Evaluation of adsorption energy distribution of microporous materials by a multivariant identification

The applicability of new BET-like adsorption models (uLBET and LBET formulas) for the examination of microporous materials of irregular structure is discussed. The models are intended to handle the main effects of random pore geometry on the adsorption isotherm shape. They are based on a generalized BET theory and employ simple (but realistic) adsorption energy-pore geometry relationships. To overcome numerical problems inherent in fitting any more advanced adsorption model, a multivariant identification procedure is proposed with a subset of parameters being fixed in each variant. The reliability assessment of the system parameters evaluation is stressed. The paper outlines formal and numerical bounds for adsorption measurements-based identifiability of irregular porous structures. The LBET-type formulas are recommended for the examination of such materials, as a relatively simple alternative versus DFT-based techniques. Numerous simulation results and exemplary examinations of an empirical isotherm are discussed.     

爆轰裂解法制备纳米石墨粉的吸附性能研究

 在酸含量不同的原材料中,通过爆轰的方法制备纳米石墨粉,并利用BET方程以及BJH方法对所得纳米石墨粉进行比表面积和孔径分布分析。分析结果表明,所得爆轰产物中有的比表面积大致为天然鳞片石墨的5.3~9.2倍,而且随酸含量的增大逐渐增大,产物的等温线中存在吸附滞后现象。其中,增大的比表面积主要由分布在3 nm至7 nm之间的孔引起的,而且在爆轰后,孔径4 nm左右的孔,其数量达到最大值。通过对纳米石墨粉的研究,分析了酸在爆轰过程中的积极作用,并为纳米石墨粉的进一步应用提供了结构信息。     

Characterization of synthetic carbons activated with phosphoric acid

The structural heterogeneity of synthetic phosphoric acid activated carbons has been analyzed using pore-size distributions (PSDs) obtained from nitrogen at −196 °C and carbon dioxide at 0 °C isotherms. PSDs where obtained by the BET–Kelvin method. It is shown that the BET–Kelvin method is in good agreement with DFT and provides a fast means for assessment of the porous structure of adsorbents. PSDs obtained by the BET–Kelvin method using different adsorbates give results consistent with each other. Due to the restricted pressure range for carbon dioxide adsorption isotherm the PSD gives information only about pores in the micropore range. The agreement between different methods is better for small micropores.     

Adsorption characterization of surfactant-templated ordered mesoporous silicas synthesized with and without hydrothermal treatment

This work reports a systematic study of ordered mesoporous silicas (OMSs) synthesized with and without hydrothermal treatment at 373 K for a series of surfactants of different alkyl chain length (from C10 to C18). For these samples nitrogen adsorption and small angle X-ray scattering (SAXS) data were measured to characterize their adsorption and surface properties. Namely, nitrogen adsorption isotherms were used to evaluate their specific surface area, pore volume and pore size distribution, whereas SAXS data provided information about their structural ordering. It is shown that while the room temperature synthesis afforded OMS samples with cubic MCM-48 structure, an additional 5-day hydrothermal treatment of these samples at 373 K caused their transformation to MCM-41 (two-dimensional hexagonal structure) and improved their pore uniformity, which was manifested by reducing the width of pore size distribution.     

Characterization of PSD of activated carbons by using slit and triangular pore geometries

A mixed geometry model for activated carbons, representing the porous space as a collection of an undetermined proportion of slit and triangular pores, is developed, evaluated theoretically and applied to the characterization of a controlled series of samples of activated carbon obtained from the same precursor material. A method is proposed for the determination of the Pore Size Distribution (PSD) for such a mixed geometry model, leading to the unique determination of the proportion of pores of the two geometries fitting adsorption data. By using the Grand Canonical Monte Carlo (GCMC) simulation method in the continuum space, families of N2 adsorption isotherms are generated both for slit and triangular geometry corresponding to different pore sizes. The problem of the uniqueness in the determination of the PSD by fitting an adsorption isotherm using the mixed geometry model is then discussed and the effects of the addition of triangular pores on the PSD are analyzed by performing a test where the adsorption isotherm corresponding to the known PSD is generated and used as the “experimental” isotherm. It is found that a pure slit geometry model would widen the PSD and shift it to smaller sizes, whereas a pure triangular geometry model would produce the opposite effect. The slit and triangular geometry families of isotherms are finally used to the fit experimental N₂ adsorption data corresponding to a family of activated carbons obtained from coconut shells through a one-step chemical activation process with phosphoric acid in air, allowing for the determination of the micropore volume, the proportion of slit and triangular pores and the PSD corresponding to the mixed geometry. The same experimental data were fit using both the conventional slit pore model and the mixed geometry model. From the analysis of the effect of different preparation procedures on the resulting PSDs, it is concluded that the proposed mixed geometry model may probably better capture the morphology and energetics of activated carbons prepared by chemical activation under mild temperatures.     

Characterization of nanoporous carbons by combining CO2 and H2 sorption data with the Monte Carlo simulations

The Monte Carlo method in its grand ensemble variant (GCMC) is used in combination with experimental data in order to characterize microporous carbons and obtain the optimal pore size distribution (PSD). In particular, the method is applied in the case of AX-21 carbon. Adsorption isotherms of CO₂ (253 and 298 K) and H₂ (77 K) up to 20 bar have been measured, while the computed isotherms resulted from the GCMC simulations for several pore widths up to 3.0 nm. For the case of H₂ at 77 K quantum corrections were introduced with the application of the Feynman-Hibbs (FH) effective potential. The adsorption isotherms were used either individually or in a combined manner in order to deduce PSDs and their reliability was examined by the ability to predict the experimental adsorption isotherms. The combined approach was found to be capable of reproducing more accurately all the available experimental isotherms.     

Adsorption of nicotine from aqueous solution onto hydrophobic zeolite type USY

The isothermal adsorption of nicotine from an aqueous solution onto zeolite type USY was investigated. The adsorption isotherms of nicotine onto the zeolite at different temperatures ranging from 298 to 322 K were determined. It was found that the adsorption isotherms can be described by the model of Freundlich adsorption isotherm. Based on the adsorption isotherms the changes of adsorption heat, free energy and entropy with adsorption degree were determined. The determined decrease of adsorption heat with adsorption degree can be explained by the presence of the adsorption centers of different energy and concentration on interface of zeolite-nicotine solution. It was found that the probability function of density distribution of the heat of adsorption (DDF) has exponential form. It was concluded that the possibility of fitting the adsorption isotherms of nicotine onto the zeolite by Freundlich adsorption isotherm was a direct consequence of that. The determined increase in entropy with the increase in adsorption degree can be explained with the change of phase state of adsorbed nicotine.     

爆轰裂解法制备纳米石墨粉的吸附性能研究(英文)

在酸含量不同的原材料中,通过爆轰的方法制备纳米石墨粉,并利用BET方程以及BJ H方法对所得纳米石墨粉进行比表面积和孔径分布分析。分析结果表明,所得爆轰产物中有的比表面积大致为天然鳞片石墨的5.3~9.2倍,而且随酸含量的增大逐渐增大,产物的等温线中存在吸附滞后现象。其中,增大的比表面积主要由分布在3 nm至7 nm之间的孔引起的,而且在爆轰后,孔径4 nm左右的孔,其数量达到最大值。通过对纳米石墨粉的研究,分析了酸在爆轰过程中的积极作用,并为纳米石墨粉的进一步应用提供了结构信息。     

Effects of textural and surface characteristics of microporous activated carbons on the methane adsorption capacity at high pressures

The objective of this study is to relate textural and surface characteristics of selected microporous activated carbons to their methane storage capacity. In this work, a magnetic suspension balance (Rubotherm, Germany) was used to measure methane adsorption isotherms of several activated carbon samples. Textural characteristics were assessed by nitrogen adsorption on a regular surface area analyzer (Autosorb-MP, by Quantachrome, USA). N₂ adsorption was analysed by conventional models (BET, DR, HK) and by Monte Carlo molecular simulations. Elemental and surface analyses were performed by X-ray photoelectronic spectroscopy (XPS) for the selected samples. A comparative analysis was then carried out with the purpose of defining some correlation among the variables under study. For the system under study, pore size distribution and micropore volume seem to be a determining factor as long as the solid surface is perfectly hydrophobic. It was concluded that the textural parameters per se do not unequivocally determine natural gas storage capacities. Surface chemistry and methane adsorption equilibria must be taken into account in the decision-making process of choosing an adsorbent for gas storage.     

Theoretical studies of argon adsorption in MCM-41 mesoporous systems

Adsorption isotherms are predicted for spherical adsorbates in cylindrical channels of MCM-41 mesoporous materials over a wide range of temperatures by using the “fragment method”. This prediction shows that an equilibrium capillary condensation is impossible for pores with diameters smaller than 2.5 nm. The adsorbate distribution in relatively large pore channels was described by the quasi-chemical approximation (QCA) that takes into account direct pair correlations between interacting molecules. In order to improve the lattice-gas model in the vicinity of the critical point, a calibration function that takes into account information from the fragment method, was introduced into the QCA equations. The influence of the size factor of pores on argon adsorption isotherms was demonstrated.     

Adsorption of argon and nitrogen in cylindrical pores of MCM-41 materials: application of density functional theory

Adsorption of argon and nitrogen at their respective boiling points in cylindrical pores of MCM-41 type silica-like adsorbents is studied by means of a non-local density functional theory (NLDFT), which is modified to deal with amorphous solids. By matching the theoretical results of the pore filling pressure versus pore diameter against the experimental data, we arrive at a conclusion that the adsorption branch (rather than desorption) corresponds to the true thermodynamic equilibrium. If this is accepted, we derive the optimal values for the solid-fluid molecular parameters for the system amorphous silica-Ar and amorphous silica-N₂, and at the same time we could derive reliably the specific surface area of non-porous and mesoporous silica-like adsorbents, without a recourse to the BET method. This method is then logically extended to describe the local adsorption isotherms of argon and nitrogen in silica-like pores, which are then used as the bases (kernel) to determine the pore size distribution. We test this with a number of adsorption isotherms on the MCM-41 samples, and the results are quite realistic and in excellent agreement with the XRD results, justifying the approach adopted in this paper.     

Molecular simulation of adsorption of NO and CO2 mixtures by a Cu-BTC metal organic framework

Environmental problems due to the discharge of gases, including NO and CO₂, in addition, diseases caused by improper concentration of NO and CO₂ in vivo must be resolved. In this study, Grand canonical Monte Carlo (GCMC) simulations are combined with density functional theory (DFT) to calculate the adsorption of NO and CO₂ from a dual-component mixture to the Cu-BTC metal organic framework. The results show that the adsorption isotherms for various molar ratios of the gaseous mixture followed a Langmuir distribution. At higher pressures more CO₂ than NO was adsorbed by Cu-BTC, with NO showing a tendency to desorb. However, better results for adsorption of NO were observed at lower pressures. For the different pressure and molar ratios of the gaseous mixture examined, more CO₂ than NO was always adsorbed. Compared with three-way catalysts, Cu-BTC offers benefits to adsorption of CO₂ and NO from gaseous mixtures without increased durability problems.     

Physical Characteristics and Sintering Behavior of MgO-Doped ZrO2nanoparticles

Nanosized particles of 13mol% MgO-doped ZrO₂ with a narrow distribution of pore sizes were prepared by the coprecipitation technique using optimized parameters of synthesis. Transmission electron microscopy analysis of the calcined powder reveals that the majority of the particles have grain sizes in the 10–20nm range. From nitrogen adsorption analysis an average particle size of 13nm was estimated, which is similar to the average pore size diameter (12nm). Besides the unimodal distribution of pore sizes, the linear shrinkage curve of a powder compact exhibits several inflexions indicating different rates of densification up to 1600°C. After sintering at 1600°C for 2h, the microstructure features of a compact are characteristics of the intermediate stage with interconnected porosity preferentially observed at grain boundaries. These results are explained as a size effect of nanoparticles of magnesia-doped zirconia during sintering.     

Comparing the Basic Phenomena Involved in Three Methods of Pore‐size Characterization: Gas Adsorption,Liquid Intrusion and Thermoporometry

Gas adsorption at 77 K is probably the most widely used method to characterize mesoporous adsorbents via capillary condensation. Mercury intrusion is also commonly used for the characterization of such materials with the possibility to extend the pore width distribution measurement to the macropore range. More recently water intrusion was proposed to study hydrophobic porous solids. Thermoporometry is a further characterization method, now based on the study of the solid/liquid transition. We can consider that all these methods rely on the modification of the phase diagram of a pure substance under the effect of confinement. For each of them, the equations and models derived are based either on classical thermodynamics or on statistical approaches. In most cases, the derivation of the pore width distribution is carried out with the assumption that the porous system is made up of non‐connected cylinders, whereas the actual complexity of the structure is derived from the hysteresis shape. Our aim here is to compare the four approaches above (gas adsorption, mercury intrusion, water intrusion, thermoporometry) on samples giving rise, in their gas adsorption isotherms, to the typical hysteresis loops of the IUPAC classification. Our comparison is in great part carried out after the shape of the hysteresis loops obtained with the various methods and after the information it brings on the mechanism of replacement of a phase by the other within the porous system.     

Simulation study of argon adsorption on (0 0 1) faces of phyllosilicates

Grand Canonical Monte Carlo molecular simulations have been performed for argon and nitrogen adsorption on the basal surfaces of phyllosilicates without surface cations. The results have been compared with derivative isotherms analysis of experimental data. An optimization of the surface-Ar interaction has been performed by varying the oxygen atom LJ ?/k_B parameter and the optimized value was used to perform the nitrogen adsorption simulations. The analysis of the argon adsorption simulation indicates that adsorption mechanisms are more complex than may be suggested by experimental results obtained by low-pressure adsorption. The structure of the adsorbed film has a marked dynamic behaviour and the monolayer capacity strongly depends on the equilibrium relative pressure. For nitrogen adsorption, while high pressure behaviour is simulated adequately, some deviation is observed in low-pressure region of the isotherms suggesting that additional simulation and perhaps the use of a more sophisticated potential to model the nitrogen molecule can be necessary to understand fully the behavior of this gas on clay minerals.     

Ion exchange and adsorption equilibrium studies on clinoptilolite, bentonite and vermiculite

In the present paper three natural minerals used in many industrial and environmental applications namely zeolite clinoptilolite and the clays bentonite and vermiculite are studied by utilising ion exchange and adsorption. In particular, the Dubinin-Astakhov adsorption isotherm is used modified by introducing a solubility-normalized adsorption potential for studying the ion exchange process. The equation, is applied in experimental isotherms in order to determine adsorption energy and heterogeneity parameter for the ion exchange of Pb²⁺ in the natural minerals. The results indicate that the modified Dubinin-Astakhov adsorption isotherm represents the experimental data well and at the same time provides the heterogeneity parameter of the materials, which is an important adsorbent physical parameter as well as the adsorption energy. In order to deepen the study and link the results to the pore structure BET analysis is presented as well.     

Equilibrium partitioning of a simple fluid in a matrix-filled cylindrical pores with adsorbing walls: A grand canonical Monte Carlo study

We have studied a model of a hard sphere fluid adsorbed in a cylindrical pore filled with quenched disordered matrix of hard sphere particles using Grand canonical Monte Carlo simulations. The interactions between matrix species and pore walls are assumed of a hard sphere type. However, the pore walls exert a short-range attraction upon adsorbed fluid particles. We discuss the adsorption isotherms and the density profiles of fluid particles in pores with different microporosity for several values of the pore radius. We have observed that like in homogeneous microporous media the adsorption increases with increasing porosity. However, trends of behavior of the isotherms also reflect layering of adsorbed fluid. The data obtained in this study may serve as a benchmark for the development of the theory of confined quenched-annealed systems and for computer simulation investigation of models permitting phase transitions in pores. This project has been supported in parts by DGAPA of the UNAM under research grant IN111597, by the National Council for Science and Technology (CONACyT), grant No. 25301-E.