Surface chemistry of ordered mesoporous carbons

Ordered mesoporous carbons (OMC) were produced by pyrolysis of hydrocarbons adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in either hydrofluoric acid or sodium hydroxide. Some carbons were subsequently heat treated at temperatures of up to 1600 °C. The chemistry of the external surface was studied by X-ray photoelectron spectroscopy (XPS) and static secondary ion mass spectroscopy (SIMS). Information on the graphitic order of the surface of the mesopores was obtained from low-pressure nitrogen adsorption data. The external and internal surface of the OMC has a polyaromatic, graphite-like character. This character increases considerably with increasing pyrolysis and/or post-pyrolysis temperature, as expected. According to the XPS and the nitrogen adsorption data, this increase was especially pronounced for temperatures above 1100 °C. In spite of the different pore structures, only small differences in the polyaromatic character were found for OMC synthesised either in a MCM-48 or in a SBA-15 matrix. Differences exist for the non-carbon elements. When hydrofluoric acid is used for dissolution of the silica matrix, organic fluorine compounds are formed. Their concentration is higher when a MCM-48 matrix as opposed to a SBA-15 matrix was used. Dissolution of the silica matrix in sodium hydroxide yielded a less contaminated OMC as compared to dissolution in hydrofluoric acid.     

Methane sorption on ordered mesoporous carbon in the presence of water

An ordered mesoporous carbon was synthesized using SBA-15 as the template. The sorption isotherms of methane on the synthesized carbon material were collected. Its ordered structure was confirmed by the XRD, SEM and TEM examinations. The BET surface area is 1100-1200 m²/g, the total pore volume is 1.24-1.30 cm³/g, and the pore size distribution is very narrow and centered at 2-5 nm. As high as 41.2 wt.% of methane was stored per unit mass of carbon at 275 K and pressures less than 7 MPa in the presence of 3.86 times more water. This sorption amount is 31% higher than the largest sorption capacity reached by activated carbon in the presence of water, which was equal to or higher than the storage capacity of compression till 20 MPa. The enthalpy change corresponding to the sudden change of isotherms was equal to the enthalpy change of methane hydrate formation; therefore, the mechanism of the enhanced methane storage was considered due to the formation of methane hydrate in the porous carbon material.     

Templating synthesis of ordered mesoporous carbon particles

This research reports the synthesis of spherical ordered mesoporous carbon particles using mesoporous silica particles as a template. The silica particles with ordered cubic and lamellar mesostructures were synthesized from an aerosol-assisted self-assembly process using block copolymer surfactants as the structural directing agent. Infiltrating the pores of the silica particles with a sucrose solution followed by carbonization, and silica removal results in mesoporous carbon particles with replicated mesostructures. The particles were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and nitrogen sorption techniques.     

Superior electric double layer capacitors using ordered mesoporous carbons

This paper reports for the first time superior electric double layer capacitive properties of ordered mesoporous carbon (OMCs) with varying ordered pore symmetries and mesopore structure. Compared to commercially used activated carbon electrode, Maxsorb, these OMC carbons have superior capacitive behavior, power output and high-frequency performance in EDLCs due to the unique structure of their mesopore network, which is more favorable for fast ionic transport than the pore networks in disordered microporous carbons. As evidenced by N₂ sorption, cyclic voltammetry and frequency response measurements, OMC carbons with large mesopores, and especially with 2-D pore symmetry, show superior capacitive behaviors (exhibiting a high capacitance of over 180 F/g even at very high sweep rate of 50 mV/s, as compared to much reduced capacitance of 73 F/g for Maxsorb at the same sweep rate). OMC carbons can provide much higher power density while still maintaining good energy density. OMC carbons demonstrate excellent high-frequency performances due to its higher surface area in pores larger than 3 nm. Such ordered mesoporous carbons (OMCs) offer a great potential in EDLC capacitors, particularly for applications where high power output and good high-frequency capacitive performances are required.     

Thermal reactivation of ammonia-tailored granular activated carbon exhausted with perchlorate

Thermal reactivation with CO₂ or NH₃ at temperatures higher than 700 °C effectively restored the perchlorate adsorption capacity of ammonia-tailored carbon. In contrast, steam regeneration restored only a portion of the perchlorate adsorption capacity, and these distinctions were attributed to the change in surface chemistry that was induced by regeneration. After perchlorate loading, regenerating the ammonia-tailored GAC via CO₂ or NH₃ preserved the nitrogen content and positive surface charge density of the initial ammonia-tailored activated carbon. In contrast, steam regeneration caused a decrease in nitrogen content and positive surface charge. Perchlorate breakthrough was monitored in rapid small-scale column test (RSSCT) operations with either CO₂ or NH₃ regeneration. 4000-4500 bed volumes of perchlorate adsorption life could be achieved through at least three cycles of RSSCT operation and regeneration. This compared favorably to the 4500 bed volumes that had been achieved when using the initial ammonia-tailored carbon.     

Nano-structured carbon obtained by chlorination of NbC

New carbon materials have been synthesized by chlorinating niobium carbide at different temperatures. During the reaction, the volatile niobium chloride, mainly NbCl₅, is formed and eliminated leaving the carbon in the shape of spherical particles. TEM examination of the remaining particles revealed that at 400 °C and 700 °C they are composed of a NbC core with an amorphous carbon shell outside. The NbC cores are very small (∼20 nm) at 700 °C and they are not observed at 900 °C. Carbon particles without the NbC core present a more ordered structure composed of concentric wavy graphene layers. This structure seems intermediate between carbon onions and carbon blacks. The 900 °C sample presents a very high BET surface area (1292 m² g⁻¹) and the lower temperature samples exhibit a hysteresis phenomenon that can be attributed to the existence of large pores in the interspace between the NbC core and the carbon shell.     

Structure of nanoporous carbon produced from boron carbide

The structure of nanoporous carbon produced by chlorination of powdered boron carbide at 600, 800, 1000, 1300, 1500, and 1800 °C has been studied by scanning electron microscopy, X-ray diffraction analysis, helium pycnometry, and low-temperature adsorption of nitrogen. On the basis of the results obtained, suggestions are made concerning the type of organization of the nanoporous structure of these materials. The evolution of the structure of nanoporous carbon is analyzed in relation to the synthesis temperature of nanoporous carbon. It is shown that, as the chlorination temperature increases, the structure of nanoporous carbon becomes more perfect: it changes from paracrystalline to turbostratic. The specific surface area decreases from 2200 to 36 m²/g, the volume of micropores decreases from 0.93 to 0.01 cm³/g, and that of mesopores first increases from 0.15 cm³/g (600 °C) to 0.57 cm³/g (1000 °C) and then decreases to 0.19 cm³/g (1800 °C). The total pore volume decreases from 1.08 to 0.20 cm³/g.     

Adsorption of vitamin B12 on ordered mesoporous carbons coated with PMMA

Ordered mesoporous carbons CMK-3 and CMK-1 were prepared from SBA-15 and MCM-48 materials with pore diameters 3.9 nm and 2.7 nm, respectively. When both mesoporous carbons were coated with about 10 wt.% poly(methyl methacrylate) (PMMA), the pore diameters decreased from 3.9 nm to 3.4 nm for CMK-3 and from 2.7 nm to 2.5 nm for CMK-1. These mesoporous carbons containing about 10 wt.% PMMA were studied as adsorbents of Vitamin B 12 (VB12) from water solutions, and their performances were compared with that of pristine CMK-3, CMK-1. Compared with CMK-1, CMK-3 showed higher vitamin B12 adsorption due to a larger mesopore volume, a higher BET surface and a larger pore diameter. After coated with PMMA, both mesoporous carbons showed higher adsorption capacity than pristine materials. The adsorption properties were influenced by the pore structure and surface properties of mesoporous carbons.     

Modification of multi-walled carbon nanotubes with fatty acid and their tribological properties as lubricant additive

Multi-walled carbon nanotubes (MWNTs) were treated with mixture of sulfuric acid and nitric acid. The surface modification of the oxidized MWNTs was achieved by refluxing the MWNTs with stearic acid (SA). The modified MWNTs were examined by the transmission electron microscope, scanning electron microscope, Raman spectroscopy and Infrared spectroscopy. Furthermore, the modified MWNTs were added to base lubricant and the tribological properties of resultant MWNTs lubricant were investigated by using a pin-on-plate wear tester. The results indicated that an esterification was formed in the oxidized MWNTs and SA, and the modification led to an improvement in the dispersion of MWNTs and the tribological properties of MWNTs as lubricant additive.     

Electrochemical energy storage in ordered porous carbon materials

Highly ordered porous carbon materials obtained by a replica technique have been used for supercapacitor application and electrochemical hydrogen storage. For the preparation of the well-tailored carbons, MCM-48, SBA-15 and MSU-1 molecular sieves served as templates, whereas a sucrose solution, propylene and pitch were the carbon source. A careful physico-chemical characterization (CO₂ and N₂ adsorption, X-ray diffraction, electron microscopy observations) allowed to estimate the total surface area, the pore size distribution, the micro/mesopore volume as well as the structure and the microtexture of the investigated carbons. The specific capacitance (F/g) and the hydrogen adsorption capacity in the carbon nanopores were correlated with the microtextural properties. Especially, a linear dependence has been found between the capacitance or the amount of electrochemically stored hydrogen and the ultramicropores (pores smaller than 0.7 nm) volume. It clearly indicates that in these carbons: (a) the major part of the electrical double layer is charged with non-solvated ions; (b) ultramicropores play a determinant role for hydrogen storage.     

Nitrogen-doped porous carbon microspherules as supports for preparing monodisperse nickel nanoparticles

N-doped porous carbon microspherules were developed through controlled carbonization of the copolymer of vinylidene chloride and acrylonitrite. The carbon precursors were prepared by an inorganic-organic hybrid route. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) analysis showed the formation of N-doped carbon spherules. N₂ sorption analysis showed that the resultant carbon materials have a BET surface area of 692 m²/g. Nickel nanoparticles supported over them are kept in a well-dispersed state. Electron Probe Microanalysis (EPMA) and Transmission Electron Microscopy (TEM) showed nickel nanoparticles are quite monodisperse. Analysis of XPS spectra of the samples with different surface nitrogen atomic concentration demonstrated that nitrogen species on the carbon surfaces have a great impact on the dispersion state of the mounted metal nanocrystals.     

Synthesis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide

Synthesis of nanoporous carbide-derived carbon, CDC, by extraction of titanium and silicon from Ti₃SiC₂ by chlorine is discussed in this work. Thermodynamic simulations using a Gibbs free energy minimization program provided general guidelines to the experimental design. Raman spectroscopy, X-ray diffraction, and electron microscopy studies showed that the structure of CDC depends on the chlorination temperature. The low temperature synthesis resulted in an amorphous CDC structure. Noticeable graphite formation starts above 800 °C and well ordered graphite ribbons of 1-3 nm in thickness form at 1200 °C. The macroscopic volume and shape of Ti₃SiC₂ preform were preserved during the transformation. However, the chlorination resulted in the formation of cracks between the former grains of the polycrystalline Ti₃SiC₂ preform. These cracks are believed to be caused by a contraction in the direction perpendicular to the basal planes of Ti₃SiC₂. The synthesized nanoporous carbon demonstrated excellent sorption properties. Energy dispersive X-ray spectroscopy studies showed that Ti₃SiC₂ material chlorinated at 400 °C is capable of trapping over 40 wt.% of Cl₂.     

Low temperature synthesis of carbon nanofibres on carbon fibre matrices

Carbon nanofibres are grown on a carbon fibre cloth using plasma enhanced chemical vapour deposition from a gas mixture of acetylene and ammonia. A cobalt colloid is used as a catalyst to achieve a good coverage of nanofibres on the surface of the carbon fibres in the cloth. The low temperature growth conditions that we used would allow growth on temperature sensitive polymers and fibres. The nanofibres grown by a tip growth mechanism have a bamboo-like structure. A significant increase of the bulk electrical conductivity of the carbon cloth was observed after the nanofibre growth indicating a good electrical contact between carbon nanofibres and carbon fibres. The as-grown composite material could be used as high surface area electrodes for electrochemical applications like fuel cells and super-capacitors.     

Fabrication of superhydrophobic surfaces with non-aligned alkyl-modified multi-wall carbon nanotubes

Films of superhydrophobic multi-wall carbon nanotubes (MWCNTs) have been obtained by using alkyl-modified MWCNTs (MWCNT(COOC₁₈H₃₇)_n) and a simple and effective preparation method. The films show both a high contact angle and a small sliding angle for water droplets. A particular characteristic is that on the superhydrophobic surface the alkyl-modified MWCNTs are not intentionally aligned, thus avoiding the preparation techniques using aligned carbon nanotubes to produce the same effect.