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, Ln2O2SO4 (Ln = La, Pr, Nd and Sm), which utilize the nonmetallic element (S) as a redox site instead of metallic cations. The reduction by H2 or hydrocarbons and subsequent reoxidation by O2 between Ln2O2SO4(S6+) and Ln2O2S(S2-) achieved an oxygen storage of 2 (mol of O2)·mol-1, which is 8 times larger than that of the conventional CeO2−ZrO2 material. Although the reversible redox cycle of thermostable Ln2O2SO4 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 SO2/O2 from the bulk crystallites of sulfate precursors yields the macroporous texture of Ln2O2SO4 and Ln2O2S 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.