Solar hydrogen generation with H2O/ZnO/MnFe2O4 system

Abstract The hydrogen generation reaction in the H 2 O/ZnO/MnFe 2 O 4 system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H 2 . At 1273 K, the mixture of ZnO and MnFe 2 O 4 reacted with water to generate H 2 gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn II (divalent manganese ion) in the A-site of MnFe 2 O 4 was substituted with Zn II (divalent zinc ion) and that chemical formula of the solid product was estimated to be Zn 0.58 Mn II 0.42 Mn III 0.39 Fe 1.61 O 4 (Mn III : trivalent manganese ion). Its lattice constant was smaller than that of the MnFe 2 O 4 (one of the two starting materials). From the chemical composition, the reaction mechanism of the H 2 generation with this system was discussed. Since the Mn ions in the product solid after the H 2 generation reaction are oxidized to Mn 3+ , which can readily release the O 2− ions as O 2 gas around 1300 K, the two-step of H 2 generation and O 2 releasing seem to be cyclic.

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