Microporosity and nanostructure of activated carbons: characterization by X-ray diffraction and scattering, Raman spectroscopy and transmission electron microscopy (original) (raw)
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Characterization of the Microporous Structure of Activated Carbons through Different Approaches
Industrial & Engineering Chemistry Research, 1999
The characterization of the porosity of three samples of commercial activated carbons, from adsorption isotherms of N 2 (77 K), obtained by the volumetric method are presented. Special care was taken in the measurement of the isotherms in the range of low relative pressures (10-6) in order to characterize the microporosity of these carbons by different approaches: Dubinin-Radushkevich (D-R), R plots, Jaroniec-Choma (J-Ch), and Pfeifer et al. Microporosity characterization was completed by measurement of the specific surface area (BET) of the adsorbents in which different amounts of benzene at 295 K had been preadsorbed. It was established that the three samples are basically microporous, with a strongly pronounced structural and energetic heterogeneity.
Comparative study of nanopores in activated carbons by HRTEM and adsorption methods
Carbon, 2012
Detailed analysis of nanopores (IUPAC micropores at pore half-width x < 1 nm) of carbonised porous phenolformaldehyde resin microbeads used as a precursor of activated carbon (AC) and CO 2 activated carbon (at 50% burn-off) has been performed on the basis of highresolution transmission electron microscopy (HRTEM) image analysis and nitrogen adsorption data analysed using several density functional theory (DFT) methods. The results of quenched solid DFT (QSDFT) and nonlocal (NLDFT) are in agreement with the pore size distributions of nanopores based on the HRTEM image analysis. Development of porosity with progressive activation degree in a series of ACs leads to enhancement of the deviation of the pore shape from the used pore models. The TG/DTA data and Raman spectra show nonlinear but weak changes in the AC characteristics with increasing burn-off degree.
Some remarks on the calculation of the pore size distribution function of activated carbons
Journal of Colloid and Interface Science, 2006
Different authors investigated the effects of geometric and energetic heterogeneities on adsorption and on carbon characterization methods. In most theoretical studies carbon structure is modeled as parallel infinite graphite walls that form ideal slit-shaped pores of the fixed widths. In the literature there is the lack of systematic studies showing the influence of pore structural and Lennard-Jones (LJ) potential parameters on the poresize distribution functions. Moreover, the parameters characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent surface should be taken into account. The Nguyen and Do method with proposed by us ASA algorithm, were utilized for the assessment of the porosity from the series of almost few thousands numerically generated local adsorption isotherms. The values of the mentioned-above parameters are varied over the wide range (ca. ±20%) of the reference ones. Different types of the theoretical and experimental adsorption isotherms (nitrogen at 77 K) were taken into account as the global ones. They were related to the mechanism of the primary, secondary or mixed micropore filling. The variations in some above-mentioned parameters have significant effects only for PSDs (and for average pore widths) corresponding to the primary micropore filling mechanism. On the other hand, for the process of the secondary micropore filling, the influence of these parameters (without the BET coefficient for adsorption on a "flat" surface, c s,B ) is rather insignificant. Nevertheless the differences between local and global adsorption isotherms (in the whole range of relative pressures) the absence of micropores having pore half width equal to ca. 1 nm on PSDs was observed for studied adsorbate-adsorbent systems with exceptions of the strictly microporous adsorbents and/or the low values of c s,B . Comparison of the experimental data with the generated theoretical isosteric enthalpy of adsorption indicates that the phenomenal uptake observed from experiment can be explained in terms of the reasonable solid-fluid interaction parameters. Therefore, we varied the heterogeneity of the adsorbent surface via the strength and the range of the solid-fluid potential and the parameter c s,B in order to reproduce the experimental data of enthalpy of adsorption. Note that similar procedure was applied by Wang and Johnson to reproduce some hydrogen adsorption data measured for carbon nanofibres. The analysis of the obtained results shows that the selection of the values of the parameters of the intermolecular interactions and the quantities characterizing the properties of the adsorbed phase and the heterogeneity of the adsorbent walls for molecular simulations should be made with care and the influence of possible errors should be considered.
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.
Use of Liquid Phase Adsorption for Characterizing Pore Network Connectivity in Activated Carbons
""A simple percolation theory-based method for determination of the pore network connectivity using liquid phase adsorption isotherm data combined with a density functional theory (DFT)-based pore size distribution is presented in this article. The liquid phase adsorption experiments have been performed using eight different esters as adsorbates and microporous–mesoporous activated carbons Filtrasorb-400, NoritROW0.8 andNoritROX0.8 as adsorbents. The density functional theory (DFT)-based pore size distributions of the carbons were obtained using DFT analysis of argon adsorption data. The mean micropore network coordination numbers, Z, of the carbons were determined based on DR characteristic plots and fitted saturation capacities using percolation theory. Based on this method, the critical molecular sizes of themodelcompounds used in this study were also obtained. The incorporation of percolation concepts in the prediction of multicomponent adsorption equilibria is also investigated, and found to improve the performance of the ideal adsorbed solution theory (IAST) model for the large molecules utilized in this study.""
Adsorption
In this paper we study a method for the determination of the micropore volume distribution function of activated carbons. This method is based on the Integral Adsorption Equation concept (IAE). The micropore volume distribution function is assumed to be a Gaussian of which the parameters are unknown. These parameters are determined using adsorption isotherms of carbon dioxide on a given activated carbon (F30/470 CHEMVIRON CARBON) at 278, 288, 298, 303, 308, 318 and 328 K and for pressures up to 100 kPa. Several local adsorption models are used (Langmuir, Volmer, Fowler-Guggenheim, Hill-de Boer). The influence of the choice of the local model on the pore volume distribution function is discussed. The physical validity of this function and the performances of the different models are presented. It appears that the effect of the temperature on the adsorption isotherms is difficult to model over a wide range of relative pressure. The Hill-de Boer and the Langmuir local models are the most efficient (average errors respectively equal to 3.53% and 2.80% in the studied range of temperature and pressure). They provide the most meaningful parameters for the pore volume distribution function.
Adsorption Science & Technology, 2014
We determined the pore-size distributions (PSDs) of 12 activated carbons (ACs) using a combination of density functional theory method from Kierlik and Rosinberg applied to slit-like pores and a regularization method. This combination of methods was applied to nitrogen adsorption isotherms at 77.35 K on the selected ACs, prepared by heat treatment of lignin impregnated with orthophosphoric acid. The effect of three variables, namely, activation temperature, orthophosphoric acid/lignin weight ratio and time of impregnation, was discussed with respect to the resultant PSDs.
Size effect in the characterization of microporous activated nanostructured carbon
Microporous and Mesoporous Materials, 2010
Two kinds of microporous activated nanostructured carbons were studied by X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and electrical conductivity measurement. They have different specific surface areas: 1500 m 2 /g (AC I) and 2500 m 2 /g (AC II). From X-ray diffraction analysis the average values of the nanodomain sizes of the AC I and AC II carbon particles are 1.7 nm and 1.5 nm, respectively. The Raman spectra and conductivity measurements have consistently confirmed the gradation of the crystallite sizes. It was observed also that the EPR spectra shapes and their temperature evolution between 300 K and 4 K were depending on the types of carbon sample. Room temperature spectra show dissimilar line-widths DB = 0.5 mT and 1.6 mT for AC I and AC II, respectively. The AC I carbon EPR spectra show: (a) a broadening at the beginning of the cooling process, similar to the EPR behavior of graphite and, subsequently, (b) a quick narrowing below 130 K. For the super-activated carbon (AC II), a monotonic narrowing process is observed on decreasing the temperature. These different behaviors were attributed to the size effect of graphenes in both activated carbons.
Langmuir, 2000
We present a unified approach to pore size characterization of microporous carbonaceous materials such as activated carbon and carbon fibers by nitrogen, argon, and carbon dioxide adsorption at standard temperatures, 77 K for N2 and Ar and 273 K for CO2. Reference isotherms of N2, Ar, and CO2 in a series of model slit-shaped carbon pores in the range from 0.3 to 36 nm have been calculated from the nonlocal density functional theory (NLDFT) using validated parameters of intermolecular interactions. Carbon dioxide isotherms have also been generated by the grand canonical Monte Carlo (GCMC) method based on the 3-center model of Harris and Yung. The validation of model parameters includes three steps: (1) prediction of vapor-liquid equilibrium data in the bulk system, (2) prediction of adsorption isotherm on graphite surface, (3) comparison of the NLDFT adsorption isotherms in pores to those of GCMC simulations, performed with the parameters of fluid-fluid interactions, which accurately reproduce vapor-liquid equilibrium data of the bulk fluid. Pore size distributions are calculated by an adaptable procedure of deconvolution of the integral adsorption equation using regularization methods. The deconvolution procedure implies the same grid of pore sizes and relative pressures for all adsorbates and the intelligent choice of regularization parameters. We demonstrate the consistency of our approach on examples of pore structure characterization of activated carbons from adsorption isotherms of different gases and from different models (NLDFT and GCMC). Since the CO2 isotherms measured up to 1 atm are not sensitive to pores wider then 1 nm, the NLDFT method for CO2 has been extended to high-pressure CO2 adsorption up to 34 atm. The methods developed are suggested as a practical alternative to traditional phenomenological approaches such as DR, HK, and BJH methods.
Applied Surface Science, 2002
The concept of a comparative method to analyse the porous structure of activated carbons by means of solid-liquid excessadsorption isotherms is presented. This method is based on a comparison of the excess-adsorption isotherm for benzene (1) cyclohexane (2) mixtures on a nanoporous active carbon with that measured on a non-porous reference adsorbent. The proposed approach gives the possibility for estimating the maximum amounts adsorbed in the micropores and on the surface of mesopores within the nanoporous material. Consequently, the total surface phase capacity can be determined. The partial excesses for micro-and mesopores may be extracted from the total adsorption excess. As the comparative method is based on a linear equation (the so-called ''n/x''-plot), a suitable statistical analysis of errors can been applied. The energy-distribution function is used to characterize heterogeneity effects. A regularization method has been employed, which takes into account the ill-posed character of the integral equation. The partial excess-adsorption isotherm, evaluated for micropores may be analyzed with regard to structural heterogeneity properties of nanoporous solids. Two series of modified active carbons, i.e. active carbons prepared from plum stones and synthetic active carbons, were used in these studies.