Micropore Size Distribution of Activated Carbons Impregnated After Carbonization

Impregnated active carbons were prepared to be used as catalysts of complete oxidation, aiming the reduction of atmospheric emission of volatile organic compounds. The onversion efficiency is regulated by the catalyst dispersion in the porous structure where pollutants can access to be converted, which means that a good dispersion is required. Therefore, microporore size distribution has an important role on the catalyst efficiency, either because the access to the catalyst can be made through micropores, or because micropores can offer a significant deposition surface, if they are wide enough. When the impregnation is performed on the raw material or after activation, the micropore size distribution is already well studied. This paper aims to analyse that distribution when impregnation is performed after carbonization, because this knowledge is yet very scarce. Nut and almond shells were used as raw materials to prepare carbons impregnated with CoO and Co₃O₄. The comparison between the micropore size distributions was performed using Medek model. When impregnation is made after activation, it was concluded that, for both raw materials, the catalysts block part of the initial microporous structure, leading to a decrease in micropore volume and to an increase of the mean equivalent radius. When impregnation step is conducted after carbonization, the total micropore volume can increase or decrease in relation to the non-impregnated carbons, depending on the catalyst distribution and type of carbonized support. Nevertheless, all the carbons analysed showed that impregnation after carbonization increases micropore volume of wider micropores.     

利用标准等温线分析活性炭的完全孔分布

利用我们从标准等温线性质发展的孔分布计算两个新方法——完全无模型孔分布计算法和改进的微孔分析法对有代表性的九个活性炭样品进行了完全孔分布分析.用吸附仪测量样品的低温氮吸附等温线,用Pickett方程关联了这些吸附等温线,发现n值在1.1—1.4之间.因常数C值在100—300之间,选用标准等温线n_2作为完全孔分布分析的依据.获得了总表面积、微孔的表面积和体积、非微孔的表面积和体积以及平均微孔水力半径等主要孔结构参数,并获得了所有样品的完全孔分布.两个比值SR和VR在0.9—1.1之间,结果是令人满意的.分析结果指出,DX-09-1-1,7S及J三个样品基本上全是微孔,而其余的炭毡及BPL炭样品都有相当数量的非微孔.这些结果说明,利用标准等温线进行活性炭样品的完全孔分布分析是实用的.     

二苯并呋喃在活性炭上的吸附相平衡和动力学

The adsorption of dibenzofuran on three commercial granular activated carbons （ACs） was investigated by dynamic experiment to correlate the adsorption equilibrium and kinetics with the structure of activated carbons. Physical properties including surface area, average pore diameter, micropore area and micropore volume of the activated carbons were characterized by N2 adsorption experiment on ASAP2010. To calculate the adsorption parameters, adsorption isotherm data were fitted to the Langmuir equation, and adsorption kinetic data were fitted to the linear driving force （LDF） diffusion model. From the correlation results, it is concluded that the adsorption equilibrium and diffusion coefficient of dibenzofuran on activated carbon are controlled respectively by the total adsorbent surface area and the adsorbent pore diameter.     

Comparisons of pore properties and adsorption performance of KOH-activated and steam-activated carbons

Carbonaceous adsorbents with controllable pore sizes derived from carbonized pistachio shells (i.e., char) were prepared by the KOH activation and steam activation methods in this work. The pore properties including the BET surface area, pore volume, pore size distribution, and pore diameter of these activated carbons were characterized by the t-plot method based on N₂ adsorption isotherms. Through varying the KOH/char ratios from 0.5 to 3, the KOH-activated carbons exhibited BET surface areas ranging from 731 to 1687 m²/g with a similar micropore content (80–92%). The carbons activated by steam at 830 °C for 2 h had a BET surface area of 821 m²/g with the micropore content of 42%. The micropore/total pore volume ratio (V_micro/V_pore) and average pore size (D_pore) were independent of the KOH/char ratio, revealing that KOH activation is a powerful method in developing and controlling the number of micropores with a very similar pore size distribution. The adsorption equilibria and kinetics of methylene blue, basic brown 1, acid blue 74, 2,4-dichlorophenol, 4-chlorophenol, and phenol from water on all activated carbons at 30 °C were investigated to demonstrate the fact that adsorption of organics is not only dependent upon the BET surface area but is also determined by the relative size between pores and molecules. The adsorption isotherms were subjected to the model fitting according to Langmuir and Freudlich equations. By comparing the projected area of adsorbates, the surface coverage of phenols is about 3.6 times of that of dyes (based on unit gram of activated carbon). The Elovich equation was found to suitably describe the adsorption process of all KOH-activated carbons while the adsorption behavior on the steam-activated carbon was reasonably fitted with the intraparticle diffusion model.     

Water vapour adsorption on lignin‐based activated carbons

Lignocellulosic wastes are interesting precursors for carbon products. The high amount of Na observed in kraft lignin makes it a promising precursor for the preparation of activated carbons for desiccant applications. Water adsorption capacity and kinetics of kraft lignin‐based chars and activated carbons with different burn‐off and inorganic matter content have been studied. CO₂ partial gasification of lignin char develops a wide porous structure. An increase of the micropore volume can be observed at low to medium burn‐offs. At degrees of higher activation the mesoporous structure develops. For very high burn‐off the porous structure is destroyed by coalescence of the pores and reduction of the carbon material. The carbons obtained show atomic surface concentrations of sodium from 7.6–15.4%, as revealed by XPS analysis. Water vapour adsorption isotherms have been obtained in a thermogravimetric system and have been fitted by a DS model, which properly represents the experimental data. The kinetics of water vapour adsorption follows a linear driving force mass transfer (LDF) model. The presence of sodium and oxygen surface groups on the carbon surface enhances water vapour adsorption at low relative pressure. Activated carbon produced at 41% burn‐off shows the highest water vapour adsorption at low relative pressures, as a consequence of the high sodium dispersion on its surface. The sodium dispersed over the carbon surface undergoes clustering as gasification proceeds, decreasing the number of active centres. For burn‐off higher than 41%, this behaviour produces a decrease in the water adsorbed at low relative pressures. Copyright © 2007 Society of Chemical Industry     

Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons

We present a new model of adsorption on micro-mesoporous carbons based on the quenched solid density functional theory (QSDFT). QSDFT quantitatively accounts for the surface geometrical inhomogeneity in terms of the roughness parameter. We developed the QSDFT models for pore size distribution calculations in the range of pore widths from 0.4 to 35 nm from nitrogen at 77.4 K and argon at 87.3 K adsorption isotherms. The QSDFT model improves significantly the method of adsorption porosimetry: the pore size distribution (PSD) functions do not possess gaps in the regions of ∼1 nm and ∼2 nm, which are typical artifacts of the standard non-local density functional theory (NLDFT) model that treats the pore walls as homogeneous graphite-like plane surfaces. The advantages of the QSDFT method are demonstrated on various carbons, including activated carbons fibers, coal based granular carbon, water purification adsorbents, and mirco-mesoporous carbon CMK-1 templated on MCM-48 silica. The results of PSD calculations from nitrogen and argon are consistent, however, argon adsorption provides a better resolution of micropore sizes at low vapor pressures than nitrogen adsorption.     

水蒸气活化法制备椰壳活性炭的孔结构特征

以农林废弃物椰壳在600℃炭化2h后的炭化料为原料,以水蒸气为活化剂,研究了活化温度、活化时间、水蒸气用量对活性炭的比表面积、微孔容积和收率等的影响。结果表明：椰壳炭化料的比表面积仅为185m^2／g,且以中孔为主。在活化过程中,通过提高活化温度和水蒸气用量缩短了活化时间,扩宽了孔径;当水蒸气用量和活化温度较为适宜时,延长活化时间,有利于微孔的形成。活性炭的比表面积、总孔容积、微孔容积可达：1465m^2／g,0．9703cm^3/g,0．7519cm^2／g。并通过非定域密度函数理论（NLDFT）对活性炭的孔径分布进行了表征。     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.     

Quenched solid density functional theory method for characterization of mesoporous carbons by nitrogen adsorption

Quenched solid density functional theory (QSDFT) model for characterization of mesoporous carbons using nitrogen adsorption is extended to cylindrical and spherical pore geometries. The kernels of theoretical isotherms in the range from 0.4 to 50 nm are constructed accounting for different possible variations of the pore shapes in micropore and mesopore regions. The results of QSDFT method are illustrated with experimental data on adsorption on novel CMK-3 and 3DOm carbons. The proposed method is recommended for pore size distribution calculations for micro–mesoporous carbons obtained through various templating mechanisms.     

Role of microporosity and surface functionality of activated carbon in methylene blue dye removal from water

Activated carbons have been prepared from jute stick by both chemical and physical activation methods using zinc chloride and steam, respectively. They were characterized by evaluating surface area, iodine number, pore size distribution, and concentration of surface functional groups. The chemically activated carbon largely featured micropore structure, while the physically activated carbon mainly featured macropore structure. The specific surface area of chemically and physically activated carbons was 2,325 and 723 m ² /g, while the iodine number was 2,105 and 815mg/g, respectively. The concentration of surface functional groups was determined by Boehm titration method, which suggested that different types of surface functional groups are randomly distributed on chemical activated carbons, while it is limited for physical activated carbon. The microporosity along with surface functional groups provided a unique property to chemically activated carbon to adsorb Methylene Blue dye to a large extent. The adsorption of dye was also affected by the adsorption parameters such as adsorption time, temperature and pH. Comparatively, higher temperature and pH significantly facilitated dye adsorption on chemically activated carbon.     

On the methane adsorption capacity of activated carbons: in search of a correlation with adsorbent properties

BACKGROUND: There exists now a widely held view that the methane storage capacity on an activated carbon is not related to any of the routinely determined properties of the adsorbent, such as surface area or micropore volume. This has been confirmed and a correlation pursued with other physical and/or chemical properties of both commercially available carbons and those prepared in the laboratory. Textural characteristics (from nitrogen adsorption isotherms at 77 K) considered were BET‐equivalent specific surface area, DR micropore volume and Horvath–Kawazoe micropore size distribution. Chemical properties were evaluated using Fourier transform infrared (FTIR) spectroscopy, thermal programmed decomposition (TPD) and Boehm titrations. Both kinetic and equilibrium methane adsorption experiments were performed at 273 and 298 K and up to 3.5 MPa. RESULTS: Using phosphoric acid to activate peach stones together with additional thermal treatment enabled the production of activated carbons with 137 v/v methane adsorption capacity at 298 K. CONCLUSIONS: The presence of acidic surface functional groups has a detrimental influence on methane uptake, due to the chemical inertness of the adsorbate and/or to pore blockage of the adsorbent. Basic surface functional groups (pyrone), together with a desirable pore size distribution centered at ca 0.8 nm, are thought to be responsible for improved methane adsorption capacity on such activated carbons. Copyright © 2009 Society of Chemical Industry     

酸改性活性炭对甲苯、甲醇的吸附性能

分别用1 mol·L^-1硝酸、1 mol·L^-1盐酸、1 mol·L^-1硫酸对商业活性炭进行浸渍改性。采用比表面积及孔径分析仪、Boehm滴定、傅里叶转换红外光谱(FTIR)对活性炭的物化性质进行表征。以甲苯、甲醇为吸附质,在283 K下进行了固定床吸附实验。研究表明:酸改性能去除表面碱性基团,显著增加表面酸性含氧官能团的含量;酸改性活性炭的吸附量与其比表面积、总孔容、微孔孔容、表面总酸性官能团呈现出良好的线性关系;Langmuir方程比Freundlich方程更加适合描述甲苯、甲醇在活性炭上的吸附;甲醇在活性炭上为物理吸附,甲苯在活性炭上以物理吸附为主,与表面官能团之间的化学键作用能增强甲苯吸附量;甲苯、甲醇在活性炭上的微孔有效扩散系数的大小顺序为:AC-N>AC-1>AC-S>AC-C;并且甲醇的微孔有效扩散系数大于甲苯。     

A novel method for production of activated carbon from waste tea by chemical activation with microwave energy

This study presents the production of activated carbon from waste tea. Activated carbons were prepared by phosphoric acid activation with and without microwave treatment and carbonisation of the waste tea under nitrogen atmosphere at various temperatures and different phosphoric acid/precursor impregnation ratios. The surface properties of the activated carbons were investigated by elemental analysis, BET surface area, SEM, FTIR. Prior to heat treatment conducted in a furnace, the mixture of the waste tea and H₃PO₄ was treated with microwave heating. The maximum BET surface area was 1157 m²/g for the sample treated with microwave energy and then carbonised at 350 °C. In case of application of conventional method, the BET surface area of the resultant material was 928.8 m²/g using the same precursor and conditions. According to the Dubinin–Radushkevich (DR) method the micropore surface area for the sample treated with microwave energy was higher than the sample obtained from the conventional method. Results show that microwave heating reasonably influenced the micropore surface area of the samples as well as the BET surface area.The samples activated were also characterised in terms of the cumulative pore and micropore volumes according to the BJH, DR and t-methods, respectively.     

Microporous structures of carbonaceous adsorbents

The simplest model of the porous structure of carbonaceous adsorbents is proposed, in which the microporous zones are formed by the totality of contacting and merged carbon crystallites. The micropores limited in extent resulted from the burnout on activation of several atomic layers of carbon in the crystallites by gaseous oxidizers (intercrystallite micropores) and from some development of the spaces between contacting crystallites (intercrystallite micropores). The micropore zones are shaped into regular cubes. The slit-like spaces between part of the micropore zones represent the micropore volume, while the total surface area of the walls of such pores is the mesopore surface. The parameters of the model for typical samples of microporous carbonaceous adsorbents are calculated and discussed. The formation of intercrystallite slit-like micropores as a result of the burnout of carbon atomic layers in crystallites on avtivation is considered. It is shown that only a wide micropore distribution leading to the formation of the supermicropores practically results in deviation from the applicability of the one-term adsorption equation of the theory of volume filling of micropores.     

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m²/g), do not seem to be effective for double layer capacitors.     

Formation of the pore structure of brown coal upon thermolysis with potassium hydroxide

The pore-structure characteristics of active carbons prepared by the thermolysis (800°C) of brown coal impregnated with potassium hydroxide were studied. The dependence of the specific surface area, total pore volume, micropore volume, micropore fraction, and micropore size distribution on the KOH/coal weight ratio R _KOH ≤ 1.0 g/g was found. Condensation processes with the formation of a low-porosity material were predominant at low ratios of R _KOH ≤ 0.1 g/g. The development of a micropore structure was observed at R _KOH ≥ 0.1 g/g, and it increased as R _KOH was increased to 1.0 g/g. It was hypothesized that pore formation was due to the thermally initiated reactions of the structural fragments of coal with KOH molecules, which occurred within the framework of coal.     

The comparison of experimental and calculated pore size distributions of activated carbons

Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO₂ isotherms have been measured at 273 K on seven well-characterized microporous carbons with average micropore widths between 0.65 and 1.5 nm and analysed with model isotherms obtained with standard Monte Carlo simulations. The resulting PSD are in good agreement with those obtained from a modified Dubinin equation, from liquid probes of molecular dimensions between 0.4 and 1.5 nm, from STM and from modelling based on CH₄ adsorption at 308 K. The present study validates the determination of micropore distributions in active carbons based on CO₂ isotherms, provided that no gate effects are present.     

Densification of ordered microporous carbons and controlling their micropore size by hot-pressing

Ordered microporous carbons synthesized by the template method using zeolite Y were hot-pressed at 573 K up to 147 MPa, aiming at densification of the templated carbons for the future use as energy storage media. Upon the hot-pressing, the powdery templated carbons were easily pelletized without any binder and the bulk density was significantly increased from 0.2 g/cm³ to 0.7-0.9 g/cm³. As a result, both the surface area and micropore volume per unit volume were increased. Surprisingly, it was found that the hot-pressing treatment reduced the average micropore size and it was tunable by simply adjusting a pressure in the treatment. In contrast, such changes in the density and the pore structure were not observed for commercial KOH-activated carbons when they were hot-pressed under the same conditions. The observed peculiar behavior of the templated carbons upon the hot-pressing can be explained by their unique molecular structure as compared to the conventional activated carbons.     

Inhomogeneous microporous structures of carbonaceous adsorbents

Ways for theoretical calcultion of the geometric surface area of the micropore walls of carbonaceous adsorbents with inhomogeneous microporous structures are considered for the slitlike micropore model. Two calculations methods have been worked out based on the theory of volume filling of micropores (TVFM) from the parameters of the adsorption equation proposed by Stoeckli and from the parameters of the binomial TVFM equation. An independent method for determining the geometric surface area of the micropore walls from water vapor adsorption isotherms served as a reference. When we used an active carbon with a wide micropore distribution all the three methods led to practically identical results, which indicated that the concept of the geometric surface area of carbonaceous adsorbents is physically feasible. The physical unfeasibility of a specific surface determined by the BET method for a microporous adsorbent is shown.     

Role of activated carbon structural properties and surface chemistry in mercury adsorption

In this work the performance of activated carbons prepared from raw and demineralised lignite for gas-phase Hg° removal was evaluated. A two-stage activation procedure was used for the production of the activated samples. In order to study the effect of mineral matter on pore structure development and surface functionality of the activated carbons, a demineralisation procedure involving a three-stage acid treatment of coals, was used, prior to activation. Hg° adsorption tests were realized in laboratory-scale unit consisted of a fixed-bed reactor charged with the tested activated samples. The examined adsorbent properties that may affect removal capacity were the pore structure, the surface chemistry and the presence of sulphur on the surface of activated carbons. The obtained results revealed that activated carbons produced from demineralised lignite posses a high-developed micropore structure with increased total pore volume and BET surface area. These samples exhibit enhanced Hg° adsorptive capacity. In all cases, mercury removal efficiency increased by sulphur addition. Finally, the starting material properties and activation conditions affect the concentration and the type of the oxygen groups on activated carbon surface, that have been determined with TPD-MS experiments.