Competitive adsorption of nitrogen and sulphur compounds on a multisite model of NiMoS catalyst: A theoretical study

• Adsorption of various N- and S-compounds on NiMoS sites quantified by DFT-D2.
• Effects of dispersion, temperature and entropic corrections are discussed.
• Protonation of basic N-compounds on Brønsted –SH sites favoured at high T.
• Detailed vibrational analysis of pyridine adsorption modes.
• Multi-site (Lewis Ni or Mo and Brønsted –SH) inhibition of thiophene and 4,6-DMDBT by N-compounds.

Using density functional theory including dispersion corrections, we establish a scale of enthalpies and Gibbs energies of adsorption of various organo-nitrogen molecules on a multisite model of the NiMoS active phase, which includes coordinatively unsaturated Lewis sites (Mo or Ni) and Brønsted –SH sites located on the M-edge and S-edge. Dispersion corrections and entropic effects are shown to impact significantly Gibbs energies of adsorption. In particular, protonation of pyridine derivatives is enhanced at high temperature on the Brønsted –SH sites of the M-edge and S-edge. For pyridine, a vibrational frequency analysis of the adsorption modes is undertaken. Stable adsorption configurations are identified for thiophene and 4,6-DMDBT on Lewis and Brønsted sites, and compared to N-containing molecules. While pyrrole derivatives are weak poisons of adsorption sites, pyridine derivatives are the stronger inhibitors. Furthermore, ammonia exhibits a non-negligible inhibitor character. We finally discuss the impact of inhibitors on 4,6-DMDBT hydrodesulphurization pathways.

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