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Platinum group metal oxides for heterogeneous catalysis: Novel synthesis and advanced characterization


Platinum group metals (PGMs) are well established and widely used for catalytic processes. It has been demonstrated that PGMs can be superior catalysts for hydrocarbon conversion reactions compared to the industry standard. However, the findings in academic labs cannot always translate directly to industrial usage. One limitation to using PGMs for large scale processes is sometimes cost. Access to an inexpensive and highly efficient catalyst could be a step towards using recovered hydrocarbons, in the form of natural gas and shale gas, more widely for energy production.

While platinum group metal species have been intensively examined,

less is know about reactivity associated with ionic PGMs in oxides.

Using ionic species could be a route to achieving more efficient conversion of mixed hydrocarbon feedstocks to fuels and commodity chemicals.The work presented in this dissertation focuses on Pd-substitution in binary and complex oxides along with model compounds containing noble metals, with the aim of preparing an inexpensive and

robust C-H bond activation catalyst. With an emphasis on preparation methods and careful characterization, it has been a goal of this work to establish structure-property relationships in oxide catalysts.

Initial work on Pd--substitution in CeO2 has lead to the development of ultrasonic spray pyrolysis (USP) as a method for preparing substituted oxides having relatively high surface area. Phase pure Pd--substituted perovskites were also prepared using this technique. Methane partial oxidation reactions on Pd-substituted CeO2 provided some understanding of Pd substitution in oxides. It was determined that ionic Pd when substituted in CeO2 is readily reduced to fcc-Pd. Investigation of more complex oxides that could stabilize ionic Pd under reducing conditions through inductive effects became the target of subsequent research.

Pd-substituted LnFeO3 (Ln = Y, La) showed promising results for increased stabilization of Pd ions under methane partial oxidation conditions. Microwave-assisted heating methods were employed to to prepare these materials very rapidly. With just several minutes of microwave-assisted heating, Pd-substituted perovskite materials were prepared for characterization and testing.

With the help of our collaborators, Pd--substituted LaFeO3 was applied as a catalyst precursor material for aryl chloride coupling under mild conditions.

The focus was shifted to model compounds, noble metal complex oxides, La2BaPdO5 and La2BaPtO5, already having unique noble metal sites. The thermal stability of these complex oxides compared to binary oxides was both an attractive property for probing ionic PGM catalysis and a fascinating feature, worthy of further investigation.

Using density functional theory, the electronic structures of La2BaPdO5 and La2BaPtO5 were compared

to those of the binary PdO and PtO oxides. It was determined that a shift in the O 2p band is responsible for the increased stability in complex oxides.

Through this study of Pd ions, we have developed two novel methods for preparation of substituted and stiochiometric oxide materials. A combination of characterization methods, including X-ray diffraction and X-ray photoelectron spectroscopy, are required to understand the structure of these complicated materials. Often times, more advanced characterization, such as neutron diffraction and extended X-ray absorption fine structure measurements, provide the necessary insights to understanding the structures and properties of oxide catalysts. In this dissertation, preparation and characterization are emphasized for ionic PGM and oxide catalysts.

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