Platinum Metals Review - Volume 57, Issue 4, 2013

This review constitutes a detailed but non-exhaustive examination of the directed ortho metallation (DoM)–cross-coupling fusion in its many flavours. Special attention is paid to the application of the concept of the linked reactions and the synthetic utility that it endows, particularly in the case of one-pot reactions that can greatly increase the ease and efficiency of the process. Personal experience of particular issues that can arise from these reactions and examples of their solutions are given, as well as illustrations of the rapid access to complex molecules that the technique encourages.

Proton exchange membrane fuel cells (PEMFCs) dominate the transportation fuel cell market and platinum (Pt) is the catalyst material used for both anode and cathode. This review sets out the fundamentals of activity, selectivity, stability and poisoning resistance which make Pt or its alloys the best available materials to use in this application. It is clear that Pt is the only element which can meet the requirements for performance while avoiding slow reaction kinetics, proton exchange membrane (PEM) system degradation due to hydrogen peroxide (H2O2) formation and catalyst degradation due to metal leaching. Some of the means by which the performance of Pt can be enhanced are also discussed.

The price differential between platinum and palladium has driven the industry to adopt emissions control catalyst formulations for gasoline engines that contain higher levels of Pd than Pt, and in most cases no Pt. In addition fluctuations in the price of rhodium have led to thrifting of this metal. This study compares the performance of ten different catalyst compositions with varying ratios of Pt, Pd and Rh for a Euro 5 vehicle and under bench test conditions. The results show that a system with low Rh loading can readily be improved by increasing the Rh loading and there is a relatively large effect of doing this by a small amount. Increasing the Pd or Pt loading also improves emissions performance but by a significantly smaller amount than the effect of changing the Rh loading. Conversely it may be possible to decrease the Pt or Pd loading with only a small effect on emissions. Furthermore it was found that Pd outperforms Pt under most conditions, although not significantly. The difference appears greater under more stressful conditions such as high-speed driving or wide perturbation amplitude.

Activated carbon-supported palladium catalysts are liable to progressive deactivation even in the absence of any gaseous contaminants during the oxidation of hydrogen under ambient conditions. The high value of palladium coupled with environmental considerations means that new, efficient and cost effective methods for the quantitative recovery of palladium from such materials are required. In the present study, a process for extracting precious metals from spent catalyst or inorganic waste was developed. Palladium was extracted from the spent catalyst with an acid solution containing dilute hydrochloric acid and hydrogen peroxide at leaching temperatures of around 90°C. Palladium in the leached solution was then precipitated by use of sodium borohydride solution. The effectiveness of the method for recovery of precipitated palladium was investigated by ultraviolet-visible (UV-vis) spectrophotometry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). The recovered metallic palladium was of a suf cient grade to manufacture fresh activated carbon-supported palladium catalysts.

In the flotation of platinum group minerals (PGMs) it has generally been assumed that they will behave similarly to base metal sulfides and thus sulfide reagent regimes are generally used in such flotation plants. However the tellurides and arsenides of platinum and palladium contribute about 50% of the PGMs present in the Platreef ore, located in the northern limb of the Bushveld Complex of South Africa, and there is evidence of these minerals reporting to the flotation tailings. The present investigation was aimed at studying the flotation behaviour of tellurides, arsenides and sulfides of Pt and Pd and relating these observations to their surface characteristics. Microflotation, zeta potential determinations, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) analyses were used to study the synthesised samples after various treatments. It was shown that in almost all cases the addition of a typical sulfide collector, sodium isobutyl xanthate (SIBX), increased the recovery of all the Pt and Pd minerals investigated but that the presence of copper sulfate widely used to activate sulfide minerals caused the recoveries to decrease. These results may question the value of adding copper sulfate in the treatment of Platreef PGM-bearing ore.