Tungsten sulfur effect

The main cause of anode wear is electrochemical oxidation or sulfur attack of anodic surfaces. As copper is not sufficiently resistant to this type of attack, thin caps of oxidation and sulfur-resistant material, such as platinum, are bra2ed to the surface, as shown in Eigure 15a. The thick platinum reinforcement at the upstream corner protects against excessive erosion where Hall effect-induced current concentrations occur, and the interelectrode cap protects the upstream edge from anodic corrosion caused by interelectrode current leakage. The tungsten undedayment protects the copper substrate in case the platinum cladding fails. 

Before going on to describe the functions of the metals we observe that among heavier non-metals only selenium, chlorine and other halogens need any further comment. Selenium is found in some hydrogenases in even the most primitive life forms and may be it was required initially since it is a more effective catalytic centre than sulfur although much less available. (Compare tungsten with molybdenum later.) Its amino acid selenomethionine is coded in early DNA Later it is involved... 

The cores of the W6 clusters are very similar to those of the Mo6 analogues, and they are composed of the regular octahedra of six tungsten atoms capped by eight sulfur atoms (41, 43, 44). The W-W distances are almost the same, and difference in terminal ligands has little effect on the geometry (Table I). Only in the case of tert-butylpyridine complexes has a very small compression of the octahedron been observed in the c-axis direction (44). The bond order for the W—N bonds in the pyridine complexes is much weaker than that for the triethyl-phosphine analogue (43). 

Dimethylsulfide was added to the n-heptane to give 1000 ppm S in the gas phase. Figure 22.3 shows the effect of sulfur on conversion. The platinum catalyst lost its activity in about 3 h, whereas the tungsten carbide catalyst was very slightly affected during the experiment. Sulfur tolerance is important in petroleum refineries, since it may allow the substitution of the costly noble metals with carbides in streams containing sulfur. 

The mechanism of the catalyst activation by sulfur is not understood. The amount of sulfur compounds necessary to maintain or increase the catalyst activity depends in some cases on the stability of the heavy-metal sulfide component of the catalyst. Thus molybdenum sulfide seems to require a higher hydrogen sulfide concentration than tungsten sulfide. However, some catalysts that do not contain elements that can form sulfides under reaction conditions also showed an increased activity when sulfur compounds were added to the feed. Hydrogen sulfide in many cases decreases the catalyst sensitivity to nitrogen compounds and thus causes an activity increase. Sufficient data for pure compounds are not available to permit segregation of these effects. 

Extension of this understanding to metal/silicate systems has come through studies of Jana and Walker (1997a,b) in which it was observed that carbon and sulfur both have large effects on the magnitude of liquid metal-liquid silicate Ds. For example, for tungsten, dissolved carbon increases metal/silicate Ds, whereas sulfur... 

Figure 7 The effect of sulfur content of metallic liquid on the magnitude of the solid metal/liquid metal (SM/LM) partition coefficient. Note that copper and silver have an affinity for S-bearing liquid, whereas nickel, gallium, tungsten, osmium, and rhenium all prefer the solid metal. The connection to core formation is that the latter group of elements will have a lower metal/silicate partition coefficient if the metal is liquid and contains sulfur. Similar effects have been documented for carbon (Willis and Goldstein, 1982) (sources Chabot et al., 2003 Malvin et al., 1986 Jones and Drake, 1983 Liu and Reel, 2001 Fleet et al., 1999).

Examples of sulfur oxidation constitute the sulfone of the ruthenium complex and the sulfoxide of the tungsten derivative [37]. In the latter, an excess of dioxirane effected the oxidation to the corresponding sulfone. Oxyfunctionalizations of this type are unprecedented. 

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