Platinum Metals Review - Volume 41, Issue 4, 1997

The platinum metals alloys, DOP-26 iridium and platinum-30 per cent rhodium, have been successfully used to encapsulate plutonia fuel pellets for the Cassini Spacecraft. The iridium-encapsulated heat sources provide approximately 900 watts of electrical power for the spacecraft and its experiments, whereas the platinum alloy clad pellets will supply about 150 watts of heat to various parts of the spacecraft and its lunar probe, Huygens. The particular alloys used on this mission have been selected to fulfil the critical function of maintaining fuel containment during normal service and for projected malfunction or accident scenarios. Their ability to perform satisfactorily has been demonstrated through extensive testing of their mechanical, physical and impact properties. The Cassini heat source manufacturing yields were significantly higher than those obtained for previous missions.

Work on the synthesis of ethanol from carbon dioxide over a rhodium-selenium catalyst is reported, and related reactions and characterisation studies are briefly reviewed. In order to inhibit the formation of methane (complete reduction of carbon dioxide) and simultaneously activate carbon-carbon bond formation by the reaction of CHx with carbonyl derivatives, it is necessary to control the active rhodium sites. Based on a study of single crystal rhodium surfaces it is proposed that acetyl and acetate intermediates are formed. Recently it has been discovered that supported Rh/TiO2, promoted by selenium from inside the rhodium metal sites, is a potential catalyst for ethanol synthesis from carbon dioxide. The action of this catalyst is compared to related studies.

With the exception of osmium, iridium is the least abundant of the six platinum group metals. It is of crucial importance in a number of high technology applications and, at least in the past, has been regarded as a strategic material and stockpiled by major governments. In recent years it has become more readily available, prompting renewed interest by researchers, including those seeking to develop improved catalysts. In this paper we discuss the results of our investigations on the use of iridium as an hydrogenation catalyst. Data are presented on the characteristics of an iridium-based catalyst developed here for the effective and selective synthesis of substituted N-aryl-hydroxylamines and chloro-substituted anilines, symmetric and asymmetric azoxybenzenes, and unsaturated alcohols. These demonstrate that iridium is unique among the platinum metals.

The crystallographic properties of rhodium at temperatures from absolute zero to its melting point are assessed following a review of the literature published during the period 1915 to date. Selected values of thermal expansion coefficients and length change measurements have been used to calculate the variations with temperature of lattice parameters, interatomic distances, atomic and molar volumes and density. Literature values are compared graphically with the selected values.