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

A novel oxygen storage material consisting of Ce-substituted La-oxysulfate, (La 1− x Ce x ) 2 O 2 SO 4 (0 ≤ x ≤ 0.2), was synthesized via a surfactant-assisted route. The layered mesophase templated by dodecyl sulfate anion (DS = C 12 H 25 OSO 3 − ) 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 Ce 3+ /Ce 4+ compared to that of La 3+ . Although part of Ce was deposited as CeOSO 4 and/or CeO 2 on the oxysulfate, subsequent reduction in H 2 yielded single phases of Ce-substituted oxysulfides, (La 1− x Ce x ) 2 O 2 S, 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 SO 4 unit of La 2 O 2 SO 4 . 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 Ce 3+ /Ce 4+ not only in oxysulfates but also in oxysulfides. The Ce 3+ /Ce 4+ species on the surface are therefore considered to facilitate the reduction/reoxidation of sulfur. The resulting smooth oxygen release/storage of (La 1− x Ce x ) 2 O 2 SO 4 enhanced the catalytic activity for anaerobic CO oxidation at 600 °C under the unsteady cycled feed stream conditions, where the activity of unsubstituted La 2 O 2 SO 4 was negligible.