Abstract
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•The single sulfur vacancy is preferable to be formed at the S-edge of unpromoted MoS2/WS2.•The formation process of single sulfur vacancy is relatively difficult over the unpromoted WS2.•The existence of Co/Ni atom at the S-edge promotes the formation of single sulfur vacancy.•The double sulfur vacancies present at a very low frequency over the unpromoted MoS2/WS2.•The temperature and pressure are two important factors affecting the sulfur vacancy formation.
The formations of sulfur vacancies over different active sites of Mo-based and W-based active phases are investigated under the traditional hydrodesulfurization (HDS) reaction conditions using DFT calculations. It can be found that under the HDS reaction conditions (600 K, p(H2) =4 MPa and p(H2S)/p(H2) = 0.001), the single sulfur vacancy can be easily formed at the S-edge rather than the M-edge/W-edge. Comparably, with the promotions of Co or Ni atoms occurring at the S-edge of the MoS2/WS2 active phase, the formation of single sulfur vacancy will become more favorable at this site. Moreover, it is interesting that when Ni atoms further replace Mo atoms at the S-edge of NiMoS active phases, the formations of double sulfur vacancies become more favorable, which can expose more accessible Ni sites for the adsorptions of sulfur-containing compounds. And temperature and pressure are two important factors affecting the sulfur vacancy formation.