Catalytic graphitization of carbons by borons

Boron is known as a unique graphitization catalyst, because it accelerates the homogeneous continuous graphitization process of the entire carbon without any formation of specific carbon components such as graphitic carbon. This study uses various amounts of boron and several kinds of carbon in an attempt to reveal whether the boron exhibits other kinds of catalytic effect. For the non-graphitizing phenol-formaldehyde resin carbon, a boron addition of 1 wt % accelerates the homogeneous continuous graphitization process of the entire carbon at heat-treatment temperatures from 1800 to 2600 °C. As the amount was increased to 5 and 10 wt %, the boron catalysed the formation of specific turbostratic carbon (no three-dimensional ordered structure, d₀₀₂ 3.38 Å, L_c 200 Ź) at 2200 °C, and graphitic carbon (three-dimensional ordered structure, $\text{d}{}_{002}\text{3.36}\text{A}\text{?}\text{, L}{}_{\mathrm{c}}\text{1000}\text{A}\text{?}$) above 2400 °C, besides the homogeneous effect, which was enhanced by increased heat-treatment temperature and an increase in the amount of boron. For the graphitizing 3,5-dimethylphenol-formaldehyde resin carbon, only the homogeneous acceleration was catalysed by 1 wt % boron. Additions of 5 and 10 wt % boron resulted in the formation of graphitic carbon above 2400 °C in addition to the homogeneous effect observed above 1800 °C. Turbostratic carbon was never found. The catalytic mechanisms for these effects are discussed.     

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.     

Catalytic graphitization of fibrous and particulate carbons

Catalytic graphitization of carbon fibers, carbon black and anisopropic mesophase spheres by chromia and chromium compounds was studied to examine the influences of the amount of catalyst, procedures of catalyst dispersion and heating on the extent of graphitization. Catalyst addition as a liquid solution provided better dispersion than mixing of powdered catalyst with the carbons by grinding, and permitted graphitization of intact carbon fibers. However, the catalytic action caused restructuring of the fibers and the carbon black. Sequential heat treatments at temperatures < 2000°C were more effective for graphitization than higher temperature treatments in which the catalyst sublimed. In all cases the amount of graphitization increased sharply with the amount of catalyst. The mechanism of catalytic graphitization is briefly discussed.     

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.     

Catalytic graphitization of cokes by indigeneous mineral matter

Indigeneous mineral matter in coals acts catalytically towards graphitization during heat treatment of coals to 2273 K. Nineteen coals of a wide range of rank were demineralized by acid extraction. Original and demineralized coals were carbonized in the range 1073–2273 K, and the resulting cokes examined by optical microscopy, X-ray diffraction and phase-contrast high resolution electron microscopy. Optical microscopy indicated the extent of formation of anisotropic carbon in the resultant cokes. The (002) X-ray diffraction profiles indicated three types of catalytic effect, for which electron microscopy demonstrated different crystallite structures and interrelations. The importance of catalytic graphitization in metallurgical cokes in relation to their strength and reactivity is discussed.     

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.     

Development of biobased microwave absorbing composites with various magnetic metals and carbons

The preparation and characterization of a biobased electromagnetic absorbing composites derived from natural lacquer as a renewable resource with microwave‐absorption fillers, including Ni–Zn ferrite and carbonyl iron (CI) as magnetic metals and soot and carbon nanotube (CNT) as carbon materials, were investigated in terms of the gel content, hardness, drying properties, and electromagnetic absorption properties. Interestingly, composites with ferrite and CI contained up to 320 and 550 wt %, respectively, of these compounds. This quite high loading capacity of the metal fillers in a natural‐lacquer base could have been due to the high compatibility between the filler and the natural lacquer; this indicated that the natural lacquer worked as a binder for these metals. The morphology of the biobased composite was characterized by scanning electron microscopy. The electromagnetic absorption properties of composites were characterized in the frequency range from 0.05 and 20 GHz by the reflection loss (RL) measurement method in terms of the kind of fillers and filler loading. The natural lacquer did not affect the absorption properties of the fillers. Biobased composites showed over 99% electromagnetic absorption in the frequency range 3.0–4.0 GHz for 280 wt % ferrite and 8.9–9.7 GHz for 200 wt % CI. Conversely, 10 and 20 wt % soot exhibited good performance (RL < ?20 dB) between 16.5 and 17.3 and between 8.8 and 9.2 GHz, respectively. The areas with RL values of less than ?20 dB of the CNT composites were 10.4–11.0 GHz for 5 wt % and 14.6–15.2 GHz for 10 wt %. Hence, natural lacquer can be used as a binder material for electromagnetic absorption composites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44131.     

Catalytic graphitization of model compound chars by aluminum and beryllium

The use of metal additives for the catalytic graphitization of chars prepared by high temperature, high pressure carbonization of anthracene, phenanthrene or biphenyl has been investigated. In particular, the extent and type of catalysis initiated by aluminum or beryllium has been examined. Quantitative X-ray intensity measurements have been performed to show the existence of at least two concurrent mechanisms of graphitization catalysis. Evidence for the preferential attack on the disorganized carbon by the graphitization catalyst has been found.     

Catalytic graphitization of non-graphitizable carbon by chromium and manganese oxides

Carbon prepared from the benzene-insoluble fraction of a solvent refined coal (non-fusible), an active carbon, a charcoal and PAN carbon fibres have been heat-treated with oxides of chromium, manganese, molybdenum and vanadium to investigate catalytic graphitization in the temperature range 1673–2773 K. Resultant materials were examined by X-ray diffraction, SEM and phase contrast high resolution electron microscopy. The chromia is an effective catalyst at 2273 K at concentration of chromium of 30%, no changes being observed at higher temperatures. With other oxides the extent of catalytic graphitization (using L_c) increased with HTT to 2773 K, values of C₀ being less sensitive to HTT. Soak times are important, equilibration taking 2 hr at 2073 K (SRC-BI: Cr₂O₃) and 10 hr at 1673 K (SRC-BI; MnO₂). Large concentrations of additives (up to 30% of metal) are required. The microscopy reveals the development of flaky graphite (SRC-BI) and the layered stacking arrangements of graphite planes in the SRC-BI graphites.     

Magnetic susceptibility and kinetics of “graphitization” of glass-like carbons

The room temperature magnetic susceptibility (X) of glass-like carbons (GLC) from several sources has been determined as a function of heat treatment temperature (HTT) over the range 1000–3000°C. Effects of dilute ferromagnetic impurities were observed for HTT < 1500°C. The impurity (probably Fe) exists in a non-magnetic form in the carbonized GLC; transforms to a ferromagnetic state with 1000 ? HTT ? 1400°C; then disappears at higher HTT. The dependence of the diamagnetism of pure GLC on HTT is characterized by a slope decrease and inflection near 1500°C and increases smoothly thereafter. X ～- 5.2 × 10^?6emu/g and is still increasing at HTT = 3000°C, and the values for different GLCs with the same HTT differ by < 10%. The evolution of X as a function of isothermal residence time (HTt) over the range 2400–3000°C for three GLCs was analyzed by the superposition method. Very high effective activation energies, 360–420 kcal/mole (~ 1500–1750 kJ/mole) were obtained and attributed to successive-activation processes of structural development required by the microstructural constraints inherent to difficult-to-graphitize carbons. Evidence was found for a small Xincrease due to the plastic volume dilation (density decrease) processes that occur in some GLCs at high HTT.     

Catalytic graphitization of novolac resin for refractory applications

This study investigated how to induce graphite generation from the carbonization process of novolac resins using conditions that can be adopted for carbon-containing refractories (CCRs) production. The effect of boron oxide or boric acid (graphitizing agents), cross-linking additive (hexamethylenetetramine) and some processing parameters (mixing technique, vacuum degassing, heating rate and thermal treatments) on carbon graphitization from a commercial novolac resin were evaluated. The X-ray diffraction (XRD) technique was selected to measure the graphitization level and crystal parameters of the prepared samples. Based on the attained results, adding graphitizing agents prior to the pyrolysis of resin resulted in carbon crystallization. The best graphitization level was obtained when the mixtures containing 6 wt% B₂O₃ or 10 wt% H₃BO₃ were fired up to 1000 °C for 5 h using a heating rate of 3 °C/min. Although the reproducibility of the obtained results was ascertained, heterogeneous graphitization could be observed based on the XRD profiles, as well as some discrepancies in the calculated graphitization level values. This phenomenon was attributed to the additives susceptibility to agglomeration, preferential graphitization starting from lower binding energy sites and heat treatment temperature, among others.     

Catalytic performance of porous carbons obtained by chemical activation

Porous carbons were prepared by the pyrolysis of pinewood in nitrogen at 773 K, followed by chemical activation using different concentrations of KOH in nitrogen at 973 K. Both water-washed and acid-washed samples exhibited much higher specific surface areas than unactivated carbon. The water-washed sample showed strong basicity and a high catalytic performance in the decomposition of isopropanol, even higher than superbase 26%KNO₃/γ-Al₂O₃. Moreover, these porous carbons can act as water-resistant solid bases. The formation of the insoluble basic sites is most possibly related to the intercalation of potassium in the carbon during the chemical activation.     

Catalytic graphitization of graphitizable carbon by chromium,manganese and molybdenum oxides

Catalytic graphitization of a graphitizable carbon from Kureha pitch is investigated in the temperature range 1200–2800 K, using chromia and other chromium compounds. Two distinct stages of catalytic graphitization are detected at temperatures of 1500–1800 K and above 2000 K. Values of L_c(002) and C₀(002) are 23 nm and 673.0 pm at 1800 K and 80 nm and 670.9 pm at 2800 K. Consideration of the phase diagram of chromium-carbon system suggests that graphitization at 2800 K may proceed via a partial dissolution-precipitation process of carbon. The known catalytic graphitization of non-graphitizable carbons at 2800 K supports this mechanism. Graphitization at 1500–1800 K occurs only with a graphitizable carbon, suggesting that graphitization may proceed via the elimination of structural defects in the carbon although catalytic chemical or physical transformations may also be possible. The catalytic activity of other oxides was investigated and the high activity of manganese and molybdenum oxides is noted.     

Catalytic graphitization of carbon using titanium and zirconium

The role of titanium and zirconium as graphitization catalysts was studied using anisotropic carbons prepared from Gilsonite pitch and acenaphthylene (HHT 1173°K), and an isotropic carbon prepared from a phenol-hexamine resin (HTT 1273°K). Heat-treated mixtures of metal and carbon, HTT 2673 and 3073°K were examined by polarized-light optical microscopy, line-broadening analyses of X-ray diffraction profiles and electron probe measurements. Both metals are effective as catalysts, producing crystallites with dimensions of about 200 nm at 3073°K. The extent of graphitization increases with metal content to about 10% of metal. Diffusion of titanium and zirconium within the carbon matrix is significant. Graphitic material is precipitated from solution of carbon in titanium and zirconium. There is no evidence of carbide formation.     

Influences of particle size of metal on catalytic graphitization of non-graphitizing carbons

It is known that addition of certain metals or inorganic compounds into carbon accelerates the graphitization process at elevated temperature through formation of graphitic carbon. Recently, however, it became apparent that some other kinds of catalytic graphitization effects result from varying the particle size of metal catalysts. Studies on this subject are summarized.     

Catalytic graphitization of electroless Ni-P coated PAN-based carbon fibers

Catalytic graphitization of electroless Ni-P coated PAN-based carbon fibers is reported. PAN-based carbon fibers with and without electroless Ni-P coatings were heat treated and the structural changes were followed by X-ray diffraction and Raman spectroscopy, both of which indicate that the graphitization of PAN-based carbon fibers was enhanced in the presence of the coating. The graphitization was shown to be better for electroless Ni-P coated PAN-based carbon fibers heat treated at 1400 °C than for uncoated fibers heat treated at 2400 °C.     

Catalytic graphitization of electrospun cellulose nanofibres using silica nanoparticles

Electrospun cellulose nanofibres have been graphitized in the presence of silica (SiO₂) nanoparticles. The structure of the resultant SiC/C hybrids was characterised using transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Bamboo-like silicon carbide (SiC) nanostructures were observed emanating from the nanofibres treated at 1500 °C which were thought to grow through a vapour–liquid–solid process. The formation of SiC was also thought to lead to a higher degree of graphitization for the electrospun cellulose fibres. These porous and graphitized nanofibres might find applications in electrochemical energy storage.