The effect of Co-promotion on MoS2 catalysts for hydrodesulfurization of thiophene: A density functional study
Graphical abstract
Density functional theory (DFT) calculations of the hydrogenation (HYD) and direct desulfurization (DDS) pathways of thiophene hydrodesulfurization over cobalt-promoted MoS2. The Co–Mo–S edge is reactive toward both hydrogenation and C–S bond scission.
Introduction
MoS2-based catalysts are widely used as hydrodesulfurization (HDS) catalysts and the need to optimize these catalyst is driven by both the need to process heavier feeds and the increasing demand for ultra clean diesel products. Typically, Co- or Ni-promoted MoS2 is used for industrial HDS catalysts, since promotion of MoS2 with Co or Ni increases the activity of the catalyst [1]. In this work, we focus on Co-promotion, and the structure responsible for catalytic activity in Co-promoted catalysts is the Co–Mo–S-type structure [2], which consists of MoS2 particles in which Mo-edge atoms are substituted by Co or Ni. The Co–Mo–S structure was initially identified by using a series of in situ techniques [1]; and recent joint theoretical and experimental investigations have revealed that Co substitutes Mo atoms at the edge of MoS2 [3], [4], [5], [6], [7], [8]. This edge is commonly denoted as the S-edge, whereas the other low-index edge is denoted as the Mo-edge. Despite the increased understanding of the Co–Mo–S structure, very little is known about the nature of the active site and the reaction mechanism of HDS over Co–Mo–S catalysts. Furthermore, there is still much debate on the effect of promotion with regard to the relative increase in hydrogenation and C–S-scission activity [9], [10], [11], [12], [13], [14], [15] and the resistivity toward inhibitors such as H2S [9], [11], [16].
For a long time it has been a common belief that the catalytic activity of HDS is related to its ability to form sulfur vacancies, i.e., coordinatively unsaturated sites (CUS) at which sulfur can be bound subsequent to carbon–sulfur bond scission [1], [17], [18]. The present results show that this simple picture has to be modified somewhat. Therefore, this concept warrants further discussion. A natural starting point for each edge structure is its stoichiometric termination. However, this may not be the structure dominating under different experimental or reaction conditions, but such structures can be determined from DFT-based phase diagrams [6], [18], [19], [20], [21]. These phase diagrams reveal a large variety of structures with different sulfur and hydrogen coverages depending on temperature and H2 and H2S partial pressures. It is interesting that we found [22] that in several cases the equilibrium structures “as is” are reactive toward hydrogenation and/or sulfur scission. For example, we found in our previous study that the relevant Mo-edge structure can adsorb thiophene and catalyze HDS on unpromoted MoS2. In other cases, sulfur has to be removed first for the edge to be reactive at all, as was calculated for the S-edge in our previous study of MoS2 [22], [23]. For the unpromoted Mo-edge, we would thus consider this edge to be “born with sulfur vacancies”, which means that sulfur vacancies are already present in the equilibrium structure. In contrast, for the unpromoted S-edge, sulfur vacancies have to be created first. For promoted structures, Byskov et al. showed some years ago that the introduction of Co and Ni to the MoS2 edges increases the tendency to form vacancies [4]. In contrast, the incorporation of Fe, which is known not to be an efficient promoter, did not decrease vacancy formation energy [4]. More recently, scanning tunneling microscopy (STM) [24] revealed that MoS2 nano-clusters have bright brim structures (the so-called brim sites) which were subsequently identified by DFT to be formed by metallic edges states [25] and combined STM and DFT studies showed that these sites may play a role in adsorption, hydrogenation, and C–S-scission [26], [27]. These brim sites may be reactive without removal of sulfur in the first step, i.e. they may already contain sulfur vacancies in their equilibrium structure. In the present paper, we demonstrate a similar reaction mechanism for the Co–Mo–S edge. In its equilibrium structure, the sulfur coordination number is four, but additional sulfur atoms can be bound easily subsequent to C–S-scission.
For the case of unpromoted MoS2 catalysts, we have recently performed DFT calculations of the complete thiophene HDS pathway [22], [23]. In the present paper, we extend these mechanistic studies to Co-promoted systems and present the pathways for thiophene HDS over Co–Mo–S structures. In Section 3.1, we present the equilibrium edge configuration of the Co-promoted S-edge under typical HDS conditions and introduce the different investigated elementary reactions. In Sections 3.2 HYD pathway, 3.3 DDS pathway, we investigate the hydrogenation (HYD) and direct desulfurization (DDS) pathways, with the two pathways being defined in the same way as in Ref. [22]. Specifically, the DDS pathway is defined as the pathway which is initiated by an initial addition of an hydrogen atom to carbon 2 forming 2-hydro-thiophene-3-yl which reacts further by direct C–S-scission. In the HYD pathway,thiophene is hydrogenated such that 2,5-dihydrothiophene is formed prior to the initial C–S bond scission. In Section 3.4 we compare the results for Co–Mo–S for the two pathways to those for the unpromoted MoS2 catalyst. In Section 3.5, we discuss the overall effect of promotion.
Section snippets
Computational details
An infinite stripe model, which had previously been used successfully to model MoS2-based systems [4], [19], [22], [28], is used to investigate the edges of Co–Mo–S. The infinite stripe exposes both the Mo-edge and the S-edge. We use a unit cell consisting of 4 Mo atoms in the x-direction and 4 Mo atoms in the y-direction. The stripes are separated by 14.8 Å in the z-direction and 9 Å in the y-direction. This model represents MoS2 structures with no support interactions and can be regarded as a
The active sites in equilibrium structures
We investigate the Co-promoted S-edge, since experiment and theory have found that Co atoms preferentially substitute Mo atoms at the S edge [3], [5], [6], [7], [8]. We focus on the fully promoted S-edge which in addition to providing insight into the fully promoted MoS2 also provides a reference point for understanding partially substituted edges such as the structures proposed in reference [3], [44], [45] and observed for NiMoS [7]. We find that the Co–Mo–S edge at HDS conditions is
Conclusion
Co-promotion of MoS2 catalysts leads to the formation of the Co–Mo–S phase, in which Co atoms are incorporated into the S-edges of MoS2 particles, forming the Co–Mo–S edge and significantly changing both the catalytic and structural properties of the catalyst [2], [7], [58]. An important consequence of Co-promotion is that the resulting Co–Mo–S equilibrium structure contains sulfur vacancies, i.e., that no additional sulfur vacancies need to be formed at the start of the catalytic
Acknowledgments
The Center for Atomic-scale Materials Design is sponsored by the Lundbeck Foundation. The Danish Center for Scientific Computing is acknowledged for granting the computer resources needed for this project.
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