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

Abstract

Substitution of Sn⁴⁺ ion in CeO₂ creates activated oxygen in Ce_0.8Sn_0.2O₂ leading to higher oxygen storage capacity compared to Ce_0.8Zr_0.2O₂. With Pd ion substitution in Ce_0.8Sn_0.2O₂, 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 Ce_1-xSn_xO₂ 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.