Composition and structure of Pt chloride complexes in hydrothermal solutions, according to X-ray absorption spectroscopy

Abstract

The local atomic environment of Pt in chloride solutions is studied at 25 and 300–350°C via X-ray absorption spectroscopy. A comparison of the Pt L₃-edge XANES spectra for aqueous chloride solutions and reference compounds (K₂PtCl₄ and K₂PtCl₆) shows that platinum is in oxidation state +2 at all temperatures, and the complex composition remains unchanged in the experimental range of temperatures (25–350°С) and solution compositions (m(Cl_tot) > 0.4 mol kg⁻¹ of H₂O). Based on EXAFS spectral analysis, the composition of the complex being dominant in chloride solutions is found to be PtCl₄^2- with interatomic Pt–Cl distances of 2.31 ± 0.01 Å, regardless of the temperature. It is concluded that the local environments of Pt and Pd in hydrothermal solutions are similar, and the main form of transfer for these metals is a square-planar complex with four Cl ligands and identical interatomic distances.