Hydrogen trapping: Synergetic effects of inorganic additives with cobalt sulfide absorbers and reactivity of cobalt polysulfide

The biphasic product CoS2 + Co(OH)2 obtained by oxidation of cobalt sulfide is known to trap hydrogen at room temperature and low pressure according to a balanced reduction equation. Adding various inorganic compounds to this original absorber induces their reduction by hydrogen in the same conditions at a significant rate: (i) excess cobalt hydroxide is reduced to metallic cobalt; (ii) nitrate ions are reduced to ammonia; (iii) sulfur and sodium thiosulfate are reduced to H2S or NaHS and Na2S, respectively. Without a hydrogen absorber these inorganic compounds are not reduced by H2, suggesting synergetic effects involving H2 and the hydrogen absorber. Amorphous cobalt polysulfide, CoS5, is also reduced by hydrogen at room temperature and releases H2S gas. In the presence of a base to neutralize H2S gas, the reaction rate is initially slower than with the CoS2 + Co(OH)2 mixture due to the higher stability of polysulfide chains but the H2 trapping yield is improved, making CoS5 a good candidate for H2 trapping.

► The hydrogen absorber is a mixture of Co(OH)2 and amorphous CoS2.
► In the presence of H2 at room temperature, the absorber produces CoS and Co9S8.
► A supplementary cobalt hydroxide is reduced to metallic cobalt.
► Added nitrates ions are reduced to ammonia.
► Cobalt polysulfide CoS5 is reduced by hydrogen at room temperature and releases H2S.