Carbon monoxide adsorption, surface speciation and particle size distributions have been studied in platinum, palladium, and rhodium hydrosol systems using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. In transmission electron micrographs of unprotected platinum and palladium hydrosols, particle necklacing believed to arise from sintering during preparation, is apparent. The average particle sizes of all hydrosols studied were in the range of 2 to 12 nm. X-ray photoelectron spectra of the metal hydrosols revealed evidence for (Pt-O)ads, Pt(II) and Pt(IV) oxides on platinum hydrosol particles whereas Pd(II) and Pd(IV) oxides were detected on the surfaces of palladium hydrosol particles. These surface oxides are found to be important in influencing hydrosol surface processes such as CO adsorption as a function of pH, inhibition of CO adsorption by alcohols and surface corrosion products resulting from the addition of iodide and cyanide. Fourier transform infrared spectra of CO-treated metal hydrosols revealed bands due to CO linearly adsorbed on the metal particles at ca. 2070 cm-1 (Pt), 2067 cm-1 (Pd) and 2045 cm-1 (Rh) whereas bands due to bridge-adsorbed (B2) CO were detected at ca.1950 cm-1 (Pd) and 1890 cm-1 (Rh). The use of CO as a spectroscopic probe molecule enabled the study of changes in the surface properties of the metal hydrosols which were induced by changes in the dispersion medium. For example, v(CO)ads was observed to decrease in infrared spectra of CO-treated platinum and rhodium hydrosols as pH was increased by KOH or other dissolved salts yielding alkaline solutions. This suggested a reduction in CO coverage resulting from hydroxyl adsorption and consequent increased oxide growth on the particles. In contrast, CO adsorption on platinum and rhodium hydrosols was enhanced in acidic media possibly as a result of the neutralisation of surface hydroxyls. The spectroscopic behaviour of adsorbed CO on platinum and rhodium hydrosols was only comparable to that of CO adsorbed at an electrode surface in acidic media when protecting agent was present which prevented aggregation of the hydrosol in such media. Inhibition of CO adsorption on platinum hydrosols was induced by the addition of aliphatic alcohols, poly(vinyl alcohol) and poisoning anions such as CN- and SH-. Correlations of v(CO)ads with CO coverage suggested that island formation of adsorbed CO was occurring for CO adsorption on unprotected palladium hydrosols and protected platinum and rhodium hydrosols as a function of pH. In allied investigations, an infrared spectroelectrochemical study of corrosion of a nickel electrode in aqueous cyanide media has revealed that [Ni(CN)4]2- is detected at potentials more cathodic than 200 mV vs. SCE. Cyanide was oxidised to cyanate (OCN-) and then successively to carbon dioxide at potentials more anodic than 200 mV vs. SCE. The appearance of features at 2094 cm-1 (HCN) and 2256 cm-1 (HNCO) were attributed to pH changes associated with the oxidation of cyanide to cyanate. The appearance of a band at ca. 2218 cm-1 in infrared spectra of the thin layer at very high potentials (> 1000 mV vs. SCE) was believed to arise from an unstable nickel(II) isocyanate complex.
| Identifer | oai:union.ndltd.org:ADTP/276011 |
| Date | January 1991 |
| Creators | Mucalo, Michael Roger |
| Publisher | ResearchSpace@Auckland |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated., http://researchspace.auckland.ac.nz/docs/uoa-docs/rights.htm, Copyright: The author |
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