In situ x-ray absorption spectroscopic study of the adsorption of Ni 2+ on Fe 3O 4 nanoparticles in supercritical aqueous fluids

Abstract

In situ X-ray absorption spectroscopy (XAS) measurements were made on Fe 3O 4 nanoparticles in supercritical aqueous fluids to study their reactivity with Ni 2+ ions and to investigate the structural properties of the reacted nanoparticles. XAS spectra were measured from Ni 2+ added to an aqueous solution containing Fe 3O 4 nanoparticles at temperatures up to 500 °C. The XANES and the EXAFS data indicate that reactivity of Fe 3O 4 nanoparticles with Ni 2+ ions is negligible to 300 °C but becomes significant in the 400-500 °C temperature range. Analysis of Ni K-edge extended X-ray absorption fine structure (EXAFS) using an atomic cluster model based on the spinel structure shows that Ni 2+ adsorbs predominantly on octahedral sites of the surface of nanoparticles in aqueous fluids at 400-500 °C. The EXAFS structural results are consistent with diffusion of Ni into the near-surface region of the Fe 3O 4 nanoparticles in the aqueous fluid at 400 and 500 °C. © 2011 American Chemical Society.