Search

An ultrathin (thickness 1.0 nm) 2D nitrogen-doped hierarchically porous carbon (2DNHPC) film is developed by the nanocasting method; for comparison, a 3D nitrogen-doped ordered mesoporous carbon (3DNOMC) is also synthesized. Characterizations reveal that 2DNHPC is featured by an extremely high aspect ratio (several hundred) and a bimodal pore distribution. Such a 2D hierarchically porous structure is found to facilitate both the mass transfer of the reactive species and the utilization of active site in the electrode. First, 2DNHPC yields a larger limiting current than does 3DNOMC for the oxygen reduction reaction (ORR), revealing the key role of the low dimensional structure to facilitate the mass transfer. Second, at the loading of 500 μg cm −2 , 2DNHPC shows the same kinetic current with 3DNOMC, indicating that the two catalysts have the same active site and turnover frequency. In comparison, at a lower loading of 250 μg cm −2 , the kinetic current of 2DNHPC remains unchanged, which however seriously deteriorates for 3DNOMC. This result strongly highlights the effect of the carbon dimension on the utilization efficiency of the active site. Finally, it is noted that 2DNHPC yields a comparable ORR electrocatalytic activity and long-term stability with commercial Pt catalyst in both alkaline and acid media. [ABSTRACT FROM AUTHOR]

Covalent organic framework (COF) is a novel platform to develop efficient and green safe heterogeneous photocatalysts. Additionally, multiple photoredox reactions and mechanism studies over porous catalysts remain to be further exploited. In this context, two benzofuran-embedded COFs (BF-COFs) with favorable solvent tolerance were constructed via tautomerism strategy. Attributing to the benzofuran-hetero[6]radialene knots, BF-COFs displayed as donor-acceptor properties, and exhibited wide visible light response range and well separation efficiency for photogenerated charges. Under visible-light, BF-COFs could promote the C-S bond formation reaction between β-ketoate ester and its analogues with NH 4 SCN, affording multi-substituted olefins (28 examples, upto 98% yield) efficiently. Moreover, the atomic economic strategy was also applicative to prepare L -proline derived olefins (≥ 99% ee), which could be further converted into the chiral amino-substituted thiazoles (99% yield and ≥ 99% ee). The research about BF-COFs sheds light on the green construction of C-S bond, (chiral) olefins and chiral amino thiazoles. [Display omitted] • Two benzofuran-heteroradialene joints-embedded BF-COFs were facilely constructed and proved to be donor-acceptor structures. • BF-COFs exhibit remarkable photocatalytic activity for the synthesis of multi-substituted olefins with upto 98% yield (28 examples). • BF-COFs can promote the synthesis of L -proline-derived olefins and amino-substituted thiazoles with maintained chirality (≥ 99% ee). • The photocatalytic mechanism for multi-substituted olefins synthesis were elaborated in detail. [ABSTRACT FROM AUTHOR]

Nitrogen-doped ordered mesoporous carbon (NOMC) is studied as the support to synthesize a Pt/C N-doped catalyst for methanol oxidation reaction (MOR). The effects of carbon dimension and metal loading are investigated by nitrogen ad/desorption isotherms, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical methods. Both TEM and XRD results show that platinum nanoparticles (Pt NPs) are highly dispersed on NOMC with a uniform and narrow distribution, while the dimension of NOMC has a considerable effect on the dispersion of Pt. For 10 wt% metal loading, the average diameter of Pt NPs on 2D-NOMC is 1.40 nm; smaller than that on 3D-NOMC (1.90 nm). XPS results reveal that a strong electronic interaction exists between Pt and NOMC, indicating the formation of Pt–N bonds at the interface. Such an interaction gets more pronounced in the case of low Pt loadings and low-dimensional supports; the reason is that the 2D support is more accessible to load Pt and the metal-support interface is better developed at low metal loadings. The electrochemical results are well correlated with the physiochemical characterizations. 1) The electrochemically active surface area of Pt is higher on the 2D-support than that on the 3D-one, confirming the better dispersion of Pt on NOMC-2D. 2) A positive shift in the potential is observed for the adsorption of oxygen-containing species onto Pt, which is indicative of the charge transfer from Pt to the support and the formation of Pt–N bonds. 3) Both onset and peak potentials are negatively shifted by ca . 50 mV for MOR on 10 wt% Pt/NOMC-2D, as compared with the commercial Pt/XC-72. The ratio of forward to backward current, a measure of poisoning tolerance, is 1.1 on Pt/NOMC-2D and 0.80 on Pt/XC-72R. This enhancement can be attributed to the bifunctional mechanism at the Pt/C N-doped interface. In MOR, CO-like intermediate species on Pt can be effectively stripped off by the adjacent active –OH species on carbon, generated at lower potentials. [ABSTRACT FROM AUTHOR]