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.     

Water vapor adsorption onto activated carbons prepared from cattle manure compost (CMC)

Activated carbons were prepared from cattle manure compost (CMC) using zinc chloride activation. The structural and surface chemical characteristics of CMC-based activated carbons were determined by N₂ adsorption-desorption and Boehm titration, respectively. The water vapor adsorption properties of the prepared activated carbons with various pore structure and surface nature were examined, and the mechanism of water adsorbed onto activated carbon was also discussed. The results show that the adsorption of water vapor on carbons begins at specific active sites at low relative humidity (RH), followed by micropore filling at medium RH through the formation of pentamer cluster of water molecules in the narrow micropores. The water vapor adsorption capacity of activated carbon is predominantly dependent on its pore volume and surface area. Although capillary condensation is not the mechanism for water adsorption onto activated carbon, water can adsorb on narrow mesopore to some extent.     

Characteristics of adsorption phase with water/organic mixtures at a surface of activated carbons possessing intraparticle and textural porosities

The behaviour of water and water/organic mixtures adsorbed onto activated microporous carbons or a carbon adsorbent with narrow intraparticle micropores and broad mesopores and macropores between nanoparticles was studied using low-temperature adsorption method and ¹H NMR spectroscopy with layer-by-layer freezing-out of liquids at 190-273 K. These investigations revealed concentration-dependent effects of benzene, DMSO, acetone, chloroform, methane and acetonitrile on the characteristics of adsorbed water and the influence of this water on the interfacial behaviour of adsorbed organics. The influence of organics causes the structural and energetic differentiations of adsorbed water. The latter can be displaced by organics from micropores into broader pores and/or form mixture with polar solvents in meso and macropores. Freezing of adsorbed water can affect the adsorbent structure because ice crystallites have a larger size than that of liquid water droplets that lead to changes in the behaviour of adsorbed water/organic mixtures observed by the ¹H NMR and adsorption methods.     

Porosity of microporous zeolites A, X and ZSM-5 studied by small angle X-ray scattering and nitrogen adsorption

Small-angle X-ray scattering (SAXS) using synchrotron radiation and nitrogen adsorption have been applied to characterizations of porosities and microporous structures for the zeolites of NaA, KA, CaA, NaX and ZSM-5. Besides the information on the external morphology of the particles of the zeolites, the complementation of the two techniques has revealed rich and consistent structural and surface information on the molecular scale crystalline pores of these zeolites. Analyses of the data suggest that the determined sizes of the micropores imply the pore spaces occupied by the probe molecules of water in the SAXS and nitrogen in adsorption techniques, respectively. The microporous information of NaA and KA are difficult to obtain from nitrogen adsorption, due to the blocking of nitrogen by their narrow channels, but have been satisfactorily measured by SAXS. The factors causing variations of the measured values of the parameters in different analysis methods have been discussed.     

Adsorption of water—methanol mixtures in carbon and aluminosilicate pores: a molecular simulation study

A theoretical study is reported of the adsorption behaviour of water—methanol mixtures in slit carbon and in uncharged alumino-silicate micropores. The adsorption isotherms are obtained for a pore of width of 2 nm and at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The results show that the graphite and uncharged silicate surfaces are covered by a dense layer of flatly adsorbed water and methanol molecules having weaker hydrogen bonding. In the interior of the pore, the fluid exhibits bulk-like behaviour with a stronger hydrogen bonded structure. Solvation forces are also calculated as a function of pore size. The positive values found for the solvation force for all pore sizes reflect the hydrophobic interactions of the mixture with the carbon and uncharged alumino-silicate walls.     

STUDY OF HOMOGENEOUS ADSORPTION KINETICS OF CRYSTAL VIOLET

The adsorption kinetica of crystal violet (CV) on silver mirror and two different kinds of silver sols is studied. The adsorption of CV on silver is homogeneous. It is confirmed that the topography of the surface affects only adsorption rate constant, while it does not change heterogeneity parameter. The similar behavior of enhancement factor and adsorption rate constant of CV in the two silver sols is discussed. The adsorption energy of CV and the concentration of adsorbed sites on the surface are estimated by adsorption isotherm. It is found that the enhancement effect on the silver mirror is determined by the concentration of adsorbed sites on surface.     

Observation of micropores in hard-carbon using Xe NMR porosimetry

The existence of micropores and the change of surface structure in pitch-based hard-carbon in xenon atmosphere were demonstrated using ¹²⁹Xe NMR. For high-pressure (4.0 MPa) ¹²⁹Xe NMR measurements, the hard-carbon samples in Xe gas showed three peaks at 27, 34 and 210 ppm. The last was attributed to the xenon in micropores (<1 nm) in hard-carbon particles. The NMR spectrum of a sample evacuated at 773 K and exposed to 0.1 MPa Xe gas at 773 K for 24 h showed two peaks at 29 and 128 ppm, which were attributed, respectively, to the xenon atoms adsorbed in the large pores (probably mesopores) and micropores of hard-carbon. With increasing annealing time in Xe gas at 773 K, both peaks shifted and merged into one peak at 50 ppm. The diffusion of adsorbed xenon atoms is very slow, probably because the transfer of molecules or atoms among micropores in hard-carbon does not occur readily. Many micropores are isolated from the outer surface. For that reason, xenon atoms are thought to be adsorbed only by micropores near the surface, which are easily accessible from the surrounding space.     

方解石纳米孔隙内二氧化碳毛细凝聚的分子模拟

在二氧化碳地质封存、增产非常规油气以及孔隙材料表征测量方面,纳米孔隙中二氧化碳相态的准确预测具有重要意义.然而,由于纳米尺度下毛细力、分离压等作用力占据主导因素,流体在孔隙中的相行为与体相流体存在根本不同.已有实验和模拟表明,Kelvin毛细凝聚理论无法预测特征尺度10 nm下的,孔隙内流体凝聚压力与体相饱和蒸气压的偏...     

Removal of organic contaminants from aqueous solution by cattle manure compost (CMC) derived activated carbons

The activated carbons (ACs) prepared from cattle manure compost (CMC) with various pore structure and surface chemistry were used to remove phenol and methylene blue (MB) from aqueous solutions. The adsorption equilibrium and kinetics of two organic contaminants onto the ACs were investigated and the schematic models for the adsorptive processes were proposed. The result shows that the removal of functional groups from ACs surface leads to decreasing both rate constants for phenol and MB adsorption. It also causes the decrement of MB adsorption capacity. However, the decrease of surface functional groups was found to result in the increase of phenol adsorption capacity. In our schematic model for adsorptive processes, the presence of acidic functional groups on the surface of carbon is assumed to act as channels for diffusion of adsorbate molecules onto small pores, therefore, promotes the adsorption rate of both phenol and MB. In phenol solution, water molecules firstly adsorb on surface oxygen groups by H-bonding and subsequently form water clusters, which cause partial blockage of the micropores, deduce electrons from the π-electron system of the carbon basal planes, hence, impede or prevent phenol adsorption. On the contrary, in MB solution, the oxygen groups prefer to combine with MB⁺ cations than water molecules, which lead to the increase of MB adsorption capacity.     

Influence of structural heterogeneity of nanoporous sorbent walls on hydrogen storage

Heterogeneity is an ubiquitous aspect of adsorption, often modifying substantially the observed behaviour of the adsorbate-adsorbent system. In this paper, the influence of heterogeneity is explicitly analyzed for the case of the adsorption of molecular hydrogen onto nanoporous carbon. Grand Canonical Monte Carlo simulations were used to study the mechanism of adsorption in the models of the adsorbate that include both energetic and structural modifications of graphene-based slit pores. In particular, a partial substitution of carbons by boron modifies both the symmetry of the energy landscape and the strength of hydrogen physisorption; which results in considerable increases of the amount of adsorbed gas without major modification of the mechanism of adsorption. Additional heterogeneity arises from structural modifications of the adsorbent by neutron irradiation of boron-doped samples, where the boron fission products generate additional surface area for adsorption. Simulations of adsorption in such pores show that hydrogen uptake is strongly dependent on the chemical nature of the modified pore walls.     

Constant pressure Gibbs ensemble Monte Carlo simulations of adsorption into narrow pores

An extension to the Gibbs ensemble method for the study of adsorption of fluids into pores is proposed. Since equality of pressure is not a necessary condition for full thermodynamic equilibrium between the bulk fluid and that confined in a narrow cavity, previous studies have been performed with the volume of both simulation cells fixed. More naturally, the pressure of the bulk fluid should be constrained and the volume (and hence density) allowed to attain its equilibrium value. Thus we propose a scheme in which volume fluctuations within the bulk box are permitted; the volume of the pore remains constant. The pressure is an input parameter and the amount of adsorption, as a function of applied pressure, is obtained directly. We demonstrate that the novel moves obey microscopic reversibility. Such a method is most useful when the equation of state for the model fluid is unknown. Thus, we illustrate the method by simulating an associating model of water adsorbed into a slit-shaped carbon pore with activated sites. Good qualitative correspondence with experiment is obtained; further refinement of our model is expected to yield quantitative agreement.     

小角x射线散射方法研究甲基改性氧化硅干凝胶的孔结构

以正硅酸乙酯［Si(OC2H5)4，TEOS］为前驱体制备SiO2悬浮液，分别以甲基三乙氧基硅烷［CH3Si(OC2H5)3，MTES］和二甲基二乙氧基硅烷［(CH3)2Si(OC2H5)2，DDS］为前驱体制备硅氧烷聚合物溶液，通过混合法得到两种不同的甲基改性氧化硅凝胶-测量了凝胶的散射强度，计算了凝胶的孔径分布、平均孔径DSAXS、界面层厚度E等参数，结合氮气吸附实验，分析了凝胶的孔结构-发现SiO2一次簇团被MTES聚合物或DDS聚合物连接为二次簇团时产生微孔，同时甲基随聚合物连接于凝胶骨架上形成与 关键词： 小角x射线散射 甲基改性氧化硅 干凝胶 孔结构     

Monodisperse spherical meso–macroporous silica particles: Synthesis and adsorption of biological macromolecules

Monodispersed spherical silica particles, including large mesopores (over 10 nm) and macropores (up to 100 nm) were obtained by chemical etching in an autoclave. A method for introducing globular protein myoglobin molecules into the pores is developed. The method of filling is based on a high adsorption capacity of the developed internal pore structure of the particles. The structure and adsorption properties of the materials are studied.     

The adsorption of water in finite carbon pores

Grand canonical Monte Carlo simulations were applied to the adsorption of SPCE model water in finite graphitic pores with different configurations of carbonyl functional groups on only one surface and several pore sizes. It was found that almost all finite pores studied exhibit capillary condensation behaviour preceded by adsorption around the functional groups. Desorption showed the reverse transitions from a filled to a near empty pore resulting in a clear hysteresis loop in all pores except for some of the configurations of the 1.0?nm pore. Carbonyl configurations had a strong effect on the filling pressure of all pores except, in some cases, in 1.0?nm pores. A decrease in carbonyl neighbour density would result in a higher filling pressure. The emptying pressure was negligibly affected by the configuration of functional groups. Both the filling and emptying pressures increased with increasing pore size but the effect on the emptying pressure was much less. At pressures lower than the pore filling pressure, the adsorption of water was shown to have an extremely strong dependence on the neighbour density with adsorption changing from Type IV to Type III to linear as the neighbour density decreased. The isosteric heat was also calculated for these configurations to reveal its strong dependence on the neighbour density. These results were compared with literature experimental results for water and carbon black and found to qualitatively agree.     

Kinetic energy of neon atoms adsorbed on activated carbon

Direct measurements of the kinetic energy of Ne atoms adsorbed on activated carbon (AC) at submonolayer coverage, is reported. The electronvolt spectrometer of the ISIS neutron source that utilizes the neutron Compton scattering technique, was employed. The specific adsorption area of the AC was ∼3000 m²/g. The momentum distribution of adsorbed Neon was measured at 12 K. The kinetic energy of Ne was found to be 66.3±6.2 K, and is much higher than that of solid neon, at the same temperature, being 45.8±4.8 K. This result is considered to be an evidence for the occurrence of a confinement effect of Ne atoms within the pores of the AC. The result was also used for deducing the average pore size of the AC adsorber assumed to have slit shaped pores.     

Influence of chemical and physical surface heterogeneity on chemical reaction equilibria in carbon micropores

Recent simulation results are presented for the equilibrium yield of the ammonia synthesis reaction in various model microporous carbons. It is found that the reaction equilibria within the micropores is affected by many factors, including pore size, pore shape, connectivity, surface roughness, and surface chemical activation. In order to probe these effects, reactive Monte Carlo simulations of the reaction were performed in several microporous carbon models: smooth slit-shaped carbon pores, a realistic carbon model generated from experimental diffraction data, single-walled carbon nanotubes, and smooth slit-shaped pores activated by carboxyl surface groups. The simulations show that the ammonia conversion is most sensitive to the carbon pore width and to the amount of surface chemical activation. Effects of surface corrugation and pore connectivity on the equilibrium reaction yield are minimal.     

Molecular mechanism of adsorption/desorption hysteresis: dynamics of shale gas in nanopores

Understanding the adsorption and desorption behavior of methane has received considerable attention since it is one of the crucial aspects of the exploitation of shale gas. Unexpectedly, obvious hysteresis is observed from the ideally reversible physical sorption of methane in some experiments. However, the underlying mechanism still remains an open problem. In this study, Monte Carlo (MC) and molecular dynamics (MD) simulations are carried out to explore the molecular mechanisms of adsorption/desorption hysteresis. First, a detailed analysis about the capillary condensation of methane in micropores is presented. The influence of pore width, surface strength, and temperature on the hysteresis loop is further investigated. It is found that a disappearance of hysteresis occurs above a temperature threshold. Combined with the phase diagram of methane, we explicitly point out that capillary condensation is inapplicable for the hysteresis of shale gas under normal temperature conditions. Second, a new mechanism, variation of pore throat size, is proposed and studied. For methane to pass through the throat, a certain energy is required due to the repulsive interaction. The required energy increases with shrinkage of the throat, such that the originally adsorbed methane cannot escape through the narrowed throat. These trapped methane molecules account for the hysteresis. Furthermore, the hysteresis loop is found to increase with the increasing pressure and decreasing temperature. We suggest that the variation of pore throat size can explain the adsorption/desorption hysteresis of shale gas. Our conclusions and findings are of great significance for guiding the efficient exploitation of shale gas.     

Quantitative analysis of pore space of moraine clay soils by SEM images

Problems of studying the pore space of moraine clay soils are considered by analyzing SEM images of the surface of samples prepared using a special technique. A new algorithm for estimating the pore shape is described. The study of the microstructure of moraine clay soils showed that they are characterized by five pore types. Of particular interest is the existence of large micropores with specific anisometric crescent shape in such soils. These micropores are formed at the interface between the surface of large sand-dust quartz or feldspar grains and the finely divided unoriented clay matrix. Such pores can be ways of localized groundwater filtration through soils, significantly affecting the soil properties.