Perovskite BaCeO 3 as a New Redox Host for Catalytic Pd Nanoparticles
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0988-QQ09-10
Perovskite BaCeO3 as a New Redox Host for Catalytic Pd Nanoparticles Jun Li and Ram Seshadri Materials Research Laboratory, University of California, Santa Barbara, CA, 93106
ABSTRACT We have examined the ability of a carefully chosen perovskite, BaCeO3, to act as a redox host for noble metals, notably Pd, in the hope of producing an "intelligent" catalyst where palladium is absorbed into the host lattice as ions under oxidative conditions and released as elemental Pd under reducing conditions. Pd-substituted perovskites BaCe1-xPdxO3-δ (0 ≤ x ≤ 0.1) were prepared by solid-state reactions in pure oxygen. The crystal structure was refined in the orthorhombic space group Pnma. Palladium was found to be driven in and out of the perovskite lattice upon repeated redox cycles by detailed XRD study. SEM/EDX revealed the formation of perovskite nanowires and nanorods when reducing Pd-substituted BaCeO3 up to 1000 °C. INTRODUCTION Catalytic converter, a key component of modern automobiles, has been used to reduce air pollution caused by auto exhaust gases since 70’s. The catalysts inside the catalytic converters are generally ceramic supported precious metal fine particles. One of the drawbacks of these conventional catalysts is that their activity tends to decrease with time in use due to the sintering of the metal particles when exposed to heat. Perovskite-type mixed oxides have been known for their catalytic properties for many years [1,2]. Recently Nishihata and coworkers demonstrated using X-ray diffraction and absorption that LaFe0.57Co0.38Pd0.05O3 is a self-regenerative Pdperovskite catalyst, or an “intelligent catalyst” [3,4], wherein palladium can reversibly move into and out of the host oxide in response to the redox fluctuation encountered in exhaust gas atmosphere. The self-regeneration process seems to suppress the sintering of the metallic Pd particles, conserve their high catalytic surface area and hence improve the durability of the catalyst. LaFe0.95Pd0.05O3 was found to be similarly self-regenerative by the same group [5,6]. In this work we chose BaCeO3 as a new perovskite host for palladium because Ba and Ce are large for Pd incorporation, the oxygen atoms in BaBO3 are very oxidizing and therefore help to stabilize Pd2+, and Ce4+/Ce3+ couple allows some redox flexibility. Our XRD study clearly shows that palladium incorporates in the host perovskite lattice as cations under oxidizing conditions, and that it is released as elemental Pd nanoparticles under reducing conditions. The regeneration of Pd is repeatable through many redox cycles. EXPERIMENT The BaCe1-xPdxO3-δ (0 ≤ x ≤ 0.1) perovskite samples were prepared by mixing 1 ∼ 2 g of BaO2 (99%, Cerac), CeO2 (99.9%, Cerac) and PdO (99.95%, Cerac) in a stoichiometric ratio. The powder mixtures were ground, pelletized and calcined in oxygen flow at 1000 ºC for 10 h,
with intermediate grinding and reheated for another 10 h. The as-prepared samples are also referred to as oxidized samples. Pd metal was used in place of PdO as well for substituti
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