Low-pressure Argon Adsorption Research Articles

Study on Multifractal of Coal Pore Structure Based on Low Pressure Argon Adsorption and Its Effect on Gas Adsorption/Desorption

Study on Multifractal of Coal Pore Structure Based on Low Pressure Argon Adsorption and Its Effect on Gas Adsorption/Desorption

Experimental study on geometric parameters of pore structure and quantitative evaluation of complexity of pulverized coal with different metamorphic degrees

In order to quantify pore structure complexity of coal, the low pressure argon adsorption method (LP-ArGA) was used to study geometric parameters of pore structure and quantitative evaluation of complexity of pulverized coal. The results showed that the geometric parameters of pore structure of most single-particle coals showed an obvious downward trend with the decrease of particle size, indicating that the pore structure became simpler in the pulverization process. The weight value of equivalent matrix scale is the largest, which is 0.4871, indicating that it has the greatest influence on pore structure complexity. With the decrease of particle size, the composite evaluation index of complexity of 1#, 2#, 3#, 4#, 5# and 6# samples decreased by 52.4%, 61.1%, 86.3%, 42.1%, 74.6% and 39.9%, respectively. It indicated that particle size has an extremely influence on pore complexity characteristics of single-particle coal. With the decrease of particle size, the pore complexity characteristics of coal samples tended to be simple, on the contrary, they showed medium or complex characteristics. The model has important guiding significance for evaluating the same coal sample with different particle sizes, but there are still some deficiencies for different coal samples.

Comparative analysis of pore structure parameters of coal by using low pressure argon and nitrogen adsorption

The accurate characterization of coal pore structure is of great significance for in-depth understanding of interior properties and gas adsorption, desorption and diffusion characteristics. In this study, the low pressure argon adsorption (LP-ArGA) and low pressure nitrogen adsorption (LP-N2GA) were all used to analyze the pore parameters of coal with different metamorphism. The results indicated that the adsorption isotherm type and desorption hysteresis type of coal samples obtained by the LP-ArGA and LP-N2GA belong to the same type, but the adsorption capacity of the former is higher than that of the latter. The pore size distribution (PSD) of coal samples shows a significant multi-peak distribution feature. Compared with the LP-N2GA, the LP-ArGA can accurately analyze the pore parameters between 2 and 4 nm or part of 2–7 nm. The mesopore volumes obtained by the LP-ArGA is 1.66–2.84 times that of the LP-N2GA, and there is no obvious law of macropore volume. The macropore volumes of QN and PM samples decreased by 17.2% and 50.9%, respectively, while GHS sample increased by 166.0%. The corresponding specific surface area (SSA) showed the same properties. Fractal curve fitting results obtained by the two methods are highly correlated, but compared with LP-ArGA, D1 and D2 obtained by the LP-N2GA can well reflect pore characteristics. The fractal dimension obtained by the two methods and the variation trend with the metamorphic degree are all different, which is mainly due to the difference of adsorption volume. The purpose of this paper is to introduce a new method for the analysis of coal pore structure in order to better understand the pore characteristics.

Study of Low-Pressure Argon Adsorption on Synthetic Nontronite: Implications for Smectite Crystal Growth

AbstractBecause relatively little information about the crystal-growth process of smectite is available, the process was assessed here by studying the size and shape of nontronite particles synthesized at six different temperatures from 75 to 150°C over a period of 4 weeks.The morphology of nontronites was studied using low-pressure isotherms of argon adsorption at 77 K, a method which enables the measurement of the basal and edge surface areas of the nontronite particles and of their mean diameter and thickness. During the crystal growth of nontronite, the mean particle length increased whereas their thickness (and the number of stacked layers) did not vary significantly.A specific two-dimensional crystal-growth process was observed for smectite via the lateral extension of the layers. This process also appears to occur during the growth of neoformed natural smectite.

Morphological properties of vermiculite particles in size-selected fractions obtained by sonication

The objective of this study is to determine the morphological properties of vermiculite particles obtained by sonication followed by separation into different size fractions (0.1–0.2, 1–2 and 10–20μm). These size fractions were chosen because they represent the wide range of particle sizes encountered in natural media. As previously reported, the crystal-chemistry of this vermiculite from Santa Olalla (Huelva, Spain) was found to be unaffected by the sonication process, although a loss of crystallinity and an increase in structural disorder might have occurred for the finest fraction. Using low-pressure argon adsorption analyses and a suite of microscopy techniques (i.e., atomic force, optical and transmission electron microscopy), the geometrical properties of the particles, including the distribution of the basal and lateral surfaces and aspect ratios, were obtained. Good agreement was observed between the gas adsorption experiments when applying the derivative isotherm summation procedure and the atomic force or optical microscopy analyses. However, the surface area was overestimated by transmission electron microscopy, which was attributed to the sample preparation technique. All of these data demonstrated that the particle aspect ratios are similar for all fractions, and the finest particles tend to show slightly more elongated surface morphologies than the coarser particles. Finally, a comparison of the distribution of lateral, basal and interlayer adsorption sites to the experimental cation-exchange capacity (CEC) showed that only the basal and interlayer sites are available in the conditions of CEC analysis. Based on these findings, the pertinence of using these size fractions as models to better understand the interactions between swelling clay minerals and water/solutes is discussed.

Berthierine-like mineral formation and stability during the interaction of kaolinite with metallic iron at 90 C under anoxic and oxic conditions

The interaction between metallic iron and kaolinite was studied in conditions relevant to those that may be encountered in a high-level radioactive waste disposal facility in geological formation. Experiments were carried out under anoxic atmosphere at 90 °C and in chloride solutions to simulate conditions close to disposal facilities. KGa-2 kaolinite was put in contact with powdered metallic iron in batch experiments for durations of 1, 3, and 9 months. Solutions extracted from the end-products were analyzed (pH, Eh, conductivity, and cation concentrations). End-products were characterized by a set of chemical (oxide analyses, CEC, EDXS) and mineralogical techniques (SEM, TEM, XRD, and FTIR), textural analyses (nitrogen adsorption and low-pressure argon adsorption), XPS, and Mossbauer spectroscopy. In another set of experiments the system was changed from anoxic to oxic conditions to evaluate the stability of the system in the presence of O2. The interaction between metallic iron and kaolinite led to a fast initial reaction as major modifications took place during the first month. The partial oxidation of metallic iron resulted in a pH increase and negative Eh values. Iron was not found in solution but in two new Fe-rich phases: magnetite in very low amounts and a Fe-rich clay phase, belonging to the berthierine family. The Si and Al of the berthierine are derived from the partial alkaline dissolution of kaolinite, mostly along edge faces. TEM-EDXS local analyses showed that the composition of resulting particles consisted in mixtures of berthierine and kaolinite layers. Clay particles became thicker with the epitaxial growth of berthierine layers on the basal surfaces of pristine kaolinite. Neoformed berthierine was not stable in the presence of O2 at 90 °C. Berthierine layers dissolved, iron was mobilized to form iron oxides and oxyhydroxides while kaolinite layers recrystallized from released Al and Si.

Textural and hydration properties of a synthetic montmorillonite compared with a natural Na-exchanged clay analogue

An aluminous montmorillonite has been synthesized at hydrothermal conditions (623 K, 120 MPa) and compared to the natural Na-exchanged Wyoming montmorillonite (Na-SWy2). The synthetic product is characterized by a high cation exchange capacity (83 meq/100 g) and ethylene glycol specific surface area (764 m 2/g) similar to Na-SWy2 but with a higher purity with one crystalline phase. Compacted smectite samples were percolated with water by using an oedometer cell equipped with a separated injection system, in order to simulate the behaviour of an engineered clay barrier in the context of subsurface waste landfill. Percolation experiments showed very low hydraulic conductivities (10 − 12 m/s) for both synthetic and natural smectites, in addition to the non-macroscopic swelling of the synthetic montmorillonite, in contrast to Na-SWy2. At the nanometric scale, this synthetic clay mineral is characterized by higher hydration states and a better organization in the distribution of water molecules in particles, as proven by X-ray diffraction technique and continuous water vapor gravimetry method. Moreover, adsorption–desorption isotherms of nitrogen revealed the classical feature of swelling clay minerals, and the Brunauer–Emmet–Teller calculation gives a nitrogen specific area of 87 m 2/g for the synthetic smectite, twice higher than for the Wyoming clay, due to its higher mesoporosity. Finally, low-pressure argon adsorption coupled with the Derivative Isotherm Simulation procedure highlighted classical adsorption energy distributions of natural phyllosilicates, and thus the synthetic montmorillonite was characterized by an average of 11 layers per particle in the dry state, with basal and edge surface areas different from Na-SWy2.

Role of Exchangeable Cations on Geometrical and Energetic Surface Heterogeneity of Kaolinites

Textural and energetic proprieties of kaolinite were studied by low-pressure argon adsorption at 77 K. The heterogeneity of four kaolinites (two low-defect and two high-defect samples) modified on their surface by cation exchange with Li+, Na+, or K+ was studied by DIS analysis of the derivative argon adsorption isotherms. The comparison between the derivative adsorption isotherms shows that the nature of the surface cation influences the adsorption phenomena on edge and basal faces. In the case of basal faces, two adsorption domains are observed: for the first one, argon adsorption is slightly sensitive to the nature of the surface cation; for the second one, argon adsorption energy depends on the nature of surface cation suggesting their presence on theoretically uncharged basal faces. This study also shows that the shape of elementary particles, as derived from basal and edge surface areas, changes with the nature of cation. This anomalous result is due to the decrease of edge surface area with increasing the size of the cation. This surface cation dependence can be accounted for the area occupied by the edge surface cations in the first argon monolayer.

AFM and low-pressure argon adsorption analysis of geometrical properties of phyllosilicates

The distribution of edge and basal surface areas of phyllosilicate particles is an essential parameter for understanding the interaction mechanisms at solid/gas or solid/liquid interfaces. Among the techniques proposed to determine the geometrical heterogeneities of flat solids, low-pressure argon adsorption and AFM analysis are the most promising to derive the weight-averaged values of specific surface areas. A series of publications have recently been dedicated to the combination of both methods showing the correlation between the two approaches. As obtaining a large set of high-resolution AFM images is time-consuming, it is necessary to test the ability of AFM routine analysis to derive surface areas and aspect ratio systematically and statistically, with all possible experimental and instrumental artefacts. In the present study, the expected agreement was found between AFM and argon adsorption determination for total, basal, and edge-specific surface areas of nonswelling clay minerals, except for one kaolinite, which is very heterogeneous in size. In addition, it was observed that for a given sample, individual particles present similar shapes, whatever their size, making it possible to derive a statistical relationship between AFM basal and total surface areas. On the basis of the obtained results, recommendations are given to derive accurate edge, basal, and total specific surface areas of phyllosilicates by combining conventional gas adsorption (nitrogen BET) and routine AFM techniques.

Low-pressure argon adsorption assessment of micropore connectivities in activated carbons

Low-pressure argon adsorption has been used to study the energetic distribution of microporous activated carbons differing by their burn-off. The collected isotherms were analyzed using the derivative isotherm summation method. Some oscillations on the experimental curves for very low partial pressures were detected. The results are analyzed and discussed according to the literature and could be attributed to local overheating caused by spontaneous mass transfer of argon through constrictions between former pores and the new opening pore or deadend pores. We used the dynamic character of the experimental method and mainly the discrepancy of the quasi-equilibrium state to deduce key parameters related to the porosity topology.

Nanopore Structure and Sorption Properties of Cu−BTC Metal−Organic Framework

Grand canonical Monte Carlo simulations in conjunction with high-resolution low-pressure argon adsorption experiments were employed to study adsorption mechanisms on the copper(II) benzene-1,3,5-tricarboxylate metal-organic framework (Cu−BTC). We constructed a molecular structural model of Cu−BTC. The pore network of Cu−BTC has a simple cubic symmetry. It consists of main channels of a square cross-section of ca. 0.9 nm diameter and tetrahedral side pockets of ca. 0.5 nm, which are connected to the main channels by triangular windows of ca. 0.35 nm diameter. Using a parameterized united-atom force field, we have determined the preferential adsorption sites and the sequence of adsorption mechanisms from a gradual filling of the side pockets to a stepwise adsorption and condensation in the main channels. The simulation results agree quantitatively with the experimental isotherm of argon up to almost complete filling of the pore network.

Morphology and surface heterogeneities in synthetic goethites

In the framework on a study of the acido-basic and sorption properties of iron oxides, a thorough characterization of two types of goethite powders was performed in several laboratories joined in a common project. Chemical analysis by ICPAES; high-resolution SEM, TEM, and AFM observations; XRD with line width analysis; and argon and nitrogen sorption isotherms were used for that purpose. The main crystallographic faces of goethite particles could be identified as {001}, {101}, and {121}, and their abundance correlated with the distribution of low-pressure argon adsorption local isotherms. These results will be very useful for further studies on the relationship between surface reactivity in aqueous solution and orientation of solid surfaces.

Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis

Imaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment.

Adsorption of Spherical Molecules in Probing the Surface Topography. 1. Patchwise Heterogeneity Model

The Fowler−Guggenheim local isotherm corresponding to the lattice model of localized adsorption on a patchwise surface was used to evaluate the statistical distribution of adsorption sites with respect to their adsorption energies from an experimental adsorption isotherm. An accurate low-pressure argon adsorption isotherm at 77 K on muscovite was used as initial information. The calculations were carried out in two ways that imply different surface constructions. The first one is based on the classical hypothesis of Langmuir, who assumed a finite manifold of adsorption sites and wrote an overall isotherm as a series summing the additive contributions of the different sites. The second is derived on the assumption of an infinite manifold of sites, when the sum turns into an integral. Both representations are equivalent only with the exact adsorption model. However, as shown, in a number of cases they can give similar results even with the approximate model. The influence of the ω parameter expressing the reduced energy of lateral interactions was tested for different ω values ranging between 0 and 4 in units of RT. Muscovite possesses crystallinity. Therefore, one may expect that its surface consists of recurring adsorption sites. Peaks on the energy distribution could mean at least a short range ordering (SRO) on this surface. A derivative of a local isotherm for sites of each kind is known to be a bell-shaped curve. The greater the energy of lateral attraction is, the narrower is the bell; that is, the Langmuir local isotherm (the case of ω = 0) corresponds to the widest bell. Hence, if the hypothesis concerning SRO holds true, an energy distribution obtained with ω = 0 must be discrete, inasmuch as a broadening of the calculated partial distributions can lead only to a deteriorating of the approximation accuracy of local derivatives and, correspondingly, of the overall derivative and the overall isotherm. Indeed, the distributions calculated with a small ω represent sums of δ-like peaks. With a small ω, both algorithms show the same results; therefore, any numerical artifacts are excluded. However, SRO on the surface is not proven yet, because with a large ω the algorithms lead to continuous distributions rather than to discrete ones. The relative approximation error of the overall isotherm with ω = 0 is not too large (≅2−3%). However, the large approximation error of the overall derivative (≅15%) unambiguously indicates the unacceptability of the assumption ω = 0 (local Langmuir isotherm). Thus, to make final conclusions about the structure of the surface and validate such an analysis, it is necessary to improve the patchwise model, to test the method by using an independent molecular simulation, and to determine realistic values of the energies of lateral interactions being different and not equal for adsorption sites of each kind.

Characterization of modified mesoporous silicas using argon and nitrogen adsorption

Argon adsorption isotherms were measured at 87 K for a series of MCM-41 samples chemically modified with several kinds of organosilane ligands. Low-pressure argon adsorption on these materials was found to be highly dependent on the type of bonded ligands, which opens an opportunity for using it to identify bonded moieties on the silica surface. BET calculations performed for argon adsorption systems in a commonly used relative pressure range were found to provide good estimations of the monolayer capacity for octyldimethylsilyl (ODMS)-modified silicas. After a proper calibration of the statistical film thickness for argon on ODMS-modified materials, it was possible to obtain essentially artifact-free mesopore size distributions from argon adsorption isotherms on the modified samples of similar surface properties. The mesopore size distributions calculated from argon data were highly similar to those determined from nitrogen adsorption isotherms. It was concluded that similarly to nitrogen adsorption, argon adsorption is suitable to determine the specific surface areas and pore size distributions for chemically modified silicas, including functionalized mesoporous molecular sieves, as well as to characterize their surface properties. However, a reliable and self-consistent characterization of porous solids on the basis of argon data requires the application of proper methods, for instance, those calibrated using MCM-41 materials with a wide range of pore sizes.

Characterization of dealuminated zeolites using 129Xe n.m.r. spectrometry and lowpressure argon adsorption

This work compares the use of 129Xe n.m.r. spectrometry and argon adsorption techniques for characterization of the micropore region of dealuminated Y and CSZ-1 zeolites. Low-pressure argon adsorption provides a measure of zeolite aperture sizes, whereas 129Xe n.m.r. spectrometry gives an indication of dimensions of internal void volumes. The two analytical techniques detect subtle differences in the pore structure of Y zeolites due to dealumination method and steam treatment severity. Both techniques predict the presence of two microcrystalline phases in CSZ-1 zeolite and the collapse of one of the phases following severe steaming. 129Xe n.m.r. spectrometry and argon adsorption techniques are useful for distinguishing small differences in pore structures of similar materials. The correlation between the two techniques suggests that both techniques are influenced by the energetics of the sorption process in addition to pore geometry.