Molybdenum
phosphide (MoP) has been extensively experimentally
shown to possess high and surprisingly increasing hydrodesulfurization
(HDS) activities during the HDS process. In order to understand the
HDS mechanism, we investigate the HDS of thiophene on clean and single-sulfur-atom-modified
MoP(001) using self-consistent periodic density functional theory
(DFT). Thiophene strongly prefers flat adsorption,
which is slightly weakened in the presence of a surface S atom. Thermodynamic
and kinetic analyses of the elementary steps show that the HDS of
thiophene takes place along the direct desulfurization (DDS) pathway
on both clean and S-modified MoP(001), because of the very low C–S
bond activation barriers as well as very high exothermicities involved.
More importantly, the surface S atom does not elevate the C–S
bond activation barriers but opens a new concerted pathway for the
simultaneous rupture of both C–S bonds in thiophene. These
results indicate that the presence of a surface S atom could be helpful
for thiophene desulfurization. For comparison, we also investigate
the influence of a surface S atom on the HDS of thiophene on Pt(111).
The results show clearly a negative effect of the surface S atom,
in accordance with the lower sulfur resistance of noble metals.
The hydrodesulfurization (HDS) of thiophene on clean and S-modified MoP(010) is investigated to understand the HDS mechanism as well as the surface sulfur (S) atom effect using periodic density functional theory (DFT). The results show that thiophene prefers strongly flat adsorption on both the clean and S-modified surfaces, in either the molecular state or the dissociative state breaking simultaneously one C-S bond, and the adsorption of thiophene can be slightly weakened by the surface S atom. Thermodynamic and kinetic analysis indicates that the HDS of thiophene in both the molecular and dissociative adsorption states prefers to take place along the direct desulfurization (DDS) pathway rather than hydrogenation on both the clean and S-modified MoP(010) surfaces. Surface S shows a promotion effect on the HDS catalytic activity of MoP(010), because the energy barrier/rate constant of the rate-determining step on the DDS pathway is decreased/enlarged under the S modification. Compared with the situation of MoP(001), MoP(010) should have relatively low HDS activity, since a higher energy barrier as well as weaker exothermicity is involved in the reaction on the latter surface.
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