Porous graphitic carbons prepared by combining chemical activation with catalytic graphitization

Porous graphitic carbons were prepared by combining chemical activation (ZnCl₂) with catalytic graphitization (metal Fe and Ni). The activating agent ZnCl₂ increased the surface area of porous carbon, and the catalyst (Fe, Ni) accelerated the graphitization. With increasing heat treatment temperature (600–900 °C), metal Fe and Ni promoted the degree of graphitization which was characterized by powder X-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy. The surface area of samples prepared using Fe and Ni catalysts varied from 275 to 787 m² g⁻¹ and from 83 to 121 m² g⁻¹, respectively.     

Hierarchically porous phenolic resin-based carbons obtained by block copolymer-colloidal silica templating and post-synthesis activation with carbon dioxide and water vapor

A series of hierarchically porous carbons was synthesized by self-assembly of polymeric carbon precursors and block copolymer template in the presence of tetraethyl orthosilicate (TEOS) and colloidal silica under acidic conditions. Resorcinol and formaldehyde were used as carbon precursors, poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer was employed as a soft template, and TEOS-generated silica and colloidal silica were used as hard templates. The carbon precursors were polymerized in hydrophilic domains of block copolymer, followed by carbonization and silica dissolution. This resulted in carbons possessing cylindrical (∼12 nm) and spherical (20 or 50 nm) mesopores created by thermal decomposition of the soft template and by the dissolution of colloidal silica, respectively; fine pores were also formed by the dissolution of the TEOS-generated silica (∼2 nm). A further increase in fine porosity was achieved by post-synthesis activation of the carbons with carbon dioxide and/or water vapor, which resulted in hierarchical carbons with a surface area and pore volume approaching 2800 m²/g and 6.0 cm³/g, respectively.     

Synthesis of nitrogen-doped porous graphitic carbons using nano-CaCO3 as template,graphitization catalyst,and activating agent

Nitrogen-doped porous graphitic carbons (NPGCs) with controlled structures were synthesized using cheap nano-CaCO₃ as template, melamine-formaldehyde resin as carbon precursor, and dilute HCl as template removing agent. In addition to its use as a template, the nano-CaCO₃ acted as an internal activating agent to produce micro- and mesopores, as an adsorbent to remove the released hazardous gases (i.e. HCN, NH₃), and as a mild graphitization catalyst. The obtained NPGCs with hierarchical nanopores contained as high as 20.9 wt% of nitrogen, had surface areas of up to 834 m² g^–1, and also exhibited high thermal stability with respect to oxidation. Using carbohydrate or phenolic resin as the carbon precursor, this simple approach was also capable of producing hierarchical porous graphitic carbons with high surface area (up to 1683 m² g^–1) and extremely large pore volumes (>6 cm³ g^–1). X-ray diffraction and infrared spectroscopy suggested that the intermediate CaCN₂ or CaC₂ generated during the carbonization plays a critical role in the formation of the graphitic structure.     

The influence of carbon source on the wall structure of ordered mesoporous carbons

A series of ordered mesoporous carbons (OMCs) have been synthesized by filling the pores of siliceous SBA-15 hard template with various carbon precursors including sucrose, furfuryl alcohol, naphthalene and anthracene, followed by carbonization and silica dissolution. The carbon replicas have been characterized by powder XRD, TEM and N₂ adsorption techniques. Their electrochemical performance used as electric double-layer capacitors (EDLCs) were also conducted with cyclic voltammetry and charge-discharge cycling tests. The results show that highly ordered 2D hexagonal mesostructures were replicated by using all these four carbon sources under the optimal operation conditions. Physical properties such as mesoscopic ordering, surface areas, pore volumes, graphitic degrees, and functional groups are related to the precursors, but pore sizes are shown minor relationship with them. The sources, which display high yields to carbons, for example, furfuryl alcohol and anthracene are favorable to construct highly ordered mesostructures even at high temperatures (1300 °C). OMCs prepared from non-graphitizable sources such as sucrose and furfuryl alcohol display amorphous pore walls, and large surface areas and pore volumes. The functional groups in the precursors like sucrose and furfuryl alcohol can be preserved on carbon surfaces after the carbonization at low temperatures but would be removed at high temperatures. The graphitizable precursors with nearly parallel blocks and weak cross-linkage between them like anthracene are suitable for deriving the OMCs with graphitic walls. Therefore, the OMCs originated from sucrose and furfuryl alcohol behave the highest capacitances at a carbonization of 700 °C among the four carbons due to the high surface areas and plenty of functional groups, and a declination at high temperatures possibly attribute to the depletion of functional groups. Anthracene derived OMCs has the lowest capacitance carbonized at 700 °C, and a steady enhancement when heated at high temperatures, which is attributed to the graphitization. The OMCs derived from naphthalene have the stable properties such as relatively high surface areas, few electroactive groups and limited graphitizable properties, and in turn medium but almost constant capacitances.     

Highly porous graphitic materials prepared by catalytic graphitization

Ultrathin graphitic nanostructures are grown inside solid activated carbon particles by catalytic graphitization method with the aid of Ni. The graphitic nanostructures consist of 3–8 graphitic layers, forming a highly conductive network on the surface of disordered carbon frameworks. Owing to the ultrathin characteristic of the produced graphitic nanostructures, the resulted porous graphitic carbons show a high specific surface area up to 1622 m²/g. A detailed investigation reveals that the features of the growing graphitic nanostructures are strongly associated with the catalytic temperature as well as the state of Ni nanoparticles. Some well-dispersed fine Ni particles with diameter below 15 nm are found to be the key to form the ultrathin graphitic nanostructures at appropriate catalytic temperature. Also, a novel mechanism is proposed for the catalytic formation of the ultrathin graphitic nanostructures. As the electrode material of electrochemical capacitors, the porous graphitic carbon exhibits much higher high-rate capacitive performance compared to its activated carbon precursor.     

Dual-template synthesis of magnetically-separable hierarchically-ordered porous carbons by catalytic graphitization

Magnetically-separable hierarchically-ordered porous carbons with graphitic structures (HPC-G) have been directly synthesized by one-pot dual-templating with evaporation-induced self-assembly at calcination temperatures ranging between 600 and 1000 °C. Polystyrene latex spheres and triblock copolymer F127 were used as macro- and meso-porous structure-directing agents, while phenol–formaldehyde resins and Ni species were added as the carbon source and graphitization catalyst, respectively. The microstructures in terms of morphology, surface area, pore texture, thermal stability, degree of graphitization and magnetic properties were characterized by scanning and transmission electron microscopy, small angle X-ray scattering, X-ray powder diffraction, surface area analysis, Raman spectroscopy, thermogravimetric analysis, and superconducting quantum interference device magnetometry. Addition of nickel species catalyzes the graphitization of HPC-G at relatively low carbonization temperatures under different atmospheres (N₂ or H₂/N₂). The HPC-G exhibits well-crystallized graphitic domains, excellent magnetic properties, uniform and interconnected porous structures, and high surface area. The magnetically-separable HPC-G shows a high adsorption capacity for methylene blue and improved electrocatalytic activity towards and I₂ reductions in dye-sensitized solar cells. Results obtained in this study allow us to develop an environmentally friendly technique for fabrication of HPC with well-crystallized graphitic carbon and magnetically-separable properties for novel applications.     

Carbide-derived-carbons with hierarchical porosity from a preceramic polymer

Synthesis of carbon by extraction of metals from carbides has been successfully used to produce a variety of micro-porous carbide-derived carbons (CDCs) with narrow pore size distributions and tunable sorption properties. This approach is of limited use when larger mesopores are targeted, however, because the relevant synthesis conditions yield broad pore size distributions. Here we demonstrate the porosity control in the 3-10 nm range by employing preceramic polymer-derived silicon carbonitride (SiCN) precursors. Polymer pyrolysis in the temperature range 600-1400 °C prior to chlorine etching yields disordered or graphitic CDC materials with surface area in the range 800-2400 m² g⁻¹. In the hierarchical pore structure formed by etching SiCN ceramics, the mesopores originate from etching silicon nitride (Si₃N₄) nano-sized crystals or amorphous Si-N domains, while the micropores come from SiC domains. The etching of polymer-derived ceramics allows synthesis of porous materials with a very high specific surface area and a large volume of mesopores with well controlled size, which are suitable for applications as sorbents for proteins or large drug molecules, and supports for metal catalyst nanoparticles.     

The use of organometallic additives to promote graphitization of carbons derived from furfuryl alcohol resins

The promotion of graphitization of carbons derived from furfuryl alcohol resins and phenolic resins, which had been modified by the addition of various organometallic (OM) compounds, was investigated. Of 15 different OM additives evaluated, the most effective at promoting graphitization were those which contained Ti, V or Zr. Resins doped with these additives yielded carbons whose L_c values were between 150 and 250 Å, as compared to values of less than 30 Å for control specimens. The OM compounds of Co, Fe, and Ni yielded carbons with L_c values of approximately 80 Å; the remaining additives had little, if any, effect. Because of the efficient dispersal of the dissolved OM compounds, additions representing as little as 0·1% metal in the precursor resin were usually sufficient to promote sample graphitization. Also investigated were mixtures containing OM-doped furfuryl alcohol resins and glassy carbon filler. Electron and optical microscopy revealed that reorganization of the amorphous filler particles takes place in preference to the moderately graphitic binder residue. The experimental data suggest that the promotion of graphitization is not the result of low-temperature structural modification to the precursor resins during crosslinking or carbonization, but that promotion occurs at higher temperatures and is consistent with the mechanism of catalytic graphitization of amorphous carbons by metals or metal carbides as proposed by Gillot et al. and Fitzer and Kegel.     

Synthesis of High Surface Area Silica Gels Using Porous Carbon Matrices

Porous silica was prepared using the sol-gel synthesis with porous carbon matrices as a pore-forming support. Tetraethoxysilane (TEOS) was hydrolyzed in an acid medium in the presence of a substoichiometric amount of water. Various carbon materials were used, among them Sibunit and catalytic filamentous carbon. Carbon matrices were impregnated with hydrolyzed TEOS and dried, then carbon was removed by burning out in air at 873 K. The obtained porous silica samples were studied by adsorption and electron microscopic methods. The specific surface area as high as 1267 m²g and pore volume as high as 5.7 cm³/g were determined for some silica samples. Thus deposited SiO₂ was found to cover the carbon surface copying its surface. With CFC used as carbon matrix, silica nanotubes were obtained. Thermostability of such silica is significantly greater as compared to silica gels reported earlier.     

IrO2-SiO2 binary oxide films: Preparation, physiochemical characterization and their electrochemical properties

Mixed IrO₂-SiO₂ oxide films were prepared on titanium substrate by the thermo-decomposition of hexachloroiridate (H₂IrCl₆) and tetraethoxysilane (TEOS) mixed precursors in organic solvents. The solution chemistry and thermal decomposition kinetics of the mixed precursors were investigated by ultra violet/visible (UV/vis) spectroscopy and thermogravimetry (TGA) and differential thermal analysis (DTA), respectively. The physiochemical characterization of the resulting materials was conducted by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. It is shown from the UV/vis spectra that the electronic absorption intensity of IrCl₆²⁻ complexes in the precursors decreases in the presence of TEOS, indicating the interaction between these two components. Thermal analysis shows the decomposition reaction of H₂IrCl₆ is inhibited by TEOS in the low temperature range, but the further oxidation reaction at high temperatures of formed intermediates is independent of the presence of silane component. Physical measurements show a restriction effect of silica on the crystallization and crystal growth processes of IrO₂, leading to the formation of finer oxide particles and the porous morphology of the binary oxide films. The porous composite films exhibit high apparent electrocatalytic activity toward the oxygen evolution reaction. In addition, the long-term stability of Ti-supported IrO₂ electrodes is found to apparently improve with appropriate amount of SiO₂ incorporation, as tested under galvanostatic electrolysis.     

Synthesis of graphitic mesoporous carbons with different surface areas and their use in direct methanol fuel cells

Using resorcinol and formaldehyde as carbon precursors, graphitic mesoporous carbons (GMC) with different surface areas were synthesized by adjusting the amount of iron nitrate used as a graphitization catalyst. The BET surface area of the GMC decreases with increasing pore diameter and the material becomes more graphitic with increasing amount of catalyst. Using the GMC as the support, a series of PtRu electrocatalysts (PtRu-GMC) were synthesized. The electrocatalytic activities of the PtRu-GMC toward the methanol oxidation reaction were investigated in both half cells and single cells. The results indicate that the pore size of the GMC is especially important in determining the performance of direct methanol fuel cells.     

Catalytic graphitization of templated mesoporous carbons

Graphitic porous carbons with a wide variety of textural properties were obtained by using a silica xerogel as template and a phenolic resin as carbon precursor. The synthetic procedure used to prepare them was as follows: (a) infiltration of the porosity of silica by a solution containing phenolic resin, (b) carbonization of the silica-resin composite, (c) removal of the silica skeleton, (d) impregnation of the templated porous carbon with a metallic salt and (e) catalytic graphitization of the impregnated carbon by heat treatment at 900 °C. The graphitization of the carbons thus prepared varies as a function of the carbonization temperature used and the type of metal employed as catalyst (Fe, Ni or Mn). The porous characteristics of these materials change greatly with the temperatures used during the carbonization step. These graphitized carbons exhibit high electrical conductivities up to two orders larger than those obtained for the non-graphitized samples.     

Reducing Cr6+ emissions from gas tungsten arc welding using a silica precursor

Hexavalent chromium (Cr⁶⁺) emission from stainless steel welding operations poses a serious threat to worker safety and ambient air quality. In this study, tetraethyloxysilane (TEOS) was used as a silica precursor additive to welding shield gas during gas tungsten arc welding (GTAW) operations to determine the feasibility of using these chemicals for Cr⁶⁺ exposure reduction. Fume aerosol samples were analyzed for Cr⁶⁺ concentration using ion chromatography (IC) and for total Cr by inductively coupled plasma with atomic emission spectroscopy (ICP-AES).At high temperature, silica precursors are pyrolyzed to form amorphous silica (SiO₂) which can condense on the existing metal aerosols. The inert silica layer surrounding the aerosols can prevent further chromium oxidation by insulating chromium aerosols. Experimental results showed approximately 45% Cr⁶⁺ reductions when 3.0% TEOS was added to the shield gas. Nitrate concentration also decreased by 53%, indicating that reactive oxygen species were also reduced. Transmission electron microscopy (TEM) images of collected fume aerosols showed SiO₂ coating on metal particles, verifying the proposed mechanism.     

Processing of strong and highly conductive carbon foams as electrode

Porous carbons were processed by the foaming of two-part polymer precursors with pre-loaded carbon powder (graphitic or amorphous), and then resin impregnation and carbonization to control both porosity and mechanical strength of the resulting foam. Electrical conductivity of the foams was improved by nickel-catalyzed graphitization. Different levels of graphitization were obtained for varied concentrations of nickel to the amorphous carbon foams. The presence of graphitic carbon improves the electrical conductivity by a factor of 50, compared to the amorphous counterparts. Electrochemical studies showed that graphitization of the amorphous structures increased the specific electrochemical surface area and electron transfer rate of the carbon electrodes.     

Synthesis of porous graphitic carbon from mesocarbon microbeads by one-step route

We developed a one-step route to synthesize porous graphitic carbon (PGC) from mesocarbon microbeads. The MCMB was ground with NaOH and FeCl₃ in an agate mortar, and then the mixture was heated in an Argon flow. Finally the PGC was obtained after washing with acid and deionized water. The sample was characterized by nitrogen adsorption, X-ray diffraction, SEM, TEM and Raman spectroscopy. The results showed that NaOH acted as activation reagent to increase surface area, meanwhile FeCl₃ worked as the catalyst to accelerate the graphitization. In order to balance the two effects, the orthogonal experimental design was used to optimize the experiment parameters including temperature, the amounts of NaOH and FeCl₃. The optimized result showed a surface area of 929 m²/g and a high degree of crystallization (L _c  = 32.6 nm). As a result, the combination of chemical activation and catalytic graphitization upon MCMB created one kind of PGC with a surface graphitization structure which was helpful to contribute a conductivity network.     

Graphitic mesoporous carbons synthesised through mesostructured silica templates

In this paper the fabrication and characterization of graphitizable and graphitized porous carbons with a well-developed mesoporosity is described. The synthetic route used to prepare the graphitizable carbons was: (a) the infiltration of the porosity of mesoporous silica with a solution containing the carbon precursor (i.e. poly-vinyl chloride, PVC), (b) the carbonisation of the silica–PVC composite and (c) the removal of the silica skeletal. Carbons obtained in this way have a certain graphitic order and a good electrical conductivity (0.3 S cm⁻¹), which is two orders larger than that of a non-graphitizable carbon. In addition, these materials have a high BET surface area (>900 m² g⁻¹), a large pore volume (>1 cm³ g⁻¹) and a bimodal porosity made up of mesopores. The pore structure of these carbons can be tailored as a function of the type of silica selected as template. Thus, whereas a graphitizable carbon with a well-ordered porosity is obtained from SBA-15 silica, a carbon with a wormhole pore structure results when MSU-1 silica is used as template. The heat treatment of a graphitizable carbon at a high temperature (2300 °C) allows it to be converted into a graphitized porous carbon with a relatively high BET surface area (260 m² g⁻¹) and a porosity made up of mesopores in the 2–15 nm range.     

Graphitic carbon growth on crystalline and amorphous oxide substrates using molecular beam epitaxy

We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp² amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.PACS: 81.05.uf; 81.15.Hi; 78.30.Ly.     

Synthesis and characterization of microporous carbons templated by ammonium-form zeolite Y

Emerging applications such as gas storage require porous carbon materials with tailored structural and surface properties. Template synthesis approach to porous carbons offers opportunities for tailoring these properties. In this study, ammonium-form zeolite Y (NH₄Y) was used as a template and furfuryl alcohol (FA) was employed as a carbon precursor to prepare microporous carbons by simple impregnation method. The effects of synthesis conditions such as carbonization temperatures and heating rates on the pore structure of the microporous carbons were investigated. The thermal behaviors of FA-NH₄Y mixtures and zeolite/carbon composites were studied by thermogravimetric analysis (TGA). The physical, structural, and surface properties of the microporous carbons were characterized with X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM), elemental analysis, and physical adsorption of nitrogen. Microporous carbons with high surface areas, pore volumes and nitrogen-containing surface functional groups can be readily synthesized.     

Review od ADVANCES IN DRYING,Vol. 3,

RF hydrogels were synthesized by the sol-gel polycondensation of resorcinol with formaldehyde and RF cryogels were prepared by freeze drying of the hydrogels with t-butanol. The cryogels were characterized by nitrogen adsorption, density measurements, and scanning electron microscope. Their porous properties were compared with those of the aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >5.8× 10^?4m³/kg. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Aerogel-like carbons (carbon cryogels) were obtained by pyrolyzing RF cryogels in an inert atmosphere. The carbon cryogels were mesoporous materials with high surface areas >8.0× 10⁵m²/kg and large mesopore volumes >5.5× 10^?4m³/kg. When pyrolyzed, micropores were formed inside the cryogels more easily than inside the aerogels.     

Carbon formation promoted by hydrogen peroxide in supercritical water

A limited use of hydrogen peroxide in supercritical water has produced graphitic carbons from hydrocarbons at the low temperature of 400 °C. The choice of precursor hydrocarbons leads to morphological and microstructural variations. The use of n-hexane has provided chain-like interlinked nanoparticles comprised of graphitic layers, while benzene has been converted to a fine spherical shape of colloidal carbons with sub-micron size. The microstructure of the colloidal carbons is comprised of smaller aromatic clusters than the hexane-derived graphitic layers. Furthermore, the graphitization of the colloidal carbons at 2300 °C has induced not closed shell-like graphitic layers with concentric arrangement but ribbon-like graphitic layers growing in no particular direction. The arbitrary direction of the graphitization demonstrates the smallness of the aromatic clusters allowing for their flexible rearrangement.