Hydrogenation of nanostructured graphite by mechanical grinding under hydrogen atmosphere

Orimo et al. reported that nanostructured graphite, prepared by using the mechanical grinding under hydrogen atmosphere, contained more than 7 mass% of hydrogen whose thermal desorption spectrum (TDS) showed characteristic two peaks; one is at around 700 K and the other around 1000 K [Appl. Phys. Lett. 75 (1999) 3093; Appl. Phys. A72 (2001) 167; J. Appl. Phys. 90 (2001) 1545]. We confirmed this claim; namely, c.a. 4.5 mass% of hydrogen was detected by TDS in the desorbed gas from graphite powder mechanically ground under hydrogen in a Cr/Ni steel mortar. Yet the mechanism of hydrogenation and the physico-chemical state of adsorbed hydrogen are not known well. We found that the amount of contained hydrogen depends significantly on the grinding mortar. When a Cr steel mortar was used, we obtained 2 mass%; and when an agate mortar was used, only a trace amount of hydrogen was detected. The transmission electron microscopy and the X-ray powder diffractometry indicated that the nanostructured graphite ground in steel mortars contained a large quantity of cementite, Fe 3C, to which the iron element was supplied by wearing out of mortar walls during the grinding. We examined the influence of metal particles by intentionally adding iron and nickel powder into graphite during the grinding in the metal-free agate mortar. Although in the agate mortar with metallic additives the hydrogenation did not proceed as much as in the steel mortar, the TDS spectrum showed characteristic features. The presence of catalytic metal particles seems to be a prerequisite for the hydrogenation of graphite under hydrogen by mechanical grinding.