High-temperature and pressure spectroscopic cell for in-situ XAS study of supercritical fluids at the Australian synchrotron

Abstract

The X-ray Absorption Spectroscopy (XAS) method, including X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) allows to characterise ion hydration and the structure of solutions up to supercritical conditions, and to obtain direct information about the molecular structure and thermodynamic properties of aqueous complexes over wide ranges of temperature, pressure, and fluid composition. In this paper we present the design and implementation of the mAESTRO (Australian Extreme SpecTROscopy) system, a high temperature-pressure (to 60 MPa, 800 °C) autoclave developed for in-situ XANES and EXAFS spectroscopy of supercritical fluids. The system has been tested at the XAS beamline of the Australian Synchrotron, where high quality XAS data have been collected for aqueous Ni(II) chloride complexes at up to 427 °C and 40 MPa. These data show that octahedral Ni(II) complexes (e.g., Ni(H2O)62+) are replaced by tetrahedral or square planar complexes containing a larger number of chloride ligands with increasing salt concentration and temperature. The potential applications of the mAESTRO system include, but are not limited to: ore deposit research and mineral exploration, corrosion, geothermal systems, CO2 geosequestration, chemical engineering systems (e.g., hydrometallurgy) and biological / environmental studies.