Preparation of mesoporous carbon by freeze drying

Resorcinol–formaldehyde (RF) cryogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde and freeze drying with t-butanol. The cryogels were characterized by nitrogen adsorption and density measurements. Their porous properties were compared with those of RF aerogels prepared by supercritical drying with carbon dioxide. RF cryogels were mesoporous materials with large mesopore volumes >0.58 cm³/g. Although surface areas and mesopore volumes of the cryogels were smaller than those of the aerogels, the cryogels were useful precursors of mesoporous carbons. Carbon cryogels were obtained by pyrolyzing RF cryogels in an inert atmosphere. Carbon cryogels were mesoporous materials with high surface areas >800 m²/g and large mesopore volumes >0.55 cm³/g. When pyrolyzed, micropores are formed inside the cryogels more easily than inside the aerogels.     

Preparation and characterization of activated carbon spheres from polystyrene sulphonate beads by steam and carbon dioxide activation

The polymeric precursor polystyrene sulphonate beads were used to produce activated carbon spheres (ACSs). ACSs were prepared by carbonization of polymeric precursor at 800°C followed by activation of resultant char with steam and carbon dioxide activation processes. The resulting ACSs were characterized for N₂ adsorption, Raman spectrometry, and scanning electron microscope (SEM). The adsorption properties such as, BET surface area (S_BET), pore volume (V_pore), and micropore volume (V_micro) of ACSs produced at different gasification time and temperature with steam and carbon dioxide activation were investigated in this study. It is found that porosity of ACSs produced from steam and carbon dioxide activation increases with increasing activation time. The results exhibited that ACSs produced from above carbon dioxide activation have shown high S_BET and V_pore 1266 m²/g and 1.13 cm³/g respectively compared to ACSs from steam activation S_BET 949 m²/g and V_pore 0.98 cm³/g, respectively. SEM study revealed that ACSs produced from carbon dioxide activation have exhibited a smooth surface and better microstructure as compared to ACSs from steam activation process. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

PREPARATION OF ORGANIC MESOPOROUS GEL BY SUPERCRITICAL/FREEZE DRYING

ABSTRACT

Resorcinol-formaldehyde (RF) aerogels were synthesized by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The control of mesoporous structure of the aerogels was studied by changing the amounts of resorcinol, formaldehyde, distilled water, and sodium carbonate (basic catalyst) used in the polycondensation. As a result of characterization by nitrogen adsorption, the mesopore radius of the RF aerogel was controlled in the range of 2.5 – 9.2 nm. After the hydrogels were immersed in excess of t-butanol, RF cryogels were prepared by freeze drying. The cryogels prepared were mesoporous materials with high surface areas > 500 m²/g and large mesopore volumes > 0.8 cm³/g. Although the surface areas and mesopore volumes of RF cryogels were smaller than those of RF aerogels, the cryogels were useful precursors of mesoporous carbons.     

Control of mesoporous properties of carbon cryogels prepared from wattle tannin and furfural

Wattle tannin–furfural (TFu) carbon cryogels are synthesized by sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide (NaOH) as a catalyst, dried by freeze-drying technique and then pyrolyzed under inert atmosphere, respectively. The amounts of wattle tannin (T), furfural (Fu), NaOH (C) and distilled water (W) are changed for preparing the mesoporous TFu carbon cryogels. The mole ratio of tannin to catalyst T/C plays a crucial role for the synthesis of TFu organic and carbon cryogels. The results suggest that the T/C ratio should be above 0.25 but <1.0 to prepare the mesoporous and homogeneous cryogels. Although TFu carbon cryogels have the broad mesopore size distribution, the mesoporous structure is controllable by the synthesis conditions. The carbon cryogels possess the mesopore volume less than 0.56 cm³/g and the BET surface area less than 600 m²/g. Moreover, the ratio of catalyst to water C/W can be used to prepare the homogeneous and mesoporous carbon cryogels, and to control the mesopore radius of carbon cryogels in the range of 1.6–9.6 nm.     

Alkaline activation of the donbass coals of different ranks

The effect of the rank of coal (C ^daf = 80?95.2%) on the yield and characteristics of activated carbons prepared under the conditions of alkaline activation (800°C, 1 h, Ar) at KOH/coal ratios of 1 g/g was studied. Under these conditions, the ability of coals to form porous materials decreased in the metamorphic series. Grade D coal (C ^daf = 80%) exhibited a maximum activation ability to form a material with S _BET = 1560 m²/g, V _Σ = 0.71 cm³/g, and V _mi = 0.51 cm³/g. A minimum activation ability was found in anthracite (C ^daf = 95.2%), which forms activated carbon with poorly developed porosity (S _BET = 306 m²/g, V _Σ = 0.15 cm³/g, and V _mi = 0.11 cm³/g).     

Activated carbon nanotubes-supported catalyst in fuel cells

The electrochemical property of platinum loaded on activated carbon nanotubes (Pt/ACNTs) was investigated by cyclic voltammograms (CVs) recorded in H₂SO₄ and H₂SO₄/CH₃OH aqueous solutions, respectively. Compared to 0.0046 A/cm² of Pt-loaded on pristine carbon nanotubes (Pt/CNTs) with a S_BET of 164 m²/g and 0.0042 A/cm² of conventional carbon black (Pt/C, Vulcan XC-72) with a S_BET of ∼250 m²/g, a better electrochemical activity (a high current density of 0.0070 A/cm² for weak-H₂ adsorption/desorption) of the Pt/ACNTs with high specific surface area (S_BET) of 830-960 m²/g was obtained. Furthermore, the highest current density of 0.079 A/cm² at 0.65 V in anodic sweep was observed during the methanol oxidation. On the basis of Pt size, utility ratio, and electro-active specific surface area (EAS), the Pt/ACNTs with a high Pt-loading of 50 wt.% exhibited the best electrochemical activity. The present ACNTs may be an excellent support material for electrochemical catalyst in proton exchange membrane and direct methanol fuel cells.     

Hierarchical porous carbon obtained from animal bone and evaluation in electric double-layer capacitors

Animal bone, an abundant biomass source and high volume food waste, had been converted into a hierarchical porous carbon in a simple two-step sustainable manner to yield a highly textured material. The structures were characterized by nitrogen sorption at 77 K, scanning electron microscopy and X-ray diffraction. The electrochemical measurement in 7 M KOH electrolyte showed that the porous carbon had excellent capacitive performances, which can be attributed to the unique hierarchical porous structure (abundant micropores with the size of 0.5–0.8 and 1–2 nm, mesopores and macropores with the size of 2–10 and 10–100 nm), high surface area (S_BET = 2157 m²/g) and high total pore volume (V_t = 2.26 cm³/g). Its specific capacitance was 185 F/g at a current density of 0.05 A/g. Of special interest was the fact that the porous carbon still maintained 130 F/g even at a high current density of 100 A/g.     

Template synthesis,structure, sorption properties and acidity of micro-mesoporous materials obtained from sol-precursor of zeolite BEA

Partially zeolitized micro-mesoporous materials (the total specific surface area S _BET  = 460–645 m²/g, mesopore volume V _meso  = 0.30–0.57 cm³/g, mesopore diameter D _meso  ≈ 5.6 nm, micropore volume V _micro  = 0.11–0.16 cm³/g, micropore diameter D _micro  ≈ 0.72 nm) and X-ray amorphous micro-mesoporous materials with uniform mesoporous structure (mesopore specific surface area S _meso  = 820–890 m²/g, total pore volume V _t  = 0.71–0.86 cm³/g, V _meso  = 0.53–0.67 cm³/g, D _meso  = 1.8–2.4 nm) were obtained by micellar templating of sol-precursor containing primary products of crystallization of zeolite BEA under conditions typical for forming of mesoporous molecular sieve MCM-41 (hydrothermal treatment at 100–130 °C for 3–5 days). It was found that the obtained X-ray amorphous micro-mesoporous materials contain secondary building units of zeolite BEA (five-membered rings of Si–O tetrahedra of BEA) and show acidic properties comparable to zeolite BEA.     

Nanoporosity development in the thermal-shock KOH activation of brown coal

Thermal-shock KOH activation of brown coal (800 °C, KOH/coal ratio 1 g/g) was shown to produce nanoporous activated carbon with more developed surface area than thermally-programmed heating (S_BET up to 1700 vs 1000 m²/g). Increasing the KOH/coal ratio (up to 1 g/g) in the activated mixture increases the total pore volume (0.14–1.0 cm³/g), the micropore volume (0.03–0.71 cm³/g), and also the volume of subnanometer pores (0.01–0.40 cm³/g). Thermal shock produces nanoporosity at lower KOH/coal ratios (0.5-1.0 g/g) than respective low-rate heating KOH activation.     

Optimization of mesoporous alumina sol-gel synthesis by Taguchi and response surface methodology

Taguchi method (TM) and response surface methodology (RSM) have been employed to optimize three parameters, including the amounts of P123, the amounts of nitric acid and calcination temperature, in order to define an optimal setting for sol-gel synthesis of high surface area mesoporous alumina powder (MA). Herein, the comparison of the both statistical approaches has been examined and discussed considering the nitrogen adsorption as the response variable because this important character for mesoporous materials is exceedingly sensitive to the synthesis parameters. The BET surface area (S_BET) and pore volume of MA under Taguchi optimal condition were 323.5 m² g⁻¹ and 0.551 cm³ g⁻¹, respectively, by conducting confirmation test. Furthermore, the confirmation test showed high S_BET of MA (363.4 m² g⁻¹), which was in a good agreement with calculated S_BET result (431.25 m² g⁻¹) by a quadratic model under RSM optimal condition. Moreover, 3D response surface plots and 2D contour plots of desirability have been discussed to visualize the influence of input factors on response variable. It is also concluded that RSM shows more appropriate (12.33% higher S_BET than TM) and efficient optimal condition with determining a quadratic function as the relationship between S_BET and synthesis parameters.     

Chemistry and kinetics of chemical vapour deposition of pyrocarbon: VII. Confirmation of the influence of the substrate surface area/reactor volume ratio

Carbon deposition from a methane–hydrogen mixture (p_CH4=17.5 kPa, p_H2=2.5 kPa) was studied at an ambient pressure of about 100 kPa and a temperature of 1100°C, using deposition arrangements with surface area/reactor volume ratios, [A_S/V_R], of 10, 20, 40 and 80 cm⁻¹. Steady-state deposition rates and corresponding compositions of the gas phase as a function of residence were determined. The deposition rates in mol/h increase with increasing [A_S/V_R] ratio at all investigated residence times up to 1 s. However, surface-related deposition rates in mol/m²h decreased. As the same results have been obtained in a preceding study using pure methane at a partial pressure of 10 kPa, it has been confirmed that all the kinetics can be determined by changing the [A_S/V_R] ratio.     

Influence of the type of sepiolite on the modification of the pore-size distribution in γ-Al2O3 supports

γ-Al₂O₃ modified supports with bimodal pore-size distributions were prepared by the addition of different types of natural sepiolites (α or β) into alumina. The supports were characterized by nitrogen physisorption, mercury porosimetry, X-ray diffraction, HRTEM and DTA techniques. A wide range of S_BET (94–238 m² g^− 1), pore volumes (0.3–0.82 cm³ g^− 1), and pore sizes were obtained in the supports depending on the type of sepiolite and its concentration added into alumina. The pore sizes were distributed as follows: mesopores around 1.8 nm in radius, mesopores in the radius range 3.0–25 nm and macropores between 25 and 300 nm in radius. The shape of the pore-size distributions depended on the type of sepiolite: the modal peak for pores larger than 3.0 nm was broad with β-type sepiolites and narrow with α-type sepiolites. The mesopore and macropore sizes can be controlled by the type of sepiolite as well as its concentration added to alumina.     

Hierarchical porous carbon templated with silica spheres of a diameter of 14 nm from pure chitosan or a chitosan/ZnCl<Subscript>2</Subscript> solution

Nitrogen-containing mesoporous carbons with the use of colloidal silica spheres of (14 nm) and chitosan as a carbon precursor were obtained. A removal of such small template particles from carbonized silica–chitosan composite is difficult and HF with a minimum concentration of 15 wt% should be used. By varying the silica-to-chitosan ratio, the porous characteristic of products is controlled. The modification by ZnCl₂ with a molar Zn-to-C (in chitosan mass) ratio of ‘6’ results in the development of microporosity; however it is accompanied by a significant reduction of mesopore volume (V_mes). The addition of ZnCl₂ in a ratio of ‘5.25’ and pH adjustment to 5.8 increase the volumes of micropores, small mesopores, BET surface area to 1975 m²/g, and preserve V_mes of 4.15 cm³/g. The novelty of the presented strategy is the creation of microporosity in the hard-templated materials by incorporating ZnCl₂ into the mixture of Ludox HS-40 template and chitosan precursor, as well as the investigation on how the pH of synthesis influences the final porosity. The pH of a silica–chitosan–zinc solution, equal to 3.9, provides some coordination of Zn²⁺ by –OH and –NH₂ groups, whereas pH adjustment to 5.8 results in the precipitation of a new template—Zn(OH)₂.     

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.     

Characterization of reticulate,three — dimensional electrodes

A study has been made of the mass transfer characteristics of a reticulate, three-dimensional electrode, obtained by metallization of polyurethane foams. The assumed chemical model has been copper deposition from diluted solutions in 1 M H₂SO₄. Preliminary investigations of the performances of this electrode, assembled in a filter-press type cell, have given interesting results: with 0.01 M CuSO₄ solutions the current density is 85 mA cm^–2 when the flow rate is 14 cm s^–1.List of symbols a area for unit volume (cm^–1) - C copper concentration (mM cm^–3) - c _L copper concentration in cathode effluent (mM cm^–3) - c ₀ copper concentration of feed (mM cm^–3) - C ₀ ⁰ initial copper concentration of feed (mM cm^–3) - d pore diameter (cm) - D diffusion coefficient (cm²s^–1) - F Faraday's constant (mcoul me _q ^–1 ) - i electrolytic current density on diaphragm area basis (mA cm^–2) - I overall current (mA) - K _m mass transfer coefficient (cm s^–1) - n number of electrons transferred in electrode reaction (me_q mM^–1) - P ] volumetric flux (cm³s^–1) - Q total volume of solution (cm³) - (Re) Reynold's number - S section of electrode normal to the flux (cm²) - (Sc) Schmidt's number - (Sh) Sherwood's number - t time - T temperature - u linear velocity of solution (cm s^–1) - V volume of electrode (cm³) - divergence operator - void fraction - u/K _m a(cm) - electrical specific conductivity of electrolyte (^–1 cm^–1) - _S potential of the solution (mV) - density of the solution (g cm^–3) - v kinematic viscosity (cm²s^–1)     

Theoretical evaluation of pillared clay adsorbents: Part III: The total porosity and the macrostructure of Al-pillared montmorillonite and hectorite

As a new class of porous materials, pillared clays have been more and more investigated as a potential substituent or complement to zeolite materials. Despite of intensive research, real industrial applications are still lacking. Somehow, the porosity of these materials is lower than what was expected. To evaluate the potential of the pillared clays as adsorbent, theoretical calculations were carried out to obtain a better insight of the influence of the PILC's macrostructure on the micro- and mesoporosity.Assuming the layers to stack in a 3-dimensional brick-like fashion, the micro- and mesoporosity and interpillar free distance were calculated taking into account: the clay layer dimensions, interlayer free spacing, lateral distance between adjacent layers, partial pillaring/collapse and pillar symmetry. The calculations were performed for Al-pillared montmorillonite and hectorite and validated with experimental results.Nomenclature a height of the mesopore - Al-pilM aluminium pillared montmorillonite - Al-pilH aluminium pillared hectorite - b lateral distance between two stacks - CEC cation exchange capacity meq/g - d _pp interpillar free distance - H height of a clay layer Å - IFS interlayer free spacing Å - L length and width of a clay layer Å - P pillar height Å - pil/g number of pillars in 1 g clay - (pil/g)_in number of pillars, in 1 g clay, intercalated between the layers - (pil/g)_out number of pillars, in 1 g clay, not intercalated - (pil/g)_tot total number of pillars in 1 g clay - pl/g number of clay layers in 1 g clay - s number of layers in a stack - SA^av layer surface area available for pillaring m²/g - SA_BET BET surface area m²/g - SA_max maximum surface area of the clay layers m²/g - SA _meso ^av available surface area in the mesopores m²/g - Ustr unit structure - Ustr/g number of unit structures in 1 g clay - V_free free volume in the unit structure - V _meso ^av calculated available mesopore volume cc/g - V _meso ^ult calculated ultimate mesopore volume cc/g - V _micro ^av calculated available micropore volume cc/g - V _micro ^ult calculated ultimate micropore volume cc/g - V_1pil volume of one pillar cc/g - V_pil(in) total volume of the pillars between the layers cc/g - V_pil(out) total volume of the pillars outside the layers cc/g - V_tot total pore volume atp/p ₀ = 0.98 cc/g - W_pil width of a pillar Å - Wt%_pil weight percentage Al₂O₃ present as pillars     

Design characteristics of an aluminium-air battery with consumable wedge anodes

An attempt was made to optimize a mechanically rechargeable bipolar-cell battery, exemplified by an aluminium-air battery with self-perpetuating wedge anodes. The optimization involved current density of battery operation and some design parameters such as the anode thickness and the cell dimensions. It was shown that these parameters depend on the energy-to-power ratio selected by the user. The saline electrolyte aluminium-air battery was found to be essentially a low power-density/high energy-density power source. Energy densities of up to over 1500 W h kg^–1 are achievable for low power needs, indicating very long operations between recharging. It was also shown that aluminium should render significantly cheaper electric energy than any of the high-energy density metals.Nomenclature d anode plate thickness (cm) - d _p thickness of end-plates (cm) - d thickness of cell walls (cm) (see Fig. 1) - E energy density (W h kg^–1) - E _B total energy contained in the battery (k W h) - F the Faraday constant 26.8 A h mol^–1 - g _c weight of the air cathode per unit anode area (g cm^–2) - g _e excess electrolyte per unit electrode area (g cm^–2) - g _h weight of the hardware per unit anode area (g cm^–2) - g _m weight of metal per unit electrode area (g cm^–2) - _m g excess of unconsumable metal per unit electrode area (g cm^–2) - g ₀ sum of all the weights except that of consumable metal (g cm^–2) - g _ox weight of oxygen consumed withg _m (g cm^–2) - G total weight of battery (g) - G _m total amount of reserve metal per cell and per cm width (kg cm^–1) - G _m total weight of the wedges (kg) - G _r total weight of the reserve anode container except the metal (kg) - G free energy of oxidation of the metal (kW h mol^–1) - h _a height of the wedge (cm) - h _r reserve anode height (cm) - j current density (mA cm^–2) - J total current drawn from the battery (mA) - n number of electrolyte replacements between anode replacement - n _c number of cells in a battery - M atomic weight of the metal (kg mol^–1) - P power density (W kg^–1) - Q _e cost of metal in the cost of unit energy produced ($ kW^–1 h^–1) - Q _e ⁰ theoretical figure of merit of a metal ($ kW^–1 h^–1) - Q _m cost of metal per unit weight ($ kg^–1) - S _a total anode surface area (cm²) - U cell voltage without ohmic drop (V) - V cell voltage (V) - x width of battery (cm) - z number of electrons exchanged per atom of metal dissolved - interelectrode spacing (cm) - spacing between cover and top of a new reserve anode (cm) - _f material efficiency - _v voltage efficiency - _e conductivity of electrolyte (ohm^–1 cm^–1) - _e electrolyte density (g cm^–3) - _m density of metal (g cm^–3) - _p density of end-plates (g cm^–3) - _w density of cell-walls (g cm^–3)     

Improvement of mesoporosity of carbon cryogels by acid treatment of hydrogels

Wattle tannin–furfural (TFu) gels are synthesized by the sol–gel polycondensation of wattle tannin with furfural by using sodium hydroxide as a catalyst and cured at 363 K. After cured, the TFu hydrogels are treated in hydrochloric acid (HCl) solutions with various variables as follows: aging temperature, HCl concentration and pH. TFu cryogels aged are then freeze-dried and pyrolyzed under an inert atmosphere to obtain TFu carbon cryogels. The TFu and carbon cryogels were characterized by N₂ adsorption and scanning electron microscope. The high HCl concentration yields the increase of mesopore volume almost twice especially at high temperature and the mesopore size distribution can be greatly developed sharper and narrower when the high concentration of HCl or/and the high temperature are used. Moreover, this method is the versatile approach for developing the mesopore structure of the low and high porous TFu hydrogels and has no serious problem of weight loss after treatment.     

Preparation of granular activated carbon from oil palm shell by microwave-induced chemical activation: Optimisation using surface response methodology

In this study, waste palm shell was used to produce activated carbon (AC) using microwave radiation and zinc chloride as a chemical agent. The operating parameters of the preparation process were optimised by a combination of response surface methodology (RSM) and central composite design (CCD). The influence of the four major parameters, namely, microwave power, activation time, chemical impregnation ratio and particle size, on methylene blue (MB) adsorption capacity and AC yield were investigated. Based on the analysis of variance, microwave power and microwave radiation time were identified as the most influential factors for AC yield and MB adsorption capacity, respectively. The optimum preparation conditions are a microwave power of 1200 W, an activation time of 15 min, a ZnCl₂ impregnation ratio of 1.65 (g Zn/g precursor) and a particle size of 2 mm. The prepared AC under the optimised condition had a BET surface area (S_BET) of 1253.5 m²/g with a total pore volume (V_tot) of 0.83 cm³/g, which 56% of it was contributed to the micropore volume (V_mic).     

Effect of anodically evolved gas bubbles on the rate of cathodic mass transfer in a vertical cylinder cell

Mass transfer coefficients were measured for the deposition of a copper from acidified copper sulphate solution at a vertical cylinder cathode stirred by oxygen evolved at a coaxial vertical cylinder lead anode placed upstream from the cathode and flush with it. The cathodic mass transfer coefficient was increased by a factor of 2.75–6.7 over the natural convection value depending on the rate of oxygen discharge at the lead anode and height of the cathode. The data were correlated by the equation:J=0.66(F_rR_e)^–0.21 An electrochemical reactor built of a series of vertical coaxial annular cells stirred by the counter electrode gases is proposed as offering an efficient way of stirring with no external stirring power consumption.Nomenclature a, b, c constants - C concentration of copper sulphate, mol cm^–3 - d cylinder diameter, cm - D diffusivity, cm² s^–1 - F Faraday's constant - g acceleration due to gravity, cm² s^–1 - h electrode height, cm - i current density at the oxygen generating anode, A cm^–2 - I _L limiting current density, A cm^–2 - K mass transfer coefficient, cm s^–1 - P gas pressure, atm - R gas constant, atm cm³ mol^–1 K^–1 - T temperature, K - u solution viscosity, poise - V oxygen discharge rate as defined by Equation 9, cm³ cm^–2 s^–1 or cm s^–1 - Z number of electrons involved in the reaction - J mass transferJ factor (S _tS_c ^0.66) - S _t Stanton number (K/V) - S _c Schmidt number (v/D) - S _h Sherwood number (Kh/D) - R _e Reynold's number (/Vh/u) - F _r Froude number (V ² /hg) - G_r Grashof number [gh ³/v² (1–)] - density of the solution, g cm^–3 - kinematic viscosity, cm² s^–1 - void fraction of the gas in the liquid-gas dispersion