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Abstract
This thesis investigates the synthesis, characterization, and catalytic behavior of metal oxide-modified platinum nanoparticles. Synthesis of the Pt nanoparticles used a traditional method for reduction of a Pt salt precursor. This H2(g) driven reduction was performed in the presence of precursors to, or fully synthesized, polyoxometalates. Molybdate, molybdate at high concentration and low pH, or phosphododecamolybdate served as stabilizers in these studies.
Characterization with electron microscopy revealed that nanoparticle samples consisted of Pt cores surrounded by molybdenum-oxide overlayers. Sizes for the central Pt core were typically in the 7–12 nm range depending on the identity of molybdenum-oxide stabilizer. The thickness of the amorphous metal-oxide overlayer was also found to vary with the stabilizer identity. Standard molybdate syntheses produced small, yet detectable, amounts of molybdenum on the Pt nanoparticle. On the other hand, the high molybdenum concentration–low pH samples had thick (up to 4 nm) metal-oxide overlayers with global Mo content as high as 31%. For all samples, the oxidation state determined with XPS were predominately Mo6+ and Pt0. Voltammetric characterization revealed electrochemical signatures for both the oxometalate overlayer and Pt core.
Catalytic studies using all types of molybdenum oxide-modified nanoparticles were performed. The studies focused on methanol electro-oxidation and gas-phase dehydrogenation of cyclohexene. Molybdate-modified particles exhibited lower peak currents in methanol oxidation but appear to be more resistant to catalyst poisoning than bare Pt particles. In cyclohexene dehydrogenation, the naked Pt nanoparticles exhibited the highest absolute conversion to benzene. However, the modified Pt nanoparticles with high molybdenum content had higher selectivity for benzene (>80%) then did the naked Pt sample (∼60%).
To explore surface enhancement effects in Raman spectroscopy, colloidal gold particles (11.2 nm) were coated with platinum overlayers resulting in bimetallic particles with 1–8 mole percentage Pt. These particles were characterized using electron microscopy, energy-dispersive X-ray spectroscopy and electrochemistry, confirming the Au-Pt core-shell structure. The UV-visible spectra showed only minimal red shifts with increasing Pt content. Raman spectra, on the other hand, changed dramatically with slight increases in Pt, resulting in a loss of appreciable surface-enhancement effects by 4 mole percent Pt.
