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

Abstract

Xenon isotope–nuclear magnetic resonance (¹²⁹Xe–NMR) spectroscopy is a powerful technique for investigating the pore sizes of porous carbon materials, particularly those with narrow pore size distributions. However, the ¹²⁹Xe–NMR spectra of complex and disordered porous materials often exhibit peak broadening and overlapping owing to broad pore size distributions and the presence of differently shaped pores (e.g., slit-shaped and cylindrical). This in turn makes accurate assessment of the pore sizes difficult. To overcome this issue and enable selective and individual evaluations for each pore, this study proposes a combined method based on molecular masking and ¹²⁹Xe–NMR spectroscopy. Fullerene C₆₀ was used as the molecular masking agent. A model porous carbon material, comprised of a mixture of microporous and mesoporous carbons, was vacuum-impregnated with a C₆₀–toluene solution. Subsequently, the toluene was removed, resulting in the selective masking of micropores smaller than 1 nm by the C₆₀ molecules. In addition, ¹²⁹Xe–NMR were 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. These spectra showed that C₆₀ masking totally eliminated the spectral contributions from micropores smaller than 1 nm, while leaving the signals from larger mesopores essentially unchanged, enabling an individual evaluation of unmasked mesopores. N₂ adsorption and desorption isotherms confirmed these observations. Taken together, these results demonstrate the potential of the combined method for selectively evaluating the pore sizes of disordered and complex porous carbon materials.