Solar hydrogen generation with H_2O/ZnO/MnFe_2O_4 system

摘要

The hydrogen generation reaction in the H_2O/ZnO/MnFe_2O_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 H2. At 1273 K, the mixture of ZnO and MnFe_2O_4 reacted with water to generate H_2 gas in 60% yield. X-ray dif-fractometry and chemical analysis showed that 48 mol% of Mn~(Ⅱ) (divalent manganese ion) in the A-site of MnFe_2O_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_(0.42)~(Ⅱ)Mn_(0.39)~(Ⅲ)Fe_(1.61)O_4 (Mn~(Ⅲ): trivalent manganese ion). Its lattice constant was smaller than that of the MnFe_2O_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.