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

In situ X-ray absorption spectroscopy (XAS) measurements were made on Fe O nanoparticles in supercritical aqueous fluids to study their reactivity with Ni ions and to investigate the structural properties of the reacted nanoparticles. XAS spectra were measured from Ni added to an aqueous solution containing Fe O nanoparticles at temperatures up to 500 °C. The XANES and the EXAFS data indicate that reactivity of Fe O nanoparticles with Ni 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 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 O nanoparticles in the aqueous fluid at 400 and 500 °C. © 2011 American Chemical Society.