Ultra-Trace Analysis of Palladium: State-of-the-Art and Future Challenges

Abstract

Due to the introduction of the three-way catalytic converter (TWC) in Europe in the mid-eighties, determination of platinum group elements (PGE) in environmental compartments has received elevated interest over the past two decades. While improving air quality by converting hydrocarbons, carbon monoxide, and nitrogen oxides into the less harmful substances carbon dioxide, nitrogen, and water, the use of these devices constitutes a new source for the widespread emission of PGE into the environment. Catalytic converters are composed of a monolithic support surrounded by a mat and tightly packed into a stainless steel housing. The monolith has a honeycomb structure traversed by straight, square channels and consists of a ceramic or sometimes metallic material, coated with the so called washcoat. To carry out the catalytic reactions, PGE are dispersed throughout the washcoat, which typically has a thickness of about 10-30 μm at the walls and 100-150 μm at the corners of the support channels and contains as major constituents alumina and other oxides. While in the early years the classical Pt/Rh catalyst dominated the market, Pd was successively used to replace Rh and later Pt, leading finally to the development of the Pd-only catalyst. In 2008, 136.2 tonnes of Pd was used for catalytic converters, accounting for 63.9% of the worldwide demand.