Debao Li

Controlled Synthesis of N‐Doped Carbon Nanospheres with Tailored Mesopores through Self‐Assembly of Colloidal Silica

Limited strategies have been established to prepare monodisperse mesoporous carbon nanospheres (MCNs) with tailored pore sizes. In this work, a method is reported to synthesize MCNs by combining polymerization of aniline with co-assembly of colloidal silica nanoparticles. The controlled self-assembly behavior of colloidal silica enables the formation of uniform composite nanospheres and convenient modulation over mesopores. After carbonization and removal of sacrificial templates, the resultant MCNs possess tunable mesopores (7-42 nm) and spherical diameters (90-300 nm), as well as high surface area (785-1117 m(2)  g(-1) ), large pore volume (1.46-2.01 cm(3)  g(-1) ) and abundant nitrogen moieties (5.54-8.73 at %). When serving as metal-free electrocatalysts for the oxygen reduction reaction (ORR), MCNs with an optimum pore size of 22 nm, compared to those with 7 and 42 nm, exhibit the best ORR performance in alkaline medium.

Nitrogen-Doped Nanoporous Carbon Membranes with Co/CoP Janus-Type Nanocrystals as Hydrogen Evolution Electrode in Both Acidic and Alkaline Environments

Self-supported electrocatalysts being generated and employed directly as electrodes for energy conversion has been intensively pursued in the fields of materials chemistry and energy. Herein, we report a synthetic strategy to prepare freestanding hierarchically structured, nitrogen-doped nanoporous graphitic carbon membranes functionalized with Janus-type Co/CoP nanocrystals (termed as HNDCM-Co/CoP), which were successfully applied as a highly efficient, binder-free electrode in the hydrogen evolution reaction (HER). Benefited from multiple structural merits, such as a high degree of graphitization, three-dimensionally interconnected micro/meso/macropores, uniform nitrogen doping, well-dispersed Co/CoP nanocrystals, as well as the confinement effect of the thin carbon layer on the nanocrystals, HNDCM-Co/CoP exhibited superior electrocatalytic activity and long-term operation stability for HER under both acidic and alkaline conditions. As a proof-of-concept of practical usage, a 5.6 cm × 4 cm × 60 μm macroscopic piece of HNDCM-Co/CoP was prepared in our laboratory. Driven by a solar cell, electroreduction of water in alkaline conditions (pH 14) was performed, and H2 was produced at a rate of 16 mL/min, demonstrating its potential as real-life energy conversion systems.

Nickel and manganese co-modified K/MoS2 catalyst: high performance for higher alcohols synthesis from CO hydrogenation

Abstract Mn and Ni co-modified K/MoS2 catalyst for higher alcohols synthesis from syngas was prepared by co-precipitation, and its performance in CO hydrogenation was tested. The results indicated a synergistic effect between Mn and Ni on co-modified K/MoS2 catalyst, which led to high performance for higher alcohols synthesis.

Novel preparation of nitrogen-doped graphene in various forms with aqueous ammonia under mild conditions

Nitrogen-doped graphene (N-G), in the form of a stable dispersion, a strong hydrogel and a macroporous aerogel, was synthesized by simultaneous nitrogen doping and reduction of graphene oxide with aqueous ammonia under mild conditions. The N content and thermal stability of the resultant samples proved to be high.

Synthesis of single-crystal-like nanoporous carbon membranes and their application in overall water splitting

Nanoporous graphitic carbon membranes with defined chemical composition and pore architecture are novel nanomaterials that are actively pursued. Compared with easy-to-make porous carbon powders that dominate the porous carbon research and applications in energy generation/conversion and environmental remediation, porous carbon membranes are synthetically more challenging though rather appealing from an application perspective due to their structural integrity, interconnectivity and purity. Here we report a simple bottom–up approach to fabricate large-size, freestanding and porous carbon membranes that feature an unusual single-crystal-like graphitic order and hierarchical pore architecture plus favourable nitrogen doping. When loaded with cobalt nanoparticles, such carbon membranes serve as high-performance carbon-based non-noble metal electrocatalyst for overall water splitting.

The Effect of Nitrogen on the Autoreduction of Cobalt Nanoparticles Supported on Nitrogen-Doped Ordered Mesoporous Carbon for the Fischer–Tropsch Synthesis

Nitrogen‐doped ordered mesoporous carbons (OMCs) were prepared by using a post‐synthetic method with cyanamide as a nitrogen source; they were used as supports for the fabrication of the cobalt‐based Fischer–Tropsch synthesis (FTS) catalysts. The obtained composites were well characterised by using nitrogen physisorption, Raman spectroscopy, high‐angle annular dark‐field scanning transmission electron microscopy, hydrogen chemisorption, X‐photoelectron spectroscopy, thermogravimetry–MS, and in situ XRD to investigate the effects of nitrogen on the dispersion of cobalt species and successive autoreduction behaviour of cobalt oxide as well as the catalytic performance in the FTS. The results indicate that the doped nitrogen atoms, especially the pyridine‐like nitrogen, actually serve as the anchoring sites for cobalt species. Consequently, the more uniform cobalt particle size is observed for the catalysts with nitrogen‐doped OMCs as supports in comparison with their counterparts based on the pristine OMCs. In contrast, the autoreduction temperature of cobalt oxide in the as‐synthesised catalysts lowers considerably after nitrogen doping, although slightly increased autoreduction temperature is observed for the catalysts with relatively high nitrogen content owing to the metal–support interaction. Dictated by the balance between decreasing particle size of cobalt and increasing strength of the metal–support interaction, the cobalt specific activity of the nitrogen‐doped catalysts reaches a maximum and then decreases in the FTS with increasing nitrogen content. Notably, under optimum conditions, the cobalt specific activity on the nitrogen‐doped sample with medium nitrogen content is 1.5 times higher than its analogue on the pristine OMC without compromising the selectivity to C5+ hydrocarbons.

XPS study of potassium-promoted molybdenum carbides for mixed alcohols synthesis via CO hydrogenation

Abstract The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface characteristic of potassium-promoted or un-promoted both β-Mo2Cand α-MoC1−x pretreated by syngas at different temperatures, and the promotional effect of potassium on the catalytic performance was also studied. XPS results revealed that the content of surface Mo and its valence distribution between β-Mo2Cand α-MoC1−x were quite different. Promoted by potassium, the remarkable changes were observed for surface composition and valence of Mo distribution over β-Mo2C. Potassium had strong electronic effect on β-Mo2C, which led to a higher Mo4+ content. On the contrary, potassium had little electronic effect on α-MoC1−x, and K-Mo interaction was weak. Therefore, Mo0 and Mo2+ became the dominant species on the catalyst surface, and the Mo4+ content showed almost no increase as the pretreatment temperature enhanced. In terms of catalytic performance of molybdenum carbides, the increase in Mo0 most likely explained the increase in hydrocarbon selectivity, yet Mo4+ might be responsible for the alcohols synthesis.

The intrinsic effects of shell thickness on the Fischer–Tropsch synthesis over core–shell structured catalysts

A series of core–shell catalysts with cobalt nanoparticles coated by silica shells were prepared to provide an insight into the effects of the shell thickness on the Fischer–Tropsch synthesis. The catalysts displayed uniform silica shell thicknesses in the range of 4.3–18.2 nm as ascertained by TEM. From the H2 chemisorption results, increasing the shell thickness did not reduce the number of active sites due to the similar active cobalt surface areas. Even though the reducibility determined by H2-TPR decreased rapidly with the increase in shell thickness, the catalytic activity was not evidently reduced. The hydrocarbon products shifted to shorter chains and the C15–C18 selectivity had a volcano-type dependence as the shell thickness increased, which is probably because thicker shells contribute to more severe diffusion limitations of the reactants.

Constructing Hierarchically Hollow Core–Shell MnO2/C Hybrid Spheres for High‐Performance Lithium Storage

Hierarchical MnO2 /C hybrid spheres (MCS@MnO2 ), consisting of numerous hollow core-shell MnO2 @C nanospheres, are developed via a facile deposition process. The well-defined inner voids and robust carbon framework endow MCS@MnO2 with excellent mechanical stability, efficient utilization of MnO2 , and enhanced reaction kinetics for Li-ion batteries, therefore leading to large specific capacities, superior rate capability, and long-term cycling stability.

Studies of Cobalt Particle Size Effects on Fischer–Tropsch Synthesis over Core–Shell‐Structured Catalysts

A series of core–shell‐structured catalysts that consist of different‐sized Co3O4 nano‐particles and silica shells were prepared by an in situ coating method. The reduced catalysts displayed uniform core sizes that ranged from 5.5–12.7 nm as ascertained by TEM, which concurred well with XRD analysis. The BET results revealed the highly mesoporous nature of the silica shell, which contributes to the facile access of the reactant gas to the active sites on the core particles. The degree of reduction of the calcined catalysts studied by H2 temperature‐programmed reduction was enhanced with increased Co particle size. In the Fischer–Tropsch synthesis, a volcano‐like curve was plotted as the CO conversion and Co‐time‐yield revealed a rapid growth if the particle size increased from 5.5 to 8.7 nm and then decreased with further increased particle size to 12.7 nm, which is an effect of the combination of Co dispersion and reducibility. However, the turnover frequency remained invariant for catalysts with particle sizes larger than 8.7 nm. If we consider the product selectivity, generally, larger particles led to a longer chain length of hydrocarbons with a larger chain‐growth probability. The selectivity towards methane decreased and the corresponding heavy hydrocarbons (C19+) increased continuously with the increase of particle size. The catalyst with a particle size of 8.7 nm exhibited the highest selectivity and the maximum space‐time‐yield towards middle distillates (C5–C18) because of the modest chain‐growth probability.