Periodic DFT Study on the Adsorption and Catalytic Desulfurization of

Article pubs.acs.org/JPCC

Periodic DFT Study on the Adsorption and Catalytic Desulfurization of Thiophene over VC(001) and VN(001) via Hydrogenation and Direct Pathways Eugenio Furtado de Souza,*,† Teodorico C. Ramalho,*,‡ and Ricardo Bicca de Alencastro† †

Instituto de Química, Programa de PG em Química, Universidade Federal do Rio de Janeiro, Ilha do Fundão, Rio de Janeiro, CEP 21941-909, Brazil ‡ Departamento de Química, Universidade Federal de Lavras, Campus Universitário - UFLA - CEP 37200-000, Lavras, MG 3037, Brazil S Supporting Information *

ABSTRACT: Periodic ab initio DFT calculations based on ultrasoft pseudopotentials and a plane-wave basis set were carried out to shed some light on the adsorption and reaction mechanisms of thiophene over vanadium carbide and vanadium nitride cubic face-centered (001) surfaces and its implications for hydrodesulfurization (HDS). Direct (DDS) and hydrogenating (HYD) routes were considered for further desulfurization reactions. Furthermore, to identify their role in the adsorption and desulfurization process, various initial configurations for the adsorption complexes were investigated. Our results suggest that ring hydrogenation does not necessarily lead to a preference for the HYD pathway of thiophene desulfurization. Moreover, surface and electronic effects due to adsorption were also evaluated. We have found that under ideal conditions, vanadium nitride and carbide surfaces present similar activity over desulfurization of thiophene, in qualitative agreement with experiments.

1. INTRODUCTION The imposition of increasingly stringent environmental restrictions on the sulfur content of petroleum derivative fuels, associated with its ever-growing global utilization, makes it necessary to begin efforts in research and development of new active phases capable of dealing with such a demand.1 The combustion of sulfur-containing compounds releases principally oxidized species (SOx), which are well-known catalyst poisons and air pollutants.2 Accordingly, among the most challenging chemical reactions is the desulfurization of S-aromatic molecules like thiophene (C4H4S)3−5 and its polyaromatic derivatives,6−9 which is generally carried out through the breaking of the ring S−C bonds. In the petroleum industry, one of the most widely used and efficient catalytic desulfurization processes is known as hydrodesulfurization (HDS).4 In this process, the S-containing feedstock is generally exposed in the presence of alumina-supported CoMoS or NiMoS catalysts to high temperatures and hydrogen gas (H2). Despite their efficiency, the use of these catalysts generally results in a reduction of the octane rating due to the saturation of olefins, and to lesser quality fuels and high consumption of hydrogen,4,5 which in turn opens new perspectives and motivates the development of cost-efficient and more active catalysts. The study of desulfurization properties of transition metal carbides and nitrides (TMCNs) is of great academic and technological interest. TMCNs possess extraordinary chemical © XXXX American Chemical Society

and physical properties arising from a combination of characteristics of ceramics and metals such as high melting point and excellent electric and thermal conductivities, which makes them good candidates for practical applications.10−13 They have also been considered as promising active phases11−13 including interesting HDS activity,1 qualitatively resembling those of expensive noble metals.7 However, despite the fact that a considerable number of studies on the interaction of thiophene with metal nitrides and carbides have been published,4,14,15 the most substantial work has been devoted to catalytic applications on systems containing Mo and/or W atoms, while we face relative scarceness when it comes to other TMCNs. Although vanadium nitrides and carbides were found to be catalytically active presenting good performance in numerous reactions,16−18 and therefore promising materials, experimental and especially theoretical studies on the surface properties, reaction mechanisms, and catalytic activity of vanadium carbides and nitrides on desulfurization processes are quite scarce.19,20 In line with these issues, DFT studies have made valuable contributions to the elucidation of the complex phenomena resulting from the interactions of S-containing species and surfaces providing qualitative and even quantitative Received: November 9, 2015 Revised: February 16, 2016

A

DOI: 10.1021/acs.jpcc.5b10993 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

for that reason, they were not taken into account in the present work. The surfaces used to investigate thiophene adsorption were modeled employing periodic boundary conditions (PBC), in which the elementary 2-atomic primitive unit cell vectors (rocksalt structure) were periodically translated twice along the main crystallographic axes. A thiophene molecule was then included on only one side corresponding to a coverage of 0.062 monolayer (ML). The slab was divided into four atomic layers, and a vacuum region of ∼15 Å along the z-axis, able to minimize spurious interactions between adjacent periodic replicas, was applied. In each case, surface relaxation was taken into account, and the two topmost layers together with the thiophene molecule were allowed to fully relax, while the bottom two layers were kept fixed in their bulk positions so as to represent a semi-infinite bulk crystal. As implemented in QE, dipole corrections were applied to cancel long-range dipole interactions caused by a charge rearrangement on the surface due to adsorption and interactions between the two surfaces of the slab. Adsorption energies, EAds, were calculated according to EAds = ESurf+Th − (ESurf + ETh). The first term, ESurf+Th, denotes the calculated energy of the surface slab plus the adsorbed thiophene molecule, whereas ESurf and ETh refer to the total energy of the carbide and nitride substrates and the free thiophene, respectively. An exothermic adsorption is characterized by a negative Eads. A single k point was used in the calculations for the gas-phase thiophene, embedded in a large cell (15 × 15 × 15 Å3), ensuring negligible intermolecular interaction, and providing good agreement between the calculated and experimental structural data.24 It is important to point out that the commonly used DFT exchangecorrelation functionals (PBE, for example) tend to underestimate adsorption energies because formally they do not take van der Waals (vdW) dispersion forces into consideration. Sony et al., for example, observed that the inclusion of vdW interactions could significantly influence the adsorption energy of thiophene over Cu(110).25 On the other hand, it was observed that the inclusion of long-range dispersion forces does not substantially improve the resulting orientations and other properties like charge density, electronic states, and work function when data for some aromatics like furan and thiophene, obtained by standard DFT-GGA calculations, are compared to those obtained from experimental data.25,26 Moreover, Moses et al.27 showed that the inclusion of vdW interactions in the adsorption and reaction profile of thiophene on MoS2 led to a constant lowering of the energy differences of the entire reaction path, which in turn is not affected by vdW interactions. Hence, the results reported here suggest that the chosen theoretical method is adequate and the results are reliable. Vibrational frequencies of thiophene adsorbed over VN(001) and VC(001) surfaces were computed by considering only the displacement of the molecule’s individual atoms, which means that surface atoms were explicitly kept frozen during the computations of the frequencies. The climbing-image nudged elastic band (CI-NEB) method28,29 was used as implemented in Quantum-ESPRESSO21 along with the quasi-Newton optimization scheme from Broyden to locate energy barriers and transitions states (TS) through the minimum energy path (MEP) by connecting two minima, initial (IS) and final (FS) states. Because of the high computational demand of the NEB calculations, we made use of the energetically most stable adsorption complexes (lower EAds) to assess the elementary steps involved in the direct

insights. Regarding TMCNs, some theoretical studies have been devoted to Mo and W compounds. Much less work, however, has been addressed to vanadium carbide and nitride, and therefore not much is known about their catalytic and surface properties.10 The adsorption of thiophene represents the first step in the catalytic cycle of desulfurization, and in many cases the S−C bond weakening/breaking mechanism is still not well understood.19,33 Hence, a fundamental understanding is necessary to develop refined or new active phases as well as to allow extensions to other TMCNs. With this in view, we carried out a detailed computational investigation on the aspects involved in thiophene adsorption and desulfurization processes over VC(001) and VN(001) surfaces. The (001) plane was first chosen because it generates a nonpolarized surface defined by an equal number of metal and nonmetal atoms, and also because it is energetically the most stable cleavage plane.20 However, a systematic study of the lowerindex surfaces of (110) and (111) should be explored in the future. Here, electronic and structural properties of the Thiophene@VC(001) [Th@VC(001)] and Thiophene@ VN(001) [Th@VN(001)] adsorption complexes were evaluated using the plane-wave periodic density functional theory by testing more than 20 different 73-atom supercell structures. Furthermore, the effects of thiophene hydrogenation were also studied, and the possible mechanisms involved in bond breaking were also discussed. We believe that the results reported here are very important because they provide qualitative trends that can help to predict and give support for future experiments, and shed some light on the molecular steps involved in thiophene desulfurization on carbides and nitrides. To the best of our knowledge, this is the first theoretical study devoted to the catalytic desulfurization properties of vanadium carbide and nitride surfaces.

2. COMPUTATIONAL MODELS AND METHODS Total-energy calculations were performed within the framework of the periodic Density Functional Theory (DFT) using the PWSCF computational code as implemented in the QuantumESPRESSO (QE) suite of programs.21 The Kohn−Sham electronic states were expanded in plane-waves up to a kinetic energy cutoff of 30 Ry (1 Ry ≈ 13.6 eV) and 240 Ry for the charge density cutoff. The Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation was used to calculate the exchange and correlation contributions. Geometry optimizations of the atomic positions were performed by using the BFGS algorithm. Integrals over the Brillouin zone were performed by sum of a 2 × 2 × 1 Monkhorst−Pack22 k-mesh grid, and the Marzari−Vanderbilt cold-smearing scheme23 was employed with a broadening of 0.01 Ry. Self-consistency was achieved when the variation of the total energy between two consecutive iterations was on the order of 0.0001 Ry, and the force applied on each atom due to structural relaxation was less than 0.001 Ry/Bohr. To check the convergence, we compared the total energy of systems calculated with finer k-mesh and one extra layer, which produced only small changes in the adsorption energy and no appreciable changes in the geometric parameters. Bulk calculations using a 12 × 12 × 12 grid show that the lattice parameters obtained for carbide and nitride, 4.154 and 4.112 Å, respectively, are quite close to the experimental data (4.172 and 4.126 Å, respectively).58 Spinpolarized computation tests (not presented here) showed convergence to a solution with negligible spin-polarization, and, B


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The Journal of Physical Chemistry C desulfurization process (DDS) of thiophene and in the hydrodesulfurization (HDS) pathways of thiophene Hderivatives, relying principally on the possible structures present under HDS conditions; the reaction steps investigated here were successfully explored in a variety of other computational studies.19,30 To locate the configurations of maximum energy along the MEP, which climb uphill to the saddle point and are identified as the TS, we specified eight images of the elementary steps that span the space between the optimized reactant and product configurations. The activation energy (EA) was estimated as the energy difference between TS and IS. Because we were first interested in the determination of the energetically most stable adsorbate@surface configuration, it was necessary to compare different adsorption orientations and their respective energies. Figure S.1 gives an overview of several possible active site geometries. In the first type, termed “flat” or “η5”, the thiophene molecular ring is parallel to the surface bonded through its π-system. On the other hand, the terminology “upright” or “η1” characterizes thiophene in a vertical position relative to the surface and bonded through the sulfur (S) lone pair of electrons. Thus, after different adsorption sites were tested in a total of 20 different structures with 73 atoms each (slab+thiophene), the most stable adsorption complex was found among 10 in η1 and 10 in η5 configurations. The adsorption energies of the evaluated configurations are found in Table S1.

3. RESULTS AND DISCUSSION 3.1. Energetics and Structural Properties of Thiophene Adsorption. Although the surfaces under study are able to accommodate a great number of possible adsorption configurations (see Figure S.1), Figure 1A and Table 1 indicate the most important properties of the more energetically favorable Th@VN(001) system. In this case, thiophene adsorbs preferentially in an upright (η1) fashion, centered above a surface hollow site with a corresponding adsorption energy of −0.68 eV. As seen, the molecular plane lies pointed to the surface nitrogen (nitridic) atoms, a feature that might be associated with the surface properties.36 The rocksalt (NaCl) structure is formed by interpenetrating layers of metal and nonmetal atoms, and a reduction of the surface energy is caused by a rippling relaxation of the surface atoms from which nonmetals are usually displaced outward, whereas the transition metal atoms move in the opposite direction. Our DFT calculations, in good agreement with LEED analysis, indicate that the VN(001) top layer is rippled and formed by two separated V- and N-subplanes, while the second layer is nearly planar (see Figure 1A).36 LEED experiments show that the top N-subplane is displaced 0.17 Å outside the V-subplane, which is in turn 1.92 Å away from the second layer. Both results agree well with our DFT calculations of ∼0.26 and ∼1.97 Å, respectively. During the adsorption process, to accommodate the thiophene molecule, we observed a considerable increase in the neighboring surface N−N, V−V, and V−N interatomic distances from 2.883, 2.896, and 2.066 Å to 4.148, 4.228, and 2.969 Å, respectively. Moreover, it is possible to observe that the thiophene intramolecular bond distances were only slightly elongated after adsorption, from 1.716 Å in the gas phase to 1.732 and 1.731 Å for S−C2 and S−C5 bonds, respectively, and from 1.420 Å (gas) to 1.429 Å for C3−C4. On the other hand, both C2−C3 and C4−C5 were in turn slightly shortened from 1.373 to 1.365 Å, respectively. Despite the structural variations, the adsorption process apparently caused only a minor damage

Figure 1. Configurations of the most stable structures for the thiophene Th@VN(001) (A) and the Th@VC(001) (B) adsorption systems. The optimized configurations are referred to the initial positions 1 (η1) and 3 (η1) over carbide and nitride, respectively (see Figure S.1).

to the ring aromaticity, because changes in bond distances were not substantial. Importantly, it can be seen in Table S1 that even though the adsorption complex with the η 1 (3) configuration type (see Figure S.1) is the energetically more stable, it is also almost isoenergetic with respect to thiophene in η5 (4) configuration, and therefore it is quite plausible that both coexist on the same surface. Accordingly, a more detailed analysis of the desulfurization process regarding thiophene (and some of its H-derivatives) in η5 (4) configuration can be found in the Supporting Information. Because of the high computational demand needed for the study of ring-opening reactions, the remaining minimum configurations shown in Figure S.1 will not be considered. Figure 1B and Table 1 depict the most stable structure found for the Th@VC(001) adsorption complex. Different from vanadium nitride, here thiophene adsorbs preferably with the molecular ring in a partially tilted configuration with the S atom covalently binding toward a surface V(Surf) site. Interestingly, before optimization, the thiophene S atom was directly bonded (η1) to a surface C atom (see Figure S.1, position 1 - η1). However, after successive self-consistent iterations, the molecular ring was tilted and displaced further away from its initial position. This feature has been also observed in the adsorption process of thiophene over niobium carbide.34 C


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Table 1. Adsorption Energies (eV) and Structural Parameters (Å) of the Energetically Most Stable Adsorption Complexesa surface VC(001)

molecule Th

2,5-DHTh

VN(001)

Th

Eads −0.44 gas ads. −0.76 gas ads. −0.68 ads.

dS−C2

dC2−C3

dC3−C4

dC4−C5

dC5−S

1.716 1.727

1.372 1.367

1.420 1.428

1.372 1.366

1.716 1.726

2.654

3.804

3.713

1.835 1.838

1.496 1.492

1.332 1.332

1.496 1.493

1.835 1.840

2.579

3.735

3.690

1.732

1.366

1.429

1.365

1.731

4.242/4.238

4.206/4.211

dS−V

3.070/3.082 dS−N

dC2−V

dC5−V

2.975/2.946 2,5-DHTh a

−1.07 ads.

1.837

1.494

1.333

1.493

1.837

2.789

3.765

3.862

Gas stands for the gas-phase parameters of thiophene and H-derivatives; ads. corresponds to the same molecules in the adsorbed phase.

VC(001) surfaces as well as the PDOS of the molecule C (2p) and S (2p) atoms (orbitals), in both gas and adsorbed phases, are shown in Figures 2 and 3, respectively.

Clearly, the molecular ring does not fully interact with the surface, thus producing an adsorption energy of −0.43 eV and an S−V(Surf) bond distance of 2.654 Å. The partially tilted η1 configuration over VC(001) produces negligible changes in the thiophene intramolecular bond structure, stretching the S−C2, S−C5, and C3−C4 bonds to 1.727, 1.726, and 1.428 Å, respectively, whereas C2−C3 and C4−C5 were slightly shortened from 1.372 Å (gas) to 1.366 Å. Our calculations showed no indications of substantial changes in ring aromaticity after adsorption, qualitatively analogous to theoretical results reported for thiophene adsorption on δ-MoC(001) and TiC(001) .14,42 Our calculations have also indicated a rippled relaxation in which the surface C atoms moved toward vacuum while the metal atoms moved to the inside, corroborating a general trend for this kind of TMCN surface.32,43,49 Therefore, the approximation of the thiophene molecule to the VC(001) clean surface may be also difficult to achieve, because the surface C atoms corrugate toward the outside, in some extension causing a steric hindrance.14 This effect might explain why, after being optimized from a surface C site, thiophene prefers to bind via its S end to a V(Surf) atom in a bent fashion. This configuration would represent a geometry that maximizes the interaction of the aromatic π-system with the VC(001) surface. On the experimental side, it was observed that VN-based catalysts tend to be tolerant to sulfidation. VC-based catalytic surfaces are more likely to interact with S under HDS conditions than VN. It has been observed that the concentration of S on the VN surface after HDS reaction is low, nearly 3%.18 Our DFT adsorption-energy calculations obtained through the equation Eads(S) = (E(S@VN\VC) + E(H2)) − (E(VN\VC) + E(H2S)) confirm this observation, and show that S can bind on the VN surface with an adsorption energy of [Eads(S)] −1.38 eV. On the other hand, a higher S/V ratio (0.10) was found on VC after a thiophene desulfurization reaction,18 and our DFT calculations are also in line with these experimental findings, because we have encountered a more exothermic value of Eads(S) (−1.60 eV) for the adsorption of S on VC. 3.2. Electronic Structure. 3.2.1. Density of States. The study of the orbital-projected partial density of states (PDOS) and its variations is useful to evaluate the contribution of a particular atom or electronic state to the thiophene adsorption process. The computed PDOS spectra projected on the 3d, 2p, and 2p states of selected V, C, and N atoms involved in the adsorption of thiophene on both the VN(001) and the

Figure 2. Partial electronic density of states (PDOS) for the vanadium nitride surfaces and thiophene before (VN(001) and Thgas) and after the adsorption process takes place. We consider the energetically most stable adsorption structure.

The analysis of PDOS (Figure 2) shows that adsorption acts on the electronic structure of the whole thiophene molecule. First, the region crossing the nitride’s Fermi level is dominated by V 3d states; thus the main effect of adsorption was to alter slightly the concentration of electronic states in the Fermi level vicinity (highlighted). This effect suggests a moderate to weak molecule−metal interaction due to the surface V 3d and the thiophene π-states (1a2,2b1), confirmed by the disappearance of the corresponding thiophene peak’s (gas) highest occupied orbitals located below the Fermi level. It is also possible to observe a partial filling of the molecule’s unoccupied (C−S antibonding) π*-states (3b1) due to interaction with metal states, in close similarity with the adsorption of aromatic D


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3.2.2. Difference Charge Density. To shed more light on the details of the interaction between thiophene and both the nitride and the carbide surfaces, charge density difference plots were evaluated according to eq 1: Δρ = (ρTh@Surf ) − (ρSurf + ρTh )

(1)

Here, ρSurf, ρTh, and ρ(Th@Surf) are the charge densities for the clean surface, thiophene, and adsorption complex, respectively. Depending on whether an atom gains or loses charge, the charge density can be either positive (Δρ > 0.003 e/Å3) or negative (Δρ < 0.003 e/Å3). Accordingly, red areas correspond to electron accumulation zones, whereas blue areas are electron depletion zones. The charge density difference contours of the selected systems are shown in Figure 4.

Figure 3. Partial DOS for the vanadium carbide surfaces and thiophene molecule before (VC(001) and Thgas) and after the adsorption process takes place.

molecules on metallic surfaces.30,44−47 An increase in the density of states in the region corresponding to the S (2p)-lone pair (9a1) state is also observed and might be associated with the interaction with the surface N 2p states. Accordingly, the analysis of PDOS shows that the main features of the electronic structure of the selected surface N atoms for both the Th@ VN(001) and the clean VN(001) systems are rather different, suggesting that the N atoms do not act merely like spectators during the adsorption, but are instead active participants in the process: the interaction with the surface N(Surf) p-states consists mainly of the electronic states of thiophene C−C and the S−C (2p) π-states (8a1,5b2,6b2,1b1), with a small contribution from V 3d states, shifting the surface N 2p states to higher energy levels, which in turn contributes to the relative stability of the η1 adsorption in a hollow position. The PDOS (Figure 3) of the VC(001) surfaces projected onto the V 3d states shows that the metallic states are important for the molecule−surface interaction. The interaction between V 3d and surface C 2p states characterized by low energy states within the range from −4 to −2 eV is attributed to chemical bonds between the V and C atoms.35,48 The V 3d states fall mainly in the Fermi level vicinity for the clean surface. After the thiophene adsorption, the density of the states around the Fermi level has been slightly perturbed. In other words, considering the absence of vdW forces, the change in the PDOS profile near the Fermi level after the adsorption indicates that the electronic states corresponding to the πsystem (bonding C−C and S−C π-states) orbitals, plus a contribution from the S-lone pair, are responsible for the interaction of thiophene with the surface through the overlapping with the V 3d states. On the other hand, the examination of the surface C 2p electronic states indicates a slightly unperturbed profile during thiophene adsorption. Thus, carbidic C(Surf) atoms also play a role in the adsorption process, according to qualitative differences of electronic structure before and after the adsorption takes place.

Figure 4. Differential electron density isosurfaces of adsorbed thiophene plus substrate minus clean substrate minus free thiophene. The charge density difference contour is shown where red (blue) corresponds to gain (depletion).

As can be seen in Figure 4A, regions of charge accumulation (red) and depletion (blue) suggest that the interaction of thiophene with VN(001) induces a local charge redistribution. It is possible that π*-states accept, via backdonation mechanism, charge density from the occupied V 3d states.51 The donor properties of thiophene, adsorbed on a variety of surfaces, have already been observed by other authors.52,53 The analysis of the Δρ contour of the Th@VC(001) system is distinctly different from the nitride case. Figure 4B shows that charge density reduces considerably in the region underneath the molecular ring (blue) and accumulates mainly in the region (red) between S and V atoms and V−C(Surf) bond, revealing the formation of a chemisorptive S−V bond. The red volume E


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Figure 5. Reaction pathway and relative energy profile for the direct desulfurization of the thiophene molecule on the carbide surface. The reference energy is thiophene on the VC(001) and thiophene in the gas phase. The insets show the possible structures of the calculated transition states (TS1,2DDS−VC), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the DDS pathway, whereas EPES refers to the relative energy on PES.

Figure 6. Reaction pathway and relative energy profile for the direct desulfurization of the thiophene molecule on the nitride surface. The reference energy is thiophene on the VN(001) and thiophene in the gas phase. The insets shows the possible structures of the calculated transition states (TS1,2DDS−Nitr), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the DDS pathway, whereas EPES refers to the relative energy on PES. F


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nitridic N(Surf), the participation of surface nonmetal atoms in the S−C bond-breaking process was found to be pronounced over VN(001). Moreover, according to our previous analyses and observed in the literature,12,57 V−N bonding in vanadium nitride has also an ionic character with charge transferring from metal to nonmetal atoms. As seen in Figure 6, the transition state (TS1DDS−VN) is further stabilized by an interaction between C2 and a slightly projected out-of-plane N(Surf) with C2−N(Surf) distance of 1.833 Å. Such a motion was expected because the N(Surf) subplane of a (100) VN surface also adopts a 1 × 1 LEED pattern due to atomic displacements normal to the surface plane,36 facilitating the participation of these atoms in the bond-breaking process. From the beginning of the S−C2 bond scission, the molecular ring was tilted down at an angle of ∼39° toward the surface, where S points to a V(Surf) atom. In the TS1DDS−VN, the S−C2 bond has a length of 2.109 Å. Energetically, this exothermic (−0.40 eV) elementary step is characterized by an activation barrier of 1.59 eV. The reaction proceeded through TS2DDS−VN from which we observed a C5− N(Surf) interaction of 2.306 Å and a low energy barrier of 0.17 eV. The final state is marked by the formation of a hydrocarbon fragment (C4H4) interacting only with the N(Surf) atoms, forming thus C2−N(Surf) and C5−N(Surf) bonds, whereas the S atom remains bound to a V(Surf). The whole process was found to be exothermic (−0.42). Unfortunately, to the best of our knowledge, there are no further experimental works available to validate these data, preventing quantitative correlations with the theoretical data. Overall, we noticed that the VN(001) desulfurization properties under ideal conditions might be to some extent compared to those of VC(001). Despite being slightly endothermic (0.33 eV), the desulfurization reaction on VC(001) would have a slightly lower kinetic barrier to overcome, while the breaking of thiophene S−C2 was found to be the rate-limiting step over VN(001). Experimentally, the capability of VC and VN-based catalysts in HDS reactions was found to be quite similar, albeit a slightly higher activity of VC catalysts was observed,18,19 which in turn might be associated with the presence of surface carbidic C-vacant active sites (nonmetal vacancies). It is important to mention that the existence of surface vacancies was not taken into account in our supercell approach. It is known that, although the NaCl-type crystal structure of VC remains stable over a wide range of nonstoichiometric surface imperfections and defects,16 the desirable catalytic properties and reactivity might be substantially affected by their presence. The study of the influence of these features on the catalyst electronic structure still represents a challenge to theorists, and it is beyond the scope of this Article. 3.4. Desulfurization of Thiophene Hydrogenated (H−) Derivatives: HYD Pathway. Under realistic HDS conditions, there might be a competition between direct S−C rupture and previous ring hydrogenation mechanisms.1,26 In this section, DFT studies were performed to assess the desulfurization properties of vanadium carbide and nitride by taking into account the case of hydrogenated thiophenes. Additionally, a description in greater detail of the H2 adsorption and dissociation over the surfaces, hydrogenation paths, and hydrogenated derivatives can be found in the Supporting Information. Here, we describe the desulfurization and hydrogenation processes of some hydrogenated derivatives: 2and 3-monohidrothiophene (2- and 3-MHTh), 2,3- and 2,5dihidrothiophene (2,3- and 2,5-DHTh), and tetrahidrothio-

indicates that charge transfers from S and from occupied V dstates. Furthermore, it is also possible to observe charge transfers from V d-states to a neighbor carbidic surface C(Surf) atom, in agreement with NEXAFS investigations,37,56 showing that the V−C bond of vanadium carbide is characterized by significant ionic contribution due to the difference in metal− nonmetal electronegativity.48 3.3. Thiophene Desulfurization: The DDS Pathway. 3.3.1. DDS of Thiophene on VC(001). The DDS pathway includes two subsequent S−C bond breakings and no participation of hydrogen species (Figure 5). Here, the pathway was investigated taking into consideration the relative energies of the Th@VC(001) system, and the zero energy fixed at the initial adsorbed state (Figure 4). As suggested by previous experiments, the S−C bond scission can occur before the ring hydrogenation through the inclusion of surface atoms via covalent bond to stabilize the resulting fragments.38,54 Thus, the DDS double-step mechanism of the S−C bond scission includes the formation of a thiolate-like (first step) and separate species (second step), C4H4S, and S + C4H4, respectively. It is acceptable that under typical HDS conditions, the C4H4 species formed via DDS are readily hydrogenated to form hydrocarbon gaseous products.1,19 It is also proposed in the literature that highly reactive species such as thiolates behave as key intermediates in a variety of desulfurization mechanisms.1,54 In the present work, for the first step, our findings indicate an endothermic reaction energy (1.42 eV) with a corresponding activation barrier of 1.49 eV. At the transition state (TS1DDS−VC), the S−C2 bond is partially broken at a bond distance of 2.578 Å, while the intramolecular bond distances, C2−C3, C3−C4, C4−C5, and S−C5 were practically unchanged in comparison with the corresponding initial phase. Despite the low barrier for the reverse reaction (∼0.07 eV), we assume that the formation of thiolate is likely to occur because at the TS1DDS−VC the dissociating S−C2 bond already interacts with a V(Surf) atom via thiophene C2 and with a bond length of 2.196 Å. As seen in the second step of the DDS pathway (Figure 5), the TS2DDS−‑VC was stabilized by S−V interaction with a distance of 2.275 Å and bonding of thiophene C5 with a carbidic C(Surf) atom (C5−C(Surf) = 2.165 Å). As shown by our previous analyses (section 3.2) and by other authors in similar systems,50,55,56 the surface nonmetal atoms can play a role over surface−molecule interaction.12 The effective participation of carbidic C(Surf) atoms in surface VC chemistry was also observed in a variety of different studies.48 Moreover, LEED 1 × 1 patterns show that the structure of the substoichiometric VC(001) surface presents an outward movement of the first layer C(Surf) atoms,43 in accordance with our surface model (Figure 1). Furthermore, we noticed an extra protruding motion of the carbidic C(Surf) atom (∼0.04 Å) involved in the S−C5 bond-breaking process, which in turn would facilitate the interaction with the thiophene C5 atom. Consequently, with regards to the forward reaction, a relatively low kinetically favorable reaction barrier of 0.46 eV was found, while the whole process was found to be slightly endothermic (0.33 eV). 3.3.2. DDS of Thiophene on VN(001). We now explore the relative energies of S−C bond-breaking calculated with the zero of the energy set at the initial adsorbed state, Th@VN(001), as shown in Figure 6. Despite the existence of vacancies in the bulk, Gauthier et al. demonstrated that the VN surface structure is essentially different from the bulk because the presence of vacancies was not detected on it.36 So, due to the nature of the thiophene adsorption (η1@hollow) and the interaction with G


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Figure 7. Relative energy diagram for the hydrodesulfurization of 2,3-dihydrothiophene (2,3-DHTh) on the carbide surface. The reference energy is 2,3-dihydrothiophene on the VC(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Carb), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

Figure 8. Relative energy diagram for the hydrodesulfurization of 2,5-dihydrothiophene (2,5-DHTh) on the carbide surface. The reference energy is 2,5-dihydrothiophene on the VC(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Carb), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

be the rate-determining step in desulfurization reactions,19 and for that reason in all systems an equilibrium between H2(gas) and adsorbed H atoms is assumed. 3.4.1. HYD Pathway over VC(001). First Hydrogenation. Figures S.7 and S.8 show the optimized adsorption structures of the hydrogenated thiophene compounds, 2-MHT and 3MHTh@VC(001), respectively, from which the relative energy

phene (THT). For simplicity’s sake, we limited ourselves to the description of the desulfurization process through the hydrogenation pathway (HYD) by considering only S−C2 and S−C5 bond-breaking, because it has been demonstrated by studies in similar systems that the scission of ring C−C bonds is expected to be kinetically more difficult than the former cases.61 It is worth mentioning that the dissociation of H2 is not believed to H


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Figure 9. Relative energy diagram for the hydrodesulfurization of THT on the carbide surface. The reference energy is the THT@VC(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Nitr), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

values were obtained. The hydrogenation process of the C3 ring presented a higher hydrogenation barrier (2.16 eV) when compared to the C2 position (1.44 eV). In both cases, the ring plane is distorted, while the 2-MHTh adsorption energy (−1.32 eV) is lower in comparison with 3-MHTh (−1.15 eV). Regarding the 2-MHTh ring-opening (S−C2H2) reaction (first step), we found an energy barrier of 1.09 eV and endothermic reaction energy of 0.36 eV. On the other hand, the same process involving the 3-MHTh molecule possesses a considerably higher activation barrier (2.51 eV) for the first step (S−C2H). Interestingly, the activation barrier for the first step in 2-MHTh is lower when compared to those found for thiophene via the DDS mechanism (section 3.3.1), suggesting that the hydrogenation in C2 has a destabilizing effect over thiophene S−C bond. However, the second elementary step of 2-MHTh desulfurization (S−C5H bond-breaking) has a relatively high activation barrier (3.13 eV) and configures an endothermic reaction (0.10 eV). The second step of the bond S−C (S−C5H) bond scission in 3-MHTh resulted in an activation barrier of 1.72 eV and an endothermic reaction energy of 0.09 eV. Second Hydrogenation. Here, the scission process of the thiophene S−C bonds was evaluated regarding both 2,5-DHT (Figure S.5) and 2,3-DHT@VC(001) (Figure S.10) adsorption complexes due to the fact that the difference in the adsorption energies found between them is small (less than 0.01 eV). In the first case (2,3-MHT), ring-opening starts with the breaking of the S−C2H2 bond followed by a rupture of the nonhydrogenated S−C5H bond, as shown in Figure 7. This

elementary step accounts for a surprisingly high activation energy of 4.71 eV and exothermic reaction energy of −0.16 eV. The scission of the non-hydrogenated part (S−C5H) is even less kinetically favorable (activation barrier of 5.55 eV) as well as endothermic (0.95 eV). Regarding the second case (2,5DHTh), the scission of the hydrogenated S−C2H2 bond in 2,5DHTh is the starting point of the desulfurization process, followed by the rupture of S−C5H2, forming the coadsorbed species S and C4H6 (butadiene). These products are wellknown in the literature, and are formed in a great variety of HDS reactions.19,39−41 The literature shows that these species generally undergo fast hydrogenation and subsequent isomerization yielding butenes and butane as well as small quantities of methane, ethane, and propane, among other products.40 For the reasons highlighted above, reactions involving 2,5-DHTh have been analyzed in more detail. Accordingly, Figure 8 depicts the reaction energies, structures, and transition states for the first and second bond-breaking stages (TS1HYD−VC and TS2HYD−VC, respectively). The cleavage of the S−C2H2 bond forms cis-2-butenethiolate after a high activation barrier of 4.54 eV is overcome, with an endothermic reaction energy of 0.65 eV. A similar situation was observed in the second step: in TS2HYD‑2‑VC, the rupture of the S−C5H2 bond causes a rotation of the C2H2 group, and this elementary step accounts for an activation energy of 2.36 eV and a total endothermic reaction energy of 0.05 eV. As seen, overall the activation barriers regarding S−C bond scission via HYD mechanism for both dihidrogenated thiophene are significantly higher than those of the DDS mechanism. Tominaga and Nagai59 have also found I


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Figure 10. Relative energy diagram for the hydrodesulfurization of 2,3-dihydrothiophene (2,3-DHTh) on the nitride surface. The reference energy is 2,3-dihydrothiophene on the VN(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Carb), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

C4H6 from the cleavage of 2,5-DHTh S−C bonds would be hampered because the formation of 2,5-DHTh would not be a straightforward event. Obviously, this is speculative, and more studies are needed to elucidate the effect of ring hydrogenation on the rise of bond-breaking energetic barriers. Although the literature shows that under realistic HDS conditions subsequent hydrogenations to form tetrahydrothiophene (THT) have equilibrium limitations and hence THT is expected to be an intermediate only at high H2 pressure and low temperatures,19 the S−C2H2 and S−C5H2 bond-breaking processes in THT (shown in Figure 9 and Figure S.12) were also explored in this Article. Here, we found that ring-opening reactions follow the same tendency as shown for previous hydrogenated systems, being therefore difficult mainly due to the chemical stability of the molecule. The scission of the S− C2H2 bond presents a high activation barrier (5.22 eV) and an endothermic reaction energy of 0.47 eV; the second elementary step (S−C5H2) was also found to be kinetically difficult because high activation energy (4.66 eV) has been identified, whereas the total reaction energy is endothermic (0.58 eV). Hence, both the adsorption configuration (Figure S.12), which keeps the ring C atoms apart from the surface, and the high energy barriers for S−C scission suggest that if formed THT would not easily dissociate over vanadium carbide. 3.4.2. HYD Pathway over VN(001). First Hydrogenation. Here, we noticed that the adsorption of 2-MHTh is energetically more favorable than that of 3-MHTh and that the difference in adsorption energies is 0.24 eV. Considering

considerably high activation barriers of the thiophene desulfurization process over clean and sulfided β-M2C(001) under hydrogenating conditions. They pointed out that during some steps the molecule’s torsion angles largely changed, which was due to the high activations. In this case (2,5-DHTh), the hydrogenation affects the S−C bond lengths to a very small extent (see Table 1), and for that reason it would contribute weakly to the S−C bond-breaking. Moreover, both dihydrogenated molecules possess no unpaired electrons, and for that reason they should be more stable. Thus, from the calculated energy barriers, it is clear that in general the HYD pathway disfavors the S−C bond-breaking process, which in turn proceeds more readily via DDS mechanism. Such results indicate that the main HDS products (butanes and butenes60) are formed via hydrogenation of the C4H4 fragment;30 such process was not evaluated in this Article. Even though the reasons for the substantial increase in the energy barriers are not yet fully known, we have observed that the extra H atoms are not “fixed” in the ring C2 and C5 atoms during the bond scission process, causing intramolecular distortions and in turn increasing the activation barriers. Thus, the scission of S−C bonds in hydrogenated thiophenes might in turn involve more steps than those accounted in the present work, like the activated breaking of C−H bonds, which leads to H migration. The literature shows that the partially ionic character of V−C (V−N) bonds in vanadium carbides (nitrides) might somehow explain their selectivities in dehydrogenation reactions with C4 hydrocarbons.12,16 Consequently, in our case, the formation of J


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Figure 11. Relative energy diagram for the hydrodesulfurization of 2,5-dihydrothiophene (2,5-DHTh) on the carbide surface. The reference energy is 2,5-dihydrothiophene on the VN(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Carb), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

fore it is important to understand how S−C bond scissions occur in both. Concerning the 2,3-DHTh@VN(001) adsorption complex, we identified high a energy barrier (4.84 eV) for the first step of S−C bond breaking and a reaction energy of −0.95 eV, as depicted in Figure 10. Interestingly, the second elementary step presents a considerably lower activation barrier (1.72 eV), and the final reaction energy has been found to be slightly exothermic by −0.19 eV. The HYD pathway for the 2,5DHTh@VN(001) adsorption complex is shown in more detail in Figure 11. The calculated activation energy for this elementary step was 4.81 eV. Again, a substantial increase in the activation barrier was observed when compared to the DDS pathway. During such step, the molecular ring tilts slightly down toward the surface parallel plane by an angle of ∼44°. At the end of this exothermic step (−0.46 eV), the C2H2 group is covalently linked to a N(Surf) site with a bond distance of 1.477 Å, and the length of the S−V(Surf) bond is shortened from 2.643 to 2.352 Å. For the formation of the coadsorbed S+C4H6 species, the system has to overcome a high energy barrier of 4.43 eV at TS2HYD‑2‑VN with a slightly endothermic reaction energy of 0.01 eV. At TS2HYD‑2‑Nitr, the breaking initiates at the S−C5 bond along with the forming of C5−V. Moreover, as observed for vanadium carbide, once the thiophene ring was fully hydrogenated forming THT on vanadium nitride (as represented in Figure 12 and Figure S.12), even if the reaction energies were found to be exothermic (first S−C bond scission, −0.58; and second S−C bond scission, −0.62 eV), it proved to be kinetically troublesome for the THT molecule to dissociate over VN(001) due to the high energetic barriers involved in both the first (5.20 eV) and second (4.12 eV) elementary steps of the thiophene desulfurization reaction. Furthermore, the ring

the energetics involved in thiophene ring hydrogenation over VN(001), our theoretical calculations have indicated that this process takes place preferentially on the C2 ring to form the 2MHTh@VN(001) (Figure S.9) system; the corresponding activation barrier for the first hydrogenation is 1.28 eV. When we turned to the 3-MHTh@VN(001) system, it was not possible to estimate the activation barrier due to a lack of convergence of NEB calculations. Such a situation would be expected if one takes into consideration that thiophene is adsorbed in a perpendicular configuration regarding the surface plane (see Figure 1A), making difficult the hydrogenation of C3 through the mechanism we are evaluating here (Langmuir− Hinshelwood), in which we have considered the presence of hydrogenating species previously adsorbed on the surface. So, C3 hydrogenation might occur through a distinct mechanism (not evaluated), and for that reason a TS could not be identified for S−C bond scissions in 3-MHTh. Regarding the first system (2-MHTh), the energy barrier and the reaction energy found for the S−C2H2 ring opening are 1.77 and 0.38 eV, respectively, while the second step (S−C5H bond breaking) has an activation energy of 0.83 eV and an exothermic reaction energy of −0.78 eV. Second Hydrogenation. As for the second step of thiophene hydrogenation on the VN(001) surface, the difference in the adsorption energies between 2,5- and 2,3-DHTh is relatively small (0.04 eV); the adsorption configurations are shown in Figures S.4 and S.11, respectively. As previously shown (3MHTh), here we also did not identify a transition state relative to the second hydrogenation step of C3 to form the 2,3-DHTh system. However, due to the small difference of adsorption energies between the adsorbed 2,3 and 2,5-DHTh, they are probably thermodynamically competitive systems, and thereK


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Figure 12. Relative energy diagram for the hydrodesulfurization of THT on the carbide surface. The reference energy is the THT@VN(001) surface and the same molecule in the gas phase. The inset shows the possible structures of the calculated transition states (TS1,2HDS−Nitr), intermediate, and final product. Ea2 stands for the activation barrier in the second step of the HYD pathway, whereas EPES refers to the relative energy on PES.

nitride (001) surfaces. The theoretical catalytic performance of these surfaces toward desulfurization reactions has been accessed by considering different adsorption models. We have observed that, over VC(001), thiophene molecules in a partially tilted η1 configuration are favored. In this fashion, the energy is minimized by the ring S atom, which forms a covalent bond with a V(Surf) atom, although only small modifications in the intramolecular structure could be observed. On the VN(001) surface, thiophene molecules prefer, in principle, to interact with the ring orthogonally located on a surface hollow site, although other adsorption configurations might coexist due to small differences in adsorption energies. Our calculations confirm that the surfaces present certain resistance to sulfidation deposition due to the relatively low calculated adsorption energies of sulfur atoms o the surfaces. Furthermore, the analysis of the electronic properties confirms to some extent the participation of the nonmetal surface atoms in the adsorption and reactions. In addition, the ring hydrogenation and S−C bond-breaking processes of thiophene and some of its most important H-derivatives have been evaluated via HYD mechanism. The theoretical investigation of the S−C bond breaking of these derivatives revealed that the HYD pathway does not favor the desulfurization process because activation barriers have in general increased for such reaction after ring hydrogenation. Qualitatively, for both surfaces, this increase of the energy barriers caused by ring hydrogenation reveals a preference for the DDS pathway instead of HYD for ringopening reaction and complete desulfurization of thiophene in resemblance to noble metal surfaces. Therefore, important

carbons were found to be considerably separated from the surface, hindering any kind of interaction with the surface. Under real conditions, it was observed that vanadium carbide and nitride catalysts present lower HDS activities when compared to other TMCNs and other commonly used catalysts,60 although we have noticed that qualitatively they seem to resemble the main characteristics of the noble metal platinum surfaces, in which the DDS pathway is, in principle, preferred over HYD in desulfurization processes.30,31 Overall, in theory, our results provide some indications as to why, experimentally, such compounds have not been observed as efficient catalysts for the activation of S compounds in desulfurization processes. On the other hand, the results could be used to tailor the catalytic properties of these important materials, although more studies are necessary for proper description and comprehension of the systems, especially in the presence of hydrogenating species, and notably on the effect of vacancies and other atoms possibly present in the crystal lattice (oxygen, for example, or the formation of alloys like cobalt and molybdenum among others) as well as the presence of non carbidic C, O, S, and H on the surface and the effect of vdW forces on the theoretical side.

4. CONCLUDING REMARKS Periodic ab initio DFT calculations have been used to study the adsorption process and desulfurization reactions of the thiophene molecule (Th) and some of its hydrogenated derivatives (2-MHTh, 3-MHTh, 2,3-DHTh, 2,5-DHTh, and THT), over the cubic face-centered vanadium carbide and L


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products of HDS such as butanes and butenes would be formed through hydrogenation of the C4H4 fragment obtained from DDS of thiophene.

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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.5b10993. A schematic configuration for the thiophene starting positions on the vanadium carbide and nitride surfaces as well as adsorption energies for 20 different thiophene@ surface structures; supplementary data and discussion on the vibrational modes of the free and adsorbed thiophene molecule on both VN(001) and VC(001) surfaces; further discussion on the hydrogenation reactions and adsorbed H-derivatives; and additional results and discussion on the ring-opening process involving the η5(4) thiophene configuration (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +55-21-3938-7132. E-mail: [email protected]. *Phone: +55-35-38291552. Fax: +55-35-38291271. E-mail: [email protected]fla.br. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Conselho Nacional de ́ Desenvolvimento Cientifico e Tecnológico (CNPq), Cooŕ Superior denaçaõ de Aperfeiçoamento de Pessoal de Nivel (CAPES), Fundaçaõ de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), and Fundaçaõ de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).

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REFERENCES

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