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A study on rhodium and palladium mixed-media sols

Historically the majority of precious metal sols have been prepared in an aqueous medium, while a much smaller proportion have been prepared in various non‐aqueous solvents. Few however have been prepared in mixtures of the two, this paucity being the raison d’être of this study. The goal therefore was to perform a systematic investigation of the changes in the properties of rhodium and palladium sols generated in mixtures of water and another organic cosolvent across a range of volumetric compositions. Six cosolvents were investigated: methanol, dimethylsulfoxide, acetonitrile, N,N‐dimethylformamide, acetone, and tetrahydrofuran. To stabilise the resulting sols against aggregation four protecting agents were used: poly(vinyl pyrrolidone), poly(vinyl alcohol), poly(acrylic acid), and arabinogalactan, which was a newly‐available polysaccharide extracted from the Western Larch tree. To reduce the metals from their precursor salts, sodium borohydride and hydrazine hydrate were used. The resulting colloids were analysed by Transmission Electron Microscopy (TEM), Photon Correlation Spectroscopy (PCS) and Ultraviolet/Visible Spectrophotometry. The size of the primary particles in the resulting sols was shown by TEM to be inversely proportional to the proportion of cosolvent in which they were generated. A theory was postulated to account for these changes based on temporary stabilisation of the metal nuclei by adsorption of cosolvent molecules. The characteristics of the aggregates observed in the TEM micrographs were shown to be independent of the cosolvent proportion, changing progressively but unpredictably across the composition ranges studied for most systems. This behaviour was believed to be caused by changes in the conformations of the protecting agents, which depended nonlinearly on the composition of the dispersion medium in which they were used. The measurements collected in‐situ by both PCS and UV/Vis were considered to be good indicators of the levels of aggregation observed in the sols ex‐situ by TEM, with any disparities between the two sets of measurements rationalised with relative ease. PCS measurements gave average hydrodynamic diameters of approximately 20 nm – 80 nm for sols generated in purely aqueous dispersion media, depending on the metal and protecting agent used. Infrared spectra were collected of carbon monoxide adsorbed to the surface of the particles of the sols in mixed‐media to determine the effect of the cosolvent proportion on ν(CO)ads. Sols generated for the aforementioned work were found to be of insufficient density for this purpose, so a series of high‐density sols were synthesised especially. Adsorbed CO was successfully detected on rhodium sols in mixtures of water and either MeOH, DMF, DMSO or MeCN. In these cases υ(CO)ads and its intensity decreased as the cosolvent proportion was increased. This behaviour was rationalised by competitive adsorption of the cosolvent molecules onto the surface of the colloidal particles, precluding the adsorption of CO molecules. The consequent decrease in surface coverage was presumed to have lead to a loss of dipolar coupling, affecting the stretching frequency. These data were used to support the theory postulated for the decrease in the average particle size of the sols generated in mixed‐media.
Type of thesis
Jaine, J. E. (2011). A study on rhodium and palladium mixed-media sols (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/5295
University of Waikato
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