In-situ DRIFTS study of CeO2 supported Rh catalysts for N2O decomposition

Ceria supported Rh catalysts exhibit superior activities for the decomposition of N2O in the presence of water and excess 02 comparing with other oxides supported Rh catalysts. In contrast to Rh/gamma-AL(2)O, hydrothermal aging (750 degrees C for 20 h with 10% H2O in air) significantly enhances the catalytic activity of Rh/CeO2 for the decomposition of N2O. The presence of H2O, on the other hand, inhibits the reaction rate on all Rh catalysts, but this inhibition effect is less on the aged Rh/CeO2 than on its fresh state. The focus of this work is to explore the nature of this activity enhancement on the aged Rh/CeO2 catalyst and the effect of water. Ceria and alumina supported Rh catalysts were characterized by in-situ DRIFTS with CO and N2O as probe molecules to investigate the synergism between Rh species and ceria support and the interaction between water molecules and the Rh/CeO2 catalysts. The results suggest that Rh supported on ceria has more CO adsorption sites and better hydrothermal stability than that on alumina. On Rh/CeO2 catalysts, the surface Rh species was found to facilitate the reduction of Ce4+ ions to Ce3+ at the interfacial area and as a result increased the concentration of Ce3+ species relative to the CeO2 support after a reduction treatment. Ce3+ ions at the interface between Rh and CeO2 support can be easily oxidized to Ce4+ by N2O at room temperature and played an important role in the disassociation of N2O. After a hydrothermal aging treatment on Rh/CeO2, more surface Rh sites were found that can adsorb CO and N2O. In addition, on the aged Rh/CeO2 catalyst, a different type of Rh species was found to be situated in a more electron-rich environment. These combined factors may be responsible for its enhanced N2O decomposition activity on the aged Rh/CeO2 catalyst. DRUFS were also collected on fresh and aged Rh/CeO2 catalysts with different degrees of sample hydration. The intensity of the OH group at 3658 cm(-1) over an aged catalyst was found to decrease with increasing temperature at a faster rate than that on the fresh catalyst. This finding may explain their difference in H2O sensitivity for the N2O decomposition reaction.