Metal—sulphur reactions

The method of construction of this type of diagram will be illustrated using the general case of the three component system metal-sulphur-oxygen (M-S-O) whose values of AG for the reactions between the various condensed phases are given in Talile 7.37 on page 7 191. 

Fig. 7.67 Phase stability diagram for a metal-sulphur-oxygen (M-S-O) system at I 000 K. (For the thermodynamic data AC 000 for the various across-boundary reactions, see Table 7.37)...

Reactions of carbon subsulphide and of elementary phosphorus, sulphur and selenium with complexes of the platinum metals Sulphur dioxide insertion reactions of transition metal alkyls and related complexes... 

Reactions (6), (7) and (9) occur with substitution-labile oxidants and (6), (7) and (8) with substitution-inert oxidants. It is postulated that the species (MS03)" involves metal-oxygen coordination for labile M", and metal-sulphur coordination for inert M". The latter mode is thought to prevent sulphur-sulphur bond formation. 

Clearly, any abrasion of surface oxide by the hard crystal edges of the molybdenum disulphide is likely to be particularly important in exposing free metal to reaction with the sulphur atoms. In addition, the depletion of surface oxide in sliding in high vacuum should make it easier for molybdenum disulphide to attach to a worn surface, but the potential of this for re-supply of a molybdenum disulphide film in high vacuum applications has apparently not been studied. 

In reactions of sulphur with hydrogen and hydrocarbons over transition-metal sulphides rate constants decreased in the order H>C2H4 >C2Hg. The rate-determining step for the hydrocarbons, as for hydrogen (cf. Section 2), was cleavage of the metal-sulphur bond. 

Reactions in metal-sulphur systems have been studied, as model systems, to verify mechanisms that are proposed for the high-temperature oxidation processes. The excellent correspondence between the initial theories of Wagner and observation in the case of sulphidation established confidence in the theory, which was then applied widely to the more practically important metal-oxidation processes. 

Stage III The metal-sulphur (Me-S) bonds formed in stage II are active ring openers for Sg molecules and sulphur insertion occurs. It has been postulated that rubber-soluble zinc plays a role in these insertion reactions by forming a chelate with the nitrogen and sulphur atoms of the MET [28]. The net result is the formation of surface molecules of the type (see Figure 6.3, stage III) ... 

Gases which are high in FIjS are subject to a de-sulphurisation process in which H2S is converted into elemental sulphur or a metal sulphide. There are a number of processes based on absorption in contactors, adsorption (to a surface) in molecular sieves or chemical reaction (e.g. with zinc oxide). 

Sulphur can be reduced directly to hydrogen sulphide by passing hydrogen through molten sulphur the reversible reaction H2 -I-S H2S occurs. In the laboratory the gas is most conveniently prepared by the action of an acid on a metal sulphide, iron(II) and dilute hydrochloric acid commonly being used ... 

Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidise non-metals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products. 

These materials have been prepared by polymerisation of p-halothiophenoxide metal compounds both in the solid state and in solution. They have also been prepared by condensation of p-dichlorobenzene with elemental sulphur in the presence of sodium carbonate while the commercial polymers are said to be produced by the reaction of p-dichlorobenzene with sodium sulphide in a polar solvent. 

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