Structure and dynamics of hydrogen sorption in mesoporous MCM-41

Abstract

Adsorption isotherms taken at temperatures ranging from 20 to 77 K show a large pore volume and surface area of 980 m2 g^-1 for the physical adsorption of molecular hydrogen on MCM-41. The adsorbed hydrogen behaves more like a solid than a liquid and isosteric heats of adsorption reveal a heterogenous surface. The evaporation behaviour of the adsorbed hydrogen indicates that the hexagonally packed tubes in MCM-41 may be effectively interconnected into a single void space. Neutron inelastic scattering shows that molecular hydrogen exists in two sites in the pores of MCM-41. We designate as surface states those with a variety of weakly hindered rotational excitations centred at 11.8(2) meV and as bulk states, at high doping of H₂, those which have narrow rotational excitations at 14.7(3) meV. These excitations are hardly changed from those of the free crystal in energy, energy width or momentum transfer dependence. Each type accounts for ca. 50% of the total pore volume. Strong hydrogen recoil scattering is also observed at momentum transfers above ca. 3 Å^-1. Neutron diffraction from the filled sample at 1.9 K shows a single peak at 3.1 Å, characteristic of the H₂–H₂ correlations in bulk hydrogen. This peak is much weaker for the surface adsorbed species. The concentration dependence of this peak also shows a 1:1 division of void space between surface-adsorbed and bulk-like species. In addition we observe a separate peak at 14.38(3) Å, the hexagonal (21) of MCM-41 involving the silica framework, whose diffracted intensity changes dramatically with hydrogen doping, in a way inconsistent with a smooth walled, hexagonally packed mesopore. These data, together with previous X-ray diffraction data, provide information on silica density, absorbing surface and free volume projected onto the basal plane. The detail and complexity of the projection were unexpected. The data agree well with a model for which the silica in the walls is loosely interwoven, and occupies only 40% of the wall volume. The 35% silica surrounding the empty 7 Å hole as a 12.7 Å lining has a projected density of ca. 90% that of the walls. Either highly divided, ‘hairy’ walls or a structure like a highly defective variant of the MCM-48 structure would fit the data. The projection of smooth-walled but highly twisted channels onto a horizontal plane could give a density distribution equivalent to the highly porous silicate walls in straight channels.

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