Adsorption of SO2 by typical carbonaceous material: a comparative study of carbon nanotubes and activated carbons

Applying activated carbons for SO₂ adsorption and conversion to H₂SO₄, as a dry process, has been considered the development direction of desulfurization technology. Coal-based activated carbon, coconut shell activated carbon, single wall carbon nanotube and multi-wall carbon nanotubes were used as typical carbonaceous materials to study the SO₂ adsorption mechanism. SEM, N₂ adsorption, XPS and fixed-bed reaction system were employed to study the morphology, pore structure, surface functional groups and SO₂ adsorption behaviors of the four adsorbents. The fixed-bed experiment was carried out at normal pressure and SO₂ concentration was set 1,000 ppm. According to SEM and N₂ adsorption results, hierarchical pore structure was an important characteristic of activated carbon. Aggregation was an important characteristic of CNTs. Mesopores and macropores took the dominance of pore structure in CNTs. According the SO₂ adsorption data and correlation analysis, it can be concluded that the dominant adsorption type on activated carbons does not alter with adsorption temperature changing. However, the adsorption type of SO₂ adsorption on CNTs changes with adsorption temperature varying. With adsorption temperature increasing, the dominant adsorption type transforms to chemisorption by physisorption. Higher-density π–π* in carbon nanotubes may be the active sites for the SO₂ chemical adsorption. Micropores with the diameter smaller than 0.7 nm were the best SO₂ adsorption place for both activated carbons and carbon nanotubes. The results provided a profound insight into the microstructure and SO₂ adsorption mechanism of the two kinds of carbonaceous materials.     

A TPD-MS study of the adsorption of ethanol/cyclohexane mixture on activated carbons

The adsorption of ethanol/cyclohexane binary mixtures on different types of activated carbons was studied in this work by temperature programmed desorption coupled with mass spectroscopy (TPD-MS). The texture, morphology and surface chemistry of the carbons were evaluated by N₂ adsorption, scanning electron microscopy (SEM) and TPD-MS techniques. The ethanol and cyclohexane TPD-MS desorption profiles showed that specific interactions between the carbon material and the adsorbate are involved during the adsorption. Most of the activated carbons adsorb strongly ethanol on the surface, leading to desorption temperatures above 100 °C. Only one carbon exhibits an affinity for cyclohexane. These observations were correlated to the different surface chemistry of the materials.     

Sorption of trifluoromethane in activated carbon

Sorption isotherms for trifluoromethane (R-23) in activated carbon have been measured at ca. 298 and 323 K using a gravimetric microbalance. High-resolution TEM images of the activated carbon show a very uniform microstructure with no evidence of any contaminants. The adsorption in the activated carbon reaches about 22.8 mol kg^?1 at 2.0 MPa and 298 K or 17.6 mol kg^?1 at 2.0 MPa and 323 K. Three different adsorption models (Langmuir, multi-site Langmuir, and BET equations) have been used to analyze the activated carbon sorption data, with a particular interest in the heat of adsorption (?ΔH). The heat of adsorption for R-23 in the activated carbon was about 29.78 ± 0.04 kJ mol^?1 based on the multi-site Langmuir model and is within the range of typical physical adsorption. According to the IUPAC classification, the activated carbon exhibits Type I adsorption behavior and was completely reversible. Compared with our previous work for the sorption of R-23 in zeolites (5A (Ca,Na-A), 13X (Na-X), Na,K-LSX, Na-Y, K,H-Y, Rb,Na-Y) and ionic liquids ([omim][TFES] and [emim][Tf₂N]) the activated carbon had the highest adsorption capacity. The adsorption process in the activated carbon also took less time than in the zeolites or the ionic liquids to reach thermodynamic equilibrium.     

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.     

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.     

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 of carbonyl sulfide on modified activated carbon under low-oxygen content conditions

Activated carbon sorbents impregnated with KOH, Fe(NO₃)₃, Cu(NO₃)₂, Zn(NO₃)₂ or Co(NO₃)₂ and their applications in catalytic oxidation reaction of COS were investigated. The results showed that the activated carbon modified with 10 % (mass percentage) KOH enhanced the adsorption ability significantly. And it was also found that the oxygen content and temperature were the two most important factors in the COS adsorption. Further investigation on the pore structures of the samples with X-ray photoelectron spectroscopy indicated that an adsorption/oxidation process happened in the KOH modified activated carbon in which the major existing forms of sulfur were SO₄ ^2? and S species. The oxidation of COS suggested that KOH in the micropores may play a catalytic role during the adsorption. On the other hand, we found that the desorption activation energy from KOHW was higher than that from AC by the CO₂-TPD spectra, which indicated the adsorption of CO₂ on KOH impregnated activated carbon was stronger. The strong adsorption could be attributed to the basic groups on the activated carbon surface. In conclusion, the activated carbon impregnated with KOH promises a good candidate for COS adsorbent.     

Calorimetric evaluation of activated carbons modified for phenol and 2,4-dinitrophenol adsorption

Two commercial activated carbons with differences in their superficial chemistry, one granular and the other pelletised, were modified for use in phenol and 2,4-dinitrophenol adsorption. In this paper, changes to the activated carbon surface will be evaluated from their immersion calorimetry in water and benzene, and they will then be compared with Area BET, chemical parameters, micropore size distributions and hydrophobicity factors of the modified activated carbons. The activated carbons were modified using 60 % solutions of phosphoric acid (H₃PO₄), nitric acid (HNO₃), zinc chloride (ZnCl₂) and potassium hydroxide (KOH); the activated carbon/solution ratio was 1:3 and impregnation was conducted 291 K for a period of 72 h before samples were washed until a constant pH was obtained. Water immersion calorimetry showed that the best results were obtained from activated carbons modified with nitric acid, which increased from ?10.6 to ?29.8 J g^?1 for modified granular activated carbon, and ?30.9 to ?129.3 J g^?1 for pelletised activated carbon. Additionally, they showed the best results in phenol and 2.4-dititrophenol adsorption. Those results indicate that impregnation with nitric acid under the employed conditions could generate a greater presence of oxygenated groups on their surface, which favours hydrogen bond formation and the increased adsorption of polar compounds. It should also be noted that immersion enthalpy in benzene for modified activated carbon with nitric acid is the method with the lowest value, which is consistent with the increased presence of polar groups on its surface. Regarding hydrophobicity factors, it was observed that granular carbons modified with nitric acid and potassium hydroxide have the lowest ratios, indicating greater interaction with water.     

Influence of free-space calibration using He on the measurement of adsorption isotherms

It is well known that helium (He) molecules that remain inside micropores after free-space calibration at a low temperature (77.4 K) affect the shape of an adsorption isotherm, especially in a very low relative pressure region. This negative effect of the remaining He leads to a misunderstanding of the porous characteristics, such as micropore size distribution and surface properties. However, it is still believed that such erroneous interpretations are limited to narrow microporous materials such as activated carbon and measurements at low temperatures, namely the measurement of the adsorption of N₂ and Ar at their boiling points. Here we report a systematic investigation of the influence of free-space calibration using He on microporous, mesoporous and non-porous materials. Zeolite Y, mesoporous silica, carbon black and aerosil 200 were used for the measurements. N₂, H₂O and CO₂ adsorption isotherms were measured at 77.4, 298 and 298 K, respectively. Free-space calibration was carried out before and after the isotherm measurement for each sample. Although the influence of the He that remained in the sample was small for the non-porous sample, the shape of the isotherms for the other samples in a low relative pressure region was rather affected by the timing of the free-space calibration even for the mesoporous sample, and at an ambient temperature.     

Influence of activated carbon in zeolite X/activated carbon composites on CH<Subscript>4</Subscript>/N<Subscript>2</Subscript> adsorption separation ability

A series of zeolite X/activated carbon composites with different ratio of zeolite X and activated carbon were prepared, which were adjusted by adding solid pitch powder and silicon dioxide as additional carbonaceous and silica source, respectively. The corresponding modified samples were obtained by treatment with the ammonium chloride solution. CH₄ and N₂ adsorption isotherms on all composites were determined within the pressure of 0–100 kPa at 298 K, and fitted with Henry model and Freundlich model. The results showed the adsorption separation abilities for CH₄ and N₂ were strongly influenced by activated carbon content, micropore structure and surface properties. The increase of activated carbon content increased the BET surface area, micropore surface area and micropore volume, leading to an enhanced CH₄ adsorption capacity and CH₄/N₂ adsorption selectivity. Compared with the unmodified composites, the modified composites showed higher CH₄/N₂ adsorption selectivity, and CH₄ adsorption capacity decreased slightly, which can be attributed to the reduction of the micropore structure parameters, the surface basic amount and basic strength. Furthermore, the modified composite HAX-3 presented the highest CH₄/N₂ selectivity of 3.4, and high CH₄ adsorption capacities, which is favorable for application in pressure swing adsorption processes.     

Preparation and characterization of waste ion-exchange resin-based activated carbon for CO<Subscript>2</Subscript> capture

Waste ion-exchange resin was utilized as precursor to produce activated carbon by KOH chemical activation, on which the effects of different activation temperatures, activation times and impregnation ratios were studied in this paper. The CO₂ adsorption of the produced activated carbon was tested by TGA at 30 °C and environment pressure. Furthermore, the effects of preparation parameters on CO₂ adsorption were investigated. Experimental results show that the produced activated carbons are microporous carbons, which are suitable for CO₂ adsorption. The CO₂ adsorption capacity increases firstly and then decreases with the increase of activation temperature, activation time and impregnation rate. The maximum adsorption capacity is 81.24 mg/g under the condition of 30 °C and pure CO₂. The results also suggest that waste ion-exchange resin-based activated carbons possess great potential as adsorbents for post-combustion CO₂ capture.     

Preparation and characterization of activated carbon from pine cone by microwave-induced ZnCl2 activation and its effects on the adsorption of methylene blue

In this work, activated carbon prepared from pine cone (PCAC) with ZnCl₂ as an activation agent under microwave radiation was investigated. The activation step was performed at the microwave input power of 400 W and radiation time of 5 min. The properties of activated carbon were characterized by N₂ adsorption Brunauer–Emmett–Teller (BET), scanning electron microscopy and Fourier transform infrared spectroscopy. Results showed that the BET surface area, Langmuir surface area, and total pore volume of PCAC were 939, 1,486 m²/g and 0.172 cm³/g, respectively. Adsorption capacity was demonstrated by the iodine numbers. The adsorptive property of PCAC was tested using methylene blue dye. Equilibrium data was best fitted by the Langmuir isotherm model, showing a monolayer adsorption capacity of 60.97 mg/g. The pseudo-first- and pseudo-second-order kinetic models were examined to evaluate the kinetic data, and the rate constants were calculated. Adsorption of the dyes followed pseudo-first order kinetics. Thermodynamic parameters such as free energy, enthalpy and entropy of dye adsorption were obtained.     

Assessing surface chemistry and pore structure of active carbons by a combination of physisorption (H2O, Ar, N2, CO2), XPS and TPD-MS

In order to address open questions concerning the surface chemistry and pore structure characterization of nanoporous carbons, we performed extensive experiments by combining various experimental techniques on a series of commercially available activated carbons which exhibit diverse surface chemistry characteristics. Pore size analysis was performed on Ar (87 K), N₂ (77 K) and CO₂ (273 K) adsorption isotherms using state-of-the art methods based on density functional theory, including the recently developed quenched solid density functional theory (QSDFT). A detailed study of the surface chemistry was obtained by applying temperature programmed desorption coupled with mass spectrometry (TPD-MS) as well as XPS (X-Ray-Photoelectron Scattering). This information together with the pore structure information leads to a reliable interpretation of systematic water adsorption measurements obtained on these materials. Our results clearly suggest that water adsorption is indeed a sensitive tool for detecting differences in surface chemistry between chemically and physically activated active carbon materials with comparable ultramicropore structure. The occurrence of sorption hysteresis associated with the filling of micro- and narrow mesopores (in a range where nitrogen and argon isotherms are reversible) provides additional structural information, complementary to the insights from argon/nitrogen/carbon dioxide adsorption.     

Preparation of activated carbon from cotton stalk and its application in supercapacitor

High specific capacitance and low cost are the critical requirements for a practical supercapacitor. In this paper, a new activated carbon with high specific capacitance and low cost was prepared, employing cotton stalk as the raw material, by using the phosphoric acid (H₃PO₄) chemical activation method. The optimized conditions were as follows: the cotton stalk and activating agent with a mass ratio of 1:4 at an activation temperature of 800 °C for 2 h. The samples were characterized by nitrogen adsorption isotherms at 77 K. The specific surface area and pore volume of activated carbon were calculated by Brunauer–Emmett–Teller (BET) and t-plot methods. With these experimental conditions, an activated carbon with a BET surface area of 1,481 cm²?g^?1 and micropore volume of 0.0377 cm³?g^?1 was obtained. The capacitance of the prepared activated carbon was as high as 114 F?g^?1.The results indicate that cotton stalk can produce activated carbon electrode materials with low cost and high performance for electric double-layer capacitor.     

Enthalpic characterisation of activated carbon monoliths obtained from lignocellulosic materials

In this study, energetic interactions between activated carbon monoliths and various liquids were evaluated by determining immersion enthalpies in C₆H₆, H₂O and aqueous solutions of NaOH and HCl. Immersion enthalpies depend on both the surface chemistry and the interactions between specific groups, and were compared with results from volumetric titrations. Immersion enthalpies of activated carbon monoliths were between ?95.85 and ?176.5 J g^?1 for C₆H₆ and between ?11.19 and ?68.31 J g^?1 for H₂O; whereas immersion enthalpies in NaOH and HCl solutions were between ?20.36 and ?82.25 J g^?1 and ?18.81 and ?96.16 J g^?1, respectively. In support of these results, a high level of acidic groups was found on the surface of the activated carbon monoliths by Boehm volumetric titrations, with values between 719 and 1,290 g mol^?1, in agreement with the higher immersion enthalpies observed in NaOH. Correlations were established between immersion enthalpies in the liquids and the surface chemistry properties of the activated carbon monoliths determined by volumetric titrations, demonstrating that immersion enthalpy is a useful parameter for characterisation of these materials in specific liquids.     

Carbon dioxide sequestration and methane removal from exhaust gases using resorcinol–formaldehyde activated carbon xerogel

The world is faced with intrinsic environmental issues. Among these issues, the minimization of greenhouse gas emission to acceptable levels presents a high priority. This study seeks to help to reduce the greenhouse effect in sustainable manner. A resorcinol–formaldehyde xerogel was synthesized at specific conditions and used to prepare an activated carbon xerogel (RF-ACX). RF-ACX exhibited micropores in range of 1.2–1.4 nm, a surface area of 496 m²/g and a cumulative pore volume of 0.81 cm³/g. Scanning electron microscopy showed that it is made of microspherical particles with an almost uniform particle size of 1.3 ± 0.2 μm. Equilibrium and kinetic studies for the adsorption of CO₂, CH₄ and N₂ on RF-ACX were conducted at five temperatures (293, 303, 313, 323, and 333 K) and pressures of up to 1 MPa. The adsorption capacity on RF-ACX was highest for CO₂, followed by CH₄ and then N₂. Isosteric heats of adsorption and adsorption rates were investigated. The measured adsorption equilibria were fitted with the extended multisite Langmuir adsorption model and further used to predict adsorption equilibria of their corresponding binary systems.     

Experimental and numerical study on water sorption over modified mesoporous silica

–SO₃H modified mesoporous silica adsorbent with water sorption capacity and fast desorption kinetics for water sorption was synthesized and studied via a combined experimental and numerical approach. Mesoporous silica was synthesized using sol–gel method in H₂SO₄ medium. The water adsorption isotherms and kinetics over the silica were evaluated by a dynamic water vapor sorption analyzer. Mesoporous silica was modeled using annealing simulation with CVFF forcefield. –SO₃H modified mesoporous silica was modeled by the attachment of –SO₃H to the surface hydroxyl groups and validated. Simulation results show water sorption capacity at low relative humidity (RH) increases with –SO₃H loading on mesoporous silica. Energy distribution of intermolecular interaction and micro-view of water sorption over –SO₃H modified mesoporous silica reveal that although strong interaction (intermolecular interaction of ?40 to ?20 kcal/mol) between hydrophilic groups (–SO₃H) with water can increase water sorption capacity at low RH, weak H₂O–H₂O interaction (intermolecular interaction of ?20 to ?10 kcal/mol) dominated water sorption capacity at both low and high RH.     

Adsorption of carbon disulfide on activated carbon modified by Cu and cobalt sulfonated phthalocyanine

A series of adsorbents were studied for removal efficiency of carbon disulfide (CS₂) under micro-oxygen conditions. It was found that activated carbon modified by Cu and cobalt sulfonated phthalocyanine (CoSPc) denoted as ACCu–CoSPc showed significantly enhanced adsorption ability. Reaction temperature was found to be a key factor for adsorption, and 20 °C seems to be optimal for CS₂ removal. Samples were analyzed by N₂-BET, XRD, XPS, SEM–EDS and CO₂-TPD. The characterization results demonstrated that large quantities of SO₄ ^2? anions were formed and adsorbed in the reaction process. SO₂, CS₂ and COS were detected in the effluent gas generated from the temperature programmed desorption of ACCu–CoSPc–CS₂. Therefore, it can be concluded that ACCu–CoSPc most likely acted as a catalyst in the adsorption/oxidation process on the surface of the impregnated sample. The generated sulfide and sulfur oxide can cover the active sites of adsorbents, resulting in pronounced reduction of adsorbent activity. Finally, the exhausted ACCu–CoSPc can be regenerated by thermal desorption.     

Adsorption isotherms of CO2, CO,N2, CH4, Ar and H2 on activated carbon and zeolite LiX up to 1.0 MPa

The adsorption isotherms of CO₂, CO, N₂, CH₄, Ar, and H₂ on activated carbon and zeolite LiX were measured using a volumetric method. Equilibrium experiments were conducted at 293, 308, and 323 K and pressures up to 1.0 MPa. The adsorption isotherm and heat of adsorption were analyzed for two pressure regions of experimental data: pressures up to 0.1 MPa and up to 1.0 MPa. Each experimental isotherm was correlated by the Langmuir, Sips, Toth and temperature dependent Sips isotherm models, and the deviation of each model was evaluated. The Sips and Toth models showed smaller deviation from the experimental data of adsorbents than the Langmuir model. Isosteric heats of adsorption were calculated by the temperature dependent Sips model and are presented along with surface loading. From deviation analysis, it is recommended that the isotherm in the proper pressure range be used to appropriately design adsorptive processes.     

Why the pore geometry model could affect the uniqueness of the PSD in AC characterization

In this paper we discuss why the pore geometry can affect the unicity of the pore size distribution (PSD) of a given activated carbon (AC) sample, when different probe gases are used in adsorption measures. In order to characterize the solid sample we used grand canonical Monte Carlo simulation and the independent pore model with slit or triangular pore geometry, focusing our analysis on the possibility of representing the adsorptive processes of a triangular pore of defined size by means of a combination of slit pores of different sizes. This representation is tested on experimental adsorption data of N₂ (77 K) on AC samples and acceptable results were obtained. Finally, we have performed a theoretical test, which consisted of analyzing a virtual porous solid with this approach and different probe gases (N₂ at 77 K and CO₂ at 273 K), showing that the differences between the pore representations can cause differences between the solid representations for the adsorptive properties, for these different gases. The analysis presented here can be extended to other pore geometries and other adsorbates, and provide arguments to further explain results presented in our previous paper, which refers to cases when different adsorbates yield different PSDs for a given sample and the same pore geometry model.