Hydrogen adsorption on model adsorbents from the viewpoint of the theory of volumetric filling of micropores

Hydrogen adsorption on model microporous adsorbents with slit-shaped pores was calculated on the basis of Dubinin’s theory of volumetric filling of micropores using the property of linearity of adsorption isosters. Model adsorbents with micropore widths of 0.5, 0.9, and 1.2 nm obtained by the successive exclusion of one, two, and three layers of hexagonal carbon in the crystalline lattice of graphite were used. Hydrogen adsorption was calculated in the structures with single-layer and two-layer carbon walls at temperatures 20, 33, 77, 200, 300, and 400 K and pressures up to 20 MPa. The maximal hydrogen desorption for the AU structure (1:3) with the pressure drop from 20 to 0.1 MPa was 8 wt.% at 200K. The parameters of the porous adsorbent structure were calculated.     

Relation between characteristic adsorption energy and internal pressure in the theory of volume filling of micropores

An equation for the internal pressure acting on an adsorbate in micropores was obtained on the basis of the assumption that the chemical potential of an adsorbate in micropores is equal to that in an equilibrium gas phase and using the Dubinin-Radushkevich equation. The empirical relation between the characteristic adsorption energy and the half width of pores was expressed in terms of internal pressure and diameter of adsorbate molecules. The two-dimensional pressure was calculated for micropores with plane-parallel walls, where the width of a micropore coincides with the diameter of an adsorbate molecule. The results obtained were compared with the two-dimensional pressure of a monolayer on a free planar surface for an adsorbate and adsorbent of the same nature.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1928–1930, October, 1995.     

Estimation of adsorption of ethane on the superactive microporous carbon adsorbent using the theory of volume filling of micropores

Russian Chemical Bulletin - Using the main statements of the Dubinin theory of volume filling of micropores (TVFM) the amount of adsorbed ethane was estimated at pressures up to 4 MPa in the...     

Theory of volume filling of micropores applied to the description of methane adsorption on the microporous carbon adsorbent AUK

Methane adsorption on the microporous carbon adsorbent AUK was calculated on the basis of Dubinin’s theory of volume filling of micropores in the temperature range 177.7—393 K and at pressures from 1 Pa to 6 MPa. The calculated isotherms of absolute adsorption were compared with the isotherms of methane obtained experimentally. Good agreement between the calculated and experimentally measured amounts adsorbed is observed in the area of applicability of the theory at micropore ranging in coverage θ from 0.25 to 0.95. The adsorption isosters were calculated for the same pressure and temperature ranges. The adsorption isosters satisfactorily represent the temperature dependence of the amount of methane adsorbed obtained experimentally. The calculations gave for the thermal coefficient of limiting adsorption a value of 6h_calc = 1.64• 10^-3 K^-1., which exceeds the experimental value by ∼15%.     

An attempt to describe the plastification of polyvinylchloride in terms of the theory of volume filling of micropores

The significant phenomenological analogy between physical adsorption from liquid solutions on microporous adsorbents and plastification of PVC has been observed. This justified the use of the solution analog of the adsorption isotherm equations of the theory of volume filling of micropores as being adequate to describe the process of plastification of PVC by the composition of plasticizer and modifying agent. The qualitative agreement between isotherm predicted by model and experimental data allowed to formulate the mechanism of the process of interest.     

Hydrogen adsorption on a carbon adsorbent with slitlike micropores below and above the critical temperature

Hydrogen adsorption is calculated for model microporous adsorbents with slitlike micropore widths of 0.538, 0.878, and 1.218 nm obtained by the consecutive exclusion of one, two, and three layers of hexagonal carbon from graphite structure taken as a model cell. Calculations are performed using the basic concepts of the theory of volume filling of micropores, Dubinin-Radushkevich equation, and linear adsorption isosteres. For structures with one-and two-layer carbon walls, the calculation is carried out at temperatures of 20, 33, 77, 200, 300, and 400 K and pressures up to 20 MPa. For AC3:5 structure, the maximum hydrogen adsorption amounts to 7.9 wt % at 20 MPa and 300 K. The parameters of adsorbent porous structure are established. Hydrogen adsorption is shown to be governed by the capacity and the energy of adsorption.     

Determination of the adsorption volume and surface of carbon adsorbents with developed mesoporosity

The isotherms of excess adsorption of CH₄ (atP=0.001–160 MPa), SF₆ (atP=0.001–2.4 MPa), and C₆H₆ (atP=0.0001–0.1 MPa) on carbon adsorbents—microporous carbons CMS and FAS with developed mesoporosity and graphitized soot—were measured in the 298–408 K temperature region. Calculation of the isotherms of absolute adsorption of the total content of these substances requires knowledge of the adsorption volume, which was determined by different methods: by the Dubinin—Radushkevich equation; by the experimental isotherm of excess adsorption and the equation of absolute adsorption; by the method using the intersection of nonlinear isosteres of excess adsorption and isosteres of absolute adsorption; by the comparative plot of values of the excess C₆H₆ adsorption Γ_FAS—Γ_soor; by the method using the difference of molecular radii of adsorptives and the surface of the specific adsorbent; and by the calculation of the adsorption layer thickness using the FHH equation for mesoporous systems. The results of determination of the adsorption volume for microporous systems of these carbons agree well with each other and with the passport data for the adsorbents. Analysis of the results revealed the peculiarity of the sulfur hexafluoride adsorption related to the formation of associates on the surface of the carbon adsorbents. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 688–696, April, 2000.     

The heat of adsorption of water on nonporous hydrophilic carbon adsorbents in the model of two-stage adsorption

The behavior of integral and differential heats of adsorption of water in the region of transition from the first stage of adsorption (formation of clusters) to the second stage (formation of a stretched liquid film) was considered. The curve of integral heat of adsorption has an inflection at the transition point, and the differential heat of adsorption changes jumpwise. The values of these effects were estimated by the simplest model of formation of one and two hydrogen bonds between a water molecule and an adsorption center on the surface of the carbon adsorbent. Curves of differential heat of adsorption with transition points for real systems are presented. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1479–1483, August, 1999.     

Hydrogen adsorption in carbon nanostructures: comparison of nanotubes, fibers, and coals

Single-walled carbon nanotubes (SWNT) were reported to have record high hydrogen storage capacities at room temperature, indicating an interaction between hydrogen and carbon matrix that is stronger than known before. Here we present a study of the interaction of hydrogen with activated charcoal, carbon nanofibers, and SWNT that disproves these earlier reports. The hydrogen storage capacity of these materials correlates with the surface area of the material, the activated charcoal having the largest. The SWNT appear to have a relatively low accessible surface area due to bundling of the tubes; the hydrogen does not enter the voids between the tubes in the bundles. Pressure-temperature curves were used to estimate the interaction potential, which was found to be 580+/-60 K. Hydrogen gas was adsorbed in amounts up to 2 wt % only at low temperatures. Molecular rotations observed with neutron scattering indicate that molecular hydrogen is present, and no significant difference was found between the hydrogen molecules adsorbed in the different investigated materials. Results from density functional calculations show molecular hydrogen bonding to an aromatic C[bond]C that is present in the materials investigated. The claims of high storage capacities of SWNT related to their characteristic morphology are unjustified.