Ag-incorporated macroporous CeO₂ catalysts for soot oxidation: Effects of Ag amount on the generation of active oxygen species

A series of CeO₂-based catalysts were investigated for soot oxidation with O₂. The macroporous CeO₂ catalyst (M-CeO₂) showed higher soot oxidation activity than mesoporous CeO₂ due to the enhanced contact between catalyst and soot caused by the large pore size of M-CeO₂. Moreover, various amounts of Ag (2–20 wt.%) were introduced to M-CeO₂ to increase the activity, and the Ag-incorporated macroporous CeO₂ catalysts (Ag(x)_M-CeO₂) were characterized. Raman spectra showed that the ratio of active oxygen species (O_xⁿ⁻) were different according to amount of Ag. The ratio of highly reactive superoxide (O₂⁻) was largest for the Ag(5)_M-CeO₂ catalyst, and then it was decreased as the Ag amount increased further. In addition, XPS analysis showed that the difference in O_xⁿ⁻ generation could be attributed to diverse surface oxygen vacancies in the catalysts. Hence, the amount of loaded Ag affected the surface oxygen vacancies of Ag(x)_M-CeO₂ and consequently the ratio of O_xⁿ⁻ was different according to the surface oxygen vacancies of the catalysts. Ag(5)_M-CeO₂ with the appropriate surface oxygen vacancies induced the promotion of O₂⁻ generation, resulting in the best soot oxidation activity. It was concluded that the amount of Ag on CeO₂ has a great influence on the catalytic soot oxidation activity, and proper surface oxygen vacancies facilitated O₂⁻ generation, whereas excessive surface oxygen vacancies hindered the formation of highly reactive O_xⁿ⁻.