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

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Abstract

Substitution of Sn4+ ion in CeO2 creates activated oxygen in Ce0.8Sn0.2O2 leading to higher oxygen storage capacity compared to Ce0.8Zr0.2O2. With Pd ion substitution in Ce0.8Sn0.2O2, activation of oxygen is further enhanced as observed from the H2/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 H2/TPR experiment of Ce1−xSnxO2 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 Pd2+ ion. Substitution of Pd2+ ion thus facilitates synergistic reduction of the catalyst at lower temperature. We have shown that simultaneous reduction of the Ce4+ and Sn4+ ions by Pd0 is the synergistic interaction leading to high oxygen storage capacity at low temperature.

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