La (1− x) Sr xMnO 3− δ perovskites as redox materials for the production of high purity hydrogen

Abstract

Materials of the perovskite structure and of the general formula La 1− x Sr x MnO 3 ( x = 0, 0.3, 0.7) are investigated as redox catalysts for the two-step steam reforming of methane towards the production of high purity hydrogen. During the activation step, methane is oxidized with lattice oxygen to carbon dioxide and carbon monoxide, while oxygen is withdrawn from the material until a maximum deficiency level which depends on the strontium content and the reaction temperature. During the reaction step water is splitted to gaseous hydrogen and lattice oxygen that fills the oxygen vacancies. It appeared that, after the achievement of a characteristic oxygen deficiency level, La 1− x Sr x MnO 3 materials exhibit good activity for the water-splitting reaction. The activity is further found to be proportional to the oxygen vacancy concentration. At high activity levels, initial water conversions per 15 μmol pulse of up to 70% are achieved at 1000 °C. The cumulatively produced hydrogen during the water-splitting step, per injected water, increases with increasing strontium content, reaching a production of 60 μmol H 2 per 500 μmol water passed over 200 mg La 0.3Sr 0.7MnO 3 at 1273 K and no coke formation. The materials exhibit stable behavior after eight successive oxidation–reduction cycles. The relations between the redox behavior and the material defect chemistry are discussed. Finally the energy efficiency of the process, future prospects and ways for its optimization are discussed.