An electrochemical study of the oxidation of platinum employing ozone as oxidant and chloride as complexing agent

Abstract

Motor car exhaust catalysts are some of the most important users of platinum, and much attention is given to the recycling of scrap platinum from spent exhaust systems. The dissolution of platinum from waste exhausts was previously only possible by pyrometallurgical processes or by the use of aggressive chemicals, such as aqua regia and cyanide, all of which, however, cause pollution problems. Recently the potential for the development of hydrometallurgical processes was identified. These processes are more efficient and more environmentally friendly than traditional processes. It was the aim of this study to investigate the oxidation of platinum with ozone as oxidizing agent in the presence of chloride as complexing agent. The influence of various factors, such as chloride ion concentration, pH and temperature were studied thermodynamically and electrochemically in order to achieve efficient leaching. The thermodynamic investigation, leading to the construction of Pourbaix diagrams of platinum in the presence of chloride, confirmed the possibility of the formation of stable aqueous complexes, as well as platinum oxides. From the electrochemical results obtained it can be concluded that ozone may be useful as an oxidizing agent, but according to the leaching results obtained, ozone holds few benefits compared to aqua regia and cyanide, although they still present some environmental challenges. The relatively low percentages of recovery obtained with leaching may be attributed to the rapid decomposition of ozone, which is affected by several factors, such as temperature, pH, initial concentration of ozone, ionic strength and stirring rate. Further optimisation of these factors can possibly prove ozone to be a useful oxidizing agent for the recovery of platinum.