A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the parameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.     

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.     

Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons

Characterization of microporous solids (activated carbons and carbon molecular sieves) has been carried out by N₂ (subatmospheric pressures) and CO₂ adsorption (at subatmospheric and high pressures) at 77 and 273 K, respectively. Because the relative fugacity range covered by our CO₂ study is similar to the relative pressure range covered with N₂, a suitable comparison of both adsorptives can be made. The results of such comparison show that both adsorptives give the same micropore size distribution (MPSD) for open porosity activated carbons. This observation confirms that the adsorption mechanism of both adsorptives is similar. However, carbon molecular sieves, with very narrow microporosity, cannot be characterized by N₂ at 77 K, due to the existence of diffusional problems. This is also extensive to many other carbon materials, such as carbon fibers and activated carbons with low degree of activation. As a consequence, in this type of samples, N₂ adsorption at 77 K is useless to determine neither the micropore volumes of the narrowest porosity nor their micropore size distributions (MPSD). In this work, the usefulness of CO₂ for the characterization of carbon molecular sieves and activated carbons with different activation degrees is demonstrated. In addition, examples of applications that cannot be explained from N₂ adsorption but yes by CO₂ are presented. As a result, we strongly encourage the use of CO₂ (i.e. at 273 K) as a complement to N₂ adsorption at 77 K.     

Temperature Programmed Desorption of n-nonane; the method to measure the micropore size distribution

TPD of n-nonane gives a micropore size distribution of micropores in the range of .4 to 2 nm. This method is independent of the shape of the pore entrances. It is proven that n-nonane fills these micropores. The Temperature Programmed Desorption patterns of five activated microporous carbons and three zeolites will be shown.The difference in adsorption behaviour of CO₂ and SF₆ at 273 K and of CH₄ at 315 K on the carbons is explained with the aid of the obtained micropore size distributions.     

The Characterization of the Micropore Structures of Some Activated Carbons of Plant Origin by N2 and CO2 Adsorptions

Abstract

In this study, active carbons prepared from almond and hazelnut shells under various experimental conditions were investigated. Merck-2514 and Merck-2184 active carbons were used for comparison. N₂ (77 K) gas and CO₂ (273 and 195 K) gas adsorptions were determined as comparison criteria. Regarding the specific surface area and micropore volume results obtained from these adsorption data, it is concluded that N₂ (77 K) adsorption by itself is inadequate in the characterization of active carbons which are low-sized microporous dominated. In addition, it is concluded that it would be useful to investigate CO₂ (195 and 273 K) adsorption. The iodine and methylene blue tests at 298 K were also applied for the characterization of the carbon adsorbents mentioned. From these data it was seen that the iodine test can be applied as a total porosity indicator and that the methylene blue test can be used as a developed microporosity indicator. These results indicate that the best adsorbents were those prepared from hazelnut shells. Depending on the preparation conditions, the physically activated carbon has an activation time up to 4 hours and has adsorption properties on the level of Merck commercial carbons.     

Water adsorption by activated carbons in relation to their microporous structure

The present paper examines the adsorption of water by microporous carbons in the absence of specific interactions. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), shows that the type V isotherms follow the Dubinin-Astakhov (DA) equation and fulfill the requirement for temperature invariance. Furthermore, the parameters of the DA equation can be related in a simple way to structural properties of the model carbons. For a number of well-characterized carbons, the type V isotherms generated by combining model isotherms with the corresponding PSDs are in good agreement with the limiting isotherms at 293 and 310 K derived on the basis of a recent development of Dubinin’s theory. This approach will provide the basis for further studies including specific interactions.     

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.     

Adsorption of CO2 and SO2 on activated carbons with a wide range of micropore size distribution

The adsorption isotherms of N₂ at 77K, CO₂ at 251, 273 and 298K, and SO₂ at 262 and 273K have been determined on a series of physically activated carbons with a wide range of micropore size distributions. Since the series includes carbons with very high burn-off, it shows the problems involved in the characterization of microporsity in superactivated carbons. On the other hand, the results show that the carbon surface-adsorbate interactions for SO₂ at low relative pressures are weaker than for N₂ and CO₂, as a result of the strong adsorptive-adsorptive interactions in the bulk gas phase.     

Microporosity in carbon blacks

Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO₂ adsorption isotherm at 273 K, combined with computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N₂ (77 K), CO₂ (273 K) and C₆H₆ (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.     

Adsorption of phenol by oil–palm-shell activated carbons in a fixed bed

Steam-activated carbons from oil–palm shells were prepared and used in the adsorption of phenol. The activated carbon had a well-developed non-micropore structure which accounted for 55% of the total pore volume. The largest Brunauer–Emmett–Teller (BET) surface area of the activated carbon was 1183 m²/g with a total pore volume of 0.69 cm³/g using N₂ adsorption at 77 K. Experimental tests on the adsorption of phenol by the activated carbons were carried out in a fixed bed. The aqueous phase adsorption isotherms could be described by the Langmuir equation. The effects of the operation conditions of the fixed bed on the breakthrough curve were investigated. A linear driving force model based on particle phase concentration difference (LDFQ model) was used to simulate the fixed bed adsorption system. The model simulations agreed with the experimental data reasonably well.     

The controlled reaction of active carbons with air at 350°C—II: Evolution of microporosity

The evolution of the porosity in two series of samples prepared by the reaction of two activated carbons (from almond shells and olive stones) with air at 350°C has been followed by adsorption of CO₂ (273 and 298 K) and n-butane (273 K). The results have been compared with the adsorption of N₂ at 77 K [1] and the anomalous behaviour of samples prepared from almond shells in respect to the adsorption of CO₂ is discussed. The analysis of the isotherms in terms of percentages of pores filled at different relative pressures gives a good picture of the evolution of the microporosity in the two series of carbons.     

Dual gas analysis of microporous carbons using 2D-NLDFT heterogeneous surface model and combined adsorption data of N2 and CO2

Knowledge of the pore structure of carbon materials including micropores is crucial for applications such as double layer supercapacitors, gas separation, and other applications requiring high specific surface area materials. High surface area is always associated with fine micropores. The pore size distribution (PSD) of microporous carbons is usually evaluated from nitrogen adsorption isotherms measured at 77 K in the relative pressure range from 10⁻⁷ to 1. Due to the very slow gas diffusion into fine pores at cryogenic temperatures and low pressures, the adsorption measurements may be extremely time consuming and sometimes inaccurate when the adsorption equilibrium is difficult to achieve during the measurement. In this work, we discuss an approach in which the carbon PSD is calculated from the combined N₂ and CO₂ data measured in the pressure range from 1 to 760 Torr. Under such conditions, the diffusion into micropores is usually fast and equilibration times are short for both measurements. In the PSD calculations we use 2D-NLDFT models for carbons with heterogeneous surfaces (J. Jagiello and J.P. Olivier, Adsorption 19, 2013, 777–783). We show that both isotherms can be fitted simultaneously with their corresponding models and as a result the unified PSD can be obtained.     

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。     

Activated carbons from almond shells—II: Characterization of the pore structure

Several series of activated carbons have been prepared from almond shells by carbonization in nitrogen followed by activation in a flow of carbon dioxide. The adsorption of CO₂ at 195 and 273 K and n-C₄H₁₀ at 273 K confirms that the carbonized materials are essentially microporous with dimensions or constrictions in the range 0.3–0.5 nm. Upon activation with carbon dioxide there is a considerable increase in the aperture of micropores and an increase in the apparent surface area. The effect of preparation conditions on the adsorptive capacity of the carbons are also discussed.     

Effect of solution pH on the removal of clofibric acid by cork-based activated carbons

Clofibric acid adsorption from the aqueous phase was studied using cork-based activated carbons (CAC: chemically activated with K₂CO₃; CPAC: physical activation of sample CAC with steam). CPAC outperformed the uptake of water treatment commercial carbons. Results highlight the importance of pH in clofibric acid adsorption: the highest removals were obtained for pH 2.0 and decrease for higher pHs. The sigmoidal adsorption isotherms obtained were fitted to the Dubinin-Astakhov equation. The characteristic adsorption energy revealed that CAC has the highest affinity for the solute, in accordance with its narrow micropore size distribution. The molecular and electronic structure showed that the solvation energy of the undissociated and dissociated forms of clofibric acid is the key factor to explain the isotherm shape and the dependence of the pH. For pH 3.6 the dissociated form is dominant and the uptake significantly decreases, showing that the solvent shields the interaction of the dissociated specie (higher solvation energy) with the carbon surface. The results show that once the solvation energies of the undissociated and dissociated forms of clofibric acid are known, a complete characterization of an activated carbon allows one to predict with confidence its behavior for the adsorption of this compound from the aqueous phase.     

Characterization of accessible and inaccessible pores in microporous carbons by a combination of adsorption and small angle neutron scattering

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size <20 Å), but not for carbons which have significant mesopore populations of sizes up to 48 Å and high mass fractal degrees. The pore size distribution (PSD) results based on SANS data reveal significant populations of micropores of size <6.1 Å, and mesopores of size >20 Å, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO₂ 273 K. This inaccessible porosity becomes accessible to CO₂ and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.     

Studies on metal impregnated activated carbon: Complete pore structure analysis

Coconut shell activated carbon was impregnated with 2.08, 3.01, 6.41, 8.24 and 9.39% (w/w) copper. Mercury porosimetry was used to analyse pores ranging between 50 and 75,000 Å. Benzene adsorption isotherm and the Kelvin equation were used to analyse pores of radii 15–50 Å. Pores below 15 Å radius (micropore region) were analysed employing Kadlec's method and the Dubinin-Astakhov equation. Impregnants affect micro and transitional pores reducing the pore volume but without altering the basic structure.     

Use of n-nonane pre-adsorption for the determination of micropore volume of activated carbon aerogels

When the α_s or t methods and the DR (Dubinin-Radushkevich) or DA (Dubinin-Astakhov) Equations are used to analyse nitrogen adsorption isotherms determined at 77 K on activated carbon aerogels it is found that, in contrast to the behaviour generally found with other carbon materials, the α_s or t estimate of micropore volume is significantly less than the DR or DA estimate. In this paper the reasons for the overestimation of the DA micropore volume and underestimation of the α_s micropore volume are explained and it is shown how use of the n-nonane pre-adsorption method enables consistent values of micropore volume to be obtained.     

Characterization of Porous Carbonaceous Sorbents Using High Pressure CO2 Adsorption Data

In this paper we present adsorption isotherms of carbon dioxide on five different activated carbons from CHEMVIRON CARBON Belgium (Centaur HSV, BPL 410, F30-470, WS 42, Reactivated) and on a carbon molecular sieve from BERGBAU FORSCHUNG Gmbh (CMS II). The temperature is 303 K and the pressure ranges from 100 kPa up to 4000 kPa. Such conditions correspond to relative pressures ranging from 0.01 to 0.5. We also provide, for the same six sorbents, the nitrogen isotherms at 77 K (pressure: 0.001 to 100 kPa, relative pressure: 10^-5 to 1). A theoretical treatment based on the Dubinin-Radushkevich and Stoeckli concept is presented and applied to the experimental results in order to obtain the micropore size distribution function (considered as Gaussian) of each sorbent. Using the CO₂ data, it is possible to point out important structural differences between the six carbons. The theoretical treatment provides micropore size distribution functions in agreement with what is physically expected. Using N₂ data, the structural differences are not so well marked. As a consequence, the structural parameters provided by the theoretical treatment are not reliable: except for the total micropore volume, they fluctuate strongly when changing the relative pressure domain of the used data.     

Highly selective CO2 capture by S-doped microporous carbon materials

S-doped microporous carbon materials were synthesized by the chemical activation of a reduced-graphene-oxide/poly-thiophene material. The material displayed a large CO₂ adsorption capacity of 4.5 mmol g⁻¹ at 298 K and 1 atm, as well as an impressive CO₂ adsorption selectivity over N₂, CH₄ and H₂. The material was shown to exhibit a stable recycling adsorption capacity of 4.0 mmol g⁻¹. The synthesized material showed a maximum specific surface area of 1567 m² g⁻¹ and an optimal CO₂ adsorption pore size of 0.6 nm. The microporosity, surface area and oxidized S content of the material were found to be the determining factors for CO₂ adsorption. These properties show that the as synthesized S-doped microporous carbon material can be more effective than similarly prepared N-doped microporous carbons in CO₂ capture.