Noble of metal

Catalytic oxidation ia the presence of metals is claimed as both nonspecific and specific for the 6-hydoxyl depending on the metals used and the conditions employed for the oxidation. Nonspecific oxidation is achieved with silver or copper and oxygen (243), and noble metals with bismuth and oxygen (244). Specific oxidation is claimed with platinum at pH 6—10 ia water ia the presence of oxygen (245). Related patents to water-soluble carboxylated derivatives of starch are Hoechst s on the oxidation of ethoxylated starch and another on the oxidation of sucrose to a tricarboxyhc acid. AH the oxidations are specific to primary hydroxyls and are with a platinum catalyst at pH near neutraUty ia the presence of oxygen (246,247). Polysaccharides as raw materials ia the detergent iadustry have been reviewed (248). 

Table 4 shows a galvanic series for some commercial metals and alloys. When two metals from the series are in contact in solution, the corrosion rate of the more active (anodic) metal increases and the corrosion rate of the more noble (cathodic) metal decreases. 

Catalysts vary both in terms of compositional material and physical stmcture (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support stmcture (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

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). 

The possible effects of fluid velocity on galvanic corrosion are sometimes overlooked. Fluid velocity can affect the apparent potential of metals in a given environment. Depending on the environment, a metal under the influence of relatively rapid flow may assume either a more noble or a more active character than that indicated by the galvanic series. Occasionally, this shift in potential may result in galvanic corrosion that would not occur under stagnant or low-flow conditions. 

The galvanic potential of metals can vary in response to environmental changes such as changes in fluid chemistry, fluid-flow rate, and fluid temperature. For example, at ambient temperatures steel is noble to zinc (as in galvanized steel). In waters of certain chemistries, however, a potential reversal may occur at temperatures above 140°F (60°C), and the zinc becomes noble to the steel. 

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 ... 

When they have served their purpose or become damaged the noble metals will realise a very high proportion of their initial cost. No matter in what form they are utilised, very efficient processes are available for effecting their complete recovery. This factor often makes noble metals the most economic in use in the chemical and engineering industries. For many applications, no other metal or group of metals can fulfil their function as efficiently, combined with such a low net cost to their user. 

A slight degree of acceleration can be obtained by introducing traces of metals which are more noble than iron, for example nickel, copper, cobalt, silver and mercury. These metals are deposited electrochemically over the... 

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