Estimation of the influence of structural elements of activated carbons on the energetic components of adsorption (original) (raw)
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Carbon, 2007
The energetic surface heterogeneity of four different activated carbons was assessed by the parallel probing at the solid/liquid and solid/gas interfaces. At the solid/liquid interface a method of inverse liquid chromatography, frontal analysis by characteristic point was applied using phenylalanine in water solution as a probe molecule. At the solid/gas interface, argon was used as a probe in the low pressure quasi equilibrium volumetry method. The treatment of the adsorption isotherms by the derivative isotherm summation procedure revealed similar adsorption energy distributions for both argon and phenylalanine. Such an agreement between both methods suggests that, at the solid/liquid interface and on the solid/gas interface, the adsorption was mainly controlled by geometric parameters and no specific interaction was observed and physisorbed water did not played a significant role in adsorption process on three from four studied carbons.
Study of Hexane Adsorption on Activated Carbons with Differences in Their Surface Chemistry
Molecules, 2018
The study of aliphatic compounds adsorption on activated carbon can be carried out from the energetic change involved in the interaction; the energy values can be determined from isotherms or by the immersion enthalpy. Vapor phase adsorption isotherms of hexane at 263 K on five activated carbons with different content of oxygenated groups and the immersion enthalpy of the activated carbons in hexane and water were determined in order to characterize the interactions in the solid-liquid system, and for calculating the hydrophobic factor of the activated carbons. The micropore volume and characteristic energy from adsorption isotherms of hexane, the BET (Brunauer-Emmett-Teller) surface area from the adsorption isotherms of N 2 , and the area accessible to the hexane from the immersion enthalpy were calculated. The activated carbon with the lowest content of oxygenated groups (0.30 µmolg −1) and the highest surface area (996 m 2 g −1) had the highest hexane adsorption value: 0.27 mmol g −1 ; the values for E o were between 5650 and 6920 Jmol −1 and for ∆H im were between −66.1 and −16.4 Jg −1. These determinations allow us to correlate energetic parameters with the surface area and the chemical modifications that were made to the solids, where the surface hydrophobic character of the activated carbon favors the interaction.
Journal of Industrial and Engineering Chemistry, 2019
This study provided the surface characteristics (surface energy and the acid-base properties) of surface treated activated carbons (ACs) using inverse gas chromatography. The surface energy and the acid-base characteristic of studied adsorbents are determined by the retention time using several non-polar and polar probes at 140 °C. It is observed that the dispersive surface energy dominates for all AC samples. The treatment of Maxsorb III with H2 exhibits the highest basicity due to lower oxygen content on the surface. This study provides further insights and useful information in functionalizing the activated carbon for various industrial applications.
Characterization of Activated Carbons Using Liquid Phase Adsorption
"A modification of the Dubinin–Radushkevich pore filling model by incorporation of the repulsive contribution to the pore potential, and of bulk non-ideality, is proposed in this paper for characterization of activated carbon using liquid phase adsorption. For this purpose experiments have been performed using ethyl propionate, ethyl butyrate, and ethyl isovalerate as adsorbates and the microporous–mesoporous activated carbons Filtrasorb 400, Norit ROW 0.8 and Norit ROX 0.8 as adsorbents. The repulsive contribution to the pore potential is incorporated through a Lennard–Jones intermolecular potential model, and the bulk-liquid phase non-ideality through the UNIFAC activity coefficient model. For the characterization of activated carbons, the generalized adsorption isotherm is utilized with a bimodal gamma function as the pore size distribution function. It is found that the model can represent the experimental data very well, and significantly better than when the classical energy–size relationship is used, or when bulk non-ideality is neglected. Excellent agreement between the bimodal gamma pore size distribution and DFT-cum-regularization based pore size distribution is also observed, supporting the validity of the proposed model."
The use of organic vapour preadsorption to understand water adsorption on activated carbons
Microporous and Mesoporous Materials, 2017
In this work, n-octane preadsorption coupled with water vapour sorption has been assessed as a useful technique for the advanced characterization of porous carbon materials. First, two microporous activated carbons with different oxygen contents were gradually preloaded with the n-alkane and then characterized by different adsorbates. As the micropore system is being filled, a narrowing of the hysteresis loop and a decrease in the slope of the water isotherms at low and medium relative pressures was observed. The latter result indicates that the available surface oxygen content gradually decreased with the blockage of the micropores, thus suggesting that the primary water adsorption centers of activated carbons are located in the narrow pores. Moreover, correlations between the available surface oxygen content and the micropore volumes measured by N 2 and CO 2 adsorption were found. Then, the microporous system of two activated carbons with a mainly mesoporous structure was nearly completely blocked. The surface chemistry of these two carbons was modified in order to have a deeper analysis of the role played by the surface oxygen groups on the water adsorption mechanism. The obtained results showed that the water sorption in the mesopores is highly dependent on the available surface oxygen content.
Adsorption properties of two different activated carbons, steam and chemically activated, respectively, were examined by inverse gas chromatography (IGC). The influence of acid, basic and combined washings, carried out in order to remove ashes, on the adsorption properties of these materials was also tested using this technique. Chemical and textural characterization was carried out by nitrogen adsorption, ICP-MS and temperature programmed desorption, whereas thermodynamic properties (enthalpy of adsorption, surface free energy characteristics) have been determined by IGC. Washing procedures (specially those involving acid washing) removes almost completely the mineral ashes of the carbons. Concerning to the effect on porous structure, this procedures only affect significantly to the properties of chemically activated carbon, with increases of micropore volume of up to 43%. The steam activated carbon shows the best adsorptive behaviour, with an enthalpy of adsorption up to 16% higher than the corresponding to chemically activated one, as well as a large amount of micropores. The removal of these ashes enhances the adsorption of studied compounds, especially for alkanes and chlorinated ones. Washing procedures also modify the chemical structure of the organic functionalities, but this effect does not seem to be very important on the adsorption properties of these materials.
Three sets of activated carbons (ACs) were prepared with the same precursor but activated differently (by CO 2 or water vapour) with various burn-off levels. The ACs demonstrate increased deviation of the pore shape from the slitshaped model with increasing burn-off and contributions of pores of different sizes depending on the activation type. Significant rearrangement of adsorption complexes, especially of the Van der Waals type characteristic for nonpo-lar or weakly polar adsorbates (H 2 , CH 4 , CH 2 Cl 2 , CHCl 3), occurs in both micropores and mesopores of ACs with decreasing temperature. The behaviour of their mixtures with Electronic supplementary material The online version of this article water and DMSO can strongly differ from that of individual adsorbates.
Langmuir, 1999
The adsorption of polluted water is performed by activated carbon fibers. Three kinds of material are compared: microporous and mesoporous cloths and a microporous granular activated carbon. These porous solids are characterized by scanning electron microscopy and atomic force microscopy. BET surface areas and porous volumes are determined. Adsorption of a large number of organic compounds is carried out in water onto activated carbon cloths and granules. Kinetic and equilibrium data are expressed in terms of initial velocities and classical model parameters (Freundlich). These adsorbability parameters are then discussed according to solute molecular structures and activated carbon characteristics. The results obtained show that fluid-solid transfer is directly related to surface and porous structure (pore size distribution and pore connection with the external surface of adsorbents). The adsorption data of several aromatics and aliphatics onto a microporous activated carbon cloth are discussed statistically. A quantitative structureproperty relationship method is used. Multiple linear regression and neural networks enable the assessment of correlations between the Freundlich adsorption parameter (log K) and molecular structure defined by molecular connectivity indexes. The neural network architecture is optimized, and results determined by the two statistical methods are compared: the neural network approach seems to give better results than multiple linear regression to assess this kind of relationship between adsorption and adsorbate molecular structure, even though its predictive ability is low. From a variable analysis, the results are discussed in terms of adsorbate positions at the adsorbent surface and mechanisms of interaction between solutes and an activated carbon surface are proposed.
Journal of Physical Chemistry C, 2007
Activated carbons have both hydrophilic surface oxygen functional groups, which act as primary adsorption centers for water vapor and hydrophobic graphene layers on which non-polar species are primarily adsorbed. The aim of this research was to investigate the effects of oxygen surface functional groups, in activated carbons, on the adsorption characteristics of water vapor. Activated carbon G, was oxidized using nitric acid and then heat treated in the range 387-894 K to produce a suite of adsorbents with varying oxygen contents in the range 0•4-21•5 %, but very similar porous structure characteristics, thereby minimizing effects due to changes in porous structure. The type and concentration of surface oxygen groups present on each sample was assessed using TPD, FTIR and Boehm titration methods. Water vapor adsorption at low relative pressure was dramatically enhanced by the presence of functional groups, in particular, carboxylic groups. Kinetic profiles for each pressure increment were modeled using a set of nested kinetic models, which allow the adsorption kinetics to be interpreted in relation to the adsorption mechanism. The results establish a clear relationship between water adsorption kinetics and the type and concentration of oxygen surface functional groups. A linear relationship was observed between the rate constants in the low pressure region and the inverse of the Henry's Law constant. This indicates the importance of adsorbate-adsorbent interactions in water adsorption kinetics and is consistent with a site-to-site hopping mechanism between functional groups.