Reaction between Molybdenum Disulphide and Water

Abstract

MOLYBDENUM disulphide, MoS₂, undergoes oxidation in bulk at temperatures above 450° C, which may be demonstrated¹ by gravimetry. However, the substance must ordinarily be covered with a surface layer of oxide, since heating samples in a good vacuum leads to the evaporation of molybdenum trioxide. It is possible that such layers are involved in the frictional transients observed² when molybdenum disulphide is used as a lubricant in wet atmospheric environments. It is germane to inquire as to the origin of the oxide layers, and we wish to comment on the possible extent of the hydrolytic decomposition of molybdenum disulphide to yield hydrogen sulphide and molybdenum dioxide. Haltner found² that when an MoS₂ film, supported on certain metals, and particularly copper, was exposed to rubbing and shearing forces in a wet atmosphere, hydrogen sulphide could be detected. If no metal is present, however, the reaction appears to be different, for Cannon³ found that the gravimetry of adsorption of water on fine particle molybdenum disulphide showed a strong and extensive irreversible interaction at 0° C. The latter also found that for very finely dispersed samples, if the specific surface area, rather than the mass of the solid, were taken as the extensive variable, the observed gravimetry conformed most closely with the reaction : rather than a hydrolysis. While no search for hydrogen evolution was made in the earlier experiments^1–3, an examination of the thermal behaviour³ of the putative oxysulphide indicated that a temperature of 350° C could be attained before any appreciable loss in weight was observed, and that this loss appeared to be associated with the formation of sulphur dioxide rather than hydrogen sulphide. It thus seemed likely that the reaction of molybdenum disulphide with water, studied in the absence of metals under conventional high vacuum conditions, was not a hydrolytic process of sulphide to oxide plus hydrogen sulphide, but a net oxidation. We have now extended the work to higher temperatures, with the following interesting results. The water adsorption work was repeated at 180° C, 20µ water vapour pressure, in quartz microbalance equipment like that described before^1,3. The surface-area limited reaction with a permanent weight increase proportional to the specific surface area was seen again. 20 g samples of molybdenum disulphide (Σ = 15 m²/g) were then exposed to 20 mm water vapour at progressively higher temperatures between 200° and 500° C. The experiments were conducted in a quartz envelope to permit pre-degassing the solid. Pressures were monitored through the seven-day holding period of each of the experiments and gas samples were withdrawn for mass spectrometric study after one day each at various temperatures. The results are summarized in Table 1.