Quantum effects in the sorption of hydrogen by mesoporous materials

Abstract

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 Ea ≈ 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 Em ≈ 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.

The effect of graphene oxide reduction temperature on the kinetics of low-temperature sorption of hydrogen

“…The latter of the above-mentioned factors has the most pronounced effect on the structure and sorption properties of the sample heat-treated at the highest temperatura of 900 °С. 17 The previous studies of the low-temperature hydrogen sorption-desorption by thermally reduced graphene oxide 23 revealed that the highest sorption capacity exists for samples treated at 300 and 900 °С ( Fig. 1).…”

“…In order to eliminate this effect from the measurement results, the procedure provided for the measurement of the characteristic sorption and desorption times to be done over short periods of time in which the nuclear spin conversión could hardly have time to occur. 25 Figure 3 shows the temperatura dependences of the characteristic times of Н 2 sorption (desorption) by a graphite oxide sample [ Fig. 3(a)], and by thermally reduced TRGO samples [ Figs.…”

“…The procedure used to manufacture the initial graphite oxide powder (hereinafter referred to as GtO) was described in Ref. 17. The thermal exfoliation of the graphite oxide powder thus obtained was carried out for five samples at different temperatures (200, 300, 500, 700, and 900 °С) for 15 min under argon.…”

“…Fast water vaporization results in the exfoliation of graphite oxide, i.e., sharp increase in the distance between the carbon sheets, which markedly increases the sorption capacity of the material. 17 Heating above 200 °С initiates two competing processes that determine the features of the sample structure: the reduction of the graphite carbon structure, which decreases the interlayer spacing via removal of oxygencontaining groups and mechanical stress relaxation, and the formation of defects via detachment of carbon atoms directly coupled with oxygen atoms in oxygen-containing groups by strong covalent bonds. The latter of the above-mentioned factors has the most pronounced effect on the structure and sorption properties of the sample heat-treated at the highest temperatura of 900 °С.…”

“…It is known that structural defects can cause at least a two-fold increase in the graphene area accessible for sorption. 16,17 Furthermore, thermal heating does not imply the use of reducing agents and therefore rules out the appearance of chemical impurities on the graphene surfaces.…”