1H NMR Study of Water Adsorbed on Carbon Powders

Abstract

1H NMR relaxation measurements are performed for H2O molecules adsorbed on the surface of carbon powders. Spin-lattice relaxation time T1 shows a minimum at T ∼ 185 K, where fast decrease in spin-spin relaxation time T2 occurs, and this corresponds to the slowing of intramolecular rotational motions. The T2 shows a maximum at a higher temperature, T ≍ 250 K. At the temperature where the T2 maximum occurs, the spin-lattice relaxation time in the rotating frame, T1ρ, also shows a maximum. The Carr-Purcell-Meiboom-Gill and stimulated echo measurements indicate the presence of nonconventional diffusion effects. These anomalies are interpreted as originating from the restricted diffusion of H2O on the surface of carbon particles with the size of restriction about 20 Å, the diffusion constant equal to (6 ± 2) × 10−12 cm2/s, and a field gradient of (4 ± 1.5) × 105 Oe/cm, measured at room temperature.