular supramolecular effects on the spin transition. Discrete hydrogen bonding exists between cations and cations and anions in both compounds. However, the hydrogen bonding in the crystals of II is much stronger than in I. The relatively close packing arrangement of the [Mn(5-Br-sal-N-1,5,8,12)]+ cations probably precludes their spin transformation.

5.
Angew Chem Int Ed Engl ; 58(23): 7767-7772, 2019 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-30957924

RESUMO

In this study, a series of Sr(LiAl3 )1-x (SiMg3 )x N4 :Eu2+ (SLA-SSM) phosphors were synthesized by a solid-solution process. The emission peak maxima of SLA-SSM range from 615 nm to 680 nm, which indicates structural differences in these materials. 7 Li solid-state NMR spectroscopy was utilized to distinguish between the Li(1)N4 and Li(2)N4 tetrahedra in SLA-SSM. Differences in the coordination environments of the two Sr sites were found which explain the unexpected luminescent properties. Three discernible morphologies were detected by scanning electron microscopy. Temperature-dependent photoluminescence and decay times were used to understand the diverse environments of europium ions in the two strontium sites Sr1 and Sr2, which also support the NMR analysis. Moreover, X-ray absorption near-edge structure studies reveal that the Eu2+ concentration in SLA-SSM is much higher than that in in SrLiAl3 N4 :Eu2+ and SrSiMg3 N4 :Eu2+ phosphors. Finally, an overall mechanism was proposed to explain the how the change in photoluminescence is controlled by the size of the coordinated cation.

6.
ACS Appl Mater Interfaces ; 10(44): 38547-38557, 2018 Nov 07.
Artigo em Inglês | MEDLINE | ID: mdl-30360110

RESUMO

In this work, we report a novel application of chemical vapor deposition (CVD) in which the calcination and reduction of Cu(thd)2 deposited onto 4.9 wt % Cu/SiO2 induces significant decomposition of 28 nm crystalline Cu into ultrasmall ∼2 nm particles (5.1 wt % Cu/SiO2). The Cu loading slightly increased, but the particle size dramatically decreased. The deposition of Cu(thd)2 onto the Cu surface can initially affect the size reduction of the metallic Cu particles due to charge transfer between Cu(thd)2 and the Cu surface. Thermal treatments, including calcination in air and reduction in H2, can further influence the Cu particle decomposition. The mechanism of change in the Cu particle decomposition was investigated by a variety of experiments, such as X-ray diffraction and in situ X-ray absorption spectroscopy. CVD treatment of Cu/SiO2 can create Cu-rich sites, which effectively enhance the conversion and acrolein yield in selective propylene oxidation. The intermediate associated with propylene oxidation on the Cu catalysts was also examined by IR spectroscopy.

7.
Chem Sci ; 9(33): 6785-6795, 2018 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-30310611

RESUMO

We have synthesized and structurally characterized a series of centred cuboctahedral copper clusters, namely [Cu13{S2CNR2}6{C[triple bond, length as m-dash]CR'}4](PF6), 1a-d (where a: R = n Bu, R' = CO2Me; b: R = n Bu, R' = CO2Et; c: R = iPr, R' = CO2Et; d: R = n Pr, R' = 3,5-(CF3)2C6H3); [Cu12(µ12-S){S2CNR2}6{C[triple bond, length as m-dash]CR'}4], 2a-c; [Cu12(µ12-Cl){S2CNR2}6{C[triple bond, length as m-dash]CR'}4](PF6), 3a-e (where e: R = n Bu, R' = Ph); [Cu12(µ12-Br){S2CN n Bu2}6{C[triple bond, length as m-dash]CPh}4](PF6), 4e; and [Cu12(µ12-Cl)(µ3-Cl){S2CN n Bu2}6{C[triple bond, length as m-dash]CCO2Me}3]+ 5a. Cluster 1a is the first structurally characterized copper cluster having a Cu13 centered cuboctahedral arrangement, a miniature of the bulk copper fcc structure. Furthermore, the partial Cu(0) character in the 2-electron superatoms 1 was confirmed by XANES. Inverse coordination clusters 2-5 are the first examples of copper clusters containing main group elements (Cl, Br, S) with a hyper-coordination number, twelve. A combined theoretical and experimental study was performed, which shows that the central copper (formally Cu1-) in nanoclusters 1 can be replaced by chalcogen/halogen atoms, resulting in the formation of clusters 2-5 which show enhanced luminescence properties and increase in the ionic component of the host-guest interaction as Br ≈ Cl > S > Cu, which is consistent with the Cu-X Wiberg indices. The new compounds have been characterized by ESI-MS, 1H, 13C NMR, IR, UV-visible, emission spectroscopy, and the structures 2a-b, 3d-e, 4e and 5a were established by X-ray diffraction analysis.

8.
Nanoscale ; 10(30): 14719-14725, 2018 Aug 02.
Artigo em Inglês | MEDLINE | ID: mdl-30043036

RESUMO

In the present work, chitin microspheres were impregnated at different concentrations of palladium salt solution to generate the precursor-Pd2+/chitin, and then a series of size-controllable palladium nanocatalysts (Pd@chitin) were successfully constructed by calcining the composite microspheres. Transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) provided reliable evidence for well-dispersed and ultrafine palladium nanoparticles (Pd NPs) with mean diameters from about 1 to 3 nm. Chitin microspheres, as the supporting framework of these catalysts, played a significant role for stabilizing the highly dispersed Pd NPs based on their abundant functional groups and large surface areas. Moreover, the chitin matrix acted as a reductant for the precursor-Pd2+ during calcination, and the calcination process made Pd@chitin more stable. These Pd@chitin catalysts were further tested for the hydrogenations of styrene and benzaldehyde, and they displayed superior catalytic activities compared to commercial Pd/C and unsupported homogeneous Pd(OAc)2 catalysts. Notably, the most active catalyst of 1.2 wt% Pd@chitin had a highly competitive turnover frequency (TOF) of 50 000 h-1 in the hydrogenation of styrene, and the catalyst could be repeatedly used for more than 10 cycles with no decay of the catalytic activity, suggesting potential industrial applications.


Assuntos
Quitina/química , Nanopartículas Metálicas/química , Paládio/química , Hidrogenação , Microesferas
9.
Chem Commun (Camb) ; 54(12): 1481-1484, 2018 Feb 11.
Artigo em Inglês | MEDLINE | ID: mdl-29359210

RESUMO

Environmentally benign iron catalysts promote a wide variety of chemical transformations; however, insight into the mechanism and active intermediates is far from satisfactory, and the main difficulties lie in directly "seeing" the active species under "live" catalytic conditions. Herein, an unknown sextet Ph(THF)FeCl2 species was well-characterized in a live FeCl3-PhZnCl reaction system for the first time by Raman, in situ IR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopic (XAS) and density functional theory (DFT) calculations. This work provides insight into the structure and reactivity of catalytically relevant σ-aryliron(iii) species, and shall provide useful guidelines for understanding iron chemistry.

10.
Nanotechnology ; 29(6): 064002, 2018 Feb 09.
Artigo em Inglês | MEDLINE | ID: mdl-29176050

RESUMO

The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (α-Fe2O3) nanorods was investigated with reference to electronic structures by in situ synchrotron x-ray absorption spectroscopy (XAS). Current density-potential measurements demonstrate that the PEC activity of α-Fe2O3 nanorods depends strongly on the species and concentrations of dopants. The doping of α-Fe2O3 nanorods with a low level of Nb or Ta can improve their electrical conductivity and thereby facilitate charge transport and reduced electron-hole recombination therein. The photoconversion effects of Nb and Ta-doped α-Fe2O3 by in situ XAS in the dark and under illumination revealed opposite evolutions of the spectral intensities of the Fe L-edge and Nb/Ta L-edge, indicating that charge transfer and a conduction pathway are involved in the photoconversion. Analytic in situ XAS results reveal that the α-Fe2O3 that is doped with a low level of Nb has a greater photoconversion efficiency than that doped with Ta because Nb sites are more active than Ta sites in α-Fe2O3. The correlation between PEC activity and the electronic structure of Nb/Ta-doped α-Fe2O3 is examined in detail using in situ XAS and helps to elucidate the mechanism of PEC water splitting in terms of the electronic structure.

11.
Small ; 14(2)2018 01.
Artigo em Inglês | MEDLINE | ID: mdl-29134759

RESUMO

One of the key challenges in artificial photosynthesis is to design a photocatalyst that can bind and activate the CO2 molecule with the smallest possible activation energy and produce selective hydrocarbon products. In this contribution, a combined experimental and computational study on Ni-nanocluster loaded black TiO2 (Ni/TiO2[Vo] ) with built-in dual active sites for selective photocatalytic CO2 conversion is reported. The findings reveal that the synergistic effects of deliberately induced Ni nanoclusters and oxygen vacancies provide (1) energetically stable CO2 binding sites with the lowest activation energy (0.08 eV), (2) highly reactive sites, (3) a fast electron transfer pathway, and (4) enhanced light harvesting by lowering the bandgap. The Ni/TiO2[Vo] photocatalyst has demonstrated highly selective and enhanced photocatalytic activity of more than 18 times higher solar fuel production than the commercial TiO2 (P-25). An insight into the mechanisms of interfacial charge transfer and product formation is explored.

12.
Chem Commun (Camb) ; 53(64): 8984-8987, 2017 Aug 08.
Artigo em Inglês | MEDLINE | ID: mdl-28744532

RESUMO

In this work, we develop an oxidation-induced C-H functionalization strategy, which not only leads to a new avenue to build C-N bonds, but also leads to different site-selectivity compared with "classic directing-groups". The high selectivity of our new catalytic system originates from the heterogeneous electron-density distribution of the radical cation species which are induced by single electron transfer between the aromatics and oxidant-Cu(ii) species.

13.
Chem Commun (Camb) ; 53(50): 6736-6739, 2017 Jun 20.
Artigo em Inglês | MEDLINE | ID: mdl-28589197

RESUMO

In this study, we broke through the directing function of the amide group. The coordination interaction between metal and directing-group enhanced the reactivity of the substrate. Using this strategy, we realized the selective amination of 8-aminoquinolines at the C5 position via employing azoles as the source of amine. Various kinds of 8-aminoquinolines and different substituted azoles were compatible to afford the corresponding C-N coupling products.

14.
ACS Appl Mater Interfaces ; 9(28): 23995-24004, 2017 Jul 19.
Artigo em Inglês | MEDLINE | ID: mdl-28605911

RESUMO

Searching for a non-rare-earth-based oxide red-emitting phosphor is crucial for phosphor-converted light-emitting diodes (LEDs). In this study, we optimized a blue and UV-light excited Sr4Al14O25:Mn phosphor exhibiting red emission peaked at ∼653 nm, which was successfully synthesized by solid-state reaction. The crystal structure, micromorphology, and luminescent properties of Sr4Al14O25:Mn phosphors were characterized by X-ray Rietveld refinement, high-resolution transmission electron microscopy, and photoluminescence spectra. The band gap and electronic structure of Sr4Al14O25 were analyzed by density functional theory calculations using the hybrid exchange-correlation functional. The crystal field environment effect of Al sites from introducing activator Mn ions was investigated with the aid of Raman 27Al nuclear magnetic resonance spectra and electron spin resonance. The pressure dependent luminescent properties and decay time of this compound were presented. The tricolor display spectrum by combining blue InGaN chips, commercial ß-SiAlON:Eu2+ green phosphor, and Sr4Al14O25:Mn red phosphor were evaluated for commercial applications: using the present Sr4Al14O25:Mn red phosphor converted LED as a backlighting source.

15.
Org Lett ; 19(9): 2330-2333, 2017 05 05.
Artigo em Inglês | MEDLINE | ID: mdl-28421769

RESUMO

The first example of DMSO serving not only as a solvent but also as an oxidant to promote the oxidation of Cu(I) to Cu(II) has been demonstrated. X-ray absorption and electron paramagnetic resonance evidence revealed a single-electron redox process where DMSO could oxidize Cu(I) to Cu(II). The novel discovery guided the rational design of copper-catalyzed oxidative cyclization of aryl ketones with styrenes to furans, providing a new method for the synthesis of multiaryl-substituted furans from cheap and readily available starting materials.

16.
Org Lett ; 19(8): 2170-2173, 2017 04 21.
Artigo em Inglês | MEDLINE | ID: mdl-28406640

RESUMO

A cobalt-catalyzed reaction protocol is developed to achieve the intramolecular oxidative C(sp3)-H/N-H carbonylation of aliphatic amides with CO. Various substituted propanamides are selectively transformed into corresponding succinimides in good to high yields. Notably, predominant selectivity for the carbonylation at the α-methyl groups of linear aliphatic amides is observed in this reaction system.

17.
Nat Commun ; 8: 14794, 2017 03 16.
Artigo em Inglês | MEDLINE | ID: mdl-28300072

RESUMO

As a versatile metal, copper has demonstrated a wide application in acting as both organometallic reagent and catalyst. Organocuprates are among the most used organometallic reagents in the formation of new carbon-carbon bonds in organic synthesis. Therefore, revealing the real structures of organocuprates in solution is crucial to provide insights into the reactivity of organocuprates. Here we provide several important insights into organocuprate chemistry. The main finding contains the following aspects. The Cu(0) particles were detected via the reduction of CuX by nBuLi or PhLi. The Cu(II) precursors CuX2 (X=Cl, Br) could be used for the preparation of Gilman reagents. In addition, we provide direct evidence for the role and effect of LiX in organocuprate synthesis. Moreover, the EXAFS spectrum provides direct evidence for the exact structure of Li+ CuX2- ate complex in solution. This work not only sheds important light on the role of LiX in the formation of organocuprates but also reports two new routes for organocuprate synthesis.

18.
Chemistry ; 22(51): 18331-18334, 2016 Dec 19.
Artigo em Inglês | MEDLINE | ID: mdl-27862461

RESUMO

Transition-metal mediated C-S bond formation using thiol compounds has been widely used in recent years. However, there has been less focus on the interaction between the metal and thiol compounds. In this work, we have successfully evidenced the single-electron transfer between CuX2 and thiophenol utilizing EXAFS. The fitting EXAFS results reveal that two halide anions are coordinated with the CuI center, whereas no sulfur atom is observed in the first coordination sphere. This CuI ate complex serves as the key intermediate for the proton transfer in the application of Markovnikov-type hydrothiolation reactions.

19.
ACS Appl Mater Interfaces ; 8(45): 30677-30682, 2016 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-27802017

RESUMO

A new concept called "full-spectrum lighting" has attracted considerable attention in recent years. Traditional devices are usually combined with ultraviolet-light-emitting diode (LED), red, green, and blue phosphors. However, a cyan cavity exists in the 480-520 nm region. Hence, cyan phosphors are needed to compensate for the cavity. (Sr,Ba)5(PO4)3Cl:Eu2+ phosphors feature an extremely unique and tunable photoluminescence spectrum. Nevertheless, the tuning mechanisms of these phosphors remain unclear. In this study, we elucidate the mechanism of the cation size-controlled activator uneven-occupation and reoxidation in (Sr,Ba)5(PO4)3Cl:Eu2+ phosphors. This mechanism could help tune the optical properties of related apatite families and structures with multiple cation sites and strongly uneven occupation of activators and cations. Finally, the package of the LED device is constructed to show that both color rendering index Ra and R9 are higher than 95. Thus, the device could be a potential candidate for full-spectrum lighting.

20.
ACS Appl Mater Interfaces ; 8(30): 19612-7, 2016 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-27403812

RESUMO

Red phosphors (e.g., SrLiAl3N4:Eu(2+)) with high thermal stability and narrow-band properties are urgently explored to meet the next-generation high-power white light-emitting diodes (LEDs). However, to date, synthesis of such phosphors remains an arduous task. Herein, we report, for the first time, a facile method to synthesize SrLiAl3N4:Eu(2+) through Sr3N2, Li3N, Al, and EuN under atmospheric pressure. The as-synthesized narrow-band red-emitting phosphor exhibits excellent thermal stability, including small chromaticity shift and low thermal quenching. Intriguingly, the title phosphor shows an anomalous increase in theoretical lumen equivalent with the increase of temperature as a result of blue shift and band broadening of the emission band, which is crucial for high-power white LEDs. Utilizing the title phosphor, commercial YAG:Ce(3+), and InGaN-based blue LED chip, a proof-of-concept warm white LEDs with a color rendering index (CRI) of 91.1 and R9 = 68 is achieved. Therefore, our results highlight that this method, which is based on atmospheric pressure synthesis, may open a new means to explore narrow-band-emitting nitride phosphor. In addition, the underlying requirements to design Eu(2+)-doped narrow-band-emitting phosphors were also summarized.

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