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Edge promotion and basal plane activation of MoS2 catalyst by isolated Co atoms for hydrodesulfurization and hydrodenitrogenation
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Edge promotion and basal plane activation of MoS2 catalyst by isolated Co atoms for hydrodesulfurization and hydrodenitrogenation ⁎
Xiaowan Bai, Qiang Li , Li Shi, Chongyi Ling, Jinlan Wang
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School of Physics, Southeast University, Nanjing 211189, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrodesulphurization Hydrodenitrogenation CoMoS Metal vacancy interface Density functional theory
CoMoS or NiMoS catalysts have been commercially used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions for decades. However, insight into the reaction mechanism is still under debate. Meanwhile, the basal planes are generally regarded as inactive for HDS or HDN reactions, leaving large surface areas of the catalysts useless. Therefore, six edge and four surface models as HDS and HDN catalysts are constructed to investigate the reactivity and selectivity of possible active sites systematically. In addition to active edge sites, our results demonstrate that the reactivity of basal planes of MoS2 catalysts can be largely improved by incorporating isolated Co atoms, which promote vacancy generation and form CoeSeMo bonds. In particular, a novel metal vacancy interface is proven to be a promising catalytic model for HDS and HDN reactions, in which the synergistic effect of Co atom and sulfur vacancy can promote the breakage of S/NeC bond and hydrogenation process. This detailed study paves the way of maximization of the efficiency of MoS2 based catalysts by combining both edge and surface active sites.
1. Introduction With the increasing demand for fuels and the consuming of light crude oil, the oil-refining industry must convert deep hydrogenation of heavier oil and vacuum residue to remove impurities, such as sulphur and nitrogen heteroelements [1–5]. This transformation is crucial not only to decrease SOx and NOx emissions upon combustion but also to improve the performance of downstream processes [3,6]. In the past decades, the nickel or cobalt promoted molybdenum sulfide (so-called CoMoS and NiMoS) hydrotreating catalysts have been widely used in industrial hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes [7–13]. Experimental and theoretical studies have uncovered that the coordinately unsaturated sites (CUS) as the active sites located at S-edges, Mo-edge, and component Co/Ni-Mo-S edge of these catalysts for HDS and HDN reactions [10,14–23]. However, the active sites at the edges are reduced with the decrease of the thickness of MoS2 crystal [24], resulting in lower efficiency for HDS and HDN. Therefore, searching for new or further improvement of current stateof-the-art HDS and HDN catalysts is of significant importance. A large number of studies have shown that the basal planes of twodimensional MoS2 crystals are catalytically inert for HDS reactions due to poor ability of C–S scissions and dissociating H2 (formation of H2S) [18,25–27]. Recently, MoS2 basal plane decorated with isolated Co
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atoms as the catalyst of hydrodeoxygenation (HDO) reaction was experimentally synthesized and exhibited superior activity, selectivity and stability [24]. More importantly, experimental analysis and density functional theory (DFT) calculations revealed that a large number of CUS can be formed on the MoS2 basal planes, and these sulfur vacancies may contribute to the high activity for HDO. Further theoretical studies revealed that the isolated Co atoms can promote the generation of sulfur vacancies on the basal plane of MoS2 by H2 activation and form a metal-vacancy model, which reduces the activation barriers of key steps of the HDO process considerably [28]. Consequently, the inert basal plane of MoS2 for HDS and HDN reactions is also expected to be activated by incorporating isolated Co atoms. Inspired by these experimental and theoretical works, we intend to extend possible catalytic models for HDO to HDS and HDS as well. We systematically investigate the reactivity and selectivity of the edge and surface active sites for HDS and HDN reactions, including six edge models and four surface models. The simplest alkanethiol (CH3SH) and methylamine (CH3NH2) are chosen as sulphur/nitrogen-organic molecules to study the S/NeC bond breaking, S/NeH bond breaking and the formation of CH4. The minimum energy paths and the rate-determining steps in the desulfurization and denitrification reactions are determined by thermodynamic reaction energies and kinetic barriers. Our results show that in addition to edge domain catalytic model, the inert surface
Corresponding authors. E-mail addresses: [email protected] (Q. Li), [email protected] (J. Wang).
https://doi.org/10.1016/j.cattod.2019.07.049 Received 16 May 2019; Received in revised form 24 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Xiaowan Bai, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.049
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(S-edge and Co-S-edge). The (4 × 4) supercells of basal plane of singlelayer MoS2, which contains 4 Mo atoms in the x direction, 4 Mo atoms in the y direction, and one layer in the z direction, were constructed from the MoS2 (001) surface based on a recently experimental report [24]. Fig. 1b shows the four possible sites of Co atom adsorption on MoS2 (001) surface, such as the hollow site, the top site of Mo atom, the top site of S atom and Co-filled S vacancy site. The Co atom on the top site of Mo atom has the strongest binding energy except for Co-filled S vacancy site in Supporting Information, as shown in Table S1. Therefore, the four basal plane models of single-layer MoS2 were chosen with and without Co atom or S vacancy initially: (1) only one S vacancy site (SV-surface), (2) Co filled S vacancy site (Co-fill-SV-surface), (3) Co on the top site of Mo atom without S vacancy (Co-surface), (4) Co on the top site of Mo atom with S vacancy formed a metal vacancy interface model (Co-SV-surface). Fig. 1c shows the optimized CH3SH and CH3NH2 configurations, in which activation barriers of the S/N–C scission, S/N-H scission and the formation of CH4 were used to elucidate reaction mechanisms.
of MoS2 can be activated by incorporating isolated Co atoms and metal vacancy interface as active sites can promote the breakage of S/NeC bond and CH4 formation, which may allow the HDS and HDN reactions to operate at a relatively low temperature. 2. Computational details All spin-polarized DFT calculations were performed by using the Vienna ab-initio simulation package (VASP) software [29,30]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional [31] was used to describe the exchange and correlation energies, while the projector augmented wave (PAW) method [32] was employed to describe the interactions between ion cores and valence electrons. The cutoff energy for the plane-wave basis was set to be 400 eV. During the geometry optimization, the convergence threshold was 10−5 eV and 0.02 eV/Å for energy and force, respectively. The calculated lattice constant of bulk MoS2 using the PBE functional is 3.19 Å, in agreement with experimental data (3.16 Å) [33–35] and other theoretical results [7,36]. All edges and surfaces of MoS2 calculations were performed using a periodic 4 × 4 unit cell, in which the Brillounin zone was sampled using a Monkhorst-Pack k-point [37] set of 4 × 1×1 and 3 × 3×1, respectively. To avoid an unphysical interaction between two periodic units, a vacuum space of at least 15 Å was used in edge and surface models. The semiempirical dispersioncorrected DFT force-field approach, (DFT-D2) [38] was used to describe the weak Van der Waals interaction. The energies of the CH3SH and CH3NH2 adsorbates in vacuum were calculated using a 15 × 15 × 15 Å unit cell. The minimum energy pathways of S/NeC bond breaking, S/ NeH bond breaking and the formation of CH4 were determined by using the climbing nudged elastic band method (CI-NEB) [39,40], and each transition state was further verified by frequency analysis. To systematically investigate differences of catalytic activity for HDS and HDN reactions on the edge and basal plane of single-layer MoS2 with and without Co-prompted, the (4 × 4) supercells of both (100) Mo- and (010) S-edges, which contains 4 Mo atoms in the x direction, 4 Mo atoms in the z direction, and one layer in the y direction, were constructed from the MoS2 (100) surface based on previous experimental [16] and theoretical studies [20,41–43]. Fig. 1a shows the six edge models used initially: (1) unpromoted and Co-promoted (100) Mo-edge without sulfur coverages (Mo-edge (0% S) and Co-Mo-edge (0% S)), (2) unpromoted and Co-promoted (100) Mo-edge with the 50% sulfur coverages (Mo-edge (50% S) and Co-Mo-edge (50% S)), (3) unpromoted and Co-promoted (010) S-edge with the 50% sulfur coverages
3. Results and discussion 3.1. The formation of S vacancy on edge and surface of MoS2 Previous studies have suggested that activity for HDS reaction mainly depends on the morphology structures (Mo(100) edge or S(010) edge) and CUS as active sites (S vacancies) of MoS2 catalysts [20,22,43]. Therefore, the H2 adsorption and the S vacancy generation were calculated in detail on the Mo-edge and Co-Mo-edge with 50% sulfur coverage. As shown in Fig. 2a, the formation of two -SH groups by the splitting of H2 and the S vacancy generation by formation of H2S molecule require activation barriers of 1.07 eV and 0.92 eV on the Moedge (50% S) structure, which agree well with previous results [22,43]. For Co promoted edge, i.e. Co-Mo-edge (50% S) structure in Fig. 2b, the activation barriers of both steps are 0.76 and 0.61 eV, which are lower than unpromoted Mo-edge (50% S). This confirms that Co promoters decrease the activation barrier of hydrogen activation and facilitate the formation of S vacancies at edge sites [44,45]. Similarly, formation energy of a single S vacancy can reach up to 2.04 eV for pristine MoS2 surface, while the energy barrier for CUS formation on a Co promoted surface requires as much as 1.80 eV. [24,28] Therefore, one of the promotion effects by isolated Co atoms can be certain: the formation of large number of S vacancy, as observed from experiments [24]. Both DFT calculations and experiments show that the sulfur vacancy has a Fig. 1. (a) Unprompted and Co-prompted edge models of MoS2 (100) surface: Mo-edge, Mo-edge (50% S coverage) with Sulfur vacancy (SV) and S-edge. The CUS sites were labelled on Mo-, Mo (50% S)-, and S- edges. (b) Possible sites of Co atom adsorption on MoS2 (001) surface. (c) Methanethiol (CH3SH) and Methanamine (CH3NH2) molecules. The Mo, S, C, N and H atoms are represented by light green, yellow, gray, blue and white spheres.
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Fig. 2. Calculated splitting of H2 and formation of H2S: (a) Mo-edge (50% S) and (b) Co-Mo-edge (50% S). The Mo, Co, S and H atoms are represented by light green, purple, yellow and white spheres.
Fig. 3. DFT optimized structure, density of states (DOS) and electron-density distribution of MoS2 (001) surface with one sulfur vacancy denoted as SV (a), and with one Co atom adsorption at Mo top site combined with SV as Co-SV-surface (b). Electron-density are plotted at 0–0.8 e/bohr3. The Mo, Co and S atoms are represented by light green, purple and yellow spheres.
hence the better the catalytic performance [47]. However, the high concentration of vacancy sites is not always wanted for different reaction mechanisms. For instance, a sulfur vacancy on the Mo edge, the higher the concentration is, the stronger the adsorption of the intermediate state is, the more difficult the CH4 formation is [20]. Besides,
certain effect on the catalytic activity, for example, the vacancy effect on HER [46]. In the case of HDS and HDN reactions, the vacancy sites can offer chemisorption sites to the sulfur- and nitrogen- containing compounds and lower the activation barriers [19,36]. In general, the higher the vacancy concentration, the more active sites exposed, and 3
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the surface than the H atom and CH3NH fragments (CH3NH2-CH3NH +H: reaction energy of −0.23 eV). Therefore, NeH scission is preferred than direct NeC scission in HDN reaction on Mo-edges (0% S). Although the fully exposed Mo-edge is active, the sites can be highly unstable and are prone to reconstruction. In general, the pure Mo-edge model can be considered as the cases of connected two or more S vacancies, on which HDN reaction can readily occur while HDS reaction is limited. In the case of Co-Mo-edge (0% S), we first consider one Co atom replacing one Mo atom at Mo edge and compare the reactivity without Co promoters. The geometry optimization shows that the S-containing groups are incline to bridge position of two Mo atoms because Mo edge atoms will tend to maintain the full sulfur coordination of 6 by extensive edge reconstruction (Fig. S3b). However, N-containing groups intend to adsorb on Mo centers [51], on which the HDN reaction occurs subsequently (Fig. S4b). The activation barriers of the rate-determining steps are lower than Mo-edge without Co promoters. These results once again confirm that the incorporated Co atoms promote HDS and HDN reactions, in accordance with earlier results for Co-Mo-edge model with one sulfur atom [20]. The rest four edge models all involves S vacancies, i.e. Mo edge (50% S) with and without Co promotion and S edge with and without Co promotion. In all cases, CH3SH and CH3NH2 tend to adsorb on the S vacancy site and lead to SeH or NeH breaking. The lowest energy reaction pathway for complete HDS and HDN reactions are shown in Fig. 4 and other pathways can be seen in Supporting Information. For HDS reaction in Fig. 4a, CH3SH first adsorbs on the S vacancy position via breaking the SeH bond and forming the H atom at Mo atom and CH3S at S vacancy site with a slightly exothermic energy of −0.08 eV and a small activation barrier of 0.43 eV. Considering the HDS process at SV sites, an essential step is the hydrogenation of CH3 to form CH4, and the H can be subtracted from adjacent -SH groups [28]. In the following, a relatively high activation barrier of 1.68 eV is required for SeC scission and CH4 formation in the study case of CH3SH, and 2.71 eV for the Butadiene formation on the S-edge [22]. Thereafter, removing S atom requires an external H2 dissociation with one H atom at remaining S atom and another H atom at S site of S-edge. Then, H atom and SH fragment forms H2S and S vacancy site, which proceed to the next cycle for HDS reaction. The activation barriers for the two steps are 0.50 and 1.43 eV, respectively. Therefore, rate determining steps of the whole HDS process are 1.68 eV and 1.43 eV for SeC scission and vacancy regeneration, suggesting that high temperature is required for the HDS in the experiments. Comparing all the edge models for HDN reaction, the optimal process seems to proceed on a pure Mo-edge (Fig. 4b), which is highly active as we discussed above. After the methane formation, external H2 molecule was introduced to promote NH3 formation and S vacancy generation. Hence, the rate determining steps of the whole HDN process are 1.36 eV and 1.95 eV for NeC scission and vacancy regeneration with NH3 formation.
the vacancy concentration can have large influence on the physical and chemical properties of the materials, such as carrier mobility, conductivity, stability, and so on [47–49]. 3.2. Electronic properties of six edge and four surface models of CoMoS catalyst Understanding the correlation between the electronic structure and reactivity on hydrotreating catalysts is an important prerequisite for the rational design of new catalysts with higher reactivity and selectivity for HDS and HDN reactions. Therefore, the density of states (DOS) and electron-density distribution of ten catalytic models were calculated and shown in Supporting Information. It is well known that the pristine monolayer MoS2 is a semiconductor with direct band gap of 1.61 eV [48], while the edges present metallic properties as shown in Fig. S1. Therefore, MoS2 related catalytic reactions, hydrogen evolution reaction (HER) for example, are attributed to the edge sites, and the electrons are delocalized along the edge chains. In the presence of an S vacancy site, deep acceptor states inside the band gap are created, and the localized state distribute mainly on the Mo atoms in the vicinity of the SV (Fig. 3a), thus the unsaturated Mo atoms on the surface of MoS2 become active and have a tendency to react with sulfur/oxygen/nitrogen-containing groups. As a Co atom is attached on the surface, the metallic Co contributes to DOS crossing the Fermi level (Fig. 3b), and thus can be used for hydrogenation process. Therefore, the synergistic effect of the metal-vacancy catalysts based on atomically dispersed Co on MoS2 can promote the superior activity and selectivity of HDS, HDO and HDN [28]. Next, we will discuss in detail the reactivity and selectivity of HDS and HDN on different edge and surface models of MoS2. 3.3. Dissociation of CH3SH and CH3NH2 on the six edge models Although MoS2 catalyst has been commercially used for HDS and HDN reactions, the active sites and reaction pathways are continuously debating for years [41,50]. It is generally accepted that the CUS located at the S-edge and the Mo-edge serve as active sites for HDS reactions. However, some studies showed that sulfur vacancy at different edges of MoS2 has different effects for HDS reaction [20,22,43]. Therefore, the Mo-edge (0% S), Mo-edge (50% S) and S-edge with and without Co promoters are considered as six edge models for HDS and HDN reactions in this study. We start with CH3SH and CH3NH2 molecules absorbed on the pure Mo-edges (0% S). After geometric optimization, the S atom from CH3SH intends to adsorb on a bridge position between two Mo atoms while N atom of CH3NH2 is preferably adsorbed on the top site of Mo atom. Once the CH3SH molecule is attached, the HDS reaction can start either from SeH breaking or direct SeC scission (Fig. S3a). In the former case, the formation of the H atom and CH3S fragment has an activation barrier of 0.43 eV and is exothermic with reaction energy of −1.22 eV. Then, the formation of CH4 from the adsorbed H and CH3S fragments has an activation barrier (2.53 eV), due to the strong SeC and Mo–H bonds at bridge position, and the high energy barrier is also confirmed by previous calculations [20]. In the latter case, the activation barriers of SeC scission and CH4 formation are 0.57 and 2.68 eV, respectively, as shown in Fig. S3a. Similarly, the HDN reaction also has two reaction pathways with NeH scission andNeC scission on the pure Mo-edges (0% S). The activation barriers for the NeH and NeC scission are 1.18 eV in Fig. 4b and 1.83 eV in Fig. S4a, respectively. Obviously, the HDN reaction is more difficult than the HDS reaction because the bond energy of NeH/C is higher than that of SeH/C bond. After the NeH and NeC bonds break, the formation of CH4 for two reaction pathways needs to overcome activation barriers of 1.36 in Fig. 4b and 2.77 eV Fig. S4a, respectively. The energy difference of NeH and NeC paths (1.41 eV) for CH4 formation is caused by the energies of the intermediate states, in which the NH2 and the CH3 fragments (CH3NH2CH3+NH2: reaction energy of −1.20 eV) are more strongly bound to
3.4. Dissociation of CH3SH and CH3NH2 on the four basal plane models It is generally accepted that the basal plane of MoS2 is inert for HDS and HDN reactions, but the basal planes can be activated by existing vacancy sites, which are proven to be active for HER and surface functionalization of MoS2 [47,52]. Besides, the incorporated Co atoms that sitting on the basal planes of MoS2 can be served as single atom catalyst [24]. Therefore, we consider four surface models including SV site, single Co atom site, Co filled SV site and the proposed metal vacancy interface for HDS and HDN catalytic reactions. For SV-surface model, two very low activation barriers of 0.47 and 0.61 eV are found for the SeH scission and CH4 formation for HDS reaction in Fig. S5a, respectively. The activation of NeH bond requires higher energy comparing with SeH, and the activation barriers for NeH scission and CH4 formation are 1.35 and 0.99 eV for HDN reaction, as shown in Fig. S5b. As a result, the desulfurization and denitrogenation processes are 4
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Fig. 4. Minimum energy path for (a) the HDS reaction of CH3SH to CH4 and H2S by SeH bond scission on the unpromoted S-edge of MoS2 and (b) the HDN reaction of CH3NH2 to CH4 and NH3 by NeH bond scission on the unpromoted Mo-edge of MoS2. Structures and energies of the intermediates and transition states (TS) are indicated. The red and black values in curve graphs are activation energies and reaction energies, respectively. The red values in structures are bond distances. Shading indicates spontaneous (blue) versus non-spontaneous (red) steps.
Fig. 5. Minimum energy paths for (a) the HDS reaction of CH3SH to CH4 and H2S by SeC bond scission on the Co-SV-surface and (b) the HDN reaction of CH3NH2 to CH4 and NH3 by NeC bond scission on the Co-SV-surface.
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Fig. 6. Activation energies of S-C scission in HDS (a), N-C scission in HDN (b), CH4 formation in HDS (c), and CH4 formation in HDN (d). The S-C or N-C scission reactions is at CUS on the unprompted Mo-edge, Co-prompted Mo-edge of MoS2 and Co-prompted MoS2 surface. CH4 formation in HDS and HDN is at CUS on all considered structures.
accomplished by filling the SVs with either sulfur or nitrogen atoms. In this way, the regeneration of the catalytic centres of SVs seems to be blocked, since the formation of SV sites on the surface requires an ultrahigh formation energy (2.04 eV). Next, we consider promoter atoms locate at surface sites and SV sites as reaction centres (Fig. S5). Direct SC breaking on single Co atoms requires activation barriers of 1.52 and 1.08 eV for Co-fill-SV model and Co-surface model, and the following SH scission and CH4 formation bear activation barriers of 0.81 and 1.60 eV, respectively. It seems that the rate determining steps for the HDS on the surface models are comparable with edge models. As expected for HDN process, the NeC scission on the two Co surface models requires activation energy as much as 1.80 eV, as shown in Fig. S5b. Finally, we consider the synergetic effect of promoter atoms and SV to create a metal and vacancy interface as HDS and HDN reaction centres, as shown in Fig. 5. The isolated Co atoms and the adjacent SV sites are both considered as active sites for SeC and NeC scission, and the dual active sites lead to a considerable decrease with activation barriers of 0.89 eV for SeC scission and 1.48 eV for N-C scission. After SeC and NeC bonds break, the SV sites are filled with SH and NH2 groups, while isolated Co atoms can capture the CH3 fragments for further hydrogenation process. Under H2 reaction condition, Co atoms continue with adsorption and activation of H2 molecules, and provide proton for CH4, H2S and NH3 production. The calculated activation barriers for the methane formation are 0.13 and 0.17 eV for HDS and HDN, respectively, while H2S and NH3 formation are 0.88 and 1.00 eV. Therefore, the rate-determining steps are SeC and NeC scission on the Co-SVsurface and lower than all of the edge models of MoS2.
3.5. Discussions As we noticed from all possible reaction pathways above for ten catalytic models, we emphasize the HDS and HDN reactions on two general rate-determining steps, S/N-C scission and CH4 formation, and summarized in Fig. 6 and Tables S2 and S3. Clearly, the reactivity and selectivity of both HDS and HDN reactions highly rely on the catalytic models. In general, the HDS and HDN processes can sustainably occur both on the edges and Co modified surfaces under work conditions of ˜300 ℃, according to the semiempratical formula 1012exp(―Eb/kT) ≈ 1 [53]. Therefore, the basal plane of MoS2 catalysts can be activated by metal incorporation and thus increase the active sites for HDS and HDN reactions. Compared with the edge models, the activation energies on the metal vacancy interface model are the lowest for S/N-C scission and CH4 formation steps except for the Mo-edge (0% S). Although the SeC bond is easy to break on the Mo-edge (0% S), it is difficult to form CH4 and needs to overcome a high activation barrier of 2.68 eV. On the whole, the catalytic performance of the MoS2 surface modified by Co atoms is better than that of MoS2 edges. For the surface models, the interactions between the sulfur/nitrogen-containing compounds and the basal plane of MoS2 are too weak to proceed for further reaction (see Fig. S6). In addition, the activation barriers of other three surface models, namely, the pure sulfur vacancy, Co filled sulfur vacancy and Co on the top site of Mo atom without sulfur vacancy, are higher than that of the metal vacancy interface model. In particular, the methane formation only needs to overcome 0.13 and 0.17 eV for HDS and HDN on the Co-SV-surface model, respectively. Therefore, the metal vacancy interface model stands out from all other catalytic models (green marks in Fig. 6) and largely improves the catalytic performance. 6
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Fig. 7. Priorities of R-H and R-C scission (R = O, S, N) are on SV-surface and Co-SV-surface, respectively. The TS for N-H scission should be higher than the FS of 1.65 eV, and is formidable to locate by NEB simulation.
As a prerequisite for high selectivity of the catalysts, the reaction pathways of the catalytic reactions should be limited. In this respect, we consider competition reactions based on the metal vacancy interface, i.e. R-C scission and R-H breaking, where R denotes for S, O or N. As shown in Fig. 7, OeC and NeC scission process are favorable over OeH and NeH cleavages. The energy of the NeH scission product leads to a large endothermic of +1.63 eV, thus we failed to locate the transitional state for the reaction step due to highly unstable configuration. Alternatively, the SeH breaking process is energetically favorable comparing with the SeC scission, due to the weak SeH bond, and this leads to the refill of the SV sites with S atoms. Although S atoms fill in the vacancy, the presence of metal atom can further promote the regeneration of SV sites in that case [28]. Therefore, the high selectivity and stability (recycle of the catalyst) of the HDS reaction on metal vacancy model can be expected. 4. Conclusions In conclusion, we have investigated the detailed reaction mechanisms of CoMoS catalyst for HDS and HDN reactions based on six edge and four basal plane models. Our results show that in addition to edge domain catalytic model, the inert surface of MoS2 can be activated by incorporating isolated Co atoms, which also promotes vacancy generation and formation of CoeSeMo bonds. Specifically, a metal vacancy interface model has been verified with high reactivity and selectivity for both HDS and HDN reactions, in which the synergistic effect of Co atom and S vacancy urges NeC and SeC breakage. Therefore, both edge and surface models can be served as active sites for HDS and HDN reactions. In addition, the metal vacancy model shows lower energy barriers for elementary steps in HDS and HDN steps, and may allow the reactions to operate at a relatively low temperature. This systematic study provides new insights into the HDS and HDN mechanisms and theoretical guidelines on the design of efficient catalysts for HDS and HDN reactions. Acknowledgements This work is supported by the National Natural Science Foundation for Distinguished Young Scholar (21525311), National Key R&D Program of China (Grant No. 2017YFA0204800), National Natural Science Foundation of China (Grant Nos. 21773027, 21703032), Jiangsu 333 project (BRA2016353) and the Scientific Research Foundation of Graduate School of Southeast University (YBPY1920) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin. 7
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Edge promotion and basal plane activation of MoS2 catalyst by isolated Co atoms for hydrodesulfurization and hydrodenitrogenation ⁎
Xiaowan Bai, Qiang Li , Li Shi, Chongyi Ling, Jinlan Wang
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School of Physics, Southeast University, Nanjing 211189, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrodesulphurization Hydrodenitrogenation CoMoS Metal vacancy interface Density functional theory
CoMoS or NiMoS catalysts have been commercially used for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions for decades. However, insight into the reaction mechanism is still under debate. Meanwhile, the basal planes are generally regarded as inactive for HDS or HDN reactions, leaving large surface areas of the catalysts useless. Therefore, six edge and four surface models as HDS and HDN catalysts are constructed to investigate the reactivity and selectivity of possible active sites systematically. In addition to active edge sites, our results demonstrate that the reactivity of basal planes of MoS2 catalysts can be largely improved by incorporating isolated Co atoms, which promote vacancy generation and form CoeSeMo bonds. In particular, a novel metal vacancy interface is proven to be a promising catalytic model for HDS and HDN reactions, in which the synergistic effect of Co atom and sulfur vacancy can promote the breakage of S/NeC bond and hydrogenation process. This detailed study paves the way of maximization of the efficiency of MoS2 based catalysts by combining both edge and surface active sites.
1. Introduction With the increasing demand for fuels and the consuming of light crude oil, the oil-refining industry must convert deep hydrogenation of heavier oil and vacuum residue to remove impurities, such as sulphur and nitrogen heteroelements [1–5]. This transformation is crucial not only to decrease SOx and NOx emissions upon combustion but also to improve the performance of downstream processes [3,6]. In the past decades, the nickel or cobalt promoted molybdenum sulfide (so-called CoMoS and NiMoS) hydrotreating catalysts have been widely used in industrial hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) processes [7–13]. Experimental and theoretical studies have uncovered that the coordinately unsaturated sites (CUS) as the active sites located at S-edges, Mo-edge, and component Co/Ni-Mo-S edge of these catalysts for HDS and HDN reactions [10,14–23]. However, the active sites at the edges are reduced with the decrease of the thickness of MoS2 crystal [24], resulting in lower efficiency for HDS and HDN. Therefore, searching for new or further improvement of current stateof-the-art HDS and HDN catalysts is of significant importance. A large number of studies have shown that the basal planes of twodimensional MoS2 crystals are catalytically inert for HDS reactions due to poor ability of C–S scissions and dissociating H2 (formation of H2S) [18,25–27]. Recently, MoS2 basal plane decorated with isolated Co
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atoms as the catalyst of hydrodeoxygenation (HDO) reaction was experimentally synthesized and exhibited superior activity, selectivity and stability [24]. More importantly, experimental analysis and density functional theory (DFT) calculations revealed that a large number of CUS can be formed on the MoS2 basal planes, and these sulfur vacancies may contribute to the high activity for HDO. Further theoretical studies revealed that the isolated Co atoms can promote the generation of sulfur vacancies on the basal plane of MoS2 by H2 activation and form a metal-vacancy model, which reduces the activation barriers of key steps of the HDO process considerably [28]. Consequently, the inert basal plane of MoS2 for HDS and HDN reactions is also expected to be activated by incorporating isolated Co atoms. Inspired by these experimental and theoretical works, we intend to extend possible catalytic models for HDO to HDS and HDS as well. We systematically investigate the reactivity and selectivity of the edge and surface active sites for HDS and HDN reactions, including six edge models and four surface models. The simplest alkanethiol (CH3SH) and methylamine (CH3NH2) are chosen as sulphur/nitrogen-organic molecules to study the S/NeC bond breaking, S/NeH bond breaking and the formation of CH4. The minimum energy paths and the rate-determining steps in the desulfurization and denitrification reactions are determined by thermodynamic reaction energies and kinetic barriers. Our results show that in addition to edge domain catalytic model, the inert surface
Corresponding authors. E-mail addresses: [email protected] (Q. Li), [email protected] (J. Wang).
https://doi.org/10.1016/j.cattod.2019.07.049 Received 16 May 2019; Received in revised form 24 July 2019; Accepted 29 July 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Xiaowan Bai, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.07.049
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(S-edge and Co-S-edge). The (4 × 4) supercells of basal plane of singlelayer MoS2, which contains 4 Mo atoms in the x direction, 4 Mo atoms in the y direction, and one layer in the z direction, were constructed from the MoS2 (001) surface based on a recently experimental report [24]. Fig. 1b shows the four possible sites of Co atom adsorption on MoS2 (001) surface, such as the hollow site, the top site of Mo atom, the top site of S atom and Co-filled S vacancy site. The Co atom on the top site of Mo atom has the strongest binding energy except for Co-filled S vacancy site in Supporting Information, as shown in Table S1. Therefore, the four basal plane models of single-layer MoS2 were chosen with and without Co atom or S vacancy initially: (1) only one S vacancy site (SV-surface), (2) Co filled S vacancy site (Co-fill-SV-surface), (3) Co on the top site of Mo atom without S vacancy (Co-surface), (4) Co on the top site of Mo atom with S vacancy formed a metal vacancy interface model (Co-SV-surface). Fig. 1c shows the optimized CH3SH and CH3NH2 configurations, in which activation barriers of the S/N–C scission, S/N-H scission and the formation of CH4 were used to elucidate reaction mechanisms.
of MoS2 can be activated by incorporating isolated Co atoms and metal vacancy interface as active sites can promote the breakage of S/NeC bond and CH4 formation, which may allow the HDS and HDN reactions to operate at a relatively low temperature. 2. Computational details All spin-polarized DFT calculations were performed by using the Vienna ab-initio simulation package (VASP) software [29,30]. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional [31] was used to describe the exchange and correlation energies, while the projector augmented wave (PAW) method [32] was employed to describe the interactions between ion cores and valence electrons. The cutoff energy for the plane-wave basis was set to be 400 eV. During the geometry optimization, the convergence threshold was 10−5 eV and 0.02 eV/Å for energy and force, respectively. The calculated lattice constant of bulk MoS2 using the PBE functional is 3.19 Å, in agreement with experimental data (3.16 Å) [33–35] and other theoretical results [7,36]. All edges and surfaces of MoS2 calculations were performed using a periodic 4 × 4 unit cell, in which the Brillounin zone was sampled using a Monkhorst-Pack k-point [37] set of 4 × 1×1 and 3 × 3×1, respectively. To avoid an unphysical interaction between two periodic units, a vacuum space of at least 15 Å was used in edge and surface models. The semiempirical dispersioncorrected DFT force-field approach, (DFT-D2) [38] was used to describe the weak Van der Waals interaction. The energies of the CH3SH and CH3NH2 adsorbates in vacuum were calculated using a 15 × 15 × 15 Å unit cell. The minimum energy pathways of S/NeC bond breaking, S/ NeH bond breaking and the formation of CH4 were determined by using the climbing nudged elastic band method (CI-NEB) [39,40], and each transition state was further verified by frequency analysis. To systematically investigate differences of catalytic activity for HDS and HDN reactions on the edge and basal plane of single-layer MoS2 with and without Co-prompted, the (4 × 4) supercells of both (100) Mo- and (010) S-edges, which contains 4 Mo atoms in the x direction, 4 Mo atoms in the z direction, and one layer in the y direction, were constructed from the MoS2 (100) surface based on previous experimental [16] and theoretical studies [20,41–43]. Fig. 1a shows the six edge models used initially: (1) unpromoted and Co-promoted (100) Mo-edge without sulfur coverages (Mo-edge (0% S) and Co-Mo-edge (0% S)), (2) unpromoted and Co-promoted (100) Mo-edge with the 50% sulfur coverages (Mo-edge (50% S) and Co-Mo-edge (50% S)), (3) unpromoted and Co-promoted (010) S-edge with the 50% sulfur coverages
3. Results and discussion 3.1. The formation of S vacancy on edge and surface of MoS2 Previous studies have suggested that activity for HDS reaction mainly depends on the morphology structures (Mo(100) edge or S(010) edge) and CUS as active sites (S vacancies) of MoS2 catalysts [20,22,43]. Therefore, the H2 adsorption and the S vacancy generation were calculated in detail on the Mo-edge and Co-Mo-edge with 50% sulfur coverage. As shown in Fig. 2a, the formation of two -SH groups by the splitting of H2 and the S vacancy generation by formation of H2S molecule require activation barriers of 1.07 eV and 0.92 eV on the Moedge (50% S) structure, which agree well with previous results [22,43]. For Co promoted edge, i.e. Co-Mo-edge (50% S) structure in Fig. 2b, the activation barriers of both steps are 0.76 and 0.61 eV, which are lower than unpromoted Mo-edge (50% S). This confirms that Co promoters decrease the activation barrier of hydrogen activation and facilitate the formation of S vacancies at edge sites [44,45]. Similarly, formation energy of a single S vacancy can reach up to 2.04 eV for pristine MoS2 surface, while the energy barrier for CUS formation on a Co promoted surface requires as much as 1.80 eV. [24,28] Therefore, one of the promotion effects by isolated Co atoms can be certain: the formation of large number of S vacancy, as observed from experiments [24]. Both DFT calculations and experiments show that the sulfur vacancy has a Fig. 1. (a) Unprompted and Co-prompted edge models of MoS2 (100) surface: Mo-edge, Mo-edge (50% S coverage) with Sulfur vacancy (SV) and S-edge. The CUS sites were labelled on Mo-, Mo (50% S)-, and S- edges. (b) Possible sites of Co atom adsorption on MoS2 (001) surface. (c) Methanethiol (CH3SH) and Methanamine (CH3NH2) molecules. The Mo, S, C, N and H atoms are represented by light green, yellow, gray, blue and white spheres.
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Fig. 2. Calculated splitting of H2 and formation of H2S: (a) Mo-edge (50% S) and (b) Co-Mo-edge (50% S). The Mo, Co, S and H atoms are represented by light green, purple, yellow and white spheres.
Fig. 3. DFT optimized structure, density of states (DOS) and electron-density distribution of MoS2 (001) surface with one sulfur vacancy denoted as SV (a), and with one Co atom adsorption at Mo top site combined with SV as Co-SV-surface (b). Electron-density are plotted at 0–0.8 e/bohr3. The Mo, Co and S atoms are represented by light green, purple and yellow spheres.
hence the better the catalytic performance [47]. However, the high concentration of vacancy sites is not always wanted for different reaction mechanisms. For instance, a sulfur vacancy on the Mo edge, the higher the concentration is, the stronger the adsorption of the intermediate state is, the more difficult the CH4 formation is [20]. Besides,
certain effect on the catalytic activity, for example, the vacancy effect on HER [46]. In the case of HDS and HDN reactions, the vacancy sites can offer chemisorption sites to the sulfur- and nitrogen- containing compounds and lower the activation barriers [19,36]. In general, the higher the vacancy concentration, the more active sites exposed, and 3
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the surface than the H atom and CH3NH fragments (CH3NH2-CH3NH +H: reaction energy of −0.23 eV). Therefore, NeH scission is preferred than direct NeC scission in HDN reaction on Mo-edges (0% S). Although the fully exposed Mo-edge is active, the sites can be highly unstable and are prone to reconstruction. In general, the pure Mo-edge model can be considered as the cases of connected two or more S vacancies, on which HDN reaction can readily occur while HDS reaction is limited. In the case of Co-Mo-edge (0% S), we first consider one Co atom replacing one Mo atom at Mo edge and compare the reactivity without Co promoters. The geometry optimization shows that the S-containing groups are incline to bridge position of two Mo atoms because Mo edge atoms will tend to maintain the full sulfur coordination of 6 by extensive edge reconstruction (Fig. S3b). However, N-containing groups intend to adsorb on Mo centers [51], on which the HDN reaction occurs subsequently (Fig. S4b). The activation barriers of the rate-determining steps are lower than Mo-edge without Co promoters. These results once again confirm that the incorporated Co atoms promote HDS and HDN reactions, in accordance with earlier results for Co-Mo-edge model with one sulfur atom [20]. The rest four edge models all involves S vacancies, i.e. Mo edge (50% S) with and without Co promotion and S edge with and without Co promotion. In all cases, CH3SH and CH3NH2 tend to adsorb on the S vacancy site and lead to SeH or NeH breaking. The lowest energy reaction pathway for complete HDS and HDN reactions are shown in Fig. 4 and other pathways can be seen in Supporting Information. For HDS reaction in Fig. 4a, CH3SH first adsorbs on the S vacancy position via breaking the SeH bond and forming the H atom at Mo atom and CH3S at S vacancy site with a slightly exothermic energy of −0.08 eV and a small activation barrier of 0.43 eV. Considering the HDS process at SV sites, an essential step is the hydrogenation of CH3 to form CH4, and the H can be subtracted from adjacent -SH groups [28]. In the following, a relatively high activation barrier of 1.68 eV is required for SeC scission and CH4 formation in the study case of CH3SH, and 2.71 eV for the Butadiene formation on the S-edge [22]. Thereafter, removing S atom requires an external H2 dissociation with one H atom at remaining S atom and another H atom at S site of S-edge. Then, H atom and SH fragment forms H2S and S vacancy site, which proceed to the next cycle for HDS reaction. The activation barriers for the two steps are 0.50 and 1.43 eV, respectively. Therefore, rate determining steps of the whole HDS process are 1.68 eV and 1.43 eV for SeC scission and vacancy regeneration, suggesting that high temperature is required for the HDS in the experiments. Comparing all the edge models for HDN reaction, the optimal process seems to proceed on a pure Mo-edge (Fig. 4b), which is highly active as we discussed above. After the methane formation, external H2 molecule was introduced to promote NH3 formation and S vacancy generation. Hence, the rate determining steps of the whole HDN process are 1.36 eV and 1.95 eV for NeC scission and vacancy regeneration with NH3 formation.
the vacancy concentration can have large influence on the physical and chemical properties of the materials, such as carrier mobility, conductivity, stability, and so on [47–49]. 3.2. Electronic properties of six edge and four surface models of CoMoS catalyst Understanding the correlation between the electronic structure and reactivity on hydrotreating catalysts is an important prerequisite for the rational design of new catalysts with higher reactivity and selectivity for HDS and HDN reactions. Therefore, the density of states (DOS) and electron-density distribution of ten catalytic models were calculated and shown in Supporting Information. It is well known that the pristine monolayer MoS2 is a semiconductor with direct band gap of 1.61 eV [48], while the edges present metallic properties as shown in Fig. S1. Therefore, MoS2 related catalytic reactions, hydrogen evolution reaction (HER) for example, are attributed to the edge sites, and the electrons are delocalized along the edge chains. In the presence of an S vacancy site, deep acceptor states inside the band gap are created, and the localized state distribute mainly on the Mo atoms in the vicinity of the SV (Fig. 3a), thus the unsaturated Mo atoms on the surface of MoS2 become active and have a tendency to react with sulfur/oxygen/nitrogen-containing groups. As a Co atom is attached on the surface, the metallic Co contributes to DOS crossing the Fermi level (Fig. 3b), and thus can be used for hydrogenation process. Therefore, the synergistic effect of the metal-vacancy catalysts based on atomically dispersed Co on MoS2 can promote the superior activity and selectivity of HDS, HDO and HDN [28]. Next, we will discuss in detail the reactivity and selectivity of HDS and HDN on different edge and surface models of MoS2. 3.3. Dissociation of CH3SH and CH3NH2 on the six edge models Although MoS2 catalyst has been commercially used for HDS and HDN reactions, the active sites and reaction pathways are continuously debating for years [41,50]. It is generally accepted that the CUS located at the S-edge and the Mo-edge serve as active sites for HDS reactions. However, some studies showed that sulfur vacancy at different edges of MoS2 has different effects for HDS reaction [20,22,43]. Therefore, the Mo-edge (0% S), Mo-edge (50% S) and S-edge with and without Co promoters are considered as six edge models for HDS and HDN reactions in this study. We start with CH3SH and CH3NH2 molecules absorbed on the pure Mo-edges (0% S). After geometric optimization, the S atom from CH3SH intends to adsorb on a bridge position between two Mo atoms while N atom of CH3NH2 is preferably adsorbed on the top site of Mo atom. Once the CH3SH molecule is attached, the HDS reaction can start either from SeH breaking or direct SeC scission (Fig. S3a). In the former case, the formation of the H atom and CH3S fragment has an activation barrier of 0.43 eV and is exothermic with reaction energy of −1.22 eV. Then, the formation of CH4 from the adsorbed H and CH3S fragments has an activation barrier (2.53 eV), due to the strong SeC and Mo–H bonds at bridge position, and the high energy barrier is also confirmed by previous calculations [20]. In the latter case, the activation barriers of SeC scission and CH4 formation are 0.57 and 2.68 eV, respectively, as shown in Fig. S3a. Similarly, the HDN reaction also has two reaction pathways with NeH scission andNeC scission on the pure Mo-edges (0% S). The activation barriers for the NeH and NeC scission are 1.18 eV in Fig. 4b and 1.83 eV in Fig. S4a, respectively. Obviously, the HDN reaction is more difficult than the HDS reaction because the bond energy of NeH/C is higher than that of SeH/C bond. After the NeH and NeC bonds break, the formation of CH4 for two reaction pathways needs to overcome activation barriers of 1.36 in Fig. 4b and 2.77 eV Fig. S4a, respectively. The energy difference of NeH and NeC paths (1.41 eV) for CH4 formation is caused by the energies of the intermediate states, in which the NH2 and the CH3 fragments (CH3NH2CH3+NH2: reaction energy of −1.20 eV) are more strongly bound to
3.4. Dissociation of CH3SH and CH3NH2 on the four basal plane models It is generally accepted that the basal plane of MoS2 is inert for HDS and HDN reactions, but the basal planes can be activated by existing vacancy sites, which are proven to be active for HER and surface functionalization of MoS2 [47,52]. Besides, the incorporated Co atoms that sitting on the basal planes of MoS2 can be served as single atom catalyst [24]. Therefore, we consider four surface models including SV site, single Co atom site, Co filled SV site and the proposed metal vacancy interface for HDS and HDN catalytic reactions. For SV-surface model, two very low activation barriers of 0.47 and 0.61 eV are found for the SeH scission and CH4 formation for HDS reaction in Fig. S5a, respectively. The activation of NeH bond requires higher energy comparing with SeH, and the activation barriers for NeH scission and CH4 formation are 1.35 and 0.99 eV for HDN reaction, as shown in Fig. S5b. As a result, the desulfurization and denitrogenation processes are 4
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Fig. 4. Minimum energy path for (a) the HDS reaction of CH3SH to CH4 and H2S by SeH bond scission on the unpromoted S-edge of MoS2 and (b) the HDN reaction of CH3NH2 to CH4 and NH3 by NeH bond scission on the unpromoted Mo-edge of MoS2. Structures and energies of the intermediates and transition states (TS) are indicated. The red and black values in curve graphs are activation energies and reaction energies, respectively. The red values in structures are bond distances. Shading indicates spontaneous (blue) versus non-spontaneous (red) steps.
Fig. 5. Minimum energy paths for (a) the HDS reaction of CH3SH to CH4 and H2S by SeC bond scission on the Co-SV-surface and (b) the HDN reaction of CH3NH2 to CH4 and NH3 by NeC bond scission on the Co-SV-surface.
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Fig. 6. Activation energies of S-C scission in HDS (a), N-C scission in HDN (b), CH4 formation in HDS (c), and CH4 formation in HDN (d). The S-C or N-C scission reactions is at CUS on the unprompted Mo-edge, Co-prompted Mo-edge of MoS2 and Co-prompted MoS2 surface. CH4 formation in HDS and HDN is at CUS on all considered structures.
accomplished by filling the SVs with either sulfur or nitrogen atoms. In this way, the regeneration of the catalytic centres of SVs seems to be blocked, since the formation of SV sites on the surface requires an ultrahigh formation energy (2.04 eV). Next, we consider promoter atoms locate at surface sites and SV sites as reaction centres (Fig. S5). Direct SC breaking on single Co atoms requires activation barriers of 1.52 and 1.08 eV for Co-fill-SV model and Co-surface model, and the following SH scission and CH4 formation bear activation barriers of 0.81 and 1.60 eV, respectively. It seems that the rate determining steps for the HDS on the surface models are comparable with edge models. As expected for HDN process, the NeC scission on the two Co surface models requires activation energy as much as 1.80 eV, as shown in Fig. S5b. Finally, we consider the synergetic effect of promoter atoms and SV to create a metal and vacancy interface as HDS and HDN reaction centres, as shown in Fig. 5. The isolated Co atoms and the adjacent SV sites are both considered as active sites for SeC and NeC scission, and the dual active sites lead to a considerable decrease with activation barriers of 0.89 eV for SeC scission and 1.48 eV for N-C scission. After SeC and NeC bonds break, the SV sites are filled with SH and NH2 groups, while isolated Co atoms can capture the CH3 fragments for further hydrogenation process. Under H2 reaction condition, Co atoms continue with adsorption and activation of H2 molecules, and provide proton for CH4, H2S and NH3 production. The calculated activation barriers for the methane formation are 0.13 and 0.17 eV for HDS and HDN, respectively, while H2S and NH3 formation are 0.88 and 1.00 eV. Therefore, the rate-determining steps are SeC and NeC scission on the Co-SVsurface and lower than all of the edge models of MoS2.
3.5. Discussions As we noticed from all possible reaction pathways above for ten catalytic models, we emphasize the HDS and HDN reactions on two general rate-determining steps, S/N-C scission and CH4 formation, and summarized in Fig. 6 and Tables S2 and S3. Clearly, the reactivity and selectivity of both HDS and HDN reactions highly rely on the catalytic models. In general, the HDS and HDN processes can sustainably occur both on the edges and Co modified surfaces under work conditions of ˜300 ℃, according to the semiempratical formula 1012exp(―Eb/kT) ≈ 1 [53]. Therefore, the basal plane of MoS2 catalysts can be activated by metal incorporation and thus increase the active sites for HDS and HDN reactions. Compared with the edge models, the activation energies on the metal vacancy interface model are the lowest for S/N-C scission and CH4 formation steps except for the Mo-edge (0% S). Although the SeC bond is easy to break on the Mo-edge (0% S), it is difficult to form CH4 and needs to overcome a high activation barrier of 2.68 eV. On the whole, the catalytic performance of the MoS2 surface modified by Co atoms is better than that of MoS2 edges. For the surface models, the interactions between the sulfur/nitrogen-containing compounds and the basal plane of MoS2 are too weak to proceed for further reaction (see Fig. S6). In addition, the activation barriers of other three surface models, namely, the pure sulfur vacancy, Co filled sulfur vacancy and Co on the top site of Mo atom without sulfur vacancy, are higher than that of the metal vacancy interface model. In particular, the methane formation only needs to overcome 0.13 and 0.17 eV for HDS and HDN on the Co-SV-surface model, respectively. Therefore, the metal vacancy interface model stands out from all other catalytic models (green marks in Fig. 6) and largely improves the catalytic performance. 6
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Fig. 7. Priorities of R-H and R-C scission (R = O, S, N) are on SV-surface and Co-SV-surface, respectively. The TS for N-H scission should be higher than the FS of 1.65 eV, and is formidable to locate by NEB simulation.
As a prerequisite for high selectivity of the catalysts, the reaction pathways of the catalytic reactions should be limited. In this respect, we consider competition reactions based on the metal vacancy interface, i.e. R-C scission and R-H breaking, where R denotes for S, O or N. As shown in Fig. 7, OeC and NeC scission process are favorable over OeH and NeH cleavages. The energy of the NeH scission product leads to a large endothermic of +1.63 eV, thus we failed to locate the transitional state for the reaction step due to highly unstable configuration. Alternatively, the SeH breaking process is energetically favorable comparing with the SeC scission, due to the weak SeH bond, and this leads to the refill of the SV sites with S atoms. Although S atoms fill in the vacancy, the presence of metal atom can further promote the regeneration of SV sites in that case [28]. Therefore, the high selectivity and stability (recycle of the catalyst) of the HDS reaction on metal vacancy model can be expected. 4. Conclusions In conclusion, we have investigated the detailed reaction mechanisms of CoMoS catalyst for HDS and HDN reactions based on six edge and four basal plane models. Our results show that in addition to edge domain catalytic model, the inert surface of MoS2 can be activated by incorporating isolated Co atoms, which also promotes vacancy generation and formation of CoeSeMo bonds. Specifically, a metal vacancy interface model has been verified with high reactivity and selectivity for both HDS and HDN reactions, in which the synergistic effect of Co atom and S vacancy urges NeC and SeC breakage. Therefore, both edge and surface models can be served as active sites for HDS and HDN reactions. In addition, the metal vacancy model shows lower energy barriers for elementary steps in HDS and HDN steps, and may allow the reactions to operate at a relatively low temperature. This systematic study provides new insights into the HDS and HDN mechanisms and theoretical guidelines on the design of efficient catalysts for HDS and HDN reactions. Acknowledgements This work is supported by the National Natural Science Foundation for Distinguished Young Scholar (21525311), National Key R&D Program of China (Grant No. 2017YFA0204800), National Natural Science Foundation of China (Grant Nos. 21773027, 21703032), Jiangsu 333 project (BRA2016353) and the Scientific Research Foundation of Graduate School of Southeast University (YBPY1920) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin. 7
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