The Noble Metals

PART I Ruthenium, Osmium, Rhodium, and Iridium by L. A. P. Kane-Maguire 

The adduct Ru3(CO)12,AlBr3 is formed via treatment of Ru3(CO)12 with AlBr3 in toluene.5 This diamagnetic red solid reverts to the parent on exposure to air or acetone. It displays a low-frequency carbonyl stretching band at 1535 cm-1 characteristic of Lewis-acid-co-ordinated bridging carbonyls. Reaction of Ru3(CO)12 with l,2-bis(dimethylsilyl)ethane has been shown6 to yield the stable white chelate complex (1 M = Ru) 13C n.m.r. studies indicate that this species is stereochemi-cally rigid. 

Ruthenium-( —n), -(0), and -(i).—A single-crystal A-ray analysis of the complex [Ru(NO)2(PPh3)2] has revealed a distorted tetrahedral co-ordination about the 

General. —Values of AHf 29s have been obtained for several oxides, hydroxides, and halides of ruthenium using a method of comparative calculations. Heating the metal with EuH2 under an atmosphere of hydrogen produces the dark red, air-stable Eu2RuHg, which has a fluorite structure. 

Heating a solution of Ru3(CO)i2 under H2 briefly at 393 K yields H4Ru4(CO)i2.  

Rudieniom(o).—The u.v. photoelectron spectrum of Ru(Pp3)5 has been recorded PF3 is considered to have a greater overall electron-withdrawing effect than CO. 

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The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration, hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. 

Catalytic hydrogenation of furan to tetrahydrofuran is accompHshed in either Hquid or vapor phase. Hydrogenation of the double bonds is essentially quantitative over nickel catalysts but is generally accompanied by hydrogenolysis over the noble metals. 

E. E. Beamish, The Analytical Chemisty of the Noble Metals, Pergamon Press, New York, 1966, p. 162. 

In the Parkes process, a quantity (1—2%) of 2inc is added to lead which is in excess of the saturation value. This creates insoluble intermetaUic compounds consisting of 2inc and the noble metals that precipitate from the lead on cooling (22). 

Electroplating. Aluminum can be electroplated by the electrolytic reduction of cryoHte, which is trisodium aluminum hexafluoride [13775-53-6] Na AlE, containing alumina. Brass (see COPPERALLOYS) can be electroplated from aqueous cyanide solutions which contain cyano complexes of zinc(II) and copper(I). The soft CN stabilizes the copper as copper(I) and the two cyano complexes have comparable potentials. Without CN the potentials of aqueous zinc(II) and copper(I), as weU as those of zinc(II) and copper(II), are over one volt apart thus only the copper plates out. Careful control of concentration and pH also enables brass to be deposited from solutions of citrate and tartrate. The noble metals are often plated from solutions in which coordination compounds help provide fine, even deposits (see Electroplating). 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

Correct application of this principle can lead to what would appear to he peculiar recommendations. For example, if just one member of a couple is to be coated, it should be the noble member. Most coating systems leave holidays or tiny openings where the metal is exposed. If the active metal is coated, the area of exposure at the holidays can be quite small compared to the area of the noble metal, resulting in an unfavorable area ratio. On the other hand, if the noble metal is coated, the holidays provide a small cathodic area and hence a highly favorable area ratio with respect to the active metal. Similarly, if dissimilar metal fasteners must be used, they should be noble relative to the metals being fastened (see Case History 16.1). 

Corrosion of the noble metal will be slight or nonexistent, even though it would corrode in the given environment if it were not coupled to the active metal. 

In general, corrosion of the active metal will be most severe at its junction with the noble metal (Fig. 16.2) and will decrease with increased distance from the junction. 

When possible, avoid coupling materials having widely dissimilar galvanic potentials. If this cannot he avoided, make use of favorable area ratios by giving the active metal a large exposed area relative to the noble metal. For example, copper or copper-based alloy tubes may be joined to a steel tube sheet. Because of the favorable area ratio in this case, a relatively inexpensive steel tube sheet may be intentionally substituted for a bronze or a brass tube sheet if thickness specifications allow for a small amount of galvanic corrosion of the steel. 

Starting with a ceramic and depositing an aluminum oxide coating. The aluminum oxide makes the ceramic, which is fairly smooth, have a number of bumps. On those bumps a noble metal catalyst, such as platinum, palladium, or rubidium, is deposited. The active site, wherever the noble metal is deposited, is where the conversion will actually take place. An alternate to the ceramic substrate is a metallic substrate. In this process, the aluminum oxide is deposited on the metallic substrate to give the wavy contour. The precious metal is then deposited onto the aluminum oxide. Both forms of catalyst are called monoliths. 

An alternate form of catalyst is pellets. The pellets are available in various diameters or extruded forms. The pellets can have an aluminum oxide coating with a noble metal deposited as the catalyst. The beads are placed in a tray or bed and have a depth of anywhere from 6 to 10 inches. The larger the bead (1/4 inch versus 1/8 inch) the less the pressure drop through the catalyst bed. However, the larger the bead, the less surface area is present in the same volume which translates to less destruction efficiency. Higher pressure drop translates into higher horsepower required for the oxidation system. The noble metal monoliths have a relatively low pressure drop and are typically more expensive than the pellets for the same application. 

The measurement ranges for the base-metal thermocouples are 0 to +750 °C (type J), -200 to +1200 °C (type K), and -200 to +350 °C (type T). The noble-metal thermocouples can be used at higher temperatures up to 1700 °C. The dynamic response of sheathed thermocouples is not very fast however, a probe made from bare, thin wires can have very fast dynamic properties. One of the best features of thermocouples is the simplicity of making new probes by soldering or welding the ends of two wires together. 

Base metals frequently are used in nonsupported form, but noble metals rarely are, except in laboratory preparations. Supporting the noble metals makes a more efficient catalyst on a weight of metal basis and aids in recovery of the metal. Neither of these factors is of much importance in experimental work, but in industrial processing both have significant impact on economics. 

Eventually all catalysts become spent. At this stage they can be discarded, itself sometimes a problem, or returned to a refiner for recovery of metal values. In commercial use, noble-metal catalysts are always returned to a refiner. At the refinery, the catalyst is destroyed and the noble metals are recovered and converted to high-purity metal. In a loop system, the pure metal is converted to a suitable salt and again used for catalyst manufacture. In the entire loop, some metal will be lost and must be replaced with fresh metal. Refining is nowadays very efficient, and most metal loss will occur in the process itself, The total cost of a catalyst used in a loop is accordingly given by ... 

Clark et al.n recently discovered another FS related inechanisni in CuPt, different from the above mentioned nesting. In this case, the relevant contribution to the coneentration waves suseeptibility is due to the contemporary presenee of the noble metal-like neck at the L point and the d hole pocket at X. Ifie connecting vector of these Van Hove singularities belongs to the star 1,1,1 and is commensurate with the Lli ordering. In fact, it produces a phase characterised by alt ate hexagonal Cu and Pt planes, in the direction perpendicular to (1,1,1). 

Figure 2. Calculated length of the Fenni wavevector along the line, kp, for AgPd, CuPt and CnPd random alloys versns the noble metal atomic concentration, c. Dashed lines are drawn as a gnide for the eyes. The solid line indicates the value, V2/2, at which kp is exactly commensurate with LIq or LI2 orderings.

With copper alloys containing more noble metals the oxide will be substantially pure copper oxide since the oxides of the noble metals have higher dissociation pressures than the copper oxides. With alloys containing baser metals, however, the alloying element will appear as an oxide in the scale, often in greater concentration than in the alloy itself, and sometimes to the exclusion of copper oxides. The dissociation pressures of many oxides have been calculated by Lustman... 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

Although in the majority of their applications the choice of noble metals is determined by their chemical rather than by their physical and mechanical properties, some consideration of the latter is necessary. The relevant information for the noble metals as a whole is given in Tables 6.1 and 6.2, and details relating to the individual metals will be found in the following paragraphs. 

The factors leading to the high resistance of the noble metals to chemical attack, i.e. their thermodynamic stability over a wide range of conditions and the possibility of the formation of very thin protective films under oxidising conditions, have already been mentioned. A factor tending to reduce corrosion resistance in aqueous solutions is the tendency of these metals to form complexes with some anions. 

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