An atomic- and molecular-level understanding of heterogeneous catalysis is required to characterize the nature of active sites and improve the rational design of catalysts. Achieving this level of ...characterization requires techniques that can correlate catalytic performances to specific surface structures, so as to avoid averaging effects. Tip-enhanced Raman spectroscopy combines scanning probe microscopy with plasmon-enhanced Raman scattering and provides simultaneous topographical and chemical information at the nano/atomic scale from ambient to ultrahigh-vacuum and electrochemical environments. Therefore, it has been used to monitor catalytic reactions and is proposed to correlate the local structure and function of heterogeneous catalysts. Bimetallic catalysts, such as Pd-Au, show superior performance in various catalytic reactions, but it has remained challenging to correlate structure and reactivity because of their structural complexity. Here, we show that TERS can chemically and spatially probe the site-specific chemical (electronic and catalytic) and physical (plasmonic) properties of an atomically well-defined Pd(sub-monolayer)/Au(111) bimetallic model catalyst at 3nm resolution in real space using phenyl isocyanide as a probe molecule (Fig. 1a). We observe a weakened NC bond and enhanced reactivity of phenyl isocyanide adsorbed at the Pd step edge compared with that at the Pd terrace. Density functional theory corroborates these observations by revealing a higher d-band electronic profile for the low-coordinated Pd step edge atoms. The 3nm spatial resolution we demonstrate here is the result of an enhanced electric field and distinct electronic properties at the step edges.
Covalent organic frameworks (COFs) have recently emerged as a new generation of porous polymers combining molecular functionality with the robustness and structural definition of crystalline solids. ...Drawing on the recent development of tailor-made semiconducting COFs, we report here on a new COF capable of visible-light driven hydrogen generation in the presence of Pt as a proton reduction catalyst (PRC). The COF is based on hydrazone-linked functionalized triazine and phenyl building blocks and adopts a layered structure with a honeycomb-type lattice featuring mesopores of 3.8 nm and the highest surface area among all hydrazone-based COFs reported to date. When illuminated with visible light, the Pt-doped COF continuously produces hydrogen from water without signs of degradation. With their precise molecular organization and modular structure combined with high porosity, photoactive COFs represent well-defined model systems to study and adjust the molecular entities central to the photocatalytic process.
•The thermal stability and pyrolysis mechanism of cured BPR are investigated.•The high char yield of BPR results from the formed phenyl borates during curing.•Boron oxide is formed on the surface of ...carbonized product during pyrolysis.•The formed boron oxide revealed the cleavage of O–C bonds from phenyl borates.•The graphitization degree and graphite crystallites of PR are improved by introducing boron.
Boron-containing phenolic resin (BPR) is a kind of the ablative resins with high-performance. Due to the lack of the exact knowledge concerning the pyrolysis mechanism of BPR, its development and application are greatly impeded. In the present paper, the chemical structure of the cured BPR and its structural evolution at high temperatures are investigated to clarify the reason for the high char yield of BPR. The results indicate that the high char yield of BPR is mainly attributed to the phenyl borates formed during curing, which can block parts of phenolic hydroxyl groups, and effectively inhibit their thermal decomposition reaction. Boron oxide is formed on the surface of carbonization products by the cleavage of O–C bonds from phenyl borates via pyrolysis, which avoids the release of volatile carbon dioxide and reduces the development of micro-structural defects of carbonization products. Introducing boron into PR improves the graphitization degree and graphite crystallites of carbonization products, which promotes the formation of a more ordered glassy carbon during pyrolysis. This study provides a new vision for the understanding of the high char yield of BPR, which makes it possible to develop a new ablative resin through molecular design.
Research on aggregation-induced emission (AIE) has been a hot topic. Due to enthusiastic efforts by many researchers, hundreds of AIE luminogens (AIEgens) have been generated which were mainly based ...on archetypal silole, tetraphenylethene, distyrylanthracene, triphenylethene, and tetraphenyl-1,4-butadiene,
To enlarge the family of AIEgens and to enrich their functions, new AIEgens are in high demand. In this work, we report a new kind of AIEgen based on tetraphenylpyrazine (TPP), which could be readily prepared under mild reaction conditions. Furthermore, we show that the TPP derivatives possess a good thermal stability and their emission could be fine-tuned by varying the substituents on their phenyl rings. It is anticipated that TPP derivatives could serve as a new type of widely utilized AIEgen, based on their facile preparation, good thermo-, photo- and chemostabilities, and efficient emission.
The performance of anion exchange membrane fuel cells (AEMFCs) employing Pt or PtRu electrocatalyst and ionomers with different polyaromatic backbones is correlated with the density functional theory ...(DFT)-calculated adsorption energies of the ionomer fragments on the metal surfaces. The performance of the AEMFCs tested in this work significantly changes depending on the backbone structure of polyaromatic ionomer or the type of the catalyst used at the anode. For the same anode catalyst, the performance decreases in the order poly(fluorene) > poly(p-biphenyl alkylene) > poly(terphenyl alkylene)s, which is in excellent agreement with the decrease in the DFT-calculated interaction energies between the catalyst surface and the corresponding ionomer fragment. Namely, DFT-calculated adsorption energies decrease in the order: p-terphenyl ≥ m-terphenyl > biphenyl > diphenyl ether > benzene ≥ o-terphenyl > 9,9-dimethyl fluorene. The trend in the adsorption energies is explained on the basis of the structural and conformational features of the ionomer fragments. Namely, strong adsorption of the polyaromatic ionomer fragments correlates with the number of benzene rings with a low rotational barrier that can bind parallel to the metal surfaces, leading to strong interaction and hybridization of the aromatic π-orbitals with metal electronic states. The results of this work suggest, therefore, that the interaction between the ionomer and electrocatalyst should be taken into account when designing high-performing ionomers even before considering other factors such as hydroxide conductivity, gas permeability, and water uptake of the ionomeric binder.
Synthesis of a carbon nanobelt Povie, Guillaume; Segawa, Yasutomo; Nishihara, Taishi ...
Science (American Association for the Advancement of Science),
04/2017, Letnik:
356, Številka:
6334
Journal Article
Recenzirano
Odprti dostop
The synthesis of a carbon nanobelt, comprising a closed loop of fully fused edge-sharing benzene rings, has been an elusive goal in organic chemistry for more than 60 years. Here we report the ...synthesis of one such compound through iterative Wittig reactions followed by a nickel-mediated aryl-aryl coupling reaction. The cylindrical shape of its belt structure was confirmed by x-ray crystallography, and its fundamental optoelectronic properties were elucidated by ultraviolet-visible absorption, fluorescence, and Raman spectroscopic studies, as well as theoretical calculations. This molecule could potentially serve as a seed for the preparation of structurally well-defined carbon nanotubes.
Fundamental insight into excimer formation has been gained by using 9,10-bis4-(9-carbazolyl)phenylanthracene (
) as a probe.
exhibits a highly emissive blue fluorescence in solution and is found to ...emit a panchromatic white light spectrum (400-750 nm) in film, powder and single crystal, in which an additional excimer band appears at ∼550 nm. Detailed structural analyses, emission relaxation dynamics and a theoretical approach conclude the formation of an anthracene*/phenyl ring excimer through an overlap between π* (anthracene) and π (phenyl ring) orbitals in a face-to-edge stacking orientation. The rate of excimer formation is determined to be 2.2 × 10
s
at room temperature, which requires coupling with lattice motion with an activation energy of 0.44 kcal mol
. Exploiting
as a single emitter, a fluorescent white organic light emitting diode (WOLED) is fabricated with a maximum external quantum efficiency (
) of 3.6% at 1000 cd m
(4.2 V) and Commission Internationale de L'Eclairage (CIE) coordinates of (0.30, 0.33). The white-light
reveals a preferred orientation of the transition dipole moment in the emitting layer to enhance light outcoupling. This non-doped, single component (
) WOLED greatly reduces the complexity of the fabrication process, rendering a green and cost-effective alternative among the contemporary display/lighting technologies.
A visible-light initiated oxidative cyclization of phenyl propiolates with sulfinic acids has been developed. The arylsulfonylation of alkynes was performed at room temperature under metal-free ...conditions to generate coumarin derivatives with wide functional group tolerance, good yields and high regioselectivity.
2,3-Diphenylcyclopropenone (1) reacts with N-imidoylthioureas 2a-e to form the pyrimidin-4(3H)-ones 5a-e. The reaction mechanism can be described as due to stepwise addition accompanied by ...elimination of phenyl isothiocyanate.
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