Reevaluation of the\nSuitability of <sup>129</sup>Xe\nNuclear Magnetic Resonance Spectroscopy for Pore Size Determination\nin Porous Carbon Materials

Xenon isotope nuclear magnetic resonance (<sup>129</sup>Xe-NMR)\nspectroscopy has been widely used to evaluate the pore structure of\nmaterials. However, determining how to apply this technique to investigate\nporous carbon materials is sometimes challenging, partly due to the\nstructural disorder and heterogeneity of the surface properties of\nthese materials, and partly due to the lack of reliable methods for\ncontrolling and assessing the density of adsorbed Xe. In this study,\nwe designed and constructed a temperature- and pressure-controllable <sup>129</sup>Xe-NMR system to evaluate the interaction between activated\ncarbon (AC) and adsorbed Xe molecules. Based on a confirmation of\nsurface-covering adsorption form of Xe molecules in AC pores, the\nextrapolation to the ordinate in plots of the surface area-normalized\nXe adsorption amount (ρ<sub>Xe</sub>) and measured <sup>129</sup>Xe-NMR chemical shifts of adsorbed Xe molecules, δ(Xe), to\nremove the influence of the Xe–Xe interaction allowed us to\nestimate the interaction between the AC pore surface and a single\nXe molecule. These data confirmed that interactions between the pore\nsurface and adsorbed Xe molecules depend on a parameter related to\nthe AC pore size, regardless of the content of oxygen-containing surface\nfunctional groups, and an empirical equation to estimate the average\npore size of ACs from the <sup>129</sup>Xe-NMR results was proposed.\nDownward deviations of the linear correlation between ρ<sub>Xe</sub> and δ(Xe) were attributed to the influence of paramagnetism\npresumably derived from oxygen-containing functional groups on the\nsurfaces of ACs and to changes in the adsorption form in low- and\nhigh-ρ<sub>Xe</sub> regions, respectively. These findings confirm\nthe suitability of <sup>129</sup>Xe NMR for pore size determination\nin porous carbon materials.