Hydrogen physisorption by potassium-graphite intercalation compounds prepared from mesocarbon microbeads

Potassium is intercalated into mesocarbon microbeads (MCMBs) with different heat-treatment temperatures (HTTs) and the hydrogen physisorption of the resulting compounds (KCx; 8 ≤ x ≤ 36) is investigated. The hydrogen-physisorption behavior depends largely on the HTT of MCMB. Particularly, KCx compounds prepared from MCMB with a HTT of 1500 °C (KCx(HTT-1500)) show anomaly in the composition range from KC8 to KC12; the saturated amount of physisorbed hydrogen is remarkably large compared to that observed in the usual KCx compounds prepared from graphite, and the H2D2 separation coefficient is also very large. It is found by X-ray diffraction measurement that KCx(HTT-1500; 8 ≤ x ≤ 11) compounds have a stage-1 structure. This is in contrast to KCx(HTT-2600; 8 ≤ x ≤ 11) compounds, which have a mixture of stages 1 and 2 as is the case with KCx prepared from graphite. This means that the density of the potassium layer in KCx(HTT-1500; 8 < x ≤ 11) is smaller than that of usual stage-1 KC8. It is considered that nanospaces of this ‘dilute’ stage-1 structure can accommodate large amounts of hydrogen and that it is suitable for the hydrogen-isotope separation.