Origin of activation of Lattice Oxygen and Synergistic Interaction in Bimetal-Ionic Ce_0.89Fe_0.1Pd_0.01O_2−δ Catalyst

Flourite-type nanocrystalline Ce 0.9 Fe 0.1 O 2−δ and Ce 0.89 Fe 0.1 Pd 0.01 O 2−δ solid solutions have been synthesized by solution combustion method, which show higher oxygen storage/release property (OSC) compared to CeO 2 and Ce 0.8 Zr 0.2 O 2 . Temperature programmed reduction and XPS study reveal that the presence of Pd ion in Ce 0.9 Fe 0.1 O 2−δ facilitates complete reduction of Fe 3+ to Fe 2+ state and partial reduction of Ce 4+ to Ce 3+ state at temperatures as low as 105 °C compared to 400 °C for monometal-ionic Ce 0.9 Fe 0.1 O 2−δ . Fe 3+ ion is reduced to Fe 2+ and not to Fe 0 due to favorable redox potential for Ce 4+ + Fe 2+ → Ce 3+ + Fe 3+ reaction. Using first-principles density functional theory calculation we determine M−O (M = Pd, Fe, Ce) bond lengths, and find that bond lengths vary from shorter (2.16 Å) to longer (2.9 Å) bond distances compared to mean Ce−O bond distance of 2.34 Å for CeO 2 . Using these results in bond valence analysis, we show that oxygen with bond valences as low as −1.55 are created, leading to activation of lattice oxygen in the bimetal ionic catalyst. Temperatures of CO oxidation and NO reduction by CO/H 2 are lower with the bimetal-ionic Ce 0.89 Fe 0.1 Pd 0.01 O 2−δ catalyst compared to monometal-ionic Ce 0.9 Fe 0.1 O 2−δ and Ce 0.99 Pd 0.01 O 2−δ catalysts. From XPS studies of Pd impregnated on CeO 2 and Fe 2 O 3 oxides, we show that the synergism leading to low temperature activation of lattice oxygen in bimetal-ionic catalyst Ce 0.89 Fe 0.1 Pd 0.01 O 2−δ is due to low-temperature reduction of Pd 2+ to Pd 0, followed by Pd 0 + 2Fe 3+ → Pd 2+ + 2Fe 2+, Pd 0 + 2Ce 4+ → Pd 2+ + 2Ce 3+ redox reaction.