Jungang Wang

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.

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.

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.

The oxidizing pretreatment-mediated autoreduction behaviour of cobalt nanoparticles supported on ordered mesoporous carbon for Fischer–Tropsch synthesis

Ordered mesoporous carbons (OMCs) were employed to support cobalt species for insight into the autoreduction behaviour of cobalt oxide on carbon substrates and their catalytic performance during the Fischer–Tropsch synthesis (FTS). The as-synthesized samples were subjected to the characterization of N2-physisorption, XPS, in situ XRD, TPR, TG-MS, HAADF-STEM and H2-chemisorption. The results show that the oxidizing pretreatment of OMC substantially improves the dispersion of cobalt species. Consequently, more uniform cobalt particles are observed on the pretreated supports in comparison with their counterparts on pristine OMCs, which enables the autoreduction of cobalt oxide on the carbon supports to occur at a lower temperature owing to the weak metal–support interaction. As compared to the hydrogen-reduced samples, the morphology of cobalt particles suffers from a significant change after the autoreduction and a mechanism with respect to the anti-oriented diffusion of oxygen atoms is proposed to account for the formation of ellipsoidal or quasi-ellipsoidal particles. During the evaluation of FTS, the autoreduced catalysts exhibit higher activity than the hydrogen-reduced ones owing to more cobalt atoms being exposed on the surface. Dictated by the decreasing cobalt particles, the cobalt-specific activity of pretreated samples is ca. 2.2 times higher than its analog on the pristine OMCs under optimum conditions without any influence on the TOF or at the expense of the C5+ hydrocarbon selectivity.

Influence of the bimodal pore structure on the CO hydrogenation activity and selectivity of cobalt catalysts

A series of Co-based catalysts supported on different silica-based bimodal mesoporous materials for Fischer–Tropsch synthesis (FTs) were prepared by the incipient wetness impregnation (IWI) method. The results showed that Co-based catalysts presented a bimodal mesoporous structure. Catalysis and characterization results showed that the bimodal structure strongly influenced the dispersion of cobalt species and the F–T catalytic performance. Moreover, the F–T synthesis results showed that the catalysts with a bimodal pore size distribution of 2.5 and 8 nm or 2.5 and 11 nm had a lower methane selectivity than those with larger pores. The catalyst with a 2.5 and 22 nm pore size distribution showed the highest activity and the highest selectivity to C5–11.

Effects of macropores on reducing internal diffusion limitations in Fischer–Tropsch synthesis using a hierarchical cobalt catalyst

Internal diffusion limitations in Fischer–Tropsch catalysts strongly affects their catalytic activities and product selectivities. Large pellet catalysts demonstrate especially severe internal diffusion limitations in fixed bed reactors. In order to overcome this problem, macropores were introduced into cobalt catalysts, and the resulting effects on reaction activity and selectivity were studied. Meso–macroporous silica (S1) with mesoporous walls was prepared by a sol–gel process and was used to prepare the Co/S1 catalyst. A bimodal mesoporous silica (S2) support with an equivalent mesopore diameter to the S1 support was also prepared for comparison. The effects of internal diffusion limitations in the S1 and S2 supports with different pellet sizes on FT synthesis were investigated. The results showed that the macropores played an important role in reducing internal diffusion limitations, especially for large pellet catalysts.

Effect of hierarchical meso–macroporous structures on the catalytic performance of silica supported cobalt catalysts for Fischer–Tropsch synthesis

A series of meso–macroporous silica supports with the same macroporous diameter but different mesoporous diameters were prepared by introducing phase separation into a sol–gel process and used to prepare cobalt catalysts for Fischer–Tropsch synthesis. The mesoporous diameter could be controlled in the range 6.5–35.0 nm while the macroporous diameter was kept at approximately 500 nm. The mesoporous porosity of the meso–macroporous silica supports greatly influenced the size, reducibility and dispersion of cobalt nanoparticles, and therefore resulted in different catalytic performances for Fischer–Tropsch synthesis. The meso–macroporous catalyst with an appropriate mesoporous size of 8.5 nm displayed a higher catalytic activity due to the best combination of the Co dispersion and reduction degree. The product distribution strongly depended on the mesoporous diameter due to the following two reasons: 1) the difference in the H2/CO ratio on the active sites due to the diffusional limitations of CO in the mesopores; 2) the Co crystallite size effect. In addition, large pellet catalysts (800–1700 μm) exhibited similar product distributions to small pellet catalysts (180–250 μm), which indicated that the macropores played an important role in reducing internal diffusion limitations for large pellet catalysts.

Self-Activating, Capacitive Anion Intercalation Enables High-Power Graphite Cathodes

Developing high-power cathodes is crucial to construct next-generation quick-charge batteries for electric transportation and grid applications. However, this mainly relies on nanoengineering strategies at the expense of low scalability and high battery cost. Another option is provided herein to build high-power cathodes by exploiting inexpensive bulk graphite as the active electrode material, where anion intercalation is involved. With the assistance of a strong alginate binder, the disintegration problem of graphite cathodes due to the large volume variation of >130% is well suppressed, making it possible to investigate the intrinsic electrochemical behavior and to elucidate the charge storage kinetics of graphite cathodes. Ultrahigh power capability up to 42.9 kW kg-1 at the energy density of >300 Wh kg-1 (based on graphite mass) and long cycling life over 10 000 cycles are achieved, much higher than those of conventional cathode materials for Li-ion batteries. A self-activating and capacitive anion intercalation into graphite is discovered for the first time, making graphite a new intrinsic intercalation-pseudocapacitance cathode material. The finding highlights the kinetical difference of anion intercalation (as cathode) from cation intercalation (as anode) into graphitic carbon materials, and new high-power energy storage devices will be inspired.

Silicon carbide supported cobalt for Fischer–Tropsch synthesis: probing into the cause of the intrinsic excellent catalytic performance

The thin SixOy layer on a SiC surface is changed to Al2O3 to form Al2O3@SiC. Co/Al2O3@SiC shows distinct different catalytic behaviour with Co/SiC, indicating that the SixOy layer on the surface of SiC plays a great role in the intrinsic excellent catalytic performance of Co/SiC.