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

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 Bronsted -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 Bronsted -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 Bronsted 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. (C) 2015 Elsevier Inc. All rights reserved.