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A DFT study of the effects of oxygen on the hydrodesulfurization of sulfur macromolecules during the direct hydrodesulfurization process
Molecular Catalysis 485 (2020) 110803
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A DFT study of the effects of oxygen on the hydrodesulfurization of sulfur macromolecules during the direct hydrodesulfurization process
T
Yi Zhenga, Weixia Zhoua, Yang Liua, Chenyang Zhanga, Suya Chua, Yongjun Liua,b,* a b
Department of Chemical Engineering, Huaqiao University, Xiamen, 361000, PR China Xi`an Jiaotong University Suzhou Academy, Xi`an Jiaotong University, Suzhou, 21500, PR China
ARTICLE INFO
ABSTRACT
Keywords: Co-Mo-S Oxygen effects Density functional theory DDS route 4,6-DMDBT
Periodic density functional theory (DFT) calculations were employed to study the effects of oxygen on the direct desulfurization (DDS) performance of Co-Mo-S catalyst in this work. The structural differences between Co-Mo-S and oxygen-containing Co-Mo-S (Co-Mo-O-S), as well as the oxygen effects on dibenzothiophene (DBT) and 4,6dimethyl-dibenzothiophene (4,6-DMDBT) hydrotreatment during DDS, were calculated. The calculation results show that oxygen reduces the electronic quantity in metal Co and Mo atoms. During the DDS process, Co-Mo-S has stronger adsorption capacity for the reactants than Co-Mo-O-S. The dissociation of hydrogen was promoted by oxygen, which helps to provide active hydrogen more easily during the reaction, but oxygen slightly increases the activation energy of hydrogen transfer. Due to the asymmetric structure, two possible reaction pathways exist for the CeS bond scission on Co-Mo-O-S. On the S-edge and M-edge of Co-Mo-O-S, CeS bond cleavage is more inclined to react on the oxygen-rich side. Meanwhile, oxygen changes the priority site of the reaction. On Co-Mo-S, the S-edge is the priority reaction site. On Co-Mo-O-S, CeS bond cleavage is favorable on the M-edge. This work paves a new pathway for improving HDS catalytic activity via synergistic structure and charge characteristics.
1. Introduction Hydrodesulfurization (HDS) is currently the most studied and widely used process in refineries during the production of low-sulfur fuels [1,2]. With the ever-increasing fuel quality standards, which require sulfur content to be lower than 10 ppm, and the ever-increasing sulfur content in crude oil, hydrodesulfurization catalysts are being vigorously promoted [3–5]. With an abundant composition and high activity, MoS2 is widely used in HDS [6]. In addition, doping with transition metals, especially Co and Ni, will greatly improve its catalytic activity. In recent years, numerous studies have been conducted to elucidate the catalytic mechanism of Co-Mo-S catalyst for hydrodesulfurization [7–9]. Both theoretical [10] and experimental [11] works have confirmed that the HDS activity originates from the edge active sites of the Co-Mo-S layer, while the base is catalytically inert. However, in the process of catalyst preparation, a large amount of oxygen is involved, which is mainly from the molybdenate precursors and oxide supports [12–15]. Oxygen remains in the catalyst if the sulfuration is insufficient and has a strong interaction with the Mo atoms [16–18]. Meanwhile, the presence of oxygen also affects the morphology of the catalyst, which may lead to a bend structure of
⁎
nanoclusters [19]. Xie’s group prepared MoS2 catalyst with moderately disordered oxygen doping, and the catalyst exhibited superior HER (hydrogen evolution reaction) performance [20]. Xue et al. prepared sensors with high sensitivity and detection range by doping oxygen into MoS2/graphene. Additionally, the authors successfully applied the sensors for actual detection and determination in gas detection [21]. These studies indicate that doping oxygen into the MoS2 phase structure may have important application prospects. In addition, previous studies have demonstrated that doping with heteroatoms (B, N, O, Co, Ni, or others) could increase the catalytic activity of the MoS2 [3,22–24]. However, to the best of our knowledge, the effect of oxygen doping on the properties of Co-Mo-S catalyst, particularly its hydrotreating properties, has not yet been reported. Numerous works [25–28] have indicated that the key to reducing the sulfur content in crude oil is to remove the sulfur-containing macromolecules such as 4,6-dimethyl-dibenzothiophene (4,6-DMDBT), which have large steric hindrance effects. Generally, for the HDS of 4,6DMDBT, it is well recognized that there are two parallel paths: hydrogenation (HYD) and direct desulphurization (DDS) [29]. HDS kinetic studies have proved that due to the two methyl groups of 4,6-DMDBT, the adsorption of 4,6-DMDBT at the active center is much weaker than
Corresponding author at: Department of Chemical Engineering, Huaqiao University, Xiamen, 361000, PR China. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.mcat.2020.110803 Received 28 October 2019; Received in revised form 16 January 2020; Accepted 29 January 2020 2468-8231/ © 2020 Elsevier B.V. All rights reserved.
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HDS process. The success of regulating the oxygen content of the catalyst will pave a new pathway for improving HDS catalytic activity by synergistic structure and charge characteristics. 2. Experimental and computation 2.1. Preparation of catalyst Generally, 2 mmol (NH4)6Mo7O24.4H2O, 66 mmol thiourea and 0.7 mmol Co(NO3)∙6H2O were dissolved in 60 ml distilled water and stirred vigorously for 1 h to obtain a homogeneous solution. The solution was transferred to a 100 ml Teflon-lined stainless steel autoclave, which was kept at 200 °C for 24 h. The reactor was then naturally cooled to room temperature. A filter was used to obtain the product, which was washed with distilled water and ethanol three times and dried overnight at 60 °C in vacuum. The materials was then sulfurized at 623 K, the partial pressure of 9:1 between H2 and H2S for 2 h. 2.2. Characterization
Fig. 1. TEM image of bending structure in Co-Mo-O-S catalyst.
The catalysts were investigated by high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2100UHR microscope (JEOL Ltd., Akishima, Tokyo, Japan) with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) and were employed to detect the content of various elements in the catalyst.
that of DBT. This also makes desulfurization of 4,6-DMDBT through the DDS route difficult [30–32], and the dissociation of hydrogen is hindered [27,28]. Some recent theoretical works have proved that on some active sites of the catalyst, 4,6-DMDBT and DBT show some similar behavior during the adsorption process. The benzene-ring and the edge of the catalyst are at an angle, which effectively reduces the substituent effect caused by methyl groups [33,34]. In this work, we have successfully prepared oxygen-containing CoMo-S catalysts (Co-Mo-O-S). To further study the oxygen effect on the sulfur-containing macromolecular HDS process, DFT calculations were used to obtain an efficient and accurate result [35,36]. First, we compared the structure and charge properties of both oxygen-containing and none-oxygen containing Co-Mo-S catalysts edge models. After that, the reaction details of DBT and 4,6-DMDBT on active center during the DDS process were discussed. In addition, the frequencies analysis of transition states and the calculation of Gibbs free energy under reaction conditions were used to better understand the oxygen effects on the
2.3. Analysis and modeling The oxygen in the catalyst mainly came from molybdenate precursors [12,13]. Oxygen affects the microstructure of the catalyst and the properties of the active center, but further research is needed to discuss the accurate location of oxygen in Co-Mo-S. The HRTEM image of oxygen-containing Co-Mo-S nanoclusters is shown in Fig. 1. It could be found that some of the crystal surfaces of the catalyst were curved, which is consistent with the results of other studies [37,38]. From the XPS elemental analysis, the atomic ratio of S:Co and Mo was 1.63:1, and that of S:O was 1.28:1. It could be predicted that the catalyst was not Fig. 2. Model images of Co-Mo-S and Co-MoO-S edge structure. Optimized structures of (a) S-edge of Co-Mo-S, (b) M-edge of Co-Mo-S, (c) S-edge of Co-Mo-O-S, (d) M-edge of Co-Mo-O-S (yellow color: sulfur atoms; red color: oxygen atoms; blue color: molybdenum atoms; purple color: cobalt atoms). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 1 Charge transfer on framework of edge structure.
completely sulfurized, and some sulfur atoms were replaced by oxygen atoms. A previous study had proved that oxygen would cause a bent structure, which was thermodynamically stable. In this work, DFT calculations for thermodynamic analysis of Co-Mo-S bending and flat edge structure are listed in Table S2. The calculation results show a dense packing of atoms in the bent structure, which made the repulsive force between atoms greater, and ultimately it led to a higher binding energy of the bent structure. When half of the sulfur atoms on Co-Mo-S were replaced with oxygen, the bent structures were more thermodynamically stable because of the more reasonable OeMo bond length. Hence, the oxygen-containing Co-Mo-S is more consistent with the bent structure in the TEM image (Fig. 1). Therefore, the edge models of oxygen containing Co-Mo-S nanocluster in this work was established by replacing half the sulfur atoms with oxygen atoms, whose configurations were consistent with the bent structure in TEM image. The dominant crystal phase structure of molybdenum disulfide is 2H-MoS2 with a space group of no.194, p63/mmc, and lattice constants of a = b = 3.15 Å and c = 12.3 Å in experimental data (according to ICDD/JCPDS-PDF#37-1492). In our work, the lattice parameters are a = b = 3.19 Å and c = 12.39 Å, which are in good agreement with the
experimental values. The cif file of the optimized 2H-MoS2 is found in the supporting material. Subsequent modeling was based on the optimized 2H-MoS2 cell. In this work, the edge structure of the Co-Mo-S nanocluster was based on previous studies [4,10,39]. Fig. 2 shows the models of Co-MoS and Co-Mo-O-S edge structures. In this work, the coverage of Co and S on the active S-edge (Co-S-edge) was 100 % and 50 %, respectively, while the coverage was 50 % and 25 % on the Mo-edge (Co-Mo-edge), respectively. To overcome the illusory force between the adsorbed molecules and the adjacent slab, the supercell parameters used for CoMo-S edge were 12.8 Å × 18 Å × 36 Å, which had a more than 20 Å vacuum layer in the z-direction. The oxygen-containing Co-Mo-S (CoMo-O-S) exhibited a bent structure [19], but the edge structure, especially regarding S coverage, required further investigation. To better understand the effect of oxygen on the substituents of sulfur-containing macromolecules, the same coverage of Co and S as Co-Mo-S was used in this work. The supercell parameters of 11.9 Å × 18 Å × 36 Å were used for both S-edge and Mo-edge of Co-Mo-O-S. Additionally, the calculations were focused on the effect of oxygen on the direct desulfurization of DBT and 4,6-DMDBT.
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Fig. 3. Total DOS spectra of different active sites of Co-Mo-S and Co-Mo-O-S. (a) S-edge of Co-Mo-S, (b) M-edge of Co-Mo-S, (c) S-edge of Co-Mo-O-S, (d) M-edge of Co-Mo-O-S. The Fermi level is the reference energy.
2.4. DFT computations
Dispersion energy may reach −90 kJ/mol, while traditional exchange and correlation functionals have less impact in this aspect. Hence, DFTD3 [44] was used in our calculation to estimate the dispersion interaction Edisp . The adsorption energy with dispersion correction ( EDFT D ) was calculated by the following formulation:
The periodic density functional theory calculations in this work were performed through the Vienna Ab-initio simulation package (VASP) [40,41]. The projector augmented-wave (PAW) potential [42] was used to deal with electron–ion interactions, and the general gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) [43] was employed to account for the electronic structure. The cut-off energy was set at 500 eV for the plane-wave basis. For geometry optimizations, all the edge structures were set on a 3, 2, 1 k-points of the Brillouin zone, and Methfessel-Paxton method was used with σ =0.1 eV. Dipole corrections were employed to each structure to overcome the influence of non-symmetrical structures. For the calculation of DOS (Density of States), higher k-points of 9, 6 and 3 was used to obtain more accurate results. Additionally, spin polarization was considered, due to the present of Co atoms. Geometry optimizations were completed when all atom forces were lower than 0.02 eV/ Å. The geometric optimization process was divided into the following steps: (i) all the layers were relaxed when no adsorption was performed, and (ii) when adsorption occurred, the two bottom layers were fixed, while the top three layers were relaxed. All molecules were placed in the center of the boxes with enough space in all directions. The absorption energy without dispersion correction ( EDFT ) was calculated by the following formula:
EDFT = EMol – edge
EMol
Eedge
EDFT
D
=
Edisp +
EDFT
(2)
Climbing image nudged elastic band method (CINEB) [45–47] combined with the Dimer method [48,49] was used to search for the transition states in order to calculate the surface reaction activation energies of the reaction process. CINEB provided a good initial guess of the transition state. Subsequently, the Dimer method was used to accurately converge upon a saddle point. The calculation results of transition frequencies and Gibbs free energy corrections are found in the supporting information. The calculation results of transition frequencies and Gibbs free energy corrections could be found in supporting information. 3. Results and discussions 3.1. Structure analysis of Co-Mo-S and Co-Mo-O-S The properties of catalysts are closely related to their structures, element compositions, and especially the electronic structures. The edge structures were calculated to better understand the effect of oxygen. In general, oxygen has a smaller atomic radius than sulfur. Introducing oxygen into the Co-Mo-S phase structure contributes to the formation of a bent structure. Additionally, it has been proved that this structure could exist stably [19]. Perhaps the electronic properties from DFT calculations was a much more important information, which dominates the performance of the
(1)
EMol is the energy of the adsorbed molecule; Eedge is the energy of edge structure without adsorbate; and EMol–edge represents the energy of molecule absorbed on the edge. Dispersion interactions play an important role in the process of absorption, especial for macromolecules such as 4,6-DMDBT.
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Table 2 Adsorption of DBT and 4,6-DMDBT.
The units of energy and length are kJ mol−1 and Å, respectively.
catalyst. The results of Bader analysis on the edge structures are listed in Table 1. The introduction of oxygen led to redistribution of electron density [50]. Relative to Co-Mo-S, the Mo atoms and Co atoms of CoMo-O-S carried more positive charges due to the incorporation of electronegative oxygen. Meanwhile, sulfur atoms carried less negative charges, with the exception of the unsaturated coordination sulfur atoms on M-edge. Those sulfur atoms carried more negative charges. Density of States information is listed in Fig. 3. It could be seen that the DOS spectra of S-edge and M-edge exhibited something different after oxygen incorporation. This indicated that the presence of oxygen affected the electronic structure, which dominated the chemical adsorption and the chemical reaction processes. Specifically, on both S-edge and M-edge, the containing of oxygen introduced new states at the lower-lying valence bands, whose main contribution was from the s orbital of the oxygen atoms. Meanwhile, some peaks at the energy level
of −2.5–8.0 eV were offset, and the expansion of DOS at the conduction bands increased. Combined with the date from Bader analysis, it could be concluded that when oxygen was introduced into the Co-Mo-S crystal phase, oxygen atoms with strong electronegativity were used to replace S site. On S-edge, oxygen enhanced the mentality of Co and Mo atoms and reduced non-metallicity of sulfur atoms. On M-edge, oxygen enhanced the metallicity of Co and Mo atoms, and also enhanced the non-metallic properties of sulfur atoms. It could be predicted that oxygen may have different effects on different structural edges. 3.2. The effects of oxygen on the adsorption of reactants and their intermediates In this part, we described the adsorption of reactants (DBT and 4,6DMDBT) and their intermediates (C12H9S and C14H13S) on the active
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Fig. 4. Adsorption models of DBT and 4,6-DMDBT: (a) and (b) are on the M-edge of Co-Mo-S, (c) and (d) are on the M-edge of Co-Mo-O-S.
center. This comparative analysis would help us to better understand the impact of oxygen during the adsorption process. A previous study proved that the promoter atoms are the most accessible atoms to participate in the reaction [51]. Hence, this work only considered the adsorption process of the promoter Co atom. The adsorption configurations of reactants on the S-edge and M-edge of Co-Mo-S and Co-MoO-S are reported in Table 2. Moreover, it could be found in Table 2 that all the adsorbed molecules were symmetrically adsorbed at the active site of the S-edge. Meanwhile the incorporation of oxygen caused a decrease in total adsorption energy and dispersion energy (27.11 kJ/ mol and 6.97 kJ/mol lower for DBT, 27.44 kJ/mol and 3.33 kJ/mol lower for 4,6-DMDBT). Comparing the calculation results of DBT and 4,6-DMDBT on the S-edge of Co-Mo-S and Co-Mo-O-S, it could be found that as there was enough space in CUS (coordinatively unsaturated site) to contain two methyl groups from 4,6-DMDBT, the presence of methyl groups did not weaken the adsorption. The adsorption energies of DBT and 4,6-DMDBT were basically equal, which agreed with a previous report by S. Ding et al. [52]. Comparing the absorption of DBT and 4,6DMDBT on the M-edge of Co-Mo-S and Co-Mo-O-S, the result showed that the incorporation of oxygen led to the changes in adsorption energy and adsorption configuration. Oxygen obviously weakened the
adsorption of DBT and 4,6-DMDBT, 34.64 kJ/mol and 37.75 kJ/mol lower in total energy respectively. As shown in Fig. 4, DBT and Mo-edge were at the angle of 31° when DBT adsorbed on the active site of Co-MoS. As a result of the substituent effects from two methyl groups, 4,6DMDBT was found at a smaller angle of 26° when its S atom adsorbed on the Co atom. On the M-edge of Co-Mo-O-S, the situation was slightly different. Due to a shorter O-Mo bond and a bend structure, the adsorbed molecules were tilted at an angle perpendicular to the M-edge. Additionally, DBT and 4,6-DMDBT were adsorbed on the metal-edge at a larger angle and had a smaller angle difference, 40° for DBT and 37.5° for 4,6-DMDBT, which indicated that oxygen slightly weakened the substituent effects of two methyl groups. It should be noted that Edisp measurements indicated the dispersion interaction contributed significantly to the total energy, especially on the M-edge. The adsorption of intermediates plays an important role in the DDS process; hence, the details of C12H9S and C14H13S adsorption processes were considered in this work. The calculation results of C12H9S and C14H13S adsorption process were listed in Tables 3 and 4. On the S-edge of Co-Mo-S and Co-Mo-O-S, as there was enough space in CUS site, the steric effects of the methyl groups were almost negligible, and the adsorption energies for the two intermediates were
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Table 3 Adsorption of C12H9S on the Co-Mo-S and Co-Mo-O-S.
The units of energy and length are kJ mol−1 and Å, respectively.
almost the same. They only differed by 3–5 kJ/mol. Additionally, the small atomic mass of oxygen caused a decrease in dispersion interaction. On the M-edge, the presence of more electronegative oxygen atoms led to a higher chemical adsorption energy difference. The adsorption energies of the intermediates were 39–51 kJ/mol higher than those of the reactants on Co-Mo-S. Interestingly, the M-edge of Co-MoO-S showed an even higher energy difference of 99–139 kJ/mol. Additionally, the dispersion correction for C12H9S on the M-edge of CoMo-O-S was much higher than that of Co-Mo-S, which could be the result of reinforcement of metallic of metal edge and the structural changes. Combined with the calculation results, it was obvious that the incorporation of oxygen led to some changes in adsorption energy and adsorption configuration. In general, catalysts containing oxygen have lower adsorption energies of the reactants and a higher adsorption energy difference between the reactant and intermediate. This higher adsorption energy difference may contribute to CeS bond cleavage [53]. Meanwhile, as the result of the bent structure and the change in surface properties, the adsorption configurations of reactants on the metal-edge had similar angles of inclination with the edge structure (maintaining the same curvature as Co-Mo-O-S) and a larger inclination angle with the horizontal plane of the M-edge.
3.3. The effects of oxygen on hydrogen dissociation and transfer Hydrogen dissociation plays an important role in DDS. The competitive adsorption of sulfur-containing molecules influences hydrogen dissociation; hence, hydrogen dissociation without absorption of DBT and 4,6-DMDBT was considered in this work. The calculation results of hydrogen dissociation on the S-edge and M-edge are listed in Tables 5 and 6, respectively. The hydrogen dissociation on the S-edge is exothermic, which is consistent with previous research [54]. While on CoMO-O-S, the reaction appears to be a decrease in heat release or endotherm. On the clean S-edge of Co-Mo-S and Co-Mo-O-S, the reaction energy and activation energy of hydrogen dissociation were almost the same, with a difference of only 2.22 kJ/mol in reaction energy and a difference of 1.74 kJ/mol in activation energy. However, the oxygen effects on hydrogen dissociation could be diversified when DBT or 4,6DMDBT occupied the CUS site. On the S-edge of Co-Mo-S, the adsorption of DBT and 4,6-DMDBT caused the reactions to be micro-endothermic, and a higher activation energy was measured. On the S-edge of Co-Mo-O-S, the competitive adsorption of S-containing molecules made the hydrogen dissociation reaction more exothermic, and a lower activation energy (25.34–27.43 kJ/mol lower than that on the S-edge of Co-Mo-S) was measured. On the M-edge of Co-Mo-S and Co-Mo-O-S, all
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Table 4 Adsorption of C14H13S on the Co-Mo-S and Co-Mo-O-S.
The units of energy and length are kJ mol−1 and Å, respectively.
hydrogen dissociation reactions were exothermic. Meanwhile, because the DBT and 4,6-DMDBT were adsorbed obliquely on the active site, the dissociation of hydrogen was not obviously influenced by the adsorbates. Compared to the calculation results of Co-Mo-S and Co-Mo-OS, the incorporation of oxygen slightly reduced the activation energy of the reaction by approximately 11 kJ/mol. The change in activation energy was not obvious but played an important role during the process of hydrogen dissociation. The ability of hydrogen transfer at the metal sites of the M-edge also played an important role in the process of DDS. Table 7 presents the details of hydrogen transfer on the M-edge of Co-Mo-S and Co-Mo-O-S. On both Co-Mo-S and Co-Mo-O-S, when DBT was adsorbed, hydrogen was easier to transfer compared to when there was nothing adsorbed on the M-edge. Additionally, the reaction energy barrier was lower by about 2–4 kJ∙mol−1, which indicated that hydrogen atoms preferred to transfer after the reactants adsorbed. When 4,6-DMDBT was adsorbed on the M-edge of Co-Mo-S, hydrogen transfer became even easier. However, when 4,6-DMDBT was absorbed on the M-edge of Co-Mo-O-S, the activation energy of hydrogen transfer was slightly higher than when DBT was absorbed. From Table 7, the activation energy of hydrogen transfer on the M-edge of Co-Mo-O-S was slightly higher than for Co-Mo-S. Combining the data of Tables 1 and 7, it could be predicted that oxygen atoms with strong electronegativity had higher
electron density, which would form a strong interaction with adsorbed hydrogen and blocked the transfer of hydrogen. However, the difference in activation energy was only slight. In brief, the hydrogen dissociation energies on different edges of CoMo-S and Co-Mo-O-S were clearly affected by oxygen. Owing to the strong electron-negative oxygen incorporation, hydrogen dissociation was relatively easy, as a lower activation energy is needed during this reaction. Moreover, the activation barrier on the S-edge decreased significantly. There were also some negative effects: the hydrogen that adsorbed on the M-edge after dissociation required more energy to transfer to the adjacent active center before it could participate in a subsequent reaction. Ultimately, oxygen is beneficial for the dissociation of hydrogen but slightly hinders the transfer of hydrogen. 3.4. The effect of oxygen on DDS To analyze the effect of oxygen on the DDS route, we calculated the CeS bond cleavage that occurred on the S-edge and M-edge. Due to the asymmetric structure of the Co-Mo-O-S nanocluster, there are two possible routes of CeS bond cleavage, as shown in Fig. 5. Using the formation of C12H9S as an example, the CeS bond cleavage occurred on the O-Mo side (route one). Route two involved the CeS bond scission on the S-Mo side.
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Table 5 Hydrogen dissociation on S-edge of Co-Mo-S and Co-Mo-O-S.
3.4.1. Oxygen effect on the CeS bond scission of S-edge The calculation results of CeS bond cleavage on the S-edge are listed in Table 8. In particular, for all CeS bond cleavage transition states, the carbocyclic ring was saturated with a hydrogen ring to generate a benzene ring, after that final product was formed though a short reaction path. Combining the data of CeS bond scission for DBT and 4,6-DMDBT on Co-Mo-S, it could be found that the CeS bond scission of the reactants on the S-edge was an exothermic reaction. Due to the substituent effects of the two methyl groups from 4,6-DMDBT, higher activation energy was required for 4,6-DMDBT CeS bond scission than for DBT. Due to the structural asymmetry, there are two possible pathways for CeS bond cleavage on Co-Mo-O-S: CeS bond cleavage on the O-Mo side and S-Mo side. The data presented in Table 8 show that the activation energy of CeS bond scission on the O-Mo side is much lower than that on the S-Mo side, meaning the incorporation of
oxygen facilitates CeS bond cleavage on the oxygen-rich side. There may be two reasons for this: one reason is that the asymmetric bending structure of Co-Mo-O-S provides more space for the reactant to undergo CeS bond scission at the O-Mo side. In this case, the substituent effects are smaller, and the reaction is more favorable. The second reason is that during CeS bond cleavage, a hydrogen atom is transferred, and although oxygen increases the difficulty of hydrogen transfer, the incorporation of oxygen facilitates CeS bond cleavage and the rotation of the benzene ring. CeS bond cleavage plays a more important role than hydrogen transfer, after all. However, the activation energy of the CeS bond scission on the O-Mo side is still slightly higher than that on the Sedge of Co-Mo-S. In short, for CeS bond cleavage on the S-edge, the CeS bond of the reactant cleaved under the participation of active hydrogen. Subsequently, a new benzene ring was formed, and it rotated to
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Table 6 Hydrogen dissociation on M-edge of Co-Mo-S and Co-Mo-O-S.
generate a new product. Due to the structural asymmetry of Co-Mo-O-S, there were two ways to break the CeS bond, in which the activation energy of the CeS bond scission on the O-Mo side was lower. This was mainly due to changes in structure and charge properties induced by oxygen. However, the incorporation of oxygen increased the difficulty of CeS bond scission on the S-edge overall.
adsorbates. Hence, when 4,6-DMDBT was adsorbed on the M-edge, higher activation energy was required for cleavage of the CeS bond than when DBT was adsorbed. On Co-Mo-O-S, there were also two possible reaction pathways. Similarly, the oxygen-rich side was more conducive to reaction of CeS bond cleavage. However, the incorporation of oxygen on the M-edge decreased the activation energy, indicating that the increase in metallic properties caused by oxygen made the M-edge more favorable for the reaction. Oxygen has different effects on DDS at different active sites. For both the S-edge and M-edge, the incorporation of oxygen provided active hydrogen stably, because oxygen atoms are more attracted to hydrogen atoms than sulfur atoms. This also led to an increase in the difficulty of hydrogen transfer. Non-metallic elemental sulfur is terminal on the S-edge, while Co and Mo are terminal on the M-edge. Hence, oxygen has various effects on CeS bond scission of the S-side and the Mside. Relative to Co-Mo-S, CeS bond scission of the S-edge of Co-Mo-O-S is unfavorable, while M-edge exhibits good reactivity. According to the relationship between the electronegativity of basal elements, the incorporation of different electronegative non-metallic elements into the catalyst is one way to effectively regulate catalyst function.
3.4.2. The effects of oxygen on the CeS bond scission of the M-edge The reaction process on the M-edge was similar to the S-edge, which involved the transfer of hydrogen, scission of the CeS bond and the rotation of the benzene ring. Similarly, due to the asymmetric structure of Co-Mo-O-S, there are two possibilities for CeS bond cleavage. The calculation results for CeS bond scission of DBT and 4,6-DMDBT on the M-edge are listed in Table 9. The results show that bond cleavage and benzene ring rotation pushed the sulfur atom that was adsorbed on the active center during the reaction process, which resulted in the deformation of the original Co-S bond. On Co-Mo-S, the reactions were exothermic, indicating that the force between the carbon atom and sulfur atom on the M-edge was weaker than that on the S-edge. The two methyl groups of 4,6-DMDBT weakened the adsorption Co-S bond of
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Table 7 Hydrogen transfer on the M-edge of Co-Mo-S and Co-Mo-O-S.
Fig. 5. Reaction pathway route of DBT to C12H9S on the S-edge of Co-Mo-O-S.
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Table 8 CeS bond cleavage on the S-edge of Co-Mo-S and Co-Mo-O-S.
4. Conclusion
modifications.
In summary, the basic structure, electronic properties and hydrodesulfurization properties reflected by DBT and 4,6-DMDBT on Co-MoS and Co-Mo-O-S were comparatively analyzed to further determine the effects of oxygen on the Co-Mo-S catalyst during DDS. The Co-Mo-S catalyst containing oxygen has a bent structure, which is consistent with HRTEM imaging. During DDS, oxygen facilitates the dissociation of hydrogen but is not conducive to the transfer of active hydrogen. The presence of oxygen also affects the preference of catalytic reaction sites. C–S bond scission is more likely to happen on the M-edge, while the activation energy of CeS bond scission on S-edge is increased when oxygen is incorporated into the catalyst structure. From comparative analysis, it could be inferred that oxygen could enhance the performance of the Co-Mo-S catalyst. This work allows us to improve the activity of catalysts via synergistic structural and electronic
CRediT authorship contribution statement Yi Zheng: Conceptualization, Formal analysis, Writing - original draft. Weixia Zhou: Data curation. Yang Liu: Investigation. Chenyang Zhang: Methodology. Suya Chu: Project administration, Supervision. Yongjun Liu: Resources, Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Table 9 CeS bond cleavage on the M-edge of Co-Mo-S and Co-Mo-O-S.
Acknowledgements
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This work was financially supported by the Industry-UniversityResearch Collaborative Innovation Project of Xiamen (Grant No. 3502Z20183024), the National Natural Science Foundation of China (Grant No. 21603077, 51603077), and Postgraduates’ Innovative Fund in Scientific Research of Huaqiao University. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2020.110803. References [1] C. Song, An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel, Catal. Today 86 (2003) 211–263. [2] I.V. Babich, J.A. Moulijn, Science and technology of novel processes for deep desulfurization of oil refinery streams: a review, Fuel 82 (2003) 607–631. [3] Z.N. Jaf, M. Altarawneh, H.A. Miran, Z.-T. Jiang, B.Z. Dlugogorski, Hydrodesulfurization of Thiophene over γ-Mo2N catalyst, Mol. Catal. 459 (2018) 21–30. [4] E. Krebs, B. Silvi, A. Daudin, P. Raybaud, A DFT study of the origin of the HDS/ HydO selectivity on Co(Ni)MoS active phases, J. Catal. 260 (2008) 276–287. [5] C. Zuriaga-Monroy, J.-M. Martínez-Magadán, E. Ramos, R. Gómez-Balderas, A DFT study of the electronic structure of cobalt and nickel mono-substituted MoS2
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A DFT study of the effects of oxygen on the hydrodesulfurization of sulfur macromolecules during the direct hydrodesulfurization process
T
Yi Zhenga, Weixia Zhoua, Yang Liua, Chenyang Zhanga, Suya Chua, Yongjun Liua,b,* a b
Department of Chemical Engineering, Huaqiao University, Xiamen, 361000, PR China Xi`an Jiaotong University Suzhou Academy, Xi`an Jiaotong University, Suzhou, 21500, PR China
ARTICLE INFO
ABSTRACT
Keywords: Co-Mo-S Oxygen effects Density functional theory DDS route 4,6-DMDBT
Periodic density functional theory (DFT) calculations were employed to study the effects of oxygen on the direct desulfurization (DDS) performance of Co-Mo-S catalyst in this work. The structural differences between Co-Mo-S and oxygen-containing Co-Mo-S (Co-Mo-O-S), as well as the oxygen effects on dibenzothiophene (DBT) and 4,6dimethyl-dibenzothiophene (4,6-DMDBT) hydrotreatment during DDS, were calculated. The calculation results show that oxygen reduces the electronic quantity in metal Co and Mo atoms. During the DDS process, Co-Mo-S has stronger adsorption capacity for the reactants than Co-Mo-O-S. The dissociation of hydrogen was promoted by oxygen, which helps to provide active hydrogen more easily during the reaction, but oxygen slightly increases the activation energy of hydrogen transfer. Due to the asymmetric structure, two possible reaction pathways exist for the CeS bond scission on Co-Mo-O-S. On the S-edge and M-edge of Co-Mo-O-S, CeS bond cleavage is more inclined to react on the oxygen-rich side. Meanwhile, oxygen changes the priority site of the reaction. On Co-Mo-S, the S-edge is the priority reaction site. On Co-Mo-O-S, CeS bond cleavage is favorable on the M-edge. This work paves a new pathway for improving HDS catalytic activity via synergistic structure and charge characteristics.
1. Introduction Hydrodesulfurization (HDS) is currently the most studied and widely used process in refineries during the production of low-sulfur fuels [1,2]. With the ever-increasing fuel quality standards, which require sulfur content to be lower than 10 ppm, and the ever-increasing sulfur content in crude oil, hydrodesulfurization catalysts are being vigorously promoted [3–5]. With an abundant composition and high activity, MoS2 is widely used in HDS [6]. In addition, doping with transition metals, especially Co and Ni, will greatly improve its catalytic activity. In recent years, numerous studies have been conducted to elucidate the catalytic mechanism of Co-Mo-S catalyst for hydrodesulfurization [7–9]. Both theoretical [10] and experimental [11] works have confirmed that the HDS activity originates from the edge active sites of the Co-Mo-S layer, while the base is catalytically inert. However, in the process of catalyst preparation, a large amount of oxygen is involved, which is mainly from the molybdenate precursors and oxide supports [12–15]. Oxygen remains in the catalyst if the sulfuration is insufficient and has a strong interaction with the Mo atoms [16–18]. Meanwhile, the presence of oxygen also affects the morphology of the catalyst, which may lead to a bend structure of
⁎
nanoclusters [19]. Xie’s group prepared MoS2 catalyst with moderately disordered oxygen doping, and the catalyst exhibited superior HER (hydrogen evolution reaction) performance [20]. Xue et al. prepared sensors with high sensitivity and detection range by doping oxygen into MoS2/graphene. Additionally, the authors successfully applied the sensors for actual detection and determination in gas detection [21]. These studies indicate that doping oxygen into the MoS2 phase structure may have important application prospects. In addition, previous studies have demonstrated that doping with heteroatoms (B, N, O, Co, Ni, or others) could increase the catalytic activity of the MoS2 [3,22–24]. However, to the best of our knowledge, the effect of oxygen doping on the properties of Co-Mo-S catalyst, particularly its hydrotreating properties, has not yet been reported. Numerous works [25–28] have indicated that the key to reducing the sulfur content in crude oil is to remove the sulfur-containing macromolecules such as 4,6-dimethyl-dibenzothiophene (4,6-DMDBT), which have large steric hindrance effects. Generally, for the HDS of 4,6DMDBT, it is well recognized that there are two parallel paths: hydrogenation (HYD) and direct desulphurization (DDS) [29]. HDS kinetic studies have proved that due to the two methyl groups of 4,6-DMDBT, the adsorption of 4,6-DMDBT at the active center is much weaker than
Corresponding author at: Department of Chemical Engineering, Huaqiao University, Xiamen, 361000, PR China. E-mail address: [email protected] (Y. Liu).
https://doi.org/10.1016/j.mcat.2020.110803 Received 28 October 2019; Received in revised form 16 January 2020; Accepted 29 January 2020 2468-8231/ © 2020 Elsevier B.V. All rights reserved.
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HDS process. The success of regulating the oxygen content of the catalyst will pave a new pathway for improving HDS catalytic activity by synergistic structure and charge characteristics. 2. Experimental and computation 2.1. Preparation of catalyst Generally, 2 mmol (NH4)6Mo7O24.4H2O, 66 mmol thiourea and 0.7 mmol Co(NO3)∙6H2O were dissolved in 60 ml distilled water and stirred vigorously for 1 h to obtain a homogeneous solution. The solution was transferred to a 100 ml Teflon-lined stainless steel autoclave, which was kept at 200 °C for 24 h. The reactor was then naturally cooled to room temperature. A filter was used to obtain the product, which was washed with distilled water and ethanol three times and dried overnight at 60 °C in vacuum. The materials was then sulfurized at 623 K, the partial pressure of 9:1 between H2 and H2S for 2 h. 2.2. Characterization
Fig. 1. TEM image of bending structure in Co-Mo-O-S catalyst.
The catalysts were investigated by high-resolution transmission electron microscopy (HRTEM) using a JEOL JEM-2100UHR microscope (JEOL Ltd., Akishima, Tokyo, Japan) with an acceleration voltage of 200 kV. X-ray photoelectron spectra (XPS) were recorded on a Thermo ESCALAB 250Xi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) and were employed to detect the content of various elements in the catalyst.
that of DBT. This also makes desulfurization of 4,6-DMDBT through the DDS route difficult [30–32], and the dissociation of hydrogen is hindered [27,28]. Some recent theoretical works have proved that on some active sites of the catalyst, 4,6-DMDBT and DBT show some similar behavior during the adsorption process. The benzene-ring and the edge of the catalyst are at an angle, which effectively reduces the substituent effect caused by methyl groups [33,34]. In this work, we have successfully prepared oxygen-containing CoMo-S catalysts (Co-Mo-O-S). To further study the oxygen effect on the sulfur-containing macromolecular HDS process, DFT calculations were used to obtain an efficient and accurate result [35,36]. First, we compared the structure and charge properties of both oxygen-containing and none-oxygen containing Co-Mo-S catalysts edge models. After that, the reaction details of DBT and 4,6-DMDBT on active center during the DDS process were discussed. In addition, the frequencies analysis of transition states and the calculation of Gibbs free energy under reaction conditions were used to better understand the oxygen effects on the
2.3. Analysis and modeling The oxygen in the catalyst mainly came from molybdenate precursors [12,13]. Oxygen affects the microstructure of the catalyst and the properties of the active center, but further research is needed to discuss the accurate location of oxygen in Co-Mo-S. The HRTEM image of oxygen-containing Co-Mo-S nanoclusters is shown in Fig. 1. It could be found that some of the crystal surfaces of the catalyst were curved, which is consistent with the results of other studies [37,38]. From the XPS elemental analysis, the atomic ratio of S:Co and Mo was 1.63:1, and that of S:O was 1.28:1. It could be predicted that the catalyst was not Fig. 2. Model images of Co-Mo-S and Co-MoO-S edge structure. Optimized structures of (a) S-edge of Co-Mo-S, (b) M-edge of Co-Mo-S, (c) S-edge of Co-Mo-O-S, (d) M-edge of Co-Mo-O-S (yellow color: sulfur atoms; red color: oxygen atoms; blue color: molybdenum atoms; purple color: cobalt atoms). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
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Table 1 Charge transfer on framework of edge structure.
completely sulfurized, and some sulfur atoms were replaced by oxygen atoms. A previous study had proved that oxygen would cause a bent structure, which was thermodynamically stable. In this work, DFT calculations for thermodynamic analysis of Co-Mo-S bending and flat edge structure are listed in Table S2. The calculation results show a dense packing of atoms in the bent structure, which made the repulsive force between atoms greater, and ultimately it led to a higher binding energy of the bent structure. When half of the sulfur atoms on Co-Mo-S were replaced with oxygen, the bent structures were more thermodynamically stable because of the more reasonable OeMo bond length. Hence, the oxygen-containing Co-Mo-S is more consistent with the bent structure in the TEM image (Fig. 1). Therefore, the edge models of oxygen containing Co-Mo-S nanocluster in this work was established by replacing half the sulfur atoms with oxygen atoms, whose configurations were consistent with the bent structure in TEM image. The dominant crystal phase structure of molybdenum disulfide is 2H-MoS2 with a space group of no.194, p63/mmc, and lattice constants of a = b = 3.15 Å and c = 12.3 Å in experimental data (according to ICDD/JCPDS-PDF#37-1492). In our work, the lattice parameters are a = b = 3.19 Å and c = 12.39 Å, which are in good agreement with the
experimental values. The cif file of the optimized 2H-MoS2 is found in the supporting material. Subsequent modeling was based on the optimized 2H-MoS2 cell. In this work, the edge structure of the Co-Mo-S nanocluster was based on previous studies [4,10,39]. Fig. 2 shows the models of Co-MoS and Co-Mo-O-S edge structures. In this work, the coverage of Co and S on the active S-edge (Co-S-edge) was 100 % and 50 %, respectively, while the coverage was 50 % and 25 % on the Mo-edge (Co-Mo-edge), respectively. To overcome the illusory force between the adsorbed molecules and the adjacent slab, the supercell parameters used for CoMo-S edge were 12.8 Å × 18 Å × 36 Å, which had a more than 20 Å vacuum layer in the z-direction. The oxygen-containing Co-Mo-S (CoMo-O-S) exhibited a bent structure [19], but the edge structure, especially regarding S coverage, required further investigation. To better understand the effect of oxygen on the substituents of sulfur-containing macromolecules, the same coverage of Co and S as Co-Mo-S was used in this work. The supercell parameters of 11.9 Å × 18 Å × 36 Å were used for both S-edge and Mo-edge of Co-Mo-O-S. Additionally, the calculations were focused on the effect of oxygen on the direct desulfurization of DBT and 4,6-DMDBT.
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Fig. 3. Total DOS spectra of different active sites of Co-Mo-S and Co-Mo-O-S. (a) S-edge of Co-Mo-S, (b) M-edge of Co-Mo-S, (c) S-edge of Co-Mo-O-S, (d) M-edge of Co-Mo-O-S. The Fermi level is the reference energy.
2.4. DFT computations
Dispersion energy may reach −90 kJ/mol, while traditional exchange and correlation functionals have less impact in this aspect. Hence, DFTD3 [44] was used in our calculation to estimate the dispersion interaction Edisp . The adsorption energy with dispersion correction ( EDFT D ) was calculated by the following formulation:
The periodic density functional theory calculations in this work were performed through the Vienna Ab-initio simulation package (VASP) [40,41]. The projector augmented-wave (PAW) potential [42] was used to deal with electron–ion interactions, and the general gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) [43] was employed to account for the electronic structure. The cut-off energy was set at 500 eV for the plane-wave basis. For geometry optimizations, all the edge structures were set on a 3, 2, 1 k-points of the Brillouin zone, and Methfessel-Paxton method was used with σ =0.1 eV. Dipole corrections were employed to each structure to overcome the influence of non-symmetrical structures. For the calculation of DOS (Density of States), higher k-points of 9, 6 and 3 was used to obtain more accurate results. Additionally, spin polarization was considered, due to the present of Co atoms. Geometry optimizations were completed when all atom forces were lower than 0.02 eV/ Å. The geometric optimization process was divided into the following steps: (i) all the layers were relaxed when no adsorption was performed, and (ii) when adsorption occurred, the two bottom layers were fixed, while the top three layers were relaxed. All molecules were placed in the center of the boxes with enough space in all directions. The absorption energy without dispersion correction ( EDFT ) was calculated by the following formula:
EDFT = EMol – edge
EMol
Eedge
EDFT
D
=
Edisp +
EDFT
(2)
Climbing image nudged elastic band method (CINEB) [45–47] combined with the Dimer method [48,49] was used to search for the transition states in order to calculate the surface reaction activation energies of the reaction process. CINEB provided a good initial guess of the transition state. Subsequently, the Dimer method was used to accurately converge upon a saddle point. The calculation results of transition frequencies and Gibbs free energy corrections are found in the supporting information. The calculation results of transition frequencies and Gibbs free energy corrections could be found in supporting information. 3. Results and discussions 3.1. Structure analysis of Co-Mo-S and Co-Mo-O-S The properties of catalysts are closely related to their structures, element compositions, and especially the electronic structures. The edge structures were calculated to better understand the effect of oxygen. In general, oxygen has a smaller atomic radius than sulfur. Introducing oxygen into the Co-Mo-S phase structure contributes to the formation of a bent structure. Additionally, it has been proved that this structure could exist stably [19]. Perhaps the electronic properties from DFT calculations was a much more important information, which dominates the performance of the
(1)
EMol is the energy of the adsorbed molecule; Eedge is the energy of edge structure without adsorbate; and EMol–edge represents the energy of molecule absorbed on the edge. Dispersion interactions play an important role in the process of absorption, especial for macromolecules such as 4,6-DMDBT.
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Table 2 Adsorption of DBT and 4,6-DMDBT.
The units of energy and length are kJ mol−1 and Å, respectively.
catalyst. The results of Bader analysis on the edge structures are listed in Table 1. The introduction of oxygen led to redistribution of electron density [50]. Relative to Co-Mo-S, the Mo atoms and Co atoms of CoMo-O-S carried more positive charges due to the incorporation of electronegative oxygen. Meanwhile, sulfur atoms carried less negative charges, with the exception of the unsaturated coordination sulfur atoms on M-edge. Those sulfur atoms carried more negative charges. Density of States information is listed in Fig. 3. It could be seen that the DOS spectra of S-edge and M-edge exhibited something different after oxygen incorporation. This indicated that the presence of oxygen affected the electronic structure, which dominated the chemical adsorption and the chemical reaction processes. Specifically, on both S-edge and M-edge, the containing of oxygen introduced new states at the lower-lying valence bands, whose main contribution was from the s orbital of the oxygen atoms. Meanwhile, some peaks at the energy level
of −2.5–8.0 eV were offset, and the expansion of DOS at the conduction bands increased. Combined with the date from Bader analysis, it could be concluded that when oxygen was introduced into the Co-Mo-S crystal phase, oxygen atoms with strong electronegativity were used to replace S site. On S-edge, oxygen enhanced the mentality of Co and Mo atoms and reduced non-metallicity of sulfur atoms. On M-edge, oxygen enhanced the metallicity of Co and Mo atoms, and also enhanced the non-metallic properties of sulfur atoms. It could be predicted that oxygen may have different effects on different structural edges. 3.2. The effects of oxygen on the adsorption of reactants and their intermediates In this part, we described the adsorption of reactants (DBT and 4,6DMDBT) and their intermediates (C12H9S and C14H13S) on the active
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Fig. 4. Adsorption models of DBT and 4,6-DMDBT: (a) and (b) are on the M-edge of Co-Mo-S, (c) and (d) are on the M-edge of Co-Mo-O-S.
center. This comparative analysis would help us to better understand the impact of oxygen during the adsorption process. A previous study proved that the promoter atoms are the most accessible atoms to participate in the reaction [51]. Hence, this work only considered the adsorption process of the promoter Co atom. The adsorption configurations of reactants on the S-edge and M-edge of Co-Mo-S and Co-MoO-S are reported in Table 2. Moreover, it could be found in Table 2 that all the adsorbed molecules were symmetrically adsorbed at the active site of the S-edge. Meanwhile the incorporation of oxygen caused a decrease in total adsorption energy and dispersion energy (27.11 kJ/ mol and 6.97 kJ/mol lower for DBT, 27.44 kJ/mol and 3.33 kJ/mol lower for 4,6-DMDBT). Comparing the calculation results of DBT and 4,6-DMDBT on the S-edge of Co-Mo-S and Co-Mo-O-S, it could be found that as there was enough space in CUS (coordinatively unsaturated site) to contain two methyl groups from 4,6-DMDBT, the presence of methyl groups did not weaken the adsorption. The adsorption energies of DBT and 4,6-DMDBT were basically equal, which agreed with a previous report by S. Ding et al. [52]. Comparing the absorption of DBT and 4,6DMDBT on the M-edge of Co-Mo-S and Co-Mo-O-S, the result showed that the incorporation of oxygen led to the changes in adsorption energy and adsorption configuration. Oxygen obviously weakened the
adsorption of DBT and 4,6-DMDBT, 34.64 kJ/mol and 37.75 kJ/mol lower in total energy respectively. As shown in Fig. 4, DBT and Mo-edge were at the angle of 31° when DBT adsorbed on the active site of Co-MoS. As a result of the substituent effects from two methyl groups, 4,6DMDBT was found at a smaller angle of 26° when its S atom adsorbed on the Co atom. On the M-edge of Co-Mo-O-S, the situation was slightly different. Due to a shorter O-Mo bond and a bend structure, the adsorbed molecules were tilted at an angle perpendicular to the M-edge. Additionally, DBT and 4,6-DMDBT were adsorbed on the metal-edge at a larger angle and had a smaller angle difference, 40° for DBT and 37.5° for 4,6-DMDBT, which indicated that oxygen slightly weakened the substituent effects of two methyl groups. It should be noted that Edisp measurements indicated the dispersion interaction contributed significantly to the total energy, especially on the M-edge. The adsorption of intermediates plays an important role in the DDS process; hence, the details of C12H9S and C14H13S adsorption processes were considered in this work. The calculation results of C12H9S and C14H13S adsorption process were listed in Tables 3 and 4. On the S-edge of Co-Mo-S and Co-Mo-O-S, as there was enough space in CUS site, the steric effects of the methyl groups were almost negligible, and the adsorption energies for the two intermediates were
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Table 3 Adsorption of C12H9S on the Co-Mo-S and Co-Mo-O-S.
The units of energy and length are kJ mol−1 and Å, respectively.
almost the same. They only differed by 3–5 kJ/mol. Additionally, the small atomic mass of oxygen caused a decrease in dispersion interaction. On the M-edge, the presence of more electronegative oxygen atoms led to a higher chemical adsorption energy difference. The adsorption energies of the intermediates were 39–51 kJ/mol higher than those of the reactants on Co-Mo-S. Interestingly, the M-edge of Co-MoO-S showed an even higher energy difference of 99–139 kJ/mol. Additionally, the dispersion correction for C12H9S on the M-edge of CoMo-O-S was much higher than that of Co-Mo-S, which could be the result of reinforcement of metallic of metal edge and the structural changes. Combined with the calculation results, it was obvious that the incorporation of oxygen led to some changes in adsorption energy and adsorption configuration. In general, catalysts containing oxygen have lower adsorption energies of the reactants and a higher adsorption energy difference between the reactant and intermediate. This higher adsorption energy difference may contribute to CeS bond cleavage [53]. Meanwhile, as the result of the bent structure and the change in surface properties, the adsorption configurations of reactants on the metal-edge had similar angles of inclination with the edge structure (maintaining the same curvature as Co-Mo-O-S) and a larger inclination angle with the horizontal plane of the M-edge.
3.3. The effects of oxygen on hydrogen dissociation and transfer Hydrogen dissociation plays an important role in DDS. The competitive adsorption of sulfur-containing molecules influences hydrogen dissociation; hence, hydrogen dissociation without absorption of DBT and 4,6-DMDBT was considered in this work. The calculation results of hydrogen dissociation on the S-edge and M-edge are listed in Tables 5 and 6, respectively. The hydrogen dissociation on the S-edge is exothermic, which is consistent with previous research [54]. While on CoMO-O-S, the reaction appears to be a decrease in heat release or endotherm. On the clean S-edge of Co-Mo-S and Co-Mo-O-S, the reaction energy and activation energy of hydrogen dissociation were almost the same, with a difference of only 2.22 kJ/mol in reaction energy and a difference of 1.74 kJ/mol in activation energy. However, the oxygen effects on hydrogen dissociation could be diversified when DBT or 4,6DMDBT occupied the CUS site. On the S-edge of Co-Mo-S, the adsorption of DBT and 4,6-DMDBT caused the reactions to be micro-endothermic, and a higher activation energy was measured. On the S-edge of Co-Mo-O-S, the competitive adsorption of S-containing molecules made the hydrogen dissociation reaction more exothermic, and a lower activation energy (25.34–27.43 kJ/mol lower than that on the S-edge of Co-Mo-S) was measured. On the M-edge of Co-Mo-S and Co-Mo-O-S, all
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Table 4 Adsorption of C14H13S on the Co-Mo-S and Co-Mo-O-S.
The units of energy and length are kJ mol−1 and Å, respectively.
hydrogen dissociation reactions were exothermic. Meanwhile, because the DBT and 4,6-DMDBT were adsorbed obliquely on the active site, the dissociation of hydrogen was not obviously influenced by the adsorbates. Compared to the calculation results of Co-Mo-S and Co-Mo-OS, the incorporation of oxygen slightly reduced the activation energy of the reaction by approximately 11 kJ/mol. The change in activation energy was not obvious but played an important role during the process of hydrogen dissociation. The ability of hydrogen transfer at the metal sites of the M-edge also played an important role in the process of DDS. Table 7 presents the details of hydrogen transfer on the M-edge of Co-Mo-S and Co-Mo-O-S. On both Co-Mo-S and Co-Mo-O-S, when DBT was adsorbed, hydrogen was easier to transfer compared to when there was nothing adsorbed on the M-edge. Additionally, the reaction energy barrier was lower by about 2–4 kJ∙mol−1, which indicated that hydrogen atoms preferred to transfer after the reactants adsorbed. When 4,6-DMDBT was adsorbed on the M-edge of Co-Mo-S, hydrogen transfer became even easier. However, when 4,6-DMDBT was absorbed on the M-edge of Co-Mo-O-S, the activation energy of hydrogen transfer was slightly higher than when DBT was absorbed. From Table 7, the activation energy of hydrogen transfer on the M-edge of Co-Mo-O-S was slightly higher than for Co-Mo-S. Combining the data of Tables 1 and 7, it could be predicted that oxygen atoms with strong electronegativity had higher
electron density, which would form a strong interaction with adsorbed hydrogen and blocked the transfer of hydrogen. However, the difference in activation energy was only slight. In brief, the hydrogen dissociation energies on different edges of CoMo-S and Co-Mo-O-S were clearly affected by oxygen. Owing to the strong electron-negative oxygen incorporation, hydrogen dissociation was relatively easy, as a lower activation energy is needed during this reaction. Moreover, the activation barrier on the S-edge decreased significantly. There were also some negative effects: the hydrogen that adsorbed on the M-edge after dissociation required more energy to transfer to the adjacent active center before it could participate in a subsequent reaction. Ultimately, oxygen is beneficial for the dissociation of hydrogen but slightly hinders the transfer of hydrogen. 3.4. The effect of oxygen on DDS To analyze the effect of oxygen on the DDS route, we calculated the CeS bond cleavage that occurred on the S-edge and M-edge. Due to the asymmetric structure of the Co-Mo-O-S nanocluster, there are two possible routes of CeS bond cleavage, as shown in Fig. 5. Using the formation of C12H9S as an example, the CeS bond cleavage occurred on the O-Mo side (route one). Route two involved the CeS bond scission on the S-Mo side.
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Table 5 Hydrogen dissociation on S-edge of Co-Mo-S and Co-Mo-O-S.
3.4.1. Oxygen effect on the CeS bond scission of S-edge The calculation results of CeS bond cleavage on the S-edge are listed in Table 8. In particular, for all CeS bond cleavage transition states, the carbocyclic ring was saturated with a hydrogen ring to generate a benzene ring, after that final product was formed though a short reaction path. Combining the data of CeS bond scission for DBT and 4,6-DMDBT on Co-Mo-S, it could be found that the CeS bond scission of the reactants on the S-edge was an exothermic reaction. Due to the substituent effects of the two methyl groups from 4,6-DMDBT, higher activation energy was required for 4,6-DMDBT CeS bond scission than for DBT. Due to the structural asymmetry, there are two possible pathways for CeS bond cleavage on Co-Mo-O-S: CeS bond cleavage on the O-Mo side and S-Mo side. The data presented in Table 8 show that the activation energy of CeS bond scission on the O-Mo side is much lower than that on the S-Mo side, meaning the incorporation of
oxygen facilitates CeS bond cleavage on the oxygen-rich side. There may be two reasons for this: one reason is that the asymmetric bending structure of Co-Mo-O-S provides more space for the reactant to undergo CeS bond scission at the O-Mo side. In this case, the substituent effects are smaller, and the reaction is more favorable. The second reason is that during CeS bond cleavage, a hydrogen atom is transferred, and although oxygen increases the difficulty of hydrogen transfer, the incorporation of oxygen facilitates CeS bond cleavage and the rotation of the benzene ring. CeS bond cleavage plays a more important role than hydrogen transfer, after all. However, the activation energy of the CeS bond scission on the O-Mo side is still slightly higher than that on the Sedge of Co-Mo-S. In short, for CeS bond cleavage on the S-edge, the CeS bond of the reactant cleaved under the participation of active hydrogen. Subsequently, a new benzene ring was formed, and it rotated to
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Table 6 Hydrogen dissociation on M-edge of Co-Mo-S and Co-Mo-O-S.
generate a new product. Due to the structural asymmetry of Co-Mo-O-S, there were two ways to break the CeS bond, in which the activation energy of the CeS bond scission on the O-Mo side was lower. This was mainly due to changes in structure and charge properties induced by oxygen. However, the incorporation of oxygen increased the difficulty of CeS bond scission on the S-edge overall.
adsorbates. Hence, when 4,6-DMDBT was adsorbed on the M-edge, higher activation energy was required for cleavage of the CeS bond than when DBT was adsorbed. On Co-Mo-O-S, there were also two possible reaction pathways. Similarly, the oxygen-rich side was more conducive to reaction of CeS bond cleavage. However, the incorporation of oxygen on the M-edge decreased the activation energy, indicating that the increase in metallic properties caused by oxygen made the M-edge more favorable for the reaction. Oxygen has different effects on DDS at different active sites. For both the S-edge and M-edge, the incorporation of oxygen provided active hydrogen stably, because oxygen atoms are more attracted to hydrogen atoms than sulfur atoms. This also led to an increase in the difficulty of hydrogen transfer. Non-metallic elemental sulfur is terminal on the S-edge, while Co and Mo are terminal on the M-edge. Hence, oxygen has various effects on CeS bond scission of the S-side and the Mside. Relative to Co-Mo-S, CeS bond scission of the S-edge of Co-Mo-O-S is unfavorable, while M-edge exhibits good reactivity. According to the relationship between the electronegativity of basal elements, the incorporation of different electronegative non-metallic elements into the catalyst is one way to effectively regulate catalyst function.
3.4.2. The effects of oxygen on the CeS bond scission of the M-edge The reaction process on the M-edge was similar to the S-edge, which involved the transfer of hydrogen, scission of the CeS bond and the rotation of the benzene ring. Similarly, due to the asymmetric structure of Co-Mo-O-S, there are two possibilities for CeS bond cleavage. The calculation results for CeS bond scission of DBT and 4,6-DMDBT on the M-edge are listed in Table 9. The results show that bond cleavage and benzene ring rotation pushed the sulfur atom that was adsorbed on the active center during the reaction process, which resulted in the deformation of the original Co-S bond. On Co-Mo-S, the reactions were exothermic, indicating that the force between the carbon atom and sulfur atom on the M-edge was weaker than that on the S-edge. The two methyl groups of 4,6-DMDBT weakened the adsorption Co-S bond of
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Table 7 Hydrogen transfer on the M-edge of Co-Mo-S and Co-Mo-O-S.
Fig. 5. Reaction pathway route of DBT to C12H9S on the S-edge of Co-Mo-O-S.
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Table 8 CeS bond cleavage on the S-edge of Co-Mo-S and Co-Mo-O-S.
4. Conclusion
modifications.
In summary, the basic structure, electronic properties and hydrodesulfurization properties reflected by DBT and 4,6-DMDBT on Co-MoS and Co-Mo-O-S were comparatively analyzed to further determine the effects of oxygen on the Co-Mo-S catalyst during DDS. The Co-Mo-S catalyst containing oxygen has a bent structure, which is consistent with HRTEM imaging. During DDS, oxygen facilitates the dissociation of hydrogen but is not conducive to the transfer of active hydrogen. The presence of oxygen also affects the preference of catalytic reaction sites. C–S bond scission is more likely to happen on the M-edge, while the activation energy of CeS bond scission on S-edge is increased when oxygen is incorporated into the catalyst structure. From comparative analysis, it could be inferred that oxygen could enhance the performance of the Co-Mo-S catalyst. This work allows us to improve the activity of catalysts via synergistic structural and electronic
CRediT authorship contribution statement Yi Zheng: Conceptualization, Formal analysis, Writing - original draft. Weixia Zhou: Data curation. Yang Liu: Investigation. Chenyang Zhang: Methodology. Suya Chu: Project administration, Supervision. Yongjun Liu: Resources, Funding acquisition, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Table 9 CeS bond cleavage on the M-edge of Co-Mo-S and Co-Mo-O-S.
Acknowledgements
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