Molybdenum and Tungsten Sulfides

Molybdenum disulfide, M0S2, can be prepared by chemical vapor deposition using M0CI5 [116, 117], MoFf, [117] or Mo(CO)6 (9) [118] as the molybdenum source. The sulfur source in each case is hydrogen sulfide. The molybdenum tetrakis(thiolate) compound, Mo(S-f-Bu)4, is a unimolecular precursor to M0S2 and decomposes at temperatures as low as 100°C [111]. 

Tungsten disulfide shows properties similar to M0S2 and has been prepared by the CVD of W(CO)h (9) in the presence of H2S [118]. 

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Rosa Llusar was born on September 20th, 1960 in Almenara (Spain). She studied chemistry at the University of Valencia (Spain) where she graduated in 1983 with the highest honors. Her doctorate work was devoted to the chemistry of cubane-type molybdenum and tungsten sulfides and she received Ph.D. degrees from Valencia University in 1987 and from Texas A M University (USA) in 1988, under the guidance of Prof. F. Albert Cotton. After working in the Research and Development Department of a Caprolactam Production Plant in Castello (Spain) for three years, she spent one year (1992) with Prof John D. Corbett at the... 

A similar relationship can be observed with promoted M0S2. Each family of catalysts has its own linear correlation, which cannot be compared to each other directly because of the corrosivity problem. More recently, low-temperature oxygen chemisorption has been claimed to be more reliable, but it also lacks a well-determined stoichiometry (52). Oxygen chemisorption has also been applied to tungsten and rhenium sulfides, as well as promoted molybdenum and tungsten sulfides. In the isotropic class, it has been applied only to ruthenium sulfide, in which case it gives approximately the same result as a BET measurement due to the isotropic nature of this sulfide (41). 

The usual catalysts are based on cobalt, nickel, molybdenum and tungsten sulfides, generally combined and deposited on alumina. The most widely used formula is a composite sulfide of molybdenum and colbalt oo alumina. Run length and catalyst life are longer than those of tbe catalytic systems employed in brst step hydrogenation, Le. 6 to 12 months and 3 to 5 years, and the regeneration method is identical... 

Phthalocyanines are excellent lubricants at temperatures of 149—343°C (191). Combinations with other lubricants, like grease, molybdenum, or tungsten sulfides, have found appHcations in the automotive industry or professional drilling equipment (192—195). Further uses include indicators for iron(Il), molybdenum(V), and uranium(IV) (196) or redox reactions (197), medical appHcations like hemoglobin replacements (198) or sterilisation indicators (199), or uses like in gas filters for the removal of nitrogen oxides from cigarette smoke (200). 

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

Although all of the above elements catalyze hydrogenation, only platinum, palladium, rhodium, ruthenium and nickel are currently used. In addition some other elements and compounds were found useful for catalytic hydrogenation copper (to a very limited extent), oxides of copper and zinc combined with chromium oxide, rhenium heptoxide, heptasulfide and heptaselen-ide, and sulfides of cobalt, molybdenum and tungsten. 

The promotion of bulk binary sulfide is limited exclusively to the promotion of molybdenum and tungsten by the first-row transition metal. The effect of the structural promotion (creating more of the same sites) is always coupled to the electronic promotion (creating more active sites). One ap-... 

In addition to simple CEPT being a favored mechanistic possibility for poly sulfide reduction and formate oxidation, CEPT is almost certainly involved, in all cases, in the reactivation of the molybdenum and tungsten cofactor sites... 

Formate dehydrogenase and poly sulfide reductase function by what could be called simple CEPT (see Section VI.B.2). However, if CEPT is an important component of substrate reactivity in most of the molybdenum and tungsten enzymes, it must involve not only substrate activation but also water addition or elimination. Involvement of the metal in these processes could rely on oxidation state-coupled pA a changes of aqua, hydroxido, or hydrosulfido ligands. Whether the metal is directly involved in substrate binding and water activation or whether... 

In a recent publication zinc was considered for reactions with molybdenum and tungsten sulfides4. When compared with the elements discussed above, zinc has the highest affinity to sulfur, and if the pure elements, e.g., Mo or W, Zn and S are allowed to react with each other, ZnS forms immediately, prior to the formation of Mo2-06S3, MoS2, or WS2, respectively. Zinc is so aggressive that molybdenum or tungsten sulfides are reduced at once at elevated temperatures,... 

Substantial advances in the chemistry of early transition metal trimers with halide, 0X0, and sulfide ligands (no 7c-acid ligands) have been made since a summary of this area was published in 1980 Molybdenum and tungsten are treated in separate sections to clarify differences in their chemistry in spite of some general similarities between... 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

Furthermore, under conditions of kinetic control, the rate of saturation of fused polyaromatic rings has been found to be faster than for monoaromatics over sulfided molybdenum and tungsten catalysts, and it appears that relative rates can be generalized as... 

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