(La1−xCax)MnO3−δ (x = 0, 0.2, 0.3, 0.4) Perovskites as Redox Catalysts in Chemical Looping Hydrogen Production Process: The Relation between Defect Chemistry and Redox Performance

Abstract

The interaction between point defects in (La1−xCax)MnO3−δ (x = 0, 0.2, 0.3, 0.4) perovskites and their redox catalytic properties in a three-reactor chemical looping hydrogen production process is investigated. During the reduction step with CH4, the behavior of the materials is extrinsically determined and strongly depends on the Ca content. At small oxygen deficiencies, CH4 becomes oxidized to CO2. As the deficiency increases, partial oxidation to CO and H2 at a molar ratio of approximately 2 is favored. During the water-splitting step, the dependency on the Ca content is much weaker since it is intrinsically determined by the Mn2+→Mn3+ oxidation with simultaneous annihilation of oxygen vacancies that are not required to compensate for the extra negative charge of the Ca dopant. Hydrogen productivities in the order of 13 cm3 (STP) H2/g solid could be achieved during the water-splitting step at 1000 °C. The materials exhibited reproducible catalytic behavior during 10 cycles of the complete three-step process and were found to retain their perovskite structure.