Condensation In Pores Research Articles

Adsorption of organophosphates from solution by porous organosilicates: Capillary phase-separation

The adsorption characteristics for paraoxon, diazinon, and structurally related compounds by hierarchical diethylbenzene-bridged organosilicate sorbents are reported here. Adsorption behavior was found to deviate from the expected Langmuir–Freundlich binding isotherm. Instead, capillary phase-separation within the pores of the sorbent structure occurred when presented with these targets in aqueous solution. This behavior has not been widely reported, but it is similar to the pore condensation observed in gas adsorption studies. The behavior is a result of the pore sizes, the availability of interaction sites on the pore walls, and the miscibility of the targets (solutes) with water (solvent). Adsorption of targets from aqueous solution was observed to follow a hybrid Freundlich–BET binding isotherm. Adsorption of the targets from octanol solutions was observed to follow the Langmuir–Freundlich binding isotherm as was adsorption of more soluble targets such as p-nitrophenol. With the increasing numbers of applications described for mesoporous materials, this phenomenon will likely be more frequently encountered. The range of conditions utilized in characterization of sorbents should be considered carefully.

Capillary Condensation of Water in Mesoporous Carbon

To examine the mechanisms of condensation and evaporation of water in mesoporous carbons without surface functional groups and micropores, we measured gravimetrically the adsorption–desorption isotherms of water on ordered mesoporous carbons with and without surface functional groups and micropores at various temperatures between 265 and 295 K. Water wets partially the pore walls of the mesoporous carbons. Nevertheless, pore condensation of water occurs very slowly at pressures close to or equal to saturation, depending on both the properties of pore walls and the pore size. The uptake of water in the mesoporous carbons without micropores decreases with a decrease in temperature, although the uptake in the mesoporous carbon with micropores does not change at all with temperature. Capillary evaporation of water occurs at pressures expected from the Kelvin equation that describes the link between wetting and capillary condensation.

Deposition nucleation viewed as homogeneous or immersion freezing in pores and cavities

Abstract. Heterogeneous ice nucleation is an important mechanism for the glaciation of mixed phase clouds and may also be relevant for cloud formation and dehydration at the cirrus cloud level. It is thought to proceed through different mechanisms, namely contact, condensation, immersion and deposition nucleation. Conceptually, deposition nucleation is the only pathway that does not involve liquid water, but occurs by direct water vapor deposition onto a surface. This study challenges this classical view by putting forward the hypothesis that what is called deposition nucleation is in fact pore condensation and freezing (PCF) occurring in voids and cavities that may form between aggregated primary particles and host water at relative humidity RHw < 100% because of the inverse Kelvin effect. Homogeneous ice nucleation is expected to occur below 235 K when at least one pore is filled with water. Ice nucleation in pores may also happen in immersion mode but with a lower probability because it requires at least one active site in a water filled pore. Therefore a significant enhancement in ice nucleation efficiency is expected when temperature falls below 235 K. For a deposition nucleation process from water vapor no discontinuous change in ice nucleation efficiency should occur at T = 235 K because no liquid water is involved in this process. Studies on freezing in confinement carried out on mesoporous silica materials such as SBA-15, SBA-16, MCM-41, zeolites and KIT have shown that homogeneous ice nucleation occurs abruptly at T = 230–235 K in pores with diameters (D) of 3.5–4 nm or larger but only gradually at T = 210–230 K in pores with D = 2.5–3.5 nm. Pore analysis of clay minerals shows that kaolinites exhibit pore structures with pore diameters (Dp) of 20–50 nm. The mesoporosity of illites and montmorillonites is characterized by pores with Dp = 2–5 nm. The number and size of pores is distinctly increased in acid treated montmorillonites like K10. Water adsorption isotherms of MCM-41 show that pores with Dp = 3.5–4 nm fill with water at RHw = 56–60% in accordance with an inverse Kelvin effect. Water in such pores should freeze homogeneously for T < 235 K even before relative humidity with respect to ice (RHi) reaches ice saturation. Ice crystal growth by water vapor deposition from the gas phase is therefore expected to set in as soon as RHi > 100%. Pores with D > 7.5 nm fill with water at RHi > 100% for T < 235 K and are likely to freeze homogeneously as soon as they are filled with water. Given the pore structure of clay minerals, PCF should be highly efficient for T < 235 K and may occur at T > 235 K in particles that exhibit active sites for immersion freezing within pores. Most ice nucleation studies on clay minerals and mineral dusts indeed show a strong increase in ice nucleation efficiency when temperature is decreased below 235 K in accordance with PCF and are not explicable by the classical view of deposition nucleation. PCF is probably also the prevailing ice nucleation mechanism below water saturation for glassy, soot, and volcanic ash aerosols. No case could be identified that gives clear evidence of ice nucleation by water vapor deposition onto a solid surface.

Physical adsorption characterization of nanoporous materials: progress and challenges

Within the last two decades major progress has been achieved in understanding the adsorption and phase behavior of fluids in ordered nanoporous materials and in the development of advanced approaches based on statistical mechanics such as molecular simulation and density functional theory (DFT) of inhomogeneous fluids. This progress, coupled with the availability of high resolution experimental procedures for the adsorption of various subcritical fluids, has led to advances in the structural characterization by physical adsorption. It was demonstrated that the application of DFT based methods on high resolution experimental adsorption isotherms provides a much more accurate and comprehensive pore size analysis compared to classical, macroscopic methods. This article discusses important aspects of major underlying mechanisms associated with adsorption, pore condensation and hysteresis behavior in nanoporous solids. We discuss selected examples of state-of-the-art pore size characterization and also reflect briefly on the existing challenges in physical adsorption characterization.

Condensation and Evaporation in Slit-Shaped Pores: Effects of Adsorbate Layer Structure and Temperature

We have carried out an extensive computer simulation study of the effects of temperature on adsorption and desorption of argon in two slit mesopores; one of which has both ends open to the surroundings, and the other with one end closed, to explore the fundamental reasons for the disappearance of the hysteresis loop as the temperature approaches the critical hysteresis temperature, Tch. Detailed mechanisms are presented for adsorption and desorption. At temperatures below Tch, both adsorption and desorption branches of the isotherm are metastable resulting in a hysteresis loop. As the temperature is increased, waves, due to thermal fluctuations, appear at the boundary between the dense adsorbed phase close to the pore walls and the gas-like phase in the core. For temperatures above Tch, these thermal fluctuations override the formation and subsequent movement of the meniscus (interface); adsorption is entirely due to the densification of the adsorbate, and desorption proceeds by rarefaction of the adsorba...

ChemInform Abstract: Density Functional Theory Methods for Characterization of Porous Materials

AbstractReview: 209 refs.

Chemical deactivation by phosphorous under lean hydrothermal conditions over Cu/BEA NH3-SCR catalysts

To obtain a better understanding of the deactivation of SCR catalysts that may be encountered due to the presence of P-containing impurities in diesel exhausts, the effects induced by P over Cu/BEA NH3-SCR catalysts were studied as functions of the temperature of poisoning and P concentration in the feed. Cu/BEA catalysts with different Cu loadings (4 and 1.3wt% Cu) were exposed to P by controlled evaporation of H3PO4 in the presence of 8% O2 and 5% H2O at 573 and 773K. The reaction studies were performed by NH3-storage/TPD, NH3/NO oxidation and standard NH3-SCR. In addition, a combination of several characterisation techniques (ICP–AES, BET surface area, pore size distribution, H2-TPR and XPS) was applied to provide useful information regarding the mechanism of P deactivation. Pore condensation of H3PO4 in combination with pore blocking was observed. However, the measured overall deactivation was found to occur mostly by chemical deactivation reducing the number of the active Cu species and hence deteriorating the redox properties of the Cu/BEA catalysts. The process of P accumulation on the surface preferentially occurs on the “over exchanged” Cu active sites with the formation of phosphate species. This is likely the reason for the more severe deactivation of the 4% Cu/BEA compared to 1.3% Cu/BEA. Further, the higher NOx reduction performance at 773K of the P-poisoned Cu/BEA catalysts was found to originate from the lower selectivity towards NH3 oxidation, which occurs predominately on the “over-exchanged” sites.

Dynamics of capillary condensation in lattice gas models of confined fluids: A comparison of dynamic mean field theory with dynamic Monte Carlo simulations

This article addresses the accuracy of a dynamic mean field theory (DMFT) for fluids in porous materials [P. A. Monson, J. Chem. Phys. 128, 084701 (2008)]. The theory is used to study the relaxation processes of fluids in pores driven by step changes made to a bulk reservoir in contact with the pore. We compare the results of the DMFT to those obtained by averaging over large numbers of dynamic Monte Carlo (DMC) simulation trajectories. The problem chosen for comparison is capillary condensation in slit pores, driven by step changes in the chemical potential in the bulk reservoir and involving a nucleation process via the formation of a liquid bridge. The principal difference between the DMFT results and DMC is the replacement of a distribution of nucleation times and location along the pore for the formation of liquid bridges by a single time and location. DMFT is seen to yield an otherwise qualitatively accurate description of the dynamic behavior.

Pore Condensation in Glycerol Dehydration

Pore condensation followed by polymerization is proposed as an explanatory model of several observations reported in the literature regarding the dehydration of glycerol to acrolein. The major conclusion is that glycerol pore condensation in the micro- and mesopores, followed by polymerization in the pores, play a role in catalyst deactivation.

Density functional theory methods for characterization of porous materials

This review presents the state-of-the-art of adsorption characterization of mesoporous and microporous materials by using the density functional theory (DFT) methods. The DFT methods have found numerous applications for calculating pore size distributions in traditional and newly discovered nanoporous solids. We discuss the foundations of the non-local (NLDFT) and quench solid (QSDFT) density functional theories applied for modeling adsorption and capillary condensation in pores of different geometry and surface chemistry. Special attention is paid to the limitations of the theoretical models and critical analysis of the obtained data. The methods are demonstrated on a wide variety of systems, including microporous and mesoporous carbons and silicas, zeolites, mesoporous crystals of MCM and SBA families, metal–organic frameworks, and other designer nanoporous materials. Illustrated with many typical examples and detailed discussions of the advantages and limitations of the NLDFT and QSDFT methods, this review provides guidance for the practitioners interested in getting a better understanding of the current capabilities and limitations of the adsorption methods for characterization of porous solids

Capillary Condensation of Ionic Liquid Solutions in Porous Electrodes

We use classical density functional theory to investigate room temperature ionic liquid + solvent mixtures in porous electrodes. We consider those mixtures that display a miscibility gap in the bulk fluid and hence have the capacity to undergo capillary condensation in pores which preferentially attract the ionic liquid component. A novel aspect of this transition is that, when the system is at or near the critical point for this transition, we find an extraordinary increase in the capacitance, which derives from the fact that the capacitance is a thermodynamic response function, diverging at spinodal points.

Adsorption, intrusion and freezing in porous silica: the view from the nanoscale

This review presents the state of the art of molecular simulation and theory of adsorption, intrusion and freezing in porous silica. Both silica pores of a simple geometry and disordered porous silicas which exhibit morphological and topological disorders are considered. We provide a brief description of the numerical models of porous silicas available in the literature and present the most common molecular simulation and theoretical methods. Adsorption in regular and irregular pores is discussed in the light of classical theories of adsorption and capillary condensation in pores. We also present the different evaporation mechanisms for disordered systems: pore blocking and cavitation. The criticality of fluids confined in pores, which is still the matter of debate, is then discussed. We review theoretical results for intrusion/extrusion and freezing in silica pores and discuss the validity of classical approaches such as the Washburn-Laplace equation and Gibbs-Thomson equation to describe the thermodynamics of intrusion and in-pore freezing. The validity of the most widely used characterization techniques is then discussed. We report some concluding remarks and suggest directions for future work.

Capillary condensation, freezing, and melting in silica nanopores: A sorption isotherm and scanning calorimetry study on nitrogen in mesoporous SBA-15

Condensation, melting, and freezing of nitrogen in a powder of mesoporous silica grains (SBA-15) has been studied by combined volumetric sorption isotherm and scanning calorimetry measurements. Within the mean-field model of Saam and Cole for vapor condensation in cylindrical pores, a liquid nitrogen sorption isotherm is well described by a bimodal pore radius distribution. It encompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, and a significantly broader peak characteristic of micropores, located at 1 nm. The material condensed in the micropores as well as the first two adsorbed monolayers in the mesopores do not exhibit any caloric anomaly. The solidification and melting transformation affects only the pore condensate beyond approximately the second monolayer of the mesopores. Here, interfacial melting leads to a single peak in the specific-heat measurements. Homogeneous and heterogeneous freezing along with a delayering transition for partial fillings of the mesopores result in a caloric freezing anomaly similarly complex and dependent on the thermal history to that observed for argon in SBA-15. The axial propagation of the crystallization in pore space is more effective in the case of nitrogen than previously observed for argon, which we attribute to differences in the crystalline textures of the pore solids.

Unusual mechanism of capillary condensation in pores modified with chains forming pillars

Density functional approach is applied to study the phase behavior of Lennard-Jones(12,6) fluid in pillared slit-like pores. Our focus is in the evaluation of phase transitions in fluid adsorbed in the pore of a fixed width. If the length of pillars is sufficiently large, we observe additional phase transitions of the first and second order due to the symmetry breaking of the distribution of chain segments and fluid species with respect to the slit-like pore center. Re-entrant symmetry changes and additional critical, critical end points and tricritical points then are observed. The scenario of phase changes is sensitive to the energy of fluid-solid interaction, the amount, and the length of the pillars. Quantitative trends and qualitative changes of the phase diagrams topology are examined depending on the values of these parameters.

Solar cooling with aluminium pillared clays

Aluminium modified clays were prepared, characterised and tested for their potential application as solar coolers of roof surfaces. The water adsorption isotherm of the samples with restored cation exchange capacity was of hydrophilic type II, indicating multilayer adsorption with large heat of adsorption and pore condensation of water vapour at the pressures of the proposed application. In addition, high moisture adsorption capacity (more than 0.1 g of moisture per g of material at 60–70% of relative humidity) and fast kinetics for night sorption (comparable to silica gel) were determined for the freeze-dried pillared sample. In cyclic experiments with low irradiation during the day and night relative humidity of 55%, the maximum temperature inside the pillared montmorillonite was 6.5 °C lower than the corresponding temperature inside a typical soil sample. The primary mechanism for the reduced temperature elevation at aluminium modified clays was evaporative cooling and desorption with minor influence of solar reflection. These results indicate the suitability of aluminium pillared clays for lowering the roof surface temperatures.

Phase behavior of the molecular medium in nanopores and vibrational spectra structure transformation

A model description of molecular medium spectra during phase transitions in cylindrical nanopores is developed based on the thermodynamic concept of surface adsorption and capillary condensation. Good agreement of the calculated and experimental spectra measured during carbon dioxide adsorption and condensation in pores of nanoporous Vycor glass with a diameter of 2 nm is obtained. The constructed model connects the behavior of the medium spectra with the characteristics of the porous structure of a material and can be used for their determination based on spectroscopic data.

Study of heat of adsorption across the capillary condensation in cylindrical pores

We have investigated the isosteric heat of argon adsorption in MCM-41 and carbon nanotubes using grand canonical Monte Carlo (GCMC) simulation, and find that the isosteric heat during the capillary condensation is practically constant irrespective of temperature, pore-curvature, and surface strength. We also studied the microscopic behavior during capillary condensation by analyzing the histogram of the number of particles, the local number fluctuation and the layer compressibility. The histograms exhibit increasingly broader peaks close to condensation which subdivide into two separate peaks, indicating the relative probability of two co-existing states. Local number fluctuation allows us to identify the boundary between the adsorbed phase and the gas phase inside the pore. The layer compressibility can be used to identify the states of the adsorbate in each layer within the adsorbed phase. Finally we propose a simple model for the heterogeneous surface of MCM-41 to simulate the effects of surface heterogeneity on the isosteric heat. This model is able to describe the experimental isosteric heat of argon adsorption at 87.3 K in MCM-41.

The stability and deactivation of Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation: an effect of moisture

The catalytic stability of a Pd–Cu–Clx/Al2O3 catalyst for low temperature CO oxidation was investigated in different conditions. Under ∼0.1% moisture, the CO conversion over the Pd–Cu–Clx/Al2O3 catalyst can be maintained at 100% for 30 h even at 0 °C, but it deactivated reversibly at 25 °C with ∼0.6% moisture and irreversibly deactivated at 25 °C with ∼3.1% moisture or 0 °C with ∼0.6% moisture. The reversible deactivation is resulted from physical capillary condensation in the small pores of the catalysts. The irreversible deactivation is due to a breakage of the close-knit structure of Pd–Cu–Cl leading to an aggregation or transformation of the active copper phase, and the formation of a carbonate species on the catalyst surface, meaning that the inactive Pd0 species over the surface of the Pd–Cu–Clx/Al2O3 catalyst is hard to re-oxidize back to the active Pd+ sites by copper species in the high moisture reaction conditions.

Capillary condensation in cylindrical pores: Monte Carlo study of the interplay of surface and finite size effects

When a fluid that undergoes a vapor to liquid transition in the bulk is confined to a long cylindrical pore, the phase transition is shifted (mostly due to surface effects at the walls of the pore) and rounded (due to finite size effects). The nature of the phase coexistence at the transition depends on the length of the pore: for very long pores, the system is axially homogeneous at low temperatures. At the chemical potential where the transition takes place, fluctuations occur between vapor- and liquidlike states of the cylinder as a whole. At somewhat higher temperatures (but still far below bulk criticality), the system at phase coexistence is in an axially inhomogeneous multidomain state, where long cylindrical liquid- and vaporlike domains alternate. Using Monte Carlo simulations for the Ising/lattice gas model and the Asakura-Oosawa model of colloid-polymer mixtures, the transition between these two different scenarios is characterized. It is shown that the density distribution changes gradually from a double-peak structure to a triple-peak shape, and the correlation length in the axial direction (measuring the equilibrium domain length) becomes much smaller than the cylinder length. The (rounded) transition to the disordered phase of the fluid occurs when the axial correlation length has decreased to a value comparable to the cylinder diameter. It is also suggested that adsorption hysteresis vanishes when the transition from the simple domain state to the multidomain state of the cylindrical pore occurs. We predict that the difference between the pore critical temperature and the hysteresis critical temperature should increase logarithmically with the length of the pore.

Physical Adsorption Characterization of Nanoporous Materials

AbstractDuring recent years, major progress has been made in the understanding of the adsorption, pore condensation and hysteresis behavior of fluids in novel ordered nanoporous materials with well defined pore structure. This has led to major advances in the structural characterization by physical adsorption, also because of the development and availability of advanced theoretical procedures based on statistical mechanics (e.g., density functional theory, molecular simulation) which allows to describe adsorption and phase behavior of fluids in pores on a molecular level. Very recent improvements allow even to take into account surface geometrical in‐homogeneity of the pore walls However, there are still many open questions concerning the structural characterization of more complex porous systems. Important aspects of the major underlying mechanisms associated with the adsorption, pore condensation and hysteresis behavior of fluids in micro‐mesoporous materials are reviewed and their significance for advanced physical adsorption characterization is discussed.