An electrochemical investigation of the dissolution of platinum employing AICI3/HNO3

MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

Industrial activities of mankind are feared to damage the environment irretrievably. Especially the release of huge amounts of harmful gases causes concern. In this regard the environmental pollution caused by the one billion motor vehicles on earth is particularly important. The platinum-group metals (PGM) are well known for their catalytic activity. They are used extensively for reducing the amounts of hydrocarbons, carbon monoxide and nitrogen oxides from the exhausts gas emitted by automobiles. In 2012 20% of platinum and 27% of palladium produced were used in the manufacture of catalytic converters. With the increasing use of PGM-containing autocatalysts, the reclaiming of PGMs from spent catalysts has become essential. Particularly attractive hydrometallurgical methods are those based on the use of halide ions e.g. sodium chloride, as complexing agent in conjunction with nitric acid as oxidant. The chemical reactions between mixtures of aluminium chloride and nitric acid have been studied, but the electrochemical reactions have received little attention. The research reported in this dissertation is aimed at providing data relating to the electrochemical behaviour of platinum in mixtures of aluminium chloride and nitric acid. The construction of Pourbaix diagrams of platinum in chloride environments confirmed that the stable chloro-complexes [PtCl4]2- and PtCl6]2-, as well as platinum oxides (PtO and PtO2) could play a role under the experimental conditions employed in this study. From the thermodynamic results it can be concluded that the systems deserving consideration favour high chloride concentrations and high temperatures. Notable anodic reactions found were the adsorption of chloride on the platinum surface and the gradual formation of [PtCl6]2-, followed by the formation of platinum oxides at 1.00 to 1.01 V. The results show that anodic currents diminished with lower chloride concentrations. A seemingly anomalous anodic behaviour at 35 °C and 45 °C could be explained in terms of a competition between platinum oxide formation and the formation of platinum chloro-complexes. Evidence for the following cathodic reduction reactions was found: hydrogen evolution, reduction of dissolved oxygen to hydrogen dioxide (-1.3 V SHE), nitrate ion reduction to nitrite ions (-0.01 V SHE), nitrite ion reduction to nitric oxide (-0.85 V SHE) and reduction of PtO and PtO2 to Pt (at -1.00 V and 1.01 V SHE, respectively). A brief study was undertaken in an attempt to relate the electrochemical results to the leaching of platinum from a virgin automotive exhaust catalyst. The recovery was low for mixtures with low chloride concentrations, which could be expected from the electrochemical polarisation curves obtained in electrolytes with different chloride concentrations. The maximum platinum recovery attained, was 60% at 45 °C in a mixture containing 0.6 M AlCl3 and 0.9 M HNO3.

Masters