The Platinum Group Metals

The choice of metal or metals is dependent on the fuel used. Platinum shows a 

Palladium is the most often used PGM in catalytic combustion applications for natural gas. Palladium exists in the form PdO at low temperatures, probably at least on the surface of the particle. As the temperature increases there is a reduction of PdOx to metallic Pd, around ca. 800 The reaction is reversible 

Kikuchi and co-workers have shown that Pd supported on a-alumina exhibited higher catalytic activity for combustion of methane than Pd supported on y- or 0-alumina. Pd supported on alumina and the influence of the calcination temperature of alumina from 900 to 1600 °C was studied. The catalytic activity of Pd supported on alumina calcined at 1600°C was 100 times higher compared to Pd supported on alumina calcined at 1200 °C. Temperature-programmed oxidation studies showed that Pd on alumina calcined up to 1600 °C is oxidized to PdO at temperatures lower than for Pd on alumina calcined up to 1200°C. Hence, low surface area alumina was able to stabilize a more active Pd-species (PdO ) than high surface area alumina. 

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Commercial metal anodes for the chlorine industry came about after the late 1960s when a series of worldwide patents were awarded (6—8). These were based not on the use of the platinum-group metals (qv) themselves, but on coatings comprised of platinum-group metal oxides or a mixture of these oxides with valve metal oxides, such as titanium oxide (see Platinum-GROUP metals, compounds Titanium compounds). In the case of chlor-alkaH production, the platinum-group metal oxides that proved most appropriate for use as coatings on anodes were those of mthenium and iridium. 

Many competitive programs to perfect a metallic anode for chlorine arose. In one, Dow Chemical concentrated on a coating based on cobalt oxide rather than precious metal oxides. This technology was patented (9,10) and developed to the semicommercial state, but the operating characteristics of the cobalt oxide coatings proved inferior to those of the platinum-group metal oxide. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

The principal applications of the PGMs are summarized in Table 17. Table 17. Applications of the Platinum-Group Metals... 

There are several exceUent sources of information about the platinum-group metals. The exceUent reference work G. Wilkinson, R. D. GiUard, and J. A. McCleverty, eds.. Comprehensive Coordination Chemistry Pergamon Press, Oxford, U.K., 1987, contains iadividual chapters devoted to descriptive chemistry of each element. 

The Platinum Group Metal Reviews is a specialized review series focusiag on the new developments and uses of PGMs. Each issue also provides a brief description of recent patents issued ia the field. 

A small amount of the platinum-group metals is also used in jewelry. The high cost of these metals is an obvious deterrent. 

F. R. Hartley (ed.) Chemistry of the Platinum Group Metals, Elsevier, Amsterdam, 1991, 642 pp. 

The active ingredients of the promoter are typically the platinum group metals. The platinum, in the concentration of 300 ppm to 800... 

High-temperature Applications of the Platinum-group Metals... 

The most widely used methods for the application of coatings of gold, silver and the platinum group metals (platinum, palladium, rhodium, iridium, ruthenium, osmium) to base metals are mechanical cladding and electroplating. 

Thermochemistry and oxidation potentials of the platinum group metals and their compounds. R. N. Goldberg and L. G. HepleT, Chem. Rev., 1968, 68,229-252 (212). 

The hydrogen reduction of the metal halides, described in Sec. 1.2, is generally the favored reaction for metal deposition but is not suitable for the platinum-group metals since the volatilization and decomposition temperatures of their halides are too close to provide efficient vapor transport. 1 1 For that reason, the decomposition of the carbonyl halide is preferred. The exception is palladium which is much more readily deposited by hydrogen reduction than by the carbonyl-halide decomposition. 

The platinum-group metals comprise ruthenium (Ru), rhodium (Rh) and palladium (Pd) from the second transition series and osmium (Os), iridium(Ir), and platinum (Pt) from thethird transition series. Little or no C VD investigation of palladium and osmium have been reported and these metalsarenotincludedhere. The properties of the other platinum-group metals are summarized in Table 6.9. 

The platinum group metals form several binary, pseudo-binary, and ternary chalcogenides. The outstanding features of these compounds as related to catalysis and materials science have been widely reported and reviewed [88],... 

In their electrochemical surface properties, a number of metals (lead, tin, cadmium, and others) resemble mercury, whereas other metals of the platinum group resemble platinum itself. Within each of these groups, trends in the behavior observed coincide qualitatively, sometimes even semiquantitatively. Some of the differences between mercury and other. y- or p-metals are due to their solid state. Among the platinum group metals, palladium is exceptional, since strong bulk absorption of hydrogen is observed here in addition to surface adsorption, an effect that makes it difficult to study the surface itself. 

Unlike the cathodic reaction, anodic oxidation (ionization) of molecular hydrogen can be studied for only a few electrode materials, which include the platinum group metals, tungsten carbide, and in alkaline solutions nickel. Other metals either are not sufficiently stable in the appropriate range of potentials or prove to be inactive toward this reaction. For the materials mentioned, it can be realized only over a relatively narrow range of potentials. Adsorbed or phase oxide layers interfering with the reaction form on the surface at positive potentials. Hence, as the polarization is raised, the anodic current will first increase, then decrease (i.e., the electrode becomes passive see Fig. 16.3 in Chapter 16). In the case of nickel and tungsten... 

The oxygen reactions occur at potentials where most metal surfaces are covered by adsorbed or phase oxide layers. This is particularly true for oxygen evolution, which occurs at potentials of 1.5 to 2.2 V (RHE). At these potentials many metals either dissolve or are completely oxidized. In acidic solutions, oxygen evolution can be realized at electrodes of the platinum group metals, the lead dioxide, and the oxides of certain other metals. In alkaline solutions, electrodes of iron group metals can also be used (at these potentials, their surfaces are practically completely oxidized). 

Palladium gave the highest activity of all the platinum group metals evaluated platinum, rhodium and ruthenium exhibited very poor activity. The choice of support was also demonstrated to be very important the activated carbon supported Pd catalyst showed a nearly fourfold increase in activity than did Pd supported on alumina. 

Koch, K. R. Sacht, C. Grimmbacher, T. Bourne, S. New Ligands for the platinum-group metals deceptively simple coordination chemistry of N-acyl-N -alkyl and N-acyl-N, N -dialkyl-thioureas. S. Afr. J. Chem. 1995, 48, 71-77. 

Cleare, M. J. Grant, R. A. Charlesworth, P. Separation of the platinum-group metals by use of selective solvent extraction techniques. Extr. Metall. 81, Pap. Symp. 1981, 34-41. 

Koch, K. R. New chemistry with old ligands N-alkyl- and N,N-dialkyl-N -acyl(aroyl)thioureas in co-ordination, analytical and process chemistry of the platinum group metals. Coord. Chem. Rev. 2001, 216, 473 188. 

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