Study of active sites and mechanism for soot oxidation by silver-loaded ceria catalyst

The catalytic behaviors of metal oxides and Ag-loaded CeO2 have been studied for soot oxidation by O2 in tight contact condition. Ag2O, as the most active oxide among 30 kinds of metal oxides tested, showed high activity even under inert atmosphere. However, Ag2O completely deactivated after the first catalytic run because of its conversion to Ag metal particle, indicating that Ag2O does not act as a catalyst but as a strong oxidant for soot combustion. Ag(20 wt%)-loaded CeO2 (Ag20Ce), consisted of silver metal nanoparticles on CeO2, showed higher activity than CeO2, and the catalyst showed high durability. Kinetic studies for soot oxidation under O2 and He showed that the activation energy changed in the order CeO2 > Ag2O ≈ Ag20Ce, and activation energy under O2 was close to those under He, indicating that surface oxygen plays an important role in catalytic soot oxidation in the presence of O2 and active oxygen species of Ag20Ce and Ag2O have similar nature. Time-resolved UV–vis analysis of Ce4+/Ce3+ redox kinetics under reducing (H2) and oxidizing (O2) atmosphere showed that the presence of Ag on CeO2 enhanced the reduction of Ce4+ to Ce3+ but did not enhance the re-oxidation of Ce3+. It is concluded that a strong interaction between supported silver nanoparticle and CeO2 surface results in the formation of highly reducible surface oxygen at silver-ceria interface, whose reactivity for soot oxidation is similar to the surface oxygen of Ag2O.