Effect of the pore geometry in the characterization of the pore size distribution of activated carbons

In this work, the characterization of Activated Carbons (AC) by using the independent pore models is discussed, with special emphasis on the issue of how the assumed pore geometry can affect the resulting Pore Size Distribution (rPSD) and on the problem of the unicity of the PSD when different probe molecules are used in adsorption experiments. A theoretical test was performed using virtual solids based in the so-called Mixed Geometry Model (MGM) (Azevedo et al. 2010). The MGM uses a kernel of adsorption isotherms generated by GCMC for different pore sizes and two pore geometries: slit and triangular. The adsorption isotherms of a virtual MGM solid were fitted with both the traditional Slit Geometry Model (SGM) and the Mixed Geometry Model (MGM). It is demonstrated that, by assuming a different pore geometry model from that of the real sample, different PSDs may be obtained by fitting adsorption isotherms of different probe gases. Finally, experimental results are shown which both point toward the MGM as an acceptable extension of the SGM and confirm that the MGM is a closer representation of the actual porous structure of most activated carbons.     

Characterization of the PSD of activated carbons from peach stones for separation of combustion gas mixtures

Controlled series of microporous carbons were prepared through chemical activation with phosphoric acid from peach stones as the precursor material, corresponding to different preparation conditions. Adsorption isotherms of N₂ at 77 K and of CO₂ at 273 K were measured to be used in the characterization of the samples. The recently proposed mixed-geometry model (MGM), which assumes that the activated carbon is better represented by a mixture of slit and triangular geometry pores, is used to obtain the PSDs of the samples, on the basis of Grand Canonical Monte Carlo (GCMC) simulated ideal isotherms, both for N₂ at 77 K and of CO₂ at 273 K. Our results emerging from the analysis of two families of activated carbons reveal a consistent picture supporting the thesis that the PSDs of the same sample obtained trough N₂ and CO₂ adsorption are different, a still controversial issue in the literature. Comparison of predictions from the MGM with those of the pure slit geometry model (PSGM) shows that the former gives a more consistent picture and more similar PSDs for the two adsorbates used.     

Experimental and theoretical study of D2/H2 quantum sieving in a carbon molecular sieve

The present work aims at providing additional insight into the crucial effect of pore size and pressure on the adsorption of H₂ and D₂ in porous carbons by means of Grand Canonical Monte Carlo simulations in model slit micropores at 77 K. In order to address the quantum behavior of the molecules the Feynman–Hibbs corrected LJ interaction potential is used for fluid–solid and fluid–fluid interactions. Based on the GCMC isotherms for the two isotopes, D₂ selectivity over H₂ is deduced for pores with different sizes as a function of pressure. Furthermore, GCMC results are coupled with experimental high pressure H₂ and D₂ adsorption data at 77 K for a commercial carbon molecular sieve (Takeda 3A).     

Hydrogen adsorption in transition metal carbon nano-structures

Templated microporous carbons were synthesized from metal impregnated zeolite Y templates. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to characterize morphology and structure of the generated carbon materials. The surface area, micro- and meso-pore volumes, as well as the pore size distribution of all the carbon materials were determined by N₂ adsorption at 77 K and correlated to their hydrogen storage capacity. All the hydrogen adsorption isotherms were Type 1 and reversible, indicating physisorption at 77 K. Most templated carbons show good hydrogen storage with the best sample Rh-C having surface area 1817 m²/g and micropore volume 1.04 cm³/g, achieving the highest as 8.8 mmol/g hydrogen storage capacity at 77 K, 1 bar. Comparison between activated carbons and synthesized templated carbons revealed that the hydrogen adsorption in the latter carbon samples occurs mainly by pore filling and smaller pores of sizes around 6 Å to 8 Å are filled initially, followed by larger micropores. Overall, hydrogen adsorption was found to be dependent on the micropore volume as well as the pore-size, larger micropore volumes showing higher hydrogen adsorption capacity.     

Synthesis and characterization of small-mesopore-added silicalite-1 zeolites using single wall carbon nanohorn templating

Small-mesopore-added silicalite-1 zeolites were prepared by using single wall carbon nanohorn (SWCNH) as a template. The samples were characterized with X-ray powder diffraction, field emission scanning electron microscopy, transmission electron microscopy (TEM) and molecular probe adsorption methods. The pore size distributions determined with N₂ adsorption at 77 K showed the presence of small mesopores in 2–4 nm pore widths, in addition to their intrinsic micropores of 0.58 nm. The mesopore volume was 0.06 cm³ g^?1. The presence of small mesopores in the SWCNH-templated silicalite-1 zeolites was supported with TEM observation as well as the liquid phase adsorption of methylene blue, which was much higher than that on a bulk (purely microporous) silicalite-1.     

Carbon dioxide and methane adsorption at high pressure on activated carbon materials

Granular and monolith carbon materials were prepared from African palm shell by chemical activation with H₃PO₄, ZnCl₂ and CaCl₂ aqueous solutions of different concentrations. Adsorption capacity of carbon dioxide and methane were measured at 298 K and 4,500 kPa, and also of CO₂ at 273 K and 100 kPa, in a volumetric adsorption equipment. Correlations between the textural properties of the materials and the adsorption capacity for both gases were obtained from the experimental data. The results obtained show that the adsorption capacity of CO₂ and CH₄ increases with surface area, total pore volume and micropore volume of the activated carbons. Maximum adsorption values were: 5.77 mmol CO₂ g^?1 at 273 K and 100 kPa, and 17.44 mmol CO₂ g^?1 and 7.61 mmol CH₄ g^?1 both at 298 K and 4,500 kPa.     

Adsorption isotherms and ideal selectivities of hydrogen sulfide and carbon dioxide over methane for the Si-CHA zeolite: comparison of carbon dioxide and methane adsorption with the all-silica DD3R zeolite

Adsorption isotherms of H₂S, CO₂, and CH₄ on the Si-CHA zeolite were measured over pressure range of 0–190 kPa and temperatures of 298, 323, and 348 K. Acid gases adsorption isotherms on this type of zeolite are reported for the first time. The isotherms follow a typical Type-I shape according to the Brunauer classification. Both Langmuir and Toth isotherms describe well the adsorption isotherms of methane and acid gases over the experimental conditions tested. At room temperature and pressure of 100 kPa, the amount of CO₂ adsorption for Si-CHA zeolite is 29 % greater than that reported elsewhere (van den Bergh et al. J Mem Sci 316:35–45 (2008); Surf Sci Catal 170:1021–1027 (2007)) for the pure silica DD3R zeolite while the amounts of CH₄ adsorption are reasonably the same. Si-CHA zeolite showed high ideal selectivities for acid gases over methane at 100 kPa (6.15 for H₂S and 4.06 for CO₂ at 298 K). Furthermore, H₂S adsorption mechanism was found to be physical, and hence, Si-CHA can be utilized in pressure swing adsorption processes. Due to higher amount of carbon dioxide adsorbed and lower heats of adsorption as well as three dimensional channels of Si-CHA pore structure, this zeolite can remove acid gases from methane in a kinetic based process such as zeolite membrane.     

Cluster-associated filling of water molecules in graphene-based mesopores

Graphene monoliths were prepared through unidirectional freeze-drying method of graphene oxide colloids-KOH mixed solution and successive reduction by heating at 573 K in Ar. The porosity- and crystallinity-controlled graphene monoliths were prepared by the KOH activation at different temperature and the post-heating in Ar. These activated graphene monoliths were characterized by N₂ adsorption at 77 K, X-ray diffraction and Raman spectroscopy. Water adsorption isotherms show a typical hydrophobicity below P/P ₀ = 0.5 and a marked hydrophilicity above P/P ₀ = 0.6, which depends on the pore width. In the water adsorption isotherms of porous graphene monoliths activated at different temperature, the higher the activation temperature, the larger the rising P/P ₀. No essential change in the shape of the water adsorption isotherm for the post-heated nanoporous graphene monoliths is observed except for the decrease in water adsorption amount with higher post-heating temperature. The linear relationship between the saturated water adsorption and pore volume whose width is smaller than 4 nm indicates clearly that water molecules are adsorbed in small mesopores by the cluster-associated filling mechanism.     

Chemical modification of activated carbon monoliths for CO2 adsorption

The development of materials with potential application for CO₂ capture is a topic of great scientific interest. Activated carbons (AC) can be conveniently used as CO₂ adsorbents thanks to their microporous structure and tunable chemical properties. In this work, two AC honeycomb monoliths were synthesized starting from African palm stones through activation either with H₃PO₄ or with ZnCl₂ solution. Surface functionalization was performed in order to add nitrogen groups, aiming at an enhancement of CO₂ adsorption capacity. This chemical modification was performed either with ammonia in gas phase or a with 30 % ammonium hydroxide aqueous solution on both AC monolith samples. The original and modified monoliths were characterized by N₂ adsorption at 77 K, infrared spectroscopy, Boehm titration, and immersion calorimetry in benzene and water. CO₂ adsorption on both raw and functionalized AC monoliths was evaluated in volumetric equipment at a temperature of 273 K and until 1 bar, and adsorption capacity ranging between 120 and 220 mg_CO2 g _AC ^?1 was obtained. The experimental results indicated that both methods of chemical modification determined an increase in the content of superficial nitrogen groups and thus an increase in CO₂ adsorption capacity, the treatment with ammonium hydroxide being slightly preferable.     

Fluorinated Porous Poly(spirobifluorene) Via Direct C‒H Arylation: Characterization,Porosity, and Gas Uptake

Considering intrinsic properties of conjugated polyfluorenes and special functions of porous polymers, synthesis of fluorinated porous poly(spirobifluorene) via direct C?H arylation polycondensation is explored. Owing to the contorted structure and cross-linking nature, the obtained polymer FPSBF shows permanent porosities with Brunauer–Emmett–Teller specific surface area up to 700 m² g^?1 and exhibits a narrow pore size distribution with the dominant pore size at about 0.63 nm, which is more suitable for adsorption of small gas molecules. Based on the measured gas physisorption isotherms with pressure up to 1.13 bar, the obtained polymer shows good uptaking capacities for hydrogen (1.30 wt% at 1.0 bar and 77 K) and methane (4.80 wt% 1.0 bar and 273 K). Moreover, FPSBF has significant adsorption selectivity for CH₄ against N₂ and the estimated ideal adsorption selectivity ratio is up to 30/1 at 1.0 bar and 273 K, which makes the material possess potential application in gas separation.     

Synthesis,characterization, and KOH activation of nanoporous carbon for increasing CO2 adsorption capacity

Ordered nanoporous carbons (ONCs) were prepared using a soft-templating method. To improve the CO₂ adsorption efficiency, ONCs were chemically activated to obtain high specific surface area and micro-/mesopore volume with different KOH amounts (i.e., 0, 1, 2, 3, and 4) as an activating agent. The prepared nanoporous carbons (NCs) materials were analyzed by low-angle X-ray diffraction for confirmation of synthesized ONCs structures. The structural properties of the NCs materials were analyzed by high-angle X-ray diffraction. The textural properties of the NCs materials were examined using the N₂/77 K adsorption isotherms according to the Brunauer–Emmett–Teller equation. The CO₂ adsorption capacity was measured by CO₂ isothermal adsorption at 298 K/1 bar. From the results, the NCs activated with KOH showed that the increasing specific surface areas and total pore volumes resulted in the enhancement of CO₂ adsorption capacity.     

Calorimetric study of the CO2 adsorption on carbon materials

Given the great interest in the CO₂ removal and decreasing their impact on the environment, in this work, a calorimetric study of CO₂ adsorption on different activated carbons was performed. For this purpose, we used two methodologies for the determination heat of CO₂ adsorption: determination of CO₂ isotherms at different temperatures and adsorption calorimetry. The heats determined by these two techniques were compared. In this regard, carbonaceous materials of granular and monolithic types were prepared, characterized, and functionalized for carbon dioxide adsorption. As precursor material, African palm stones that were activated with H₃PO₄ and CaCl₂ at different concentrations was used. The obtained materials were functionalized in gas phase with NH₃ and liquid phase with NH₄OH, with the intention to incorporate the surface basic groups (amines or nitrogen groups) and subsequently were studied for CO₂ adsorption at 273 K and atmospheric pressure. For characterization of these materials, the following techniques are used: N₂ adsorption at 77 K and immersion calorimetry in different solvents. The experimental results show the obtaining of micropores and mesoporous (moderately) materials, with surface area between 430 and 1,425 m² g^?1 and pore volumes between 0.17 and 0.53 cm³ g^?1. It was determined that there is a difference between the heats of CO₂ adsorption obtained by the techniques employed. This deviation between the values corresponds to the methodological difference between the two experiments. In this work, we obtained a maximum adsorption capacity of CO₂, which is greater than 334 mg CO₂ g^?1 at 273 K and 1 bar in carbon materials with moderate surface area and pores volume.     

Thermodynamic characterisation of adsorptive gas/adsorbent systems using the ALIc-model

The ALI^c-Model is a thermodynamically consistent pore filling model which allows microporous and mesoporous adsorptive gas/adsorbent systems to be described and compared directly. Examples of this will be shown on 20 systems. To this end, the standard-molar-free-enthalpy of adsorption is divided into a material-specific concentrate term and a geometric mixing term. At standard pressure and boiling temperature, all the curves of the standard-molar-free-enthalpy of adsorption as a function of the degree of pore filling end at the point of free-enthalpy of adsorption = 0 and at the degree of pore filling = 1. From these characteristic curves, finite molar values for free-enthalpy, enthalpy and entropy of adsorption can be calculated for the adsorbate-concentrate at a negligible degree of pore filling. Alkanes on activated carbons and CO₂ on Zeolite 5A are used as demonstrating examples. These values and curves obtained from measurement of adsorption-isotherm-fields enable the interaction of the adsorbate with the adsorbent to be characterized, thus providing additional information for adsorption processes and for the development of adsorbents.     

Packing effects on argon and methanol adsorption inside graphitic cylindrical and slit pores: a GCMC simulation study

Using Grand Canonical Monte Carlo simulation, we have studied the effects of confinement on argon and methanol adsorption in graphitic cylindrical and slit pores. Linear chain, zigzag and incomplete helical packing are observed for argon adsorption in cylindrical pores. However, for methanol adsorption different features appear because the electrostatic interactions favour configurations that maximize the hydrogen bonding among methanol molecules. We have found zigzag chains with hydrogen-bonded structures for methanol adsorption in cylindrical and slit pores. To investigate how dense the adsorbed phase is and how many molecules could be packed per unit physical volume of the solid, we consider two different definitions of pore density; one based on the physical volume and the other on the accessible volume. That based on accessible volume gives a measure of the fluid density, while that based on the physical volume gives a measure of how much adsorbate can be stored per unit volume of the adsorbent. It is found that the adsorbate is denser in cylindrical pores, but that slit pores can pack more molecules per unit solid volume. We also discuss the effects on the isosteric heat of argon and methanol of pore size, pore geometry and loading.     

Study of CO2 adsorption in functionalized carbon

We evaluated the ability of CO₂ adsorption in functionalized activated carbons granular and monolithic type, obtained by chemical activation of African palm stone with H₃PO₄ and CaCl₂. We made a comparison between two methods of incorporation of nitrogen groups: the impregnation method with NH₄OH solution and NH₃ gasification. The materials were texturally characterized by N₂ adsorption at 77 K, the isotherms shows obtaining microporous materials with surface areas between 545–1425 m²?g^?1 and pore volumes between 0.22 to 0.53 cm³?g^?1. It was established that with the methodologies used for functionalization is increased content of nitrogen groups, was achieved a higher proportion of such groups when carrying out the process in liquid phase with NH₄OH. The incorporation of nitrogen groups in the material generates an increase of up to 65 % in the CO₂ adsorption capacity of the MCa2 (Monolith prepared with CaCl₂ solution at 2 %) sample. Was reached a maximum adsorption capacity of 344 mgCO²?g^?1 in the MCa2FAL (sample MCa2 functionalized with NH₄OH solution) sample.     

孔结构对活性炭吸附水溶液中铅离子的影响

选取三种表面化学性质相近的活性炭(AC),通过等温吸附实验考察活性炭对水溶液中铅离子的吸附性能,利用扫描电子显微镜(SEM)观察活性炭的表面微观形貌,通过低温(77 K)液氮吸附测定活性炭的比表面积和孔容,并分别以密度泛函理论(DFT)和Barrett-Joyner-Halenda (BJH)法计算微孔和中孔的孔径分布.结果表明:选用的三种活性炭AC1、AC2、AC3在比表面积和总孔容上呈依次下降的趋势,但表面开放孔均匀分布的AC2,具有最高的饱和吸附量,孔结构类似颗粒堆积孔的AC3,具有与表面开放孔分布集中的AC1相近的饱和吸附量;通过对孔结构与吸附量的关联分析可知,在活性炭吸附铅离子的过程中, 0.4-0.6 nm的孔是有效吸附孔, 10.5-20.6 nm、20.6-55.6 nm、5.2-10.5 nm三个区间的孔则会对吸附产生阻碍作用.     

Effect of surface functionalities on gas adsorption in microporous carbons: a grand canonical Monte Carlo study

In an attempt to offer a more realistic picture of adsorption in highly heterogeneous porous systems, such as oxygen functionalized porous carbons, we consider a series of carbon surfaces baring different amounts of oxygen functionalities (hydroxyl and epoxy). These surfaces are used to construct “oxidized” slit pores of varying width and functionality. With the aid of such inhomogeneous structures we study the interaction of Ar (87 K) inside “functionalized” pores and report grand canonical Monte Carlo adsorption simulations results. Based on our simulation data, we discuss the role of chemical heterogeneity on adsorbed/gas phase equilibrium properties such as density, heat of adsorption, and molecular packing within the pores. Comparisons are made with the case of the oxygen–free (completely homogeneous) slit pore models and conclusions on the suitability of Ar based pore size distributions for functionalized porous carbons are drawn.     

Amino-functionalized pore-expanded SBA-15 for CO2 adsorption

The adsorption of CO₂ on pore-expanded SBA-15 mesostructured silica functionalized with amino groups was studied. The synthesis of conventional SBA-15 was modified to obtain pore-expanded materials, with pore diameters from 11 to 15 nm. Post-synthesis functionalization treatments were carried out by grafting with diethylenetriamine (DT) and by impregnation with tetraethylenepentamine (TEPA) and polyethyleneimine (PEI). The adsorbents were characterized by X-ray diffraction, N₂ adsorption–desorption at 77 K, elemental analysis and Transmission Electron Microscopy. CO₂ capture was studied by using a volumetric adsorption technique at 45 °C. Consecutive adsorption–desorption experiments were also conducted to check the cyclic behaviour of adsorbents in CO₂ capture. An improvement in CO₂ adsorption capacity and efficiency of amino groups was found for pore-expanded SBA-15 impregnated materials in comparison with their counterparts prepared from conventional SBA-15 with smaller pore size. PEI and TEPA-based adsorbents reached significant CO₂ uptakes at 45 °C and 1 bar (138 and 164 mg CO₂/g, respectively), with high amine efficiencies (0.33 and 0.37 mol CO₂/mol N), due to the positive effect of the larger pore diameter in the diffusion and accessibility of organic groups. Pore-expanded SBA-15 samples grafted with DT and impregnated with PEI showed a good stability after several adsorption–desorption cycles of pure CO₂. PEI-impregnated adsorbent was tested in a fixed bed reactor with a diluted gas mixture containing 15 % CO₂, 5 % O₂, 80 % Ar and water (45 °C, 1 bar). A noteworthy adsorption capacity of 171 mg CO₂/g was obtained in these conditions, which simulate flue gas after the desulphurization step in a thermal power plant.     

Effects of pore structure and surface chemical characteristics on the adsorption of organic vapors on titanate nanotubes

Effects of pore structure and surface chemical characteristics of titanate nanotubes (TNTs) on their adsorptive removal of organic vapors were investigated. TNTs were prepared via a hydrothermal treatment of TiO₂ powders in a 10 M NaOH solution at 150?°C for 24?h, and subsequently washed with HCl aqueous solution of different concentrations. Effects of acid washing process (or the sodium content) on the microstructures and surface chemical characteristics of TNTs were characterized with nitrogen adsorption-desorption isotherms, FTIR, and water vapor adsorption isotherms. For the adsorption experiments, gravimetric techniques were employed to determine the adsorption capacities of TNTs for four organic vapors with similar heats of vaporization (i.e., comparable heats of adsorption) but varying dipole moments and structures, including n-hexane, cyclohexane, toluene, and methyl ethyl ketone (MEK), at isothermal conditions of 20 and 25?°C. The experimental data were correlated by well-known vapor phase models including BET and GAB models. Isosteric heats of adsorption were calculated and heat curves were established. Equilibrium isotherms of organic vapors on TNTs were type II, characterizing vapor condensation to form multilayers. The specific surface area (and pore volume) and hydrophilicity of TNTs were the dominating factors for the determination of their organic vapors adsorption capacity. The GAB isotherm equation fitted the experimental data more closely than the BET equation. The heats of adsorption showed that the adsorption of organic vapors on TNTs was primarily due to physical forces and adsorbates with larger polarity might induce a stronger interaction with TNTs.     

Influence of KMnO4 oxidation on the electrochemical performance of pitch-based activated carbons

In this work, activated carbons (ACs) are obtained from petroleum pitch by the combination of a chemical treatment with different potassium permanganate (KMnO₄) amounts, i.e., 0, 0.5, 1.0, and 2.0 g, and a chemical activation with KOH at a constant KOH/pitch ratio of 3/1. The effects of the chemical activating agent on the surface morphology and porosity are evaluated with scanning electron microscopy and N₂ adsorption isotherms at 77 K, respectively. The specific surface area of the pitch-based ACs is increased with increasing the amount of KMnO₄ pre-treatment and showed the highest value of 2,334 m² g^?1 at 2 g of KMnO₄ amount. The electrochemical performance of AC electrodes is examined by cyclic voltammetry and galvanostatic charge/discharge characteristics in 6 M KOH electrolyte. Among the prepared ACs, 2.0 K-ACs possesses a specific capacitance as high as 237 F g^?1 and showed excellent electrochemical performance due to its suitable porous structure and low interface resistance.