Reaction kinetics between CO2 and oil-shale residual carbon. 1. Effect of heating rate on reactivity

The reaction kinetics between CO₂ and residual carbon from Colorado oil shale (Mahogany Zone) have been investigated using both isothermal and nonisothermal methods. It was found that oil-shale residual carbon is approximately an order of magnitude more reactive than subbituminous-coal char although the surface areas are similar. The reactivity of the residual carbon was found to vary by a factor of two for samples prepared by retorting the shale at heating rates between 0.033 and 12 °C/ min. Since the surface area of the residual carbon is approximately independent of the amount of oil coking, the heating-rate effect cannot be explained by pore filling. Surface areas of the residual organic carbon in shale were estimated by comparing the surface area of retorted shale (about 3% carbon) with that of retorted shale which had been decharred by oxidation at 400 °C. Surface areas of 250–400 m²/g and 100–200 m²/g were obtained using CO₂ and N₂ respectively as the adsorbed gases. Mercury porosimetry results are also presented.     

Ordered mesoporous carbon prepared from triblock copolymer/novolac composites

Ordered mesoporous carbon is synthesized by the organic–organic self-assembly method with novolac as carbon precursor and two kinds of triblock copolymers (Pluronic F127 and P123) as template. The hexagonal structure and a worm-hole structure are observed by TEM. The carbonization temperature is determined by TG and FT-IR. Characterization of physical properties of mesoporous carbon is executed by N₂ absorption–desorption isotherms and XRD. The mass ratios of carbon precursor/template affect the textural properties of mesoporous carbon. The mesoporous carbon with F127/PF of 1/1 has lager surface area (670 m² g^?1), pore size (3.2 nm), pore volume (0.40 cm³ g^?1), smaller microporous surface area (368 m² g^?1) and wall thickness (3.7 nm) compare to that with F127/PF of 0.5/1 (576 m² g^?1, 2.7 nm, 0.29 cm³ g^?1, 409 m² g^?1 and 4.3 nm, respectively). The mesoporous carbon prepared by carbonization at high temperature (700 °C) exhibits lager surface area, lower pore size and pore volume than the corresponding one obtained at 500 °C. The structure and order of the resulting materials are notably affected with types of templates. The mesoporous carbon with P123 as template exhibits worm-hole structure compare to that with F127 as template with hexagonal structure. In general, the pore size of mesoporous carbon with novolac as precursor is smaller than that with resorcinol–formaldehyde as precursor.     

Synthesis and comparative studies of xerogels,aerogels, and powders based on the ZrO<Subscript>2</Subscript>–Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–СeO<Subscript>2</Subscript> system

The technology of liquid-phase synthesis of mesoporous xerogels and aerogels based on ZrO₂–Y₂O₃–CeO₂ is developed. Xerogels are obtained by the coprecipitation of hydroxides, while aerogels are obtained in accordance with the sol–gel technology: the average pore size is 1.5–17.2 nm and the specific surface area is 120–878 m²/g. Aerogels are characterized by a high degree of porosity: the pore volume attains 1–4 cm³/g. Based on precursor xerogels, nanopowders of a tetragonal solid solution of the (ZrО₂)_0.92(Y₂О₃)_0.03(CeО₂)_0.05 composition with a particle size of 5–9 nm and S _spec = 74 m²/g were fabricated. Due to the high values of their specific surface area, the synthesized xerogels and aerogels are promising as sorbents, catalysts, or catalyst supports.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Synthesis and characterization of mesoporous carbon through inexpensive mesoporous silica as template

Mesoporous silica materials have been synthesized through sol–gel reaction using inexpensive sodium silicate as source of silica and low cost hydroxy carboxylic acid compounds as templates/pore forming agents. The material measured surface area of 1014 m²/g, pore diameter of 65 Å and pore volume of 1.4 cc/g when parameters like time and temperature of synthesis along with mole ratio of TA/SiO₂ were optimized. Here TA stands for tartaric acid. Carbonization of sucrose inside the pores of above silica material at 900 °C followed by removal of silica framework using aqueous ethanoic solution of NaOH gave rise to mesoporous carbon material. The resulting materials were characterized by N₂-sorption, FTIR spectroscopy, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, thermal analysis and cyclic voltammetry. Three dimensional interconnecting wormhole channel arrangement of mesoporous silica template leads to mesoporous carbon replica with surface area of 1200 m²/g. X-ray photoelectron spectroscopic study (XPS) of the mesoporous carbon material shows the concentration of carbon atom in the range of 97–98% with 1–2% oxygen and negligible amount of silica. The electrochemical double layer capacitance behavior of carbon material with the specific capacitance value of 88.0 F/g at the scan rate of 1 mV/s appears to be promising.     

A two-step synthesis of ordered mesoporous resorcinol–formaldehyde polymer and carbon

A new two-step method is developed for the synthesis of resorcinol–formaldehyde polymer and carbon with highly ordered mesoporous structures. For this method, resorcinol and formaldehyde is pre-polymerized in the first step under the presence of a basic catalyst to produce resorcinol–formaldehyde resol. Then, the resorcinol–formaldehyde resol is mixed with Pluronic F127 solution followed by the addition of an acid catalyst to allow the rapid self-assembly and condensation in the second step. Compared with the early reported evaporation-induced self-assembly method as well as the one-step liquid phase self-assembly method, in the present two-step liquid method the self-assembly and condensation process can be carried out rapidly by using low amount of base and acid catalysts at room temperature. After the activation by CO₂, the carbon materials maintained ordered mesostructure, and the BET surface area enlarged to 2660 m²/g and total pore volume increased to 2.01 cm³/g. The CO₂ activation not only creates micropores within the carbon frameworks but also enlarges the mesopores by elimination of the carbon pore walls.     

Activated carbon from cherry stones by chemical activation: Influence of the impregnation method on porous structure

Cherry stones are utilized as a precursor for the preparation of activated carbons by chemical activation with phosphoric acid (H₃PO₄). The activation process typically consists of successive impregnation, carbonization, and washing stages. Here, several impregnation variables are comprehensively studied, including H₃PO₄ concentration, number of soaking steps, H₃PO₄ recycling, washing of the impregnated material, and previous semi-carbonization. The choice of a suitable impregnation methodology opens up additional possibilities for the preparation of a wide variety of activated carbons with high yields and tailored porous structures. Microporous activated carbons with specific surface areas of ～800 m² g^?1 are produced, in which > 60% of the total pore volume is due to micropores. High surface areas of ～1500 m² g^?1 can be also developed, with micropore volumes being a 26% of the total pore volume. Interestingly, using the same amount of H₃PO₄, either carbons with surface areas of 791 and 337 m² g^?1 or only one carbon with a surface area of 640 m² g^?1 can be prepared. The pore volumes range very widely between 0.07–0.55, 0.01–0.90, and 0.09–0.79 cm³ g^?1 for micropores, mesopores, and macropores, respectively.     

Zirconia with Defined Particle Morphology and Hierarchically Structured Pore System Synthesized via Combined Exo‐ and Endotemplating

Highly porous zirconia with defined particle morphology can be prepared by impregnation of spherical activated carbon as an exotemplate with a zirconia nanoparticle sol. The resulting zirconia spheres show a particle size distribution between 0.2 and 0.4 mm and exhibit high specific surface areas and specific pore volumes up to 104 m²g^–1 and 0.56 cm³g^–1, respectively. Addition of a triblockcopolymer (TBC) as an endotemplate during the synthesis leads to the formation of an additional pore system. The corresponding spherical zirconia products possess a hierarchically structured pore system with a bimodal pore size distribution with maxima at ca. 3 and 20 nm. The relative fraction of pores originating from the endotemplate can be varied by changing the endotemplate content in the zirconia nanoparticle sol. The presence of the TBC also has an influence on the specific surface area and the specific pore volume. Using the ratio of TBC to zirconium of n_TBC/n_Zr = 0.027, a material can be prepared that exhibits a specific surface area and a specific pore volume of 161 m²g^–1 and 0.62 cm³g^–1, respectively. These values are more than twice as high as for zirconia prepared by a conventional precipitation method (68 m²g^–1 and 0.11 cm³g^–1, respectively).     

3D printed carbon-ceramic structures for enhancing photocatalytic properties

3D printing creates structures from digitally designed models by bottom-up fabrication method, achieving excellent control of target structures from various materials. Compared with conventional manufacturing methods such as machining, chemical engineering and bio-template, 3D printing shows advantages in aspects of parameterization-designed structure, rapid preparation, high precision and low cost. Herein, 3D printed carbon-ceramic support with designed array patterns, square, circular and diamond, was fabricated in an inert atmosphere to obtain sophisticated pore structure with high surface area. The existence of pyrolyzed carbon from UV-curable resin suppressed the mass transfer process when sintering and was found to greatly increase pore area from 0.067 m²/g to 0.509 m²/g. Molybdenum disulfide (MoS₂) chosen as a typical catalyst was loaded on the sintered support. The photodegradation efficiency of as-printed carbon support with MoS₂ increased to 45.95% while that of pure MoS₂ was only 23.35%. The catalyst-support system showed significant stability and the efficiency decreased to 82.35% after five cycles. UV–Vis diffused reflectance spectra proved that pyrolyzed carbon increased the light adsorption efficiency at the whole range of visible light.     

Titanium Carbide and Carbide‐Derived Carbon Composite Nanofibers by Electrospinning of Ti‐Resin Precursor

TiO₂/C and TiC/C composite nanofibers were produced by electrospinning of resin/TiCl₄ precursor solution. The resulting ceramic fiber webs were porous and showed surface areas as high as 523 m²g^–1. They were further converted to carbide‐derived carbon (CDC) fibers under full retention of the fiber‐like shape and flexibility. These CDC membranes showed a hierarchical pore structure and specific surface as high as 1378 m²g^–1. Applications in the area of high temperature filtration, catalyst support and energy storage are conceivable.     

Preparation and Characterization of Activated Carbon from Eucalyptus Sawdust I. Activated by NaOH

Eucalyptus sawdust was used as a precursor to prepare activated carbon using NaOH as a chemical activation agent. The effect of preparation conditions on the characteristics of the produced activated carbon used as an adsorbent was investigated. The performance of the activated carbon was characterized by N₂ adsorption–desorption isotherms, Brunauer–Emmett–Teller equation, Barett–Joyner–Halenda equation, scanning electron microscopy and Fourier transform infrared analysis. When the eucalyptus sawdust mass was 30.00 g, with particle sizes between 0.25 and 0.42 mm, and the sawdust was heated and charred before activation by NaOH, the optimized conditions for the preparation of activated carbon was found to be as follows: mass ratio of NaOH to eucalyptus sawdust, 1:2; activation time, 30 min; and activation temperature, 700 °C. The Iodine number and BET surface area of the produced activated carbon was 899 and 1.12 × 10³ m² g^?1, respectively, with a 13.3 % yield. Activated carbon exhibits adsorption isotherms of type IV. The total pore volume, micropore volume and average pore diameter were recorded as 0.636, 0.160 cm³ g^?1 and 2.27 nm, respectively. The pore structure of the activated carbon is mainly mesoporous. Carbonyl and hydroxyl groups may also exist on the activated carbon surface.     

An activation-free method for preparing microporous carbon by the pyrolysis of poly(vinylidene fluoride)

A simple method for the preparation of microporous carbon was presented by pyrolyzing poly(vinylidene fluoride) (PVDF) at high temperature under N₂ atmosphere without activation or any other additional processes. The yield of PVDF-derived carbon is 35.0%. Its specific surface area reaches 1012 m² g with a pore volume of 0.41 cm³ g⁻¹. The carbon is microporous with unimodal pore size distribution at 0.55 nm.     

COD removal from landfill leachate using a high-performance and low-cost activated carbon synthesized from walnut shell

An activated carbon with high adsorption capacity was synthesized from walnut shell as a solid waste using different chemical reagents. It was used to mitigate chemical oxygen demand from municipal landfill leachate. The activated carbon synthesized with impregnation ratio of 4:1 (H₃PO₄ to char) at a temperature of 500°C (ACH4-500) demonstrated the best textural properties based on Brunauer–Emmett–Teller analysis, i.e., specific surface area of 1,851.1?m²/g, total pore volume of 1.03?cm³/g, and mean pore diameter of 2.24?nm and was selected for adsorption experiments. The maximum adsorption capacity and removal were 123.1?mg/g and 84.7% under optimum condition, respectively. The equilibrium data were fitted with different model isotherms and Redlich–Peterson model showed the best match with experimental data. Kinetic data were well described by pseudo-second-order equation. Study of adsorption thermodynamics revealed spontaneous and endothermic nature of adsorption. In addition, the adsorbent showed satisfactory results, e.g., 100% removal of Pb²⁺, Cd²⁺, and Mn²⁺, in diminishing heavy metals as hazardous materials from landfill leachate.     

Activated Carbon Preparation from Lignin by H3PO4 Activation and Its Application to Gas Separation

Activated carbons were produced from corn straw lignin using H₃PO₄ as activating agent. The optimal activation temperature for producing the largest BET specific surface area and pore volume of carbon was 500 °C. The maximum BET specific surface area and pore volume of the resulting carbon were 820 m²g^–1 and 0.8 cm³g^–1, respectively. The adsorption isotherm model based on the Toth equation together with the Peng‐Robinson equation of state for the determination of gas phase fugacity provide a satisfactory representation of high pressure CO₂, CH₄ and N₂ adsorption. The kinetic adsorption results show that the breakthrough difference between CO₂ and CH₄ is not obvious, indicating that its kinetic separation performance is limited.     

Microporous activated carbon spheres prepared from resole‐type crosslinked phenolic beads by physical activation

Microporous activated carbon spheres (ACSs) with a high specific Brunauer–Emmet–Teller (BET) surface area were prepared from resole‐type spherical crosslinked phenolic beads (PBs) by physical activation. The PBs used as precursors were synthesized in our laboratory through the mixing of phenol and formaldehyde in the presence of an alkaline medium by suspension polymerization. The effects of the gasification time, temperature, and flow rate of the gasifying agent on the surface properties of ACSs were investigated. ACSs with a controllable pore structure derived from carbonized PBs were prepared by CO₂ gasification. Surface properties of ACSs, such as the BET surface area, pore volume, pore size distribution, and pore diameters, were characterized with BET and Dubinin–Reduchkevich equations based on N₂ adsorption isotherms at 77 K. The results showed that ACSs with a 32–88% extent of burn‐off with CO₂ gasification exhibited a BET surface area ranging from 574 to 3101 m²/g, with the pore volume significantly increased from 0.29 to 2.08 cm³/g. The pore size and its distribution could be tailored by the selection of suitable conditions, including the gasification time, temperature, and flow rate of the gasifying agents. The experimental results of this analysis revealed that ACSs obtained under different conditions were mainly microporous. The development of the surface morphology of ACSs was also studied with scanning electron microscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Reforming of methane with carbon dioxide over supported bimetallic catalysts containing Ni and noble metal: I. Characterization and activity of SiO2 supported Ni–Rh catalysts

Cobalt catalysts supported on silica aerogel have been prepared using sol–gel chemistry followed by drying under supercritical ethanol conditions. Three different loadings of cobalt were synthesized: 2, 6, and 10% by weight. Transmission electron micrographs indicate that the metallic cobalt exists as discrete particles 50–70 nm in diameter for the 2 and 6% loadings. The 10% catalyst shows long needles of cobalt. BET and BJH measurements indicate that the catalysts retain the silica aerogel properties of high surface area (∼800 m²/g), large pore volume (∼5 cm³/g), and an average pore diameter in the mesoporous regime (∼25 nm). The catalysts were evaluated for Fischer–Tropsch activity in a laboratory-scale packed bed reactor. All three catalysts were active with the 10% Co catalyst achieving more than 20% CO conversion which corresponds to a rate of 1.53 g CO per g-cat per hour. The catalysts were selective for the C₁₀₊ hydrocarbons with more than 50% of the carbon contained within this fraction. A significant portion of the C₉–C₁₅ hydrocarbon product was observed as 1-olefins which reflects the enhanced mass transport within the very porous aerogel support.     

Preparation of ultra-thin PAN-based activated carbon fibers with physical activation

This study elucidates the stabilization and activation in forming activated carbon fibers (ACFs) from ultra-thin polyacrylonitrile (PAN) fibers. The effect of stabilization time on the properties and structure of resultant stabilized fibers was investigated by thermal analysis, X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy (SEM). Stabilization was optimized by the pyrolysis of ultra-thin PAN fibers in air atmosphere at 280°C for 15 min, and subsequent activation in steam at 1000°C for 0.75 to 15 min. Resultant ACFs were characterized by N₂ adsorption at 77 K to evaluate pore parameters, XRD to evaluate structure parameters, and field emission scanning electron microscopy (FESEM) to elucidate surface morphology. The produced ACFs had surface areas of 668–1408 m²/g and a micropore volume to total pore volume ratio from 78 to 88%. Experimental results demonstrate the surface area and micropore volume of 1408 m²/g and 0.687 cm³/g, respectively, following activation at 1000°C for 10 min. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Functionalization and Characterization of Carbon Nanohorns (CNHs) for Hydrotreating of Gas Oils

Improvement in the functionality of carbon nanohorns (CNHs), a novel carbon nanomaterial, for hydrotreating applications is investigated in the present work. The current work was carried out by using pristine CNHs synthesized by the submerged arc in liquid nitrogen and their corresponding physicochemical properties were investigated. The surface area, pore diameter and pore volume of the pristine CNHs are 129 m²/g, 23.1 nm, and 0.64 cm³/g respectively. Functionalizing the CNHs with 30 wt% HNO₃ under reflux for 15 min to 4 h at 110 °C modified the physical and chemical properties. 30 min functionalization duration was found to be the best and a co-impregnation method was used to load Ni (2.5 wt%) and Mo (13 wt%) onto the support. Techniques used to thoroughly characterize the properties of pristine CNHs, functionalized CNHs and NiMo/CNHs catalyst include: Brauner-Emmett-Teller (BET), Fourier Transform Infrared (FTIR) and Raman Spectroscopy. Type II isotherm and mesoporous pore diameter was observed for CNHs in it’s pristine, functionalized or catalyst form. An increase in surface area of over 500 m²/g was also attained under optimum functionalized conditions. The pore volume of acid treated CNH samples for hydrotreating increased by ~10 % as compared to the pore volume of the pristine CNHs. FTIR results revealed the presence of carboxylic acid (–COOH) groups on the functionalized CNHs and I _D/I _G ratios from Raman spectroscopy was used to assess the increase in defects (nanowindow) on functionalized CNHs. The enhanced properties of functionalized and catalyst-supported CNHs offers prospect for hydrotreating gas oils.     

Heteroatom-doped carbon gels from phenols and heterocyclic aldehydes: Sulfur-doped carbon xerogels

Sulfur-doped carbon xerogels were obtained through carbonization of resorcinol/2-thiophenecarboxaldehyde organic gels. The acid-catalyzed sol–gel polymerization of resorcinol and 2-thiophenecarboxaldehyde leads to organic gels whose morphology and texture is dependent on the amount of catalyst used. As a result, monolithic organic gels with sulfur content of up to 19.6 wt.% and easily tailored properties can be produced. After carbonization, a substantial amount of sulfur is retained and porous carbon xerogels with S-content of up to 10 wt.% are produced (at 800 °C). Depending on the sol–gel synthesis conditions, monolithic S-doped carbon xerogels with controllable and enhanced mesoporosity, surface areas of up to 670 m²/g and enhanced mechanical integrity were obtained. Additional KOH activation of the organic or carbon xerogels enables production of micro–mesoporous carbons with surface areas of up to 2550 m²/g while retaining over 5 wt.% of sulfur. Preliminary CO₂ adsorption measurements were performed. On the basis of resorcinol/2-thiophenecarboxaldehyde gel synthesis a more general approach towards heteroatom-doped carbon gels is proposed: sol–gel polymerization of phenols and heterocyclic aldehydes. Thus a variety of heteroatom-doped porous carbon materials with a tailored pore texture and morphology are available via this procedure.     

Novel micro-spherical Si3N4 nanowire sponges from carbon-doped silica sol foams via reverse templating method

A novel kind of nanowire sponges, namely Si₃N₄ nanowire-weaving microspheres, synthesized from a simple, convenient, high-efficient approach are proposed here. As the reverse template, three-dimensional foam skeleton structure with uniform pores and ultrathin pore walls is constructed via the effective particle-stabilized foam method, where the silica sol and carbon black are chosen as the raw materials, providing the sufficient space for the growth of nanowires during the carbothermal reduction reaction process. The formation mechanism of this novel sponge is studied via multiple characterization methods. Si₃N₄ nanowires formed microspheres possess uniform and curving morphology due to the stable environment for growing via vapor–solid mechanism, leading to the relatively high specific surface area of 86.77 m²/g. Owing to in-situ oxidation process, micro-spherical SiO₂ nanowire sponges with similar morphology are synthesized, which present diameter in range of 20-40 nm and specific surface area of 50.47 m²/g. This work provides insights for the design of high-performance nanowire sponges with promising applications in the filtration, thermal insulators, and catalyst supports fields.