Temperature dependence of water adsorption on highly graphitized carbon black and highly ordered mesoporous carbon

In previous work [1] we showed an unusual temperature dependence for water adsorption in porous activated carbons, and hypothesized that the pore size plays a critical role in water adsorption in mesopores. In this paper, we shed further light on water adsorption in mesopores by investigating its dependence on temperature for a highly graphitized non-porous carbon black and a highly ordered mesoporous carbon, Hex. They were chosen because of their distinctly different structures, with the former representing an open graphitic surface, while the latter having highly structured and graphitic mesopores of hexagonal shape. Analysis of the isotherms for these systems shows that adsorption proceeds by nucleation of water molecules around functional groups (FG) to form an embryo FG-water complex, which grows with increasing pressure to form a cluster. If the clusters are closer than a critical separation, they agglomerate to form a condensate, and provided that the pore size is small enough, the adsorbate fills the confined space. Interestingly, the amount adsorbed in the mesopores at 298 K is greater than that at 263 K, which is attributed to the formation of larger clusters at 298 K, facilitating the agglomeration of clusters followed by condensation in the mesopores.

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