'Introduction au formalisme de l'EXAFS'

Etude de nanotubes hybrides par spectroscopies vibrationnelles

We present experimental and theoretical results for the transport of ions and molecules inside carbon nanotubes.

Matériaux composites à base de silicone comprenant des charges non-linéaires

Hydrogen is very likely the ultimate future of the energetic economy of the world: it is ubiquitous in the en-vironment, it can be sustainably produced, and it holds the highest energy per unit mass of any fuel (except nuclear). If combusted, it produces only water. Therefore hydrogen, when used as energy vector, enters the natural water cycle of the Earth, with no impact on the planet’s climate. Today the major issue of the wide-spread hydrogen use is its low mass and low volume storage, as the energy density of hydrogen gas is low. Physisorption in nanoporous materials seems to be the most promising way of storage, as it does not release much energy during adsorption and in principle do not require the energy input when hydrogen is released.In this paper we will present the Grand Canonical Monte Carlo simulations of hydrogen adsorption in slit-shaped carbon nanopores. Our calculations confirm the very controversial experimental results [1-3] showing that under confinement the density of hydrogen film adsorbed on the carbon wall exceeds that of the bulk liquid at low temperature and approaches that of solid hydrogen at extreme pressures. The densification of hydrogen is restricted to the layer in direct contact with the adsorbent, and does not depend significantly on the pore size (at least for the pores of the width from 0.6 to 3.0 nm) nor on the temperature (for 77 K< T < 180 K).The simulations are confronted with the experimental isotherms of hydrogen adsorption in KOH - activat-ed nanoporous carbons [3] obtained from biomass waste [4]. The numerical and experimental results are coherent and prove that the interaction between hydrogen molecules and carbon surface is strong enough to produce adsorbed layer with solid hydrogen density.We also show that the gas confined in nanopores is structured, and its density is not uniform across the whole pore volume (Fig.1). In addition, the gas density depends on the pore size (Fig.2). This fact, never re-ported before, should be taken into account when the total (excess) amount stored is estimated from the excess (total) values.

It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other struc-tural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] and its structural transformations when adsorbed in nanoporous systems. The general analysis of methane melting in slit pores was already discussed by Mi-yahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mecha-nism of structural transformations. As an example, we discuss the mechanism of melting of methane con-fined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed. Fig. 1 presents an order pa-rameter calculated in different layers of the confined system as a function of temperature.Structural transformations of adsorbate are interesting phenomena from both fundamental and practical point of view. The mechanism of transformations is defined by the characteristic nanosize of the system and the influence of the interactions with the pore framework. This defines the unusual properties of the confined system which can be used for the system characterization.

It is well known that the nano-systems exhibit properties different from their bulk analogs. Typically, the phase diagrams are redefined because the position of coexistence lines depends on the size and shape of the nano-objects. This is related to the fact that nano-systems are characterized by high surface-to-volume ratio. The surface atoms are weakly bonded and their contribution to the latent heat is smaller. Consequently, the surface usually transforms at lower temperature and the whole transition may happen smoothly over a finite range of temperatures. This observation suggests that there is no temperature of melting (or any other structural change) in the conventional sense because the structural (phase) changes are gradual and phases are no longer distinguishable.Here we discuss mechanism of methane melting [1-3] in confined nanoporous systems. The general analysis of methane in slit pores was already discussed by Miyahara and Gubbins [1]. In this paper, we emphasize the influence of structural heterogeneity on the mechanism of structural transformations. As an example, we discuss the differences in mechanism of melting of methane confined in two different structures: first, in 3 and 4 nm slit pores, then, in 2.8 nm square channels of SURMOF porous structure. Mechanism of melting transformation in both cases will be compared and the correlation between the nano-scale and heterogeneity will be emphasized and discussed.