Pore-selective analysis of disordered and complex porous carbon materials by molecular masking and ¹²⁹Xe−NMR

<p>Xenon isotope−nuclear magnetic resonance (¹²⁹Xe−NMR) is known to be a powerful technique for probing pore size for porous carbon materials, particularly when the pore size distribution is narrow. However, the ¹²⁹Xe−NMR spectra for complex and disordered porous materials often exhibit the peak broadening and overlapping due to the broad pore size distributions and the coexistence of pores with different shapes (e.g., slit-shaped and cylindrical), giving rise to a difficulty in accurate assessment of pore size for each pore. To take this issue and enable selective and individual evaluations for each pore, in this study, we propose a combined application method of the molecular masking and ¹²⁹Xe−NMR techniques. Fullerene C₆₀ was used as a molecular masking agent: A model porous carbon material, a mixture of microporous and mesoporous carbons, were vacuum-impregnated with a C₆₀−toluene solution and then the solvent toluene was removed, resulting in the selective masking of micropores < 1 nm of the model porous carbon materials by the C₆₀ molecules. The ¹²⁹Xe−NMR spectra measured at −75°C under a Xe gas pressure of 100 kPa for the model porous carbon material before and after the C₆₀ masking revealed that the C₆₀-masking fully eliminated the spectral contributions from micropores < 1 nm while leaving the signals from larger mesopores essentially unchanged, which enabled an individual evaluation of unmasked mesopores. N₂ adsorption and desorption isotherms measurements corroborated these observations. Taken together, the potential of the proposed combination method of the molecular masking and ¹²⁹Xe−NMR techniques was demonstrated as a selective evaluation method of the pores in disordered and complex porous carbon materials.</p>