Platinum group metals properties

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). 

Rhodium. Rhodium is the most commonly plated platinum-group metal. In addition to its decorative uses, rhodium has useful properties for engineering appHcations. It has good corrosion resistance, stable electtical contact resistance, wear resistance, heat resistance, and good reflectivity. The use of rhodium for engineering purposes is covered by an ASTM specification (128). Typical formulas are shown in Table 15. The metal content is obtained from prepared solutions available from proptietary plating supply companies. Replenishment is requited because anodes are not soluble. Rhodium for decorative use may be 0.05—0.13 p.m thick for industtial use, it maybe 0.50—5.0 p.m thick. 

It is somewhat less corrosion resistant than tantalum, and like tantalum suffers from hydrogen embrittlement if it is made cathodic by a galvanic couple or an external e.m.f., or is exposed to hot hydrogen gas. The metal anodises in acid electrolytes to form an anodic oxide film which has a high dielectric constant, and a high anodic breakdown potential. This latter property coupled with good electrical conductivity has led to the use of niobium as a substrate for platinum-group metals in impressed-current cathodic-protection anodes. 

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. 

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. 

A very important characteristic of surface constitution for any metal is the position of its PZC. Table 10.1 reports values for the PZC for a number of metals. We can see that these values vary within rather wide limits. An important difference between platinum group metals and most other metals is the ability of the latter upon anodic polarization to form relatively thick superhcial oxide or salt layers. Owing to their great practical value, these layers are considered in more detail in Section 16.3. For investigations of the structure and of properties of platinum and other electrodes, many nonelectrochemical methods are also widely applied, which is discussed in more detail in Chapter 27. 

See also Potassium-sodium (NaK) alloys physical properties of, 20 604t Naked license, 25 265 AT-aldehydes, platinum-group metal catalysts and, 19 621 Naled, 4 358t... 

Aspnes DE, Heller A (1983) Photoelectrochemical hydrogen evolution and water- photolyzing semiconductor suspensions Properties of platinum group metal catalyst emiconductor contacts in air and in hydrogen. J Phys Chem 87 4919-4929... 

The commercial applications of osmium are limited and considerably fewer than other platinum group metals. Its aUoys are very hard and are used to make tips of fountain-pen nibs, phonograph needles, and pivots. The metal also exhibits effective catalytic properties in hydrogenation and other organic reactions. Such catalytic apphcations, however, are limited and osmium fads to replace other noble metals, particularly paUadium and platinum, which are more effective as catalysts and cost less. 

The properties of some rare-earth binary alloys with platinum group metals are also important in view of the role they can play in the chain of preparing ternary hydrides. Many of the alloys of the series R-M, where R is a rare earth element and M is a Group VIIIB metal, have been investigated structurally and magnetically. The alloys with iridium all have cubic structures, whereas those... 

TABLE 1. REPRESENTATIVE PROPERTIES OF PLATINUM GROUP METALS... 

Carbon monoxide is the molecule whose adsorptive properties have certainly been investigated in most detail. A review of the work prior to 1970 was published by Ford (2) in this series, but in the interim so much additional information has been obtained that a complete compilation of the data would be far beyond the scope of this article. Instead, the main emphasis will be given to a discussion of essential features that fortunately have much in common with different platinum group metals as well as with various crystal planes. 

Aspnes, D.E. and Heller, A. 1982. Photoelectrochemical Hydrogen Evolution and Water-Photolyzing Semiconductor Suspensions Properties of Platinum Group Metal Catalyst-Semiconductor Contacts in Air and in Hydrogen. J. Phy s. Chem., 87, 4919-1929. 

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