Adsorption of n-Pentane on Mesoporous Silica and Adsorbent Deformation

Development of quantitative theory of adsorption-induced deformation is important, e.g., for enhanced coalbed methane recovery by CO2 injection. It is also promising for the interpretation of experimental measurements of elastic properties of porous solids. We study deformation of mesoporous silica by n-pentane adsorption. The shape of experimental strain isotherms for this system differs from the shape predicted by thermodynamic theory of adsorption-induced deformation. We show that this difference can be attributed to the difference of disjoining pressure isotherm, responsible for the solid−fluid interactions. We suggest the disjoining pressure isotherm suitable for n-pentane adsorption on silica and derive the parameters for this isotherm from experimental data of n-pentane adsorption on nonporous silica. We use this isotherm in the formalism of macroscopic theory of adsorption-induced deformation of mesoporous materials, thus extending this theory for the case of weak solid−fluid interactions. We employ the extended theory to calculate solvation pressure and strain isotherms for SBA-15 and MCM-41 silica and compare it with experimental data obtained from small-angle X-ray scattering. Theoretical predictions for MCM-41 are in good agreement with the experiment, but for SBA-15 they are only qualitative. This deviation suggests that the elastic modulus of SBA-15 may change during pore filling

Morphologically disordered pore model for characterization of micro-mesoporous carbons

We present a new morphologically disordered slit-shaped pore (MDSP) model for simulating gas adsorption in micro-mesoporous carbonaceous materials. The MDSP model qualitatively accounts for the inherent roughness of carbon pore walls in accord with the atomistic structural model of LMA10 reference carbon material. The MDSP model is applied to pore size distribution (PSD) calculations from nitrogen adsorption isotherms measured at 77.4 K in the range of pore widths from 0.72 to 40 nm. The MDSP model improves significantly the nitrogen adsorption porosimetry and, being fully atomistic, it is transferable to study various adsorbate-adsorbent systems. Computations of PSD functions for a series of carbonaceous materials, including activated carbon fiber, granular activated carbons, synthetic activated carbons showed that MDSP generates smooth Gaussian-type PSD functions with a well-defined average pore size. Furthermore, PSD functions computed from the MDSP model are free from the artificial gaps in the region of narrow micropores (∼1 nm and ∼2 nm) predicted from the standard slit-shaped pore models with ideal graphite-like walls. MDSP is not only a complementary model to existing approaches, such as quench-solid density functional theory method, but it paves the way to efficient atomistic simulations of various compounds within morphologically disordered carbon nanopores

Super-sieving effect in phenol adsorption from aqueous solutions on nanoporous carbon beads

Removal of aromatic contaminants, like phenol, from water can be efficiently achieved by preferential adsorption on porous carbons which exhibit molecular sieving properties. Here, we present nanoporous carbon beads exhibiting an outstanding sieving effect in phenol adsorption from aqueous solution at neutral pH, which is evidenced experimentally and theoretically. The molecular sieving with pure phenol adsorbed phase is achieved by tuning the pore size and surface chemistry of the adsorbent. This study elucidates the essential role of hydrophobic interactions in narrow carbon micropores in removal and clean-up of water from organic pollutants. Furthermore, we suggest a new theoretical approach for evaluation of phenol adsorption capacity that is based on the Monte Carlo simulation of phenol adsorption with the relevance to the pore size distribution function determined by the density functional theory method from low temperature nitrogen adsorption

A Generalization of Chaplygin's Reducibility Theorem

In this paper we study Chaplygin's Reducibility Theorem and extend its applicability to nonholonomic systems with symmetry described by the Hamilton-Poincare-d'Alembert equations in arbitrary degrees of freedom. As special cases we extract the extension of the Theorem to nonholonomic Chaplygin systems with nonabelian symmetry groups as well as Euler-Poincare-Suslov systems in arbitrary degrees of freedom. In the latter case, we also extend the Hamiltonization Theorem to nonholonomic systems which do not possess an invariant measure. Lastly, we extend previous work on conditionally variational systems using the results above. We illustrate the results through various examples of well-known nonholonomic systems.Comment: 27 pages, 3 figures, submitted to Reg. and Chaotic Dy

Numerical study of multilayer adsorption on fractal surfaces

We report a numerical study of van der Waals adsoprtion and capillary condensation effects on self-similar fractal surfaces. An assembly of uncoupled spherical pores with a power-law distributin of radii is used to model fractal surfaces with adjustable dimensions. We find that the commonly used fractal Frankel-Halsey-Hill equation systematically fails to give the correct dimension due to crossover effects, consistent with the findings of recent experiments. The effects of pore coupling and curvature dependent surface tension were also studied.Comment: 11 pages, 3 figure

Surface and porous characterisation of activated carbons made from a novelbiomass precursor, the esparto grass

In the work now reported the production of activated carbons from a novel precursor, esparto grass, by activation with carbon dioxide is presented. The results show that the materials produced have interesting properties, namely BET apparent surface area and pore volume up to 1122 m(2) g(-1) and 0.46 cm(3) g(-1), respectively. The activated carbons have basic characteristics with point of zero charge between 9.25 and 10.27 and show a very fascinating structure, as shown by the SEM images. (C) 2012 Elsevier B.V. All rights reserved.The authors are grateful to the Fundacao para a Ciencia e Tecnologia (Portugal) and the European Regional Development Fund (FEDER) through the Operational Programme for Competitiveness Factors (COMPETE) and QREN for financial support through Project PTDC/CTM/66552/2006 (FCOMP-01-0124-FEDER-007142) and under the Strategic Project PEst-OE/QUI/UI0619/2011 (CQE/UE).Valente Nabais, J.; Laginhas, C.; Ribeiro Carrott, MML.; Carrott, PJM.; Crespo Amorós, JE.; Nadal Gisbert, AV. (2013). Surface and porous characterisation of activated carbons made from a novelbiomass precursor, the esparto grass. Applied Surface Science. 265:919-924. doi:10.1016/j.apsusc.2012.11.164S91992426

Using in-situ adsorption dilatometry for assessment of micropore size distribution in monolithic carbons

We demonstrate that in-situ adsorption dilatometry provides a new opportunity for structural characterization of microporous carbons. We present experimental results for CO2 adsorption at 293 K and in-situ deformation obtained by dilatometry on a synthetic monolithic carbon sample. The carbon deformation in the course of adsorption is non-monotonic: the strain isotherm shows the sample contraction at low adsorption followed by progressive expansion. To evaluate structural and mechanical properties of the sample from the experimental adsorption and strain isotherms, a kernel of theoretical adsorption isotherms is obtained with the grand canonical Monte Carlo simulation of CO2 adsorption in a series of carbon micropores ranging from 0.22 to 2.0 nm. The respective kernel of adsorption stress isotherms is constructed using the thermodynamic model of adsorption stress. The pore volume and surface area distributions were calculated independently from a) the experimental excess adsorption isotherm by deconvoluting the generalized adsorption equation and b) the experimental strain isotherm by using the kernel of adsorption stress isotherms. The proposed method of determining the pore size distribution from the strain isotherm validates the thermodynamic model of adsorption stress in micropores and provides additional information about the sample material with respect to mechanical properties of the microporous matrix

Determination of isosteric heat of adsorption by quenched solid density functional theory

The heat of adsorption is one of the most important parameters characterizing energetic heterogeneity of the adsorbent surface. Heats of adsorption are either determined directly by calorimetry or calculated from adsorption isotherms measured at different temperatures using the thermodynamic Clausius–Clapeyron equation. Here, we present a method for calculating the isosteric heat of adsorption that requires as input only a single adsorption isotherm measured at one temperature. The proposed method is implemented with either nonlocal (NLDFT) or quenched solid (QSDFT) density functional theory models of adsorption that are currently widely used for calculating pore size distributions in various micro- and mesoporous solids. The pore size distribution determined from the same experimental isotherm is used for predicting the isosteric heat. The QSDFT method has advantages of taking into account two factors contributing to the structural heterogeneity of adsorbents: the molecular level roughness of the surface and the pore size distribution. The method is illustrated with examples of low temperature nitrogen and argon adsorption on selected samples of carbons of different degree of graphitization and MCM-41 mesoporous silicas of different pore size. The isosteric heat predictions from the NLDFT and QSDFT methods are compared against relevant experiments and the results of Monte Carlo (MC) simulations, with good agreement found in the cases where the surface model adequately reflects the pore surface roughness. Analyses with the QSDFT method show that the isosteric heat of adsorption significantly depends of the molecular level roughness of the adsorbent surface, which is ignored in NLDFT and MC models. The proposed QSDFT method with further verification can be used for calculating the isosteric heat as an additional parameter characterizing the adsorbent surface in parallel with routine calculations of the pore size distribution from a single adsorption isotherm