Origin of activation of Lattice Oxygen and Synergistic Interaction in Bimetal-Ionic Ce0.89Fe0.1Pd0.01O2−δ Catalyst
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Abstract
Flourite-type nanocrystalline Ce0.9Fe0.1O2−δ and Ce0.89Fe0.1Pd0.01O2−δ solid solutions have been synthesized by solution combustion method, which show higher oxygen storage/release property (OSC) compared to CeO2 and Ce0.8Zr0.2O2. Temperature programmed reduction and XPS study reveal that the presence of Pd ion in Ce0.9Fe0.1O2−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 Ce0.9Fe0.1O2−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 Ce0.89Fe0.1Pd0.01O2−δ catalyst compared to monometal-ionic Ce0.9Fe0.1O2−δ and Ce0.99Pd0.01O2−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 Ce0.89Fe0.1Pd0.01O2−δ is due to low-temperature reduction of Pd2+ to Pd0, followed by Pd0 + 2Fe3+ → Pd2+ + 2Fe2+, Pd0 + 2Ce4+ → Pd2+ + 2Ce3+ redox reaction.
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