Platinum metal oxides, coatings

Oxidic coatings containing platinum metal oxides, in particular RuCE, and platinum metal coatings, in particular Ru... 

H. B. Beer, Electrode having platinum metal oxide coating thereon, and method of use thereof U.S. Pat. 3,711,385.1970, p. 16. 

The initial attemps to replace the graphite anodes with activated titanium anodes began as early as 1957 with platinized titanium and Pt/lr-coated anodes. However because of the short lifetimes of the anodes, they were not economic. The use of mixed metal oxides was first patented by Beer in 1965 and 1967 [150]. The initial patent described a coated metal electrode in which the active material was a mbced metal oxide coating containing one or more of the platinum metal group oxides. The second patent described coatings in which mfred metal oxide crystals contained a non-platinum metal oxide in addition to the platinum metal oxide (including Ti, Ta, and Zr 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. 

Cathodic Protection Systems. Metal anodes using either platinum [7440-06 ] metal or precious metal oxide coatings on titanium, niobium [7440-03-17, or tantalum [7440-25-7] substrates are extensively used for impressed current cathodic protection systems. A prime appHcation is the use of platinum-coated titanium anodes for protection of the hulls of marine vessels. The controUed feature of these systems has created an attractive alternative... 

Metal anodes using platinum and precious metal oxide coatings are also incorporated into a variety of designs of impressed current protection for pipeline and deep weU appHcations, as weU as for protection of condenser water boxes in power generating stations (see Pipelines Power generation). 

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

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). 

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. 

Recently it has been shown that the oxides of the platinum metals can have a higher corrosion resistance than the metals themselves , and have sufficient conductivity to be used as coatings for anodes, e.g. with titanium cores. Anodes with a coating of ruthenium dioxide are being developed for use in mercury cells for the electrolysis of brine to produce chlorine , since they are resistant to attack if in contact with the sodium-mercury amalgam. 

Precious metals and oxides platinised titanium, platinised niobium, platinised tantalum, platinised silver, solid platinum metals, mixed metal oxide-coated titanium, titanium oxide-based ceramics. 

Rare, shiny, and lightest metal of the platinum group. Hardens platinum and palladium. The presence of 0.1 % of ruthenium in titanium improves its resistance to corrosion 100-fold. The spectacular catalytic properties of ruthenium are used on industrial scales (hydrogenations, sometimes enan-tioselective, and metathesis). Titanium electrodes coated with ruthenium oxide are applied in chlorine-alkaline electrolysis. Suitable for corrosion-resistant contacts and surgical instruments. 

Titanium as a carrier metal Titanium (or a similar metal such as tantalum, etc.) cannot work directly as anode because a semiconducting oxide layer inhibits any electron transport in anodic direction ( valve metal ). But coated with an electrocatalytic layer, for example, of platinum or of metal oxides (see below), it is an interesting carrier metal due to the excellent corrosion stability in aqueous media, caused by the self-healing passivation layer (e.g. stability against chlorine in the large scale industrial application of Dimension Stable Anodes DSA , see below). 

Metal oxide coatings Commercial lead dioxide coatings, for example, on titanium, have a higher stability compared with lead or lead alloy anodes with their in situ formed oxide layer. A secure contact between Pb02 and titanium has to be guaranteed, for example, by a platinum layer or at least by a sufficiently large number of platinum crystallites. 

Smooth platinum, lead dioxide and graphite are anode materials commonly used in electrooxidation processes. All show large overpotentials for oxygen evolution in aqueous solution. Platinum coated titanium is available as an alternative to sheet platinum metal. Stable surfaces of lead dioxide are prepared by electrolytic oxidation of sheet lead in dilute sulphuric acid and can be used in the presence of sulphuric acid as electrolyte. Lead dioxide may also be electroplated onto titanium anodes from lead(Il) nitrate solution to form a non-porous layer which can then be used in other electrolyte solutions [21],... 

Transition-metal -phthalocyanines as catalysts in acid medium. To prevent carbonate formation by the carbon dioxide in the air or that produced by oxidation of carbonaceous fuels, an acid electrolyte is necessary hence it is important to find electrocatalysts for an acid medium. Independently of Jasinski, we were soon able to show 3>4> that under certain conditions the reduction of oxygen in dilute sulfuric acid proceeded better with phthalocyanines on suitable substrates than with platinum metal. The purified phthalocyanines were dissolved in concentrated sulfuric acid and precipitated on to the carbon substrate by addition of water. This coated powder was made into porous electrodes bound with polyethylene and having a geometrical surface of 5 cm2 (cf. Section 2.2.2.1.). The results obtained with compact electrodes of this type are shown in Fig. 6. 

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