Noble avoidance

In industrial appHcations it is not uncommon that the thermocouple must be coupled to the readout instmment or controUer by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wine be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wine may be prohibitive manufacturers of extension wine may suggest compromises which are less costiy. Junctions between the thermocouple leads and the extension wine should be made in an isothermal environment. The wine and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

Regeneration of noble metal catalysts to remove coke deposits can successfully restore the activity, selectivity, and stabiUty performance of the original fresh catalyst (6—17). The basic steps of regeneration are carbon bum, oxidation, and reduction. Controlling each step of the regeneration procedure is important if permanent catalyst damage is to be avoided. 

When plating any substrate less noble than copper, only a few mg/L of dissolved copper in the acid baths can adversely affect adhesion. Coatings can be too thin to be visible, yet contribute to poor adhesion. Small additions of thiourea have been used to prevent copper immersion, but it acts as a potent inhibitor, and work should be re-electrocleaned after the acid. Work should be exposed to the mildest acid treatments possible. Over-etching should be avoided. 

In the late 1980s, however, the discovery of a noble metal catalyst that could tolerate and destroy halogenated hydrocarbons such as methyl bromide in a fixed-bed system was reported (52,53). The products of the reaction were water, carbon dioxide, hydrogen bromide, and bromine. Generally, a scmbber would be needed to prevent downstream equipment corrosion. However, if the focus of the control is the VOCs and the CO rather than the methyl bromide, a modified catalyst formulation can be used that is able to tolerate the methyl bromide, but not destroy it. In this case the methyl bromide passes through the bed unaffected, and designing the system to avoid downstream effects is not necessary. Destmction efficiencies of hydrocarbons and CO of better than 95% have been reported, and methyl bromide destmctions between 0 and 85% (52). 

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. 

Catalytic processes frequently require more than a single chemical function, and these bifunctional or polyfunctional materials innst be prepared in away to assure effective communication among the various constitnents. For example, naphtha reforming requires both an acidic function for isomerization and alkylation and a hydrogenation function for aromati-zation and saturation. The acidic function is often a promoted porous metal oxide (e.g., alumina) with a noble metal (e.g., platinum) deposited on its surface to provide the hydrogenation sites. To avoid separation problems, it is not unusual to attach homogeneous catalysts and even enzymes to solid surfaces for use in flow reactors. Although this technique works well in some environmental catalytic systems, such attachment sometimes modifies the catalytic specifici-... 

In conditions where sea-water spray may be deposited regularly on magnesium articles with no alleviating mechanism for its removal, or where breaking waves may drench the components, the effect is quite different. Corrosion of bare metal will be heavy and will be intensified at junctions with other more noble metals. Unless magnesium alloys can be adequately protected in such combinations it is better to avoid their use. This matter is dealt with under the section on protection. 

Many chemical-plant vessels constructed of mild steel, copper, or other base metals may be protected from corrosive attack by their contents by means of a lining of one of the noble metals. This form of construction utilises the higher strength of the base metal —as, for example, in the walls of pressure vessels — and at the same time a minimum amount of a much more expensive noble metal is used. The use of noble metals in plant is frequently determined by the desire to avoid contamination of valuable reactants or substances by the products of corrosion of base metals. 

In spite of their high initial cost, the noble metals in industry offer substantial advantages and economies in use. They avoid the frequent and expensive... 

Paint the surfaces of both metals. Avoid painting only the less noble metal because if the coating is damaged severe attack may result at the damaged area. 

Principles Immersion plating resulting from a displacement reaction involving the metal to be coated can continue only as long as the less noble substrate remains accessible to the plating solution, and therefore as plating proceeds, the quantity of A/, deposited, and of A/j dissolved, falls. Dissolution of A/j can be avoided by coupling it with a less noble metal A/, so that only A/j dissolves, i.e, by internal electrolysis. 

Some pigments promote corrosion owing to their content of soluble salts, their reactivity, or their electrochemical action, and thus should be avoided. Rust of the spotted type can be the consequence of their presence in a paint, especially the hrst coat, e.g. of graphite (noble to steel), some red oxides of iron, gypsum, ochre or lamp black. 

However, there may be good reasons why a catalyst should not consist of particles that are too small, as we saw in the beginning of this chapter, e.g. to avoid pressure gradients in the reactor. Based on an analysis such as the above, one can decide whether it makes sense to use support particles that contain a homogeneous distribution of the catalytic phase. With expensive noble metals, one might perhaps decide to use an egg-shell type of arrangement, where the noble metal is only present on the outside of the particles. 

The ideal operating temperatures for the three-way catalyst lie between 350 and 650 °C. After a cold start it takes at least a minute to reach this temperature, implying that most CO and hydrocarbons emission takes place directly after the start. Temperatures above 800 °C should be avoided to prevent sintering of the noble metals and dissolution of rhodium in the support. 

Transition metal oxides represent a prominent class of partial oxidation catalysts [1-3]. Nevertheless, materials belonging to this class are also active in catalytic combustion. Total oxidation processes for environmental protection are mostly carried out industriaUy on the much more expensive noble metal-based catalysts [4]. Total oxidation is directly related to partial oxidation, athough opposes to it. Thus, investigations on the mechanism of catalytic combustion by transition metal oxides can be useful both to avoid it in partial oxidation and to develop new cheaper materials for catalytic combustion processes. However, although some aspects of the selective oxidation mechanisms appear to be rather established, like the involvement of lattice catalyst oxygen (nucleophilic oxygen) in Mars-van Krevelen type redox cycles [5], others are still uncompletely clarified. Even less is known on the mechanism of total oxidation over transition metal oxides [1-4,6]. 

Scuba tanks usually contain compressed air, which is essentially a mixture of nitrogen (78%) and oxygen (21%). In order to avoid a painful condition called the bends, deep-sea divers replace the nitrogen with the noble gas —... 

Nor do I deny that my son was a sinner, but he was a man of spirit also and knew that he had two half-sisters and a half-brother bom out of wedlock. He regarded Gentile s reprimands as sheer hypocrisy. She paused, as if realizing that she was avoiding the issue. Zorzi frequented courtesans, yes. But at the time of Gentile s death, he was enamored of a woman of noble birth. ... 

It must be noted that short contact time reactors are typically operated under adiabatic conditions with outlet temperatures of the order of 700-1000 °C. Under such conditions, with respect to other noble metals, Rh is believed to be especially stable due to a low vapor pressure and an increased resistance to carbon formation even under severe operating conditions. The use of low surface area oxides such as (Z-AbO j and ZrO2 as support materials has been reported to improve the catalyst stability by limiting the coarsening of Rh particles while avoiding incorporation of Rh within the oxide structure [148]. 

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