Water adsorption in nanoporous carbon characterized by in situ NMR: Measurements of pore size and pore size distribution

We report an in situ nuclear magnetic resonance (NMR) study of water adsorption in a series of activated carbon samples with pore sizes of a few nanometers down to the subnanometer scale (nanoporous carbon). Water adsorption exhibits S-shaped type V isotherms with a steep increase near a certain vapor pressure. Using a previously proposed water isotherm model, pore size and pore size distribution are derived from the in situ NMR data, and they are shown to be in good agreement with results derived from N₂ adsorption. The change of ¹H NMR spin-lattice relaxation time of adsorbed H ₂O with vapor pressure is consistent with the mechanism of water cluster formation at surface groups preceding the occurrence of pore filling. NMR spectra of high pressure H₂ gas in nanoporous carbon with preadsorbed D₂O proves unambiguously that water preferentially fills the smaller nanopores. These results suggest that water adsorption can potentially be used for the characterization of pore structures of nanoporous carbon, and that in situ NMR is a convenient method for water isotherm measurement with accompanying microscopic information.