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 DS=1.1×10−8 m2 s−1 with an estimated activation energy of 4 kJ mol−1. Any variation of DS with occupancy is within the experimental error of 50%. The isotropic rotational diffusion constant DR is of the order of 1011 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 Å. The largest fraction of pores is observed at the lower experimental limit, corresponding to a pore size of 2 Å.