Theoretical Survey of the Thiophene ... - ACS Publications

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Theoretical Survey of the Thiophene Hydrodesulfurization Mechanism on Clean and Single-Sulfur-Atom-Modified MoP(001) Guixia Li,†,‡ Houyu Zhu,*,† Lianming Zhao,† Wenyue Guo,*,† Huifang Ma,† Yanchen Yu,† Xiaoqing Lu,† and Yunjie Liu† †

College of Science, China University of Petroleum, Qingdao, Shandong 266580, P. R. China College of Science and Information, Qingdao Agricultural University, Qingdao, Shandong 266109, P. R. China

‡

S Supporting Information *

ABSTRACT: Molybdenum phosphide (MoP) has been extensively experimentally shown to possess high and surprisingly increasing hydrodesulfurization (HDS) activities during the HDS process. In order to understand the HDS mechanism, we investigate the HDS of thiophene on clean and single-sulfur-atom-modified MoP(001) using self-consistent periodic density functional theory (DFT). Thiophene strongly prefers f lat adsorption, which is slightly weakened in the presence of a surface S atom. Thermodynamic and kinetic analyses of the elementary steps show that the HDS of thiophene takes place along the direct desulfurization (DDS) pathway on both clean and S-modified MoP(001), because of the very low C−S bond activation barriers as well as very high exothermicities involved. More importantly, the surface S atom does not elevate the C−S bond activation barriers but opens a new concerted pathway for the simultaneous rupture of both C−S bonds in thiophene. These results indicate that the presence of a surface S atom could be helpful for thiophene desulfurization. For comparison, we also investigate the influence of a surface S atom on the HDS of thiophene on Pt(111). The results show clearly a negative effect of the surface S atom, in accordance with the lower sulfur resistance of noble metals.

1. INTRODUCTION The desulfurization of sulfur-containing hydrocarbons has attracted extensive attention,1−5 since effectively lowering the sulfur contents in gasoline and diesel is of great significance for reducing the pollutant emissions in automobile exhausts. Hydrodesulfurization (HDS) is an important catalytic process applied to remove sulfur from hydrocarbons in petroleum industry. The widely used conventional HDS catalysts, CoMo and NiMo sulfides, have good performances for aliphatic sulfur compounds6,7 but are less effective on aromatic thiophene (T) and its derivatives, especially 4,6-dimethyldibenzothiophene (4,6-DMDBT).6 The difficulty in desulfurization of 4,6DMDBT lies in the steric hindrance of the methyl groups near the S atom, which could be alleviated by hydrogenation.8,9 Supported noble metals (Pt, Pd) exhibit a higher HDS activity toward thiophenic compounds8,10−13 and thus are much better hydrogenation catalysts than the Mo-based sulfides but are easily poisoned by deposited S. Recent experimental observations shows that molybdenum phosphide (MoP) possesses higher HDS activity than the Mo-based sulfides14−17 and has more excellent stability when compared with noble metal catalysts.14,16,18 More interestingly, the deposited surface S atoms from the desulfurization of organic sulfur compounds might play a crucial role in improving the HDS performance of MoP.15−17,19 In Bai et al.’s experiments,16 the surfaces of bulk MoP become sulfurized during HDS of dibenzothiophene (DBT) and the HDS activity increases with time on-stream, showing that the S-modified MoP surface is more active than © XXXX American Chemical Society

the fresh MoP surface. Phillips et al. measured thiophene HDS activities of a 15 wt % MoP/SiO2 catalyst and a sulfided Mo/ SiO2 catalyst with similar Mo loadings.17 They found that the MoP/SiO2 catalyst showed similar HDS activity with the sulfided Mo/SiO2 catalyst over the initial 20 h on-stream, and then displayed an unusual monotonically increasing trend of HDS activity as a function of time on-stream and became nearly 4 times more active than the sulfided Mo/SiO2 catalyst after 150 h on-stream, indicating the promoting effect of S deposition on the HDS activity of MoP.17 Wu et al. also performed the HDS activity tests of thiophene on a MoP/SiO2 catalyst, and found that, although the surface of the MoP/SiO2 catalyst becomes partially sulfided, it is fairly stable in the initial stage of the HDS reaction.14 In spite of the experimental findings, the HDS mechanism on MoP and the influence of the surface S atom still need to be explored. Generally, there are two competing desulfurization paths for thiophenic compounds, which have been discussed in a number of reviews.3,20−24 One is termed the hydrogenation (HYD) pathway, initiated by hydrogenation and followed by S removal. The other is named the direct desulfurization (DDS) pathway, in which S is removed without prior hydrogenation. Owing to its relatively simple structure, thiophene is often used as a model molecule in the theoretical studies of the desulfurization Received: July 15, 2016 Revised: September 11, 2016

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DOI: 10.1021/acs.jpcc.6b07103 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C of thiophenic compounds.2,3,20,21,24−28 Moses et al. carried out a density functional theory (DFT) investigation of thiophene on MoS2(101̅0) and MoS2(1̅010).29 They pointed out that the HYD pathway is more important than the DDS pathway, which is in accordance with the scanning tunneling microscopy (STM) observations.30 Using the DFT calculations, we studied the HDS mechanism of thiophene on Pt(111) and Pt(110),20,21 and found that the desulfurization of thiophene proceeds predominantly via the DDS pathway, which exhibits relatively high desulfurization efficiency with low hydrogen consumption compared with the HYD pathway on MoS2. Ren et al. performed a DFT study of thiophene on clean MoP(001).31 The results showed that one of the C−S bonds in thiophene spontaneously breaks in the adsorption process and the second C−S bond scission is exothermic. However, the competitive hydrogenation reactions were not considered. To our knowledge, no theoretical reports are available to illustrate the HDS mechanism of thiophene and the influence of a surface S atom on MoP catalysts. These facts motivate us to study the hydrogenation and C−S bond cleavage of thiophene on clean and single-sulfur-atom-modified MoP surfaces. In this work, we chose the MoP(001) surface for our study, because it is one of the thermodynamically most stable surfaces according to both the X-ray diffraction (XRD) observations32 and recent DFT calculations of the surface energy of MoP surfaces.33 In addition, the Mo-terminated MoP(001) surface is widely used in theoretical studies,31,34−36 and the exposed Mo site has been characterized as the surface active site by CO adsorption with in situ Fourier transform infrared (FT-IR) spectroscopy.34 We address the adsorption geometries and energies of the involved intermediates, and present the elementary reaction steps and potential energy surfaces (PES) on both clean and S-modified MoP(001) surfaces. Compared with the situation of the Pt(111) surface, the adsorption and HDS mechanism of thiophene along with the electronic structures are analyzed and discussed to elucidate the positive effect of the surface S atom on the HDS reactions.

sulfur-atom-modified MoP(001) was simulated by placing one S atom at a hollow site on the clean (3 × 3) unit cell. A 13 Å vacuum region between the slabs was used to separate the surface from its periodic image in the direction along the surface normal. We consider a p(3 × 3) unit cell for the MoP(001) surface, and the reciprocal space was sampled by a grid of (2 × 2 × 1) k-points using the Monkhorst−Pack method.42 A single adsorbate was allowed to adsorb on one side of the slab with a surface coverage of 1/9 ML. In this coverage, the van der Waals interaction between adjacent adsorbates can be ignored because the shortest distance of adsorbed species is longer than 5.5 Å. Full-geometry optimization was performed for all relevant adsorbates and the uppermost three layers (2Mo + 1P) without symmetry restriction, and the bottom two layer atoms (1Mo + 1P) were fixed at the positions at the calculated lattice constants. The tolerances of energy, gradient, and displacement convergence were 5.442 × 10−4 eV, 1.088 × 10−1 eV/Å, and 5 × 10−3 Å, respectively. Transition state (TS) searches were performed at the same theoretical level with the complete LST/QST method.43 In this method, the linear synchronous transit (LST) maximization was performed, followed by an energy minimization in directions conjugating to the reaction pathway to obtain an approximated TS. The approximated TS was used to perform quadratic synchronous transit (QST) maximization, and then, another conjugated gradient minimization was performed. The cycle was repeated until a stationary point was located. The convergence criterion for the TS searches was set to 0.272 eV/ Å for the root-mean-square of atomic forces. Finally, the located TS was confirmed by vibrational frequency calculations. The accuracy of calculations used to produce structures and energies depends highly on the model and theoretical level. It is therefore essential to calibrate the quality of the computed parameters. The lattice parameters of bulk MoP were calculated to be a = b = 3.266 Å and c = 3.198 Å, in good agreement with the experimental results (a = b = 3.223 Å and c = 3.191 Å).44 This calculated lattice constant was subsequently used in all calculations to maintain a true energy minimum with respect to the bulk MoP reference state. Under the current computational conditions, the calculated geometrical parameters of the gasphase thiophene are almost the same as the experimental values (the largest difference is less than 0.02 Å).45 The adsorption energy for S at the most stable hcp site on clean MoP(001) was calculated to be 6.47 eV, compared with the value of 6.71 eV calculated using the GGA-RPBE functional with a basis set of double numerical plus d functions in the DMol3 code.36 Other test calculations (see section S1) further confirmed that the model, convergence criteria, and methodology we employed are appropriate and the ZPE corrections are not necessary in the present study. Note that there is a relatively large discrepancy in the adsorption energy of thiophene, which is 2.60 eV for Tbridge-fcc in our work (see Figure 1) and 1.59 eV for the same adsorption mode in the previous DFT study by Ren et al.31 In that study, the authors employed further single-point energy calculations with an all-electron basis set on the DSPP optimized structures, in which a different exchange-correlation functional (revised Perdew−Burke−Ernzerh, RPBE) and different basis sets (plane-wave basis set) and pseudopotentials (ultrasoft) were used for the unit cell optimizations. These would be responsible for the differences in the structures and energies.

2. COMPUTATIONAL DETAILS The DFT calculations were performed with the program package of DMol3 in Materials Studio of Accelrys, Inc.37−39 The exchange−correlation energy was calculated within the generalized gradient approximation (GGA) using the form of functional proposed by Perdew−Wang (PW91).40,41 To take the relativity effect into account, the density functional semicore pseudopotential (DSPP) method was employed for the Mo atoms, and the sulfur, phosphorus, carbon, and hydrogen atoms were treated with all-electron basis set. The valence electron functions were expanded into a set of numerical atomic orbitals by a double-numerical basis with polarization functions (DNP). A Fermi smearing of 0.136 eV and a real-space cutoff of 5.0 Å were used to improve the computational performance. All of the calculations were performed with spin-polarization. Further single-point energy calculations with an all-electron basis set on the structures optimized with the DSPP were carried out to calculate the electronic properties of the MoP(001) surface and adsorbates.31 The clean Mo-terminated MoP(001) surface was modeled by a five-layer slab with alternating Mo and P layers. In general, there are four high-symmetry adsorption sites, which have been shown in the previous work,35 on the clean MoP(001) surface, that is, hollow (fcc, hcp), bridge, and top sites. The singleB


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both of the MoP surfaces to gather a general view of the HDS reaction network. 3.1. Adsorption of Thiophene on Clean and SModified MoP(001). Two types of molecular adsorption of thiophene, f lat and upright, are found on clean and S-modified MoP(001), similar to the adsorption of thiophene on MoS 2, 46−50 Ni(110),27 γ-Mo 2 N(001), 31 Pt(110),20 and Pd(100).51 Flat adsorption gives the T-bridge-hollow (Tbridge-fcc and T-bridge-hcp), T-hollow (T-fcc and T-hcp), and T-cross-bridge configurations. Figure 1 shows the representative f lat adsorption configurations of thiophene on clean and Smodified MoP(001), and the adsorption energies and geometric parameters are given in Table 1. For simplification, only the results of “fcc” are shown in Figure 1 and Table 1 to represent the corresponding “hollow” for simplification, since the hcp adsorptions give similar structures and energies, as provided in Figure S1 and Table S1. We also note that spontaneous rupture of one C−S bond can occur when thiophene adsorbs, and the dissociative adsorption geometries marked with the “*” symbol are shown in Figure 2. The flat

Figure 1. Representative adsorption structures of thiophene (T) on clean and S-modified MoP(001). SS and SC represent the surface S atom sharing the surface Mo with the thiophenic S and C atoms, respectively. The C, S, H, Mo, and P atoms are shown in black, yellow, white, light-gray, and purple, respectively.

Figure 2. Adsorption structures of thiolate-fcc* and thiolate-fcc*_SC on clean and S-modified MoP(001), respectively. The notations are the same as those in Figure 1.

3. RESULTS This part is divided into two sections. In section 3.1, we characterize the adsorption of thiophene on clean and Smodified MoP(001) surfaces. In section 3.2, we investigate the desulfurization, hydrogenation, and conversion of thiophene on

adsorption behavior of thiophene on MoP(001) has a lot in common with that found on α-Mo2C(001).52 The upright adsorption of thiophene (see Figure S1 and Table S1) is less important and thus is not considered because of the much

Table 1. Adsorption Configurations, Adsorption Energies (ΔEads, eV), Structural Parameters (Å), and Mulliken Populations (e) for Thiophene on Clean and S-Modified MoP(001) sites gas bridge-fcc bridge-fcc_SS bridge-fcc_SC fcc fcc_SS fcc_SC cross-bridge cross-bridge_SS cross-bridge_SC fcc* fcc*_SC

ΔEadsa 2.60 2.38 2.05 2.57 2.42 2.33 2.54 2.32 2.28 3.92 3.31

d(S−Mo) 2.609, 2.780, 2.691, 2.515 2.570 2.488 2.478 2.497 2.500 2.437, 2.421,

2.665, 2.666 2.644, 2.734 2.481, 3.047

2.450 2.465

d(C−Mo) 2.192, 2.208, 2.207, 2.255, 2.218, 2.323, 2.390, 2.290, 2.975, 2.208, 2.212,

2.602, 2.568, 2.752, 2.365, 2.382, 2.285, 2.288, 2.296, 2.299, 2.277, 2.436,

2.598, 2.489, 2.446, 2.361, 2.397, 2.462, 2.273, 2.276, 2.299, 2.208, 2.341,

2.192 2.186 2.259 2.257 2.209 2.320 2.358 2.344 2.500 2.207 2.285

d(C2−S)

d(C5−S)

d(C2−C3)

d(C3−C4)

d(C4−C5)

charges

1.728 1.848 1.832 1.845 1.828 1.808 1.822 1.856 1.854 1.856 3.176 3.022

1.728 1.848 1.845 1.810 1.828 1.832 1.818 1.876 1.851 1.845 1.806 1.825

1.374 1.465 1.469 1.448 1.439 1.445 1.432 1.461 1.459 1.460 1.439 1.474

1.422 1.407 1.406 1.418 1.478 1.482 1.461 1.422 1.424 1.419 1.441 1.484

1.374 1.464 1.467 1.480 1.437 1.447 1.432 1.462 1.457 1.473 1.449 1.474

0 −0.504 −0.370 −0.334 −0.450 −0.363 −0.271 −0.472 −0.336 −0.295 −0.809 −0.667

ΔEads = Egas + EMoP − Egas/MoP, where Egas, EMoP, and Egas/MoP are the total energies of the gas-phase species, free (clean/S-modified) slab, and adsorption system, respectively. In this definition, a positive value of ΔEads means a stable adsorption. a

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The Journal of Physical Chemistry C lower adsorption energy compared with the flat states (0.74 vs 2.39 eV). As shown in Figure 1a, the S atom of thiophene is taken as position 1 and the rest of the carbon atoms (C2−C5) are labeled anticlockwise accordingly; the four Mo atoms beneath the adsorbed thiophene are marked as Mo1, Mo2, Mo3, and Mo4, in which Mo1, Mo2, and Mo4 form a fcc site and Mo2, Mo3, and Mo4 give a neighboring hcp site. These four metal atoms form a rhombus, with the long Mo1−Mo3 and short Mo2−Mo4 diagonals. In order to investigate the effect of a surface S atom, we consider two types of surface S atoms, i.e., sharing surface Mo atom with thiophenic S and C atoms termed SS and SC, respectively. 3.1.1. T-Bridge-fcc Configuration. In this configuration (see Figure 1a−c), the thiophene ring caps exactly the surface Mo rhombus with the C2 axis above the long diagonal; the molecular center is above the Mo2−Mo4 bridge site, the thiophenic S atom stays at the fcc site sharing the Mo2 and Mo4 atoms with adjacent C atoms, and the other two C atoms share Mo3. On the clean surface, the molecular plane tilts from the metal surface at an angle of ∼11° (see Figure 1a), facilitating the interaction via the thiophenic S lone pair orbital. In addition, two new bonds (C2−Mo2 and C5−Mo4) are formed, leaving more or less single bond character for C2−C3 and C4−C5 and double bond character between C3 and C4, quite similar to the tilted adsorption of thiophene on Pt(111).21 Correspondingly, the C−S and C2−C3/C4−C5 bonds are largely stretched (by 6.9 and 6.6%) and the C3−C4 bond is slightly shrunk (by 1.1%). This adsorption predicts an adsorption energy of 2.60 eV. On the S-modified surface, the coadsorption configurations of atomic S and thiophene are shown in Figure 1b (T-bridgefcc_SS) and Figure 1c (T-bridge-fcc_SC). Compared with the situation of the clean surface, the largest variety for the Smodified surface is the stretching of the bond of thiophenic S/ C to the Mo atom near the surface S atom (by 6.6%/3.1%), because of the formation of an extra S−Mo bond. Correspondingly, the adsorbate in T-bridge-fcc_SS is slightly pushed away from Mo1 and the C2−S bond is shortened by ∼0.9%; by contrast, the other molecular skeleton bond lengths are almost unchanged. In T-bridge-fcc_SC, the shift of the adsorbate induced by a surface S atom is also observed and also the C−C and C−S bonds change slightly (by 0.2−2.1%). In both cases, the surface S atom decreases the adsorption energy of thiophene on MoP(001), by 8.5% (SS) and 21.2% (SC). 3.1.2. T-fcc Configuration. This adsorption (see Figure 1d− f) corresponds to the slight movement of the adsorbate in the T-bridge-fcc configuration along the long diagonal of the rhombus so that the thiophene ring caps exactly the three Mo atoms forming the fcc site, in which the thiophenic S atom lies atop the Mo1 atom and each of the other two adjacent C atoms shares the neighboring Mo atom. Similar adsorption configurations have also been observed on Ni3P2-terminated Ni2P(001),31 γ-Mo2N(100),31 and α-Mo2C(001),52 in which the S atom coordinates atop one Ni/Mo atom and the thiophene ring parallels the surface. On clean MoP(001), all the skeleton bonds in thiophene are stretched (see Table 1), ∼5.7% for the C−S bonds, ∼4.7% for the C2−C3 and C4−C5 bonds ,and ∼3.9% for the C3−C4 bond. The calculated adsorption energy is 2.57 eV. On the S-modified surface, similar to the situation of Tbridge-fcc_SS, the surface S atom in T-fcc_SS (see Figure 1e) also stretches the bond of thiophenic S to the Mo atom

adjacent to SS (by 2.2%), causing the slight movement of the adsorbate toward the Mo3 side, and the molecular skeleton bonds undergo relatively small changes (by 0.2−1.1%). For Tfcc_SC (see Figure 1f), the C−Mo distance adjacent to SC stretches largely (by 3.6%), and all the skeleton bonds in thiophene (by ∼0.6%) and the S−Mo distance (by 1.1%) change slightly. SS and SC lower the thiophene adsorption energies by 5.8 and 9.3%, respectively. 3.1.3. T-Cross-Bridge Configuration. In this situation (see Figure 1g−i), the thiophene ring experiences an about π/2 rotation with respect to the T-bridge-fcc or T-fcc configuration so that the C2 axis is above the rhombus’s short diagonal (Mo2−Mo4). The molecular center is above the Mo2−Mo4 bridge site, the thiophenic S atom locates atop Mo4 (tilted to bridge site), the C3 and C4 atoms share the Mo2 atom, and the other two C atoms are over the neighboring bridge sites. This adsorption is similar to the configuration of thiophene on MoS2,46,47 in which the f lat adsorption is featured by the molecular ring centered above a coordinatively unsaturated Mo atom and the sulfur atom atop an adjacent Mo atom. As shown in Figure 1g, the adsorption on the clean surface leads to substantial elongation of some skeleton bonds of thiophene, ∼8.0% for the C−S bonds and ∼6.4% for the C2−C3 and C4− C5 bonds. The C3−C4 bond is unchanged. This adsorption accounts for a lower adsorption energy (2.54 eV) than both the T-bridge-fcc and T-fcc configurations. On the S-modified surface (see Figure 1h, i), different from the cases of T-bridge-fcc_SS and T-fcc_SS, a relatively small increase (by 0.8%) is observed for the S−Mo distance near the SS atom (see Figure 1h), because in the latter case the bond between the thiophenic S and the surface Mo is stronger, mirrored by the shorter S−Mo distance (2.478 vs ca. 2.614 Å) (see Table 1). Correspondingly, the molecular skeleton bonds also experience small varieties (by 0.1−1.3%). In the T-crossbridge_SC configuration (see Figure 1i), however, a stretching as large as 24.5% induced by SC is found for the C−Mo bond adjacent to the surface S atom, but other bonds show slight varieties, i.e., 0.9% increase for the S−Mo4 distance and less than 1.1% changes for the C−C and C−S bond lengths. The surface SS and SC atoms lower the adsorption energy of thiophene by 8.7 and 10.2%, respectively (see Table 1). 3.1.4. Thiolate-fcc* Configuration. In addition to the molecular adsorptions, we also find a dissociative adsorption configuration for thiophene on the clean and S-modified MoP(001) surfaces (see Figure 2). This adsorption corresponds to the simultaneous breaking of one C−S bond of thiophene, giving thiolate as the adsorbate with relatively high adsorption energies, i.e., 3.92 eV on a clean surface and 3.31 eV on a S-modified surface. The molecule skeleton arc surrounds a fcc site with the thiophenic S and end C atoms nearly at adjacent bridge sites. This kind of adsorption is also found for the adsorption of thiophene on the α-Mo2C(001)52 and Ni(100)26,51 surfaces as well as on the MoP(001) surface in previous DFT calculations.31 On the clean surface (see Figure 2a), the ruptured C2−S bond is stretched to 3.176 Å from the value of 1.728 Å in the gas-phase thiophene and the C5−S bond length increases by 4.5%. The other C−C bonds are elongated in the range 1.3−5.5%. On the S-modified surface (see Figure 2b), the surface SC atom results in the increase of the C−Mo distance (by 3.5%) near the surface S atom and the movement of the adsorbate toward the Mo1 atom. The C5−S bond is elongated by 1.1%, the C2−S distance is shortened by 4.8%, but the mean S−Mo D


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Figure 3. Direct desulfurization pathway of thiophene from T-bridge-fcc on clean MoP(001). The energy reference (in eV) corresponds to the total energy of T-bridge-fcc plus two atomic H adsorbed at infinitely separated sites on the slab. The atomic H’s adsorbed at the infinitely separated sites are omitted for simplicity.

stable final state (FS), thiolate-A, with an exothermicity of 2.05 eV. The rupture of the C−S bond leads to the formation of a six-member ring analogue including a Mo atom in the FS, similar to the situation of the organometallic reactions, in which one metal atom inserts into a C−S bond.53 The second step of the DDS of T-bridge-fcc is activated with the help of the stretching vibration of the C5−S bond in thiolate-A, via TS2-A, stabilizing the system by 1.31 eV. In the FS ((C4H4 + S)-A), the C5 atom occupies the fcc site adjacent to the thiophenic S in the IS and the thiophenic S moves to the adjacent hcp site in the opposite direction, resulting in a much larger C5−S separation (4.826 Å). The thiophenic S atom and C4−C5 group in the TS are located at intermediate positions between the IS and FS, that is, bridge site for thiophenic S and nearly top site for C5 at a distance of 2.620 Å. This process accounts for an activation energy of 0.44 eV. On S-Modified Surface. Figure 4a depicts the PES together with schematic structures involved in the activation of the C−S bonds in the T-bridge-fcc_SS configuration. When the rupture of the C−S bond far away from the surface S atom (C2−S bond) is regarded as the first step, quite analogous to the thiophene desulfurization on the clean surface as mentioned above, the DDS pathway involves activation energies of 0.16 and 0.61 eV and exothermicities of 1.76 and 1.62 eV for the first and second steps, respectively. In this case, the surface SS atom decreases the activation energy and destabilizes the product (thiolate-B) for the first C−S bond scission, but the overall exothermicity is almost unchanged, as compared with the situation of the clean surface. Interestingly, if the C−S bond adjacent to the surface SS atom (C5−S) is activated first, we find a simultaneous pathway of the rupture of both the C−S bonds of thiophene, giving the same FS as the two-step desulfurization (see Figure 4a). The relevant activation barrier is also very low (0.17 eV). This process involves pushing off the C4H4 group so that in TS-C the C3−C4 bond is just above the Mo atom shared by it originally and the thiophenic S atom is at the original fcc site, with C2−S and C5−S distances of 2.120 and 2.407 Å,

bond is almost unchanged. The other molecular skeleton bond lengths change in the range 1.7−3.0%. We also searched the spontaneous C−S bond ruptured configuration in which SS is involved, but the efforts failed. 3.2. Surface Reactions of Thiophene on Clean and SModified MoP(001). It is expected that the DDS and HYD pathways may compete with each other in the HDS of thiophene. In order to explore the influence of a surface S atom on HDS, we first investigate the desulfurization pathways starting from the f lat adsorption configurations of thiophene on clean and S-modified MoP(001), followed by the hydrogenation of thiophene into 2- and 3-monohydrothiophenes (MHTs) and the formation of butadiene, and finally the conversions among the representative flat adsorption configurations. The details regarding the change in the C−S distances during the desulfurization process of adsorbed thiophene are given in Table S2. The adsorption of MHTs and butadienethiolates (BDTs) on clean and S-modified MoP(001) are given in section S2; the corresponding adsorption configurations and the adsorption energies as well as geometric parameters are given in Figure S2 and Table S1, respectively. The adsorption energies of hydrogen and the hydrogenation products on clean and atomic S-modified MoP(001) and Pt(111) are given in Table S3. 3.2.1. Desulfurization from the T-Bridge-fcc Configuration. On Clean MoP(001). The relevant PES as well as the structures of the involved intermediates are shown in Figure 3. By virtue of the equivalent status of C2 and C5, here, the C2−S bond scission of thiophene is regarded as the first step of the DDS. This step experiences an “early” TS with an activation energy of only 0.18 eV, mirrored also by the fact that the structure of the TS is similar to that of the initial state (IS). In TS1-A, the C2−S bond is ruptured at a distance of 2.167 Å and correspondingly the C2−C3 bond biases slightly to the bridge site. Following the TS, the C2−C3 end experiences a large shift so that the C2 atom moves to the adjacent fcc site and C3 shifts to the top of the surface Mo atom shared by C3 and C4 in the IS, giving a quite E


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Figure 4. Direct desulfurization pathway of thiophene from (a) T-bridge-fcc_SS and (b) T-bridge-fcc_SC configurations on S-modified MoP(001). The energy reference (in eV) corresponds to the total energy of T-bridge-fcc_SS plus two atomic H adsorbed at infinitely separated sites on the slab. The atomic H’s adsorbed at infinitely separated sites are omitted for simplicity.

thiophenic S atom preserves the fcc site as in the IS and the C2 atom has slightly moved away from the thiophenic S, resulting in the C2−S bond scission at a distance of 2.126 Å. Then, the shift of the C2 atom toward the adjacent bridge site goes on producing thiolate-D with a larger C2−S distance of 3.096 Å and stabilizing the system by 1.32 eV. In TS2-D, we can observe that the C5−S bond at a distance of 2.239 Å has been fractured with the help of the movement of the thiophenic S to the adjacent bridge site from the fcc site in thiolate-D. When both of the end C atoms of C4H4 are located over adjacent bridge sites surrounding the occupied fcc site and the

respectively. Following the TS, the main change is the shift of the thiophenic S atom to the adjacent hcp site. The preferred desulfurization of T-bridge-fcc_SC is also investigated, which involves only the two-step C−S bond scissions, different from the situation of T-bridge-fcc_SS, where simultaneous rupture of both of the C−S bonds of thiophene is favored. This is because the surface SC atom in the adsorption configuration cannot activate the C5−S bond as efficiently as SC (1.810 vs 1.845 Å). The relevant activation energies are 0.10 and 0.60 eV, and the overall reaction energy is −3.64 eV; the corresponding PES is described in Figure 4b. In TS1-D, the F


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Figure 5. Direct desulfurization pathway of thiolate from (a) thiolate-fcc* (notations are the same as in Figure 3) and (b) thiolate-fcc*_SC (notations are the same as in Figure 4).

bridge_SS, the two-step desulfurization activation energies are 0.51 and 0.32 eV and the reaction energies are −2.08 and −1.13 eV, respectively. For T-cross-bridge_SC, the energy barriers are 0.42 and 0.35 eV, and the overall exothermicity is 3.41 eV. These results indicate a surface S atom can also facilitate the C−S bond scissions from both T-fcc and T-cross-bridge, but the C−S bond scissions from T-bridge-fcc are still more favorable. 3.2.3. Desulfurization from the Thiolate-fcc* Configuration. On Clean Surface. As shown in Figure 5a, the desulfurization of thiolate-fcc* gives the product of (C4H4 + S)-E. Although the IS has a configuration analogous to thiolateN (see Figure S3), the adsorbate in this case is more curving so that the end C atom is at a bridge site, and thus, thiolate-fcc* is 0.68 eV more stable than thiolate-N. The TS is also featured by the change of the thiophenic S’s position (at bridge site) compared with that in the IS (nearly at the hcp site); the C5−S distance is 2.334 Å. The movement of the thiophenic S atom predicts a relatively high energy barrier (0.77 eV). Following the TS, the movement of both the thiophenic S and C5 atoms yields the same desulfurized product (C4H4 + S)-E as the

thiophenic S atom arrives at the stable hcp site, the FS ((C4H4 + S)-D) is formed; the whole reaction is exothermic by 3.64 eV. 3.2.2. Desulfurization from the T-fcc and T-Cross-Bridge Configurations. On Clean Surface. For T-fcc, the desulfurization PES is given in Figure S3, the two-step desulfurization energy barriers are 0.75 and 0.92 eV, and the reaction energies are −0.67 and −2.85 eV for the first and second C−S bond scissions, respectively. For T-cross-bridge (see Figure S4), the two-step C−S bond activation energies are 0.38 and 0.91 eV and the reaction energies are −2.14 and −0.70 eV, respectively. On S-Modified Surface. Figure S5 shows the preferred desulfurization pathways for the T-fcc _S S and T-fcc_SC configurations. For T-fcc_SS, the desulfurization involves two sequential C−S bond scissions with activation energies of 0.35 and 0.58 eV, or 53 and 37% lower than those on the clean surface, respectively. The whole reaction is exothermic by 3.30 eV. For T-fcc_SC, the first and second C−S bond activation energies are 0.54 and 0.66 eV, respectively, lower also than the analogous processes on the clean surface. The favorable desulfurization pathways of T-cross-bridge_SS and T-crossbridge_SC are shown in Figure S6. In the case of T-crossG


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Figure 6. Hydrogenation pathways of thiophene from T-bridge-fcc. Notations are the same as in Figure 3.

Figure 7. Hydrogenation pathways of thiophene from T-bridge-fcc_SS. Notations are the same as in Figure 4.

desulfurization from T-fcc; the reaction energy is −2.17 eV. For comparison, a previous DFT study gave an energy barrier of 1.02 eV and a reaction energy of −1.28 eV for the same process31 due to the differences as mentioned above. On S-Modified Surface. The desulfurization PES of thiolatefcc*_SC is shown in Figure 5b. The process involves movements of thiophenic S to the adjacent fcc site and C5 to another fcc site. In TS-F, the C5−S bond has been ruptured with a distance of 2.266 Å, and the C2−S distance is elongated to 3.092 Å from the initial 3.022 Å. The relevant energy barrier is 0.59 eV, and the reaction energy is −2.06 eV. Compared with the desulfurization on the clean surface, the lower energy barrier for thiolate-fcc*_SC indicates that a surface S atom still facilitates desulfurization of thiolate. 3.2.4. Hydrogenation from the T-Bridge-fcc Configuration. On Clean Surface. Figure 6 shows the PES and relevant structures involved in initial hydrogenation of T-bridge-fcc.

Here, we define the paths starting with the H addition to positions 2 and 3 of thiophene as pathways G and H. In path G, the IS is the coadsorption of thiophene at the bridge-fcc site and atomic H at the hcp site near the C2 and C3 atoms. In TSG, the attacking H atom moves toward the top site adjacent to C2 with a H−Mo distance of 1.755 Å, the C2−C3 bond has biased toward the H atom from the C2 end leading to the rupture of the C2−S bond at a distance of 2.221 Å, the C2−Mo bond is slightly elongated (2.109 vs 2.099 Å), and the distance between C2 and the attacking H is 2.261 Å. Following TS-G, the C2−C3−C4 angle expands largely so that the C2−C3 bond locates above the adjacent fcc site and the attacking H atom is attached to C2, arriving at the stable FS with a long C2−S separation (3.128 Å). This process experiences an activation energy of 0.63 eV and an exothermicity of 1.35 eV. For the hydrogenation at position 3, the attacking H atom in IS-H is at the fcc site close to C3 of thiophene, unstable by 0.16 eV H


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Figure 8. Hydrogenation pathways of the C−S bond scission product of T-bridge-fcc ((C4H4 + S)-A). Notations are the same as in Figure 3.

MHT adsorption is unstable because of the dehydrogenation in the adsorption process. For T-cross-bridge on the S-modified surface (see Figure S11), the hydrogenation at position 2 is found with an energy barrier of 1.68 eV and the endothermicity of 0.40 eV; the adsorption of 3-MHT is unstable because of the dehydrogenation. To sum up, on clean and S-modified MoP(001), energy barriers for thiophene hydrogenation are higher than those for C−S bond scissions by at least 0.78 eV. Thus, DDS forming C4H4 and S is more favorable for thiophene on MoP(001) and further hydrogenation of MHT is not considered. 3.2.6. Formation of Butadiene. The final hydrogenations of the DDS products (C4H4 and S) are considered in this part, involving two-step hydrogenations producing C4H6 and onestep hydrogenation yielding SH. We take (C4H4 + S)-A produced from T-bridge-fcc as the IS, because it is more facile than those from T-fcc and T-cross-bridge on clean MoP(001). In the first step, two possible positions of H atom are considered. One is at the bridge site between C4H4 and S in (C4H4 + S)-A, leading to hydrogenation of C4H4 at C2 (path K) as well as hydrogenation of the S atom (path M); the other is at the hcp site adjacent to S and C5, resulting in only the C4H4 hydrogenation at C5 (path L). Both paths K and L end with the structurally similar FS ((C4H6 + S)-K/L), as shown in Figure 8. The two-step hydrogenation energy barriers are 0.88 and 1.01 eV in path K and 0.71 and 1.00 eV in path L. The S hydrogenation (path M), S + H → SH, has a much higher energy barrier (1.79 eV); thus, further hydrogenation of SH (SH + H → H2S) is not considered. The results indicate that path L for C4H4 is preferred, forming the coadsorbed butadiene and surface S atom. 3.2.7. Conversions among the Flat Molecular Adsorptions of Thiophene. The conversion pathways on clean and Smodified surfaces together with schematic structures of the involved species are shown in Figure S12. On a clean surface, the relative energies of T-fcc and T-cross-bridge are 0.03 and 0.06 eV, respectively, if T-bridge-fcc is set as the energy reference. It can be envisioned that these three configurations could convert into each other, in which T-bridge-fcc is a necessary intermediate. TS-B1 is the TS for the conversion of T-fcc to T-bridge-fcc, and the corresponding energy barrier is as low as 0.25 eV. However, TS-C1 corresponding to the Tcross-bridge to T-bridge-fcc conversion has an energy barrier of

compared with IS-G. In TS-H, the atomic H arrives at the Mo top site adjacent to C3 with a H−Mo distance of 1.821 Å; the C3 atom has been adjusted to the position facilitating bonding with the attacking H. This configuration predicts a high hydrogenation energy barrier of 1.48 eV. Following TS-H, 3MHT-bridge-fcc is formed, endothermic by 0.82 eV. Clearly, hydrogenation at position 2 of thiophene is preferred on clean MoP(001). On S-Modified Surface. Figure 7 shows the PES as well as the involved structures for the hydrogenation of T-bridgefcc_SS. In IS-I, the adsorbed H is at the fcc site near C2 and C3, rather than the hcp site on the clean surface; this coadsorption is 0.07 eV higher in energy than the total energy of T-bridgefcc_SS and atomic H adsorbed at infinitely separated sites on the slab. Although the IS is different, the hydrogenation at position 2 produces a similar FS (BDT) as on the clean surface but is accompanied by a higher energy barrier (0.83 eV) and a lower exothermicity (0.69 eV), indicating the surface S atom has a negative effect (0.63 vs 0.83 eV) on the hydrogenation of thiophene from T-bridge-fcc. For the hydrogenation at position 3, in IS-J, the adsorbed H is located at a fcc site, and the coadsorption is 0.27 eV lower in energy than IS-I; this hydrogenation process accounts for an energy barrier of 1.39 eV and is endothermic by 0.95 eV. For the hydrogenations at positions 2 and 3 in T-bridge-fcc_SC, the energy barriers are 0.64 and 1.48 eV and the reaction energies are −1.59 and 0.53 eV, respectively; the corresponding PES along with the involved structures is shown in Figure S7. 3.2.5. Hydrogenation from the T-fcc and T-Cross-Bridge Configurations. On Clean Surface. The initial hydrogenation PES of thiophene from the T-fcc configuration is shown in Figure S8. We can find that the initial hydrogenation producing 3-MHT-fcc is more favorable than hydrogenation at position 2 mirrored by the relevant energy barriers of 0.84 and 1.19 eV. For T-cross-bridge (see Figure S9), the hydrogenation at position 2 of thiophene is located with an activation energy of 1.70 eV and an endothermicity of 0.58 eV, and the corresponding values for the hydrogenation at position 3 are 1.33 and 1.08 eV. On S-Modified Surface. For T-fcc_SS (see Figure S10), the position 2 hydrogenation activation barrier (1.07 eV) is 0.12 eV lower than that on a clean surface, indicating the surface S atom facilitates the hydrogenation of thiophene from T-fcc. The 3I


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Figure 9. Hydrodesulfurization network of thiophene on clean and S-modified MoP(001). The data are the activation energies of the relevant steps (in eV), and the red thick arrows show the most favorable desulfurization pathways.

surface S atom decreases slightly the adsorption energies. For comparison, thiophene adsorption on clean and S-modified Pt(111) calculated by us shows that a surface S atom also reduces the adsorption energies but to a larger extent (see Figure S13). The adsorption energy of thiophene follows the order of 0.86 eV (S-modified Pt) < 1.31 eV (clean Pt) < 2.30 eV (S-modified MoP) < 2.57 eV (clean MoP), in which the average adsorption energy of the preferred configurations is used. This is in accordance with the location of the metal dband center of the relevant surfaces, that is, the closer the dband center (εd) is to the Fermi level (EF), the stronger the bond between the surface and the adsorbate would be, and vice versa.58 In this work, the d-band centers are calculated to be −2.84 eV for S-modified Pt(111), −2.55 eV for clean Pt(111), −2.30 eV for S-modified MoP(001), and −2.20 eV for clean MoP(001) (see Figure S14). Thus, the metal d-band distribution is the main factor determining the thiophene adsorption strength on either the Pt(111) and MoP(001) surfaces or the clean and S-modified surfaces. Note that the phosphorus-rich transition metal phosphides have been pointed out to be semiconductors, while the metal-rich phosphides, MP, have metallic property and are considerably more stable than the phosphorus-rich compounds.59 The metallic character of MoP has been reported by Yue et al.,33 confirmed further by our calculations (see Figure S14). Lowering the adsorption energy of thiophene on MoP(001), caused by a surface S atom, can also be mirrored by the Mulliken population analysis. The calculated charge on the Mo atom in the clean MoP(001) surface is 0.059 e, and the charge on the subsurface P atom is −0.045 e; both of them are in line with the previous DFT results (0.050 and −0.085 e31 and 0.045 and −0.077 e36). As shown in Figure S15, the surface S atom leads to significant charge redistribution on MoP(001) because the relatively high electronegativity makes S a stronger electron acceptor when compared with the situation of P.36 Remarkable electrons are transferred to the deposited S atom (ca. −0.520 e), and thus, the Mo atoms near the deposited S become more positively charged than those on a clean surface (ca. 0.132 vs 0.059 e). When thiophene f lat adsorbs on MoP(001), backdonation of electrons from the Mo 3d orbitals into the antibonding C−S states is important, resulting in net negative charges distributed on the adsorbate. Because of the depletion of electrons on the surface Mo atoms, the presence of a surface S atom reduces the absolute value of the net negative charges on thiophene (see Table 1), indicating the back-donation is

0.67 eV. On a S-modified surface, the energy barrier is as low as ∼0.35 eV for conversion from T-fcc to T-bridge-fcc and 0.57 eV for the T-cross-bridge to T-bridge-fcc conversion.

4. DISCUSSION In this section, we discuss some important points related to the influences of a surface S atom on the adsorption and HDS mechanism of thiophene on MoP(001). 4.1. Adsorption Configurations and Energies of Thiophene. Thiophene adsorption on Ni(100),2 Pt(110),20 Pt(111),21 Ni(110),27 Pd(100),51 Cu(100),51 Al(111),54 and MoS227,46−50 has been extensively investigated using DFT. On Ni(110), Pd(100), Al(111), Pt(111), and Cu(100), thiophene adsorbs molecularly in a f lat manner, with adsorption energies of 2.42, 2.20, 0.54, 1.51−1.55, and 0.47 eV, respectively; the calculated adsorption structures on these surfaces are in reasonable accordance with the results obtained in the angleresolved UV photoemission spectroscopy and X-ray absorption fine structure experiments.54−57 It is worth mentioning that thiophene could easily desorb from Cu(100) because of the weak adsorption. On Ni(100), a strong chemisorption of thiophene was observed (ΔEads = 2.57 eV), in which direct disruption of the aromatic ring is caused by the breaking of one of the C−S bonds. On Pt(110), two upright (ΔEads = 0.97−0.99 eV) and four f lat (ΔEads = 1.68−2.87 eV) adsorption configurations were found. On the clean Mo-edge of MoS2,46−50 the f lat (ΔEads = 0.79−2.84 eV) and upright (ΔEads = 0.94−1.32 eV) adsorption configurations were reported. On MoP(001), the upright and f lat configurations are also possible for thiophene adsorption with the f lat mode being preferred, similar to the situation of metals, e.g., Ni(100),2 Ni(110),27 Pd(100),51 Cu(100),51 Pt(110),20 Pt(111),21 etc. The adsorption energies of the f lat adsorbed thiophene on clean MoP(001) are ∼2.57 eV, similar to the strong adsorptions of thiophene on Pt(110)20 and the clean Mo-edge of MoS2.46−50 On the S-modified MoP(001), the f lat adsorption energies are ∼2.30 eV. Moreover, thiophene can also chemisorb on MoP(001) by breaking one of the C−S bonds with a high exothermicity (3.92 eV). The adsorption configurations of thiophene on clean MoP(001) (T-bridgehollow, T-hollow, T-cross-bridge, and thiolate-fcc*) are in good agreement with Ren’s results,31 but the adsorption energies calculated by Ren et al. are generally lower than ours. Although the adsorption configurations of thiophene on clean and S-modified MoP(001) are similar; the presence of a J


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S12) and a higher hydrogenation barrier (1.07 eV) for Tfcc_SS. T-cross-bridge_SS accounts for comparable desulfurization and conversion energy barriers (0.51 vs 0.57 eV) and a higher hydrogenation energy barrier of 1.68 eV. For T-bridgefcc_SS, the initial C−S bond scission energy barrier (0.16 eV) is also much lower than that for its hydrogenation (0.83 eV). Furthermore, a new pathway with a low energy barrier as low as 0.17 eV is opened for the simultaneous breaking of the two C− S bonds in T-bridge-fcc_SS. Therefore, a surface S atom does not change the DDS mechanism but shows somewhat of a promotion effect for the desulfurization of thiophene on MoP(001). The improved HDS performance is in agreement with the previous experimental observations.14,16,17 We also considered the influence of surface hydrogen on the HYD and DDS pathways of thiophene on MoP(001), and found that coadsorbed H atom has a small effect on the HDS mechanism, mirrored by the minor variations of all the relevant energy barriers and reaction energies (less than 0.03 eV; see Figures S17 and S18). A surface S atom on MoP(001) exerts a slight influence on the hydrogenation of the DDS product (C4H4), producing butadiene (C4H6), and the relevant energy barriers are ∼0.90 eV with or without S blocking (see Figure 8). The calculated adsorption energies of butadiene are ∼3.14 and ∼3.15 eV on clean and S-coadsorbed MoP(001), respectively (see Table S3), indicating butadiene desorption is difficult and further hydrogenation is possible. The moderate adsorption energies of cis-2and trans-2-butene (ca. 1.38 and 1.23 eV; see Table S3) indicate butenes could be desorbed from both clean and Smodified MoP(001) surfaces, in line with the experimental findings that cis-2- and trans-2-butenes are major products for thiophene HDS on MoP at 623 K after 150 h on-stream,17 or be further hydrogenated to butanes, as observed as the major products of thiophene HDS at 623 K after 6 h on-stream under different pressures.14 4.3. Comparison with the HDS of Thiophene on Pt(111). Although both noble metals and MoP possess a higher HDS activity toward thiophenic compounds, a surface S atom involves a negative effect on the catalytic activity in the former, compared to the positive effect in the latter case.15−17,19 Therefore, it is of particular importance to clarify the HDS behavior as well as the surface S atom effect on these surfaces. Just like the situation of MoP(001), the desulfurization of thiophene on Pt(111) proceeds mainly via a DDS pathway.21 However, the C−S bond scission on clean Pt(111) has an activation energy of ∼0.96 eV and a reaction energy of −0.11 eV (Figure S19), compared with the values of ∼0.31 and −3.36 eV for the analogous processes on clean MoP(001). These data strongly suggest that the MoP catalyst possesses a better DDS activity than the metal Pt catalyst both kinetically and thermodynamically. In order to study the influence of a surface S atom on the HDS of thiophene, we further consider thiophene adsorption on S-modified Pt(111). We can find that the adsorption of thiophene on Pt(111) is strongly influenced by a surface S atom. In this case, as shown in Figure S13, thiophene adsorption becomes unstable and the molecule would desorb from the surface when the surface SS atom is considered. When the surface S atom is adjacent to the C2−C3 or C4−C5 bond, thiophene can stably adsorb via the thiophenic S and two C atoms far away from the surface S atom; the other two C atoms adjacent to the surface S atom are pushed away from the surface, making the thiophene ring largely nonplanar. This

weakened. As expected, the absolute values of the net negative charges on thiophene in different adsorption states (0.504 e, bridge-fcc; 0.370 e, bridge-fcc_SS; 0.334 e, bridge-fcc_SC) are correlated to the adsorption energies (2.60; 2.38; 2.05 eV). On MoP(001), transfer of electrons from the surface Mo atoms into the deposited S atoms results in the downshift of EF, and the increase of the density of state (DOS) in the region from −7.7 to −12 eV for the S deposited surface accounts for the downshift of εd (see Figure S14). Different from the situation of MoP(001), surface metal atoms on clean Pt(111) are charged with net negative charges (−0.028 e), and the Fermi level of Pt(111) is lower than that of MoP(001) (−5.87 vs −5.09 eV); electrons would transfer from the deposited S atom into the metal (0.108 e) and thus elevate the Fermi level to −5.78 eV for the S-modified system (see Figure S14). We can find that the DOS of the S-modified system increases in the low energy region (−10 to −13 eV), which in turn leads to the downshift of εd. The S atom on Pt(111) is charged with positive charges; the Pt atoms near the surface S atom own more net negative charges than those on the clean surface (ca. −0.052 vs −0.028 e), and the Pt atoms away from the surface S atom become less net negative charged (−0.018 vs −0.028 e). Distributed on the thiophene on both clean and S-modified Pt(111) surfaces are positive charges (0.674 e, cross-bridge; 0.528 e, tilted-SC), meaning the donation of electrons from the thiophene into the Pt d orbitals. The weaker adsorption of thiophene on the S-modified surface is correlated with the smaller donation of electrons. 4.2. HDS Mechanism of Thiophene on MoP(001). The reaction networks of thiophene HDS on clean and S-modified MoP(001) are presented in Figure 9. On clean MoP(001), there are five f lat molecular adsorption states of thiophene. The conversion of T-fcc to the most stable T-bridge-fcc configuration involves an energy barrier lower than both the C−S bond scission and hydrogenation. For T-cross-bridge, because of the comparable energy barriers, both the conversion to T-bridge-fcc and direct desulfurization are possible, but hydrogenation with a higher energy barrier is impossible. Thus, in the HDS of thiophene on Mo(001), T-bridge-fcc is a key intermediate, which can be produced via either the direct adsorption of thiophene or the conversions from the T-fcc and T-cross-bridge adsorption states. The two-step C−S bond scissions from T-bridge-fcc proceed with very low energy barriers (0.18 and 0.44 eV) and very high exothermicity (3.36 eV), whereas the hydrogenation of T-bridge-fcc involves an energy barrier of ∼1.06 eV. Additionally, the hydrogenation of C4H4S (thiolate-A), the product of the initial C−S bond cleavage of T-bridge-fcc, has an energy barrier of 1.27 eV (see Figure S16), also much higher than the barrier for its C−S bond cleavage (0.44 eV). Therefore, the two-step C−S bond scissions are favored for the f lat molecular adsorption states of thiophene on MoP(001). Moreover, the dissociative adsorption of thiolate-fcc* involves also a hydrogenation energy barrier (0.99 eV; see Figure S16) higher than the C−S bond cleavage energy barrier (0.77 eV). Therefore, thiophene HDS on clean MoP(001) should proceed along the DDS pathway. For the clean Mo-edge of MoS2, Yao et al.60 and Kumar et al.48 have found that thiophene desulfurizes via the HYD pathway and the C−S bond scission energy barrier is 0.93 eV,60 much higher than that of 0.18 eV on MoP(001), in accordance with the fact that MoP processes a higher catalytic activity than MoS2. Similarly, thiophene on S-modified MoP(001) also involves a lower conversion barrier to form T-bridge-fcc_SS (see Figure K


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Figure 10. Decomposition of the adsorption energies for the IS, TS, and FS of the initial C−S bond scission of thiophene from T-cross-bridge on the clean and S-modified Pt(111) and MoP(001) surfaces.

free and adsorbed geometries, respectively. Thus, δEads and δEsurf are the distortion energies of the adsorbate and surface, reflecting the energy cost for the adjustment of the relevant species structures in the adsorption process. Clearly, the first term in formula 1 contributes positively to the adsorption energy, while the last two terms contribute negatively. The adsorption energies, distortion energies, and interaction energies for the IS, TS, and FS of the initial C−S bond scission of thiophene on the clean and S-modified Pt(111) and MoP(001) surfaces are given in Figure 10 and Table S4. For comparison, we select the T-cross-bridge configuration of thiophene as the IS because the adsorption configurations are analogous to each other. Interestingly, although the Pt and MoP surfaces have different geometrical structures, the adsorption energy differences (1.23, 1.77, and 3.09 eV) are almost equal to the interaction energy (Eint) differences (1.31, 2.02, and 3.36 eV) for the IS, TS, and FS, respectively, when going from clean MoP(001) to clean Pt(111). In other words, the large drop of adsorption energies from MoP(001) to Pt(111) is mainly due to the adsorbate−surface interaction energies (Eint), which is determined by the electronic structures, as discussed in section 4.1. The fact that the adsorption energy difference is enlarged from the IS through the TS to the FS accounts for the higher energy barrier as well as the lower exothermicity for the C−S bond scission on Pt(111) than on MoP(001). The adsorption energies of the IS and TS on the S-modified MoP(001) surface are nearly equally decreased (ca. 0.23 eV) from those of the IS and TS on the clean surface; thus, a surface S atom only slightly influences the activation energy of the process. The influence of a surface S atom is attributed to the interaction energies (Eint) because the variation of the distortion energy of both the adsorbate (δEads) and surface (δEsurf) predicts a negative effect. On the Pt(111) surface, however, a surface S atom induces a substantially larger decrease of the adsorption energy for the TS (0.76 eV) than for the IS (0.45 eV), thus elevating the C−S bond scission activation barrier and hindering thiophene desulfurization. For the IS, the influence of a surface S atom on the adsorption energy is attributed to the interaction energy (Eint) and the adsorbate distortion energy (δEads) with the former being the main factor, whereas it is due to the adsorbate distortion energy (δEads) for the TS. In this case, the importance of the adsorbate distortion energy (δEads) is in accordance with the contracted surface structure of Pt(111), which is expected to induce a

situation discounts the adsorption energy by 34% (0.86 eV). The substantial variety of the adsorption configuration may be originated from the lower adsorption strength as well as the steric effect of Pt(111) when compared to MoP(001). On the Pt(111) surface, the neighboring Pt−Pt distance is 2.775 Å, whereas it is 3.274 Å for Mo−Mo on MoP(001). Therefore, in the existence of atomic S on the Pt(111) surface, further adsorption of thiophene would become difficult and the activity of the catalyst would lose. For the HDS process, we also calculate the initial C−S bond scission of thiophene on S-modified Pt(111); the results are shown in Figure S19. Compared with the clean Pt(111) surface, the surface SC atom elevates the energy barrier of the initial C− S bond scission of thiophene by 34% (1.25 vs 0.93 eV) and makes the reaction endothermic (by 0.41 eV), indicating a negative effect of the surface S atom on the HDS of thiophene on Pt(111). This is the other reason for the different catalytic activity trend of noble metals and MoP during the desulfurization process. On Pt(111), the hydrogenation barriers of C4H4 toward C4H6 are ∼1.12 eV but can be as high as ∼2.08 eV due to the blocking of the surface S atom.21 The calculated adsorption energies of 1.03 and 0.73 eV21 mean butadiene could be desorbed from both the clean and S-coadsorbed Pt(111) surfaces, indicating the hydrogenation on Pt(111) is not as deep as that on MoP(001), in agreement with the desorption of butadiene from Pt(111) observed in the ultrahigh vacuum (UHV) experiments.61,62 4.4. Influence of Surface Atomic S on the HDS of Thiophene. In order to understand the influence of a surface S atom, we further consider the adsorption energy of the IS, TS, and FS relevant to the initial C−S bond scission of thiophene on the clean and S-modified MoP(001) and Pt(111) surfaces. The adsorption energy of the adsorbates can be partitioned into three terms using the following formulas63 ΔEads = E int − δEads − δEsurf

(1)

E int = Eads * + Esurf * − Eads/surf

(2)

δEads = Eads * − Eads

(3)

δEsurf = Esurf * − Esurf

(4)

where Eint is the interaction energy between the surface and adsorbate at the adsorbed geometries and Eads (Esurf) and Eads* (Esurf*) are the total energies of the adsorbate (surface) at the L



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larger steric effect between the adsorbate and surface S atom, resulting in a large geometric distortion of the adsorbates, as shown in Figure S13. The stability of the HDS intermediates is also a factor determining the catalytic performance. As shown in Table S4, a surface S atom decreases the adsorption energy, but the adsorption energy of thiolate on S-modified MoP(001) is still high (3.93 eV) enough for further surface reactions. Because of the weaker interaction energy (Eint), the adsorption energy of thiolate on clean Pt(111) is only 1.43 eV but is still favorable for further reactions. However, because of the distortion energies of both thiolate and the surface (δEads and δEsurf), the about 1 eV drop of the adsorption energy of thiolate on Smodified Pt(111) (giving a value of 0.45 eV) suggests desorption should be preferred and further hydrogenation and desulfurization of thiolate would become difficult.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07103. The test calculation for calculation parameters setting; the adsorption of T-bridge-hcp, T-hcp, T-upright-bridge, MHTs, and BDT; the PESs for first C−S bond scission on S-modified Pt(111), DDS, HYD, and conversion of thiophene; the C−S distances in the thiophene DDS intermediates; adsorption energies of hydrogen and the hydrogenation products; decomposition of the adsorption energies from T-cross-bridge; and partial DOSs and Mulliken populations on MoP(001) and Pt(111) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 86-532-8698-3372. Fax: 86-532-8698-3363. *E-mail: [email protected]. Phone: 86-532-8698-1334. Fax: 86-532-8698-3363.

5. CONCLUSIONS First principle periodic DFT calculations have been used to investigate the HDS of thiophene on clean and single-sulfuratom-modified MoP(001) surfaces as well as the influence of the surface S atom on the catalytic performance. Thiophene prefers strongly f lat adsorption on both clean and S-modified MoP(001) in either the molecular states (bridge-hollow, hollow, and cross-bridge) or the dissociative state (thiolatefcc*) because of the simultaneous rupture of one C−S bond. The molecular adsorption energies on the clean surface are in the range 2.54−2.60 eV and the dissociative adsorption energy is 3.92 eV, and they are slightly reduced (by 0.15−0.61 eV) in the presence of a surface S atom. From all the f lat adsorption states on both the clean and S-modified MoP(001) surfaces, thiophene HDS proceeds along the DDS routes. The T-bridgefcc adsorption states, which can be formed via either direct adsorption of thiophene or conversions from T-fcc and Tcross-bridge, are a key intermediate to produce the DDS product (C4H4 + S) with the lowest two-step desulfurization energy barriers (0.18 and 0.44 eV). Also important is the dissociative adsorption of thiolate-fcc*, which prefers to desulfurize via one C−S bond scission step. Desulfurization from T-cross-bridge may also be possible because the relevant energy barrier is lower than that for the conversion. Following the DDS, deep hydrogenations of C4H4 to butene and/or butane are possible because of the strong adsorptions of the involved intermediates as well as the moderate hydrogenation barriers. A surface S atom on MoP(001) elevates, in most cases, the hydrogenation energy barriers, decreases slightly the adsorption energies of the involved intermediates and the C− S bond scission energy barriers, and also opens a new lowbarrier (0.17 eV) pathway for the simultaneous breaking of the two C−S bonds in thiophene. Therefore, a surface S atom on MoP(001) shows somewhat of a promotion effect on thiophene HDS. For comparison, the different electronic and geometric structures predict much lower adsorption energies of the HDS adsorbates on Pt(111), which decrease substantially in the presence of a surface S atom. Also, a surface S atom elevates largely the energy barriers for both the desulfurization and hydrogenation of thiophene. Therefore, although the HDS of thiophene on Pt(111) proceeds also along the DDS routes, the hydrogenations are not as deep as on MoP(001) and the HDS activity is easy to lose because of the surface S atom.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC (21303266 and 51502348), PetroChina Innovation Foundation (2013D-5006-0406), Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2013CL031), Shandong Province Special Grant for High-Level Overseas Talents (tshw20120745), and the Fundamental Research Funds for the Central Universities (13CX05020A, 13CX02025A, 14CX02214A, 15CX06075A, 15CX05050A, 15CX08010A, 15CX05068A).

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