Characterization of the Microporous Structure of Activated Carbons through Different Approaches (original) (raw)
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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.
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, 2012
Nitrogen (N 2 , at 77 K and sub-atmospheric pressure), carbon dioxide (CO 2 , at 273 K, sub-atmospheric pressure and high pressure) and methane (CH 4 , at 298 K and high pressure) adsorption isotherms have been obtained for three microporous-activated carbons in order to characterize their micropore size distributions (MPSDs). A comparison of the results has shown that the shape of the isotherms was different, which was due in part to differences in the overall micropore volume and the MPSDs for the carbon samples studied in this work. According to the results, despite the different characteristics of CO 2 and CH 4 , a quite good consistency between the MPSDs from these two gases was obtained for the three carbon samples. Finally, it was shown that the adsorption isotherms of CO 2 and CH 4 at the higher temperatures should be considered as a complement to N 2 adsorption at 77 K for the characterization of porous texture (micropore volume and MPSDs) of activated carbon samples.
Adsorption characteristic study of activated carbons down to 4.5 k
Materials Today: Proceedings, 2018
Adsorption characteristic study of different activated carbon samples are performed at low temperature down to 5 K by using cryocooler based low temperature facility with a micropore analyzer. This facility is now operational at Institute for plasma research (IPR). It comprises of a commercial micro pore analyzer for sample characterization at 77 K and a GM cryocooler cooling unit forholding the sample at any temperature < 77 K. The sample holder is connected to the micro-pore analyzer measurement unit through a small diameter tube for gas dosing and equilibrium pressure measurement. A PID control based temperature controller unit along with temperature sensors and heatersare used to regulate and maintain the temperature of the sample cell at any temperature between 4.5 K and 77 K. Among different kinds ofactivated carbon samples studied, the surface area and pore width for carbon cloths are found to be 3438 m 2 /gm (at 4.5 K), and it increases with lowering the temperature down to 4.5 K from 77.0 K.
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.
Possible errors in microporosity in chemically activated carbon deduced from immersion calorimetry
Carbon, 2008
The microporosity of granular and disc-shaped activated carbons prepared by both ZnCl 2 and H 3 PO 4 activation has been evaluated by adsorption of nitrogen at À196°C and immersion calorimetry into liquids of different molecular dimensions (dichloromethane, benzene, 2,2-dimethylbutane, carbon tetrachloride and a-pinene). Experimental results show that immersion calorimetry into dichloromethane provides values of surface area more similar to nitrogen adsorption (BET equation) than benzene. No such effect is found for physically activated carbon. Some apparent anomalies have also been detected for the enthalpy of immersion of carbons activated with H 3 PO 4 into a-pinene due to a small amount of phosphorous remaining in the carbon after washing. This is not the case for carbons activated with ZnCl 2 , because the washing was more effective in the removal of the chemical.
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.
The Use of Liquid Phase Adsorption Isotherms for Characterization of Activated Carbons
The characterization of three commercial activated carbons was carried out using the adsorption of various compounds in the aqueous phase. For this purpose the generalized adsorption isotherm was employed, and a modification of the Dubinin–Radushkevich pore filling model, incorporating repulsive contributions to the pore potential as well as bulk liquid phase nonideality, was used as the local isotherm. Eight different flavor compounds were used as adsorbates, and the isotherms were jointly fitted to yield a common pore size distribution for each carbon. The bulk liquid phase nonideality was incorporated through the UNIFAC activity coefficient model, and the repulsive contribution to the pore potential was incorporated through the Steele 10-4-3 potential model. The mean micropore network coordination number for each carbon was also determined from the fitted saturation capacity based on percolation theory. Good agreement between the model and the experimental data was observed. In addition, excellent agreement between the bimodal gamma pore size distribution and density functional theory-cum-regularization-based pore size distribution obtained by argon adsorption was also observed, supporting the validity of the model. The results show that liquid phase adsorption, using adsorptive molecules of different sizes, can be an effective means of characterizing the pore size distribution as well as connectivity. Alternately, if the carbon pore size distribution is independently known, the method can be used to “measure” critical molecular sizes.
Adsorption-journal of The International Adsorption Society, 2023
Microporosity and structure of a set of activated carbons was studied by combination of N 2 and CO 2 adsorption, Transmission Electron Microscopy (TEM), X-ray diffraction and scattering and multiwavelength Raman spectroscopy. It is shown that correlations between measured parameteres may be established for a given set of activated carbons, most often obtained from a same precursor. Comparison of results of TEM images processing and of Small-angle scattering with adsorption data suggests that super-micropores (0.7-2 nm) are highly variable in shape and strongly deviate from the ideal slit pore model. These pores are likely located in between disordered continuous graphene stacks. It is shown that Small-angle scattering is mostly caused by supermicropores; contribution of other types of porosity is of secondary importance. For a set of carbons with similar structure, a reasonable correlation between Guinier radii and pore width obtained from N 2 adsorption can be found; however, the reason for the observed offset between the data sets remain uncertain. Sensitivity of the Raman scattering to atomic scale processes leads to poor or unclear correlations between the spectroscopic and structural data, although some notable exceptions are noted.