In situ XANES Study of Co 2+ Ion Adsorption on Fe 3 O 4 Nanoparticles in Supercritical Aqueous Fluids
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In situ XANES Study of Co2+ Ion Adsorption on Fe3O4 Nanoparticles in Supercritical Aqueous Fluids Hao Yan1, Robert A. Mayanovic1, Joseph Demster1, and Alan J. Anderson2 1 Department of Physics, Astronomy and Materials Science, Missouri State University, Springfield, MO 65897, USA 2 Department of Earth Sciences, St. Francis Xavier University, P.O. Box 5000, Antigonish, Nova Scotia, B2G 2W5, Canada ABSTRACT In situ x-ray absorption spectroscopy (XAS) measurements were made on Fe3O4 nanoparticles in supercritical aqueous fluids to 500 °C in order to study their reactivity with Co2+ aqua ions and to investigate the structural properties of the reacted nanoparticles. The analyses of the x-ray absorption near edge structure (XANES) of XAS indicate that reactivity of Fe3O4 nanoparticles with Co2+ ions is minimal to 200 °C but becomes significant in the 250–500 °C temperature range. XANES and angular momentum projected density of states (l-DOS) calculations were carried out using the FEFF8.2 code and analyses were made using multi-peak fitting to determine the origin of the features exhibited in the spectra. INTRODUCTION The chemical and physical properties exhibited by nanoscale materials due to high surface area-to-volume ratio have found novel applications. Metal oxide nanomaterials are projected to have an important role in the conversion and storage of energy under extreme conditions (e.g., supercritical water). The spalling of ferrite nanoparticles in power reactors is a specific issue that is directly relevant to this study. The spalling results from the oxidation and corrosion of steel-alloyed materials of the primary water-based cooling loops of conventional and nuclear power reactors [1]. The spalled ferric nanoparticulate matter is transported along with corrosion-product metal ions (e.g., Fe3+, Cr3+, Co2+) in the heated water of the primary cooling loops. The chemical and physical properties (e.g. reactivity, magnetism) of ferrite nanoparticles can be affected by the interactions (e.g., adsorption) between the nanoparticles and hydrated metal ions, depending upon the chemistry of the fluid, the solutes present in the fluid and upon the thermodynamic conditions of the system. In situ spectroscopic investigations of the interactions between solutes and nanoparticles in aqueous fluids are essential in order to model and control corrosion and corrosion product transport in supercritical-water-cooled reactors. Our goal in this study is to determine the in situ structural characteristics of Co2+ adsorbed on the surface region of Fe3O4 nanoparticles under varying P-T conditions in aqueous fluids. Because the adsorbed-ion-nanoparticle aqueous system lacks long-range order, localstructure techniques such as x-ray absorption spectroscopy (XAS) are ideally suited for this study. X-ray absorption near edge structure (XANES) of XAS is useful in ascertaining the chemical nature of Co2+ ions adsorbed on Fe3O4 NPs because of its sensitivity to modification in coordination and/or bonding environment surrounding the target atom.
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