Influence of surface Ce/Zr ratio on formation of k-Ce2Zr2O8 superstructure and its application in three-way catalysis

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 k-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 k-Ce2Zr2O8 formation, we investigated the effects of surface Ce/Zr ratio on the formation of k-Ce2Zr2O8 and on the supported Pdonly three-way catalysts. Raman and transmission electron microscopy (TEM) results demonstrate that partial k-Ce2Zr2O8 structure is formed in the three oxides, and the content of k-Ce2Zr2O8 structure increases with increasing surface Ce/Zr ratio, but its stability declines. After thermal aging, the proportions of Ce3+ and oxidized palladium (Pd delta++Pd2+) in the catalysts, which are shown to be positively correlated with catalytic activity, follow the same sequence as the stability of k-Ce2Zr2O8, namely Pd/M0.33ROa > Pd/M0.4ROa > Pd/CeM0.33ROa. In particular, Pd/M0.33RO, which possesses 100% ((Pd delta++Pd2+)/Pd) even after thermal aging at 900 degrees C for 4 h, owing to the strengthened metal-support interaction by the presence of stable k-Ce2Zr2O8, shows the best catalytic performance. This work is instructive to the application of k-Ce2Zr2O8 in three-way catalysis. (c) 2022 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.