The pore-wall chemistry of activated carbon fiber (ACF) was controlled by heating in Ar and H2. The ACF structures were characterized from various levels, and interaction of water vapor with the micropores of ACF was directly measured by calorimetry. Two kinds of pitch-based ACFs with different pore widths (w) (P5, w = 0.7 nm, and P20, w = 1.0 nm) were used. P20 was treated at 1273 K in a gas flow of Ar or H2 for 1 h to modify its surface properties. Adsorption isotherms of water on the two ACFs at 303 K showed different features, which are possibly caused by the pore width difference. The surface modification by the heat treatment of P20 changed its pore structure, leading to different water adsorption behavior. The mechanisms of water adsorption and desorption can be discussed through the differential or integral heat of water adsorption or desorption. Water adsorbs on the functional groups located at the surface of P20 with an adsorption heat comparable to the heat of condensation at relatively low P/P 0, causing the cluster formation of water molecules. The removal of such functional groups by heat treatment decreases the adsorption heat at low pressure. The differential heat abruptly increases at filling in all cases, indicating a structural formation of water from a clustered form to a highly ordered form.

The methane adsorption of water-preadsorbed carbons of different micropore widths w at 303 K was measured. Although the amount of adsorption of supercritical methane on microporous carbon at 303 K was less than 9.4 mg g-1 at 101 kPa, the presence of the preadsorbed water enhanced noticeably the methane adsorption at 303 K even under subatmospheric pressure. The adsorption increment of methane reached a maximum at 1−2 h after introduction of methane and decreased gradually to a steady value after 20−50 h. The adsorption increment of methane depended on the fractional filling φw of micropores by the preadsorbed water. The maximum increment of 110 mg g-1 for w = 1.1 nm at a methane pressure of 2.6 kPa was obtained at φw = 0.34, corresponding to the estimated adsorption amount at 21 MPa of methane (130 mg g-1). The methane-adsorption increment increased linearly with φw until φw = 0.35, indicating the formation of the stable methane−water clathrate of which the composition of methane to water is 1:2. Thus, the nano-order hydrates of methane should be formed in the micropore. The plausible model of the nanohydrate was proposed on the basis of the experimental results and simulation of methane adsorption.