In Situ X-ray Absorption Spectroscopic Study of Nanoparticle Catalysts
X-ray absorption spectroscopy (XAS) is sensitive to the oxidation state and coordination environment of an element and places few constraints on the samples, thus it is a useful technique for the characterization of metal nanoparticles with short-range order. With recent advances in XAS techniques, it is now possible to study materials under in situ conditions. As the synthesis and many catalytic reactions involving nanoparticles occur in solution, in situ XAS is a powerful technique to study the structures and speciation of nanoparticles under real reaction conditions. Fe@FexOy core@shell nanoparticles have been widely studied for environmental remediation and catalysis due to their low cost, relatively low toxicity and magnetic recovery advantages. Using in situ XAS, this thesis details investigations into the oxidation mechanisms of Fe@FexOy nanoparticles, the formation of Fe@FexOy/Pd and Fe@FexOy/Cu nanoparticles, the penetrability of hollow Fe oxide shells via galvanic exchange reactions between the Fe(0) core within the hollow Fe oxide shell and Pd(II) salts, and the metal speciation in these nanoparticles during catalytic reactions. First, Fe@FexOy nanoparticles were synthesized by the reduction of Fe salts in methanol or water/methanol mixtures using a NaBH4 reducing agent under nitrogen gas. Polyvinylpyrrolidone stabilizers and different volume ratios of methanol to water were used to control the sizes of the resulting Fe@FexOy nanoparticles. The relative oxidation kinetics of these different sizes of Fe@FexOy nanoparticles were monitored by in situ Fe K-edge X-ray absorption near-edge structure (XANES) spectra. These Fe@FexOy nanoparticles were also applied as catalysts for the hydrogenation of a variety of alkenes, and their catalytic abilities were compared with Fe nanoparticles synthesized in a tri(hexyl)tetradecylphosphonium chloride ionic liquid. Then, Fe@FexOy nanoparticles were reacted with different molar ratios of Pd(II) and Cu(II) by galvanic exchange reactions to form Fe@FexOy/Pd and Fe@FexOy/Cu bimetallic nanoparticles with different morphologies. The reduction processes of Pd(II) and Cu(II) toward the formation of Fe@FexOy/Pd and Fe@FexOy/Cu bimetallic nanoparticles were studied through using in situ Pd L3-edge and Cu K-edge XANES spectra. In situ XANES results also show that these Fe@FexOy nanoparticles can re-reduce oxidized Pd in Suzuki-Miyaura cross-coupling reactions. As catalysts for the hydrogenation of 2-methyl-3-buten-2-ol, Fe@FexOy/Pd nanoparticles are shown to have a higher catalytic activity in ethanol compared to water for hydrogenation reactions, and in situ XANES experiments reveal that these nanoparticles are more stable in ethanol solutions, whereas further oxidation of the Fe cores occurs in the presence of water. 50:1 or 20:1 Fe@FexOy/Pd nanoparticles could also be used as magnetically recoverable catalysts. Finally, Fe nanoparticles were also obtained by thermal decomposition of Fe pentacarbonyl in the presence of air-free 1-octadecene with oleylamine at 180 °C. Starting with these Fe nanoparticles, in situ high temperature Fe K-edge XANES spectroscopy was used to monitor the formation of hollow Fe oxide nanoparticles from Fe nanoparticles. The core-void-shell Fe-FexOy intermediates during the formation of hollow Fe oxide nanoparticles were captured. Utilizing the incompletely oxidized Fe cores in these core-void-shell structures to reduce Pd(II) to Pd(0), the penetrability of hollow Fe oxide shells was studied.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorScott, Robert; Hu, Yongfeng
CommitteeSteer, Ron; kelly, Timothy; Peak, Derek
Copyright DateMay 2017
X-ray Absorption Spectroscopy