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 O-2. The macroporous CeO2 catalyst (MCeO2) 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-x(n-)) were different according to amount of Ag. The ratio of highly reactive superoxide (O-2(-)) 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-x(n-) 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-x(n-) 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 O-2(-) 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 O-2(-) generation, whereas excessive surface oxygen vacancies hindered the formation of highly reactive O-x(n-).