Surface Properties and Gas Adsorption

It is well known that alkali metal binary GICs adsorb gaseous species (H2, N2, Ar, CH4, etc.) physisorptively at low temperatures, where physisorbed gaseous molecules are accommodated in the interstitials of the alkali metal lattice within the graphitic galleries (Lagrange and Hérold, 1975; Lagrange et al., 1972, 1976; Watanabe et al., 1971, 1972, 1973). The capacity for hydrogen adsorption, which is estimated at 144 cm3/g in KC24, for example, is large and comparable to the capacity in other adsorbers such as zeolite or activated charcoal. Interestingly, the physisorption phenomenon in alkali metal GICs has different features from that in conventional adsorbents such as zeolite or activated charcoal; that is, guest molecules in alkali metal GICs are not simply bonded to the adsorbents through weak van der Waals forces without any change in the electronic structures. Here we discuss the gas physisorption phenomenon in alkali metal GICs from general aspects, in relation to their specific features. Then in subsequent sections, we will give details of actual cases. Hydrogen is a typical gaseous molecule adsorbed in alkali metal GICs. Hydrogen physisorption takes place at low temperatures below about 200 K, where hydrogen molecules are accommodated in the graphitic galleries and are not dissociated into atomic hydrogen species. When the temperature is increased to over 200 K, the alkali metal GICs work as catalysts to hydrogen, resulting in the occurrence of hydrogen chemisorption. Hydrogen physisorption will be discussed in Section 8.1.2, hydrogen chemisorption and related issues have been discussed partly in Sections 2.2.1 and 5.4.1 from the viewpoints of structure and electronic properties, and will be discussed again in Section 8.1.2. Figure 8.1 represents the composition dependence of the amount of physisorption of hydrogen molecules in KCm at 77 K (Lagrange and Hérold, 1975). The composition of 1/m = 1/8 corresponds to the stage-1 compound and the composition 1/m = 1/24 to the stage-2 compound; intermediate compositions between 1/8 and 1/24 are considered to have a mixed structure of stage-1 and stage-2 compounds. The stage-1 compound does not adsorb hydrogen at all.