Editor’s Note: This is Part 4 of a 6-Part Series on PGMs that runs on Tuesdays.
Automotive catalytic converters (ACCs) play an essential role in exhaust systems to reduce the emission of harmful pollutants, such as carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). A typical converter is ceramic-based and consists of a substrate made of cordierite (2MgO.2Al2O3.5SiO2) formed into many fine channels similar in appearance to a honeycomb. The substrate material is wash-coated with alumina (Al2O3) to which a combination of platinum group metals (PGMs) including platinum (Pt), palladium (Pd) and rhodium (Rh) as well as rare earth oxides such as CeO2, ZrO2 are added. The PGMs are responsible for the conversion reactions that turn pollutants into harmless gases.
According to the International Platinum Group Metals Association (IPA), in 2010 ACCs accounted for 51% of total world demand for PGMs. The IPA site also reports that more than 98% of new cars sold worldwide each year are now fitted with these devices, which convert over 90% of harmful polluting gases into less harmful carbon dioxide (CO2) and water vapor.
Recovery of precious metals by recycling spent ACCs represents a substantial economical value. Most recycled catalytic converters in the U.S. contain recoverable amounts of PGMs ranging from 1-2 grams for a small car to 12-15 grams for a big truck. The value of the recovered PGMs ranges from $25 to a few hundred dollars per vehicle. In 2010, the total value of Pt, Pd and Rh reclaimed from the recycling of spent catalytic converters was $3 billion.
Energy-dispersive x-ray fluorescence (EDXRF) offers a quick and accurate determination of Pt, Pd and Rh content in spent ACCs. EDXRF is designed to analyze groups of elements simultaneously. This type of XRF instrumentation separates the characteristic x-rays of different elements into a complete fluorescence energy spectrum which is then processed for qualitative or quantitative analysis. Filters are positioned between sample and detector to improve signal, background reduction, and focus on certain regions of the spectra.
ACCs are inhomogeneous down to the microscopic level and require a thorough sample preparation before reproducible analysis is possible. A first step consists of grinding the material using a vibratory disk or ring and puck mill. It is worthwhile to check element characteristic line intensities as a function of grinding time and to assure that these have reached a constant value. This should not only be checked for the precious metals but also for the other elements present (i.e. Al, Zr, Ce, etc). Grinding sets made of tungsten carbide (WC) should be avoided because contamination of the sample by tungsten will result in characteristic lines of tungsten interfering with the lines of platinum.
Once ground, the powders can be pressed into a pellet with aid of a binder. Similar to geological samples, the inhomogeneous nature of ACCs will still lead to mineralogical effects. To avoid these, some laboratories prefer to fuse the sample but this leads to other difficulties related to the instability of precious metal oxides and their tendency to agglomerate or alloy with Pt/Au crucibles.
Read the study to see how EDXRF analysis measures up against ICP-OES to determine Rh, Pd and Pt concentrations in a set of 10 ACC samples.