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Synthesis and Oxygen Release/Storage Properties of Ce-Substituted La-Oxysulfates, (La1−xCex)2O2SO4

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journal contribution
posted on 11.11.2008, 00:00 authored by Dongjie Zhang, Fumihiko Yoshioka, Keita Ikeue, Masato Machida
A novel oxygen storage material consisting of Ce-substituted La-oxysulfate, (La1−xCex)2O2SO4 (0 ≤ x ≤ 0.2), was synthesized via a surfactant-assisted route. The layered mesophase templated by dodecyl sulfate anion (DS = C12H25OSO3) was precipitated from aqueous solutions of nitrates using ammonia as a precipitant. Calcination at ≥500 °C in air yielded the oxysulfate having the monoclinic structure, the lattice constant, b, of which was decreased with an increase of the Ce content (x) consistent with the smaller ionic radius of Ce3+/Ce4+ compared to that of La3+. Although part of Ce was deposited as CeOSO4 and/or CeO2 on the oxysulfate, subsequent reduction in H2 yielded single phases of Ce-substituted oxysulfides, (La1−xCex)2O2S, without forming impurity phases. Easier substitution of La by Ce in oxysulfide than oxysulfate could be confirmed by larger changes of lattice constants, a and c, for the rhombohedral cell. Sulfur K-edge EXAFS and IR suggested the Ce substitution probably gained distortion of the tetrahedral SO4 unit of La2O2SO4. The introduction of Ce in the structure promoted the reduction/reoxidation between oxysulfate and oxysulfide to achieve 4−8 times higher rate of oxygen release/storage. XPS analysis on the solid surface demonstrated that Ce was in the form of Ce3+/Ce4+ not only in oxysulfates but also in oxysulfides. The Ce3+/Ce4+ species on the surface are therefore considered to facilitate the reduction/reoxidation of sulfur. The resulting smooth oxygen release/storage of (La1−xCex)2O2SO4 enhanced the catalytic activity for anaerobic CO oxidation at 600 °C under the unsteady cycled feed stream conditions, where the activity of unsubstituted La2O2SO4 was negligible.