Platinum Metals Review - Volume 53, Issue 1, 2009

Under the auspices of the Platinum Development Initiative, platinum-based alloys are being developed for high-temperature and special applications requiring good corrosion and oxidation resistance. The best candidate system was found to be platinum-aluminium, after reviewing binary systems and assessing experimental ternary alloys. Ternary alloys based on Pt-Al, where the ternary additions comprised chromium, iridium, molybdenum, nickel, rhenium, ruthenium, tantalum, titanium and tungsten were tested. As well as phase characterisation work, mechanical and oxidation tests were undertaken. The best alloys were found to be Pt-Al-Cr and Pt-Al-Ru. The microstructures were similar to those of nickel-based superalloys, and comprise ~ Pt3Al precipitates in a Pt-based matrix. However, the volume fraction of the ~ Pt3Al was only ~ 40% instead of the ~ 70% found in Ni-based superalloys.

Platinum group metals (pgms) supported on a carrier material are widely applied as catalysts. These catalysts are conventionally prepared by wet-phase processes in several steps, while recently developed flame processes allow synthesis of supported pgms in a single step including the support material. Here, we describe flame processes and how finely dispersed supported pgms are made in these flames. So Pt/Al2O3, Pd/ZnO, Rh/Al2O3, Pt/Ba/Al2O3 and others are highlighted regarding their materials properties and performance as catalysts as well as in gas sensors.

The mechanism of formation of particulate matter (PM) in the diesel engine combustion process is outlined, and the increasingly stringent PM emissions limits in current and projected environmental legislation are noted in the context of the increasing use of fuel-efficient high-performance diesel engines in passenger cars. The types of filter systems for abating diesel particulates are described, as are the principles of filter regeneration – the controlled oxidation of PM retained in the filter, to prevent an accumulation which would ultimately block the filter and degrade engine performance. PM is characterised in terms of both particle size (coarse, accumulation mode, and nucleation mode nanoparticles) and chemical composition, and the filtration issues specific to the various PM types are outlined. Likely future trends in filter design are projected, including multifunctional systems combining PM filtration with NOx control catalysts to meet yet more stringent legislative requirements, including European Stage 5 and 6, and the so called ‘Bin 5’ levels in the U.S.A.