Quasi-elastic neutron scattering study of the mobility of methane in microporous silica

The dynamics of translation and rotation of methane in microporous bulk silica have been studied with quasi-elastic neutron scattering. At T = 200 K the self-diffusion coefficient of translation is D-S = 1.1x10(-8) m(2) s(-1) with an estimated activation energy of 4 kJmol(-1). Any variation of D-S with occupancy is within the experimental error of 50%. The isotropic rotational diffusion constant D-R is of the order of 10(11) s(-1), with an experimental error of a factor of 2. The pore-size distribution has been studied with small angle X-ray scattering. A broad pore-size distribution is observed, with pores smaller than 10 Angstrom. The largest fraction of pores is observed at the lower experimental limit, corresponding to a pore size of 2 Angstrom. (C) 2001 Elsevier Science B.V. All rights reserved.