Amorphous cobalt oxysulfide as a hydrogen trap

Solid amorphous cobalt oxysulfide “CoSOH” obtained by basic aqueous precipitation absorbs gaseous hydrogen up to a limit close to the H2/Co molar ratio of 0.5. To understand the nature of this phenomenon, the cobalt compound was characterized before and after hydrogen absorption (XRD, IR, TEM, XPS, inelastic neutrons scattering, and extended X-ray absorption fine structure spectroscopy). The initial oxysulfide is an amorphous solid containing hydroxide groups and sulfide moieties. Considerable amounts of hydrogen (as high as 0.3 mol of H2/Co atom) can be absorbed by this solid without any apparent changes of its structure and morphology, due to the reductive opening of S−S bridges leading to the formation of −SH groups. At higher levels of hydrogen consumption (0.3−0.5 mol of H2/Co atom), formation of crystalline Co9S8 compound was observed. Isotopic exchange and volumetric experiments demonstrated that the absorption of hydrogen is a slow and irreversible first-order solid−gas reaction.