Growth Mode of Hydrogen in Mesoporous MCM-41. Adsorption and Neutron Scattering Coupled Studies

Abstract

X-ray or neutron diffractograms of calcined MCM-41 samples are strongly modified during the sorption phenomenon. Intensity of the main (100) peak (I 100 ) could undergo a non monotonous behaviour. At the sorption beginning I 100 increases and I 100 strongly decreases in the medium and high loading regimes. These observed intensity modifications in the diffractograms are closely related to the MCM-41 sample and to the filling mechanisms of its porosity. Actually each synthesis produces MCM-41 sample with its own silica wall and porosity. Several MCM-41 structural models and adsorption filling modes were attempted to fit all the diffractograms recorded during the D 2 adsorption in a MCM-41 (Ø = 25Å) sample at T = 16.4 K. Our data agree with a MCM-41 having low density silica walls (20% voids) that is rarely reported. Consequently, the D 2 adsorption mechanism is proved to be more defined: at the low relative adsorption pressure D 2 fills the wall voids and forms a layer on the rough wall surface. Then at the relative pressure of the capillary step, D 2 fills the whole free MCM-41 mesopores. Even the solid capillary phase does not grow layer by layer on the inner pore walls but is growing up along the pore axis.

In situneutron diffraction study of adsorbed carbon dioxide in a nanoporous material: Monitoring the adsorption mechanism and the structural characteristics of the confined phase

“…According to Zickler et al, 43 their model seems to describe the film growth quite reliably because MCM-41 exhibits much smoother pore walls than SBA-15. Furthermore, Floquet et al, 32 studied the adsorption of deuterium in MCM-41 by neutron diffraction. The diffraction patterns were modeled in terms of a periodic array of hexagonal pore domains where the relative scattering intensities of the Bragg peaks, upon sorption, depend on the relative sizes of the pores and the walls.…”

“…On the other hand small-angle scattering ͑SAS͒ of x ray or neutron techniques ͓small-angle x-ray scattering ͑SAXS͒ and small-angle neutron scattering ͑SANS͔͒ are, nowadays, widely used for their structural characterization, while ͑x-ray or neutron͒ diffraction is proved to be an essential tool for the study of the molecular structure and organization of either bulk fluids or confined liquids. [20][21][22][23][24][25][26][27][28][29][30][31][32] Inelastic neutronscattering measurements have also been carried out for studying the excitations in bulk and confined phases. 33 Moreover, adsorption in conjunction with in situ SAS or diffraction can elucidate the phenomenon and resolve some of the implicated "mysteries."…”