Quantum effects in the sorption of hydrogen by mesoporous materials

The sorption and desorption of hydrogen by mesoporous MCM-41 silicate material is studied at temperatures ranging from 6.8 to 290 K. It is found that a thermally activated mechanism with an estimated activation energy E_a ≈ 466 K predominates in the H₂ sorption kinetics of an MCM-41 sample for temperatures of 60–290 K. For temperatures of 17–60 K the diffusion coefficient of H₂ molecules in MCM-41 is almost entirely temperature independent, which is typical when a tunneling diffusion mechanism predominates over the thermally activated mechanism. Within the 8–17 K range, a change in the mobility of H₂ molecules in the channels of MCM-41 is observed that appears to correspond to the formation of a monolayer (or its destruction during heating) and subsequent layers of hydrogen which have condensed on the inner surfaces of the channels. This process has an activation energy E_m ≈ 21.2 K. At temperatures below 8 K the diffusion coefficients of H₂ depend weakly on temperature. This presumably corresponds to a change in the mechanism for filling of the channels of MCM-41 from the layer-by-layer growth of film on the inner surfaces of the channels to capillary condensation of H₂ molecules. These results are compared with previously obtained data on low-temperature sorption of hydrogen by bundles of carbon nanotubes.