Study of H2 confined in the highly ordered pores of MCM-48

Anomalous Depletion of Pore-Confined Carbon Dioxide upon Cooling below the Bulk Triple Point: An In Situ Neutron Diffraction Study

The phase behavior of sorbed CO{2} in an ordered mesoporous silica sample (SBA-15) was studied by neutron diffraction. Surprisingly, upon cooling our sample below the bulk critical point, confined CO{2} molecules neither freeze nor remain liquid as expected, but escape from the pores. The phenomenon has additionally been confirmed gravimetrically. The process is reversible and during heating CO{2} refills the pores, albeit with hysteresis. This depletion was for the first time observed in an ordered mesoporous molecular sieve and provides new insight on the phase behavior of nanoconfined fluids.

Hydrogen crystallization in low-density aerogels

Crystallization of liquids confined in disordered low-density nanoporous scaffolds is poorly understood. Here, we use relaxation calorimetry to study the liquid-solid phase transition of H2 in a series of silica and carbon (nanotube- and graphene-based) aerogels with porosities ≳94%. Results show that freezing temperatures of H2 inside all the aerogels studied are depressed but do not follow predictions of the Gibbs-Thomson theory based on average pore diameters measured by conventional gas sorption techniques. Instead, we find that, for each material family investigated, the depression of average freezing temperatures scales linearly with the ratio of the internal surface area (measured by gas sorption) and the total pore volume derived from the density of aerogel monoliths. The slope of such linear dependences is, however, different for silica and carbon aerogels, which we attribute to microporosity of carbons and the presence of macropores in silica aerogels. Our results have important implications for the analysis of pore size distributions of low-density nanoporous materials and for controlling crystallization of fuel layers in targets for thermonuclear fusion energy applications.

Freezing and melting of hydrogen confined in nanoporous silica

Thermodynamic properties of condensed hydrogen in geometric confinement remain poorly understood. Here, we use relaxation calorimetry to study solidification and melting of H2 in a series of Vycor-type nanoporous silica glasses with interconnected pores with average diameters in a wide range of ∼100-3000 Å. We find that the depression of freezing and melting temperatures for this quantum system follows the classical Gibbs-Thomson-like behavior, scaling inversely with the pore size when correlated to pore diameters measured directly by electron microscopy, rather than conventional gas sorption techniques. The shapes of pore size distributions derived from hydrogen thermoporometry are, however, more complex than those measured by gas sorption. The ratio between temperatures of the depression of freezing and melting suggests that the actual pore geometry in Vycor-type nanoporous glasses deviates from cylindrical.

Growth of highly oriented deuterium crystals in silicon nanochannels

The structure of solid deuterium confined in 9 nm wide tubular silicon nanochannels has been studied by means of elastic neutron scattering techniques. As a result we report the formation of fcc D(2) as the stable solid phase in confinement in contrast to the hcp bulk structure. Further, a preferred alignment of D(2) nanocrystals with respect to the surrounding crystalline silicon matrix is discussed in terms of heteroepitaxial growth of solid D(2) on crystalline pore walls.