Large-Capacity Oxygen Storage by Lanthanide Oxysulfate/Oxysulfide Systems

Abstract

The present work has demonstrated the large-capacity oxygen storage of various isomorphous lanthanide oxysulfates, Ln 2 O 2 SO 4 (Ln = La, Pr, Nd and Sm), which utilize the nonmetallic element (S) as a redox site instead of metallic cations. The reduction by H 2 or hydrocarbons and subsequent reoxidation by O 2 between Ln 2 O 2 SO 4 (S 6+ ) and Ln 2 O 2 S(S 2- ) achieved an oxygen storage of 2 (mol of O 2 )·mol -1, which is 8 times larger than that of the conventional CeO 2 −ZrO 2 material. Although the reversible redox cycle of thermostable Ln 2 O 2 SO 4 with Ln = La, Sm, and Nd was possible only at high temperatures above 700 °C, the Pr system could work at an exceptionally low temperature of ca. 600 °C. Furthermore, the redox of the Pr system could be accelerated in the presence of impregnated noble metals (1 wt % Pd), which supply activated hydrogen as well as oxygen by spillover. Because the elimination of a large amount of sulfate species as SO 2 /O 2 from the bulk crystallites of sulfate precursors yields the macroporous texture of Ln 2 O 2 SO 4 and Ln 2 O 2 S with a high specific surface area, the resultant rapid gas diffusion as well as solid−gas reactions would facilitate the oxygen storage and release processes.