Abstract
Ceria-zirconia based (CZ) materials are still a crucial component of three-way catalysts (TWCs) now. Compared with traditional cubic or tetragonal phase, CZ material with pyrochlore-type κ-Ce2Zr2O8 phase shows an excellent oxygen storage capacity, and thus is examined as a promising material for developing TWCs. Considering the remarkable surface Ce/Zr ratio change during the κ-Ce2Zr2O8 formation, we investigated the effects of surface Ce/Zr ratio on the formation of κ-Ce2Zr2O8 and on the supported Pd-only three-way catalysts. Raman and transmission electron microscopy (TEM) results demonstrate that partial κ-Ce2Zr2O8 structure is formed in the three oxides, and the content of κ-Ce2Zr2O8 structure increases with increasing surface Ce/Zr ratio, but its stability declines. After thermal aging, the proportions of Ce3+ and oxidized palladium (Pdδ++Pd2+) in the catalysts, which are shown to be positively correlated with catalytic activity, follow the same sequence as the stability of κ-Ce2Zr2O8, namely Pd/M0.33ROa > Pd/M0.4ROa > Pd/CeM0.33ROa. In particular, Pd/M0.33RO, which possesses 100% ((Pdδ++Pd2+)/Pd) even after thermal aging at 900 °C for 4 h, owing to the strengthened metal-support interaction by the presence of stable κ-Ce2Zr2O8, shows the best catalytic performance. This work is instructive to the application of κ-Ce2Zr2O8 in three-way catalysis.
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