Studies on metal oxides suitable for enhancement of the O2-releasing step in water splitting by the MnFe2O4–Na2CO3 system

Reactivities of three metal oxides (Fe2O3, TiO2 and MnO2) with Na2CO3, and of their reaction products with CO2, have been studied to enhance the O2-releasing step in the two-step water splitting by the MnFe2O4–Na2CO3 system. X-ray diffraction analysis and thermogravimetric measurements showed that the reaction of α-Fe2O3 with Na2CO3 (mole RATIO=1:1) completed within 10 min at 1073 K to produce NaFeO2 (Fe2O3+Na2CO3→NaFeO2+CO2). Also, the regeneration of Fe2O3 and Na2CO3 proceeded readily by passing CO2 gas through NaFeO2 (71% yield). TiO2 reacted with Na2CO3 (mole RATIO=1:1) at 1073 K for 1 h to form Na8Ti5O14 and Na2TiO3 (93% yield). However, in the reaction of the products with CO2, the starting material (TiO2) was not reproduced at the temperature range from 673 K to 1073 K, but Na4Ti5O12 (having a lower Na content than the form Na8Ti5O14) was formed. In the case of MnO2 with Na2CO3 (mole RATIO=2:1), Na0.7MnO2–2.05 and NaMnO2 were produced at 1073 K for 1 h (80% yield), but the reaction between these products and CO2 hardly proceeded.