Litao Jia

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.

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.

Overview of catalyst application in petroleum refinery for biomass catalytic pyrolysis and bio-oil upgrading

Biomass is considered as an alternative source to fossil fuels for production of renewable liquid fuels and chemicals. Biomass fast pyrolysis integrated with bio-oil catalytic upgrading for liquid biofuel production has attracted much attention in recent years. Catalysts play critical roles in this process. Although many efforts have been made, the selectivity and deactivation of catalysts still remain challenges. Various catalysts have achieved great success and led to significant improvements in petroleum refining processes. The success and lessons of catalyst applications in petroleum refinery may help to make a breakthrough in biomass conversion, because there are some similarities in the reaction pathway and feedstock of the two processes. In this review, several catalysts used in petroleum refining and biofuel production are summarized and compared. Additionally, their functions and applications are discussed. The possibility of applying petroleum refining catalysts in catalytic pyrolysis and catalytic bio-oil upgrading processes are explored. The challenges and opportunities of petroleum refining catalysts for biofuel production are also summarized.

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.

Impact of tillage erosion on water erosion in a hilly landscape.

Little has been known of the interaction between tillage erosion and water erosion, while the two erosion processes was independently studied. Can tillage-induced soil redistribution lead to exaggerated (or retarded) runoff flow and sediment concentrations in steeply sloping fields? A series of simulated tillage and artificial rainfall events were applied to rectangular runoff plots (2m×8m) with a slope of 15° to examine the impacts of tillage erosion intensities on water erosion in the Yangtze Three Gorges Reservoir Area, China. Mean flow velocity, effective/critical shear stress, and soil erodibility factor K were calculated to analyze the differences in hydrodynamic characteristics induced by tillage. Our experimental results suggest that mean runoff rates were 2.26, 1.19, and 0.65Lmin(-1) and that mean soil detachment rates were 1.53, 1.01, and 0.61gm(-2)min(-1) during the 70-min simulated rainfall events for 52-, 31-, and 10-year tillage, respectively. A significant difference (P<0.05) in cumulative detachment amounts was found among different tillage intensities. Compared with the soil flux of 0kgm(-1), cumulative detachment amounts for the soil fluxes of 9.86 and 24.72kgm(-1) increased by 40.02% and 100.94%, respectively, during the 30-min rainfall event. The results imply that soil and water losses tended to increase with increasing tillage intensity. A significant difference in mean flow velocity occurred near the upper and lower slope boundaries of the field, while significant differences (P<0.05) in runoff depth and effective shear stress were observed among different slope positions. Soil erodibility factor K for the soil fluxes of 9.86 and 24.72kgm(-1) were 2.40 and 5.11 times higher, respectively, than that for the soil flux of 0kgm(-1). As tillage intensity increased, critical shear stress trended to gradually decrease for all soil fluxes. Our results indicate that tillage erosion increases soil erodibility and delivers the soil for water erosion in sloping fields, accelerating water erosion.

Determination of total phthalates in edible oils by high-performance liquid chromatography coupled with photodiode array detection

The previously reported procedure for the determination of the total phthalate in fatty food involved the extraction of phthalates using chloroform/methanol followed by the removal of the solvents before alkaline hydrolysis requiring 20 h and derivatization of phthalic acid. In this study, a phase-transfer catalyst (tetrabutylammonium chloride) was used in the liquid-liquid heterogeneous hydrolysis of phthalates in oil matrix shortening the reaction time to within 25 min. The resulting phthalic acid in the hydrolysate was extracted by a novel molecular complex based dispersive liquid-liquid microextraction method coupled with back-extraction before high-performance liquid chromatography coupled with photodiode array detection. Under the optimal experimental conditions, the linearity of the method was in the range of 0.5-12 nmol/g with the correlation coefficients (r) >0.997. The detection limit (S/N = 3) was 0.11 nmol/g. Intraday and interday repeatability values expressed as relative standard deviation were 3.9 and 7.1%, respectively. The recovery rates ranged from 82.4 to 99.0%. The developed method was successfully applied for the analysis of total phthalate in seven edible oils.

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.

The one-step oxidation of methanol to dimethoxymethane over sulfated vanadia–titania catalysts: influence of calcination temperature

Sulfated vanadia–titania catalysts were prepared by the rapid combustion method and calcined at different temperatures. The influence of calcination temperature on the physicochemical properties of the catalysts was characterized by nitrogen adsorption (BET), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2-TPR-MS), thermogravimetry (TG) and temperature programmed desorption of ammonia (NH3-TPD) techniques. The catalytic activities were evaluated by the partial oxidation of methanol to dimethoxymethane (DMM). The results showed that vanadia and sulfate were highly dispersed as the catalysts were calcined at 723 and 773 K. The reducibility of the highly dispersed vanadia was stronger than the aggregated vanadia. And the larger number of acidic sites was related to the higher dispersion of sulfate. Moreover, the higher dispersion of vanadia contributed to higher methanol conversion, and the stronger reducibility combined with the larger number of acidic sites led to high DMM selectivity. As a result, the catalysts calcined at 723 and 773 K presented higher methanol conversion and DMM selectivity than those calcined at 673 K or above 823 K.