How easy it is for MoS2 to become oxidized

I made I high pressurized $\ce{MoS2}$ pellet, using a 6 ton pressurizer. I used very thin powder to create the pellet.


I am using this pellet on beam, with some nuclear techniques. In my spectra I can see oxygen.


How easy it is for $\ce{MoS2}$ to become oxidized?




It seems that the problem has been examined even 50 years ago, probably in order to determine if/how oxidation would alter the the properties of $\ce{MoS2}$ as a lubricant.


[Here](https://archive.org/details/nasa_techdoc_19930082560) is an old NASA document from 1949, and in [Surface Oxidation of Molybdenum Disulfide](http://dx.doi.org/10.1021/j150531a020) from 1955, the authors state in the abstract, that the oxidation is limited to the outmost surface layer, which then passivates and protects the rest of the batch. 


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Just as for almost every compound of d-elements, $MoS\_2$ tends to adsorb molecules from gases on its surface. This is especially true for fine powders, that have high surface-per-gram. MoS2 is especially troublesome, because in strongly reductive atmosphere it is reduced to metallic molybdenum. So, ideally, to get a pure MoS2 you have to 


* threat powder with $H\_2S$ at about 300-400 Celsium before use
* press powder into pellet under argon
* not allow it to touch air for any significant amount of time (more that several seconds)


In general a very brief exposure to air may significantly oxidize $MoS\_2$ catalysts, so you can safely assume that if your $MoS\_2$ touched air, it contains oxygen, especially if it is in fine powder.


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