No abstract

No abstract

No abstract

The pressure dependence of a 129Xe chemical shift (δ) and the local density of xenon adsorbed in activated carbon fiber (ACF) with slit-pore widths of 0.7–1.1 nm was investigated using in-situ high-pressure 129Xe NMR. 129Xe chemical shift values below 0.025 MPa change linearly with equilibrium pressure. The initial slope of the pressure dependence of δ led to a shift value at zero pressure, δS′, which approximately reflects the xenon–wall interaction. A statistical model incorporating the xenon–wall interaction well interprets the dependence of δS′ on the pore width. Furthermore, in a higher-pressure region, the density dependence of the chemical shift led to the xenon–xenon interaction via the virial coefficients of the chemical shift up to the second order on density (the third-virial coefficient). The second-virial coefficient (a coefficient for the linear term of density) depended on the pore width. Increasing the slit width from 0.7 to 1.1 nm increased the second-virial coefficient, δ1, from 42 × 10−3 to 78 × 10−3 ppm kg−1 m3, suggesting that the space accessible by the surrounding xenon atoms increases when the slit width increases. This aspect reveals the size effect of the xenon–xenon interaction in nanospace.

Pressure (0-10 MPa) and local density dependence of 129Xe NMR chemical shift of xenon in various microporous materials was investigated using an in situ high-pressure probe. The density dependence of the chemical shift was analyzed using virial expansion of the chemical shift by xenon density. Results indicate that the second virial coefficient depends on the pore size and shape, and that the void space affects xenon-xenon interaction in both microporous and mesoporous materials. Furthermore, to interpret the magnitude of the virial coefficient in terms of the local structure of the adsorbed xenon, we analyzed the local structure of adsorbed xenon in molecular sieve 5A using Xe(n) clusters, thereby allowing description of the density dependence of the chemical shift. We also demonstrated the cluster model's validity by applying it to molecular sieves 13X and ZSM-5. The latter showed that the adsorbed xenon exists as a xenon monomer up to the filling of about 0.6 in micropores. Larger xenon clusters up to n = 4 have been grown with increasing filling of xenon. According to analyses using the Xe(n) cluster model, the second virial coefficient is related closely with the xenon cluster size, which contributes greatly to the chemical shift in the low loading region.