Toward a continuous synthesis of porous carbon xerogel beads

A continuous process for producing porous carbon xerogel beads has been developed. It consists in injecting a pre‐cured aqueous solution of resorcinol and formaldehyde on top of a column filled with hot oleic acid. The latter is pumped on the top of the column and fed at the bottom, generating an upward flow that can be adjusted to match the terminal velocity of the settling beads. Thus, the bead residence time in the column can be adjusted to match the gelation time, allowing the beads to solidify before reaching the bottom of the vessel. The obtained beads are subsequently dried and pyrolyzed. The developed experimental setup proved the continuous synthesis of porous carbon beads is possible. Nevertheless, the shaping process caused various texture changes of the porous carbon, which mainly yields macropores instead of micro and mesopores. This process also leads to the build‐up of a denser skin around the beads. © 2018 American Institute of Chemical Engineers AIChE J, 64: 1049–1058, 2018     

Template method synthesis of mesoporous carbon spheres and its applications as supercapacitors

Mesoporous carbon spheres (MCS) have been fabricated from structured mesoporous silica sphere using chemical vapor deposition (CVD) with ethylene as a carbon feedstock. The mesoporous carbon spheres have a high specific surface area of 666.8 m²/g and good electrochemical properties. The mechanism of formation mesoporous carbon spheres (carbon spheres) is investigated. The important thing is a surfactant hexadecyl trimethyl ammonium bromide (CTAB), which accelerates the process of carbon deposition. An additional advantage of this surfactant is an increase the yield of product. These mesoporous carbon spheres, which have good electrochemical properties is suitable for supercapacitors.     

One-step synthesis of iron-oxide-loaded functionalized carbon spheres

Size- and surface-tunable carbon spheres loaded with iron-oxide (α-Fe₂O₃ or Fe₃O₄) nanoparticles have been obtained from 4-ferrocenyl-butyric acid via an easy one-step method in a solvothermal system. The composites obtained have been fully characterized by X-ray powder diffraction, scanning and transmission electron microscopy, Fourier transform infrared spectrometry, Raman spectrometry and vibrating sample magnetometry. The results show that the diameters and morphologies of the spheres can be tuned by adjusting the experimental parameters including the reaction temperature, time, medium, and precursor concentration. Most of the iron-oxide nanoparticles are evenly distributed on the surface of the carbon spheres and also the surface of the iron-oxide-loaded carbon spheres has large numbers of functional groups. The α-Fe₂O₃-loaded carbon spheres show special magnetic properties with a coercivity of 101.5 Oe and a remanent magnetization of 0.203 emu/g at room temperature, while the coercivity and remanent magnetization for the Fe₃O₄-loaded carbon spheres are 58.4 Oe and 0.174 emu/g, respectively. A possible formation mechanism is proposed and discussed based on the features of the reaction system.     

Catalyst-free synthesis of multi-walled carbon nanotubes from carbon spheres and its implications for the formation mechanism

A route to producing multi-walled carbon nanotubes (MWCNTs) was reported, in which polyacrylonitrile microspheres (PANMSs) were assembled into one-dimensional strings of carbon spheres and transformed to carbon nanotubes at 1000 °C. It was found that the diameters of the MWCNTs are uniform and correlated with the size of the PANMSs used. Structures of intermediate products obtained by stopping the reaction have been examined by high-resolution transmission electron microscopy. The observations indicate that the formation of MWCNTs follows a different mechanism from the well-studied vapor–liquid–solid mode. It involves a direct self-assembly and solid-state structural transformation of PANMSs under the promotion of nitrogen atoms as shown by the X-ray photoelectron spectra of the resulting samples. On the basis of these observations, a sphere-string-tube mechanism was proposed for the MWCNT formation.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

Potassium salt-assisted synthesis of highly microporous carbon spheres for CO2 adsorption

Highly microporous carbon spheres for CO₂ adsorption were prepared by using a slightly modified one-pot Stöber synthesis in the presence of potassium oxalate. Formaldehyde and resorcinol were used as carbon precursors, ammonia as a catalyst, and potassium oxalate as an activating agent. The resulting potassium salt-containing phenolic resin spheres were simultaneously carbonized and activated at 800 °C in flowing nitrogen. Carbonization of the aforementioned polymeric spheres was accompanied by their activation, which resulted in almost five-time higher specific surface area and total pore volume, and almost four-time higher micropore volume as compared to analogous properties of the carbon sample prepared without the salt. The proposed synthesis resulted in microporous carbon spheres having the surface area of 2130 m² g⁻¹, total pore volume of 1.10 cm³ g⁻¹, and the micropore volume of 0.78 cm³ g⁻¹, and led to the substantial enlargement of microporosity in these spheres, especially in relation to fine micropores (pores below 1 nm), which enhance CO₂ adsorption. These carbon spheres showed three-time higher volume of fine micropores, which resulted in the CO₂ adsorption of 6.6 mmol g⁻¹ at 0 °C and 1 atm.     

A method for the coating of silica spheres with an ultrathin layer of pristine single-walled carbon nanotubes

We describe a method to fabricate silica gel particles coated with a monolayer of pristine single-walled carbon nanotubes (SWCNTs). SWCNTs dissolved in 1-methyl-2-pyrrolidinone (NMP) are mixed with amino-functionalized silica gels having different diameters. Strong interaction between the amino group and the SWCNT surfaces induces the adsorption of the SWCNTs on the silica, while the stable solvation in NMP hampers further adsorption of the tubes. This approach enables the production of a homogeneous, nondestructive and high-yield coating of the SWCNTs onto the silica surfaces, especially for larger sphere with a diameter over 1 μm. The density and bundling degree of the SWCNTs on the silica gel surfaces are finely controlled by simply changing the ratios of the SWCNTs to the silica gels as well as the SWCNT concentrations. We also describe the coating of the silica gels with metallic SWCNTs. The SWCNT-coated silica gels are useful for a wide range of materials, such as the stationary phase for liquid chromatography and catalyst supporting materials.     

Preparation of graphene nanowalls by a simple microwave-based method

A simple method for the preparation of graphene sheets is described which uses a conventional microwave oven to produce plasma inside a quartz tube, which is used to degrade and carbonize a carbon source (polyethylene powder) to produce graphitic nanolayers. This technique also allows deposition of the resultant graphene layers on thermally-stable surfaces in interesting morphologies. The effect of post-processing of the nanosheets by thermal treatment in air and argon was investigated.     

Synthesis of carbon xerogel particles and fractal-like structures

A variety of dense and open-architecture amorphous carbon xerogel microspheres and folded fractal-like structures were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly alkaline aqueous solution. Carbon structures were obtained by inverse emulsification of resorcinol-formaldehyde (RF) sol in cyclohexane containing a non-ionic surfactant Span-80, followed by its pyrolysis at 1173 K in nitrogen. We have investigated the effects of synthesis parameters including stirring time, resorcinol/catalyst (R/C) ratio and surfactant concentration on the structures. The average particle size of the carbon microspheres could be modulated from 5 to 46 μm by increasing the stirring time from 2 to 7 h and by varying the R/C ratio from 0.2 to 500. Particles agglomerated as the R/C ratio increases above 100. Increase in the surfactant concentration from 1% to 4% (v/v) produced smaller spherical particles with narrower size distribution. Further increase in the surfactant concentration from 10% to 50% (v/v) produced branched and folded fractal-like structures of large external area. Thus, RF sol-based precursor chemistry can be easily tuned to produce a spectrum of desired carbon particle morphologies with potential applications in printing technology, adsorbents, resonance-based solar cells, thermal detectors and carbon-based micro-electromechanical devices (C-MEMS).     

Chemical reactions between calcium carbide and chlorohydrocarbon used for the synthesis of carbon spheres containing well-ordered graphite

Carbon spheres and carbon nanospheres were prepared through the chemical reactions between calcium carbide and chlorohydrocarbon without using any catalysts. The reactants were sealed in a pressure vessel and heated to temperatures of 210-250 °C. The final products were characterized using scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS) attached to SEM, transmission electron microscopy (TEM), X-ray diffraction and Raman spectroscopy. SEM and TEM examinations show that some of the obtained products are bead-like or roe-like spheres with diameters of 100-2000 nm. The carbon nanospheres have high purity (>95%) and uniform size distribution (100-200 nm). The EDS analyses reveal that the carbon contents of the spheres are 86-94%. The results indicate that the size and the size distribution of the synthesized spheres change with the change of the reactants. The experimental yields of carbonaceous materials relative to the starting materials are about 8.0-14.4% (w/w). The yields of the spheres increase whereas their size distributions decrease with the increase of H atom and the decrease of Cl atom in chlorohydrocarbon. The average graphitization degree of sample 1 and sample 2 are 81.9% and 78.7%.     

Continuous synthesis of carbon spheres by a non-catalyst vertical chemical vapor deposition

High-purity carbon spheres were continuously produced by pyrolysis of acetylene at 1000 °C under a nitrogen atmosphere in a vertical chemical vapor deposition reactor. The produced carbon spheres had diameters in the range of 200–500 nm and were perfectly spherical in shape with rough surfaces. High resolution transmission electron microscopy analysis revealed that the carbon spheres were constructed of heavily distorted graphene layers. The results of the X-ray diffraction pattern and Raman spectra also confirmed that the presence of disordered graphene layers was due to a low graphitization degree. In addition, thermal stability and thermal oxidation of carbon spheres were studied. The results found that the surface of the carbon spheres could be modified and the amount of oxygen-containing functional groups increased after oxidation. In summary, the method provided a catalyst-free, substrate-free, hydrogen-free, and cost-effective synthesis for continuous production of carbon spheres.     

碳球的水热合成法制备及其对重金属离子的净化

以可溶性淀粉为原料,采用水热合成法制备了碳球吸附剂,研究了淀粉溶液浓度和水热时间对所制备碳球吸附铜离子和铅离子效果的影响。结果表明,扫描电镜(SEM)显示铜离子吸附在碳球表面,傅里叶红外光谱(FT-IR)显示碳球和金属离子间产生了相互作用,碳球对铜离子和铅离子的吸附具有选择性,其对铜离子的吸附效果较好,所制备的碳球在工业废水处理中具有潜在的应用前景。     

Development of carbon nanotube and carbon xerogel supported catalysts for the electro-oxidation of methanol in fuel cells

Multiwalled carbon nanotubes and high surface area mesoporous carbon xerogel were prepared and used as supports for monometallic Pt and bimetallic Pt-Ru catalysts. In order to assess the influence of the oxygen surface groups of the support, the mesoporous carbon xerogel was also oxidized with diluted oxygen before impregnation. Various reduction protocols were tested, the best results corresponding to reduction with sodium borohydride. High dispersion catalysts were obtained, which showed quite good performance in the electro-oxidation of methanol. In particular, a remarkable increase in the activity was observed when the Pt-Ru catalysts were supported on the oxidised xerogel. This effect was explained in terms of the metal oxidation state, as shown by XPS. It has been shown that the oxidised support helps to maintain the metals in the metallic state, as required for the electro-oxidation of methanol. This effect was negligible in the case of the Pt catalysts.     

Large-scale synthesis and characterization of carbon spheres prepared by direct pyrolysis of hydrocarbons

Large-scale production of pure carbon spheres, with diameters from 50 nm to 1 μm, has been achieved via direct pyrolysis of a wide range of hydrocarbons, including styrene, toluene, benzene, hexane, cyclohexane and ethene, in the absence of catalyst. Specific systematic studies using styrene as the feedstock indicate that the sizes of the resulting of carbon nanospheres can be controlled quite well by adjusting the experimental conditions. The resulting materials have been fully characterized using SEM, TEM, AFM, HRTEM, EDX, elemental analysis, density measurement, XPS, FTIR, XRD, Raman, and TGA. The results show that the spheres, which are 99% carbon, consist of concentric incompletely closed graphitic shells. The dangling bonds on the edges of the shells result in high chemical reactivity.     

Nano-sized carbon hollow spheres for abatement of ethylene

In this paper, the ethylene adsorption capacities of the nano-sized carbon hollow spheres (CNB) and active carbon (AC), the Pd (PdCl₂) impregnated CNB or AC (Pd/CNB, Pd/AC) and heat treatment under various conditions, were studied at different ethylene concentrations from 64 to 1060 ppm. The results indicated that AC had a good ethylene adsorption capacity at high ethylene concentration. Pd impregnation decreased the ethylene adsorption capacity of AC. Heat treatment and H₂ activation could increase the ethylene adsorption capacity, but also lowered than AC itself. CNB had lower ethylene adsorption capacity than AC, but heat treatment and H₂ activation could increase its ethylene adsorption capacity markedly. With activating condition from heat treatment in N₂ at 300 °C to activation in H₂/N₂ at 100 °C, to activation in H₂ at 200 °C, and to activation in H₂ at 300 °C, the ethylene adsorption capacity of Pd/CNB was increased regularly. At low ethylene concentration, viz., 64 ppm, the ethylene adsorption quantities (q _a) by Pd/CNB activated in H₂ at 200 or 300 °C were higher than any other adsorbents. So, activated in H₂ atmosphere at higher than 100 °C, Pd/CNB is particularly advantaged for adsorbing low concentration of ethylene. Amongst all the adsorbents used, Pd/CNB activated in H₂ atmosphere at 300 °C for 2 h has the highest ethylene adsorption capacity at lower concentration than 125 ppm. In addition, all the CNB, Pd/CNB, AC, and Pd/AC samples can be easily regenerated in airflow for more than 3 h.     

Hybrid carbon source for single-walled carbon nanotube synthesis by aerosol CVD method

The conductivity enhancement of single-walled carbon nanotube (SWCNT) films was achieved by increasing the bundle length in an aerosol CVD synthesis method with the help of two carbon sources. Carbon monoxide provides carbon at temperatures below 900 °C, while ethylene takes over at higher temperatures. The significant decrease in the sheet resistance at the 90% transmittance was observed from 3500 to 7500 Ω/sq. for pure CO system via 1909 and 1709 Ω/sq. for CO–H₂ system to 291 and 358 Ω/sq. in the presence of C₂H₄ at 900 and 1100 °C, respectively. Doping the film with a gold chloride solution in acetonitrile allowed us to create the transparent conductive films with the sheet resistance as low as 73 Ω/sq. at a transmittance of 90%.     

溶胶-凝胶法介孔沥青炭微球的制备及电化学性能研究

以水性中间相沥青(AMP)为前驱体,阳离子表面活性剂(CTAB)为结构导向剂,通过控制AMP的凝胶化和自组装过程,采用溶胶-凝胶法来制备介孔沥青炭微球。利用SEM、HRTEM、SAXRD、FT-IR等分析手段,对产物的形貌及结构进行表征,并研究了介孔沥青炭微球的形成机理和电化学性能。结果表明:在制备介孔沥青炭微球的过程中,CTAB与离子化的AMP可通过S+I-静电力作用形成凝胶球,经炭化后得到介孔沥青炭微球。当CTAB/AMP=1.2时(CTAB为2 g),炭化后得到的介孔炭微球之间界面明显,表面光滑,球体粒径为300~500 nm,当增加CTAB胶束在溶液中的浓度时,炭化产物中得到纳米炭棒。所得介孔沥青炭微球C-AMP1.2-800有序度差、石墨化度低,球体为乱层石墨结构,其在50 mV.s-1下的比电容为103.5 F/g。