40 research outputs found

    Choses et gens de la Martinique, ce que la Martinique demande à la France / André Delaunay-Belleville

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    Unravelling Thiol’s Role in Directing Asymmetric Growth of Au Nanorod–Au Nanoparticle Dimers

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    Asymmetric nanocrystals have practical significance in nanotechnologies but present fundamental synthetic challenges. Thiol ligands have proven effective in breaking the symmetric growth of metallic nanocrystals but their exact roles in the synthesis remain elusive. Here, we synthesized an unprecedented Au nanorod–Au nanoparticle (AuNR–AuNP) dimer structure with the assistance of a thiol ligand. On the basis of our experimental observations, we unraveled for the first time that the thiol could cause an inhomogeneous distribution of surface strains on the seed crystals as well as a modulated reduction rate of metal precursors, which jointly induced the asymmetric growth of monometallic dimers

    Atomic Resolution Imaging of Nanoscale Structural Ordering in a Complex Metal Oxide Catalyst

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    The determination of the atomic structure of a functional material is crucial to understanding its “structure-to-property” relationship (e.g., the active sites in a catalyst), which is however challenging if the structure possesses complex inhomogeneities. Here, we report an atomic structure study of an important MoVTeO complex metal oxide catalyst that is potentially useful for the industrially relevant propane-based BP/SOHIO process. We combined aberration-corrected scanning transmission electron microscopy with synchrotron powder X-ray crystallography to explore the structure at both nanoscopic and macroscopic scales. At the nanoscopic scale, this material exhibits structural and compositional order within nanosized “domains”, while the domains show disordered distribution at the macroscopic scale. We proposed that the intradomain compositional ordering and the interdomain electric dipolar interaction synergistically induce the displacement of Te atoms in the Mo–V–O channels, which determines the geometry of the multifunctional metal oxo-active sites

    Cu-TDPAT, an <i>rht</i>-Type Dual-Functional Metal–Organic Framework Offering Significant Potential for Use in H<sub>2</sub> and Natural Gas Purification Processes Operating at High Pressures

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    The separations of CO<sub>2</sub>/CO/CH<sub>4</sub>/H<sub>2</sub>, CO<sub>2</sub>/H<sub>2</sub>, CH<sub>4</sub>/H<sub>2</sub>, and CO<sub>2</sub>/CH<sub>4</sub> mixtures at pressures ranging to 7 MPa are important in a variety of contexts, including H<sub>2</sub> production, natural gas purification, and fuel-gas processing. The primary objective of this study is to demonstrate the selective adsorption potential of an <i>rht</i>-type metal–organic framework [Cu<sub>3</sub>(TDPAT)­(H<sub>2</sub>O)<sub>3</sub>]·10H<sub>2</sub>O·5DMA (Cu-TDPAT), possessing a high density of both open metal sites and Lewis basic sites. Experimental high pressure pure component isotherm data for CO<sub>2</sub>, CO, CH<sub>4</sub>, and H<sub>2</sub> are combined with the Ideal Adsorbed Solution Theory (IAST) for estimation of mixture adsorption equilibrium. The separation performance of Cu-TDPAT is compared with four other microporous materials, specifically chosen in order to span a wide range of physicochemical characteristics: MgMOF-74, MIL-101, LTA-5A, and NaX. For all mixtures investigated, the capacity of Cu-TDPAT to produce the desired product, H<sub>2</sub> or CH<sub>4</sub>, satisfying stringent purity requirements, in a fixed bed operating at pressures exceeding about 4 MPa, is either comparable to, or exceeds, that of other materials

    Investigating the Influence of Mesoporosity in Zeolite Beta on Its Catalytic Performance for the Conversion of Methanol to Hydrocarbons

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    Hierarchically porous zeolite Beta (Beta-MS) synthesized by a soft-templating method contains remarkable intracrystalline mesoporosity, which reduces the diffusion length in zeolite channels down to several nanometers and alters the distribution of Al among distinct crystallographic sites. When it was used as a catalyst for the conversion of methanol to hydrocarbons (MTH) at 330 °C, Beta-MS exhibited a 2.7-fold larger conversion capacity, a 2.0-fold faster reaction rate, and a remarkably longer lifetime in comparison to conventional zeolite beta (Beta-C). The superior catalytic performance of Beta-MS is attributed to its hierarchical structure, which offers full accessibility to all catalytically active sites. In contrast, Beta-C was easily deactivated because a layer of coke quickly deposited on the outer surfaces of the catalyst crystals, impeding access to interior active sites. This difference is clearly demonstrated by using electron microscopy combined with electron energy loss spectroscopy to probe the distribution of coke in the deactivated catalysts. At both low and high conversions, ranging from 20% to 100%, Beta-MS gave higher selectivity toward higher aliphatics (C<sub>4</sub>–C<sub>7</sub>) but lower ethene selectivity in comparison to Beta-C. Therefore, we conclude that a hierarchical structure decreases the residence time of methylbenzenes in zeolite micropores, disfavoring the propagation of the aromatic-based catalytic cycle. This conclusion is consistent with a recent report on ZSM-5 and is also strongly supported by our analysis of soluble coke species residing in the catalysts. Moreover, we identified an oxygen-containing compound, 4-methylbenzaldehyde, in the coke, which has not been observed in the MTH reaction before

    Synthesis and Gas Transport Properties of Hydroxyl-Functionalized Polyimides with Intrinsic Microporosity

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    A newly designed diamine monomer, 3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′-diol, was successfully used to synthesize two types of polyimides for membrane-based gas separation applications. The novel polymers integrate significant microporosity and polar hydroxyl groups, showing the combined features of polymers of intrinsic microporosity (PIMs) and functional polyimides (PIs). They possess high thermal stability, good solubility, and easy processability for membrane fabrication; the resulting membranes exhibit good permeability owing to the intrinsic microporosity introduced by the highly contorted PIM segments as well as high CO<sub>2</sub>/CH<sub>4</sub> selectivity that arises from the hydroxyl groups. The membranes show CO<sub>2</sub>/CH<sub>4</sub> selectivities of >20 when tested with a 1:1 CO<sub>2</sub>/CH<sub>4</sub> mixture for feed pressures up to 50 bar. In addition, the incorporation of hydroxyl groups and microporosity in the polymers enhances their affinity to water, leading to remarkable water sorption capacities of up to 22 wt % at 35 °C and 95% relative humidity

    Chiral Gold Nanowires with Boerdijk–Coxeter–Bernal Structure

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    A Boerdijk–Coxeter–Bernal (BCB) helix is made of linearly stacked regular tetrahedra (<i>tetrahelix</i>). As such, it is chiral without nontrivial translational or rotational symmetries. We demonstrate here an example of the chiral BCB structure made of totally symmetrical gold atoms, created in nanowires by direct chemical synthesis. Detailed study by high-resolution electron microscopy illustrates their elegant chiral structure and the unique one-dimensional “pseudo-periodicity”. The BCB-type atomic packing mode is proposed to be a result of the competition and compromise between the lattice and surface energy

    Strong Metal–Support Interactions Achieved by Hydroxide-to-Oxide Support Transformation for Preparation of Sinter-Resistant Gold Nanoparticle Catalysts

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    The strong metal–support interactions (SMSI) are well-known but crucial for preparation of supported metal nanoparticle catalysts, which generally occur by reduction and oxidation under harsh conditions. Here, we delineate the example of constructing SMSI without reduction and oxidation, where the key is to employ a hydroxide-to-oxide support transformation. The covering of Au nanoparticles by oxides, electronic interaction, and changes in CO adsorption tests of the catalyst are identical to those of the classic SMSI. Owing to the SMSI with oxide barriers on the Au nanoparticles, the supported Au catalysts are sintering-resistant at high temperatures, which benefit long-life reactions, outperforming the conventional supported catalysts

    Site-Specific Growth of Au–Pd Alloy Horns on Au Nanorods: A Platform for Highly Sensitive Monitoring of Catalytic Reactions by Surface Enhancement Raman Spectroscopy

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    Surface-enhanced Raman scattering (SERS) is a highly sensitive probe for molecular detection. The aim of this study was to develop an efficient platform for investigating the kinetics of catalytic reactions with SERS. To achieve this, we synthesized a novel Au–Pd bimetallic nanostructure (HIF-AuNR@AuPd) through site-specific epitaxial growth of Au–Pd alloy horns as catalytic sites at the ends of Au nanorods. Using high-resolution electron microscopy and tomography, we successfully reconstructed the complex three-dimensional morphology of HIF-AuNR@AuPd and identified that the horns are bound with high-index {11<i>l</i>} (0.25 < <i>l</i> < 0.43) facets. With an electron beam probe, we visualized the distribution of surface plasmon over the HIF-AuNR@AuPd nanorods, finding that strong longitudinal surface plasmon resonance concentrated at the rod ends. This unique crystal morphology led to the coupling of high catalytic activity with a strong SERS effect at the rod ends, making HIF-AuNR@AuPd an excellent bifunctional platform for <i>in situ</i> monitoring of surface catalytic reactions. Using the hydrogenation of 4-nitrothiophenol as a model reaction, we demonstrated that its first-order reaction kinetics could be accurately determined from this platform. Moreover, we clearly identified the superior catalytic activity of the rod ends relative to that of the rod bodies, owing to the different SERS activities at the two positions. In comparison with other reported Au–Pd bimetallic nanostructures, HIF-AuNR@AuPd offered both higher catalytic activity and greater detection sensitivity
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