Effect of various porous nanotextures on the reversible electrochemical sorption of hydrogen in activated carbons

The reversible hydrogen storage capacity of three series of activated carbons (ACs) prepared from different precursors by KOH, CO₂ and steam activation is determined by electrodecomposition of an alkaline water solution and is correlated with the nanotextural parameters of ACs. Galvanostatic charge/discharge appears as a precise quantitative method for estimating the hydrogen sorption capacity, whereas, cyclic voltammetry supplies a very useful information on the electrosorption mechanism. For the ACs studied, the hydrogen sorption capacity is not linearly related with any of the porosity parameters commonly used in other publications, such as the Dubinin-Radushkevich micropore volumes determined by nitrogen or carbon dioxide adsorption, VDR N₂ and VDR CO₂. In particular, an important discrepancy is observed for the KOH activated materials, suggesting that this treatment may provoke changes of pore shape. A better correlation is found considering the nanopore size distribution obtained from CO₂ adsorption by the DFT method. The amount of hydrogen reversibly adsorbed demonstrates a proportional trend with the volume of micropores smaller than 0.6-0.7 nm. However, in all cases, a part of the micropore volume estimated by CO₂ adsorption is ineffective, suggesting that some ultramicropores are involved in irreversible trapping of hydrogen.     

Adsorption from binary and ternary mixtures on activated carbons containing different amounts of chemically bonded oxygen

The adsorption process from the gas and liquid phase on activated carbons was investigated. Unmodified and chemically modified activated carbon Norit RKD-3 with different contents of chemisorbed oxygen were used. The surfaces were characterized by their content of surface functional groups, and the pore structure was characterized on the basis of adsorption-desorption isotherms of benzene vapor. Surface excess isotherms from binary and ternary mixtures of dioxane, n-heptane, and benzene were also determined. The influence of the chemical composition of the carbon surface on the adsorption from the gaseous and liquid phase is discussed.     

Effect of piezoelectric material on hydrogen adsorption

In hydrogen storage applications, the primary issue for physisorption of hydrogen onto solid-state materials is the weak interaction force between hydrogen molecules and the adsorbents. It is found that enhanced adsorption can be obtained under an external electric field, because it appears the electric field increases the hydrogen adsorption energy. Experiments were carried out to determine hydrogen adsorption on activated carbon using the piezoelectric material PMN-PT as the charge supplier under hydrogen pressure. Results indicate that more than 20% hydrogen adsorption enhancement was obtained. Parameters related to hydrogen adsorption enhancement include the amount of the charge and temperature. Higher voltage and lower temperature promote the increase of adsorption capacity but room temperature results are very encouraging.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Transport and sorption of water vapour in activated carbons

The adsorption of water vapour on microporous carbons derived from the carbonization of coconut shell has been studied. The adsorption and desorption characteristics of water vapour on the activated carbons were investigated over the pressure range p/p₀ 0–0.95 in a static water vapour system. In these experiments the process of water adsorption/desorption was investigated by both kinetic and equilibrium experimental data. Activated carbons differing by the degree of burn-off have shown the importance of the microstructure. A carbon enriched with nitrogen functions underlined the influence of the surface chemistry.     

Adsorption of phenol onto activated carbons having different textural and surface properties

Adsorption of phenol in aqueous phase onto activated carbons (ACs) having different textural and surface properties has been considered. Six types of ACs were used: three were commercial, and three were obtained from Kraft lignin chemically activated with sodium hydroxide, potassium hydroxide or ortho-phosphoric acid. The apparent surface areas of the commercial ACs varied from 620 to 1320 m²/g, while ACs made from lignin presented surface areas as high as 1300 m²/g and 2900 m²/g when prepared with H₃PO₄ and alkaline hydroxides, respectively; moreover, the highest proportion of microporosity was found for ACs derived from lignin. A kinetic study was carried out, showing that the phenol adsorption data may be correctly adjusted, for all the ACs tested, by an equation corresponding to a pseudo second-order chemical reaction. Freundlich, Langmuir and Tempkin equations were tested for modelling the adsorption isotherms at equilibrium, and it was concluded that Langmuir model fitted adequately the experimental data. However, Tempkin model fitted even better the adsorption data obtained with ACs derived from lignin activated with alkaline hydroxides, which are characterized by the highest number of surface groups. Remarkably high phenol adsorption capacities were found for the ACs prepared by activation of Kraft lignin with NaOH and KOH: 238 and 213 mg/g of AC, respectively. Finally, the adsorption of phenol was found to depend not only on the micropore volume, but also on the total amount of carbonyl and basic groups and on the ratio of acid to basic groups.     

Adsorption of SO2 from flowing air by alkaline-oxide-containing activated carbons

Adsorption of SO₂ under dynamic conditions from an SO₂-air mixture at 298 and 573 K on alkaline-oxide-containing activated carbons has been studied. The adsorption capacity of these samples at 298 K was, in general, lower than that in the original activated carbons and mainly governed by their microporosity accessible to benzene. However, at 573 K, the alkaline-oxide-containing activated carbons adsorbed a greater amount of SO₂ than the original activated carbon, following the order Na ≥ K > Rb. At both adsorption temperatures, part of the SO₂ adsorbed formed H₂SO₄ and Me₂SO₄, where Me = Na, K or Rb. When the SO₂ adsorption was carried out at 573 K, this gas fixed additional oxygen complexes that evolved as CO₂ under heating up to 873 K in He flow, probably by reaction of SO₂ with carbon surface atoms of a basic nature that are not able to chemisorb oxygen from the air at the same conditions.     

Correlation of methane uptake with microporosity and surface area of chemically activated carbons

Two series of activated carbon discs have been prepared by chemical activation of olive stones with ZnCl₂ and H₃PO₄. Some of the carbons have been post-treated in order to modify their porous texture and/or surface chemical composition. All carbons have been characterized by adsorption of N₂ (−196 °C) and CO₂ (0 °C) and immersion calorimetry into dichloromethane. The volume of methane adsorbed at 25 °C and 3.5 MPa is proportional to the surface area deduced from immersion calorimetry into dichloromethane. Consequently, it is possible to estimate, using a single experiment, the possibility of using activated carbons for the storage of natural gas. On the other hand, the methane uptake can be also correlated to the volume of micropores, provided by the adsorption of N₂ at −196 °C and CO₂ at 0 °C, although the correlations is not as good. Only carbons slightly activated, with low surface area and microporosity below around 0.6 nm, do not adjust the above correlations because they adsorb more methane than the expected, the effect of chemical nature of the carbon surface being almost negligible.     

Lipid adsorption capacities of magnesium silicate and activated carbon prepared from the same rice hull

This study is a comparison of the lipid adsorption capacities of synthetic magnesium silicate and activated carbon produced from rice hulls of the same origin. The lipids examined were the free fatty acids, diacylglycerols and monoacylglycerols of frying oils. Pure oleic acid, an unused sunflower frying oil and a used sunflower frying oil were used in the experiments. The produced adsorbents, magnesium silicate and activated carbon, have surface areas of 680 and 43 m²/g, respectively. The lipid adsorption capacity of the produced magnesium silicate was found as 644 mg polar compounds/g adsorbent and it is higher than the capacities of the industrial adsorbents, Magnesol XL and activated carbon. This value is only 368 mg polar compounds/g adsorbent for the activated carbon produced from the same‐origin rice hull.     

The adsorption properties of surface modified activated carbon fibers for hydrogen storages

In this study, activated carbon fibers (ACFs) with high surface area and pore volume have been modified by Ni doping and fluorination. The surface modified ACFs were characterized by BET surface area, SEM/EDS, XRD, and Raman spectroscopy. The changes in pore structure and surface properties of these modified ACFs were correlated with hydrogen storage capabilities. After fluorination treatment, although the micropore volume of ACF was decreased, amounts of hydrogen storage were found to increase. Additionally, micropore volume on ACFs was found to be unchanged with Ni doping, hydrogen storage capacities were considerably increased due to the effect of catalytic activation of nickel. Though fluorination of ACFs increases hydrogen affinity, the effect of catalytic activation of nickel is more prominent, and thus led to better hydrogen storage. Hence, it was concluded that hydrogen storage capacity was related to micropore volumes, Pore size distribution (PSD) and surface properties of ACFs as well as specific surface areas.     

Improved methane storage capacities by sorption on wet active carbons

The possibility of storing large amounts of natural gas within wet active carbons is examined. The sorption isotherms of methane at 2 °C and up to 8 MPa are built for four carbonaceous materials. Three of them originate from the same precursor (coconut shell), are physically activated at various burn-offs and are mainly microporous. The fourth material is a highly mesoporous chemically activated pinewood carbon. These adsorbents are wetted with a constant weight ratio water/carbon close to 1. The resulting isotherms all exhibit a marked step occurring near the expected formation pressure of methane hydrates, thus supporting their occurrence within the porous materials. The amount of gas stored at the highest pressures investigated then ranges from 6 to 17 mol/kg of wet adsorbent (i.e., corresponding to 10-36 mol/kg of dry carbon), depending on the material. The results are discussed on the basis of the known pore texture of each adsorbent, and stoichiometries of the formed hydrates are calculated. Considerations about adsorption/desorption kinetics and metastability are also developed.     

Characterization and adsorption properties of polymer-based microporous carbons with different surface chemistry

Polymers are promising activated carbon precursors due to their high percentage of carbon and also due to their abundance in a relatively pure state from waste recovery. Microporous activated carbons were prepared from poly(ethyleneterephthalate) (APETW, APETOX) and polyacrylonitrile (APAN). Their surface behaviour was characterized under dry and wet conditions by X-ray photoelectron spectroscopy (XPS), pH, pH_PZC and Boehm titration. The oxygen content of the nitrogen-free APETW and APETOX (S_BET = 1440 and 1509 m²/g) is 4.3 and 10.0 at.%, respectively. APAN (356 m²/g) contains 5.4 at.% oxygen and 5.3 at.% nitrogen. The ratios of the surface density of the titrated groups are approximately 20:28:65 in APETW, APETOX and APAN, respectively. The last has the most basic character. The larger oxygen content of APETOX yields greater affinity towards water, as does the presence of nitrogen functionalities in APAN. The higher the concentration of the functional groups, the higher is the water uptake at low relative humidity.

The greatest formaldehyde uptake per unit surface area was found in APAN, which is decorated with both nitrogen and oxygen functional surface groups. Due to the competitive adsorption of formaldehyde and water, increasing oxygen concentration in the APETW sample changed the kinetics of sorption but did not affect the formaldehyde uptake at saturation.     

In vitro adsorption study of fluoxetine in activated carbons and activated carbon fibres

We study the in vitro adsorption of fluoxetine hydrochloride by different adsorbents in simulated gastric and intestinal fluid, pH 1.2 and 7.5, respectively. The tested materials were two commercial activated carbons, carbomix and maxsorb MSC30, one activated carbon fibre produced in our laboratory and also three MCM-41 samples, also produced by us. Selected samples were modified by liquid phase oxidation and thermal treatment in order to change the surface chemistry without significant modifications to the porous characteristics. The fluoxetine adsorption follows the Langmuir model. The calculated Q₀ values range from 54 to 1112 mg/g. A different adsorption mechanism was found for the adsorption of fluoxetine in activated carbon fibres and activated carbons. In the first case the most relevant factors are the molecular sieving effect and the dispersive interactions whereas in the activated carbons the mechanism seams to be based on the electrostatic interactions between the fluoxetine molecules and the charged carbon surface. Despite the different behaviours most of the materials tested have potential for treating potential fluoxetine intoxications.     

Relationship between the nanoporous texture of activated carbons and their capacitance properties in different electrolytes

A series of activated carbons (ACs) with progressively changing nanotextural characteristics was obtained by heat-treatment of a bituminous coal at temperatures ranging from 520 to 1000 °C, and subsequent activation by KOH at 700 °C or 800 °C. As the pre-treatment temperature increases, the total pore volume V_T decreases from 1.28 to 0.30 cm³ g⁻¹, and the BET specific surface area from 3000 to 800 m² g⁻¹. The specific capacitance determined for each series of ACs using symmetric two electrode cells in 6 mol L⁻¹ KOH varies almost linearly with the BET surface area, suggesting that the charge accumulation is controlled primarily by the surface area development. A further analysis of the electrochemical behaviour in different electrolytic media—aqueous and organic—shows that an adequate pore size is more important than a high surface area in order to obtain high values of capacitance. Theoretical values of volumetric capacitance could be evaluated without considering the size of ions, which is always uncertain in solution, and compared with the experimental data as a function of the pore width. The efficiency of pore filling, i.e., of double layer formation, is optimal when the pore size is around 0.7 nm in aqueous media and 0.8 nm in organic electrolyte. A study of the performance of the positive and negative electrodes during the charge/discharge of the capacitor, reveals an additional pseudo-faradaic contribution due to oxygenated functionalities within the working potential window of the negative electrode. This effect is more pronounced for the ACs series obtained at 700 °C, because of their higher oxygen content.     

Application of different equations to adsorption isotherms of phenolic compounds on activated carbons prepared from cork

Activated carbons were prepared from solid cork wastes by physical activation with carbon dioxide or steam, and chemical activation by impregnation with phosphoric acid. In this work we show the possibility of using these activated carbons for the adsorption of phenolic compounds from the aqueous phase. The materials present a different response to the adsorptives used (p-nitrophenol, p-chlorophenol, p-cresol and phenol), depending on the type of activation and the parameters (burn-off, absolute concentration) used in each case. All the samples were capable of retaining the contaminants, with the best result being reached by the sample with higher burn-off and the worst with the carbonised, while intermediate values were reached with the remaining samples. The experimental isotherms were analysed with two and three parameters equations (Freundlich, Langmuir, Dubinin-Radushkevich-Kaganer and Redlich-Peterson). The results obtained from the application of the equations are similar in some aspects, but the degree of confidence is quite different. The best fit was achieved with the Redlich-Peterson equation, which can be explained by the fact that this has three adjustable parameters. However, overall the Freundlich and DRK equations appear to be more useful and provide parameters which can be correlated with the structural characteristics of the solids obtained from N₂ adsorption measurements.     

Estimation of adsorption energies using physical characteristics of activated carbons and VOCs’ molecular properties

Adsorption of volatile organic compounds (VOCs) by granular activated carbons (GACs) is a highly exothermic process and leads to temperature rises, which can be hazardous for high pollutant contents. This study points out the significant characteristics of VOCs and GACs on adsorption energies. For that purpose, adsorption energies were measured for a wide variety of VOCs, representative of different chemical groups, using eight different commercial GACs with different porous structures. Afterwards a statistical analysis was applied to the experimental database thus obtained, which enabled to enlighten the most significant variables, linked to either VOCs’ molecular properties or intrinsic characteristics of GACs. Two statistical models have been tested: multi-linear regression (MLR) and neuron networks (NN), and their efficiencies were compared in terms of prediction skill. The best results have been obtained from the MLR approach, which discriminated five different properties of the system. These explicative variables were the polarizability, the heat of vaporization, the ionization potential and the surface tension for adsorbates and the average micropore radius for GACs. The MLR model enabled to compute integral adsorption enthalpies with a precision around 10%. Thanks to the properties discriminated, we came to some conclusions on the dominant mechanisms of adsorption.     

Modified activated carbons with amino groups and their copper adsorption properties in aqueous solution

Activated carbons were prepared by two chemical methods and the adsorption of Cu (II) on activated carbons from aqueous solution containing amino groups was studied. The first method involved the chlorination of activated carbon following by substitution of chloride groups with amino groups, and the second involved the nitrilation of activated carbon with reduction of nitro groups to amino groups. Resultant activated carbons were characterized in terms of porous structure, elemental analysis, FTIR spectroscopy, XPS, Boehm titration, and pHzpc. Kinetic and equilibrium tests were performed for copper adsorption in the batch mode. Also, adsorption mechanism and effect of pH on the adsorption of Cu (II) ions were discussed. Adsorption study shows enhanced adsorption for copper on the modified activated carbons, mainly by the presence of amino groups, and the Freundlich model is applicable for the activated carbons. It is suggested that binding of nitrogen atoms with Cu (II) ions is stronger than that with H+ions due to relatively higher divalent charge or stronger electrostatic force.     

A theoretical study of activated nanostructured materials for hydrogen storage

The adsorption of molecular hydrogen on activated nanostruture materials is studied through computational simulations based on the pseudopotential density functional method. The hydrogen sorption energy and its diffusion through chemically activated pores in the materials are particularly investigated. It is found that the sorption energy of hydrogen reaches as high as about 29 kJ/mol at activated sites and can be controlled by modifying the structure and chemistry of the pores. The equilibrium pressure of hydrogen adsorption is also presented as a function of temperature by including the temperature and pressure dependence of hydrogen entropy. The desorption temperature is estimated to be close to 270 K for optimized activated nanostructures. This study demonstrates a pathway of materials search based on computational simulations for proper media that can hold hydrogen at ambient conditions through physisorption.