Origin of activation of lattice oxygen and synergistic interaction in bimetal-ionic Ce_0.89Fe_0.1Pd_0.01O_2-δ catalyst

Abstract

Flourite-type nanocrystalline Ce_0.9Fe_0.1O_2-δ and Ce_0.89Fe_0.1Pd_0.01O_2-δ solid solutions have been synthesized by solution combustion method, which show higher oxygen storage/release property (OSC) compared to CeO₂ and Ce_0.8Zr_0.2O₂. Temperature programmed reduction and XPS study reveal that the presence of Pd ion in Ce_0.9Fe_0.1O_2-δ facilitates complete reduction of Fe³⁺ to Fe²⁺ state and partial reduction of Ce⁴⁺ to Ce³⁺ state at temperatures as low as 105 °C compared to 400 °C for monometal-ionic Ce_0.9Fe_0.1O_2-δ. Fe³⁺ ion is reduced to Fe²⁺ and not to Fe0 due to favorable redox potential for Ce⁴⁺ + Fe²⁺ → Ce³⁺ + Fe³⁺ 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 CeO₂. 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/H₂ are lower with the bimetal-ionic Ce_0.89Fe_0.1Pd_0.01O_2-δ catalyst compared to monometal-ionic Ce_0.9Fe_0.1O_2-δ and Ce_0.99Pd_0.01O_2-δ catalysts. From XPS studies of Pd impregnated on CeO₂ and Fe₂O₃ oxides, we show that the synergism leading to low temperature activation of lattice oxygen in bimetal-ionic catalyst Ce_0.89Fe_0.1Pd_0.01O_2-δ is due to low-temperature reduction of Pd²⁺ to Pd⁰, followed by Pd⁰ + 2Fe³⁺ → Pd²⁺ + 2Fe²⁺, Pd⁰ + 2Ce⁴⁺ → Pd²⁺ + 2Ce³⁺ redox reaction.