Pd2+/Pd0 Redox Cycling in Hexagonal YMn0.5Fe0.5O3: Implications for Catalysis by PGM-Substituted Complex Oxides

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• 作者：Joshua A. Kurzman ; Jun Li ; Thomas D. Schladt ; Ce虂sar R. Parra ; Xiaoying Ouyang ; Ryan Davis ; Jeffrey T. Miller ; Susannah L. Scott ; Ram Seshadri
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：September 5, 2011
• 年：2011
• 卷：50
• 期：17
• 页码：8073-8084
• 全文大小：1196K
• 年卷期：v.50,no.17(September 5, 2011)
• ISSN：1520-510X

文摘

Complex oxides鈥攃ontaining at least two different cations on crystallographically distinct sites鈥攈ave recently been shown to display redox cycling of platinum group metals (PGMs), such as Pd; for example, Pd-substituted complex oxides can reversibly extrude metallic Pd under reducing conditions and then reincorporate Pd²⁺ ions into the lattice under oxidizing conditions. The title compounds, YMn_0.5Fe_0.5鈥?i>xPd_xO_{3鈭捨?/sub> (0 鈮?x 鈮?0.07) crystallizing in the noncentrosymmetric YMnO3 structure, were prepared using a sol鈥揼el process at 800 掳C, and the structures were refined from high-resolution synchrotron X-ray powder diffraction data. Their redox cycling behavior was monitored using synchrotron X-ray diffraction and EXAFS studies. In contrast to the previously studied complex oxide host compounds, YMn0.5Fe0.5鈥?i>xPdxO3鈭捨?/sub> is only modestly tolerant to cycling: repeated redox cycling leads to the formation of PdO, which, on the time-scale of the oxidation cycles, does not reincorporate in the complex oxide lattice. Both oxidized and reduced samples were tested for the oxidation of CO to CO2 under CO-lean conditions. YMn0.5Fe0.5鈥?i>xPdxO3鈭捨?/sub> performs essentially as well as previously studied YFe1鈥?i>xPdxO3鈭捨?/sub>. The CO oxidation light-off characteristics of the hexagonal hosts are very similar to finely dispersed PdO. Despite evidence that Pd is almost fully dispersed as divalent ions in the host lattice, which is presumably accompanied by the concurrent creation of oxygen vacancies (2 Pd²⁺:1 VO^2鈥?/sup>), the as-prepared hexagonal materials do not display any significant improvement in catalytic activity as a function of Pd substitution level. This suggests that the corner-connected trigonal bipyramids that characterize this structural family do not enable the transport of oxygen through the bulk of the lattice. The study casts light on factors in the solid-state chemistry of precious metal-substituted complex oxides that influence the efficacy of redox cycling of the precious metal, and catalytic performance.}