Density Functional Theory Investigation on Thiophene

Article pubs.acs.org/JPCC

Density Functional Theory Investigation on Thiophene Hydrodesulfurization Mechanism Catalyzed by ReS2 (001) Surface Yucheng Huang,* Hai Liu, Xi Chen, Danmei Zhou, Chan Wang, Jinyan Du, Tao Zhou, and Sufan Wang Center for Nano Science and Technology, College of Chemistry and Material Science, The Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Anhui Normal University, Wuhu, 241000, Peoples’ Republic of China S Supporting Information *

ABSTRACT: We present density functional theory calculations on the reaction mechanism of thiophene hydrodesulfurization (HDS) over ReS2 (001) surface under typical HDS reaction conditions. It is found that thiophene adopts an “upright” adsorption configuration with the binding energy of 1.26 eV. Considering the factors such as Bader charge, two reaction mechanisms, named direct desulfurization (DDS) to the product of butadiene and hydrogenation (HYD) to 2-butene, 1-butene, and butane, are systematically investigated. Results show that H prefers to attack thiophenic C before the first C−S bond rupture but begins to hydrogenate ST (S atom of thiophene) after ring-opening. Prehydrogenation has different effect on the activity of C−S bond breaking. When the ring is intact, it has nominal effect; but when the ring is open, appropriate prehydrogenation can dramatically decrease the energy barrier while complete hydrogenation makes the barrier rise again due to stereohindrance effect. The DDS mechanism is proved to be kinetically unfavorable while 2-butene is suggested to be a predominated product for HYD mechanism. The role of Sa (preadsorbed S) is a “ladder” which helps H approach the thiophenic molecule while ST acts as an “intermediary” for H exchange. Changing reaction conditions through partial pressure of H2 can only alter the rate-determining step but has nothing to do with the catalytic selectivity.

1. INTRODUCTION

The simplest homologue of the family of sulfur-containing molecules, which are refractory to classical HDS, is thiophene. It is often used to perform a rapid evaluation of the catalytic performance of a new catalytic formulation, although the effectiveness of such a screening is under debate.17 In recent years, with the fast development of computer, more complicated S-contained compounds were used as model catalysts to investigate the HDS mechanism by numerical calculations. However, the investigation of thiophene HDS mechanism is able to capture the key features of the catalytic performance which can provide references for understanding HDS of other thiophene-derived species. In fact, even for this simple molecule, its desulfurization mechanism is still unclear. Thus, it needs to increase insight into the structures of the active catalyst particles and the reaction mechanisms. Until now there are still some controversies for the mechanism of thiophene HDS reaction on the mature investigated catalyst of MoS2.8,18−25 On one hand, it is hard to determine what extent of prehydrogenation before the ringopening; and on the other hand, the reaction products and intermediates are also a matter of controversy, which are believed to be different depending on the reaction conditions.8,23 For example, tetrahydrothiophene is believed as a

As the environmental legislation becomes more and more strict, the upgrade of low-quality oil to clean fuel becomes extremely urgent. At present, one of the biggest challenges is the remove of refractory sulfur-containing species in crude oil.1,2 Obviously, knowledge on the detailed mechanism of hydrodesulfurization (HDS) is of great importance. It is generating increased interest in obtaining a detailed description of the catalytic mechanism which, however, has been investigated for several decades in reality.3−6 Industrial typical catalysts for HDS mainly consist of molybdenum or tungsten sulfides with the promotion of cobalt and nickel. The active phases of these catalysts and the catalytic activities have been intensively investigated.7−10 Besides these well-studied catalysts, other transition metal sulfides, which are less investigated, also show strong activity toward HDS according to a well-known theory of the “Sabatier principle”.11 One of the layered metal sulfides, ReS2 has received great interest recently because of its unique electronic properties.12 In fact, it also displays an unusual catalytic activity toward HDS as hinted by the “volcano curve”,11 from which one can see that the position of ReS2 locates on the nearly topmost point, resulting in a comparable catalytic activity with the widely used catalysts of Mo(W)S2. The use of ReS2 as active phase has been reported experimentally, and the catalytic activity has been proved to be strongly dependent on the preparation and sulfidation methods as well as the nature of the support.13−16 © XXXX American Chemical Society

Received: March 16, 2016 Revised: May 3, 2016

A

DOI: 10.1021/acs.jpcc.6b02769 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

We choose ReS2 (001) surface as the catalyst as it exposes edge which has coordinately unsaturated sites for thiophene adsorption or promoter incorporation. The same large supercell, including one elementary ReS2 unit in the x direction, two rows in the y direction, and three layers in the z direction, was constructed as reported in our previous study.38 The bottom two layers were always fixed at their bulk geometries during optimization. The tolerances of energy and force were set to 10−5 eV and 0.03 eV/ Å, respectively. Transition states were roughly located using the climbing nudged elastic band method (CNEB39,40) and then further optimized to the true saddle point using quasi-Newton algorithm. A vibrational mode analysis was used to characterize the transition states and evaluate the zero point corrections for reaction heats and energy barriers.

typical intermediate at the reaction conditions (high pressure and low temperature) because its formation is equilibriumlimited at high temperature.19,26 It is possibly that aromatic ring is hydrogenated to tetrahydrothiophene and that this compound should be considered as an intermediate because of the chemistry of thiophene.27 However, Lipsch and Schuit28 proposed that hydrogenolysis of the C−S bonds may produce 1,3-butadiene directly. Density functional theory calculations demonstrated that 2,5-dihydrothiophene is an important intermediate and it is responsible for the direct formation of butadiene.24,25 In the earliest proposal29 it was suggested that thiophene desulfurization proceeds through a double β-hydride elimination followed by a fast hydrogenation of adsorbed diacetylene to 1,3-butadiene. Therefore, it can be imagined that the hydrogenation activity of the catalyst seems to affect the concentration of the reaction products. Although the HDS mechanism has been fully investigated by both theoretical and experimental means, the mechanism on the ReS2 phase has not been reported yet. Experimental investigations on the catalytic mechanism are difficult at the cost of time-consuming and high cost, but theoretical means is very powerful which has been successfully applied toward the acceleration of new catalysis discovery and the understanding of catalytic mechanisms.30,31 In this contribution, by means of first-principles calculations, we investigate the reaction mechanism of thiophene HDS catalyzed by ReS2 (001) surface under the typical experimental condition. Considering the factors such as Bader charge, the HDS reaction possibly leads to four products, i.e., butadiene, butane, 1-butene, and 2-butene. The highest barriers for each channel always occur at the C−S bond rupture step. On the basis of kinetic analysis, we show that the formation of butadiene is unfavorable and 2-butene may be the most abundant product. Moreover, it is found that changing partial pressure of H2 cannot affect the selectivity but can alter the rate-determining steps. The paper is arranged as follows. After giving detailed computational methods, the adsorption configurations of thiophene on ReS2 (001) surface are first described, then the elementary steps involved in both direct desulfurization (DDS) and hydrogenation (HYD) pathways are confirmed, followed by a discussion regarding the role of two different S atoms and the reflection on the activity of C−S bond scission. A kinetical analysis under the quasi-equilibrium approximation is employed to provide an understanding on the rate-determining step as well as the reaction selectivity, and the final conclusions are drawn at the end of the text.

3. RESULTS 3.1. Thiophene Adsorption. In our previous work,38 we found that ReS2 (001) surface preadsorbed with 25% S was the most stable under the HDS conditions. To mimic the hydrogenation reaction of thiophene, we assume that two dissociated H atoms from H2 always exist on the surface. The rationality of this assumption is justified by the fact that experimentally H2 dissociation is found not to be a ratedetermining step.8,19 We first investigated the coadsorption of thiophene and H. As seen in Figure 1, three possible adsorption

Figure 1. Structure of thiophene adsorbed on the sulfurized ReS2 (001) surface with two coadsorded H atoms: (a) “upright” adsorption configuration; (b)“off-upright” configuration; (c) “flat” configuration; (d) numbering of thiophene.

configurations are obtained, i.e., “upright” mode with the molecular plane vertical to the surface (Figure 1a); “off-upright” mode with the plane nearly vertical to the surface (Figure 1b); and “flat” mode with the plane paralleling to the surface owing to two additional C−S bonds formation (Figure 1c). The latter two configurations have been found on MoS2 (001) surface,41 but contrary to the case of MoS2, our results show that the first one is the most stable. To conveniently describe the adsorbed geometries, we numbering the four C atoms of thiophene as C2 to C5 (Figure 1d). Thiophene adsorption causes a stretching on S−C2, S−C5, and C3−C4 bonds, they respectively elongate from 1.714, 1.715, and 1.423 Å in the gas phase to 1.742, 1.741, and 1.433 Å, but C2−C3 and C4−C5 bond lengths are slightly shortened from 1.376 in the gas phase to 1.361 and 1.364 Å (Figure 1a). The variation of C−S bond lengths is the highest, meaning that the C−S bond is activated after thiophene adsorption. This scenario was also found on the NbC/

2. COMPUTATIONAL METHODS All the calculations were performed with the Vienna ab initio simulation package (VASP).32−34 The exchange-correlation functional with generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof (PBE33) formula was adopted. The energy cutoff of the plane-wave expansion was set to 400 eV and the electron−ion interaction was described using the projector-augmented wave (PAW35,36) potentials. Brillouinzone integrations have been performed using a 4 × 2 × 1 Monkhorst−Pack grid.37 All the settings were proven to be sufficient for achieving converged results (convergence test can be seen in Table S1 of Supporting Information). The geometries and total energies of all stationary points were located with the conjugate-gradient algorithm during ionic relaxation process. B


Article


Table 1. Overview of the Reactions Involved in Thiophene HDS Including the Activation Barriers (Ea), Energy Change (ΔE), Bader Charge, and Reaction Rates (S−2) of the Reactionsa

a

The large asterisk (S−*) denotes the adsorption to the ReS2 surface and the small one denotes the dangling bond. C


Article


Figure 2. Structures of IS, TSs, IMs, and FS for thiophene DDS pathway on the ReS2 catalyst. Blue, yellow, white and purple balls represent Re, S, H, and the Sa atoms, respectively. C−H and C−S distances (in Å) are labeled with black and red.

NbN(001) surface,42 but it is in sharp contrast with that of the metallic surfaces, in which a flat configuration is preferred and the intermolecular bonds are weakened and even cracked that correlate the fact of a loss of aromaticity.43−46 Binding energy (BE) of thiophene on the ReS2(001) surface is calculated to be 1.26 eV. Inclusion of vdW dispersion forces using DFT-D2 method47 does not strongly affect the magnitude of BE. Note that a positive BE denotes a favorable adsorption. The calculated BE on ReS2 is much lower than those on the Ni(100),44 Ni(110),45 and Pt(100)46 surfaces, where the dissociative adsorption configurations were found with a strong chemisorption (BE = 2.20−2.57 eV). While on more compact surface such as Al(111),48 thiophene has a BE of only 0.54 eV. On the other hand, metal sulfide surfaces always have lower BEs, e.g., ∼0.07 eV on Co-promoted MoS2 surface at Mo and S edges (or Co−Mo−S edge) with both the flat and upright adsorption modes.24,49 Note that the magnitude of BE on ReS2 is in the range of metal surfaces and traditional commercial HDS catalysts, indicating that ReS2 may has strong activity as too weak or too strong adsorbate−substrate interaction would go against the activity. 3.2. Thiophene HDS Mechanism. The most important issue on the HDS mechanism is what extent of prehydrogenation occurs before C−S bond scission. If C−S bond breaks prior to the hydrogenation, i.e., without the need of prehydrogenation, a direct desulfurization mechanism would happen, which is called the DDS mechanism. Another situation is that thiophene first needs H to saturate and then the S−C bond breaks at a certain hydrogenated species. This one is known as the HYD mechanism. In the context of having no idea on what extent of the prehydrogenation is, lots of possible intermediates and the resulting mechanisms will be presented. Obviously, it is terribly boring to investigate all these channels; it is also computational unaffordably. Because H is positively charged, the choice of elementary reactions may be guided by Bader charge,50 where the more negative charged atoms have priority to be attacked. The rationality of this strategy will be discussed in the following. Note that the Bader charge is not the solely factor to determine where the reaction will go, sometimes spatial distance and steric hindrance may also be important. Four products, butadiene, butane, 1-butene, and 2butene, are produced which are denoted as the channels 1 to 4. Here, channel 1 is a DDS while channels 2 to 4 are the HYD mechanisms. To simplify the description of different reaction pathways, we design the notations of IS/TS/FS/IMm,n to represent the initial state/transition state/final state/intermediate state. Here m denotes one of the studied channel (m = 1− 4) and n indicates the number of the states. For example, TS1,4 denotes the fourth transition state along the channel 1. Note that in the HYD mechanisms, the prehydrogenation steps along

channels 2 to 4 are the same. At this situation, we use the symbols like IM234,2, which means it is the second common intermediate for channels 2 to 4. All the calculated reaction heats, energy barriers, Bader charges, pre-exponential factors and reaction rates are tabulated in Table 1, and the imaginary frequencies of each TS are deposited in the Table S2 of the Supporting Information. 3.2.1. DDS Pathway. As shown in Figure 2, DDS pathway begins with HS (H atom on the adsorbed S) tilting to the thiophene surface. With HS-C2 bond length decreasing to 1.474 Å, TS1,1 is formed. Vibration analysis shows that this process is a concerted mechanism that C2−H bond formation and C2−S bond breaking are simultaneously happened. The C2−S bond length in the TS1,1 is measured to be 2.407 Å. The calculated barrier is 1.76 with high endothermity of 0.71 eV (Figure 3).

Figure 3. Relative energy diagram for thiophene DDS pathway on the ReS2(001) surface.

This process leads to the formation of IM1,1, in which C2 is saturated by one H atom, at the same time the ring of thiophene is open. The high barrier and endothermity indicate this pathway is not dominant in the competition with other pathways. Note that three DDS reaction patterns are reported, one is consistent with the present one that the C−S scission needs the aid of H, the second is C−S bond directly breaks without any hydrogenation, and the last is C−S bond breaks once thiophene adsorbs.44,45 For DFT calculations, the former two mechanisms are always simultaneously considered, such as on the NbC/NbN(001),42 Pt(111),43 and Pd(111)51 surfaces. On the ReS2 surface, although the C−S bonds of thiophene are somewhat activated, we cannot locate the TS without the aid of H, probably because thiophene adopts an upright adsorption configuration. To continuous desulfurization, another H from the surface is needed to saturate the IM1,1 further. However, the surface H D


Article


Figure 4. Structures of IS, TSs, IMs, and FS for thiophene HDS pathway. Blue, yellow, white and purple balls represent Re, S, H, and the Sa atoms, respectively. C−H and C−S distances (in Å) are labeled with black and red.

As seen in Figures 4 and 5, the structures and the energy potential profiles are respectively illustrated. According to the

atom is not only far away from the IM1,1, but also cannot reach the height of IM1,1 because of the shorter Re−H bond length (Figure 2). Thus, H atom adsorbed on the surface Re would migrate to the adsorbed S atom (denotes as Sa hereafter), i.e., using Sa as a “ladder” to become the HS, and then reacts further with thiophenic atoms. This process needs a barrier of 0.75 with the reaction heat of −0.32 eV (Figure 3). Subsequently, at IM1,2, HS would select C or ST (S atom of thiophene) to attack. Because ST carries negative charge and is nearer to the HS (Table 1), thus the −STH bond is favorably formed in this stage. The ST−HS bond length shortens to 1.830 Å at TS1,3, and the obtained barrier is only 0.29 eV, accompanied by a slight heat release. The C5−S bond length at IM1,3 is 1.784 Å, which is longer than the isolated one. With the C5−S bond length elongating to 2.129 Å, the scission occurs, and the product of butadiene is produced. This step needs to surmount an activation energy of 1.85 eV, even higher than that of the initial S−C breaking route. Note that butadiene may react further by hydrogenation or intramolecular rotation. These cases were not considered here because they would occur after S removal and are not the key feature of desulfurization. One may notice that at the IM1,3, providing H of −STH transfers to C2, C3 or C4, the products such as 1-butene, 2butene, and even butane would be produced. However, these pathways are not considered here because the high barrier and endothermity of the first C−S bond rupture would prohibit the subsequent reactions. Kinetics analysis also shows the rate of this pathway (R1) is much lower than that of its competitive one (R4), which will be described below. 3.2.2. HYD Pathway. Different from the mechanism of DDS, for HYD S−C bond scission always occurs after several hydrogenation steps. Taking into account Bader charge and other factors, three channels are investigated. It is interesting that these three pathways have the same prehydrogenation steps. In the following, we first narrate these common hydrogenation processes, then each pathway will be detailedly described.

Figure 5. Relative energy diagram for thiophene HYD pathway on the ReS2(001) surface, the route of forming 1-butene and 2-butene.

Bader charge, C2 and C5 have the same charge (−0.27, Table 1). Here C2 atom is chosen for HS attacking because of the shorter H−C2 distance. When C2−HS reaches 1.404 Å, TS234,1 is formed. Compared with the first step of DDS with a concerted mechanism, the ring is well kept and the energy barrier of 0.75 eV is distinct lower. Analogue to the channel 1, the subsequently H addition needs the Sa ladder, and the barrier is calculated to be nearly 1 eV. The higher barrier may be caused by a stable IS, i.e., IM234,1 with a ring is more stable than the IM1,1 with a chain structure. Then, HS of IM234,2 would attack C5 because of its more negative charge (Table 1). At TS234,3, Hs−C5 elongates to 1.975 Å. At IM234,3 (2,5dihydrothiophene), a ring structure still maintains. This step needs a barrier of 1.31 eV, but releases a 0.93 eV heat, indicative of the enhanced stability with the hydrogenation degree. One E


Article


Formation of 1-Butene. With HS approaching to C3, ST deviates from its original position, leading to the elongation of C5−ST bond. At TS3,7, C3−H and C5−ST distances are 1.388 and 1.941 Å, respectively, corresponding to ST−H bond swinging toward C3. In FS3, 1-butene is produced, leaving the desorbed ST on the surface, indicative of the finish of one desulfurization cycle. The calculated barrier is 1.38 eV with a slight endothermicity of 0.13 eV (Table 1). Compared with the energy baseline of the IS, the formation of 1-butene releases heat of 1.36 eV, which is lower by 1.23 eV than that of 2-butene (Figure 5). The more stable feature of 2-butene is due to the effect of p-π hyperconjugation. Note that the barrier of 1.38 is largely higher than 0.38 for attacking C5, which seems to contradict with the results of Bader charge. In fact, the vibration modes are totally different for two pathways: at the TS2,7, there is only C−S stretching vibration, but at the TS3,7, C−H and C− S stretching vibrations are simultaneously presented. That is to say, the barrier of 1.38 contains the contribution from hydrogenation, not only the one from C−S bond scission. Formation of Butane. At the IM234,7, only the C2 atom is fully saturated. If the rest of the C atoms are also fully hydrogenated, a saturated alkane would be produced. The starting point of the road to butane is the movement of H atom in −STH to C4 atom. When C4−H distance reaches 1.325 Å, TS4,7 is formed. This step needs a barrier of 0.58 eV, but meanwhile needs heat of 0.36 eV. This result indicates that the formed IM4,8 is not stable: only 0.22 eV can make it return to the state of IM234,7. Thanks to another HS, the occurrence of next hydrogenation would effectively stabilize the hydrogenated species (Figure 6). Similar to the structure of IM234,5, ST atom in the IM4,8 is also closer to the HS which is negative charged. Thus, ST−H bond forms first followed by H drifting to C atom in the backbone. The formation of ST−H bond at IM4,10 needs a barrier of 0.55 eV, and 0.42 eV heat is released, which is very similar to the route from the IM234,6 to IM234,7 (Table 1). Subsequently, H begins to close to C3 atom as it only bonds to one H. Here we do not choose more negative C5 for H attacking because (i) C2 is closer to HS and (ii) attacking C5 will lead to C5−ST bond breaking and the C2−Re bond formation, which the high barrier can be expected (the actually calculated barrier is 2.07 eV). With C3−H distance being 2.389 Å, TS4,11 is formed. The barrier is substantially high, 1.59 eV, possibly because TS has a long C−H distance. The heat release is also amazing, reaches −1.85 eV, indicating the formed intermediate is strongly stabilized further. As C2, C3, and C4 atoms have already saturated by H, one more H atom is still needed for C5 to complete the full saturation. In a similar way, H needs ST as a springboard, and the structural evolution, the values of reaction heat and energy barrier resemble that of other ST−H bond formation. Then, the H atom of −STH attacks the last unsaturated C5 atom, and butane is thereby produced. This step is also a concerted mechanism, i.e., C−H formation and C5−S breaking are simultaneously taken place, and the high barrier of 1.66 eV is calculated. The high barrier may be caused by the hindrance effect from the saturated C4 atom.

may notice that at this step if HS attacks C2, C2 would be saturated and C2−S would break thereupon. However, the calculated barrier is 1.90 eV, much higher than 1.31 eV, demonstrating the strategy in the light of Bader charge is believable. As two of H atoms from H2 dissociation have been involved in the reaction, we assume that H2 is in the equilibrium state and the subsequent H addition is in a continuous manner. To avoid repeated calculation for diffusion barrier, the added H atom is directly put onto the Sa atom in IM234,4. Here, C2 is not only more negative charged, but also is closer to HS compared with C3, C4, and C5. In fact, reacting with C3 or C4 atom would lead to the formation of tetrahydrothiophene, which was proved to be equilibriumlimited at high temperatures for HDS conditions.19 The C2−H and C2−S distances are 1.612 and 2.467 Å in TS234,4, respectively and the calculated barrier is 1.70 with a heat release of 0.44 eV. At this stage, because C2 atom is saturated to methyl, C2−S bond is accordingly broken. Next, another HS begins to selectively saturate C3, C4, or C5 atom. It is interesting that Bader charge analysis pointed out ST carries negative charge again as seen in the IM1,2 (Table 1). In addition, in the newly formed IM234,5, ST is in the front of the C backbone thereby is closer to the HS. Thus, this stage H does not directly saturate C atom, but forms a −STH bond first. The HS−ST distance in TS234,6 is 2.676 Å, and a barrier of 0.27 is calculated with exothermicity of 0.50 eV. The formed IM234,7 is a metastable state, therefore ST may act as a springboard for H for next C atom hydrogenation. While considering the next attacking site for H, it is shown that C3, C4, and C5 of the IM234,7 have comparable Bader charge (Table 1). As the charge differences of these C atoms are pronounced lower than the ones in other IMs, we thus consider all these possible H intramolecular transfer which occurs through migration from ST to one of an arbitrary C atom. We illustrate the energy profiles of the pathways to 2-butene and 1butene in Figure 5, and the pathway to butane in Figure 6.

Figure 6. Relative energy diagram for thiophene HYD pathway on the ReS2(001) surface, the route of generating butane.

Formation of 2-Butene. When the ST−H bond rotates toward C5, the product of 2-butene is generated. The C5−H distance at TS2,7 is 1.576 Å, and the activation energy is only 0.38 eV. The low barrier may be caused by the fact that H is the closest to ST. In particular, this step has a huge heat release of 1.1 eV, demonstrating that the formed product is very stable (Figure 5).

4. DISCUSSION 4.1. Role of ST and Sa in the Hydrogenation Process. Hydrogenation has been proved to be a dominating reaction during HDS process,52−54 thus understanding its catalytic activity is of great significance to the hint toward industrial production. In Figure 7, the reaction network of thiophene F


Article


Figure 7. Schematic overview over the reactions involved in the HDS of thiophene. The large asterisk (S−*) denotes the adsorption to ReS2 surface and small one denotes the dangling bond.

HDS on ReS2(001) is schemed, in which the area without background color represents the prehydrogenation steps and the areas with pink background denote S−C scission while hydrogenation is highlighted with light green. It is interesting that all the hydrogenation reactions can be classified into two categories: (i) HS directly adds to the C atom; (ii) HS adds to C atom through experiencing an intermediate of −STH species. The direct hydrogenations always occur on the prehydrogenation stage, in which the ring structure keeps well; however, after the first S−C bond scission, hydrogenating C atom needs the aid of ST. The reason can be ascribed to the variation of charge state of ST. According to Bader charge results, when the ring structure is intact, ST always carries positive charge which is electrostatic repulsive to the same positive H atom (Table 1). In contrast, when the ring is open, ST carries pronounced negative charge, thereby the added H is more likely to react with it. However, after the formation of −STH species, the charge of ST is offsetted by the H atom, leading to it catching nearly neutral charge. That is to say, ST of the hydrogenated species with ring structures has neglectable bonding capability while of those with chain structures regain the chemical activities. Thus, ST gives play to different roles at different hydrogenation stage. As seen in the Table 1, the barriers for hydrogenation on these ring structures are substantially high, while those on the chain structures produced from deep hydrogenation are very lower (R2, R7, R11, and R13), indicating that the negative-charged ST is very active and thus ring breaking is beneficial to the H addition. It is also interesting that two S atoms (Sa and ST) play distinct roles in the hydrogenation process. At the experimental condition, a Sa atom is introduced into the surface.38 As the height of H−Re is not high enough, H cannot reach the C atoms. Thus, Sa just acts as a “ladder” for the next hydrogenation. On the other hand, ST acts as an “intermediary”: it receives the H atom first and then donates it to the C. One can imagine that the final desorbed ST will leave on the surface, and then its role shifts to the “ladder”. When the sulfuration degree of the catalyst is not high, the migration kinetics of H may be not affected; but when the surface is fully occupied by the S ladder, the catalyst would be deactivated. Fortunately, the activity of these sulfurated surfaces can be recovered by the reduction; i.e., H2 can take the S away by forming H2S gas.1,38 Thus, appropriate S adsorption on the surface will be beneficial to keep the activity of catalysts, but H2

atmosphere with a certain pressure is also needed to avoid complete sulfuration. 4.2. Reflection on the Activity of S−C Scission. The fact that ST carries completely distinct charge state can help us conveniently judge where the H will be added. More importantly, it may have revelation to the activity of S−C activation. To completely desulfurize, two C−S bond scissions need to be experienced. Note that whatever mechanism is adopted, C−S bond scission always needs the aid of H. The function of H on one hand, can saturate C atom in the backbone, and on the other hand, can weaken the C−S bond. Since C−S bond scission is always accompanied by the hydrogenation, the question is what is the effect of hydrogenation on the activity of S−C scission? Comparing R1 and R6 of the first C−S bond scission, the energy barriers decline by 0.06 eV by a two-step prehydrogenation. This means prehydrogenation has nominal contribution to the reactivity of C−S breaking. This result agrees with that on the Pt(111) surface, where it was believed that hydrogenation does not reduce energy barriers of C−S bond cleavage either.43 However, if thiophenic ring is broken, appropriate prehydrogenation steps can facilitate C−S bond rupture. As seen in the Table 1, the barrier of R3 is as high as 1.85 eV, but when the IS of R3 is hydrogenated to the IM234,7 (IS of R8), the barrier of C−S bond rupture dramatically declines to 0.38 eV. If it is continuously hydrogenated to the IM4,13 (IS of R14), C−S breaking barrier recovers to a high value of 1.66 eV due to stereohindrance effect. This situation means the activity of C−S bond scission is not solely determined by the factor of hydrogenation degree and is case by case. Thus, it is hard to say whether a couple of prehydrogenation steps can assist the activity of C−S bond breaking. 4.3. Kinetical Analysis under the Quasi-Equilibrium Approximation. To understand the kinetics of the DDS and HYD, the reaction rates of the hydrogenation and C−S scission reactions are calculated except for H transfer reactions (i.e, from surface to the Sa). The investigated elementary reactions are listed in the Table 1, and the rates are calculated according to ri = kiθTHi−1θH , i = 1, 2, 3, 4, 5, 6

(1)

Here, ri is the reaction rate for each hydrogenation step; ki is the forward reaction rate constant; θTHi and θH are the coverage G


Article


but when t is less than 103, the reaction rate of R3 would be slower than that of R1, thus R3 becomes a RDS. For HYD mechanism, unless the value of t is huge, otherwise the reaction rate of R5 or R6 would determine the whole kinetics. It can be simply seen when t is less/more than 0.1, R6/R5 is the RDS. Thus, the RDS is strong dependent with the value of t. On the other hand, the reaction selectivity is independent with the value of t. First, DDS is kinetically unfavorable because the rate of R1 is largely slower than that of the competitive channel of R4. In the HYD pathways, there are three competing reactions (R8, R9, and R10), which would lead to three different products. However, the relative magnitude of rates for these reactions cannot be tuned by the value of t because the powers of t in these rate expresses are the same. In a word, one can change the RDS but cannot change the reaction selectivity through changing the partial pressure of hydrogen.

of the intermediate (hydrogenated thiophene) and hydrogen, respectively. In the steady state, hydrogenation reactions are assumed in thermodynamical quasi-equilibrium on the surface under the reaction conditions. Thus, the coverage of thiophene, hydrogen, and intermediate has the following relationship: θTHiθ i * = e−Ei /(RT ) θTθHi

(2)

Then, we can deduce θTHi = e−Ei /(RT )θT

Here t is equal to

θH , θ *

θHi = e−Ei /(RT )θTt i θi *

(3)

Ei is the relative stability of THi adsorbed

on the surface with respect to the IS, which is equal to the energy difference between the adsorbed THi system and the adsorbed thiophene system. t can be deduced from the formula 4 according to the dissociation of H2 (H2 (g) + 2* = 2H (ad)): °

K 0 = e−ΔG / RT =

θH2 pH

2

p0

θ2 *

=

5. CONCLUSIONS In this study, we have presented a detailed theoretical calculation on the reaction mechanism of thiophene catalyzed by ReS2 (001) surface under the typical experimental conditions. Results showed that before ring-opening, H always attacks negative-charged C atoms and ST carries positive charge, and the calculated barriers are relatively higher; but after S−C bond breaking, H begins to attacks nearer ST atom because its charge changes to negative, and the barriers are lower. Prehydrogenation on the ring structures has nominal effect on the energy barrier of C−S bond rupture, but appropriate prehydrogenation on the ring-broken structures can dramatically decrease the barrier. Because of stereohindrance effect, overmuch prehydrogenation steps makes the barrier rise again. Moreover, DDS leads to the formation of butadiene but is kinetically unfavorable; HDS results in three products, i.e., 2butene, 1-butene, and butane. The driving force becomes stronger with the increase of hydrogenation degree and the 2butene is predicted to be a dominated product. The role of Sa provides a “ladder” for H addition and ST acts as an “intermediary” for H exchange. Changing the partial pressure of H2 can alter the rate-determining step, but it cannot alter the reaction selectivity.

t2 pH

2

p0

(4)

At the typical HDS experimental conditions (T = 650 K), t is calculated to be 1 to 10 while assuming that the hydrogen partial pressure varies in the range of 1 to 100 atm. Put the formula 3 into the formula 1, and use Arrhenius formula, the reaction rates can be further written as ri = A e−Ea,i /(RT )θTHi−1θH = A e−(Ea,i + Ei−1)/ RT t iθT , i = 1, 2, 3, 4, 5, 6

(5)

Here Ea is the zero-point energy (ZPE) corrected barrier of each step of hydrogenation; A is the pre-exponential factor. Within harmonic transition state theory,55,56 the preexponential factor of each elementary step are calculated using A=

f IS i

Π13N fiIS Π13N − 1fiTS

■

(6)

f TS i

Here are the vibrational frequencies at the IS and are the vibrational frequencies at the TS. All the calculated reaction rates as well as pre-exponential factors are tabulated in the Table 1. It should be noted that although the barriers of S−C scission are higher than those of hydrogenation, this step may not be always identified as the rate-determining step (RDS). One may notice that with the increase of hydrogenation degree, the released heat is dramatically increasing. Thus, thermodynamic driving force cancels a majority of low magnitude of the reaction rate caused by the high barrier, resulting in the order of the coefficient in the reaction rate being large (Table 1). That is to say, with the increase of hydrogenation degree, the product becomes more and more stable, in this situation even though S−C bond scission needs a high barrier, the reaction may be quickly happened. Thus, the RDS would be expected to present on the first couples of steps. As seen in the Table 1, because the power of t is different, we cannot compare the calculated reaction rates directly. For the DDS pathway, the RDS is definitely identified as the S−C bond scission, either R1 or R3, which is determined by the magnitude of t. When the magnitude of t is more than 103, R1 is the RDS;

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02769. Tables S1 and S2, giving convergence test parameters and imaginary frequencies (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*(Y.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interests.

■

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China No. 21573002, Natural Science Foundation of Anhui Province No. 1408085MKL22. The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China. H


Article


■

(22) McCarty, K.; Schrader, G. Deuterodesulfurization of Thiophene: An Investigation of the Reaction Mechanism. J. Catal. 1987, 103, 261−269. (23) Borgna, A.; Hensen, E.; Coulier, L.; de Croon, M.; Schouten, J.; van Veen, J.; Niemantsverdriet, J. Intrinsic Thiophene Hydrodesulfurization Kinetics of a Sulfided NiMo/SiO2 Model Catalyst: Volcano-Type Behavior. Catal. Lett. 2003, 90, 117−122. (24) Moses, P. G.; Hinnemann, B.; Topsøe, H.; Nørskov, J. K. The Effect of Co-Promotion on MoS2 Catalysts for Hydrodesulfurization of Thiophene: A Density Functional Study. J. Catal. 2009, 268, 201− 208. (25) Yao, X.-Q.; Li, Y.-W.; Jiao, H. Mechanism of Thiophene Hydrodesulfurization on a Mo3S9 Model Catalyst. A Computational Study. J. Mol. Struct.: THEOCHEM 2005, 726, 81−92. (26) Devanneaux, J.; Maurin, J. Hydrogenolysis and Hydrogenation of Thiophenic Compounds on a Co-Mo-Al2O3 Catalyst. J. Catal. 1981, 69, 202−205. (27) Kraus, J.; Zdražil, M. Tetrahydrothiophene as Intermediate in the Hydrodesulfurization of Thiophene. React. Kinet. Catal. Lett. 1977, 6, 475−480. (28) Lipsch, J. M. J. G.; Schuit, G. The CoO-MoO3-Al2O3 Catalyst: III Catalytic Properties. J. Catal. 1969, 15, 179−189. (29) Kolboe, S. Catalytic Hydrodesulfurization of Thiophene. VII Comparison between Thiophene, Tetrahydrothiophene, and NButanethiol. Can. J. Chem. 1969, 47, 352−355. (30) Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H. Towards the Computational Design of Solid Catalysts. Nat. Chem. 2009, 1, 37−46. (31) Zhang, J. Pem Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications; Springer Science & Business Media: 2008. (32) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (34) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (35) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758. (36) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953. (37) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B: Condens. Matter 1976, 13, 5188. (38) Huang, Y.; Liu, H.; Ling, C.; Chen, X.; Zhou, D.; Wang, S. Hydrogen Activation on the Promoted and Unpromoted ReS2 (001) Surfaces under the Sulfidation Conditions: A First-Principles Study. J. Phys. Chem. C 2015, 119, 17092−17101. (39) Henkelman, G.; Jonsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985. (40) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (41) Cristol, S.; Paul, J.-F.; Schovsbo, C.; Veilly, E.; Payen, E. DFT Study of Thiophene Adsorption on Molybdenum Sulfide. J. Catal. 2006, 239, 145−153. (42) de Souza, E. F.; Ramalho, T. C.; Chagas, C. A.; de Alencastro, R. B. Adsorption and Desulfurization Reaction Mechanism of Thiophene and Its Hydrogenated Derivatives over NbC (001) and NbN (001): An Ab Initio DFT Study. Catal. Sci. Technol. 2014, 4, 2550−2563. (43) Zhu, H.; Guo, W.; Li, M.; Zhao, L.; Li, S.; Li, Y.; Lu, X.; Shan, H. Density Functional Theory Study of the Adsorption and Desulfurization of Thiophene and Its Hydrogenated Derivatives on Pt (111): Implication for the Mechanism of Hydrodesulfurization over Noble Metal Catalysts. ACS Catal. 2011, 1, 1498−1510. (44) Mittendorfer, F.; Hafner, J. A DFT Study of the Adsorption of Thiophene on Ni(1 0 0). Surf. Sci. 2001, 492, 27−33.

REFERENCES

(1) Babich, I.; Moulijn, J. Science and Technology of Novel Processes for Deep Desulfurization of Oil Refinery Streams: A Review. Fuel 2003, 82, 607−631. (2) ISODA, D. D. W. T. Challenges in the Hydrodesulfurization of Polyaromatic Sulfur Compounds. Adv. Catal. 1998, 42, 345. (3) Kwart, H.; Schuit, G. C. A.; Gates, B. C. Hydrodesulfurization of Thiophenic Compounds: The Reaction Mechanism. J. Catal. 1980, 61, 128−134. (4) Angelici, R. J. Heterogeneous Catalysis of the Hydrodesulfurization of Thiophenes in Petroleum: An Organometallic Perspective of the Mechanism. Acc. Chem. Res. 1988, 21, 387−394. (5) Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation: Chemistry and Engineering; Wiley-VCH: 2000. (6) Tran, P. D.; Chiam, S. Y.; Boix, P. P.; Ren, Y.; Pramana, S. S.; Fize, J.; Artero, V.; Barber, J. Novel Cobalt/Nickel-Tungsten-Sulfide Catalysts for Electrocatalytic Hydrogen Generation from Water. Energy Environ. Sci. 2013, 6, 2452−2459. (7) Topsøie, H.; Candia, R.; Topsøe, N. Y.; Clausen, B. S.; Topsøe, H. On the State of the Co-Mo-S Model. Bull. Soc. Chim. Belg. 1984, 93, 783−806. (8) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis; Springer: 1996. (9) Voorhoeve, R. Electron Spin Resonance Study of Active Centers in Nickel-Tungsten Sulfide Hydrogenation Catalysts. J. Catal. 1971, 23, 236−242. (10) Voorhoeve, R.; Stuiver, J. The Mechanism of the Hydrogenation of Cyclohexene and Benzene on Nickel-Tungsten Sulfide Catalysts. J. Catal. 1971, 23, 243−252. (11) Chianelli, R. R.; Berhault, G.; Raybaud, P.; Kasztelan, S.; Hafner, J.; Toulhoat, H. Periodic Trends in Hydrodesulfurization: In Support of the Sabatier Principle. Appl. Catal., A 2002, 227, 83−96. (12) Tongay, S.; Sahin, H.; Ko, C.; Luce, A.; Fan, W.; Liu, K.; Zhou, J.; Huang, Y.-S.; Ho, C.-H.; Yan, J. Monolayer Behaviour in Bulk ReS2 Due to Electronic and Vibrational Decoupling. Nat. Commun. 2014, 5, 3252. (13) Escalona, N.; Yates, M.; Á vila, P.; López Agudo, A.; García Fierro, J.; Ojeda, J.; Gil-Llambıas, F. Effect of Re Loading on the Structure, Activity and Selectivity of Re/C Catalysts in Hydrodenitrogenation and Hydrodesulphurisation of Gas Oil. Appl. Catal., A 2003, 240, 151−160. (14) Escalona, N.; Vrinat, M.; Laurenti, D.; Gil Llambías, F. Rhenium Sulfide in Hydrotreating. Appl. Catal., A 2007, 322, 113−120. (15) Lagos, G.; García, R.; Agudo, A. L.; Yates, M.; Fierro, J.; GilLlambías, F.; Escalona, N. Characterisation and Reactivity of Re/ Carbon Catalysts in Hydrodesulphurisation of Dibenzothiophene: Effect of Textural and Chemical Properties of Support. Appl. Catal., A 2009, 358, 26−31. (16) Laurenti, D.; Thi, K.; Escalona, N.; Massin, L.; Vrinat, M.; Llambías, F. Support Effect with Rhenium Sulfide Catalysts. Catal. Today 2008, 130, 50−55. (17) Leglise, J.; Finot, L.; Van Gestel, J.; Duchet, J. Conversion of Model Sulfur Compounds to Characterize Hydrodesulfurization Como/Al2O3 Catalysts. Stud. Surf. Sci. Catal. 1999, 127, 51−65. (18) Sullivan, D. L.; Ekerdt, J. G. Mechanisms of Thiophene Hydrodesulfurization on Model Molybdenum Catalysts. J. Catal. 1998, 178, 226−233. (19) Vrinat, M. The Kinetics of the Hydrodesulfurization Process-a Review. Appl. Catal. 1983, 6, 137−158. (20) Owens, P.; Amberg, C. Thiophene Desulfurization by a Microreactor Technique. Adv. Chem. Ser. 1961, 33, 182. (21) Hargreaves, A. E.; Ross, J. R. An Investigation of the Mechanism of the Hydrodesulfurization of Thiophene over Sulfided Co-Mo/Al2O3 Catalysts: II. The Effect of Promotion by Cobalt on the C-S Bond Cleavage and Double-Bond Hydrogenation/Dehydrogenation Activities of Tetrahydrothiophene and Related Compounds. J. Catal. 1979, 56, 363−376. I


Article

The Journal of Physical Chemistry C (45) Morin, C.; Eichler, A.; Hirschl, R.; Sautet, P.; Hafner, J. DFT Study of Adsorption and Dissociation of Thiophene Molecules on Ni (110). Surf. Sci. 2003, 540, 474−490. (46) Orita, H.; Itoh, N. Adsorption of Thiophene on Ni(100), Cu(100), and Pd(100) Surfaces: Ab Initio Periodic Density Functional Study. Surf. Sci. 2004, 550, 177−184. (47) Grimme, S. Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (48) Blyth, R. I. R.; Mittendorfer, F.; Hafner, J.; Sardar, S. A.; Duschek, R.; Netzer, F. P.; Ramsey, M. G. An Experimental and Theoretical Investigation of the Thiophene/Aluminum Interface. J. Chem. Phys. 2001, 114, 935−942. (49) Moses, P. G.; Hinnemann, B.; Topsøe, H.; Nørskov, J. K. The Hydrogenation and Direct Desulfurization Reaction Pathway in Thiophene Hydrodesulfurization over MoS2 Catalysts at Realistic Conditions: A Density Functional Study. J. Catal. 2007, 248 (2), 188− 203. (50) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (51) Shi, W.; Zhang, L.; Ni, Z.; Xia, S.; Xiao, X. DFT Investigations of the Adsorption and Hydrodesulfurization Mechanism of Thiophene Catalyzed by Pd (111) Surface. RSC Adv. 2014, 4, 58315−58324. (52) Gates, B.; Topsøe, H. Reactivities in Deep Catalytic Hydrodesulfurization: Challenges, Opportunities, and the Importance of 4Methyldibenzothiophene and 4, 6-Dimethyldibenzothiophene. Polyhedron 1997, 16, 3213−3217. (53) Houalla, M.; Nag, N.; Sapre, A.; Broderick, D.; Gates, B. Hydrodesulfurization of Dibenzothiophene Catalyzed by Sulfided CoO-MoO3γ-Al2O3: The Reaction Network. AIChE J. 1978, 24, 1015−1021. (54) Egorova, M.; Prins, R. Hydrodesulfurization of Dibenzothiophene and 4, 6-Dimethyldibenzothiophene over Sulfided NiMo/γAl2O3, CoMo/γ-Al2O3, and Mo/γ-Al2O3 Catalysts. J. Catal. 2004, 225, 417−427. (55) Vineyard, G. H. Frequency Factors and Isotope Effects in Solid State Rate Processes. J. Phys. Chem. Solids 1957, 3, 121−127. (56) Wert, C.; Zener, C. Interstitial Atomic Diffusion Coefficients. Phys. Rev. 1949, 76, 1169−1175.

J


Density Functional Theory Investigation on Thiophene

Recommend Documents