Role of Surface Functional Groups in the Adsorption Kinetics of Water Vapor on Microporous Activated Carbons

Activated carbons have both hydrophilic surface oxygen functional groups, which act as primary adsorption centers for water vapor, and hydrophobic graphene layers on which nonpolar species are primarily adsorbed. The aim of this research was to investigate the effects of oxygen surface functional groups, in activated carbons, on the adsorption characteristics of water vapor. Activated carbon G was oxidized using nitric acid and then heat treated in the range 387−894 K to produce a suite of adsorbents with varying oxygen contents in the range 0.4−21.5 wt % daf, but very similar porous structure characteristics, thereby minimizing effects due to changes in porous structure. The type and concentration of surface oxygen groups present in each sample were assessed using TPD, FTIR, Boehm titration, and analytical methods. Water vapor adsorption at low relative pressure was dramatically enhanced by the presence of functional groups, in particular, carboxylic groups. Kinetic profiles for each pressure increment were modeled using a set of nested kinetic models, which allow the adsorption kinetics to be analyzed in relation to the adsorption mechanism. Relationships between water adsorption kinetics at low surface coverage and the type and concentration of oxygen surface functional groups were observed. A two component double exponential kinetic model was used when carboxylic groups were present in significant amounts with a slow kinetic component associated with adsorption on these groups. In the case of carbons where carboxylic groups were only present in, at most, relatively small amounts, a stretched exponential kinetic model was used and the rate constants in the low-pressure region decreased linearly with increasing Henry's Law constant and oxygen content. The results indicate the importance of adsorbate−adsorbent interactions in water adsorption kinetics and are consistent with a site-to-site hopping mechanism between functional groups.