Nucleation and particle growth phenomenon of potassium trans-(dioxo) tetra(hydroxo) osmate
Date
2020
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
In the past decade, there has been a strong environmental and technological initiative to refine large quantities of osmium-rich waste resulting from platinum group metal (PGM) refining endeavors. Of paramount importance as an intermediate in the synthesis of pure osmium metal is potassium trans-(dioxo) tetra(hydroxo) osmate(VI) more commonly referred to as potassium osmate. Despite a wealth of studies regarding silver and gold particle systems, there exists virtually no information on affecting the solid state properties of potassium osmate particle ensembles. The goal of this work is to explore practical means of tailoring the size and morphology of potassium osmate particles to make them attractive for catalytic applications and as precursors for value-add chemistry. The system studied was the reduction of potassium perosmate via a controlled addition of ethyl alcohol. The variables studied that were thought to most heavily impact the properties of the resulting particles were the chemistry of the reaction, the rate of addition of the reducing agent, the order in which the reagents were added, whether or not surfactants were employed, and if ripening the previously precipitated particles was a viable means of influencing properties. ☐ The amount of potassium hydroxide relative to osmium was varied from a ratio of 3 to 55 and it was found that at lower ratios, the mean particle size is roughly three times as large relative to higher ratios. Reactions were performed with ethanol addition times ranging in length from 8 seconds to 5 hours. These trials produced particles that were an order of magnitude different in mean size (7 versus 70 micron respectively) but were all octahedral in geometry and were composed of the same size crystallites. When the order of the reagents was reversed (i.e. adding potassium perosmate solution to ethanol), the material produced was smaller and more spherical in nature. When small amounts of low molecular weight polyvinylpyrolidone was added to the system, there was an improvement in the relative standard deviation associated with dump-style ethanol addition reactions and the lower [OH]/[Os] system without dramatically altering the mean size of the ensembles. Several other surfactants were explored with varying results. In a variety of ethanol, water, and potassium hydroxide matrices, potassium osmate particles were found to be slightly soluble and it was hoped that Ostwald ripening would take place if exposed for a prolonged period of time. This was observed for the reverse addition powder trials where the particles grew from around 3 micron to 5.5 while retaining the same quasi-spherical morphology. These findings were not observed in the larger reference experiment material which featured 30 micron octahedron. When suspended in ripening solvent mixtures, these particles decreased to approximately 5 micron and looked similar to the reverse addition material. These findings may suggest that there is a specific size that resides at an energetic balance between surface and bulk energetic costs. All findings in this work are interpreted in terms of applied thermodynamics and the idea of monomer solubility, supersaturation, and nucleation in continuity with similar particle system investigations recently published.
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Keywords
Osmium, Particle technology, Platinum group metals, Potassium osmate