Molybdenum sulphidation

Molybdenum sulphide, like any sulphide, reacts violently with oxidants. There was an accident reported describing a molybdenum sulphide/potassium nitrate mixture, which detonated. 

Partially Crystalline Transition Metal Sulphide Catalysts. Chiannelli and coworkers (6, 7, 8) have shown how, by precipitation of metal thio-molybdates from solution and subsequent mild heat-treatment many selective and active hydrodesulphurization catalysts may be produced. We have shown (18) recently that molybdenum sulphide formed in this way is both structurally and compositionally heterogeneous. XRES, which yields directly the variation in Mo/S ratio shows up the compositional nonuniformity of typical preparations and HREM images coupled to SAED (see Figure 2) exhibit considerable spatial variation, there being amorphous regions at one extreme and highly crystalline (18, 19) MoS at the other. 

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. 

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

Alumina-supported Co- and Ni-promoted molybdenum sulphide hydrotreating catalysts are the main workhorses in many refineries and have, therefore, attracted a lot of attention from catalytic chemists. They are usually prepared via co-impregnation, i.e. pore-volume impregnation with both Mo and the promoter atom present in solution. After drying and calcining, the catalyst manufacture is complete, but it has to be sulphided before use. Traditionally, this is done in situ... 

Alumina supported molybdenum oxide catalysts are extensively used in hydrotreating reactors. Under reaction conditions the oxides are not stable because they are transformed into molybdenum sulphides. In practice this step is carried out in a so-called sulphiding step. In this step the catalyst is pretreated with a mixture containing a sulphur compound. The sulphiding step was investigated by carrying out a TPS study. TPS is strictly analogous to TPR, except for the gas mixture which is here a H2/H2S mixture. 

Table 3.4 Properties of Less Common Molybdenum Sulphides...

The sulphidation of Mo(llO) is promoted by a series of admetals (Fe, Co, Ni, Cu, Ag and Zn) [19,20,57,58] that form sulphides that are less stable than those formed by molybdenum [42], Figure 19 displays Mo 3d XPS spectra acquired upon dosing S2 to Mo(l 10) and Mo(l 10) surfaces with similar coverages ( 1.5 ML monolayers ) of nickel [19], copper [16], zinc [20] and silver [16], These and other results [57,58] indicate that the amount of MoSx formed depends strongly on the nature of the admetal. Specifically, nickel and cobalt have a unique ability to promote Mo S interactions and the formation of molybdenum sulphide [19,57,58], Results for the reaction of S2 with a series of X/Mo(110) surfaces (X=admetal) indicate that the promotional effect of an admetal increases following the sequence Ag Zn < Cu < Fe < Co < Ni [19,57,58],... 

Molybdenum Sulphide Catalysts. Pure molybdenum sulphide is active for metathesis reactions. Preferential degenerate but-2-ene conversions of cis-io-cis and trans-Xo-trans indicate some stereoselectivity. Okuhara and Tanaka found that, over molybdenum sulphide, olefins having vinyl groups exchange vinyl hydrogens selectively and rapidly, but hydrogen scrambling in but-2-ene is extremely slow. 

The superconductivity and structure of some ternary molybdenum sulphides 258 the non-stoicheiometry of ZrS2 259 and the phase systems ZnCd-S, ZnHg-S, and CdHg-S260 have all been investigated. The reaction of carbon disulphide with the metals of the transition groups IV, V, and VI, has been studied.261 In most cases, the product of the reaction at 800—1000 °C is a sulphide (or more rarely a mixture of two sulphides), but in the case of the metals niobium and tantalum a mixture of carbides is produced. 

Subsequent deactivation is slower, and is associated with the deposition of relatively larger amounts of metal sulphides. The total amount of deposit is more than enough to block the pore volume of the original catalyst. Molybdenum sulphide is found to migrate through such deposits to the external surface and it is suggested that interactions between nickel, vanadium and molybdenum sulphides results in the formation of a deposit with significant catalytic activity. This activity would be expected slowly to decrease as further metal deposits block any porosity in the deposit itself... 

Much more interesting, however, is the observation by Simpson [36] that molybdenum is always observed on the surface of used catalysts, despite the presence of up to 85 wt% V and 20 wt% Ni on the catalyst. Evidently molybdenum sulphide can migrate readily through the deposit to the surface, a finding confirmed by at least two other studies [40, 41]. As a result, there may well be combinations of sulphided nickel, vanadium and molybdenum at the surface of the catalyst and available to promote further hydroprocessing. 

If this is the case, then nickel deposition, will also be affected by pore blocking by vanadium. As a result, subsequent removal of nickel should occur near the deposit exterior, as has been observed towards the end of the bed [35]. The resulting improvement in the catalytic activity of the deposit (as compared to the activity of vanadium sulphides) would be significant in view of possible interactions between nickel and molybdenum sulphides, and the role of nickel sulphide in facilitating the distribution of molybdenum sulphide across the surface [44]. 

Molybdenum sulphide, M0S2 (molybdenite) strong Mo-S bonds within layers, weak van der Waals interaction between... 

C.M. Sayers (1981). J. Phys. C Solid State Phys., 14, 4969-4983. The vibration spectrum of H bound by molybdenum sulphide catalysts. 

Surface modification of nanoparticles in some cases allow the formation of stable dispersions in liquid hydrocarbons [57], If molybdenum sulphide nanoparticles are considered as friction-modifying additives for liquid lubricants, then a whole range of new possibilities appears. The action of tribologically active additives as nanoparticles has been proposed as in Fig. 8.6. 

Fluorite structure (C.N. 8 4) Rutile structure (C.N. 6 3) Silica structures (C.N. 4 2) Cadmium iodide structure Molybdenum sulphide structure Chain structure... 

Fig. 8.14. Clinographic projection of the unit cell of the hexagonal structure of molybdenum sulphide, MoS2. The four S atoms represented by heavy circles are those within the unit cell the others lie outside the cell but have been added to show the co-ordination about the Mo atoms.

---