Platinum family metals

An interesting, peculiar laboratory preparative reaction may finally be mentioned. This is based on the very high stability of the intermetallic compounds of actinides (and lanthanides) with the platinum family metals. The combined reduction capability of Pt with H2 (coupled reduction, see 6.7.2 fi) can be used to obtain, from its oxide, the platinide of the actinide metal. The An-Pt intermetallic compound can then be decomposed by heating in vacuum and the actinide can be obtained by distillation. 

Platinum family metals preparation. The platinum metals (with gold and silver) are commonly found together, and a number of schemes are in current use for their separation. Platinum metals not only are extracted from ores but, due to their high value, are also recovered from a wide range of industrial residues such as Pt, Rh gauze catalysts, defective components from glass industries, etc. 

Table 5.43. Platinum family metals selected physical properties.

Table 5.45. Platinum family metals crystal structures, lattice parameters and calculated densities.

Platinum family metals and alloys. The platinum family (platinum group) metals (Ru, Os, Rh, Ir, Pd, Pt) belong (together with Au and possibly Ag and Re) to the class of precious metals. This characteristic together with their inertness to the environment (with regard to corrosion and oxidation) is the basis for a number of uses such as in the following. 

The bottom line on monocyclic aromatic compounds with re-electron sextet is that so far, among six-membered systems only benzene, the azines with 1 through 4 nitrogen atoms, phosphabenzene and arsaben-zene, pyrylium, azapyrylium, chalcogenopyrylium cations (with or without exocyclic groups such as hydroxy, amino and corresponding tautomeric or prototropic forms), and the metallabenzenes with platinum family metals have been proved to afford stable molecules under normal conditions. The list is richer for five-membered systems. 

Ruthenium is most often combined with platinum or palladium in alloys. Electrical contacts, devices for measuring very high and very low temperatures, and medical instruments are often made from ruthenium alloys. Ruthenium is also used in alloys with other platinum family metals to make jewelry and art objects. This use is limited, however, because of the high cost of ruthenium metal. 

Oxidation of hydrocarbons and alcohols If reasonably effective oxidation catalysts can be identified for aqueous electrolytes, hydrocarbon and alcohol oxidation processes would make possible promising fuel cells operating directly on quite practical fuels at moderate temperatures. The currently used platinum and platinum-family metals and alloys have substantial activity, but it is not sufficient for practical fuel cells with aqueous electrolytes. With the many electrons involved in the complete oxidation, the detailed mechanisms for the oxidation are likely to be quite complex. To avoid incomplete oxidation it is probably necessary to have the reactants remain adsorbed on the electrode surface through the complete oxidation to C02 and H20. Here again, new promising catalysts and new experimental approaches are needed. 

On platinum and platinum family metals, the O2 reduction appears to occur by the parallel mechanism with the direct four-electron reduction to water predominant in both acid and alkaline electrolyte in the absence of impurities. Evidence to support this conclusion is to be found in the influence of surface chemisorbed oxygen species (Pt-OH or Pt-O), adsorbed anions, and other adsorbed species on the extent to which the O2 reduction proceeds to water and to H2O2. In rotating disk-ring electrode studies, the intercepts in the IdIIb vs. plots have been found to be greater than unity and... 

Platinum/ platinum family metals/ and silver are classified in the second group. The oxygen electroreduction on these metals occurs both directly to water and via intermediate formation of hydrogen peroxide. The hydrogen peroxide formed in the parallel reaction is decomposed by chemical or electrochemical mechanisms. The relation between the rates of these processes depends in a complicated way on the nature of the metal, the potential, the coverage of the electrode by the chemisorbed species, and their adsorption character. The polarization curves are characterized by a single wave with a limiting current close to the diffusion current for the four-electron process. 

The analysis of the mechanism of O2 electroreduction on platinum and platinum family metals to H2O or OH is more difficult. The possible mechan-... 

ORR Mechanism on Platinum Family Metals and Their Alloys... 

Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. Because iridium is very hard and brittle, it is hard to machine, form, or work. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

The most successful class of active ingredient for both oxidation and reduction is that of the noble metals silver, gold, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Platinum and palladium readily oxidize carbon monoxide, all the hydrocarbons except methane, and the partially oxygenated organic compounds such as aldehydes and alcohols. Under reducing conditions, platinum can convert NO to N2 and to NH3. Platinum and palladium are used in small quantities as promoters for less active base metal oxide catalysts. Platinum is also a candidate for simultaneous oxidation and reduction when the oxidant/re-ductant ratio is within 1% of stoichiometry. The other four elements of the platinum family are in short supply. Ruthenium produces the least NH3 concentration in NO reduction in comparison with other catalysts, but it forms volatile toxic oxides. 

On the basis of their behaviour and of their applications, however, it may be useful to describe these metals subdividing them into two families iron family and platinum family. 

Metals of the platinum family. These metals pertain to the 2nd and 3rd transition rows. They represent, together with silver and gold, the metals also known with the common name of noble metals. 

The powder and dust of rhodium metal are flammable in air. Some of the compounds may cause skin irritations. It is best to use approved laboratory procedures when handling any of the six elements in the platinum family of metals. 

One of palladiums unique characteristics is its abihty to absorb 900 times its own volume of hydrogen gas. When the surface of the pure metal is exposed to hydrogen gas (H ), the gas molecules break into atomic hydrogen. These hydrogen atoms then seep into the holes in the crystal structure of the metal. The result is a metallic hydride (PdH that changes palladium from an electrical conductor to a semiconductor. The compound palladium dichloride (PdCl ) also has the ability to absorb large quantities of carbon monoxide (CO). These characteristics are useful for many commercial applications. Palladium is the most reactive of all the platinum family of elements (Ru, Rh, Pd, Os, Is, and Pt.)... 

Recently, the formation, structure, equilibrium, and kinetics of a family of cyano compounds containing a direct and not supported by ligands platinum-thallium metal-metal bond have been reported. The complexes are synthesized according to the reaction ... 

Iridium is in Group 9 (VIIIB) of the periodic table. The periodic table is a chart that shows how elements are related to one another. Iridium is a transition metal that is also part of the platinum family. 

The metals in the platinum family are also known as the noble metals. They have this name because they do not react well with other elements and compounds. They appear to be too superior to react with most other substances. 

Platinum family (noble metals) A group of elements that occur close to platinum in the periodic table and in Earth s surface. 

The lower selectivity of PtHFAU catalysts is due to the very rapid formation of Ce cyclic hydrocarbons (family 1). The same trend has been found in the case of acetone transformation [3]. This can be explained by the lower activity of the palladium relatively to the platinum to hydrogenate the C=0 bond. This lower activity which has been found in the case of cyclohexanone hydrogenation on platinum group metals was explained by a weaker adsorption of the ketone on Pd in comparison with Pt and Ru [9]. 

Iridium — (L. iris, rainbow), Ir at. wt. 192.217(3) at. no. 77 m.p. 2446°C b.p. 4428°C sp. gr. 22.562 (20°C) valence 3 or 4. Discovered in 1803 by Tennant in the residue left when crude platinum is dissolved by aqua regia. The name iridium is appropriate, for its salts are highly colored. Iridium, a metal of the platinum family, is white, similar to platinum, but with a slight yellowish cast. It is very hard and brittle, making it very hard to machine, form, or work. It is the most corrosion-resistant metal known, and was used in making the standard meter... 

The driving force for tlie development of non-platinum exliaust emission catalysts is the price, strategic importance and low availability of the platinum group metals. Our studies have shown that catalysts based on tin(IV) oxide (Sn02) promoted with chromium and/or copper (Cr-Sn02 and Cu-Cr-Sn02 catalysts) which exhibit excellent three-way catalytic activity - activity which is comparable to that shown by noble metals dispersed on ahunina. This family of materials offers tremendous promise as cheap and efficient catalyst systems for the catalytic conversion of noxious emissions from a variety of sources. In tliis paper we describe tliree aspects of this family of environmental catalysts (1) the synthesis, (2) tlieir characterisation, and (3) their catalytic activity. 

It is clear, therefore, that we need to investigate alternative lean NOx catalysts. One such family of catalysts comprises the platinum group metals supported on metal oxides. Promising results from such systems have already appeared in the literature [5]. Here we restrict our attention to a series of Pt/MOs catalysts of various loadings and prepared from various R precursors. 

New Applications of TCNE in Organometallic Chemistry, A. J. Fatiadi (1987). Selected reactions used in organometallic synthesis are reviewed. 311 references are given. Structure and bonding of metal-TCNE complexes as well as reactions of TCNE with main-group organometallics, with transition-metal complexes, with metal-coordinated alkenes and alkynes, and reactions of platinum-family complexes are discussed. 

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