Energy characteristics of adsorbed water in active carbons according to the NMR relaxation data

Abstract

Nuclear magnetic relaxation measurements were used to determine activation energy E act of the motion of water molecules adsorbed in active carbons. The E act value was found to depend on the filling of active carbon pores due to changes in the state of water molecules under adsorption. It was established that the E act = f(p/p s) plots, where p/p s is the relative pressure of water vapor observed for microporous active carbons (FAS-1, 2, N-15, SKT-6A), are similar in form to the corresponding plots of changes in water adsorption heats. In particular, we concluded that the plateau in the E act = f(p/p s) dependences, as in the case of adsorption heats, reflects the volumetric filling of active carbon micropores with water. We show that a linear function describes the increase in E act values for water upon the complete filling of micropores with an increase in the volume of adsorbed water clusters per one primary adsorption center (W 0/a m ). We establish that, for water in the FAS-3 sample, the deviation of E act values from this linear function was due to the contribution from the vapor phase in the mesopores (x 0 = 0.7−1.2 nm) that make up a considerable part of the active carbon’s porous system.