Sulphides of molybdenum

The only important sulphide of molybdenum is the disulphide MoSj, whose properties are described in the next chapter. Apart from the disulphide, three other sulphides have been reported , and some of their salient properties are listed in Table 3.4. 

The sesquisulphide Mo Sj is said to have been prepared by rapid heating of the disulphide in the absence of air, and extraction with cold dilute aqua regia, or by combination of molybdenum and sulphur at 1300°C °. 

The pentasulphide MojSg is said to be formed as a dark brown amorphous precipitate of the trihydrate when hydrogen sulphide is passed through an aqueous solution of a pentavalent molybdenum coompound, but like all pentavalent molybdenum compounds it is unstable. 

The trisulphide M0S3 is precipitated when hydrogen sulphide is passed through weakly acid solutions of molybdates, or can be obtained by thermal decomposition of thiomolybdates, but it has also been identified in Chilean ores. It decomposes on heating above 1000°C to give free sulphur and molybdenum disulphide. 

Compound Formula Colour Crystal Form Remarks 

---

Thiosalts and Sulphides of Molybdenum and Tungsten. Pour 2-3 ml of an ammonium molybdate solution into one test tube and 2-3 ml of a sodium tungstate solution into another one. Add 2-3 drops of a 25 % ammonia solution to each solution and pass a stream of hydrogen sulphide through them from a Kipp gas generator. What compounds form Acidify the solutions with a 10% hydrochloric... 

In addition to the above, preparation in w/o microemulsions of nanoparticles of various other types of compounds, viz. silica-coated iron oxide, Fe203-Ag nanocomposite, oxides of ytrium, erbium, neodymium, vanadium and cobalt, titanates of barium and lead, ferrites of barium, strontium, manganese, cobalt and zinc, oxide superconductors, aluminates, zirconium silicate, barium tungstate, phosphates of calcium, aluminium and zinc, carbonates of calcium and barium, sulphides of molybdenum and sodium, selenides of cadmium and silver etc. have been reported. Preparative sources and related elaboration can be found in [24]. 

Graphite was also known to the ancients, but up to the tame of Scheele no distinction was made between it and the closely analogous substance sulphide of molybdenum, HoSj, and, at that period, both those metal-like minerals whkA leave a mark... 

The most important ore of molybdenum is the sulphide molybdenite, M0S2, of which the largest known deposit is in Colorado, USA, but it is also found in Canada and Chile. Less important ores are wulfenite, PbMo04, and powellite, Ca(Mo,W)04. 

The stability of the sulphides other than those of the alkali metals, the alkaline earth metals and aluminium, ranges from that of manganese sulphide, which is easily decomposed by dilute acids and slowly by ordinary steam, to that of mercuric sulphide or molybdenum sulphide, which resist the action of concentrated hydrochloric acid solution. 

If a drop of 0-02N aqueous sodium sulphide is added to a drop of a solution containing arsenate or phosphate on a filter paper and a drop of aqueous ammonium molybdate acidified with sulphuric acid also added, a blue colour develops 2 ferro- and ferri-cyanides and thiocyanate should be absent. The formation of molybdenum blue is used in the colorimetric estimation of arsenic (see p. 321). 

Before World War II, the use of molybdenum based catalysts was popular, with molybdenum oxide (and sometimes sulphide) used for a number of isomerization processes.4 These types of catalyst were less sensitive to sulphur in the feed but were phased out owing to inferior selectivity.5... 

Some forms of molybdenum sulphide catalyse the violent decomposition of hydrazine hydrate. 

E.s.r. spectra have also been presented for molybdenum(v) centres in other iso- and hetero-polymolybdate blues479 and a theoretical description of these types of systems has been given.480 Results of an investigation of the photochromism of amine molybdates are consistent with the formation of mixed isopolyanions containing molybdenum-(v) and -(vi) centres.481 The lilac colouration formed during sulphide separations of molybdenum and tungsten appears to be due to the formation of a unit in which six tungsten(vi) atoms surround a molybdenum(v) centre.482... 

The chemical transport of molybdenum and tungsten as oxides or sulphides in the presence of iodine has been shown to proceed by the formation of the corresponding M02I2 complex.510... 

Fig. 5.19. Sulphidation kinetics of molybdenum from the gaseous phase at 750°C and a sulphur vapour pressure of 4 Pa (0.03 mm Hg). Growth kinetics of the M0S2 layer are initially linear and then parabolic. According to the experimental data by B.S. Lee and R.A. Rapp.364...

Several different processes have been used, the simplest being by the reaction of hydrogen sulphide with molybdenum pentachloride, or the reaction of sulphur vapour with molybdic oxide or molybdenum metal. The last of these processes has been called the SHS process (Self-Propagating High-Temperature Synthesis) and Russian workers have reported that the product is less contaminated with impurities and has almost identical lubricating properties to natural molybdenum disulphide. The crystal structure is considered in more detail later, but it seems probable that the initial product of syntheses has a disordered... 

The liberation of gaseous hydrogen was shown by Kalamazov to occur when water vapour was dissociated on the surface of molybdenum disulphide at 700°C. It has also been shown that hydrogen sulphide was produced during sliding of molybdenum disulphide in moist nitrogen, presumably by the reaction... 

There has been some discussion about whether pure molybdenum will in fact react with hydrogen sulphide to produce a surface film of molybdenum disulphide. The vacuum used by Bowden and Rowe to clean the surface of their molybdenum was relatively soft, and it seems probable that some oxide remained on the surface. Later work with cleaner surfaces showed that it is very difficult or even impossible to produce a molybdenum disulphide film on a really clean molybdenum surface by reaction with hydrogen sulphide. 

Probably the most widely-studied and widely-used procedure for in situ formation of molybdenum disulphide films is based on conversion of an electroplated molybdenum coating by reaction with hydrogen sulphide at 204 C (400°F), and was... 

A French paper described direct electrolytic deposition of molybdenum disulphide and molybdenum sesquisulphide from a mixed aqueous bath of ammonium molybdate and ammonium sulphide. G W Rowe and his co-workers produced a molybdenum disulphide coating on molybdenum wire by electrochemical deposition from a... 

The first reports on the incorporation of molybdenum disulphide in a metal matrix were those mentioned previously which were published by Bowden in 1950. He reported a coefficient of friction of 0.13 for a composite in sintered copper. In his other metallic composite the molybdenum disulphide was formed in situ by hydrogen sulphide in sintered molybdenum and had a coefficient of friction of 0.06. At about the same time R L Johnson et al at NACA studied the effect of molybdenum disulphide concentration in silver with 5% of copper. They reported coefficients of friction as low as 0.17 and found that the friction decreased with increasing concentration of molybdenum disulphide. Their wear rates were high, around 10 mm /Nm, but this work was the fore-runner of many studies using the same components. 

Thus any advantage of molybdenum disulphide in an oil in reducing friction will be limited to the boundary and mixed lubrication regions, where the reduction in friction may be considerable. One theoretical analysis suggested that the influence of the dispersed powder depends only on particle shape, size and concentration , or in other words that the dispersed powder is simply forming a physical barrier between the interacting surfaces. It would follow that the same effect could be produced by other dispersed solids, and this was confirmed by studies with zinc sulphide, zinc pyrophosphate and calcium hydroxide. 

Various sulphides of the chromium elements exist, and these, particularly in the case of molybdenum and tungsten, combine with alkalies and alkali sulphides to produce thio-salts thus, thio-tungstates of the types R jW03S, R WOjSj, R WOSg, and R WS, have been prepared. 

Few derivatives of molybdenum dioxide, MoOj, have been prepared, and it is doubtful whether simple salts containing tetravalent molybdenum can be formed in solution. By the electrolytic reduction of acid molybdate solutions, brownish-coloured liquids apparently containing the metal in this stage of oxidation have been obtained, but the evidence is insufficient to determine whether Mo ions are actually present, or whether the liquids merely contain mixed Mo and Mo ions. Potential measurements indicate the presence of mixed ions. The only simple substances containing tetravalent molybdenum, in addition to the oxide, are the sulphide, MoSj, the tetrachloride, M0CI4, and the tetra-bromide, MoBr. There are, however, two series of complex molybdo-eyanides, of the types R 4[Mo(OH)4(CN)J.aq. and R 4[Mo(CN)8].aq. respectively, which contain tetravalent molybdenum and yield well-cry stallised salts. Their existence is probably due to the resistance of the stable complex to hydrolytic decomposition. 

Molybdenum Tetrachloride, M0CI4, may be prepared by the action of chlorine at a high temperature upon molybdenum, or upon the oxide or sulphide mixed with carbon by heating the trichloride to redness in a stream of dry carbon dioxide, both the tetrachloride and the dichioride of molybdenum are obtained, although the former does not distil without decomposition. 

---