A DFT Study of CoMoS and NiMoS Catalysts: from Nano-Crystallite Morphology to Selective Hydrodesulfurization

Abstract

Résumé -Étude DFT des catalyseurs CoMoS et NiMoS : de la morphologie des nano-cristallites à l'hydrodésulfuration sélective -Cet article présente des résultats récents obtenus dans des études utilisant la théorie de la fonctionnelle de la densité (DFT) pour la caractérisation à l'échelle atomique des phases actives Co(Ni)MoS utilisées en catalyse d'hydrodésulfuration (HDS) sélective. L'effet des conditions du milieu réactionnel sulfo-réducteur sur la stabilité et la nature des sites actifs localisés au bord des nano-cristallites Co(Ni)MoS est mis en évidence. En combinant les calculs DFT et un modèle thermodynamique, il apparaît que la teneur en promoteur sur les bords des cristallites en conjonction avec la morphologie des cristallites dépend des conditions sulfo-réductrices. Nous montrons ensuite comment la nature des sites actifs Co(Ni)Mo influence l'HDS sélective de molécules soufrées légères (alkylthiophènes) vis-à-vis de l'hydrogénation (HydO) des oléfines légères (alkyl-butènes). La combinaison des calculs DFT d'adsorption de deux molécules modèles (2-méthyl-thiophène et 2,3-diméthyl-but-1-ène) avec la modélisation cinétique de la sélectivité HDS/HydO permet de proposer une explication de l'origine de cette sélectivité basée sur la compétition d'adsorption des réactifs. Abstract -A DFT Study of CoMoS and NiMoS Catalysts from Nano-Crystallite Morphology to Selective Hydrodesulfurization -This paper reports an overview of recent DFT studies on the atomic scale characterization of Co(Ni)MoS active phases used in selective hydrodesulfurization (HDS) catalysis. A peculiar attention is paid to the effect of the sulfo-reductive reaction conditions on the

A Rational Comparison of the Optimal Promoter Edge Decoration of HDT NiMoS vs CoMoS Catalysts

“…In both cases, the promoter-to-molybdenum atomic ratios in the nanocrystallites do not exceed 0.5, which is in agreement with nanocrystallites morphologies obtained by DFT [21,22]. The optimum obtained is the same for both promoters.…”

“…From DFT modeling [21,22], it is known that the NiMoS crystallites exhibit a deformed hexagon-like two-dimensional morphology with a proportion of M-edge of about 0.6-0.65 in HDT conditions (Fig. 8), which provides us with Equation (4).…”

“…The DFT results used in this work are presented in details in the companion paper by Krebs et al published in this issue [21] as well as in a previous paper [22]. Total energy calculations are based on the plane wave density functional theory within the generalized gradient approximation [23,24] and periodic boundary conditions.…”

Advanced Methods of Electron Microscopy in Catalysis Research

“…In both cases, the Mo-edge structure exhibits mixed Co-Mo sites while the sulfur atom is located at the top of the molybdenum atoms. Nevertheless, it was also found that sulfur atoms might also be located in bridging positions, which indicated that the mobility of sulfur atoms on the Mo-edge is high (Krebs et al, 2009;Schweiger et al, 2002). To illustrate the experimentally observed morphologies, two models for the cobaltmolybdenum-sulfur and cobalt-tungsten-sulfur catalysts were built (see Figures 4.42c and 4.42d).…”

“…In the past two decades, numerous configurations for the sulfulradsorption and the cobalt-promoter distribution at the molybdenum and sulfur active edges of MoS 2 have been intensively investigated by different theoretical and experimental groups. One of the most relevant density functional theory simulation results shows that, for typical HDS conditions, the energy of the S-edge with 100% cobalt is lower than the Mo-edge with 100% cobalt (Krebs et al, 2009;Schweiger et al, 2002;Byskov et al, 2000;Sun et al, 2004a). However, in addition to the affinity of cobalt for the S-edge under such reaction conditions, a 50% cobalt-promoter decoration of the Mo-edge reduces considerably the edge energy of the Mo-edge to a value close to the S-edge value.…”