Dissolution of metallic iridium powders in hydrochloric acid oxidizing media

Abstract

Iridium is one of the most chemically inert platinum group metals. It is extremely resistant to many reagents, including alkaline and mineral acid solutions. This, combined with the physical properties of iridium, defines the spectrum of its practical applications. This paper demonstrates that metallic iridium powders of various dispersion degrees can be dissolved in hydrochloric acid media in one stage in the presence of hydrogen peroxide. The morphology, as well as the phase and chemical compositions were studied by means of transmission and scanning electron microscopy, X-ray diffraction analysis, adsorption measurements of specific surface area and chemical analysis. The study looked at refined (30–150 μm particles), highly dispersed (150–200 nm) and nanocrystalline (8–15 nm) iridium. It also examined the dissolution kinetics of iridium(0) with different specific surface areas at the temperatures of 190 to 210 oC. For all the studied samples, the dissolution process was established to be of kinetic nature, and the obtained experimental data can be perfectly described with the shrinking core model. The dissolution activation energy for refined, highly dispersed and nanocrystalline iridium(0) in hydrochloric acid solutions in the presence of hydrogen peroxide is 145, 108 and 88 kJ/mol, correspondingly. This way, it was proved that the activation energy tends to go down as the particles get smaller. It was confirmed that iridium is present in solutions in the form of chloride complexes of quadrivalent iridium. Thus, metallic iridium was converted into chloride forms in an environmentally friendly way, which makes it easier to perform analysis for refining purposes, as well as further synthesis of iridium complexes. This research was carried out as part of a governmental assignment allocated to the Institute of Chemistry and Chemical Technology at the Siberian Branch of the Russian Academy of Sciences (Project 0287-2021-0014) and using the equipment of the Krasnoyarsk Regional Shared Knowledge Centre, a part of the Krasnoyarsk Science Center of the Siberian Branch of the Russian Academy of Sciences.