Microporous Active Carbons Research Articles

Argon Adsorption in Active Carbon at 77.3 K

Abstract The PDPT is most commonly used to describe the adsorption on microporous active carbons at 77.3 K. The micropore distribution functions were calculated assuming a slit-like model for the micropores and applied to the adsorption of argon on four active carbons.

Specific Features of the Adsorption and Nuclear Magnetic Relaxation of the Water Molecules in Active Carbons: 2. The State of Water in Active Carbon with Relatively Large Pores According to the NMR Relaxation Data

The state of water upon adsorption on FAS-3 active carbon with relatively large micropores is studied by the NMR relaxation method. The dependences of the times of spin–lattice (T1) and spin–spin (T2) NMR relaxation of adsorbed water molecules on the adsorption value are established. The character of the dependences of T1 and T2 on the number of adsorbed water molecules per primary adsorption site reflects the specific features of the volume filling of micropores and the formation of a continuous adsorption layer on the mesopore surface due to cluster coalescence on the one wall of a pore. The results obtained are compared with the data for typical microporous active carbons, as well as with the data obtained by the adsorption method.

Bioregeneration of activated carbons by bacterial degraders after adsorption of surfactants from aqueous solutions

The feasibility of activated carbon (AC) bioregeneration after adsorption of biologically resistant surface-active substances (SAS) by a community of strains of SAS-degrading bacteria of the genus Pseudomonas are analyzed. It was shown that the degree of AC equilibrium saturated with nonionic SAS under static conditions was 23–35%. The contribution of bacterial destruction of SAS to the restored capacity was 20–23% and, for all practical purposes, did not depend on the AC porosity. The efficiency of regeneration of the microporous AC after adsorption by it of the nonionic and anionic SAS under dynamic conditions was 22 and 95% of the respective capacities of fresh samples. The extent of regeneration of the mesoporous AC after dynamic sorption of nonionic SAS was 85%. Results indicate that regeneration is based on desorptive and migrational movements of the adsorbate toward the biocenosis of SAS-degrading bacteria located on the external surface of the AC grain. The adsorptive properties of the AC–SAS system and the sorbents’ porous structure determine the probability of desorption of molecules and the rate of their diffusion into the peripheral zone of the grain. The AC regeneration efficiency depends on the nature of the adsorbate distribution in its porous structure.

Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin

Carbon nutshells and hydrolytic lignin were used as starting materials for the preparation of microporous active carbons. Optimum parameters for cedar nutshell carbonization have been selected (temperature of carbonization 700–800 °C, rate of heating less than 3 °C/min) for the preparation of microporous carbons (average pore width 0.56 nm). The textural characteristics of microporous carbons made from nutshell are similar to those of a ‘Coconut’ carbon molecular sieve, but the latter has both a higher CO 2 adsorption capacity and a higher coefficient of N 2/O 2 separation. The influence of carbonization and steam-activation parameters on the microtexture and molecular-sieve properties of granular carbons made from hydrolytic lignin was also investigated. A low rate of heating (less 3 °C/min) promotes the formation of micropores with average sizes around 0.56–0.58 nm at carbonization temperature 700 °C. At the same carbonization temperature the average sizes of micropores were 0.7–0.78 nm at rates of heating more than 3 °C/min. The activation of lignin-char with steam at 800 °C resulted in the formation of active carbons with more developed micropore volume (0.3–0.35 cm 3 g −1) and with micropores of widths around 0.6–0.66 nm which are able to separate He from a He–CH 4 mixture. The size of the micropores was varied as a function of burn off value.

Specific features of the adsorption and nuclear magnetic relaxation of water molecules in active carbons. 1. Relation between the spin-spin relaxation of adsorbed water molecules and structural parameters of microporous active carbons

Relation between the spin-spin nuclear magnetic relaxation time T2 of adsorbed water molecules and parameters of microporous structure of carbon adsorbents is disclosed. The pattern of dependences of T2 on the relative pressure and the number of water molecules per one primary adsorption site (PAS) is governed by the pore sizes and the number and nature of PASs. At a complete micropore filling, the T2 value depends on the volume density of PASs in active carbons. In the absence of PASs in the micropores, T2 is equal approximately to 21 ms. The larger the volume density of PASs, the smaller the number of water molecules per one PAS at the complete filling of micropores; i.e., the looser the packing of water molecules. The results of studying active carbons by the pulsed 1H NMR method agree well with the data of the adsorption method.

Highly Mesoporous Carbon Electrodes for Electric Double-Layer Capacitors

High Brunauer-Emmett-Teller method (BET) surface area and mesoporous activated carbons (ACs) were prepared by carbonization followed by activation of vinylidene chloride copolymer containing yttrium acetylacetonate Y(acac) 3 . Their performance as electrodes for electric double-layer capacitors (EDLCs) in nonaqueous electrolyte solution was evaluated and compared with those of commercial microporous ACs. The specific EDLC capacitances of the ACs with high BET surface area are higher than those of the low BET surface area ACs at low discharge current. At high discharge current, the capacitances of the mesoporous ACs are higher than those of microporous ones. This is attributed to smooth movement of organic electrolyte ions in mesopores.

Molecular models for vapor adsorption on microporous adsorbents

The construction of a simple molecular model for the quantitative description of vapor adsorption excess isotherms on microporous active carbons is presented. It is shown that the equations quantitatively describe the experimental isotherms of various vapors in the whole range of relative pressures. The obtained parameter values are physically reliable, thus allowing the determination of the number of layers of adsorbate in micropores.

Electrochemical storage of hydrogen in activated carbons

Reversible storage of hydrogen in microporous active carbon was performed by galvanostatic electrodecomposition of water either in 6 M KOH or in 1 M H 2SO 4 electrolytic solution. The most indicative proof of hydrogen insertion into carbon is a significant decrease (from 700 to 1000 mV) of the rest potential of the carbon electrode after the charge process, as well as a distinct discharge plateau during the oxidation process. Capacity of the stored hydrogen was estimated from the Faradaic charge passed during hydrogen oxidation. The uptake of ca. 2 wt.% reached in the alkaline medium is at least four times higher than the values obtained for the same material under pressure conditions. This enhancement is interpreted by the electrolytic formation of nascent hydrogen that penetrates easily in the useful porosity of the carbon nanostructure.

Contribution of the edge effect to physical adsorption in micropores of activated carbons

The adsorption process in activated carbon micropores can be viewed as a two-dimensional condensation on micropore walls at a critical condensation pressure, p c, which evolves into volume filling. p c on a surface of micropore walls is calculated in respect to a critical condensation pressure on the reference nonporous surface, p c ref. Although theoretical treatment predicts that p c ref should be a parameter of adsorption isotherms, experimental isotherm equations are expressed in terms of the bulk saturation pressure, p s. Experimental values of p s/ p c ref for a graphitized carbon black, which is often considered as a model of activated carbon surfaces, range from 37 to 4000 for typical adsorbates and are by no means close to unity. It is proposed that this apparent discrepancy between theory and experiments has its origin in the edge effects: carbon blacks are modeled by semi-infinite slabs, whereas the magnitudes of thickness and diameters of micropore walls are of the order of a nanometre. Factors associated with edge effects reduce the adsorption energies on surfaces of finite particles resulting in shifts of condensation pressures to higher values. For particles with diameters about 1.4 and 1.8 nm, adsorption energies decrease to ≈0.72 ε 1* ∞ and ≈0.81 ε 1* ∞, respectively, where ε 1* ∞ is the energy of adsorption on an infinite plane, and p c approaches p s. This phenomenon substantiates the application of p s in adsorption equations and the success of the Dubinin equations probably owes much to this fact.

Application of Comparative Method for Estimating Microporosity of Porous Solids in Adsorption from Solutions

The concept of a comparative method in terms of the liquid adsorption on solids is presented. This method is based on comparison of the excess adsorption isotherm on a porous solid with that measured on a reference nonporous adsorbent. It is proved that proposed approach gives the possibility for separating a total excess adsorption isotherm into partial isotherms, proper for micro- and mesopores. Several experimental systems are analyzed to illustrate the usefulness of the comparative method for evaluating the surface phase capacity, which presents an important quantity in the study of adsorption from solution in the whole concentration region. The comparative method examined for adsorption systems of microporous active carbons seems to be useful for investigating other porous materials as silica gels, zeolites or metal oxides.

Molecular simulation study of water–methanol mixtures in activated carbon pores

We report a theoretical study of the adsorption behavior of water–methanol mixtures in slit activated carbon micropores. The adsorption isotherms are obtained for a pore of width 2 nm at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The water molecules are modeled using the four point transferable intermolecular potential functions (TIP4P) and methanol by the optimized potentials for liquid simulations (OPLS). Carboxyl (COOH) groups are used as active sites on a structured carbon surface. The effect of the relative contributions from dispersion and hydrogen bonding interactions of adsorbates, and of the chemical activation of adsorbents on adsorption behavior is investigated. The adsorption of the mixture components in activated carbon pores occurs by continuous filling, without the sharp capillary condensation observed in graphite pores. Water is preferentially adsorbed over methanol in activated carbon pores for a wide range of pressures, except at lower pressures. The hydrophilic nature of activated carbon pores results in the complexation of both water and methanol molecules with the active sites on the surfaces, leading to bulklike water behavior over the entire pore width. Solvation forces are also calculated as a function of pore size. The negative values found for the solvation force for all pore sizes reflect the hydrophilic interactions of the mixtures with the activated carbon surfaces.

Pore size distributions of active carbons assessed by different techniques

(3n 21) n 3 CM (Fig. 1) the frequency of the micropore widths f(L) 5 3W L a exp[2aL ] /G(n) (4) 0 observed by STM on the surface of the solid, DN /DL, is This function, like the distribution based on the DS slightly different from the volumic distribution DW /DL, equation [1,7–9], has a single maximum and it is not since the STM analysis does not take into account the suited for the description of bimodal micropore distribuactual depth of the micropores. However, the two distions, provided that they exist. In the case of carbon CM tributions are related and STM, like TEM, confirms that the pore size distribution Eq. (4) shown in Fig. 1(b), has the micropores of active carbons are locally slit-shaped. been obtained from the adsorption isotherms of CH (253, 4 This, in turn, provides the basic model for computer 273, 308 K) and of CO (253, 273, 298 K) fitted to Eq. 2 simulations. (1). This distribution is in good agreement with the In the case of microporous carbons, it has also been histogram of Fig. 1(a), derived from liquids with molecular possible to derive PSD from adsorption data within the sizes between 0.4 and 1.5 nm. As confirmed independently framework of Dubinin’s theory for the volume filling of by STM, the experimental distribution has no secondary micropores [5,6]. A possible relation is the so-called maximum around 1.5 nm. Dubinin–Stoeckli (DS) equation [1,7,8], which applies to Computer modeling of adsorption and the determination strongly activated carbons, with relatively wide distribuof pore size distributions based on standard isotherms has tions. As reported by Daley at al. [9], good agreement is become increasingly popular and it has been discussed found between the PSD derived from the DS equation and recently [10]. At the present time, modeling is frequently the experimental distribution observed by STM on strongly based on the adsorption of N [11–13], CO [14,15] and 2 2 activated carbons. CH [16–19], but a systematic comparison with pore size 4 Recently [1], a modified Dubinin isotherm has also been distributions obtained by independent techniques is still suggested to obtain PSD in the micropore range, lacking. For example, to our knowledge, no direct com-

Comparison of Four Adsorption Equations Used for the Description of the Systems: Porous Solid/Binary Liquid Non-Electrolyte Solution

The solution analogues of four single gas adsorption equations have been examined. The derived expressions have been applied to the description of the experimental adsorption isotherms on to microporous active carbons from binary liquid solutions with limited component miscibility. Statistical analyses of the individual adsorption equations, as well as comparisons of the optimized results obtained from different adsorption equations, have been undertaken.

Calorimetric Investigations of Molecular Interactions in the Adsorbate/Microporous Activated Carbon System. Towards the Mechanism of Adsorption in Micropores

This paper summarises the results of four years' investigation of the thermodynamics of adsorption in activated carbon micropores. Five adsorbates, i.e. methane, methanol, ethanol, carbon dichloride and carbon tetrachloride were adsorbed at three temperatures (308, 328 and 348 K) on two polymeric microporous carbons. Adsorption isotherms, as well as differential heats of adsorption, were measured using adsorption calorimetry. From the experimental results, the entropy as well as the average heat capacity of the adsorbed molecules were calculated. The data obtained lead to the mechanism of adsorption in carbon micropores as well as to the influence of the oxidation of the carbon surface on this mechanism. By comparing the isosteric and differential heats of adsorption, it is shown that the isosteric enthalpy cannot be applied for a correct description of the adsorption energetics. Finally, some empirical relationships describing low-coverage adsorption are proposed and discussed.

Study of the Evolution of Carbon Dioxide from Active Carbon by a Threshold Temperature-Programmed Desorption Method

A particular Temperature-Programmed Desorption (TPD) method, called ‘Intermittent TPD’, has been applied to the decomposition of oxygen groups naturally present at the surface of a microporous active carbon. It is shown that this method provides more information on the kinetics of the thermal decomposition of these species than the classical TPD technique. The main result is that the decomposition of these oxygen groups occurs in at least 7 distinct stages which have been characterized by their apparent activation energy and the corresponding frequency factor. The present study underlines the usefulness of ITPD for studying the rate of desorption of gases from powdered samples with a strongly heterogeneous surface.

Adaptation of Single Gas Adsorption Equations for the Description of Adsorption from Non-aqueous Liquid Solutions of Iodine on to Active Carbons

The solution analogs of four single gas adsorption equations have been formulated. The derived expressions have been applied for the description of the experimental adsorption isotherms from binary liquid solutions with limited component miscibility on to microporous active carbons, as well as for the derivation of standard chemical affinity distribution functions.

The Solid−Liquid Interface in Capillary Condensation. Sorption of Water by Active Carbons

Capillary condensation is associated with a decrease of the surface energy at the interface between the pore walls and the sorbate. This decrease is related to the difference in the free energy of the sorbate between the occluded state and the free liquid state. An equation based on this relation is derived. The equation is applied to condensation in pores in which only a part of the pore wall adsorbs, like in the case of the adsorption of water by microporous active carbons. The condensation pressure is determined in these systems by the parameter rk/Y, where rk is the width of the core of the pore and Y is a function of the fraction of the pore walls that adsorbs.

Diffusion Coefficients Calculated for Microporous Solids from Structural Parameters Evaluated by Fractal Geometry

This paper presents the examination of 14 microporous active carbons. The Dubinin−Astakhov equation for adsorption processes in micropores modified by a function F(z) for gamma distributed pore sizes was used for obtaining the micropore size distribution based on experimental data of benzene adsorption isotherms. This distribution can be used to determine the fractal cut-off values xmin and xmax. It is shown by means of fractal geometry how to get the structural parameters porosity ε and tortuosity τ. Both values are needed for the further calculation of diffusion coefficients according to the Knudsen diffusion equation. A comparison with experimental data shows a very good agreement between calculated and measured values.

Structural and energetic heterogeneities of microporous active carbons

Adsorption isotherms of carbon dioxide on microporous active carbons have been obtained at 195 K. The adsorption isotherms were interpreted by means of the Dubinin-Astakhov (DA) and Dubinin-Radushkevich-Stoeckli (DRS) equations. The heterogeneity of micropore structure and the energetic heterogeneity of the active carbons were determined from the differential distribution of the micropore volume with respect to the structural parameter characterizing the microporous structure and to the molar free energy change of adsorption. These distributions were obtained by the aid of the DRS and DA equations. A procedure for application of the DRS equation is described.

Separation of Water and Ethanol by the Adsorption Technique: Selective Desorption of Water from Micropores of Active Carbon

The hydrophobic micropores of active carbon were filled with a mixture of water and ethanol by adsorbing their mixed vapors (H2O/EtOH = 3/1) on active carbon. The desorption isotherm of each component from the micropores has been determined simultaneously at 25°C by a combination of volumetric and gravimetric methods. A more selective desorption of water than that of ethanol was discovered from the measured isotherms. It has been clarified from the present finding that the separation of water and ethanol could be carried out by means of an adsorption technique using a hydrophobic active carbon adsorbent. The preferential desorption of water may be attributed to the weak interaction of water with the hydrophobic micropore surface in comparison with the interaction of ethanol with the micropore surface. The adsorption energy of water on the micropore surface is 17–32 kJ mol−1at low coverage, while that of ethanol on the micropore surface is 43–52 kJ mol−1. The possibility of enrichment of ethanol from its dilute aqueous solution made by fermentation was discussed on the basis of the present desorption data.