Platinum Metals Review - Volume 51, Issue 3, 2007

Work is being done at Mintek, the University of Leeds and the University of Bayreuth to build up a platinum-aluminium-chromium-ruthenium (Pt-Al-Cr-Ru) database for the prediction of phase diagrams for further alloy development by obtaining good thermodynamic descriptions of all of the possible phases in the system. The available databases do not cover all of the phases, and these had to be gleaned from literature, or modelled using experimental data. Similarly, not all of the experimental data were known, and where there were gaps or inconsistencies, experiments had to be undertaken. A preliminary version of the database was constructed from assessed thermodynamic data-sets for the binary systems only. The binary descriptions were combined, allowing extrapolation into the ternary systems, and experimental phase equilibrium data were compared with calculated results. Very good agreement was obtained for the Pt-Al-Ru and Pt-Cr-Ru systems, which was encouraging and confirmed that the higher-order systems could be calculated from the binary systems with confidence. Since some of the phase models in earlier databases were different, these phases had to be remodelled. However, more work is ongoing for information concerning the ternary phases present in the Al-Cr-Ru, Pt-Al-Ru (two ternary compounds in each) and Pt-Al-Cr (possibly more than three ternary compounds) systems. Later in the work, problems with the thermodynamic descriptions of the Cr-Ru and Pt-Cr binary systems were found, and a programme of experimental work to overcome these has been devised, and is being undertaken.

The adoption of a low-pressure rhodium-based catalyst system in place of high-pressure cobalt for the hydroformylation of propylene by reaction with carbon monoxide and hydrogen to produce butyraldehydes (an ‘oxo’reaction) has brought large cost benefits to oxo producers. The benefits derive from improved feedstock efficiency, lower energy usage and simpler and cheaper plant configurations. The technical and commercial merits of the ‘LP Oxo^SMProcess’ for producing butyraldehydes have made it one of the best known applications of industrial-scale chemistry using a platinum group metal (pgm). Today, practically all butyraldehyde is made by rhodium catalysis, and this should provide convincing encouragement to researchers who are keen to exploit pgms as catalyst research materials, but are apprehensive as to the implications of their very high intrinsic value. It should also encourage developers and designers responsible for turning pgm chemistry into commercial processes, who may be daunted by problems such as containment and catalyst life. This article (Part I) reviews the background to the LP Oxo^SMProcess, and its development to the point of first commercialisation. Part II, covering some of the key improvements made to the process and its use in non-propylene applications, will appear in a future issue of Platinum Metals Review.

While a knowledge of the vapour pressure curve of any material is of theoretical significance in understanding its basic physical properties, it can also be of practical importance in, for example, the use of the material in high-temperature vacuum applications. Therefore readily usable equations which accurately predict values of vapour pressure over a wide range of temperatures and pressures can have an important practical use. For the platinum group metals (pgms) the vapour pressures can be immediately assessed from about 10⁻¹⁶ bar to just above the boiling point by the use of Equation (i), fitted for the solid and liquid metals separately: ln(p, bar) = A + Bln(T) + C/T + DT + ET², where p is the vapour pressure, T is the temperature in kelvin and A, B, C, D and E are constants. Although containing five coefficients, this equation can easily be evaluated by computers and scientific calculators. Although it gives values of vapour pressure at fixed temperatures, by a simple and rapid use of iteration values of temperature at fixed vapour pressure, temperature-dependent values of vapour pressure can also be obtained.

With a view to improving the catalytic performance of supported bimetallic platinum-copper catalysts in hydrogen-assisted dechlorination of halogenated alkanes, a range of catalysts were prepared by reduction of oxide precursors by the hydride anion H⁻, using both sodium and calcium hydrides (NaH and CaH2). The catalytic performance of the resulting catalyst samples in the hydrodechlorination (HDCl) of 1,2-dichloroethane at 220ºC was investigated, to gain understanding of metal alloying phenomena governing the variation in ethene selectivity with time on stream (TOS). Metal dispersion was also investigated by O2 chemisorption and transmission electron microscopy (TEM). PtCuCaH(b) catalyst, synthesised by reduction with CaH2 at 450ºC, showed a high selectivity towards ethene in comparison with that of catalysts synthesised by reduction with either NaH or hydrogen. In view of the chemisorption and TEM results, the significant high selectivity of this catalyst towards ethene was attributed to the fact that reduction by CaH2 enhanced alloying of Pt and Cu. On the other hand, the ethene selectivity of PtCuCaH(b) catalyst did not show any variation with TOS, but reached a steady state at early TOS. This suggested that Pt and Cu alloying did not take place during the course of the reaction, but might have occurred during the reduction process.