Pore geometry and isosteric heat : an analysis for the carbon dioxyde adsorption on activated carbon.

International audienceThe isosteric heat of the carbon dioxide adsorption on activated carbon is determined by grand canonical Monte Carlo simulations. The results, obtained at room temperature and low pressures for an adorbent model with a slit type porosity, show that the isosteric heat depends strongly on the slit width. The maximun of the isosteric heat is reached for a pore with a width such as cooperative effects between the adsorbed molecules enhance the adsorption. The possibility to estimate the isosteric heat of a macroscopic sample, from adsorption isotherms computed for a distribution of slit pores with given sizes, is discussed

Capillary condensation and adsorption of binary mixtures

31 pagesThe adsorption of equimolar binary mixtures of hydrogen-carbon dioxide, hydrogen-methane and methane-carbon dioxide in porous material models is determined by Grand Canonical Monte Carlo simulations. The material models have an adsorbent surface similar to that of nanofibers with a herringbone structure. Our main result, which is relevant for hydrogen purification and carbon dioxide capture, is that the adsorption selectivities calculated for the mixtures can differ significantly from those deduced from simulations of the adsorption of pure gases, in particular when one of the adsorbed gases presents a capillary condensation induced by confinement within the pore network. A comparison of our data is also made with theoretical models used in the literature for predicting the properties of the mixture adsorption

Quasi one dimensional 4^4He inside carbon nanotubes

We report results of diffusion Monte Carlo calculations for both $^4$He absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly one dimensional $^4$He. Inside the tube, the binding energy of liquid $^4$He is approximately three times larger than on planar graphite. At low linear densities, $^4$He in a nanotube is an experimental realization of a one-dimensional quantum fluid. However, when the density increases the structural and energetic properties of both systems differ. At high density, a quasi-continuous liquid-solid phase transition is observed in both cases.Comment: 11 pages, 3ps figures, to appear in Phys. Rev. B (RC

Anisotropic Condensation of Helium in Nanotube Bundles

Helium atoms are strongly attracted to the interstitial channels within a bundle of carbon nanotubes. The strong corrugation of the axial potential within a channel can produce a lattice gas system where the weak mutual attraction between atoms in neighboring channels of a bundle induces condensation into a remarkably anisotropic phase with very low binding energy. We estimate the binding energy and critical temperature for 4He in this novel quasi-one-dimensional condensed state. At low temperatures, the specific heat of the adsorbate phase (fewer than 2% of the total number of atoms) greatly exceeds that of the host material.Comment: 8 pages, 3 figures, submitted to PRL (corrected typo in abstract

Uptake of gases in bundles of carbon nanotubes

Model calculations are presented which predict whether or not an arbitrary gas experiences significant absorption within carbon nanotubes and/or bundles of nanotubes. The potentials used in these calculations assume a conventional form, based on a sum of two-body interactions with individual carbon atoms; the latter employ energy and distance parameters which are derived from empirical combining rules. The results confirm intuitive expectation that small atoms and molecules are absorbed within both the interstitial channels and the tubes, while large atoms and molecules are absorbed almost exclusively within the tubes.Comment: 9 pages, 12 figures, submitted to PRB Newer version (8MAR2K). There was an error in the old one (23JAN2K). Please download thi

Hydrogen storage in carbon nanotubes: Residual metal content and pretreatment temperature

Hydrogen storage in MWNT was enhanced via the catalytic activity of NiMgO. The magnitude of the hydrogen to metal ratio for the MWNT/NiMgO system, combined with temperature programmed adsorption and desorption studies, showed hydrogen spillover from the catalyst to the carbon surface. Metal doping combined with temperature activation studies showed that both nickel and magnesium are active in the catalytic process. The yield, quality, and carbon-metal contact were shown to affect hydrogen uptake. Higher pretreatment temperatures enhanced uptake, for both low- and high-pressure measurements, due to increased carbon-metal contact and activation of the catalyst. At 69 bar (or 1,000 psia), the hydrogen adsorption and desorption of the MWNT/NiMgO system were 3.7% and 3.6%, respectively.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34249/1/690490619_ftp.pd

Impact of the carbonisation temperature on the activation of carbon fibres and their application for hydrogen storage

Special issue: 2nd World Congress of Young Scientists on Hydrogen Energy Systems.Porous materials are gaining interest due to their potential for storing hydrogen via physisorption. In the present work, two carbon fibres, carbonised at 973 and 1273 K, have been chemically activated with KOH and NaOH, in order to obtain materials with optimised characteristics for hydrogen storage application. Highly microporous activated carbon fibres were obtained from both precursors, especially from the fibre carbonised at the lower carbonisation temperature, remarking its importance on its subsequent activation process. As activation agent, KOH is more effective for developing the narrow microporosity, and higher yields are obtained. H2 adsorption isotherms were measured at 298 K for pressures up to 20 MPa, and at 77 K up to 4 MPa. The maximum excess adsorption of hydrogen reached 1 wt% at 298 K and 3.8 wt% at 77 K. The total volumetric storage capacity is of 17 g/l at 298 K, and 32 g/l at 77 K.Financial help from the European Union (Marie Curie Research Training Network—HyTRAIN Project reference:512443), MEC (Accion complementaria; ENE2005-23824-E/CON), the Generalitat Valenciana (Accion complementaria; ACOMP06/089) and MEC-CTQ2006-08958PPQ. Samples supplied by Osaka gas Co., Ltd