Chemical transformation of additive phase in MgH2/CeO2 hydrogen storage system and its effect on catalytic performance

Highlights: • MgH₂ and CeO₂ interact chemically and make CeO_1.96 and Ce₂O₃ in situ products. • Crystalline to amorphous transformation of ceria occurs during mechanical milling. • Recrystallization occurs during de/re-hydrogenation of the ball milled samples. • The belief regarding CeH₂ as the only active in situ product is questionable. • CeO_1.66 seems to be overlooked as CeH₂ in MgH₂/CeO₂ hydrogen storage system. This study provides important information for understanding the hydrogen storage mechanism of ceria additive incorporated magnesium hydride (MgH₂). It is observed that dehydrogenation activation energy of MgH₂ can be lowered by at least 50 kJ/mol.H₂ upon incorporating a small amount of CeO₂ with MgH₂. The ceria additive partially transforms to CeO_1.96 and hcp Ce₂O₃ phases when we mechanically mix ceria with MgH₂. Subsequently, upon further dehydrogenation at 300 °C, the remaining CeO₂, CeO_1.96 and hcp Ce₂O₃ phases transform to cubic Ce₂O₃. A substantial quantity of ceria undergoes crystalline to amorphous transformation upon intense mechanical milling but the crystallinity gets restored during hydrogenation/dehydrogenation reactions. With appropriate evidence, the current study also explains, "how reduced cerium oxide phase(s) may be mistaken as cerium hydride(s)" and "why the beliefs existing in the literature regarding CeH₂ as the active in situ catalyst in MgH₂/CeO₂ hydrogen storage system needs to be further scrutinized".