12 research outputs found

    Acid Properties of Nanocarbons and Their Application in Oxidative Dehydrogenation

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    Carbon is emerging as an important metal-free catalyst for multiple types of heterogeneous catalysis, including thermocatalysis, photocatalysis, and electrocatalysis. However, the study of mechanisms for carbon catalysis has been impeded at an early stage due to the lack of quantitative research, especially the intrinsic kinetics (e.g., intrinsic TOF). In many carbon-catalyzed reactions, the surface oxygenated groups were found to be the active sites. Recently, we have shown that these oxygenated groups could be identified and quantified via poisoning by small organic molecules; however, these small molecules were toxic. As most of the oxygenated groups are acidic groups, they could also be identified and quantified with respect to the acid properties. More importantly, the method based on acid properties is very green and environmentally benign, because only inorganic bases are added. In this work, the acid properties of carbon nanotubes (CNTs) treated by concentrated HNO<sub>3</sub> were thoroughly studied by mass titration and Boehm titration. The two titration methods were also compared to the conventional methods for acidity analysis including NH<sub>3</sub> pulse adsorption, NH<sub>3</sub>-TPD, and FT-IR. Boehm titration was very effective to quantify the carboxylic acid, lactone, phenol, and carbonyl groups, and the findings were consistent with the results from XPS and NH<sub>3</sub> pulse adsorption. These CNTs were applied in the oxidative dehydrogenation (ODH) of ethylbenzene, and the activity of these catalysts exhibited a good linear dependence on the number of carbonyl groups. The value of TOF for the carbonyl group obtained from Boehm titration was 3.2 Ɨ 10<sup>ā€“4</sup> s<sup>ā€“1</sup> (245 Ā°C, atmosphere pressure, 2.8 kPa ethylbenzene, 5.3 kPa O<sub>2</sub>). For better understanding the acidity of nanocarbon, these CNTs were also applied in two acid-catalyzed reactions (Beckmann rearrangement and ring opening), and a good linear relationship between the conversion and the number of acidic sites was found

    Enhanced Chemoselective Hydrogenation through Tuning the Interaction between Pt Nanoparticles and Carbon Supports: Insights from Identical Location Transmission Electron Microscopy and Xā€‘ray Photoelectron Spectroscopy

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    Ultrasmall-sized platinum nanoparticles (Pt NPs) (āˆ¼1 nm) supported on carbon nanotubes (CNTs) with nitrogen doping and oxygen functional groups were synthesized and applied in the catalytic hydrogenation of nitroarenes. The advanced identical location transmission electron microscopy (IL-TEM) method was applied to probe the structure evolution of the Pt/CNT catalysts in the reaction. The results indicate that Pt NPs supported on CNTs with a high amount of nitrogen doping (Pt/H-NCNTs) afford 2-fold activity to that of Pt NPs supported on CNTs with oxygen functional groups (Pt/oCNTs) and 4-fold to that of the commercial Pt NPs supported on active carbon (Pt/C) catalyst toward nitrobenzene. The catalytic performance of Pt/H-NCNTs remained constant during four cycles, whereas the activity of the Pt/oCNTs was halved at the second cycle. Compared with Pt/oCNTs, Pt/H-NCNTs exhibited a higher selectivity (>99%) in chemoselective hydrogenation of halonitrobenzenes to haloanilines due to the electron-rich chemical state of Pt NPs. The strong metalā€“support interaction along with the electron-donor capacity of nitrogen sites on H-NCNTs are capable of stabilizing the Pt NPs and achieving related catalytic recyclability as well as approximately 100% selectivity. The catalyst also delivers exclusively selective hydrogenation toward nitro groups for a wide scope of substituent nitroarenes into their corresponding anilines

    Insight into the Enhanced Selectivity of Phosphate-Modified Annealed Nanodiamond for Oxidative Dehydrogenation Reactions

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    Due to a lack of fundamental understanding of the surface properties of nanocarbons, tuning their surface active sites for higher selectivity of oxidative dehydrogenation reactions has always been a great challenge for carbon catalysis. In this contribution, annealed nanodiamond was controllably grafted by phosphate, which was demonstrated to be an efficient way to adjust the nanodiamond surface and significantly improve the propene selectivity in the oxidative dehydrogenation reaction. We conducted an in-depth study to explore the role of phosphate modification in the reaction in terms of the interactions between phosphate and carbon surface, the evolution and preferential location of phosphorus species, the promotion mechanism, and the impact on the reaction pathway. The results revealed that phosphate preferentially reacts with the phenol groups initially present on the nanodiamond surface, and then it selectively blocks the defect sites that lead to COx formation with an increased propene selectivity. During this process, the catalyst active sites (ketonic carbonyl groups) were not affected. Such effects originated from the formation of covalent Cā€“Oā€“P bonds on the carbon surface, which was estimated as 15 wt % loading

    Revealing the Janus Character of the Coke Precursor in the Propane Direct Dehydrogenation on Pt Catalysts from a kMC Simulation

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    As the commercial catalyst in the propane direct dehydrogenation (PDH) reaction, one of the biggest challenges of Pt catalysts is coke formation, which severely reduces activity and stability. In this work, a first-principles DFT-based kinetic Monte Carlo simulation (kMC) is performed to understand the origin of coke formation, and an effective method is proposed to curb coke. The conventional DFT calculations give a complete description of the reaction pathway of dehydrogenation to propylene, deep dehydrogenation, and Cā€“C bond cracking. The rate-limiting step is identified as the dissociative adsorption of propane. Moreover, a comparison between different exchange-correlation functionals indicates the importance of van der Waals corrections for the adsorption of propane and propylene. The lateral interactions between the surface adsorbates are significant, which implies that mean field microkinetic modeling might not adequately describe the reaction process. There are two distinct stages in PDH, which are quick deactivation and steady state, respectively, as revealed from the kMC simulation. The precursor of coke mainly formed during the quick deactivation. The calculations indicate that the geometries of the active sites for the dehydrogenation and deep reactions are different. Therefore, the availability of surface sites is a crucial factor in the formation of propylene and side products. The active sites from quick deactivation are mainly occupied by C<sub>2</sub>/C<sub>1</sub> species, which are hard to remove. On the other hand, the surface sites that are left are mainly active toward dehydrogenation to propylene due to the geometry constraint. Therefore, a stable activity and selectivity is reached. Furthermore, the effect of hydrogen molecules in the input stream is also explored. The calculations indicate that the inclusion of hydrogen in PDH reactants not only enhances the forward reactions to the propylene formation but also reduces the consumption of the resulted propylene during the reaction. Therefore, hydrogen is very helpful to the selectivity increase in PDH in addition to other effects. Overall, the current study lays out a solid base for the future optimization of the Pt catalysts in PDH and we propose that the fine control of the surface sites on Pt has paramount importance in reducing coke formation

    Efficient Metal-Free Catalytic Reaction Pathway for Selective Oxidation of Substituted Phenols

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    Selective oxidation of substituted phenols to <i>p</i>-benzoquinones is known to be inefficient because of the competing Cā€“O coupling reaction caused by phenoxy radicals. The poor stability of conventional metal-based catalysts represents another bottleneck for industrial application. Here, we describe a metal-free reaction pathway in which onion-like carbon (OLC) as a low-cost catalyst exhibits excellent catalytic activity and stability in the selective oxidation of mono-, di- and trisubstituted phenols to their corresponding <i>p</i>-benzoquinones, even better than the reported metal-based catalysts (e.g., yield, stability) and industrial catalysts for particular substrates. Together with XPS, Raman, DFT calculations, and a series of comparative experiments, we demonstrate that the zigzag configuration as a type of carbon defects may play a crucial role in these reactions by stabilizing the intermediate phenoxy radicals

    Enhanced Stability of Immobilized Platinum Nanoparticles through Nitrogen Heteroatoms on Doped Carbon Supports

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    Catalysts in the form of dispersed platinum nanoparticles (Pt NPs) immobilized on carbon usually suffer from deactivation through sintering under reaction conditions. In this contribution, we report the enhanced stability of highly dispersed Pt NPs on surface-modified carbon nanotubes (CNTs) against thermal and electrochemical sintering by N heteroatoms in the N-doped carbon support. The improved antisintering property of Pt NPs under thermal condition is characterized by <i>in situ</i> transmission electron microscopy (TEM), while the stability in electrochemical methanol oxidation reaction (MOR) is further examined at <i>identical location</i> (IL) using an advanced IL-TEM technique. A correlation of the Pt NP growth with the electrochemical surface area (ECSA) and the mass activity in MOR has been inferred. Our results indicate that both the surface oxygen groups and nitrogen-doped species are responsible for the fine dispersion of Pt NPs on the surface-modified CNTs, while the Pt NPs can be effectively stabilized under thermal and electrochemical conditions through the strong metalā€“support interaction <i>via</i> N heteroatoms. We further reveal that the mass activity of Pt NP is closely associated with the ECSA rather than directly affected by N-doping to CNTs

    Assembly of Three-Dimensional Hetero-Epitaxial ZnO/ZnS Core/Shell Nanorod and Single Crystalline Hollow ZnS Nanotube Arrays

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    Hetero-epitaxial growth along three-dimensional (3D) interfaces from materials with an intrinsic large lattice mismatch is a key challenge today. In this work we report, for the first time, the controlled synthesis of vertically aligned ZnO/ZnS core/shell nanorod arrays composed of single crystalline wurtzite (WZ) ZnS conformally grown on ZnO rods along 3D interfaces through a simple two-step thermal evaporation method. Structural characterization reveals a ā€œ(01ā€“10)<sub>ZnO</sub>//(01ā€“10)<sub>ZnS</sub> and [0001]<sub>ZnO</sub>//[0001]<sub>ZnS</sub>ā€ epitaxial relationship between the ZnO core and the ZnS shell. It is exciting that arrays of single crystalline hollow ZnS nanotubes are also innovatively obtained by simply etching away the inner ZnO cores. On the basis of systematic structural analysis, a rational growth mechanism for the formation of hetero-epitaxial core/shell nanorods is proposed. Optical properties are also investigated <i>via</i> cathodoluminescence and photoluminescence measurements. Remarkably, the synthesized ZnO/ZnS core/shell heterostructures exhibit a greatly reduced ultraviolet emission and dramatically enhanced green emission compared to the pure ZnO nanorods. The present single-crystalline heterostructure and hollow nanotube arrays are envisaged to be highly promising for applications in novel nanoscale optoelectronic devices, such as UV-A photodetectors, lasers, solar cells, and nanogenerators

    Controllable Synthesis of Cobalt Monoxide Nanoparticles and the Size-Dependent Activity for Oxygen Reduction Reaction

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    In this work, carbon-supported cobalt monoxides with an average size of 3.5, 4.9, and 6.5 nm are synthesized via a facile colloidal method avoiding any surfactants of long chains. Along with controlling the CoO particle size, we investigate the dependence of ORR activity on particle size of the CoO/C composite. It is discovered that the turnover frequency of the ORR per CoO site is largely independent of the particle size in the range of 3ā€“7 nm, and the enhanced ORR activity for the smaller CoO particles is attributed to the enlarged interface between CoO and carbon

    Mesoporous and Graphitic Carbide-Derived Carbons as Selective and Stable Catalysts for the Dehydrogenation Reaction

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    Dehydrogenation of ethylbenzene to styrene is one of the most important catalytic processes in chemical industry. While it was demonstrated that nanocarbons like nanotubes, nanodiamond, or nanographite show high performance, especially selectivity, these powders give rise to handling problems, high pressure drop, hampered heat and mass transfer, and unclear health risks. More common macroscopic carbon materials like activated carbons show unsatisfying selectivity below 80%. In this study, mesoporous, graphitic, and easy to handle carbon powders were synthesized on the basis of the reactive extraction of titanium carbide in a novel temperature regime. This resulted in extraordinary properties like a mean pore diameter of up to 8 nm, pore volumes of up to 0.90 mL g<sup>ā€“1</sup>, and graphite crystallite sizes exceeding 25 nm. Exceptional styrene selectivities of up to 95% were observed for materials synthesized above 1300 Ā°C and pretreated with nitric acid. Furthermore, the long-term stability of these non-nanocarbon catalysts could be demonstrated for the first time during 120 h of time-on-stream
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