Carbon aerogels, cryogels and xerogels: Influence of the drying method on the textural properties of porous carbon materials

Carbon materials with tailored texture can be obtained from drying and pyrolysis of resorcinol-formaldehyde gels. The pore texture of both dried and pyrolyzed material depends on the drying process. Several more or less expensive methods (supercritical drying, freeze-drying, evaporative drying) were tested in order to determine which process is the most suitable for the synthesis of a porous carbon with a definite texture. Supercritical drying leads to the highest pore volume and the widest texture range, but residual surface tensions and shrinkage are not avoided when the pore size is small or when the material density is low; this hampers to fix both the pore volume and the pore size easily. Monoliths are very difficult to obtain by freeze-drying, and the appearance of huge channels due to ice crystal growth at high dilution ratio hinders the fabrication of low density materials. Moreover, gels with small pores do not remain frozen throughout drying, which leads to surface tensions and shrinkage. Although generally replaced by more complicated techniques, evaporative drying is suitable when dense carbons are needed or when the only selection criterion is the pore size: all pore sizes are reachable, but this parameter is in this case strongly correlated to the pore volume.     

Pore structures of multi-walled carbon nanotubes activated by air, CO2 and KOH

Multi-walled carbon nanotubes (MWNTs) synthesized by the catalytic decomposition of benzene were activated by KOH, CO₂ or air. The adsorption isotherms of the activated MWNTs were analyzed and their pore size distributions were obtained. The results showed that the specific surface areas of the MWNTs activated by KOH, CO₂ and air were increased to 785 m²/g, 429 m²/g and 270 m²/g, respectively. The MWNTs activated by KOH were rich in micropores and mesopores, especially high mesopores having volumes up to 1.04 cm³/g. The CO₂-activated MWNTs also had many micropores while the air-activated MWNTs had a much smaller micropore volume. The morphologies of the activated MWNTs were examined by transmission electron microscopy and high resolution transmission electron microscopy, and the activation mechanisms were discussed.     

HEMP-derived activated carbon fibers by chemical activation with phosphoric acid

Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m²/g and mesopore volume of 1.25 cm³/g were obtained at 550 °C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO₃ and mainly C-PO₃ and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction.     

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).     

KOH活化制备脱水污泥活性炭

以污水处理厂脱水污泥为原料,采用KOH化学活化工艺制备污泥活性炭,研究了碱泥比、活化剂浓度、活化温度及活化时间等因素对活性炭碘吸附值的影响。结果表明,最优的污泥活性炭制备条件为碱泥比3,活化剂浓度40%,活化温度500℃,活化时间60 min。用该条件下制备的污泥活性炭处理电镀废水,其对重金属吸附去除效果良好,平均去除率可以达到73.46%。     

KOH活化制备脱水污泥活性炭

以污水处理厂脱水污泥为原料,采用KOH化学活化工艺制备污泥活性炭,研究了碱泥比、活化剂浓度、活化温度及活化时间等因素对活性炭碘吸附值的影响。结果表明,最优的污泥活性炭制备条件为碱泥比3,活化剂浓度40%,活化温度500℃,活化时间60 min。用该条件下制备的污泥活性炭处理电镀废水,其对重金属吸附去除效果良好,平均去除率可以达到73.46%。     

Microporous activated carbon prepared from coconut shells using chemical activation with zinc chloride

Microporous activated carbon samples were prepared from coconut shells (low-cost lignocellulose waste), using chemical activation with zinc chloride followed by physical activation. Textural characterization was performed using nitrogen adsorption at 77 K. The sample that presented the best characterization results was then evaluated for methane adsorption at pressures between 0.1 MPa and 7 MPa and temperatures in the range 283–333 K. At 298 K and 40 bar, a capacity of ca. 122 mg of methane/g of carbon (80 v/v) was observed, just short of the target established in Brazil for ANG in remote sites transportation (100 v/v). These results suggest that activated carbons prepared from coconut shells, using chemical activation followed by physical activation, may be further developed as potential adsorbents for natural gas storage applications.     

Steam and KOH activation of biochar: Experimental and modeling studies

Biochar was used as a precursor of activated carbon using physical (steam) and chemical (potassium hydroxide) activation processes. The effects of operating conditions for each activation method on the BET surface area of the product and reaction yield were investigated using central composite design (CCD). For both activation processes, quadratic models were developed, by Design-Expert software, for BET surface area and reaction yield. The optimum operating conditions were calculated by the models to produce physically and chemically activated carbons with large surface area and product yield. The optimum BET surface area and product yield of physical activation are, respectively, 643 m²/g and 56.9 wt%, and of chemically activated carbon are, respectively, 783 m²/g and 75.3 wt%, which showed good agreement with the experimental values. Average pore diameters of physically and chemically activated carbons were in the ranges of 13–26 Å and 13–15 Å, respectively.     

Comparison of textile dyeing effluent adsorption on commercial activated carbon and activated carbon prepared from olive stone by ZnCl2 activation

Adsorption of Remazol Red B on activated carbon prepared from olive stone and commercial activated carbon from aqueous solutions was compared. Different activating agent (ZnCl₂) amounts and adsorbent particle size were studied to optimize adsorbent surface area. The adsorptive property of commercial activated carbon and activated carbon prepared from olive stone were investigated in terms of adsorbent dose, temperature, equilibrium time and pH. Then the obtained results were compared for all parameters, According to the results, the equilibrium time, optimum pH, adsorbent dosage were found 60 min, pH < 3–4 and 1.0 g/50 ml respectively. Lower adsorption capacity for RRB on activated carbon prepared from olive stone was found. The kinetic data for both adsorbents supports pseudo-second order model (r² > 0.99) and intra-particle model (r² > 0.95) but the first order kinetic model did not adequately fit to the experimental values (r² < 0.76). The equilibrium adsorption data were interpreted using Langmuir and Freundlich models. The adsorption of Remazol Red B was better represented by the Langmuir equation. In addition, the thermodynamic parameters, standard free energy (ΔG°), standard enthalpy (ΔH°), standard entropy (ΔS°) of the adsorption process were calculated for both adsorbents. To reveal the adsorptive characteristics of the produced active carbon, surface area measurements were carried out and structural analysis was performed using SEM-EDS.     

Comparative study of the textural characteristics of oil palm shell activated carbon produced by chemical and physical activation for methane adsorption

The objective of this study is to relate textural and surface characteristics of microporous activated carbon to their methane adsorption capacity. Oil palm shell was used as a raw material for the preparation of pore size controlled activated carbon adsorbents. The chemical treatment was followed by further physical activation with CO₂. Samples were treated with CO₂ flow at 850 °C by varying activation time to achieve different burn-off activated carbon. H₃PO₄ chemically activated samples under CO₂ blanket showed higher activation rates, surface area and micropore volume compared to other activation methods, though this sample did not present high methane adsorption. Moreover, it was shown that using small proportion of ZnCl₂ and H₃PO₄ creates an initial narrow microporosity. Further physical activation grantees better development of pore structure. In terms of pore size distribution the combined preparation method resulted in a better and more homogenous pore size distribution than the conventional physical activation method. Controlling the pore size of activated carbon by this combined activation technique can be utilized for tuning the pore size distribution. It was concluded that the high surface area and micropore volume of activated carbons do not unequivocally determine methane capacities.     

Preparation of high surface area activated carbon from corn by chemical activation using potassium hydroxide

Activated carbons were prepared through chemical activation of corn cob precursor, using potassium hydroxide as the chemical agent. The effect of different parameters, such as particle size, method of mixing, chemical/corn ratio, activation time and activation temperature, on weight loss and BET surface area of the produced activated carbons were discussed. The porosity of the activated carbons was evaluated through nitrogen adsorption. The storage capacity of the activated carbon was evaluated using natural gas. Under the experimental conditions investigated, the optimal conditions for production of high surface area carbons by chemical activation were identified. The results were compared with commercial activated carbons from coal.     

Porous carbon xerogels with texture tailored by pH control during sol-gel process

Despite commonly accepted ideas, evaporative drying does not always completely destroy the pore texture of phenolic gel. This work shows that very porous carbon materials can be synthesized by evaporative drying and pyrolysis of aqueous resorcinol-formaldehyde gels provided that the operating variables are correctly chosen. Moreover, in this manner monoliths can be easily produced. The pore texture of the materials was studied before and after pyrolysis in order to determine which synthesis and/or pyrolysis variables have an influence on the final texture of the carbon. Results show that it is possible to tailor the morphology of these materials by varying the initial pH of the precursors solution in a narrow range. Micro-macroporous, micro-mesoporous, microporous or totally non-porous carbon materials were obtained. The specific surface area is independent from the initial pH whereas the total void volume varies from 0.4 to 1.4 cm³/g when the initial pH decreases from 6.25 to 5.45. These materials can be used as catalysts supports or for electrochemical applications, the texture control being an interesting advantage.     

Preparation of activated carbon from paper mill sludge by KOH-activation

The purpose of this study is the preparation of activated carbon using paper mill sludge collected from biological wastewater treatment plant. The char produced from pyrolysis of paper mill sludge was chemically activated with potassium hydroxide. A systematic investigation of the effect of activation agent ratio, activation temperature and time on the properties of the char was carried out in a rotary kiln reactor. The chemically activated carbons were characterized by measuring iodine and methylene blue number and specific surface area. The activated carbon prepared from char of paper mill sludge in this study had maximum iodine and methylene blue number of 726.0 mg/g and 152.0 mg/g, and specific surface area of 1,002.0 m²/g, respectively. The result of estimation on adsorption capacities of metals, the Freundlich isotherms, yields a fairly good fit to the adsorption data, indicating a monolayer adsorption of metals onto activated carbon prepared from char of paper mill sludge using a potassium hydroxide as the activating agents.     

Molecular sieve properties obtained by cracking of methane on activated carbon fibers

Molecular sieve properties of activated carbon fibers modified by cracking treatment with methane are studied herein. The effect of methane treatment on the porous texture of the samples has been studied while varying temperature and time. These materials have been evaluated for their selectivity during CO₂ and CH₄ separation; their uptakes have been compared with non-treated activated carbon fibers (studied previously), which were considered suitable to be used as molecular sieves. Kinetics of CO₂ and CH₄ uptake have also been investigated in this research. The treatment produced materials exhibiting fast kinetics and high selectivity during CO₂ and CH₄ separation; at the same time however, the CO₂ uptake capacity was diminished.     

氯化锌活化生物质炭制备活性炭及其表征

以生物质炭为原料,采用氯化锌活化制备高比表面积微孔生物质活性炭,研究了浸渍比、活化剂浓度、活化温度与活化时间等条件对生物质活性炭吸附性能的影响,利用氮气吸附脱附、扫描电子显微镜、傅里叶红外光谱、X射线衍射等技术对生物质活性炭表面微观结构、形貌特征及化学结构进行了分析。结果表明,制备生物质活性炭的适宜工艺条件为：浸渍比为3,活化剂质量分数为40%,活化温度为600℃,活化时间为90min。在该条件下制备的生物质活性炭对亚甲基蓝的吸附值为213mg/g,超过国家水处理用活性炭一级品标准。经测试生物质活性炭的BET比表面积高达631.2m2/g,平均孔径2.23nm,总孔容为0.352cm3/g;孔隙结构发达,孔径分布狭窄,孔形状为排列整齐的蜂窝状结构,含有大量的微孔,84.4%的孔集中在2nm以内;表面存在醇羟基、羰基、醚、酚等含氧官能团。     

Preparation of activated carbon from phenolic resin by KOH chemical activation under microwave heating

Microwave irradiation was used as a heat source in the preparation of activated carbon (AC) from phenolic resin by potassium hydroxide chemical activation. The input microwave power was varied from 0.26 to 0.52 kW, while weight ratios of KOH to phenolic resin, R, were changed from 2 to 6. For a comparison, phenolic resin was also activated at R = 4 using electric furnace heating. During microwave irradiation of KOH and KOH/phenolic resin mixtures, the sample temperatures increased rapidly, which proved that activated carbon was prepared from the mixture under microwave irradiation. The most developed porosity of AC was obtained at R = 4 with a microwave power of 0.39 kW, and the activated carbon was characterized by high mesopore ratio. The development of mesopores under microwave heating was attributed to the rapid heating of the sample in contrast to slow electric furnace heating. The possibility of using the AC as a desiccant humidity conditioner was confirmed in terms of its effective water adsorptivity.