Influence of extra-framework cations on the adsorption properties of X-faujasite systems: microcalorimetry and molecular simulations

Isotherms and differential enthalpies of adsorption are obtained for nitrogen at ambient temperature on monovalent (Li(+), Na(+), K(+)) and divalent (Ca(2+), Ba(2+), Sr(2+), Mn(2+)) substituted X-faujasite systems by microcalorimetry measurements. These experimental data are compared with those obtained by combining grand canonical Monte Carlo simulations and newly derived force fields for describing the interactions between the extra-framework cations and the adsorbates obtained from a simple model based only on the intrinsic properties of the cations. It is the first time that such good qualitative agreement is reported between experiment and simulation for a series of both monovalent and divalent cations.     

Effect of degradation of montmorillonite by vibration grinding on the dTA curves in the range 20–1500°C

The effect of vibration grinding on the DTA curves of montmorillonite isolated from the most important locality of bentonite in the Slovak Republic (Jel?ovy potok, Middle Slovakia) was studied in the temperature range 20–1500°C. Interpretation is offered for 6 endothermic and 3 exothermic peaks. vibration grinding modified the course of dehydration of the mineral, suppresses the “dehydroxylation’ peak at 700°C, enables the crystallization of high-temperature quartz, cristobalite and cordierite at lower temperatures in comparison with unground natural montmorillonite. Vibration griding slows down the crystallization of mullite which—unlike quartz, cristobalite and cordierite—does not belong to high-temperature phases of Cheto-montmorillonite. The order in which these high-temperature phases occur is not influenced by vibration grinding.     

Melting of xenon monolayers - computer simulations

The results of Monte Carlo simulations of xenon monolayers deposited on graphite are presented. Systems of various surface densities and in a wide range of temperatures are investigated. The results are compared to the experimental data and to the previous results for nitrogen. The results of simulations are in good agreement with the experimental ones and confirm that melting in two dimensions is vacancy-driven. Typical real-space configurations of Xe atoms and calculated specific heat results are shown.     

Grand canonical monte carlo modeling of hydrogen adsorption on phosphorus-doped open carbon framework

Mechanism of hydrogen adsorption in high surface area carbon-based porous materials has been studied. Influence of chemical modification of the adsorbing surface has been simulated using grand canonical Monte Carlo method. Special attention has been paid to the competition between increasing the surface of open carbon frameworks and heterogeneous distribution of the energy of adsorption. Additionally, it has been shown that the molecular mass of atoms which substitute carbon atoms is an important factor determining the final hydrogen uptake.     

Calcination of SiO2-aerogel in Oxidizing Atmosphere

Differences in mass loss occurring in the course of dynamic and isothermal heating of SiO₂-aerogel and changes of specific surface and hydrophylicity during calcination were studied by thermal analysis. SiO₂-aerogel was prepared from tetramethoxysilane (TMOS) hydrolyzed by ammonia solution at 0°C with molar ratio TMOS: H₂O:NH₄OH 4:1:0.01. Differences are caused mainly by oxidation of organic matter and by diffusion of products of the oxidation. Heat transfer has none or little effect on the differences. Samples calcined at temperatures about 300°C reach maximum hydrophilicity though they still contain small amounts of residual organic matter. This revised version was published online in July 2006 with corrections to the Cover Date.     

A modification of a solution concept of the linear programming problem with interval coefficients in the constraints

We propose a modification of a concept of solving a linear programming problem with interval coefficients in the constraints. The original concept imposes the decision maker a way comparing intervals, our modification—which is an interactive approach, comprising the original one as its special case—gives him more freedom in the choice of the preference relation. Thanks to this flexibility he may be able to find a solution which better suits his needs and attitude towards risk. What is more, our modification corrects a serious drawback of the original method.     

Preconditioning trace coupled 3d‐1d systems using fractional Laplacian

Multiscale or multiphysics problems often involve coupling of partial differential equations posed on domains of different dimensionality. In this work, we consider a simplified model problem of a 3d‐1d coupling and the main objective is to construct algorithms that may utilize standard multilevel algorithms for the 3d domain, which has the dominating computational complexity. Preconditioning for a system of two elliptic problems posed, respectively, in a three‐dimensional domain and an embedded one dimensional curve and coupled by the trace constraint is discussed. Investigating numerically the properties of the well‐defined discrete trace operator, it is found that negative fractional Sobolev norms are suitable preconditioners for the Schur complement of the system. The norms are employed to construct a robust block diagonal preconditioner for the coupled problem.     

Mechanism of adsorption in cylindrical nanopores: the roles of fluctuations and correlations in stabilizing the adsorbed phase

The mechanism of the adsorption in nanometric cylindrical pores has been studied using grand canonical Monte Carlo simulations. The results have been analyzed from the point of view of microscopic correlations. It has been shown that the correlations between the energy components and between the energy and the number of adsorbed particles provide crucial information concerning the microscopic mechanism of the formation of adsorbed layers. Typical susceptibility functions have been calculated. They give the relations between the statistical correlations and the stability of the adsorbed system in different stages of adsorption. The numerical calculations have been carried out for Kr atoms adsorbed in an MCM-41 model porous material with pores of diameter d = 4 nm. The smooth-wall model as well as the model of a wall with micropores have been discussed.     

Monte Carlo Simulations of the Metastability in Molecular Crystals

A measure of the non-ergodicity has been introduced for plastic phases of molecular crystals. Monte Carlo simulations of the cubic, disordered phase of cyanoadamantane have been examined from the point of view of quasi-ergotic behavior. It has been shown that the conventional way of sampling based on the Metropolis algorithm is not efficient when temperature is lowered and the crystal is put into a quenched metastable situation. New methods are required to allow the system to overcome energy barriers which separate equally important volumes in the configuration space.