Studies on metal oxides suitable for enhancement of the O 2-releasing step in water splitting by the MnFe 2O 4–Na 2CO 3 system

Abstract

Reactivities of three metal oxides (Fe 2O 3, TiO 2 and MnO 2) with Na 2CO 3, and of their reaction products with CO 2, have been studied to enhance the O 2-releasing step in the two-step water splitting by the MnFe 2O 4–Na 2CO 3 system. X-ray diffraction analysis and thermogravimetric measurements showed that the reaction of α-Fe 2O 3 with Na 2CO 3 (mole ratio=1:1) completed within 10 min at 1073 K to produce NaFeO 2 (Fe 2O 3+Na 2CO 3→NaFeO 2+CO 2). Also, the regeneration of Fe 2O 3 and Na 2CO 3 proceeded readily by passing CO 2 gas through NaFeO 2 (71% yield). TiO 2 reacted with Na 2CO 3 (mole ratio=1:1) at 1073 K for 1 h to form Na 8Ti 5O 14 and Na 2TiO 3 (93% yield). However, in the reaction of the products with CO 2, the starting material (TiO 2) was not reproduced at the temperature range from 673 K to 1073 K, but Na 4Ti 5O 12 (having a lower Na content than the form Na 8Ti 5O 14) was formed. In the case of MnO 2 with Na 2CO 3 (mole ratio=2:1), Na 0.7MnO 2–2.05 and NaMnO 2 were produced at 1073 K for 1 h (80% yield), but the reaction between these products and CO 2 hardly proceeded.