Mechanisms and Energies of Water Gas Shift Reaction on Fe-, Co

Nov 10, 2012 - State Key Laboratory of Coal Conversion, Institute of Coal ... The reaction mechanisms and promoting effect in the water gas shift reac...
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Mechanisms and Energies of Water Gas Shift Reaction on Fe‑, Co‑, and Ni-Promoted MoS2 Catalysts Yan-Yan Chen,† Mei Dong,† Jianguo Wang,*,† and Haijun Jiao*,†,‡ †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, People's Republic of China ‡ Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ABSTRACT: The reaction mechanisms and promoting effect in the water gas shift reaction on (Fe, Co, Ni)-MoS2 have been computed at the level of density functional theory. The Mo-edge with 25% M substitution and 25% sulfur coverage is identified as the active site. On the redox reaction path (CO + H2O → CO + OH + H; OH → O + H; CO + O + 2H → CO2 + 2H), surface OH dissociation into O + H is the rate-determining step. On the carboxyl reaction path (CO + OH → COOH; COOH → CO2 + H), surface COOH dissociation into CO2 + H is the rate-determining step. On the basis of the computed effective barriers, the redox mechanism is kinetically more preferable than the carboxyl mechanism for Mo, Fe, and Co, whereas both mechanisms have very close effective barriers for Ni. Compared with pure MoS2, Ni has the largest promotion effect in reducing the effective barrier (0.72 eV), followed by Co (0.45 eV), whereas Fe has the smallest effect (0.04 eV). This promotion effect is tightly associated with the electronegativities and number of the valence electrons of the metals. As the best catalyst, NiMoS2 has the lowest adsorption energies of the surface intermediates and also the lowest effective barrier.

1. INTRODUCTION Molybdenum-based catalysts have found wide applications in hydrodesulfurization and CO hydrogenation,1−7 in synthesis of methanol and higher alcohols,8−10 and in the water gas shift (WGS) reaction (CO + H2O → CO2 + H2). Many experimental1−7 and theoretical11,12 studies have been performed to elucidate the reaction mechanisms as well as to investigate the catalytic active species and the roles of promoters and supports over MoS2-based catalysts. Two main reaction paths have been proposed for the WGS reaction: redox and associative paths. The redox path involves complete water decomposition to atomic oxygen (H2O → 2H + O), followed by CO oxidation to CO2.13−17 Hou et al.2 and Lund18 favored the redox path wherein the catalyst surface is alternately oxidized by H2O and reduced by CO from a microkinetic model for the kinetics over MoS2/Al2O3. The associative path involves the formation of surface formate (HCOO) from CO and OH, and HCOO decomposition into CO2 and H.19−22 In addition, the mechanism involving a reactive surface carboxyl (COOH) was proposed on Cu(111)23 from density functional theory (DFT) study and on Au(111)24 from near-edge X-ray adsorption fine structure, infrared adsorption spectroscopy, and DFT studies. Metal doping can enhance the catalytic activity of the WGS reaction. The WGS reaction activity of Ce(La)Ox was increased significantly by adding a small amount (2 wt %) of Cu or Ni.25 In particular, MoS2 catalysts promoted by Ni and Co are most active and widely used in the WGS reaction.26−28 Other metals (Fe, Co, Ni, Au, Pt) were used to study the role of metal in the catalysis of metal/ceria systems for WGS, and one important role of the metal is to catalyze the reduction of the surface ceria.29 For MoS2-based catalysts, it is found that Ni enhances © 2012 American Chemical Society

the reducibility of Mo species and is responsible for the synergy of Ni-MoS2-based catalysts and the higher catalytic activity,6 and nickel hydroxide from hydrolysis manifests the high catalytic activity in the WGS reaction.30,31 The higher catalytic activity of the WGS reaction on the presulfided Co-Mo/MgOAl2O3 catalyst is attributed to the optimal formation of the active Co-MoS2.32 Previous DFT study33 showed that Co plays the role of a textual promoter in creating more active sites and accelerating the reaction rate. In this study, periodic self-consistent and spin-polarized DFT calculations were performed to investigate the WGS reaction on Fe- and Ni-MoS2 catalysts. The adsorption of reactants, products, and intermediates on the surfaces has been investigated. The redox and COOH-mediated mechanisms, involving H2O dissociation, COOH formation and dissociation from CO and OH, and the formation of CO2 along with the transition states, have been studied. Previous results on CoMoS2 and pure MoS2 surfaces are compared.

2. METHODS AND MODELS Bulk MoS2 forms a hexagonal crystal lattice with a layer-type structure described by weakly bonded sulfide layers parallel to the (001) net-plane, and (100)12,34 is considered as the active surface. Figure 1a shows the ideal bulk model with four S-Mo-S rows in the z direction and four S-Mo-S units in the y direction. Figure 1b shows the most stable structures of two nonequivalent sheets terminated by sulfur, one sulfur atom per surface Mo under hydrogen-rich conditions for a H2S/H2 ratio Received: August 23, 2012 Revised: November 2, 2012 Published: November 10, 2012 25368

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equilibrium geometry, Etot(slab) is the total energy of the free slab, and Etot(X) is the total energy of the free adsorbate (X) in the gas phase. A negative Eads represents an exothermic adsorption. Eads = Etot(X/slab) − Etot(X) − Etot(slab)

(1)

Relative energies of the surfaces with different sulfur coverage were calculated according to the methods in the literature.35,36,47−49 The free energy change by adding n sulfur atoms to the reference surface at temperature T, pH2S, and pH2 were obtained from the energies of the reaction, S*1 + nH2S = S*2 + nH2, where S*1 stands for the reference surface and S*2 stands for the sulfided surface. The free energy of this reaction from eq 2 can determine the stable surfaces under experimental conditions. For H2 in the gas phase, μ = E0(H2) + G0(T) + RT ln(pH2/p0), whereas in the solid state, μ = E0. Therefore, the free energy is approximated by eq 3, where ΔE0 is the standard energy change at 0 K and ΔG0 is the temperature correction for free energy change from 0 to T K, and 575 K is used in this work. ΔG = [μ(S*2) + n(μ(H 2))] − [μ(S*1) + μ(H 2S)]

(2)

ΔG = ΔE0 + nΔG0(T ) + nRT ln(pH 2 /pH 2S)

(3)

3. RESULTS AND DISCUSSION 3.1. Energies and Geometries of M-MoS2 Edge Surfaces. Faye et.al. studied an alumina-supported MoS2 hydrotreating catalyst using the DFT approach and showed that alumina only acts as a dispersant for the active phase.50 Therefore, we chose a periodic single M-MoS2 layer to be representative of real catalyst sites for the WGS reaction. It is accepted that promoter atoms are located at the MoS2 edges, and the presence of promoter atoms at the edges is shown to lead to a significant lowering of the metal−sulfur binding energy. 34,51−54 In the previous study on the Co-MoS 2 catalyst,33 it is found that only the Co-promoted Mo-edge with 25% sulfur coverage is active for WGS, whereas the Copromoted S-edge with 50% sulfur coverage is inactive due to the positive H2O adsorption energy. Calculations on Fe- and Ni-promoted S-edges with 50% sulfur coverage also show positive H2O adsorption energies. Therefore, we present only the results on M-promoted Mo-edges with 25% sulfur coverage and discard those on M-promoted S-edges with 50% sulfur coverage. We calculated the most stable structures using eq 3. Figure 2 shows the free energy change with the increase of the pH2/ pH2S ratio; and 50% sulfur coverage is used as the reference edge. On the M-promoted Mo-edge, 25% sulfur coverage represents the most stable surface at very high pH2/pH2S ratios, and 37.5% sulfur coverage also is stable at high pH2/pH2S ratios. An equilibrium between two sulfur coverages at a given pH2/pH2S ratio may exist. In contrast, the M-promoted Moedge with 12.5% sulfur coverage has a positive free energy and is, therefore, not stable. Because H2 usually has a much higher partial pressure than H2S under real hydrogenation conditions, 25% sulfur coverage on the M-promoted Mo-edge is used as our model (Figure 3). The promoted metals relax inward to the surfaces by 0.39, 0.46, and 0.42 Å for Fe, Co, and Ni, respectively. In addition, comparing the free energy changes shows that the pH2/pH2S ratio decreases in the order of Fe (2.2 × 104) > Co (20) > Ni (0.02) at the crystal 25% sulfur coverage

Figure 1. Ideal bulk (a) and sulfur reconstructed (b) MoS2(100) surfaces (green for Mo atoms and yellow for S atoms).

< 0.05, reflecting the real reaction condition.35−38 A vacuum gap of 15 Å is used to separate the slab in y and z directions, which is found to be large enough to avoid electronic coupling between the adjacent slabs. In geometry optimization, the atoms in the two inner S-Mo-S rows are fixed as in their bulk structure and other atoms at both edge surfaces are relaxed. Plane-wave DFT calculations for the electronic properties and the interaction of reactants, intermediates, and products were performed using the Vienna Ab initio Simulation Package (VASP).39,40 The projector-augmented wave (PAW) method41 and the generalized gradient approximation with the Perdew− Wang42 exchange-correlation functional (GGA-PW91) were used. The kinetic energy cutoff for a plane-wave basis set was 400 eV. We applied Monkhorst−Pack mesh k-points of (2 × 1 × 1) for bulk and surface calculations, allowing convergence to 0.1 meV for the total electronic energy and Co > Ni, except for H, COOH, and CO2, and their adsorption on Co (−2.32, −2.97, and −0.45 eV, respectively) is stronger than that on Fe (−2.19, −2.56, and −0.37 eV, respectively). In addition, the adsorption energies of CO, H, OH, and O on the Mo2 site of M1-MoS2 are lower than those on the Mo2 site of pure MoS2, and only those of H2O on the Mo2 site of the M1-MoS2 surface are slightly higher than those on the Mo2 site of pure MoS2. 3.3. WGS Activity. It is impossible to form HCOO directly from coadsorbed CO and OH, since it has to break the C−Mo and O−H bonds and to form the O−Mo and C−H bonds. HCOO formation can be considered as the addition of surface H to free CO2, and HCOO is a side product and plays the role of a spectator and does not participate directly in the WGS reaction.33 Therefore, HCOO formation on Fe/Ni-MoS2 surfaces is not considered, while the redox and COOHmediated mechanisms are calculated. The energy barriers of all steps are shown in Table 3, and the structures of the coadsorbed reactants and products and the structures of the transition states of the elementary steps are shown in Figures 5 and 6. The adsorption and reaction sites of the reactants and

superiority on the M-promoted Mo-edge; that is, under the same conditions, the Ni-MoS2 surface has the lowest sulfur coverage and the Fe-MoS2 surface has the highest sulfur coverage. It is in line with the result that the sulfur coverage on the edge decreases in the order of MoS > CoMoS > NiMoS under the same conditions.34 The changes in electronic properties and catalytic activities of the catalysts are calculated (Figure 4). On pure MoS2, the dprojected density of states (DOS) shows that Mo1 with 4-fold coordination is closer to the Fermi level than Mo2 with 5-fold coordination; and Mo1 is more active than Mo2. On M-MoS2, the DOS of Fe1 shifts toward the Fermi level and that of Co1 shifts below the Fermi level, whereas that of Ni1 is farther away from the Fermi level. Bader charge analysis (Table 1) shows that M1 is less positively charged than Mo1 on pure MoS2, whereas Mo2 and Mo3 are more positively charged by M1 promotion, indicating electron transfer from Mo2 and Mo3 to M1. It also is found that the charge of M1 with 4-fold coordination is in the order of Mo (0.93e) > Fe (0.72e) > Co (0.43e) > Ni (0.40e), in line with the order of the Allen electronegativity55 in the order of Mo (1.47) < Fe (1.83) < Co (1.84) < Ni (1.88). This trend also is in line with the results that the DOS of Mo1 and Fe1 are located to the right of the Fermi level, whereas those of Co1 and Ni1 are located to the left of the Fermi level (Figure 4). 3.2. Adsorption of Reactants, Intermediates, and Products. Because a good catalyst is characterized by low activation energy and weak adsorption of the intermediates, the adsorption energies of the intermediates can be used to 25370

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Table 2. Adsorption Energies Eads (eV) of All Surface Intermediates at the Mo-Edge on the M1-MoS2 Surface Mo

Fe

Co

Ni

site

VASP

Dmol3

VASP

Dmol3

VASP

Dmol3

VASP

Dmol3

CO-M1 CO-Mo2 H2O-M1 H2O-Mo2 H-M1 H-Mo2 OH-M1 OH-bri-M1, Mo2 OH-Mo2 O-M1 O-bri-M1, Mo2 O-Mo2 CO2 HCOO COOH

−2.08 −1.26 −1.35 −0.57 −2.87 −2.70 −4.97 −4.35

−2.14 −1.23 −1.42 −0.64 −2.91 −2.75 −4.74 −4.15

−1.89 −1.02 −0.71 −0.61 −2.19 −2.18 −3.36

−2.01 −1.09 −0.93 −0.70 −2.16 −2.36 −3.21

−1.77 −0.90 −0.70 −0.65 −2.32 −2.19 −2.79

−1.78 −0.97 −0.82 −0.72 −2.45 −2.29 −2.62

−0.83 −1.01 −0.45 −0.68 −1.92 −2.16 −2.22

−0.88 −1.07 −0.65 −0.74 −2.05 −2.23

−7.22

−3.50 −4.77 −5.40

−3.39 −4.44 −5.19

−3.43 −4.02

−3.29 −3.79

−3.40 −2.63

−3.21

−7.49

−1.28 −4.38 −3.68

−1.11 −4.20 −3.69

−0.37 −3.64 −2.56

−0.37 −3.60 −2.48

−5.19 −0.45 −3.55 −2.97

−4.96 −0.29 −3.52 −2.81

−5.06 −0.03 −3.15 −2.27

−4.91 0.05 −3.15 −2.45

Table 3. Reaction Energies (ΔE, eV), Reaction Barriers (Ea, eV), and Forming/Breaking Distance (d, Å) at the Transition State Structures for Different Elementary Steps for the WGS Reaction at the Mo-Edge on the MoS2(100) Surface ΔE Moa Fe Co Ni Moa Fe Co Ni Mo Fe Co Ni Mo Fe Co Ni Mo Fe Co Ni a

Ea

CO + H2O → CO + OH + H 0.31 0.98 0.31 0.80 0.22 0.77 0.17 0.80 CO + OH + H → CO + O + 2H 0.60 2.10 0.86 1.56 0.79 1.34 0.87 1.50 CO + O + 2H → CO2 + 2H 0.23 0.53 0.18 0.71 0.09 0.30 0.04 0.20 CO + OH + H → COOH + H 1.06 1.27 0.62 0.71 0.36 0.47 0.14 0.42 COOH + H → CO2 + 2H 0.24 1.21 0.42 1.37 0.52 1.30 0.62 1.34

d 1.32 1.33 1.35 1.33 1.62 1.47 1.49 1.50 1.98 1.70 1.91 1.96

Figure 5. Structures of reactants, transition states, and products by the redox pathway at the Mo-edge on M-MoS2.

2.03 1.59 1.64 1.71 1.35 1.42 1.39 1.42

Calculated data on pure MoS2 in ref 12. Figure 6. Structures of reactants, transition states, and products by the COOH pathway at the Mo-edge on M-MoS2.

intermediates of all steps on Fe-MoS2 and Ni-MoS2 are the same as those on Co-MoS2.33 a. Adsorption of H2O and CO. For H2O dissociation, it is necessary to consider the coadsorption of H2O and CO at first (Table 2). For M = Mo, Fe, Co, CO and H2O prefer the M1 site over the Mo2 site, but CO on the M1 site is much stronger than H2O (−2.08, −1.89, −1.77 eV vs −1.35, −0.71, −0.70 eV, respectively). In coadsorption, CO adsorbs on M1 and H2O adsorbs on Mo2, and the coadsorption of H2O on M1 and CO

and Mo2 is not competitive on the basis of the coadsorption energies. On Ni-MoS2, however, CO and H2O prefer the Mo2 site over the Ni site (−1.01 and −0.68 eV vs −0.83 and −0.45 eV), and the coadsorption of CO on Ni and H2O on Mo2 is a little less favored than that of CO on Mo2 and H2O on Ni. In Ni-MoS2, the differences for CO and H2O coadsorption on Ni and Mo2 are much smaller than those on MoS2, Fe-MoS2, and 25371

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(COOH), which further dissociates into surface CO2 and H. Starting from the coadsorbed CO + OH + H′, the barrier of surface COOH formation is 1.27, 0.71, 0.47, and 0.42 eV, respectively, for M = Mo, Fe, Co, and Ni, and the reaction is endothermic by 1.06, 0.62, 0.36, and 0.14 eV, respectively, for M = Mo, Fe, Co, and Ni. In the transition state of surface COOH formation (TS4), the forming C−O distance is 2.03, 1.59, 1.64, and 1.71 Å, respectively, for M = Mo, Fe, Co, and Ni. For surface COOH dissociation into CO2 and H, the dissociation barrier is 1.21, 1.37, 1.30, and 1.34 eV, respectively, for M = Mo, Fe, Co, and Ni; and the reaction is endothermic by 0.24, 0.42, 0.52, and 0.62 eV, respectively, for M = Mo, Fe, Co, and Ni. In the transition state of COOH dissociation (TS5), the breaking O−H distance is 1.35, 1.42, 1.39, and 1.42 Å, respectively, for M = Mo, Fe, Co, and Ni. It is also noteworthy that the barrier for the back reaction or the recombination of COOH to CO and OH is only 0.21, 0.09, 0.11, and 0.28 eV, respectively, for M = Mo, Fe, Co, and Ni, which is much lower than that of COOH dissociation. Therefore, surface COOH can dissociate much easier back to surface CO and OH than into surface CO2 and H. To find a lower barrier pathway of COOH dissociation to CO2, we switched the adsorption sites of CO and OH, that is, CO on Mo2 and OH and Co1. After switching the position, for example, the coadsorption of CO and OH becomes higher in energy by 1.51 eV, and the adsorption of COOH becomes less stable by 1.29 eV. Therefore, switching the position of CO and OH does not result in a more stable adsorption configuration; instead, the whole potential energy surface becomes higher in energy. In addition, the COOH formation and dissociation barrier also become higher (1.28 vs 0.47 eV) and (1.81 vs 1.30 eV), respectively. 3.4. Potential Energy Surfaces. The potential energy surfaces in Figures 7 and 8 can provide the information not only about the preferred reaction path but also about the metal promoting effect. Figure 7 shows that, in the redox reaction path, surface OH dissociation into surface O + H has the highest barrier on each surface. The OH dissociation barrier is much higher than the barriers of the recombination of OH and H to H2O, CO2 formation, and the recombination of O and H

Co-MoS2. Moreover, it is calculated that the M1 site is unfavorable for H2O dissociation on the Mo-edge.33 On the basis of these comparisons, we used the coadsorption model of CO on M1 and H2O on Mo2 for H2O dissociation. b. H2O Dissociation and CO Oxidation. On the basis of the coadsorption configuration of CO + H2O (Figure 5), H2O dissociation takes place on the Mo2 site, and the coadsorption of OH and H is OH on the Mo2 site and H on the bridged S1 site. For H2O dissociation from the coadsorbed H2O + CO into the coadsorbed CO + OH + H, the computed dissociation barrier is 0.98, 0.80, 0.77, and 0.80 eV, respectively, for M = Mo, Fe, Co and Ni, and the computed dissociation energy is endothermic by 0.31, 0.31, 0.22, and 0.17 eV, respectively, for M = Mo, Fe, Co, and Ni. In the transition state of H2O dissociation (TS1), the breaking HO−H distance is 1.32, 1.33, 1.35, and 1.33 Å, respectively, for M = Mo, Fe, Co, and Ni. For OH dissociation, it is necessary to migrate the first-step dissociated H from the bridge S to the last Mo atom (which is also the Mo atom to M1 in the next cell). As shown in Figure 6, it is observed that the S1-H atom in the coadsorbed OH and H groups diffuses to the slightly more favorable Mo site, and simultaneously, the H atom in the OH group turns toward the bridged S atoms. Compared to the pure MoS2 surface,56 H diffusion on the Co- or Ni-MoS2 surfaces is easier.49 Therefore, H diffusion was not considered in this work. Although the energies (0.31, 0.31, 0.22, and 0.17 eV, respectively, for M = Mo, Fe, Co, and Ni) for H2O dissociation into the coadsorbed CO + OH + H are close, after H migration, however, the relative reaction energy of H2O dissociation into coadsorbed CO + OH + H′ becomes 0.32, 0.85, 0.66, and 0.23 eV, respectively, for M = Mo, Fe, Co, and Ni. For OH dissociation from the coadsorbed CO + OH + H′ into CO + O + 2H, the computed barrier is 2.10, 1.56, 1.34, and 1.50 eV, respectively, for M = Mo, Fe, Co, and Ni, and the computed dissociation energy is 0.60, 0.86, 0.79, and 0.87 eV, respectively, for M = Mo, Fe, Co, and Ni. In the transition state of OH dissociation (TS2), the breaking O−H distance is 1.62, 1.47, 1.49, and 1.50 Å, respectively, for M = Mo, Fe, Co, and Ni. Starting from CO + OH + H, the effective barrier is 2.13, 2.10, 1.78, and 1.56 eV, respectively, for M = Mo, Fe, Co, and Ni, and the effective dissociation energy is endothermic by 0.60, 1.40, 1.23, and 0.93 eV, respectively, for M = Mo, Fe, Co, and Ni. The OH dissociation barriers from the coadsorbed CO + OH + H′ into CO + O + 2H for H′ on the S2 site are 1.76, 1.50, and 2.13 eV, respectively, for M = Fe, Co, and Ni, which are higher than those for H′ on the Mo site. Surface O resulting from H2O dissociation will react with surface CO from CO + O + 2H to form surface CO2 + 2H via the corresponding transition state (TS3). In TS3, the computed forming C−O distance is 1.98, 1.70, 1.91, and 1.96 Å, respectively, for M = Mo, Fe, Co, and Ni. The calculated activation barrier is 0.53, 0.71, 0.30, and 0.20 eV, respectively, for M = Mo, Fe, Co, and Ni, and the oxidation reaction is endothermic by 0.23, 0.18, 0.09, and 0.04 eV, respectively, for M = Mo, Fe, Co, and Ni. For comparison, the barrier for the back reaction or the recombination of O and H to OH is 1.54, 0.70, 0.55, and 0.63 eV, respectively, for M = Mo, Fe, Co, and Ni. Therefore, the reaction of surface O with surface CO is more preferable kinetically than the recombination of surface O and H into surface OH. c. COOH Formation and Dissociation. Alternatively to OH dissociation to surface O and H in the redox path, surface OH can react with the coadsorbed CO to form surface carboxyl

Figure 7. Potential energy surfaces of H2O dissociation to produce CO2 by the redox pathway on 25% sulfur coverage of M-MoS2 surfaces. 25372

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result on M(Fe, Co, Ni)-CeO2 catalysts for the WGS reaction.58 Because Ni-MoS2 has the smallest adsorption energies of the reaction intermediates and also the lowest effective barrier, it should be the most effective catalyst for the WGS reaction. It is clearly noteworthy that such a promoting effect is associated with the electronic effect of the individual promoter, for example, the projected DOS in Figure 4. The stronger shifts of the DOS below the Fermi level, the more pronounced the promoting effect. Compared to Mo, Fe, Co, and Ni have increased the number of valence electrons, and this might indicate that a valence-electron-rich system can have a stronger promoting effect than a valence-electron-poor system. The promoting effect is also associated with the electronegativity of the promoting metals (Table 2). The Allen electronegativity increases from Mo, Fe, Co, and Ni, and this order is in line with the promoting order.This increasing electronegativity of M1 leads to more electron transfer from Mo2 site to M1 site, that is, in turn, raises the positive charge of Mo2 site, which is the site for the rate-determining step in OH dissociation. Furthermore, it is found that the C−O distance of the structure CO + OH + H′ of Figure 5 (4) is 1.168, 1.162, and 1.149 Å on Fe, Co, and Ni sites, respectively. The least activation of CO on Ni, on one hand, weakens the interaction of CO-Ni with H-Mo to lower the energy of CO + OH + H′ as well as the potential energy surface of the WGS reaction on Ni-MoS2. On the other hand, it decreases the activation of CO oxidation. Many experimental results concluded that the presence of Ni or Co enhances the reducibility of Mo species, and the catalyst consisted of a mixture of active MoIV and MoV oxidized components, which were responsible for the better catalytic performance.6,30 The experimental results are well consistent with this theoretical work that the addition of Fe, Co, and Ni promoters leads to more strongly oxidized Mo sites (Table 1), compared with pure MoS2. Therefore, it is concluded that these metal promoters play the textual role in creating more active sites to lower the effective barrier of the rate-determining step in the WGS reaction.

Figure 8. Potential energy surfaces of CO coupling with OH to produce COOH by the COOH pathway on 25% sulfur coverage of MMoS2 surfaces.

back to OH. Because H2O dissociation is slightly endothermic, surface H 2 O and OH should be the major surface intermediates, and OH dissociation is the key step for the redox mechanism. The potential energy surface in Figure 8 shows that, in the carboxyl reaction path, COOH dissociation into surface CO2 and H has the highest barrier for each surface, and it should also be the rate-determining step for the COOH pathway. Because the COOH dissociation barrier into surface CO2 and H is much higher than that into surface CO and OH, surface COOH can be a very important intermediate along with OH. The first competitive reaction between two reaction paths is the O−H dissociation into surface O and H as well as COOH formation from CO and OH. As the rate-determining step in the redox pathway, the O−H dissociation barrier is much higher than the barrier of COOH formation; therefore, COOH is the preferred intermediate along with surface OH in the first competitive reaction step. The potential energy surfaces in Figure 7 and 8 show that, for the pure MoS2 surface, the effective barrier from the coadsorbed CO + H2O to the highest point of the ratedetermining step is lower for the redox path than for the COOH path (2.45 vs 2.84 eV), and therefore, the redox path is preferred kinetically. The same results can be found for M = Fe and Co; that is, the effective barrier for the redox path is lower than that for the COOH (2.41 vs 2.59 for M = Fe, and 2.00 vs 2.32 eV for M = Co). For M = Ni, however, both redox and COOH paths have very close effective barriers (1.73 vs 1.71 eV), indicating their competitive nature. On the basis of their more favorable reaction path, it is easy to get the promoting effect. Figure 7 shows clearly that all three promoting metals can lower the effective barriers compared to the pure MoS2, and the strongest effect is found for Ni (0.72 eV), followed by Co (0.45 eV), and the weakest effect is found for Fe (0.04 eV). These results are consistent with the experimental result that the sulfided Mo-containing systems, supported on α-Al2O3, promoted with cobalt or nickel ions, with or without alkali additives, are the most active and widespread investigated catalysts in the WGS reaction.57 NiMoS2 is the most active, followed by Co-MoS2, Fe-MoS2, and pure MoS2, which is the same effect with the experimental

4. CONCLUSION DFT calculations were carried out to investigate the reaction mechanisms of the water gas shift reaction and the promoting effect on Fe-, Co-, and Ni-doped MoS2 surfaces. Under the consideration of the H2O adsorption energy, the active surface is found to have 25% M substitution on the Mo-edge with 25% sulfur coverage, and this active surface is stable at high H2/H2S ratios Compared to pure MoS2, a significant electronic effect has been observed on the basis of the computed density of states (DOS), that is, the shift of the DOS over the Fermi level to the Fermi level and finally below the Fermi level, and this shift is associated with the increasing numbers of the valence electrons and the increasing electronegativity in the order of Mo < Fe < Co < Ni. It is found that the adsorption energies of each adsorbate on the metallic sites decrease in the order of Mo > Fe > Co > Ni centers in general, and some disorders are found. For example, CO, H2O, and H prefer to adsorb on the Mo2 site instead of the Ni1 site on Ni-MoS2, whereas they prefer to adsorb on Fe and Co1 sites instead of the Mo2 site on Fe-MoS2 and CoMoS2. The redox mechanism and the COOH-mediated reaction path have been computed and compared. On the basis of the 25373

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computed effective barriers, the surface OH dissociation into O + H is the rate-determining step for the redox mechanism (CO + H2O → CO + OH + H; OH → O + H; CO + O + 2H → CO2 + 2H), and the COOH dissociation into CO2 + H is the rate-determining step of the carboxyl reaction path. Detailed comparisons show that the redox mechanism is kinetically more preferred over the COOH mechanism for pure MoS2, Fe-MoS2, and Co-MoS2, whereas both reaction paths have close effective barriers for Ni-MoS2. The calculated effective barriers of the rate-determining step show that Ni has the largest promoting effect (0.72 eV), followed by Co (0.45 eV), whereas Fe does not have such an effect (0.04 eV), compared to pure MoS2. Such an order in lowering the effective barrier is in line with the electronic effect, for example, the shift of the DOS and the Allen electronegativity as well as the number of the valence electrons of the promoters. From the adsorption configurations of the intermediates, these promoters play a textual role in creating more active sites and reducing the effective barriers. As the best promoter in the WGS reaction, Ni-MoS2 has the lowest adsorption energies of the intermediates and also the lowest effective barriers, followed by Co-MoS2.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.W.), [email protected] (H.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21073218, 21103216, 10979068), the National Basic Research Program of China (No. 2011CB201406), the Chinese Academy of Sciences, and Synfuels China Co., Ltd.

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