Platinum group metals oxides

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. 

Thermal Stability of Platinum Group Metal Oxides.115... 

Table3.1-196 Structures of platinum-group metal oxides [1.219, 277] fil...

The mechanism for CO oxidation over platinum group metals has been established from a wealth of data, the analysis of which is beyond the scope of this chapter. It is quite evident that surface science provided the foundation for this mechanism by directly showing that CO adsorbs molecularly and O2 adsorbs... 

Olefins add anhydrous acetic acid to give esters, usually of secondary or tertiary alcohols propjiene [115-07-1] yields isopropyl acetate [108-21-4], isobutjiene [115-11-7] gives tert-huty acetate [540-88-5]. Minute amounts of water inhibit the reaction. Unsaturated esters can be prepared by a combined oxidative esterification over a platinum group metal catalyst. Eor example, ethylene-air-acetic acid passed over a palladium—Hthium acetate catalyst yields vinyl acetate. 

Metals and Metallic Ions. Under appropriate conditions, ozone oxidizes most metals with the exception of gold and the platinum group. When oxidized by ozone, heavy metal ions, such as Fe and Mn , result in the precipitation of insoluble hydroxides or oxides upon hydrolysis (48—50). Excess ozone oxidizes ferric hydroxide in alkaline media to ferrate, and Mn02 to MnO. ... 

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). 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

JS/oble Metals. Noble or precious metals, ie, Pt, Pd, Ag, and Au, are ftequendy alloyed with the closely related metals, Ru, Rh, Os, and Ir (see Platinum-GROUP metals). These are usually supported on a metal oxide such as a-alumina, a-Al202, or siUca, Si02. The most frequently used precious metal components are platinum [7440-06-4J, Pt, palladium [7440-05-3] Pd, and rhodium [7440-16-6] Rh. The precious metals are more commonly used because of the abiUty to operate at lower temperatures. As a general rule, platinum is more active for the oxidation of paraffinic hydrocarbons palladium is more active for the oxidation of unsaturated hydrocarbons and CO (19). 

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 largest uses of platinum group metals in electronics are ruthenium for resistors and palladium for multilayer capacitors, both applied by thick film techniques . Most anodes for brine electrolysis are coated with mixed ruthenium and titanium oxide by thermal decomposition . Chemical vapour deposition of ruthenium was patented for use on cutting tools . 

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). 

Supported gold catalysts are, in general, less active than platinum group metal catalysts in the complete oxidation of hydrocarbons however, by choosing... 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

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. 

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... 

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