Recovery of Platinum Group Metals From Spent Catalysts

The invention of the automobile is arguably one of the most significant developments of the 20th century and will continue to shape our culture and economy well into the 21st century. The deterministic effect of the automobile has impacted every facet of our lives. Along with the economic benefits and greater mobility, go greater risks, to people, property and more importantly long-term environmental damage as a result of increased air pollution. These environmental concerns have caused the U.S. Government to act adopting increasingly stringent emission control standards. An integral part of most cars' emission control systems since about 1975 is the catalytic converter. Catalytic converters basically reduce the amount of pollutants in an automobiles exhaust gases through an exothermic chemical reaction (Toboldt, Johnson, & Gauthier, 1995). Located just downstream of the engine's exhaust manifold, the catalytic converter processes the exhaust gases by chemical reaction with the metal catalysts that change pollutants such as carbon monoxide (CO), nitrous oxides (NOX), and hydrocarbons into carbon dioxide and water. The catalysts in a typical catalytic converter consist of either a ceramic honeycomb monolithic structure or ceramic beads coated with precious metals of platinum, palladium and rhodium (commonly called platinum group metals). The cost of catalytic converters and the shear numbers of automobiles on the road make the recovery of these metals a worthwhile endeavor.
Recovery of these metals was the goal of research completed by professors at the Department of Chemistry at the University of Northern Iowa made possible by a grant from the Recycling & Reuse Technology Transfer Center also of the University of Northern Iowa. This research focused on developing a simple and economical separation and recovery method using electrochemical technology (Bartak & Woo, 1996). An electrochemical cell essentially uses a current carrying solution and two electrodes at which oxidation and reduction processes occur as current flows (Adamson, 1986).

First, the platinum group metals and other metal oxides are recovered from the ceramic substrate through the use of an acid/oxidant leaching solution. Next, an electrochemical cell was developed consisting of a cathode made of platinum and titanium and a dimensionally stable anode (Bartak et. al., 1996). The leaching solution is circulated around the cathode and an anolyte solution circulated past the anode. Here electrolysis experiments were carried out at varying current densities to bring about the optimal reduction reaction. As a result, platinum group metals were successfully extracted from the leaching solution and deposited on the cathode. These results showed that up to 80% of the platinum group metals could be recovered in this manner form spent catalysts. Also, it was discovered that the leaching solution could be reused and recycled back into the process after electrochemical deposition of the platinum group metals has occurred.

Adamson, A. (1986) Physical Chemistry. Academic Press College Division. Harcourt Brace Jovanovich, Publishers: New York.
Bartak, D., & Woo, C. (1996). Recovery of Platinum Group Metals From Spent Catalysts. Recycling & Reuse Technology Transfer Center.
Toblodt, W., Johnson, L., & Gauthier, W. 91995). Automotive Encyclopedia. The Goodheart-Wilcox Company Inc., Publishers: South Holland, Illinois.