Hysteresis-associated Pressure-shift-induced Water Adsorption in Micropores of Activated Carbon Fibers

Activated carbons fibers (ACFs) possess considerably uniform slit-shaped micropores of great pore volume. Molecules are strongly adsorbed in the micropores of the deep molecular potential well which stems from the overlapping of the molecule-pore wall interaction from opposite pore walls. Thus, a substantial adsorption of vapor begins at a very low-pressure region, giving rise to a monolayer adsorption and a further filling in residual spaces. However, the interaction of a water molecule with the hydrophobic system such as ACF is too weak to induce a predominant adsorption at a low relative pressure P/P0. Even hydrophobic ACF can induce a predominant water adsorption at high P/P0 region, giving a remarkable adsorption hysteresis for wider micropore systems, as shown in Figure 1. Recently, Ohba et al. reported that the presence of preadsorbed molecules changes the effective potential well determining a further adsorption [1]. Thus, the potential curves for adsorption and desorption must be different from each other even at the same pressure, and this difference may cause the adsorption hysteresis. Iiyama et al. showed that the adsorbed state of water in micropores of ACF upon adsorption and desorption are different from each other for the system having an adsorption hysteresis with in situ SAXS method [2]. Here, the adsorption hysteresis must provide important information on adsorption. On the other hand, it is well-known that water molecules can produce a host structure of the hydrogen-bonded network, in which includes a guest molecule. Methane, which has gathered much attention from the energy and environmental aspects, forms the clathrate compound with water, so called methane hydrate. The methane hydrate is formed at low temperature and/or at high pressure; e.g. above 80 MPa at 303 K [3]. The storage of the methane hydrate at ambient conditions has been requested. We can expect that the organized water form a stable structure with methane in the micropores of the ACF. Recently, we reported a noticeable uptake of adsorption upon the introduction of the supercritical methane on water-preadsorbed ACFs and a super-high surface-area carbon (SAC) at an ambient temperature [4]. In our previous paper, we suggested a possibility of formation of a special compound of water and methane. In this article, a transient shift mechanism of an equilibrium pressure of water vapor induced by an introduction of the supercritical methane is proposed on the basis of the precise quantitative analysis of gas phase composition [5]. 0 50