| The incorporation of Pd into crystalline complex oxide hosts (e.g., perovskite oxides) facilitates structure characterization and the direct comparison of experiment with theory. The low surface areas of perovskites, a result of their high-temperature synthesis, do not preclude high catalytic activity, and their high thermodynamic stability may be a significant advantage. Pd-substituted BaCeO3 catalyst shows much higher turnover frequency for CO oxidation than conventional catalysts, such as Pd/Al2O3. The high mobility of perovskite lattice (surface and bulk) oxygen, caused by the increased vacancy concentration associated with the presence of square-planar Pd(II) in the bulk, is the key to promoting oxidation reactions at surface Pd(II) sites. The CO oxidation mechanism is highly dependent on the ratio of CO and O2. When O2 is present in excess, the kinetics show CO inhibition, consistent with a Langmuir-Hinshelwood mechanism. When O 2 is limiting, the reaction orders for both CO and O2 show a strong dependence on P(CO)/P(O2), and eventually become independent of both P(CO) and P(O2) at high P(CO). This suggests a new, BaCeO3-mediated mechanism which dominates at high P(CO)/P(O2).;The oxidation of soot catalyzed by Pd-substituted BaCeO3 further implies that the lattice oxygen is the important oxygen source for oxidation reactions. The isotopic study of soot oxidation shows that most of the active oxygen species are obtained from the labile lattice oxygen, instead of the gaseous O2. The oxygen vacancies generated in depletion of lattice oxygen can be immediately refilled by gaseous O2 through simple heteroexchange.;Pd-substituted BaCeO3 is a superior alternative to the most studied NSR catalyst, described as a Pt/Rh/BaO/Al2O3, which is vulnerable to sintering and sulfur poisoning. Both BaO and CeO 2 are stabilized against deactivation when combined as BaCeO3, and Pd is present as cationic active sites for NOx reduction instead of metallic nanoparticles. NOx is stored on the catalyst as Ba(NO 3)2 with partial loss of the perovskite structure, and released as N2 by reduction with CO. The structure of Pd-substituted BaCeO 3 is readily re-formed upon heating to around 800 °C. Despite its low surface area (3.7 m2/g), BaCe0.9Pd0.1O 2.9 displays a very high NOx storage capacity (NSC), 1220 &mgr;mol/g. The Pd-substituted perovskite is also an excellent NOx storage-reduction (NSR) catalyst. Using CO as the reductant, the NOx conversion of fresh BaCe0.9Pd0.1O2.9 is comparable to that of a typical benchmark catalyst, 1%Pt-20%Ba/Al2O3, in the temperature range 250-600 °C.;Besides gas reactions, Pd-substituted BaCeO3 was found to be very active in Suzuki coupling (TOF ca. 50,000) and produce very low levels of residual Pd (< 0.05 ppm) in the reaction medium. The oxidative forms of the catalyst showed higher activity than the reduced form, because of the inaccessibility of the Pd(0). The catalyst exhibited little decline in activity after being recycled seven times, suggesting that perovskite may recapture Pd(II) from the reaction medium. |
