Modeling of In-Situ XANES Measured From Co 2+ Ion

1. Modeling of In-Situ XANES Measured From Co2+ Ion Adsorbed on Fe3O4 Nanoparticles in Supercritical Aqueous Fluids Joseph Demster, Robert Mayanovic, Hao Yan  Department of Physics, Astronomy and Materials Science, Missouri State University, Springfield, MO 65897 INTRODUCTION Metal oxide nanoparticles are projected to play a significant role in the conversion and storage of energy in extreme environments. This work has relevance to the spalling of ferrite nanoparticles, which results from the oxidation and corrosion of steel-alloyed pipes and reactor vessel walls of water-cooled conventional and nuclear power reactors. The reactivity, structural, magnetic and other physical properties of ferrite nanoparticles can be affected by the interactions (e.g., adsorption) between the nanoparticles and other corrosion byproducts (e.g., Fe3+, Ni2+, Co2+), depending upon the thermodynamic conditions of the system. In situ spectroscopic investigations (including XAS) of the solute and nanoparticle interactions in aqueous fluids to supercritical conditions are needed to model and mitigate corrosion and corrosion product transport in supercritical-water-cooled reactors. Magnetite (Fe3O4 ) has the inverse spinel crystal structure as shown in Fig. 2. The red atom represents a Co2+ ion within the cluster. The surrounding environment in the model includes: Fe+tetrahedrally coordinated to four oxygen atoms and Fe2+ octahedrally coordinated to six oxygen atoms. Conclusions The pre-edge peak A in the Co K-edge XANES data stems from 1s to 3d quadropole and 1s to 3d-4p hybridized state dipole electronic transitions. LDOS calculations show contribution from p and some s-DOS overlapping with the d-DOS in the energy region coinciding with Peak A. Peak B in the XANES data contains some contribution from s and d-DOS but is strongly dominated by p-DOS . From the LDOS calculations, it can be seen that the shoulder in the low energy region of the white line is a contribution of the s-state transition. The calculations show that p-DOS contributes predominantly to the white line transition (C). LDOS calculations show that point D in the XANES data is mainly dominated by d and p-DOS with a small contribution from s-DOS. The XANES modeling data are consistent with our EXAFS results showing that the Co2+ion is adsorbed in the near surface region of the Fe3O4NPs. Fig.1 The cross-section (and thus scattering amplitude) of a bound atom depends on the positions and types of neighboring atoms.3 Fig. 2. Fe3O4 cluster with Co2+ ion adsorbed on the surface. Picture courtesy of web minerals4 EXPERIMENTAL In situ x-ray absorption spectroscopy (XAS) was made at the ID20 PNC/XOR beamline of the Advanced Photon Source (APS) on 0.05m Co(NO3)2 added to Fe3O4 NPs (~24 nm dia.) in aqueous fluids to 500 °C. The XAS measurements were made in fluorescence mode on the NP samples using our modified hydrothermal diamond anvil cell. XANES MODELING Full multiple scattering was carried out using the ab initio software Feff 8.21 and an atomic model consisting of a Co2+ (x-ray photon) absorbing ion substituting for one of the octahedrally coordinated Fe atoms within a magnetite (Fe304) cluster2. For data collected at 400 oC, the model consisted of 41 atoms within the first 8 symmetric shells of nearest neighboring atoms. For data at 500 oC the model consisted of 34 atoms within the first 4 symmetric shells of nearest neighboring atoms. Fig. 3. XANES spectra of Co:Fe3O4 NPs at 400 oC. Fig. 4. XANES spectra of Co:Fe3O4 NPs at 500 oC. REFERENCES: 1. Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys. Rev. B1998, 58, 7565 2. Atoms code: www.cars9.uchicago.edu/ifeffit/BruceRavel/Horae 3. Bruce Ravel, http://feff.phys.washington.edu/~ravel/ 4. www. webmineral.com Support provided as part of the EFree, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001057. Fig. 5. Angular momentum projected local density of states for Co: Fe3O4 NPs at 400 oC. Fig. 6. Angular momentum projected local density of states for Co: Fe3O4 NPs at 500 oC.