CO Oxidation Catalyzed by Pd-doped BaCeO 3 : Coexistence of Langmuir-Hinshelwood and BaCeO 3 -mediated Mechanisms
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1217-Y02-05
CO Oxidation Catalyzed by Pd-doped BaCeO3: Coexistence of Langmuir-Hinshelwood and BaCeO3-mediated Mechanisms Xiaoying Ouyang1 and Susannah L. Scott1,2 1 Department of Chemistry & Biochemistry, University of California, Santa Barbara, CA 931065080 USA 2 Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080 USA
ABSTRACT The rate law for CO oxidation over Pd-substituted BaCeO3 was studied. Under CO-rich conditions over a range of pressures and temperatures, changing reaction orders for both CO and O2 suggest the coexistence of both Langmuir-Hinshelwood and BaCeO3-mediated mechanisms. The latter dominates at high P(CO)/P(O2), while both mechanisms contribute significantly at low P(CO)/P(O2). Under CO-lean conditions, the Langmuir-Hinshelwood mechanism dominates the kinetics. The importance of the BaCeO3-mediated mechanism increases with temperature. Steady-state isotopic transient kinetic analysis (SSITKA) using 18O2 confirmed the participation of labile lattice oxygen, thus BaCeO3 behaves as a BaO-stabilized form of CeO2.
INTRODUCTION CeO2 has been a major component of automotive exhaust catalysts since the early 1980s [1]. Both CeO2 and its mixed oxides, such as CeO2-ZrO2, are well-known for their oxygen storage capacity (OSC) [1-5] and dynamic oxygen exchange capacity (OEC) [6-9]. The Ce4+/3+ redox couple allows these materials to store and release oxygen under fuel-lean and fuel-rich conditions, respectively. Noble metals are readily dispersed on these oxides, promoting catalytic activity at the metal-support interface [10-15]. Oxidations, such as that of CO [16-20] and soot [21-23], can occur via a “CeO2-mediated mechanism”, making use of lattice oxygen. Doped barium cerate, BaCe1-xMxO3-δ, has widespread use as proton conductor in solid-oxide fuel cells [24-26]. Recently, we showed that BaCeO3 can accommodate low levels of Pd(II) substitution on the perovskite B-sites [27, 28]. Since the solid-state synthesis is conducted at 1000 °C, the B.E.T. surface area is only ca. 1 m2/g. However, the low temperature (
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