Hydrogen adsorbed molybdenum

All of the species are reacting in their adsorbed states. If enol species form as reaction intermediates as suggested for CO hydrogenation over molybdenum, the elementary surface reaction sequence may be expressed as follows ... 

Role of adsorbed hydrogen species on ruthenium and molybdenum sulfides. Characterization by inelastic neutron scattering, thermoanalysis methods and model reactions. 

For example, a simple and possibly too naive picture of the HDS reaction would be that a sulfur vacancy at the edge of an MoS2 slab is the active site. A sulfur-containing hydrocarbon such as thiophene adsorbs with its sulfur atom towards the exposed molybdenum. Next, the molecule becomes hydrogenated, the two C-S bonds in thiophene break, the sulfur-free hydrocarbon desorbs and the catalytic site is regenerated by the removal of sulfur by hydrogen. We refer to Prins et al. [50] for a discussion of reaction mechanisms. 

In summary the results show that (1) neither of the nitrided phases contains sulphur (2) the observed nitrogen contents, in both cases, are higher than the theoretical value in MoN, with the most significant difference occurring in L-MoN, as already noted. Note that these results were collected after treatment of the powders at 573 K under hydrogen, in order to remove eventual adsorbed species and (3) the low density values comparatively to the dXRD density tabulated for 5-MoN imply molybdenum atom vacancies, not an excess of N atoms. 

Adsorption on molybdenum disulphide is important because of its effect on lubrication, and Kalamazov and co-workers , studied the adsorption of oxygen, hydrogen, nitrogen and water vapour. They found that after desorption at 900°C and 10 Pa (10 Torr) subsequent re-adsorption was at a lower level, and inferred that active adsorption sites had been destroyed by the vacuum and high temperature. They found that at 700°C adsorbed water vapour was dissociated, causing oxidation and the liberation of hydrogen. 

A theoretical study of desulfurization of thiirene and methyl thiirene on the molybdenum catalysts MoS3H3 and M0S4H4 was reported to involve hydrogenation of the double bond in the first step <2003BCSE75>. The extended Huckel technique was used to determine the orientation of thiirene adsorbed on vacant sites of MoS3H3. The molecule is adsorbed preferentially in the vertical sense through the sulfur atom <2000JSOA127>. 

However, Poulet et al. (79) reported that phosphorus induces a decrease of the S/Mo ratio in a MoP/ Al catalyst after sulfidation or after a subsequent hydrogen treatment. Topspe et al (98) interpreted the shift of the IR bands of NO adsorbed on (Ni)MoP/Al catalysts toward higher frequency as indicating less sulfidation of the molybdenum (and nickel) species. These authors also confirmed, by using Mossbauer spectroscopy, the presence of a less sulfided state of molybdenum when phosphorus is present. 

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