基于密度泛函理论的活性炭孔径分布改进算法

为提高非定域密度泛函理论（NLDFT）预测活性炭孔径分布（PSD）的精度,考虑了活性炭孔壁面晶体粗糙度对结果的影响。在传统NLDFT基础上,结合吸附壁面碳原子的密度分布,推导出改进NLDFT,预测了氩在光滑及具粗糙碳晶体表面的吸附平衡,并根据87.3 K、氩在活性炭上的吸附平衡数据,在由两种NLDFT确定了不同孔径的理论等温线核后,由寻优函数确定活性炭在0.35~12 nm区间的PSD。结果表明,以改进的NLDFT预测活性炭的PSD时,确定的活性炭孔径呈连续分布,预测平衡数据的相对误差小于10%;传统NLDFT确定的孔径在1 nm处出现断点,最大的预测相对误差范围达45%。改进NLDFT能较准确预测氩在具有粗糙晶体碳表面活性炭的PSD。     

Determination of pore size distribution and adsorption of methane and CCl4 on activated carbon by molecular simulation

A combined method of grand canonical Monte Carlo (GCMC) simulation and statistics integral equation (SIE) for the determination of pore size distribution (PSD) is developed based on the experimental adsorption data of methane on activated carbon at ambient temperature, T=299 K. In the GCMC simulation, methane is modeled as a Lennord-Jones spherical molecule, and the activated carbon pore is described as slit-shaped with the PSD. The well-known Steele’s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. Covering the range of pore sizes of the activated carbon, a series of adsorption isotherms of methane in several uniform pores were obtained from GCMC. In order to improve the agreement between the experimental data and simulation results, the PSD is calculated by means of an adaptable procedure of deconvolution of the SIE method. Based on the simulated results, we use the activated carbon with the PSD as the prototype of adsorbent to investigate adsorption. The adsorption isotherms of methane and CCl₄ at 299 K in the activated carbon with the PSD are obtained. The adsorption amount of CCl₄ reaches 20 mmol/g at ambient temperature and pressure. The results indicate that the combined method of GCMC and SIE proposed here is a powerful technique for calculating the PSD of activated carbons and predicting adsorption on activated carbons.     

Quenched solid density functional theory and pore size analysis of micro-mesoporous carbons

We present a new model of adsorption on micro-mesoporous carbons based on the quenched solid density functional theory (QSDFT). QSDFT quantitatively accounts for the surface geometrical inhomogeneity in terms of the roughness parameter. We developed the QSDFT models for pore size distribution calculations in the range of pore widths from 0.4 to 35 nm from nitrogen at 77.4 K and argon at 87.3 K adsorption isotherms. The QSDFT model improves significantly the method of adsorption porosimetry: the pore size distribution (PSD) functions do not possess gaps in the regions of ∼1 nm and ∼2 nm, which are typical artifacts of the standard non-local density functional theory (NLDFT) model that treats the pore walls as homogeneous graphite-like plane surfaces. The advantages of the QSDFT method are demonstrated on various carbons, including activated carbons fibers, coal based granular carbon, water purification adsorbents, and mirco-mesoporous carbon CMK-1 templated on MCM-48 silica. The results of PSD calculations from nitrogen and argon are consistent, however, argon adsorption provides a better resolution of micropore sizes at low vapor pressures than nitrogen adsorption.     

Using DFT analysis of adsorption data of multiple gases including H2 for the comprehensive characterization of microporous carbons

Hydrogen and nitrogen adsorption isotherms at cryogenic temperatures (77 and 87 K) were used to characterize the microporosity of a series of activated carbons, representing various pore size distributions (PSD). The PSD for each carbon was calculated by simultaneous fitting of the DFT model isotherms to their experimental counterparts. The resulting PSD represents robust characteristics of the carbon structure that is consistent with all the data used in the analysis. The range of pore size analysis in this method is extended to smaller pore sizes compared to the standard nitrogen adsorption analysis. In addition, it is shown that this approach allows to detect and exclude experimental points that are not fully equilibrated due to diffusion problems in narrow micropores. The results of the analysis of a series of carbons activated with systematically increasing burn-off show that the presented approach is a useful tool for a comprehensive characterization of microporous carbons, and for obtaining detailed and reliable carbon PSDs.     

水蒸气活化法制备椰壳活性炭的孔结构特征

以农林废弃物椰壳在600℃炭化2h后的炭化料为原料,以水蒸气为活化剂,研究了活化温度、活化时间、水蒸气用量对活性炭的比表面积、微孔容积和收率等的影响。结果表明：椰壳炭化料的比表面积仅为185m^2／g,且以中孔为主。在活化过程中,通过提高活化温度和水蒸气用量缩短了活化时间,扩宽了孔径;当水蒸气用量和活化温度较为适宜时,延长活化时间,有利于微孔的形成。活性炭的比表面积、总孔容积、微孔容积可达：1465m^2／g,0．9703cm^3/g,0．7519cm^2／g。并通过非定域密度函数理论（NLDFT）对活性炭的孔径分布进行了表征。     

2D-NLDFT adsorption models for carbon slit-shaped pores with surface energetical heterogeneity and geometrical corrugation

In this work, we show that the standard slit pore model widely used for the characterization of activated carbons may be improved by introducing structural and/or energetical heterogeneity to the surface of pore walls. The existing one dimensional slit pore model assumes graphite-like energetically uniform pore walls. As a result of this assumption adsorption isotherms calculated by the non local density functional theory (NLDFT) do not fit accurately the experimental N₂ data measured for real activated carbons. Assuming a graphene-based structural framework for activated carbons and using a 2D-NLDFT treatment of the fluid density in the pores we consider two options for model pores: energetically heterogeneous (EH) and geometrically corrugated (GC). For testing, we applied these two models to the pore size analysis of porous carbons that were giving poor results of the analysis with the standard slit model. We found that the typical artifacts of the homogeneous slit pore model were eliminated. Also, the agreement of the new models with experimental data was significantly better than that of the standard slit model.     

Adsorption of N2, Ar, acetone, chloroform and acetone-chloroform mixture on carbon black in the framework of molecular layer structure theory (MLST)

Adsorption of pure nitrogen, argon, acetone, chloroform and acetone-chloroform mixture on graphitized thermal carbon black is considered at sub-critical conditions by means of molecular layer structure theory (MLST). In the present version of the MLST an adsorbed fluid is considered as a sequence of 2D molecular layers, whose Helmholtz free energies are obtained directly from the analysis of experimental adsorption isotherm of pure components. The interaction of the nearest layers is accounted for in the framework of mean field approximation. This approach allows quantitative correlating of experimental nitrogen and argon adsorption isotherm both in the monolayer region and in the range of multi-layer coverage up to 10 molecular layers. In the case of acetone and chloroform the approach also leads to excellent quantitative correlation of adsorption isotherms, while molecular approaches such as the non-local density functional theory (NLDFT) fail to describe those isotherms. We extend our new method to calculate the Helmholtz free energy of an adsorbed mixture using a simple mixing rule, and this allows us to predict mixture adsorption isotherms from pure component adsorption isotherms. The approach, which accounts for the difference in composition in different molecular layers, is tested against the experimental data of acetone-chloroform mixture (non-ideal mixture) adsorption on graphitized thermal carbon black at 50 °C.     

The comparison of experimental and calculated pore size distributions of activated carbons

Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO₂ isotherms have been measured at 273 K on seven well-characterized microporous carbons with average micropore widths between 0.65 and 1.5 nm and analysed with model isotherms obtained with standard Monte Carlo simulations. The resulting PSD are in good agreement with those obtained from a modified Dubinin equation, from liquid probes of molecular dimensions between 0.4 and 1.5 nm, from STM and from modelling based on CH₄ adsorption at 308 K. The present study validates the determination of micropore distributions in active carbons based on CO₂ isotherms, provided that no gate effects are present.     

Characterization of accessible and inaccessible pores in microporous carbons by a combination of adsorption and small angle neutron scattering

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size <20 Å), but not for carbons which have significant mesopore populations of sizes up to 48 Å and high mass fractal degrees. The pore size distribution (PSD) results based on SANS data reveal significant populations of micropores of size <6.1 Å, and mesopores of size >20 Å, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO₂ 273 K. This inaccessible porosity becomes accessible to CO₂ and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.     

An investigation of fractal characteristics of mesoporous carbon electrodes with various pore structures

Fractal characteristics of mesoporous carbon electrodes were investigated with various pore structures using the N₂ gas adsorption method and the transmission electron microscopy (TEM) image analysis method. The mesoporous carbons with various pore structures were prepared by imprinting mesophase pitch used as a carbonaceous precursor with different colloidal silica particles. All imprinted mesoporous carbons were composed of two groups of pores produced from the carbonisation of mesophase pitch and from the silica imprinting. The overall surface fractal dimensions of the carbon specimens were determined from the analyses of the N₂ gas adsorption isotherms. In order to distinguish the surface fractal dimension of the carbonisation-induced pore surface from that fractal dimension of the silica-imprinted pore surface, the individual surface fractal dimensions were determined from the image analyses of the TEM images. From the comparison of the overall surface fractal dimension with the individual surface fractal dimensions, it was recognised that the overall surface fractal dimension is crucially influenced by the individual surface fractal dimension of the silica-imprinted pore surface. Moreover, from the fact that the silica-imprinted pore surface with broad relative pore size distribution (PSD) gave lower value of the individual surface fractal dimension than that pore surface with narrow relative PSD, it is concluded that as the silica-imprinted pores comprising the carbon specimen agglomerate, the individual surface fractal dimension of that pore surface decreases.     

The role of adsorbent pore size distribution in multicomponent adsorption on activated carbon

The impact of adsorbent pore size distribution (PSD) on adsorption mechanism for the multi solute system was evaluated in this study. Anoxic and oxic adsorption equilibrium for the single solute (phenol), binary solute (phenol/2-methylphenol) and ternary solute (phenol/2-methylphenol/2-ethylphenol) systems on one granular activated carbon (GAC) F400 and two types of activated carbon fibers (ACFs), namely, ACC-10 and ACC-15, were determined. F400 has a wide PSD, while ACC-10 and ACC-15 have narrow PSD and their critical pore diameters are 8.0 Å and 12.8 Å, respectively. In single solute adsorption, the increase of adsorptive capacity under oxic conditions as compared to anoxic ones was related to the PSD of the adsorbent. Binary solute adsorption on ACC-10 and ternary solute adsorption on ACC-15 indicated no impact of the presence of molecular oxygen on the adsorptive capacity and the adsorption isotherms were well predicted by the ideal adsorbed solution theory (IAST). Significant differences between oxic and anoxic isotherms were noticed for other multicomponent adsorption systems. The narrow PSD of ACFs was effective in hampering the oligomerization of phenolic compounds under oxic conditions. Such a phenomenon will provide accurate predictions of fixed bed adsorbers in water treatment systems.     

活性炭液相吸附脱除二甲基二硫醚

通过椰壳活性炭对模拟硫酸盐松节油进行液相吸附脱硫的研究,考察了活性炭的结构和表面化学对活性炭吸附的影响。通过Gaussian03计算模拟油中各化合物的化学硬度,根据软硬酸碱理论解释表面化学对该吸附过程的影响。同时考察了活性炭吸附过程的热力学性质。结果表明,由于吸附体系中的二甲基二硫醚和莰烯的化学硬度相接近,活性炭表面的含氧官能团的变化对吸附性能影响不大,但活性炭比表面积和孔结构的变化对吸附性能的影响较为明显;等温吸附平衡数据符合Freundlich等温吸附模型,热力学数据显示该活性炭吸附过程是一个放热、混乱度降低的自发物理吸附过程。     

Water adsorption by activated carbons in relation to their microporous structure

The present paper examines the adsorption of water by microporous carbons in the absence of specific interactions. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), shows that the type V isotherms follow the Dubinin-Astakhov (DA) equation and fulfill the requirement for temperature invariance. Furthermore, the parameters of the DA equation can be related in a simple way to structural properties of the model carbons. For a number of well-characterized carbons, the type V isotherms generated by combining model isotherms with the corresponding PSDs are in good agreement with the limiting isotherms at 293 and 310 K derived on the basis of a recent development of Dubinin’s theory. This approach will provide the basis for further studies including specific interactions.     

Effect of textural property of coconut shell-based activated carbon on desorption activation energy of benzothiophene

In this work, the effect of the textural property of activated carbons on desorption activation energy and adsorption capacity for benzothiophene (BT) was investigated. BET surface areas and the textural parameters of three kinds of the activated carbons, namely SY-6, SY-13 and SY-19, were measured with an ASAP 2010 instrument. The desorption activation energies of BT on the activated carbons were determined by temperature-programmed desorption (TPD). Static adsorption experiments were carried out to determine the isotherms of BT on the activated carbons. The influence of the textural property of the activated carbons on desorption activation energy and the adsorption capacity for BT was discussed. Results showed that the BET surface areas of the activated carbons, SY-6, SY-13 and SY-19 were 1106, 1070 and 689 m²·g⁻¹, respectively, and their average pore diameters were 1.96, 2.58 and 2.16 nm, respectively. The TPD results indicated that the desorption activation energy of BT on the activated carbons, SY-6, SY-19 and SY-13 were 58.84, 53.02 and 42.57 KJ/mol, respectively. The isotherms showed that the amount of BT adsorbed on the activated carbons followed the order of SY-6 > SY-19 > SY-13. The smaller the average pore diameter of the activated carbon, the stronger its adsorption for BT and the higher the activation energy required for BT desorption on its surface. The Freundlich adsorption isotherm model can be properly used to formulate the adsorption behavior of BT on the activated carbons. __________ Translated from Journal of Functional Materials, 2007, 38(10): 1664–1668 [译自: 功能材料]     

Using the high pressure methane isotherm for determination of pore size distribution of carbon adsorbents

A method of determining pore size distribution, PSD, of carbon adsorbents based on the high pressure methane isotherm is presented. A generic software package, and an IBM compatible PC, have been used to search for a PSD in the form of a histogram. The method relies on a known local isotherm, in this case, assuming a simplified model of infinite slit shaped carbon pores.Three carbons, having very different pore structures: BPL, PX-21, and PVDC, were analyzed using the new method and the results compared with those obtained from subcritical Ar, and N₂ isotherms. The analysis from the high pressure methane isotherm gave results which are different than those from the low pressure low temperature isotherms but not significantly enough to be unrealistic.     

The effect of impregnation of activated carbon with SnCl2.2H2O on its porosity, surface composition and CO gas adsorption

Activated carbon was impregnated with different concentrations of SnCl₂.2H₂O. Unimpregnated and impregnated activated carbons were analysed by means of physical adsorption and XPS and were tested for CO gas adsorption in a PSA system. The adsorption isotherms of N₂ at 77 K were measured and showed a Type I isotherm indicating microporous carbon for all the samples. The surface area, pore volume and pore size distribution were reduced with impregnation. XPS analysis showed an increase in the intensity of Sn3d peak with impregnation. The impregnated activated carbon showed a very good adsorption ability of CO gas compared to the unimpregnated sample. The adsorptive species responsible for CO gas adsorption was confirmed to be SnO₂ instead of SnO due to the former’s comparative thermodynamic stability.     

活性炭表面热氧化对其吸附二苯并噻吩性能影响

本文主要研究活性炭表面氧化对其吸附二苯并噻吩性能的影响。将活性炭在不同低温下氧化制得表面氧化活性炭，用静态吸附法进行了二苯并噻吩在初始及氧化活性炭上的吸附等温线，应用Langmuir方程对吸附等温线进行拟合，用漫反射红外谱（DRIFTS）表征活性炭表面含氧基团，用Boehm滴定测定活性炭表面官能团含量，讨论了活性炭表面化学性质对其吸附二苯并噻吩的影响。结果表明：活性炭表面酸性含氧基团对二苯并噻吩的吸附有重要影响，酸性含氧基团越多，其吸附量越大。低温气相氧化活性炭提高了活性炭表面酸性含氧基团，提高了其对二苯并噻吩的吸附。氧化温度越高，其表面含氧基团含量越多，其对二苯并噻吩的吸附量也越大。Langmuir吸附等温线可适用于描述二苯并噻吩在活性炭表面上的吸附。     

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.