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Solid state chemistry of platinum group and noble metal oxides: Implications for heterogeneous catalysis

Kurzman, Joshua A.
Degree Grantor:
University of California, Santa Barbara.Chemistry
Degree Supervisor:
Seshadri Ram
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Inorganic chemistry

The substitution of precious metal ions in oxide hosts represents an attractive alternative to conventional metal-particle-on-oxide catalysts, potentially providing improvements in efficacy and lifetime while simultaneously reducing the loading of these scarce and expensive elements. This dissertation addresses a range of topics within a theme of understanding the solid state chemistry of platinum group and noble metal ions, with implications for their role in hetero- geneous catalysis. To inform an understanding of the crystal chemistry of precious metal-substituted oxides, we begin by examining stoichiometric complex oxide compounds of isoelectronic Au<super>3+</super> and Pd<super>2+</super>. La<sub>4</sub>LiAuO<sub>8</sub> and La<sub>2</sub>BaPdO<sub>5</sub> display exceptional resistance towards thermal dissociation and chemical reduction, serving as model compounds for understanding the crystal chemical conditions that stabilize these oxophobic elements in oxide environments. This understanding is further borne out in a neutron powder diffraction study of La<sub>4</sub>LiAuO<sub>8</sub> &ndash; the first such study of a stoichiometric (rather than lightly substituted) oxide compound with gold. Recent interest in platinum group metal- (PGM) substitution has been stimulated by findings that certain substituted complex oxides can reversibly extrude and reincorporate the PGM in response to redox changes in the atmosphere to which they are exposed. Of the PGM-substituted complex oxides studied, almost all have been perovskites. Here we present an investigation of Pd<super>2+</super>/Pd<super>0</super> redox cycling in a non-perovskite host, hexagonal YMnO<sub>3</sub>-type YMn<sub>0.5</sub>Fe<sub>0.5</sub>O<sub>3</sub>. YMn<sub>0.5</sub>Fe<sub>0.5&ndash;x</sub>Pd<sub>x</sub>O<sub>3&ndash;d</sub> is only modestly tolerant to removal and redistribution of Pd<super>2+</super>; after only a few cycle cycles PdO is formed, rather than Pd<super>2+</super> being reintegrated into the host. The final study presented here is a computational investigation of electronic structural features in compounds spanning the range of formal oxidation states adopted by gold in extended solids. Guided by the well-established importance of the position at which the transition element d band is centered (relative to the Fermi energy), we find an absence of any correlation between the charge on or oxidation state of Au and the position of the Au 5d-band center. Instead of correlating with the Au charge or oxidation state, the location of the d-band center shows a strong dependence on the degree of its filling.

UCSB electronic theses and dissertations
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