Large-capacity oxygen storage by lanthanide oxysulfate/oxysulfide systems

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)(S6+) and Ln(2)O(2)S(S2-) achieved an oxygen storage of 2 (mol of O-2)center dot mol(-1), which is 8 times larger than that of the conventional CeO2-ZrO2 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 degrees 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 SO2/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.