Structural Investigation of Activated Lattice Oxygen in Ce_1−xSn_xO₂ and Ce_1−x−ySn_xPd_yO_2−δ by EXAFS and DFT calculation

Substitution of Sn 4+ ion in CeO 2 creates activated oxygen in Ce 0.8 Sn 0.2 O 2 leading to higher oxygen storage capacity compared to Ce 0.8 Zr 0.2 O 2 . With Pd ion substitution in Ce 0.8 Sn 0.2 O 2, activation of oxygen is further enhanced as observed from the H 2 /TPR study. Both EXAFS analysis and DFT calculation reveal that in the solid solution Ce exhibits 4 + 4 coordination, Sn exhibits 4 + 2 + 2 coordination and Pd has 4 + 3 coordination. While the oxygen in the first four coordination with short M−O bonds are strongly held in the lattice, the oxygens in the second and higher coordinations with long M−O bonds are weakly bound, and they are the activated oxygen in the lattice. Bond valence analysis shows that oxygen with valencies as low as 1.65 are created by the Sn and Pd ion substitution. Another interesting observation is that H 2 /TPR experiment of Ce 1− x Sn x O 2 shows a broad peak starting from 200 to 500 °C, while the same reduction is achieved in a single step at ∼110 °C in presence Pd 2+ ion. Substitution of Pd 2+ ion thus facilitates synergistic reduction of the catalyst at lower temperature. We have shown that simultaneous reduction of the Ce 4+ and Sn 4+ ions by Pd 0 is the synergistic interaction leading to high oxygen storage capacity at low temperature.