Recycling platinum: An ionic liquid solution
- Published: Feb 1, 2012
- Author: David Bradley
- Channels: X-ray Spectrometry
Analyses using energy dispersive spectrometry (EDS) and X-ray diffraction have been used to investigate the electrochemical dissolution of platinum in an ionic liquid. Given the high value of platinum metal and its widespread use in catalysts and fuel cells this discovery could fulfil the growing need for an efficient way to recycle the metal.
The noble metals are being used more and more widely in advanced technologies as nanoscopic components for experimental devices, as catalysts, sensors, and in fuel cells. The scarcity of these metals means there is an increasing urgency to find methods to recycle the metals rather than having to obtain them from virgin sources in the ground. Platinum, in particular, is of increasing utility in proton-exchange membrane (PEM) fuel cells. While developers are investigating ways to reduce the absolute quantities of noble metals needed in these applications without degrading efficiency or performance, recycling could be an important adjunct to this exploratory work.
Now, chemist Jing-Fang Huang and his team at the National Chung Hsing University in Taichung, Taiwan, have introduced a new approach to recycling the metal using ionic liquid solvents.
A noble cause
As with most noble metals, it is difficult to dissolve platinum without recourse to highly corrosive acid mixtures, such as aqua regia, a mixture of concentrated nitric and hydrochloric acids, or "piranha", a highly oxidizing mixture of sulfuric acid and hydrogen peroxide. Electrochemical recycling of precious metals is possible but also requires toxic electrolytes or corrosive media and potentially generates large volumes of toxic gases.
Huang and Chen have side-stepped these various issues by turning to electrochemical dissolution of platinum in a mixture of zinc chloride and the ionic liquid, 1-ethyl-3-methylimidazolium chloride. Ionic liquids are simply salts that remain liquid at ambient temperature, or more loosely at temperatures below around 100 Celsius. They have several advantages over volatile organic solvents in that they are non-volatile, non-flammable and generally non-toxic. They are also thermally stable so even in use do not release toxic vapour. Moreover, because they are ionic by definition they can also conduct an electric current well making them ideally suited to electrochemical applications.
The team has demonstrated proof of principle by dissolving a used platinum electrode in the solvent mixture at 100 Celsius with an applied voltage. The process occurs rapidly over the course of 24 hours or so. The platinum thus dissolved can be removed on a carrier electrode, either as the pure metal or as a zinc alloy, without further treatment. The team suggests the same process should be equally amenable to the other noble metals.
Solving the dissolution problem
"We are doing our best to solve the problem about the effective use of precious metals," says Huang. "The recycling of precious metals is a possible strategy. Even now, we do not think we have found the best process. We will continuously modify the process in order to extend its applications or look for a much better one." There are hundreds if not thousands of possible ion combinations that could be tested to produce suitable ionic liquids based on vast numbers of organic and inorganic cations and anions available to chemists.
The team adds that the recycling process is also primed for converting the noble metal into a form usable in other technologies than the one from which the metal was initially retrieved. "This method can be used to easily prepare a source medium that contains platinum ions for the synthesis of related materials," the team says. "The preparation of catalysts that are made from platinum metal and its alloys?is popularly used in fields such as solar energy and biosensors, and may also be used to fabricate functional electrode materials for related energy or sensing applications," the researchers conclude.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.