Chromium sulfide

The high resistivity of Inconel 600 (11 OjtI 0 8 Dm) demanded the application of this material as a composite with a central aluminum core. The aluminum was totally enclosed in Inconel 600 so that the Inconel was only exposed to sulfur and polysulfides. In a test over more than three years, cells with a composite current collector of this kind suffered from a high capacity decline. Post-test analysis showed that Inconel sustained polysulfide attack with the formation of a duplex nickel and chromium sulfide layer on the current collector surface. 

Young DJ, Smeltzer WW, Kirkaldy JS (1973) Nonstoichiometry and thermodynamics of chromium sulfides. J Electrochem Soc 120 1221-1224... 

Redox reactions may cause mobile toxic ions to become either immobile or less toxic. Hexavalent chromium is mobile and highly toxic. It can be reduced to be rendered less toxic in the form of trivalent chromium sulfide by the addition of ferrous sulfate. Similarly, pentavalent (V) or trivalent (III) arsenic, arsenate or arsenite are more toxic and soluble forms. Arsenite (III) can be oxidized to As(IV). Arsenate (V) can be transformed to highly insoluble FeAs04 by the addition of ferrous sulfate. 

All sulfides (except alkali metals, ammonium, magnesium, calcium, and barium) are insoluble. Aluminum and chromium sulfides are hydrolyzed and precipitate as hydroxides. 

In some cases the methods may be combined. Examples would include the biotechnological precipitation of chromium from Cr(VI)-containing wastes from electroplating factories by sulfate reduction to precipitate chromium sulfide. Sulfate reduction can use fatty acids as organic substrates with no accumulation of sulfide. In the absence of fatty acids but with straw as organic substrate, the direct reduction of chromium has been observed without sulfate reduction [43]. 

Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS2 a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. 

The elemental sulfur may also react with chromium to form sulfides and deplete the surface in chromium. Furthermore, the chromium sulfide releases sulfur by thermal decomposition, which in turn diffuses into the metal and depletes the chromium from the surface, which would otherwise offer protection. This mechanism of depletion of chromium due to sulfidation and increase in the rate of hot corrosion can be overcome by increasing the chromium content of the alloy to a high level. 

The chromium sulfide system is very complex, with two forms of Cr2S3 and several intermediate phases between these and CrS. Rhombohedral Cr2S3 has complex electrical and magnetic properties. The mixed-valent telluride Cr3Te4 is ferromagnetic. 

Chromium sulfide is the active component for the dehydrogenation alumina which possesses desulfurization sites, confers a poor selectivity to the Cr/Al203 catalyst. We can note the very high activity of the Cr/C catalyst. The selectivity to dihydrothiophene (DHT) is always higher at a 50% conversion than at high conversion (80-100%). Results obtained at low conversion [9] confirm this tendency, the selectivity to DHT reaching 15-20% at a 10-20% conversion, in accordance with the consecutive scheme (Eqn.l) proposed for the dehydrogenation. 

Chromium sulfide is a selective catalyst of THT dehydrogenation minimizing the C-S bond cleavage. The activity seems to be correlated to the reducibility of chromium into Cril species. Carbon is an excellent support promoting both the dispersion of the active phase and favoring the creation of coordinatively unsaturated chromium species (probably CrII species). However further inve.stigations are required to improve its stability. 

The chromium sulfide preparations obtained via method I have a metallic appearance and become fused at the temperature of preparation. The CrS possesses a hexagonal superstructure of the B 8 t3qie, while at 59.7 atom% of S, a B 8 structure with the axial ratio c/a = 1.62s has been shown toexist. Thq CrgSa obtained method II consists of hexagonal black leaflets, resistant to nonoxidizing acids, easily soluble in HNO3. 

Equilibrium data are not available for 83nstems containing NiS or NisS2 in combination with molybdenum sulfide, tungsten (wolfram) sulfide, or chromium sulfide. 

Sulfur may be substituted for hydrogen in the reduction of saturated hydrocarbons over nickel sulfide catalysts. In this way, carbon disulfide may be synthesized from methane plus hydrogen sulfide over nickel sulfide catalysts at 450-750°. The same reaction takes place with other hydrocarbons at slightly lower temperatures. At still lower temperatures, hydrogen sulfide adds to the double bonds of olefins in the presence of nickel sulfide, and of nickel sulfide-chromium sulfide mixtures, to yield thiols. 

Nogl] Noguchi M., Narita T., Nonstoichiometry in Iron-Chromium Sulfide (Fe,Cr)i gS at High Temperatures (in Japanese), J. Jpn. Inst. Met, 60(6), 589-594 (1996) (Experimental, Phase Relations, 9)... 

Nog] Noguchi, M., Yamamoto, T, Narita, T, The Nonstoichiometry and Chemical Diffiisivity of Iron-Chromium Sulfide (Fe,Cr)i gS , Mater. Set Forum, 251-254,135-142 (1997) (Experimental, Phase Relations, Kinetics, Thermodyn., 7)... 

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