Geometrical and Electronic Structure of Metal Oxide Nanoparticles on Silica and Alumina

• Main Author: Rodrigo, Udaya Indike
• Other Authors: Poliakoff, Erwin
• Format: Others
• Language: en
• Published: LSU 2012
• Subjects: Chemistry
• Online Access: http://etd.lsu.edu/docs/available/etd-06302012-152750/

Recent studies demonstrated nanometer-scale combustion-generated nanoparticles cause demonstrable health effects. It is essential to understand the chemical characteristics of these nanoparticles. Since combustion-generated nanoparticles arise in complex mixtures, we employed a dendrimeric method to generate simpler, more reproducible nanoparticles, which were tethered to fumed silica and alumina to provide surrogates for fly ash. We collected XANES and EXAFS data, which are useful for characterizing disordered mixtures where the metal is a minor component. The effect of particle size on structural properties was studied with copper oxide nanoparticles on fumed silica. XANES data demonstrated copper atoms in the nanoparticles are present in the +2 oxidation state. Spectral features in the near-edge region differ between nanoparticles prepared under fully doped and severely underdoped conditions. Severely underdoped reaction conditions cause agglomeration, consistent with the long-range order seen in the radial distribution functions. Substrate effects on particle formation were studied by changing the support surface. Studies of CuO on fumed alumina suggested copper species existed with a +2 oxidation state and as CuO sites with an oxocation like structure. Similar studies for the TiO2 supported CuO confirmed formation of CuO nanoparticles. These observations suggested dendrimeric formation of copper oxide nanoparticles on support surfaces depend highly upon the selected support substrate. Mixed metal oxide nanoparticles of varied Ni and Cu concentrations were synthesized on fumed silica. X-ray studies found both ions were present with a +2 oxidation state. XANES spectra suggested copper species were slightly electropositive while nickel species were slightly electronegative. Microscopic studies confirmed the EXAFS observation of small size nanoparticles and the presence of both metals inside one nanocluster mimicking mixed metal oxide nanoclusters. The contribution of metal oxide nanoparticles in surface mediated reactions was studied by exposing size-selected nanoparticles to model polychlorinated dibenzofurans/dibenzo-p-dioxins (PCDF/PCDD) precursors. X-ray absorption data for the exposed nanoparticles were compared with the unexposed samples to characterize electronic structure and oxidation state during the process. XANES data for the temperature dependent reaction of 2-chlorophenol with CuO nanoparticles prepared at fully doped condition showed incomplete reduction at 375 oC and good agreement with the previously proposed mechanisms.