La[subscript 4]LiAuO[subscript 8] and La[subscript 2]BaPdO[subscript 5]: Comparing Two Highly Stable d[superscript 8] Square-Planar Oxides

Abstract

La{sub 4}LiAuO{sub 8} and La{sub 2}BaPdO{sub 5}, two previously known oxides, are presented as model compounds for examining the role of isolated and immobilized Au{sup 3+} and Pd{sup 2+} ions in heterogeneous catalysis. Structural characterization, stability, surface composition, and electronic structure of these compounds are presented. These are examined in studies ranging from synchrotron X-ray scattering, including pair distribution function (PDF) and maximum entropy method (MEM) analysis, to density functional calculations of the electronic structures. The exceptional stability displayed by these compounds as verified by thermogravimetric analysis can be attributed to the presence of covalent Au-O and Pd-O interactions revealed in MEM studies, which suggests a criterion for stabilizing these highly oxophobic transition metals in oxide environments. Catalytic testing of the two compounds as heterogeneous catalysts in the oxidation of CO to CO{sub 2} are presented. La{sub 2}BaPdO{sub 5} appears to be an effective catalyst for CO oxidation, despite the low surface area of the oxide being used. This is the first time that a fully ordered (rather than doped) Pd{sup 2+} oxide had been used to catalyze CO oxidation. La{sub 4}LiAuO{sub 8} on the other hand, is much less effective at catalyzing CO oxidation. Differences in the reactivities of the two compounds are discussed with respect to differences in their density functional electronic structures.