Reevaluation of the Suitability of ¹²⁹Xe Nuclear Magnetic Resonance Spectroscopy for Pore Size Determination in Porous Carbon Materials

Xenon isotope nuclear magnetic resonance (¹²⁹Xe-NMR) spectroscopy has been widely used to evaluate the pore structure of materials. However, determining how to apply this technique to investigate porous carbon materials is sometimes challenging, partly due to the structural disorder and heterogeneity of the surface properties of these materials, and partly due to the lack of reliable methods for controlling and assessing the density of adsorbed Xe. In this study, we designed and constructed a temperature- and pressure-controllable ¹²⁹Xe-NMR system to evaluate the interaction between activated carbon (AC) and adsorbed Xe molecules. Based on a confirmation of surface-covering adsorption form of Xe molecules in AC pores, the extrapolation to the ordinate in plots of the surface area-normalized Xe adsorption amount (ρ_Xe) and measured ¹²⁹Xe-NMR chemical shifts of adsorbed Xe molecules, δ(Xe), to remove the influence of the Xe-Xe interaction allowed us to estimate the interaction between the AC pore surface and a single Xe molecule. These data confirmed that interactions between the pore surface and adsorbed Xe molecules depend on a parameter related to the AC pore size, regardless of the content of oxygen-containing surface functional groups, and an empirical equation to estimate the average pore size of ACs from the ¹²⁹Xe-NMR results was proposed. Downward deviations of the linear correlation between ρ_Xe and δ(Xe) were attributed to the influence of paramagnetism presumably derived from oxygen-containing functional groups on the surfaces of ACs and to changes in the adsorption form in low- and high-ρ_Xe regions, respectively. These findings confirm the suitability of ¹²⁹Xe NMR for pore size determination in porous carbon materials.