Chemistry of carbon surface and mechanism of water molecule adsorption

Abstract

The experimental adsorption isotherms of water vapour and nitrogen on nonporous carbon adsorbents with various specific surfaces and concentrations of primary adsorption centres (PACs) have been measured. The adsorption of water molecules ends with the formation of either individual isolated clusters in equilibrium with saturated vapour or a continuous adsorption film depending on the concentration of the PACs. In the former case, the pure differential heats of adsorption are negative over the whole range of adsorption values (differential heat is lower than the heat of condensation), and the threshold adsorption value of water vapour ( a s) is proportional to the concentration of PACs and corresponds to the formation of fractions of a dense monomolecular layer on the adsorbent surface. In the second case, the value of the pure differential heat of adsorption is positive over the entire range of coverage and decreases continuously to zero at saturation, and the a s value depends on the specific surface area of the carbon adsorbents, and corresponds to the formation of a 1.7 ± 0.3 dense monomolecular layer. A method of evaluating the surface of nonporous carbon adsorbents and the mesopore surface of active carbons based on this experimental finding is proposed. The mesopore surfaces determined from the water and nitrogen (benzene) vapour adsorption isotherms are in good agreement.