CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces - The

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CO Dissociation Mechanism on Cu-Doped Fe(100) Surfaces Yonghui Zhao, Shenggang Li,* and Yuhan Sun CAS Key Laboratory of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Pudong, Shanghai 201210, China S Supporting Information *

ABSTRACT: Periodic density functional theory calculations were carried out to investigate CO dissociation pathways on the Fe(100) surfaces covered with up to one monolayer of Cu atoms, which serve as the simple models for the Cu/Fe catalysts for higher alcohol synthesis (HAS) from syngas. For all the model catalyst surfaces, H-assisted CO dissociation was predicted to have lower energy barriers than direct CO dissociation. The difference in the energy barriers between the two dissociation pathways increases as Cu surface coverage increases, suggesting reduced contribution of direct CO dissociation on Cu-rich surfaces. A further thermodynamic analysis also reaches the same conclusion. Several reaction properties for CO dissociation, including CO physisorption and chemisorption energies, and energy barriers for direct and H-assisted CO dissociations, were found to scale linearly with Cu surface coverage, and these reaction properties were predicted to depend largely on the structure of the surface layer, which can be expected to also apply to other metal alloy catalysts. Cu doping was found to reduce the activity of the Fe(100) surface in catalyzing direct and H-assisted CO dissociations, so CO dissociations should occur primarily on Fe-rich surfaces, leading to CHx formation, whereas Cu-rich surfaces are potential sources for physisorbed CO molecules. This is also expected to apply to other Cu/M catalysts and is consistent with the dual site mechanism previously proposed for these bimetallic catalysts. A synergy between these two types of active sites is beneficial for the formation of higher alcohols, which may be the reason for the superior performance of the Cu/Fe catalysts for the HAS reaction. stock.4,16,18,29,30,40 Extensive studies have been carried out recently on Cu/Fe-based catalysts to further improve the total alcohol and C2+ alcohol selectivities.4,15,18,26,35−37 Despite the extensive studies on the HAS reaction, the detailed reaction mechanism remains elusive. This is primarily due to the fact that a complex network of elementary steps are involved in the HAS reaction. There is evidence that a good HAS catalyst, such as Rh-based catalysts,2,9,12,41 should have a balanced capability in adsorbing and dissociating CO. In addition, surface hydrocarbon (CHx, x = 1−3) formation, carbon chain growth, and CO insertion are also crucial steps for higher alcohol formation, as suggested by Gupta et al.7 Xu et al.13 proposed that the active site for the Cu/M (M = Fe, Co, etc.) catalysts for higher alcohol formation is the dual functional site, which was accepted by some researchers.5,7 It is assumed that the active Fe or Co species catalyze CO dissociation, carbon chain growth, and hydrogenation, whereas the active Cu sites provide physically adsorbed CO, which are inserted into the CxHy species, resulting in the production of Cn+1 alcohols upon hydrogenation. This mechanism requires the synergy between the Cu and Fe or Co active sites in close contact for high HAS activity. The agglomeration of one of these two types of active sites, thus the separation of these two types of active

1. INTRODUCTION With persistent increasing prices and declining reserves for fossil fuels, there is significant interest in selective conversion of syngas (a mixture of CO and H2) into value-added chemicals by Fischer−Tropsch (FT) reactions.1−3 Higher alcohol synthesis (HAS), which is an important FT-type process for the production of gasoline blends, alternative transportation fuels, and chemical intermediates as feedstocks for value-added chemicals for medicines, cosmetics, lubricants, detergents, and polyesters, has attracted much attention.4−6 Extensive studies on the HAS reaction for developing active catalysts with high selectivity toward C2+ alcohols have been carried out since the late 1970s.2,4−27 Several pioneering works and excellent reviews on this subject are available in the literature.2,5−12 Several types of HAS catalysts have been developed over the past few decades, such as the Rh-based catalysts, Mo-based catalysts, and modified methanol synthesis or FT catalysts. At present, Cu-modified FT catalysts, such as Cu-modified Fe or Co catalysts, were considered as the most promising HAS catalysts.4,7,14−16,28−39 Because of poor catalyst stability and lower C2+ alcohol selectivity, Cu/Co-based catalysts are still not ready for large-scale industrial applications.5 On the other hand, Cu/Fe-based catalysts show higher catalytic activity and C2+ alcohol selectivity with excellent stability.30 In addition, Cu/Febased catalysts also exhibit higher water gas shift (WGS) activity, which makes them suitable for catalyzing the HAS reaction with biomass-derived syngas as the feed© 2013 American Chemical Society

Received: September 6, 2013 Revised: October 25, 2013 Published: October 31, 2013 24920

dx.doi.org/10.1021/jp408932y | J. Phys. Chem. C 2013, 117, 24920−24931

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The pure Fe catalyst surface is modeled by the Fe(100) surface, whereas the Cu/Fe bimetallic catalyst surfaces are modeled by partial or full replacement of one of the top two layers of Fe atoms with Cu atoms. We assume that the first two atomic layers in a metal substrate have the strongest influence on the surface reactions. We adopted the slab supercell approach with each supercell containing a five-layer slab with four metal atoms per layer representing a p(2 × 2) lateral supercell as used in ref 57. An interslab spacing of 15 Å was used to avoid interactions between repeating slabs. The k-point sampling was generated by the Monkhorst−Pack procedure69 with a (4 × 4 × 1) mesh. Adsorption is allowed only on one side of the slab to minimize the induced dipole effect. The bottom two layers are fixed at their bulk truncated positions while all remaining layers and the adsorbates are allowed to fully relax until the forces on the atoms are less than 0.02 eV/Å. The adsorption energy (Ead) of a species X (X = C, O, H, CO, CH, and HCO) on the Cu/Fe surfaces can be calculated from eq 1.

sites from each other, may lead to reduced HAS activity, as reported for the Cu/Co-based catalysts.13 However, the validity of the dual site mechanism has not been thoroughly examined by theory, and the elucidation of the catalytic mechanism for the Cu/M-catalyzed HAS reaction can provide insights for further improving these catalysts. Previous experimental and theoretical studies suggest the HAS reaction to initiate by CO dissociation, and thus a fundamental understanding of CO adsorption and dissociation is of great importance.2,9,42−46 CO adsorption and dissociation on transition metals and alloys have been carried out by experimental and theoretical studies.30,38,41,43−56 Direct or Hassisted CO dissociation on Rh, Ru, Fe, Co, Ni, and Cu catalysts to form the CHx (x = 1−3) species have been studied by theory.41−48,50−54,57,58 On the Cu(111) surface, the hydrogenation of CO to form COH or HCO was found by Sun et al. to be energetically more favored than the direct CO dissociation,54 although Grabow and Mavrikakis predicted CO hydrogenation to COH on the Cu(111) surface to have a much higher energy barrier than that to HCO in their comprehensive mechanistic study on the methanol synthesis reaction.59 Hassisted CO activation was also suggested by Ojeda and coworkers to be the kinetically predominant CO dissociation pathway on Fe and Co catalysts at typical FT reaction conditions.44 Elahifard et al.52 showed that the dominant mechanism for CO dissociation on the Fe(100) surface depends on the reaction conditions: on the bare Fe(100) surface, direct CO dissociation is the preferred pathway, whereas, under high H2 pressure and low temperatures, which favor a low number of unoccupied active sites and a considerable amount of H atoms are adsorbed on the surfaces, H-assisted CO dissociation via HCO is more favorable.52 However, the detailed CO dissociation pathways on the Cu/Febased catalysts have yet been examined by theory, and their elucidation should lead to an improved understanding on the HAS reaction mechanism on these catalysts. In this work, we use the periodic density functional theory (DFT) method to examine the adsorption and dissociation of CO on several Cu-doped Fe(100) surfaces, which serve as simple models for Cu/Fe bimetallic catalysts in the HAS reaction. The most favorable CO dissociation pathway was predicted, the effects of Cu doping on the Fe catalysts were examined, and the validity of the dual site mechanism was evaluated in light of this study. The outline of this paper is as follows: the computational details are given in section 2; section 3 presents the adsorption properties of the relevant adsorbates, including CO, H, C, O, CH, and HCO, on the Cu/Fe surfaces, and the pathways and energetics of CO dissociation with and without H-assistance; and a brief summary is given in section 4.

slab slab Ead = E X,Cu/Fe − ECu/Fe − EX

(1)

slab Here, Eslab X,Cu/Fe, ECu/Fe, and EX are the energies of X adsorbed on the Cu/Fe surfaces, the Cu/Fe surfaces, and X in the gas phase, respectively. The more negative the value of Ead, the stronger the adsorption is. To locate the transition state, the constrained minimization method70 is used. The transition states were optimized by reducing the residual force on each atom to Fe3Cu1/Fe(100) > Fe1Cu3/Fe(100) > Cu4/ Fe(100), so it becomes weaker as the Cu surface overage increases. The adsorption energy of CO on the Cu4/Fe(100) surface of −0.94 eV is only slightly more negative than the predicted value of −0.89 eV on the pure Cu(100) surface,73 and these values are much less negative than our calculated adsorption energy of CO on the Fe4/Fe(100) surface of −2.05 eV. This shows the strong effect of surface Cu doping of the Fe(100) surface on CO adsorption, and the bimetallic Cu4/ Fe(100) surface closely resembles the pure Cu(100) surface. For H, the adsorption energies differ very little for the Fe4/ Fe(100), Fe3Cu1/Fe(100), and Fe1Cu3/Fe(100) surfaces (−2.67, −2.68, and −2.61 eV, respectively), which are more negative than that on the Cu4/Fe(100) surface of −2.38 eV. 3.3. Direct CO Dissociation. Figure 3 illustrates the structures of the initial states, transition states, and final states for direct CO dissociation on Cu/Fe surfaces. The energy barriers and reaction energies for CO dissociation are shown in Table 3. The initial states for direct CO dissociation are usually chosen as the most favorable adsorption structures, as shown in Figure 2, whereas, in the final states, the C and O atoms are adsorbed at two neighboring 4-fold hollow sites, which are the most favorable adsorption sites for both atoms. For the Fe4/Fe(100) surface, in the initial state, CO is adsorbed at the 4-fold hollow site with the C−O bond titled away from the surface normal and with a predicted C−O bond distance of 1.32 Å, as shown in Figure 3a. During CO dissociation, the C atom moves toward the center of the 4-fold hollow site, whereas the O atom moves away from the C atom. In the transition state, the O atom remains at the bridge site, and the C−O bond elongates to 1.78 Å. In the final state, the O

diagram, the Cu−Fe system has a rather large miscibility range.71,72 Replacing one, three, or four Fe atoms on the surface or the second layer (subsurface) of the slab with Cu atoms leads to Cu/Fe surfaces with surface or subsurface Cu coverage of 1/4, 3/4, or 1 monolayer (ML), respectively. The relative stability of the Cu/Fe surface is determined by its formation energy as given in eq 2. slab bulk slab bulk ΔHmix = ECu/Fe + nE Fe − E Fe − nECu

(2)

slab In the above equation, Eslab Cu/Fe and EFe are the energies of the Cu/Fe surface and the pure Fe surface, respectively, Ebulk Cu and are the energies of the Cu and Fe atoms in their bulk Ebulk Fe states, and n is the number of Fe atoms replaced by Cu atoms. A negative value for ΔHmix suggests the thus-formed Cu/Fe surface to be thermodynamically stable relative to the pure Fe(100) surface and Cu and Fe atoms in their bulk states. The formation energies for the Cu/Fe surfaces with the Cu atoms present in the surface layer are always negative, −0.28, −0.48, and −0.52 eV for Cu surface coverages of 1/4, 3/4, and 1 ML, respectively. Those with the Cu atoms present in the subsurface layer are always positive, 0.52, 1.67, and 2.45 eV for Cu surface coverages of 1/4, 3/4, and 1 ML, respectively. This suggests that Cu atoms are preferably located on the surface layer of Cu/Fe surfaces. Hereafter, only Cu/Fe surfaces with surface Fe atoms replaced by Cu atoms are considered, and their top views are shown in Figure 1. For the convenience of discussion, we denote the pure Fe(100) surface as Fe4/ Fe(100), and the three Cu-doped Fe(100) surfaces as Fe3Cu1/ Fe(100), Fe1Cu3/Fe(100), and Cu4/Fe(100), respectively, and these four model surfaces will be collectively referred to as Cu/ Fe surfaces. 3.2. Adsorption of Reactants and Products. The adsorption structures and energies of the various species (C, O, H, CO, CH, and HCO) relevant to direct and H-assisted CO dissociations on the different sites of Cu/Fe surfaces were calculated to determine the most stable adsorption configurations. The optimized structures for the most stable adsorption configurations of these species are shown in Figure 2, whereas the key bonding parameters and the adsorption energies are listed in Tables 1 and 2, respectively. For C, O, H, and CH, the most favorable adsorption site is the 4-fold hollow site with four metal atoms. For CO and HCO, the most favorable adsorption site is slightly different for different Cu/Fe surfaces. For CO, the most favorable adsorption site for the Fe-rich surfaces, i.e., Fe4/Fe(100) and Fe3Cu1/Fe(100), is the 4-fold hollow site, where the C atom is located near the center of the site, and the CO molecular axis is tilted away from the surface normal, so the O atom is bonded with two metal atoms, as shown in Figure 2d,j. For the Fe1Cu3/ Fe(100) surface, the most favorable CO adsorption site is the

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As shown in Figure 3d−l, the pathways for direct CO dissociation on the Cu-doped Fe(100) surfaces are, in general, similar to that on the Fe4/Fe(100) surface. In the initial states (Figure 3d,g,j), CO is adsorbed at the 4-fold hollow site with the C−O bond lengths calculated as 1.29, 1.17, and 1.22 Å for the Fe3Cu1/Fe(100), Fe1Cu3/Fe(100), and Cu4/Fe(100) surfaces, respectively. The predicted C−O bond length is shorter on the Cu-rich Cu/Fe surfaces than on the Fe-rich surfaces, suggesting that CO is less activated on the Cu-rich surfaces. We note that, for the Fe1Cu3/Fe(100) surface, the most favorable CO adsorption configuration, where CO is adsorbed at the top site of the Fe atom (Figure 2p), was not chosen as the initial state. This is because CO first diffuses from the top site of the Fe atom to the 4-fold hollow site, which has a reaction energy of 0.09 eV with an energy barrier of only 0.29 eV. As this step has a very small energy barrier, it can be considered as quasi-equilibrated in the steady-state approximation. In the transition states for direct CO dissociation (Figures 3e,h,k), the C atom is located near the center of the 4fold hollow site, while the O atom is located at the bridge site and prefers to form bonds with as many Fe atoms as possible. The C−O bond lengths in the transition states are ∼1.80 Å for the Cu-doped Fe(100) surfaces, similar to that on the Fe4/ Fe(100) surface of 1.78 Å and substantially elongated compared to those in the initial states. The energy barrier for direct CO dissociation is the energy difference between the transition state and the initial state, as shown in Figure 3, whereas the reaction energy is the energy difference between the final state and the initial state. As shown in Table 3, the energy barrier for direct CO dissociation increases significantly as the Cu surface coverage increases, from 1.11 eV on the Fe4/Fe(100) surface, to 1.20 eV on Fe3Cu1/Fe(100), to 1.84 eV on Fe1Cu3/Fe(100), to 2.37 eV on Cu4/Fe(100). The reaction energy for direct CO dissociation, i.e., the CO chemisorption energy, becomes more positive as the Cu surface coverage increases, from −0.44 eV on the Fe4/ Fe(100) surface, to −0.11 eV for Fe3Cu1/Fe(100), to 0.64 eV on Fe1Cu3/Fe(100), to 1.07 eV on Cu4/Fe(100). Figure 4 plots the energy barrier against the reaction energy for direct CO dissociation, showing the linear relationship between these two quantities given as eq 3. Ea /eV = 0.888 × Ere /eV + 1.392 eV

(3)

2

The high R value of 0.94 for the above equation suggests a good linear fit between the energy barrier and the reaction energy, which is consistent with the Brønsted−Evans−Polanyi (BEP) relationship between the energy barrier and reaction energy.74 3.4. H-Assisted CO Dissociation. As first suggested by Schulz and co-workers,75 recent experimental and theoretical studies strongly suggest the formyl species (HCO) to be crucial in both hydrocarbon combustion and synthesis from syngas.44,45,48,57,76,77 In this pathway for hydrocarbon synthesis, CO is first hydrogenated to form HCO, which can be further hydrogenated into the CHxO (x > 1) species, and this is followed by the cleavage of the C−O bond in the HCO (or CHxO) species to form the CH (or CHx) and O species, so CO dissociation is indirectly accomplished in two steps. The structures for the initial states, transition states, and final states for H-assisted CO dissociation on Cu/Fe surfaces are shown in Figure 5. The potential energy surface diagrams are shown in Figure 6 with the energy barriers and reaction energies listed in Table 3.

Figure 3. Top views and side views of the structures of the initial states (IS), transition states (TS), and final states (FS) for CO direct dissociations on Cu/Fe surfaces. The C−O bond lengths are shown in Å.

atom diffuses to its energy-favorable 4-fold hollow site, and the C−O bond is completely broken with a calculated distance of 2.84 Å. Direct CO dissociation on the Fe4/Fe(100) surface was predicted to be exothermic by −0.44 eV with an energy barrier of 1.11 eV, in good agreement with the values of −0.29 and 1.09 eV reported by Elahifard et al.52 24924

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Table 3. Energy Barriers (Ea, eV) and Reaction Energies (Ere, eV) for Direct and H-Assisted CO Activations on Cu/Fe Surfaces Fe4/Fe(100)

Fe3Cu1/Fe(100)

Ea

Ere

CO → C + O

1.11

−0.44

CO + H → HCO HCO → CH + O

0.78 0.62

0.59 −0.97

surface

Ea

Ere

direct CO activation 1.20 −0.11 H-assisted CO activation 0.89 0.69 0.56 −0.75

Fe1Cu3/Fe(100) Ea

Ere

Cu4/Fe(100) Ea

Ere

1.84

0.64

2.37

1.07

1.03 0.67

0.67 −0.34

1.07 0.93

0.63 −0.12

O bond is also more activated upon CO hydrogenation on the three Cu-doped Fe(100) surfaces. As shown in structures f−h, k−m, and p−r in Figure 5, the C−O bond elongates upon CO hydrogenation (1.28, 1.29, and 1.38 Å for the initial state, the transition state, and the final state on Fe3Cu1/Fe(100), 1.22, 1.27, and 1.37 Å for those on Fe1Cu3/Fe(100), and 1.22, 1.26, and 1.36 Å for those on Cu4/Fe(100)). Thus, CO hydrogenation into HCO leads to a weaker C−O bond, whose cleavage can be expected to be easier than the direct CO dissociation. From Table 3, the energy barriers of HCO formation on Cu/ Fe surfaces also follow the order of Fe4/Fe(100) < Fe3Cu1/ Fe(100) < Fe1Cu3/Fe(100) < Cu4/Fe(100), although the differences in these barriers are