The features of water vapour adsorption on micro- and mesoporous activated carbons

The investigation of the adsorption behavior of water vapour on porous carbon matrices is still relevant because the humidity of the environment can significantly affect the adsorption capacity of the carbon adsorbents to certain substances. Furthermore, the measurements of water isotherms could also be used to analyze the pore structure of activated carbons. The correlation between the adsorption characteristics of series of activated carbon samples (Norit, NL) with similar values of the specific surface area, but different pore size distribution (different volumes of micro- and mesopores), towards water vapour and their internal structure, defined on the basis of nitrogen adsorption isotherms, as well as the chemical composition and the nature of primary adsorption centers of the surface has been researched in the study. By the methods of Thermogravimetric Analysis (TGA), Temperature-Programmed Desorption Mass Spectrometry (TPD-MS) and X-ray Photoelectron Spectroscopy (XPS) it has been determinated of quantitative and qualitative composition of the surface of investigated carbons; it has been shown that the mesoporous samples of activated carbon contain a greater total amount of surface oxygen-containing complexes than microporous samples. The adsorption and desorption branches of water adsorption isotherms for the microporous carbons come close to each other and give a plateau at high relative pressures wherein the water adsorption capacities for the microporous samples correspond to their micropore volumes determined from nitrogen adsorption isotherms. As opposed to this, a gradual rise of water uptake and the existence of wide hysteresis loops at high relative pressures are observed for the mesoporous carbons. Moreover, the maximum values of water adsorption in this case considerably exceed the micropore volumes, and are attributable to the capillary condensation in mesopores. Nevertheless, the total volumes of water uptake both for micro- and mesoporous activated carbons are significantly less than those determined from nitrogen adsorption. This could be due to a mechanism of cluster pore filling in which the water density is less than its bulk density. In addition, probably, the water can not condense in the larger mesopores.