First-Principles Study of C2 Oxygenates Synthesis Directly from

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First-Principles Study of C2 Oxygenates Synthesis Directly from Syngas over CoCu Bimetallic Catalysts Xin-Chao Xu, Junjie Su, Pengfei Tian, Donglong Fu, Weiwei Dai, Wei Mao, Wei-Kang Yuan, Jing Xu, and Yi-Fan Han* State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations were used to study C2 oxygenate from syngas over bimetallic Co/Cu catalysts. The thermodynamics and kinetics for all possible elementary steps involved in the formation of C2 oxygenate from syngas have been investigated on both pure and Cu-doped Co(0001) surfaces. By comparing the results on two surfaces, the role of copper in improving the selectivity toward C2 oxygenates has been identified as two aspects: (1) controlling the Co ensemble size and blocking the active sites for C−O bond cleavage, which results in inhibition of CHx coupling to hydrocarbons; (2) providing undissociated CO/ HCO as well as reducing the barriers for HCO insertion toward the formation of oxygenate precursors. With the combination of the mechanistic study and the charge analysis on the bimetallic surface, we conclude that the nature of the copper promotion is mainly a geometric effect rather than an electronic effect.

1. INTRODUCTION For the purpose of relieving the concerns about global climate change and shortage of fossil fuel resources, there is an ever increasing demand for seeking more environmentally friendly and efficient ways to utilize the traditional fossil fuels.1,2 One promising route is the conversion of fossil fuels, such as coal, biomass or natural/shale gas, to syngas (a mixture of CO and H2), which can then be catalyzed to value-added C2 oxygenates, including ethanol, acetaldehyde, and acetic acid.3,4 The study on this process has been carried out ever since Fischer and Tropsch discovered the coal-to-liquid process in 1910s.5 The catalysts for C2 oxygenate synthesis can be roughly divided into four categories:6−9 (1) Rh-based catalysts. Supported Rh shows high selectivity to C2 oxygenates under mild conditions, but the low productivity or conversion of CO (ca. 5.0%), along with the high price of Rh, prevents those catalysts from further practical applications. (2) Modified methanol synthesis catalysts (Cu-based). Cu-based catalysts generally yield branched alcohols by the so-called isobutanol synthesis, only with a poor selectivity. (3) Modified Fischer− Tropsch (F-T) type catalysts (essentially Fe- or Co-based). Those catalysts have been reported to yield mainly straight chain alcohols and hydrocarbons. (4) Mo-based catalysts. Modified Mo-based catalysts give rise to a good selectivity and productivity as well as an inherent sulfur resistance. Nevertheless, the harsh demands for reactors and reaction conditions have limited the application of this process. Thus, to achieve a better compromise between the catalytic performance and the © XXXX American Chemical Society

costs, an appropriate way is to combine two or more types of these catalysts and create new catalytic systems. Several catalysts have been proposed according to this strategy, including bimetallic Co−Cu,10−14 Rh−Cu,15,16 and Fe−Cu7,17 catalysts. Among them, Co−Cu based catalysts have attracted the most attention due to their high selectivity and low cost. The Co−Cu catalysts for oxygenate synthesis were first developed and patented in Institut Francais du Petrole (IFP) in the 1970s.18,19 It was reported that the selectivity toward oxygenates, mainly saturated straight-chain terminal alcohols, could reach as high as 90−95% over promoted catalysts. Since then, many researchers have devoted to elucidating the origin of active sites and the mechanism of the oxygenate synthesis process over Co−Cu catalysts.10,20−30 Two viewpoints have been used to explain the nature of the active sites in Co−Cu catalysts for oxygenate synthesis: (1) The formation of oxygenates may proceed via a dual-site mechanism. In this mechanism, the carbonaceous chain would be formed on large cobalt ensembles, and the terminal OH group is provided by a CO species adsorbed on alloy-modified copper atoms; (2) the production of oxygenates is attributed to a synergy between metallic copper and partially reduced cobalt species, in which the Cu mainly promotes the reduction of Co species and acts as a diluter to control the surface Co ensemble size. Received: June 30, 2014 Revised: December 2, 2014

A

dx.doi.org/10.1021/jp5065159 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Top view (a) and side view (b) of Cu6Co3 surface and possible adsorption sites on Cu6Co3. According to the number of Co and geometric configuration, the adsorption sites are denoted as (1) Co3hol, (2) Co2hol, (3) Co1hcp, (4) Co1fcc, (5) Co2brg, (6) Co1brg, and (7) Cotop.

Co(0001) surfaces were investigated first. Then, based on the results on the two surfaces, the role of Cu for C2 oxygenate synthesis is discussed. We expect this study will greatly improve the understanding of this reaction and the subsequent design of high-performance catalysts.

Mo et al.29 revealed the interactions of the different components from Co/CuZnO catalysts using steady state isotopic transient kinetic analysis (SSITKA) for CO hydrogenation. The activity for Co/CuZnO catalysts found to be significantly decreased compared to a Co/Al2O3 catalyst; however, the selectivity to oxygenates was enhanced owing to blocking a significant portion of active sites for hydrocarbon synthesis. Xiang et al.12 developed a highly selective Cu−Co catalyst that possesses a Co-rich core structure and a Cudominated mixed shell, and the selectivity to long chain alcohols has been found superior to traditional Cu−Co catalysts. This study indicates that the Co−Cu interaction, especially in the near-surface region in which a Cu-rich surface configuration is predominant, can play an important role in promoting the production of alcohols. Moreover, combining theoretical calculations and experimental study, Prieto et al.31 identified a CuCo alloy phase with Co-rich compositions as the ideal catalyst surface for long-chain alcohols. Density functional theory (DFT) calculations have demonstrated to be a powerful tool for elucidating kinetic behaviors32,33 and gaining insight into the reaction mechanism. Actually, the DFT method has been applied to investigate the F-T synthesis and oxygenate formation process over Co-based catalysts.34−38 It has been suggested that the oxygenate synthesis process is initiated by CO dissociation through a Hassisted mechanism, in which CO is first hydrogenated to produce CHxO species, followed by a C−O bond cleavage, resulting in the formation of CHx species. As for the chain growth toward higher oxygenates, a CO insertion model is the most preferred, but more recent studies also propose that a HCO insertion model may be an alternative mechanism. To the best of our knowledge, the theoretical study of the detailed reaction mechanism for this reaction system has been rarely reported. In order to elucidate the effect of Cu on Cu− Co catalysts, elementary steps involved in C2 oxygenate formation from syngas on both pure and Cu-doped

2. COMPUTATIONAL METHODS 2.1. Surface Model. To simulate the structures of bimetallic catalysts, two kinds of models were widely adopted. The first one is the bulk alloy or solid solution model, in which the two components are intimately mixed by forming a single solid phase microstructure. It has been successfully applied to the study of NOx decomposition over Rh−Ag bimetallic catalysts.39 The second one is the near surface alloy (NSA) model, which has been widely used for investigating Pt-based oxygen reduction electro-catalysts.40 In this model, the formation of alloy structure is expected to occur only at the near surface region, while the bulk region remains a pure phase. In general, the choice of the model for bimetallic catalysts is determined by how intimately the two components interact with each other. For Co−Cu catalysts, it is well established that the bulk CuCo alloy does not exist due to the low solubility of one metal into the other.41 However, a simple mechanical mixing of two metals did not change the cobalt’s selectivity.42 Actually, it has been reported that alloying phenomena may exist at the nanoscale.43 Hence, the NSA model may serve as a feasible choice. Furthermore, since Cu has a larger tendency to segregate to the surface compared to Co,44 a Cu-rich surface configuration45 is observed when exposing to syngas atmospheres. Additionally, Co atoms may either homogeneously distribute or aggregate to form local ensembles on a Cu-rich Cu−Co surface. To further identify the surface structure of a Cu−Co catalyst, we perform calculations with different Co arrangements and reveal that Co atoms prefer to aggregate rather than homogeneously distribute on the Cu6Co3 surface in B



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3. RESULTS AND DISCUSSION 3.1. Reaction Intermediates. The preferred adsorption sites and BEs of reaction intermediates involved in various reaction routes on Co9 and Cu6Co3 (Table 1). The most

thermodynamics (Supporting Information (SI), Figure S1). Thus, in our model, 2/3 of surface atoms in Co(0001) were substituted by Cu atoms, it results in a surface configuration of a triangular Co ensemble surrounded by 6 Cu atoms, Cu6Co3 (Figure 1). The model can provide the representative adsorption sites for reaction intermediates, including Co sites (Co3hol, Co2brg, and Co1top), bimetallic sites (Co2hol, Co1hcp, Co1fcc, and Co1brg) and Cu sites. The nomenclature is defined based on the number of Co involved in and the detailed adsorption sites (Figure 1). Here, we only considered a closepacked surface without step-edge sites for the following reasons: (1) As indicated by previous experimental work,68 Co terraces are the kinetically relevant sites for particles larger than 10 nm; (2) CO insertion, the most widely adopted chain growth mechanism toward C2 oxygenates, seems to be structure insensitive.34 2.2. Calculation Method. Periodic plane-wave spinpolarized DFT calculations were carried out by using the Vienna Ab-initio Software Package (VASP)46,47 with a planewave energy cutoff of 400 eV. The interactions between valence and core electrons were described by a projector augmented wave (PAW) method,48,49 and the generalized gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE) functional50 was used in the calculations. Co lattice was treated as an hcp structure, and the calculated lattice constant is 2.49 Å, agreeing well with the experimental value of 2.51 Å.51 For the pure Co catalyst, a three-layer Co(0001)-(3 × 3) slab (denoted as Co9) with a vacuum region of 10 Å between slabs was employed to simulate the surface. The top two layers and the adsorbates were allowed to relax until the energy difference between two consecutive steps was less than 1.0 × 10−4 eV, while the bottom layer was constrained at the bulk positions. A 4 × 4 × 1 Monkhorst−Pack k-point mesh52 was used to sample the Brillouin zone. For isolated molecules and atoms, calculations were carried out in a 15 Å × 15 Å × 15 Å unit cell with a k-point setting of Γ-point only. The calculation parameters for a Cu6Co3 surface are the same as that for Co9. With these settings, binding energies (BEs) were found to be converged within 0.05 eV with respect to slab thickness, interslab spacing, and k-point sampling. The BEs of adsorbates on both surfaces, Eb, were calculated as eq 1: E b = Eadsorbate/substrate − (Eadsorbate + Esubstrate)

Table 1. Preferred Adsorption Sites, and Corresponding BEs of Surface Intermediate Species on Cu6Co3 and Co9a Co9

Cu6Co3

species

Eb/eV

sites

Eb/eV

sites

C O H H2 CO CH CH2 CH3 HCO COH CH2O CHOH CH3O CH2OH CH3OH CHCO CH2CO CH3CO CHCHO CH2CHO CH3CHO CH4 C2H4

−6.91 −5.90 −2.84 −3.50 −1.81 −6.43 −4.04 −2.01 −2.26 −4.39 −0.79 −3.02 −2.88 −1.62 −0.28 −3.46 −1.14 −1.63 −4.39 −2.01 −0.33 / −0.25

C-hcp O-hcp H-fcc H-brg C-hcp C-hcp C-hcp C-hcp C-brg, O-brg C-hcp C-brg, O-top C-hcp O-hcp C-brg, O-top O-top Cα-hcp, Cβ-fcc Cα-hcp, Cβ-top Cβ-top Cα-brg, O-top Cα-top, O-top O-top / (C, C)-top

−6.89 −5.73 −2.65 −3.36 −1.79 −6.23 −3.81 −1.78 −2.13 −4.19 −0.79 −2.90 −2.62 −1.59 −0.31 −3.15 −1.10 −1.65 −4.08 −1.99 −0.32 / −0.28

C−Co3hol O−Co3hol H−Co3hol H−Co2brg C−Cotop C−Co3hol C−Co3hol C−Co3hol C−Cotop, O−Cotop C−Co3hol C−Co2brg, O−Cotop C−Co3hol O−Co3hol C−Co2brg, O−Cotop O−Cotop Cα-Co3hol, Cβ-Co2hol Cα-Co3hol, Cβ-Cotop Cβ-Cotop Cα-Co2hol, O−Cotop Cα-Cotop, O−Cotop O−Cotop / (C, C)-Cotop

a

For adsorbates containing two carbon atoms, the C atom in CHx (x = 1−3) is represented as Cα, while Cβ is used to represent the carbon atom from CO/HCO.

favorable adsorption structures of reaction intermediates are given in the SI. From these results, most of the intermediates have similar adsorption configurations on both Co9 and Cu6Co3 surfaces. In the proceeding section, the adsorption geometry and thermochemistry of all intermediates on Co9 will be described in detail. For the sake of brevity, the adsorption details on Cu6Co3 will not be deliberately presented, but a comparison between the two surfaces will be made. 3.1.1. Adsorption Geometry and Thermochemistry on Co9. 3.1.1.1. C, O, H, H2, CO, CHx (x = 1−3), CH4, C2H4. C, O, H, and H2 species prefer to bind at 3-fold hollow sites on Co9, corresponding to BEs of −6.91, −5.90, −2.84, and −3.50 eV, respectively. It should be noted that the H2 species can only molecularly bind at the hollow site, while it can easily dissociate on other sites of the surface. CO also binds at the 3-fold hollow site with a BE of −1.81 eV, via a Co−C bond of 1.97 Å. CH, CH2, and CH3 species are in favor of binding at 3-fold hollow sites as well, with BEs of −6.43, −4.04, and −2.01 eV, respectively. These results agree well with previous DFT calculations on a Co(0001) surface.56−58 Not surprisingly, CH4 is very weakly bound (−0.01 eV) on Co(0001) in a physisorption way, which is consistent with previous theoretical calculations.57,59 The adsorption of C2H4 is also moderate, only with a BE of −0.25 eV, through a configuration with both C atoms bound at the top sites.

(1)

where Eadsorbate/substrate, Eadsorbate, and Esubstrate are the total energies of the combined adsorbate/substrate system, adsorbate species in gas phase and the clean surfaces. Hence, a negative (positive) value indicates an exothermic (endothermic) adsorption, respectively. Transition states were located by combining the nudged elastic band (NEB)53,54 and Dimer methods.55 A linear interpolation between reactant and product states was used to generate the initial path (at least five images) for all NEB calculations. The highest energy structure along the converged NEB path was then used as the initial guess for the transition-state structure, which was optimized by the Dimer method along the potential energy surface to a saddle point. The Dimer separation was set at 0.01 Å, and the force on each atom was less than 0.05 eV/Å. The identified saddle points were further confirmed by a vibrational frequency calculation, in which only one imaginary frequency was obtained at the saddle point. The activation barrier (Ea) was defined as the total energy difference between the transition state (TS) and initial state (IS). C



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Figure 2. Formation of insertable monomers (CHx species) from hydrogenation and dissociation of CO on the Cu6Co3 surface (small gray, C; small red, O; small white, H). The activation barriers (left) and reaction energies (right) of the elementary steps are all shown in eV. The corresponding TS structures are shown in Figure 3.

3.1.1.2. CHxO (x = 1−3), CHxOH (x = 0−3). In general, all of CHxO (x = 1−3) and CHxOH (x = 0−3) species, except CH3OH, which is adsorbed at the top sites, prefer to bind at 3fold hollow site. Among CHxO (x = 1−3) species, CHO and CH2O prefer to bind via both C (bridge) and O (bridge/top) atoms with a BE of −2.26 and −0.79 eV, respectively, while CH3O tends to bind only via O atom with C−O bond perpendicular to the surface, and the calculated BE is −2.88 eV. The adsorption geometry of COH is quite similar to CO, namely, perpendicularly binding via its C atom, and the calculated BE is −4.39 eV. The other three CHxOH species (x = 1−3) have quite different adsorption geometries and BEs. CHOH and CH3OH tend to bind only via one atom (C for the former, while O for the latter) to the surface, whereas CH2OH is in favor of binding via both C (bridge) and O (top) atoms. The calculated BEs for CHOH, CH2OH, and CH3OH are −3.02, −1.62, and −0.28 eV, respectively. Cheng et al. have investigated the formation of oxygenates on Co(0001) using DFT calculation34 and reported data are in good agreement with our results except for the BE of CHOH species, which has a difference of 0.80 eV. The discrepancy may arise from the

different pseudopotentials chosen (Cheng’s: norm-conserving; this work: PAW). 3.1.1.3. CHxCO (x = 1−3), CHxCHO (x = 1−3). CHxCO and CHxCHO species (x = 1−3) are the key precursors for the formation of C2 oxygenates. Both the adsorption geometries and BEs vary with an increase in x. CHCO and CH2CO species tend to bind via both Cα and Cβ at the 3-fold hollow site, with BEs of −3.46 and −1.14 eV, respectively. Nevertheless, CH3CO only binds via the O atom at the top site with a BE of −1.63 eV. The adsorption configurations of CHxCHO species follow a similar trend. CHCHO and CH2CHO bind via two atoms, Cα and O, to the surface at the hollow sites, while only the O atom at the top site is involved in the binding of CH3CHO to the surface. The adsorption of CHxCHO species is weakened with the increasing of x from 1 to 3, and the corresponding BEs are −4.39, −2.01, and −0.33 eV, respectively. These results agree well with the previous data of Cheng et al.34 and Zhuo et al.56 3.1.2. Comparison of the Results on Cu9 and Cu6Co3. After summarizing the results of adsorption geometries and BEs of various intermediates on Co9, we make a comparison for these intermediates between Co9 and Cu6Co3. On Cu6Co3, all the D



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possible pathways: (i) CO dissociation followed by hydrogenation (carbide mechanism), and (ii) CO hydrogenation followed by C−O bond cleavage (H-assisted CO dissociation mechanism). We carried out calculations for CHx formation (R1−R11) based on the above mechanisms (Figure 2) and the geometries of TS (Figure 3). The potential energy diagram for these reactions is displayed in Figure 4.

species tend to bind at the Co sites rather than the Cu sites. In general, most of the species are adsorbed with the same adsorption geometries on both surfaces, except for CO and HCO. CO prefers to bind at the Cotop site on Cu6Co3, instead of the 3-fold hollow site on Co9, but interestingly, there is almost no change in BE (−1.81 eV vs −1.79 eV). Similarly, HCO also undergoes a migration from the hollow site to the bridge site when the surface switches from Co9 to Cu6Co3. On the latter, HCO prefers to bind via its C (top) and O (top) at two Co atoms with a BE of −2.13 eV, decreased by 0.13 eV compared to that on Co. The rest of the intermediate species shows the same adsorption geometries on both surfaces. Nevertheless, their BEs are different depending on the number of Co atoms involved in the adsorption. One group of intermediates prefer to bind at the 3-fold hollow sites with three surface Co atoms, and their BEs vary in the range of 0.10−0.35 eV between two surfaces. The other group of intermediates prefer to bind at the top sites merely with one Co atom, and their BEs are nearly unchanged on two surfaces. This contrary trend in the variation of BEs could attribute to the following aspects: (i) the Co−Co distance is changed from 2.489 Å on Co9 to 2.433 Å on Cu6Co3; (ii) according to the Bader charge analysis on Cu6Co3, the average charge transfer between neighboring Co and Cu atoms is only 0.05 e, in other words, there is no significant charge transfer between two components; (iii) the total magnetic moments decreased upon Cu addition, which may contribute to the weakening of adsorption. For the first aspect, it is obvious that the change in Co−Co distance can directly affect the binding of those surface species at the hollow sites, as their adsorptions are merely dependent on the Co3 ensemble. Thus, the energy difference may arise from the contraction of the ensemble. The second aspect indicates that there is nearly no modification in the electronic structure of the individual Co atom upon the addition of Cu, which leads to similar BEs on two surfaces. The differences in the adsorption on two surfaces will definitely take effect on the reaction energetics and reaction pathways, which will be presented in the following section. 3.2. Reaction Pathways on Cu6Co3. In this part, we investigated the thermodynamics and kinetics for the plausible elementary steps involving the formation of C2 oxygenate from syngas on both pure (Co9) and Cu-modified Co(0001) (Cu6Co3) surfaces. All the calculations were performed in the low coverage condition (1/9 ML), and NEB calculations were performed for all initial and final states within 0.2 eV of the most stable coadsorbed configuration within a unit cell. Most of the coadsorption configurations are in favor of locating around the Co3 ensemble with one stabilized therein while the other is in neighboring bimetallic sites. The effects of the pure Co sites, Cu-modified Co sites (namely bimetallic sites), and pure Cu sites on the reactions as well as the formation of key reaction intermediates will be presented and discussed in more detail. 3.2.1. C2 Oxygenates Formation. 3.2.1.1. CO Hydrogenation and Dissociation (R1−R11). Up to now, the CO/ HCO insertion mechanism has been widely used for elucidating C2 oxygenate synthesis on Co-based catalysts. In this mechanism, CO is assumed to be first activated to produce a chain initiator or insertable monomer, CHx species, then followed by the insertion of CO/HCO species to form the precursors for C2 oxygenates. Formation of Insertable Monomers. According to the proposed reaction mechanism, the key issue is to elucidate how CO is activated to form the CHx species. In general, there are two

Figure 3. Top view of geometries of TSs involved in the formation of CHx species on the Cu6Co3 surface (small gray, C; small red, O; small white, H).

The barrier of the CO dissociation (R1) on Cu6Co3 found as high as 2.45 eV, being comparable with our result (2.55 eV on Co9) and the reported data56,60 on pure Co surfaces. This suggests that the direct CO dissociation or carbide mechanism is unlikely to occur on Cu6Co3. As for the H-assisted mechanism, it is reported that the activation barrier for C−O bond cleavage is remarkably reduced after CO hydrogenated.61−63 Two hydrogenation pathways, hydrogenation of CO to form formyl (HCO) and formyl-isomer (COH) were considered. For HCO formation, starting from the most stable coadsorbed configuration, CO at the Co3hol site and H at a neighboring Co1hol site, the activation barrier of 1.35 eV (R2) was obtained. However, the activation barrier was reduced to 1.01 eV at the Cu site. These results suggest that HCO formation at the Cu sites is the energetically favorable pathway from CO hydrogenation. The activation barrier and transition state structure for HCO formation is in good agreement with reported results in the range of 1.31−1.50 eV.34,56,63 The preferred CO hydrogenation product, HCO, can either directly dissociate into methylidyne (CH) and O (R4) or be hydrogenated to form formaldehyde (CH2O) (R5) or hydroxymethylene (CHOH) (R6). As listed in Table 2, the C−O bond cleavage in HCO (0.98 eV) is facilitated significantly compared to that in CO (2.45 eV). The activation barrier and transition state structure are both in good agreement with reported results in the range of 0.90−1.00 eV.34,56,63 The facilitation can be explained by the fact that HCO binds to the surface via both C and O atoms, and the C− O bond is largely elongated (1.289 Å in HCO vs 1.196 Å in CO) which is an indicative of weakened C−O binding. Nevertheless, HCO dissociation is still less favorable than HCO E



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Figure 4. Potential energy diagram for the formation of CHx species on the Cu6Co3 surface. The corresponding TS structures are shown in Figure 3.

monomers formed, and this process will be presented in the following part. 3.2.1.2. CO/HCO Insertion into CHx (x = 1−3) species (R12−R17). As inspired by the homogeneous catalysis,64 the insertion of the unsaturated carbonyl-containing species into a carbon−metal bond of a CHx species has been widely adopted as the chain growth step on Co. Hence, we followed this propagation mechanism for C2 oxygenate formation on Cu6Co3, in which both CO and HCO were considered as the unsaturated carbonyl-containing species for insertion. Dif f usion of CHx (x = 1−3) and HCO. Given that there are three types of sites (Co sites, CoCu bimetallic sites, and Cu sites) on the Cu6Co3 surface, the reaction intermediates may thus undergo diffusion among these sites, which can affect the location of active centers for chain propagation processes. To expound on the effects of diffusion on the bimetallic Co−Cu surface, we have calculated the diffusion barriers of the key reaction monomers involving this process including CHx (x = 1−3) and HCO species, and the diffusion barriers are summarized in Table 3. CHx species are more likely to be formed at the Co sites. Their diffusion from initial Co sites to Cu sites are all endothermic with overall barriers in the range of 0.68−1.39 eV (Table 3). We deduce that the CHx species produced is confined at the Co sites, where subsequent reactions with neighboring reactive species may occur. On the other hand, the key carbonyl-containing species, HCO, is produced at the Cu sites during the reaction. HCO can undergo a barrierless diffusion process to Co sites or bimetallic sites. The facile diffusion of HCO species from Cu sites to Co sites would increase the concentrations of these species at the bimetallic sites. In the next section, the CO/HCO insertion into the CHx species at the bimetallic sites was investigated and summarized in Figure 5, and the corresponding TS structures (ts12−17) are illustrated in Figure 6. CO Insertion (R12−R14). We first calculated the coadsorption of CHx (x=1−3) and CO species on Cu6Co3. As indicated in Table 2, the most stable coadsorption configurations for R12−R14 are very similar, in which CHx tend to

hydrogenation, and the hydrogenation barriers for the formation of CH2O and CHOH are 0.65 and 0.83 eV, respectively. Thus, the preferred products from HCO should be CH2O, which can further undergo two processes: dissociation into CH2 and O (R7), or hydrogenation to form methoxy (CH3O) (R8) or hydroxymethyl (CH2OH) (R9). The calculation results show that CH2O dissociates exothermically (−0.07 eV) with a barrier of 1.05 eV, which is slightly higher than the barrier for HCO dissociation. As for the hydrogenation products, CH3O and CH2OH, they possess formation barriers of 0.58 and 0.68 eV, respectively. Again, dissociation is not preferred compared with hydrogenation. As a result, CH3O is more likely to be the preferred intermediate produced from the subsequent transformation of CH2O. CH3O is very stable on the surface (Eb = −2.62 eV) and binds to the surface via its O atom. The barriers for CH3O dissociation and methanol (CH3OH) formation are 1.60 and 1.48 eV (Table 2), respectively. Consequently, both dissociation (R10) and hydrogenation (R11) of CH3O is difficult to occur on Cu6Co3. Accordingly, CH3 species is unlikely to be formed via CH3O dissociation. On the basis of the above-mentioned results, we can summarize that the most favorable path for CO hydrogenation is CO → HCO → CH2O → CH3O. After CO hydrogenation, the dissociation of HCO and CH2O to produce CH and CH2 become competitive to the corresponding hydrogenation step, while the dissociation of CH3O to CH3 is unfavorable. Thus, two insertable monomers, CH and CH2, could be formed via the H-assisted CO dissociation mechanism. In addition, considering that the surface is covered by certain amounts of hydrogen under the reaction conditions, the formation channel of CHx upon hydrogenation should be involved. From R21− R22 in Table 2, CH2 and CH3 can be easily formed via CH and CH2 hydrogenation with barriers of 0.40 and 0.43 eV, respectively. Thereby, CH2 hydrogenation could act as an alternative route for CH3 formation. In summary, CH comes from HCO dissociation, whereas CH2 and CH3 come from hydrogenation. Chain growth starts immediately once the F



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Table 2. Activation Barriers (Ea), Reaction Energies (ΔH) from DFT Calculations for Elementary Reaction Steps on Co9 and Cu6Co3 no. R1 R2

R3 R4 R5 R6

R7

R8 R9 R10 R11

R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22 R23

reaction CO*+*→C*+O* CO*+H*→ HCO*+*

CO*+H*→ COH*+* HCO*+*→ CH*+O* HCO*+H*→ CHOH*+* HCO*+H*→ CH2O*+*

CH2O*+H*→ CH3O*+*

CH2O*+H*→ CH2OH*+* CH2O*+*→ CH2*+O* CH3O*+*→ CH3*+O* CH3O*+H*→ CH3OH*+*

CH*+CO*→ CHCO*+* CH2*+CO*→ CH2CO*+* CH3*+CO*→ CH3CO*+* CH*+HCO*→ CHCHO*+* CH2*+HCO*→ CH2CHO*+* CH3*+HCO*→ CH3CHO*+* CHCHO*+*→ CHCH*+O* CH2CHO*+*→ CH2CH*+O* CH2*+CH2*→ CH2CH2*+* CH*+H*→CH2*+* CH2*+H*→ CH3*+* CH3*+H*→ CH4*+*

Table 3. Diffusion path and Barriers of CHx and HCO on Cu6Co3 Surface species

diffusion path

EaCo9/ (eV)

ΔHCo9/ (eV)

EaCu6Co3/ (eV)

ΔHCu6Co3/ (eV)

CH

0.28eV Co3hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯

2.55 1.42

1.00 1.14

2.45 1.35

1.34 1.00

CH2

0.23eV Co3hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯

0.85 (Cu site) 0.88

CH3

0.30eV Co3hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯

0.84

1.01 (Cu site) 1.91

HCO

0.19eV Co2brg ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯

0.04eV

0.13eV

1.97 0.83

−0.56

0.98

0.21

1.24

0.30

0.83

0.03

0.68

0.28

0.65

0.16

−0.61

0.52 (Cu site) 0.58

−0.42 (Cu site) −0.52

1.04

0.39

0.32 (Cu site) 0.68

−0.98 (Cu site) −0.10

0.78

−0.65

1.05

−0.07

1.44

0.03

1.60

0.22

1.68

0.75

1.48

0.41

1.05

0.41

1.17 (Cu site) 0.92

−0.24 (Cu site) 0.17

0.83

0.17

0.89

0.13

1.79

0.95

1.66

0.51

0.41

−0.49

0.15

−1.15

0.33

−0.48

0.15

−1.11

1.05

−0.09

0.22

−1.14

0.79

−0.83

1.18

−0.16

1.07

−0.39

1.31

0.46

0.27

−0.95

0.30

−1.07

0.60 0.66

0.21 −0.17

0.40 0.43

−0.36 −0.35

1.05

0.05

0.85

−0.18

0.57

0.21eV

0

0.51eV Co2hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0.09eV

0.44eV Co2hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0.15eV

0.31eV Co2hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0.28eV

0.67eV Co1brg ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0

overall barrier

0.73eV Co1hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯

Cu 3hol

1.39 eV (Co3hol→ Cu3hol)

Cu 3hol


Cu 3hol


0.04eV

0.50eV Co1hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0.09eV

0.56eV Co1hol ⎯⎯⎯⎯⎯⇀ ↽⎯⎯⎯⎯⎯⎯⎯⎯⎯ 0.12eV

0 Cu 2brg ⇀ ↽⎯⎯⎯ 0

Cu 3hol

0.86 eV (Co2brg→ Cu3hol)

that HCO insertion into CH, CH2, and CH3 is highly exothermic (−1.11 to −1.15 eV) with nearly identical activation barriers in the range 0.15−0.22 eV (Table 2). In comparison with two insertion pathways, HCO insertion shows a significant superiority to CO insertion in both thermodynamics and kinetics. This superiority may arise from the smaller HOMO−LUMO gap of HCO compared with CO, which facilitates the charge transfer and hybridization with the surface.65 But it should be noted that HCO formation is highly endothermic, leading to a relatively low surface coverage, which may hinder it from serving as the main reaction channel toward C2 oxygenates. However, under realistic conditions, the presence of coadsorbed species as hydrogen may increase the surface coverage of HCO,36,67 thus making the HCO insertion as a predominant channel. In general, both CO and HCO insertion may serve as the reaction channels for the chain growth toward CHxC(H)O (x = 1−3), which are the precursors for C2 oxygenates. 3.2.1.3. CHxC(H)O Dissociation in C−O Bond (R18−R19). The CHxC(H)O species formed may either desorb as the final oxygenate product or undergo further transformation to terminate the propagation process. As listed in Table 1, CHCHO and CH2CHO both adsorb at the Cu6Co3 surface with high Bes (>1.99 eV). Therefore, those species should have a larger tendency to further react rather than desorb from the surface, while CH3CHO can desorb easily because of the low BE of 0.33 eV. Subsequently, CHCHO and CH2CHO may either be hydrogenated to produce ethanol or break the C−O bond to generate CHxCH species, which can serve as insertable monomers for higher oxygenates or hydrocarbons. It is noted that the chain terminations by hydrogenation occur easily with barriers less than 0.8 eV,36 so the related hydrogenation steps were not investigated in this study. Additionally, recent studies35−37 on Co-catalyzed Fischer−Tropsch synthesis have suggested that CHxCHO species may dissociate to produce CHxCH fragments through C−O bond cleavage with moderate activation barriers from 0.6 to 1.0 eV. We also carried out the calculations of C−O bond scission for two elementary steps, CHCH−O (R18) and CH2CH−O (R19) dissociation on Cu6Co3 surface and the TS structures (ts18 and 19) (Figure 6). CHCHO initially binds at the Co3hol site via its β-C and O atom; then, the atop bonded O atom moves to a neighboring Co1hol site with C−O bond largely elongated from 1.290 Å in IS to 1.827 Å in TS. The CHCHO dissociation is a slightly exothermic process (−0.16 eV) with a relatively high barrier of 1.16 eV. The reaction path for CH2CHO dissociation is quite similar to that for CHCHO, but the dissociation with an activation barrier of 1.31 eV becomes more difficult. The calculations suggest that both C−O bond cleavage in CHCHO

bind at the Co3hol site, while associatively adsorbed CO is stabilized at the neighboring Co1hol site. By these configurations, we found that the insertion of CO into CH, CH2 and CH3 are all endothermic with activation barriers of 0.92, 0.89, and 1.66 eV, respectively (Table 2). Generally, the insertion of CO into CH and CH2 is more favored than that into CH3. HCO Insertion (R15−R17). An alternative carbonyl-containing species for insertion is HCO and the bimetallic sites are favorable for HCO insertion. The ISs for the R15-R17 are quite similar, in which the CHx species (x = 1−3) bind at the Co3hol site, while the HCO species at the bimetallic sites. It was found G



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Figure 5. Chain propagation processes on the Cu6Co3 surface. The corresponding TS structures are shown in Figure 6.

produce CHxO species (x=1−2), which then undergo C−O bond cleavage at the Co sites, leading to the formation of insertable monomers, CH and CH2 species. Another insertable monomer, CH3, is formed via CH and CH2 hydrogenation. Upon CO/HCO insertion at the bimetallic sites, the CHx (x = 1−3) species are propagated to form the oxygenate precursors, CHxC(H)O species, and then the chain growth will be terminated by hydrogenation. Thus, it can be concluded that single metallic Co and Cu sites act as the active centers for the formation of two key intermediates, CHx (x = 1−3) and HCO species, respectively, while the bimetallic sites are the active centers for chain growth toward C2 oxygenates. 3.2.2. Formation of Methane (CH4), Ethylene (C2H4), and Methanol (CH3OH). As suggested by the experimental results10 on oxygenate synthesis over Co/Cu-based catalysts, there are mainly three undesired products formed in the process, including methane, hydrocarbons, and methanol. Here, we discuss the formation of these products and gain a more comprehensive understanding of the whole process. Moreover, C2H4 was chosen to be a representative of the hydrocarbon product as it has been proposed to possess the smallest formation barrier among the C2Hy (y = 2−6) species.66 The potential energy diagram for the subsequent transformation of CH2 species is shown in Figure 7. CH4. Figure 2 shows that CH and CH2 are initially formed through the H-assisted C−O dissociation mechanism, and then these species can undergo sequential hydrogenation processes to produce CH4. The last hydrogenation step (CH3*+H* →

Figure 6. Top view of geometries of TSs involved in the chain propagation processes on the Cu6Co3 surface.

and CH2CHO is not an energetically favorable process on Cu6Co3 surface. 3.2.1.4. Summary of the C2 Oxygenates Formation. From the above-mentioned results, we can summarize the formation route for C2 oxygenate as follows. CO is first hydrogenated to H



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enhanced upon alloying with Cu. The propagation steps behave differently according to the monomer chosen. CO insertion is slightly enhanced, while HCO insertion is largely enhanced and even becomes more favorable than CH2 coupling, which is the most energetically favorable propagation step on Co9. Finally, the C−O bond cleavage of CHxCHO species (x = 1−2) is significantly inhibited. In general, C−O dissociation and CO/ HCO insertion steps are sensitive to the addition of Cu, while hydrogenation steps are generally unaffected. 3.3.2. The Effects of Cu. Clearly, upon alloying with Cu, the selectivity to C2 oxygenate is improved significantly over Cu− Co catalysts. Herein, discussions on the role of Cu will be conducted based on the comparison between Cu6Co3 and Co9. (i) The CO dissociation at the Cu-modified Co sites (bimetallic sites) is suppressed, and thus reduces the probability for hydrocarbon formation. It is well-known that Cu alone is inactive in breaking the C−O bond, which is the prerequisite for initiating the carbon chain growth. As suggested in section 3.2, with the addition of Cu, the CHx−O (x = 1−3) bond cleavage steps are all inhibited both in kinetics and thermodynamics, which means that the addition of Cu can weaken the ability of Co in breaking C−O bond (dilution effect). This is consistent with experimental results that Cu−Co catalysts generally show lower CO conversion than pure Co catalysts.14,30 Due to the inhibition of C−O bond cleavage, the surface concentration of coupling monomers, CHx species, gets lower, and thus suppresses CHx coupling reactions. This is the reason why Cu−Co shows poor selectivity to hydrocarbons. (ii) Co−Cu ensembles serve as reservoirs for undissociated CO and HCO species. CO/HCO insertion into CH x monomers is always the key step for the formation of C2 oxygenate. On the pure Co surface, CO/HCO tends to dissociate rather than stay as undissociated species. However, with the addition of Cu, the undissociated CO/HCO species can exist at the less active Cu sites, then diffuse to the bimetallic sites and insert into the CHx species, which formed at the more active Co sites. By this way, Cu mainly serves as reservoirs for the key intermediates (CO/HCO), which is consistent with the “Dual-Site” mechanism20,24 to some extent. However, besides stabilizing the CO species on Cu sites, the addition of Cu was also found to play prevalent roles in enhancing the production of C2 oxygenates by facilitating HCO insertion in our study. With this facilitation, the relative rate of the CO/HCO insertion channel could be expected to be more favorable than that of CHx formation based on the calculated activation barriers and reaction energies, which is also similar to those results on RhCu16 and Rh.65 In addition, HCO formation at the Cu sites (in section 3.3.1) is more favorable than that at the Co sites, and HCO can easily diffuse to the bimetallic sites, which will increase the surface concentrations of HCO compared to pure Co. As a result, Cu is not only a diluter or stabilizer for undissociated CO, but also provides the active center for the formation of key intermediates (HCO), which may be the origin of the high selectivity to oxygenates. Based on discussions above, the bimetallic sites play an important role in determining the reactivity of the CuCo surfaces. Either electronic effects or geometric effects have been proposed to explain how the interaction between two components affects the catalytic behaviors for bimetallic catalysts. The Bader charge analysis shown in section 3.1.2 indicates that only negligible charge transfer (0.05e) occurs between Co and Cu. Thus, the addition of Cu takes effect on the geometry of ensembles rather than electronic property.

Figure 7. Potential energy diagram for the subsequent transformation of CH2 species on the Cu6Co3 surface. All the initial states have been supposed at the same position for clearer comparison, but the relative stabilities (based on Eb) of the four coadsorbed species are in the order: CH2(−3.81 eV) > H(−2.65 eV) > CHO(−2.13 eV) > CO(−1.70 eV). The corresponding TS structures are shown in Figure 6

CH4) has the highest activation barrier (+0.85 eV), it desorbs from the Cu6Co3 surface immediately once formed. Obviously, CH4 is very likely to be formed as a major product for CO hydrogenation on Cu6Co3. C2H4. The monomer, CH2, is formed via the H-assisted C− O dissociation mechanism, then two CH2 species couples to produce C2H4. The corresponding barrier for CH2 coupling is only 0.30 eV, and the step is also highly exothermic (−1.07 eV). From an energetic perspective, C2H4 formation at the bimetallic sites is highly favorable, even more preferred than CH4. However, as suggested in section 3.2.1, the surface concentration of CH2 species decreases with a drop in the amount of Co sites, and CH2 coupling will obviously be restricted due to the limited supply of reactant monomers. CH3OH. The formation of methanol may happen at both the Co sites and Cu sites. Both of them proceed via the sequential hydrogenation of CO. On bimetallic sites, the step with the highest barrier for the formation of CH3OH is CH3O hydrogenation (1.48 eV), even higher than that of C−O bond cleavage steps. Obviously, CH3OH formation is not favored compared to CHx formation at the Co sites of Cu6Co3. On Cu-rich sites, CH3OH formation is largely facilitated and the highest barrier (in CH3O*+H* → CH3OH) of the process is reduced to 1.15 eV. 3.3. Role of Cu. 3.3.1. Comparison of the Reaction Pathways with/without Cu. In order to clarify the role of Cu in C2 oxygenate synthesis on Co−Cu catalysts, we make a comparison of the reaction pathways for oxygenate formation on Co catalysts with or without Cu. For the chain initiation, the addition of Cu inhibits the C−O bond cleavage steps (CHxO*+* → CHx*+O* in R4 and R9) significantly, while the hydrogenation steps to form CHxO species (CHxO*+H* → CHx+1O*+* in R2, R6 and R7) are enhanced slightly. Notably, the hydrogenation of CHxO to CHxOH (x = 0−3) and hydrogenation of CHx species (x = 1−3) are largely I



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Cu6Co3 surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

(iii) The RC(H)−O bond cleavage of oxygenated intermediates is inhibited . The possibility of C−O bond cleavage in CHxC(H)O species is another factor that influences the selectivity of C2 oxygenates. From the results on Co9, RCH−O dissociation is actually easy to happen with a moderate activation barriers (less than 1.10 eV) and exothermic in thermodynamics. However, RCH−O dissociation is significantly suppressed with the addition of Cu, which can increase the selectivity to C2 oxygenates. It can be inferred that on the surfaces with larger Co ensembles, the RCH−O bond cleavage may be facilitated, leading to the formation of RCH species, which can undergo CO/HCO insertion to produce oxygenates with longer chains. Prieto et al.31 has also identified that Co-enriched surfaces are ideal for the selective production of long chain alcohols. Notably, a larger ensemble could also increase the probability for the formation of hydrocarbons from RCH species. Therefore, this study allows us gain deep insight into optimizing CoCu-based catalysts to achieve higher productivity and selectivity for oxygenates. Optimum CuCo oxygenate synthesis catalysts should be prepared to contain enough bimetallic sites, which require the intimate mixing of the two components; meanwhile, aggregated Co and Cu ensembles should also exist to provide active sites for the formation of propagation monomers. Notably, an increase in the surface cobalt ensemble size may also lead to a shift of the product distribution toward oxygenates with longer chains. Future improvements may require a microkinetic simulation to gain more insights into the promotional effects of Cu as well as the kinetic behaviors of the involved species in realistic reaction conditions, which are still ongoing in this group. Obviously, this work will provide insights and prediction for the improvement of the catalytic performance of Co−Cu catalysts and other related catalytic systems.

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*Phone: (+86) 21-64251928; fax: (86) 21-64251928; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the support from the National Science Foundation (21176071, 21106041, 21273070), the Program for New Century Excellent Talents in university (NCET-12-0852), Science and Technology Commission of Shanghai Municipality (11JC1402700), Innovation Program of Shanghai Municipal Education Commission (12ZZ051), and the Chinese Education Ministry 111 project (B08021).

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REFERENCES

(1) International Energy Outlook 2013; U.S. Department of Energy: Washington, DC, 2013. (2) Anderson, R. B. Fischer−Tropsch and Related Synthesis; Academic Press: New York, 1983. (3) Subramani, V.; Gangwal, S. K. A Review of Recent Literature to Search for an Efficient Catalytic Process for the Conversion of Syngas to Ethanol. Energy Fuel 2008, 22, 814−839. (4) Hindermann, J. P.; Hutchings, G. J.; Kiennemann, A. Mechanistic Aspects of the Formation of Hydrocarbons and Alcohols from CO Hydrogenation. Catal. Rev. 1993, 35, 1−127. (5) Fischer, F.; Tropsch, H. Brenstoff-Chem. 1926, 7, 97−116. (6) Forzatti, P.; Tronconi, E.; Pasquon, I. Higher Alcohol Synthesis. Catal. Rev. 1991, 33, 109−168. (7) Xiaoding, X.; Doesburg, E. B. M.; Scholten, J. J. F. Synthesis of Higher Alcohols from Syngas - Recently Patented Catalysts and Tentative Ideas on the Mechanism. Catal. Today 1987, 2, 125−170. (8) Spivey, J. J.; Egbebi, A. Heterogeneous Catalytic Synthesis of Ethanol from Biomass-Derived Syngas. Chem. Soc. Rev. 2007, 36, 1514−1528. (9) Gupta, M.; Smith, M. L.; Spivey, J. J. Heterogeneous Catalytic Conversion of Dry Syngas to Ethanol and Higher Alcohols on CuBased Catalysts. ACS Catal. 2011, 1, 641−656. (10) Courty, P.; Durand, D.; Freund, E.; Sugier, A. C1−C6 Alcohols from Synthesis Gas on Copper−Cobalt Catalysts. J. Mol. Catal. 1982, 17, 241−254. (11) Wang, J.; Chernavskii, P. A.; Khodakov, A. Y.; Wang, Y. Structure and Catalytic Performance of Alumina-Supported Copper− Cobalt Catalysts for Carbon Monoxide Hydrogenation. J. Catal. 2012, 286, 51−61. (12) Xiang, Y.; Chitry, V.; Liddicoat, P.; Felfer, P.; Cairney, J.; Ringer, S.; Kruse, N. Long-Chain Terminal Alcohols through Catalytic CO Hydrogenation. J. Am. Chem. Soc. 2013, 135, 7114−7117. (13) Subramanian, N. D.; Balaji, G.; Kumar, C. S. S. R.; Spivey, J. J. Development of Cobalt−Copper Nanoparticles as Catalysts for Higher Alcohol Synthesis from Syngas. Catal. Today 2009, 147, 100−106. (14) Su, J.; Mao, W.; Xu, X.-C.; Yang, Z.; Li, H.; Xu, J.; Han, Y.-F. Kinetic Study of Higher Alcohol Synthesis Directly from Syngas over CoCu/SiO2 Catalysts. AIChE J. 2014, 60, 1797−1809. (15) Zhao, Y.-H.; Yang, M.-M.; Sun, D.; Su, H.-Y.; Sun, K.; Ma, X.; Bao, X.; Li, W.-X. Rh-Decorated Cu Alloy Catalyst for Improved C2 Oxygenate Formation from Syngas. J. Phys. Chem. C 2011, 115, 18247−18256. (16) Zhang, R.; Wang, G.; Wang, B. Insights into the Mechanism of Ethanol Formation from Syngas on Cu and an Expanded Prediction of Improved Cu-Based Catalyst. J. Catal. 2013, 305, 238−255.

4. CONCLUSIONS In summary, we have used periodic DFT to study the reaction mechanism of C2 oxygenate formation from syngas on both pure and bimetallic Co(0001) surfaces. On the Cu6Co3 surface, H-assisted CO dissociation mechanism is preferable to the direct dissociation mechanism for the formation of CHx (x = 1−3) species. Starting from the insetable monomers, we have studied both CO/HCO insertion and CHx coupling. The CO/ HCO insertion has found to be the favorable pathway for chain propagation to generate the precursor of C2 oxygenate. Furthermore, a mechanism has been proposed to explain the effects of Cu on the C2 oxygenate formation on Co−Cu surfaces. First, the addition of Cu can control the Co ensemble size and block the active sites for C−O bond cleavage, resulting in inhibition of CHx coupling at the Co sites. Second, Cu can provide undissociated CO/HCO at the Cu sites as well as reduce the barriers for HCO insertion at the bimetallic sites. Combining the mechanistic study and charge analysis on the Cu−Co surface, we conclude that Cu can greatly modify the geometry of active sites. We believe that the insight derived from this study can be valuable for both the catalyst design of oxygenate synthesis and other F-T related catalytic systems.

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ASSOCIATED CONTENT

S Supporting Information *

Additional table and figure for different CuCo surfaces and the preferred adsorption configurations, corresponding BEs and bond lengths of surface intermediate species on Co9 and J



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(17) Kiennemann, A.; Barama, A.; Boujana, S.; Bettahar, M. M. Higher Alcohol Synthesis on Modified Iron Based Catalysts: Copper and Molybdenum Addition. Appl. Catal., A 1993, 99, 175−194. (18) Sugier, A.; Freund, E. US 4122110, 1978. (19) Sugier, A.; Freund, E.; Le Page, J.-F. US 4346179, 1982. (20) Bailliard-Letournel, R. M.; Gomez Cobo, A. J.; Mirodatos, C.; Primet, M.; Dalmon, J. A. About the Nature of the Co−Cu Interaction in Co-Based Catalysts for Higher Alcohols Synthesis. Catal. Lett. 1989, 2, 149−156. (21) Kiennemann, A.; Diagne, C.; Hindermann, J. P.; Chaumette, P.; Courty, P. Higher Alcohols Synthesis from CO+2H2 on CobaltCopper Catalyst: Use of Probe Molecules and Chemical Trapping in the Study of the Reaction Mechanism. Appl. Catal. 1989, 53, 197−216. (22) Sheffer, G. R.; Jacobson, R. A.; King, T. S. Chemical Nature of Alkali-Promoted Copper−Cobalt−Chromium Oxide Higher Alcohol Catalysts. J. Catal. 1989, 116, 95−107. (23) Cobo, A. G.; Mouaddib, N.; Dalmqn, J. A.; Mirodatos, C.; Perrichon, V.; Primet, M.; Chaumette, P.; Courty, P. Evidence and Role of Carbonyl Adspecies in Higher Alcohols Synthesis on Heterogeneous Cobalt−Copper Catalysts. In Studies in Surface Science and Catalysis; Holmen, A., Jens, K.-J., Kolboe, S., Eds.; Elsevier: Amsterdam, 1991; Vol. 61, pp 257−263. (24) Dalmon, J. A.; Chaumette, P.; Mirodatos, C. Higher Alcohols Synthesis on Cobalt Based Model Catalysts. Catal. Today 1992, 15, 101−127. (25) Mouaddib, N.; Perrichon, V.; Martin, G. A. Characterization of Copper−Cobalt Catalysts for Alcohol Synthesis from Syngas. Appl. Catal., A 1994, 118, 63−72. (26) Chaumette, P.; Courty, P.; Kiennemann, A.; Ernst, B. Higher Alcohol and Paraffin Synthesis on Cobalt Based Catalysts: Comparison of Mechanistic Aspects. Top. Catal. 1995, 2, 117−126. (27) Volkova, G. G.; Yurieva, T. M.; Plyasova, L. M.; Naumova, M. I.; Zaikovskii, V. I. Role of the Cu−Co alloy and Cobalt Carbide in Higher Alcohol Synthesis. J. Mol. Catal. A 2000, 158, 389−393. (28) Jacobs, G.; Ribeiro, M. C.; Ma, W.; Ji, Y.; Khalid, S.; Sumodjo, P. T. A.; Davis, B. H. Group 11 (Cu, Ag, Au) Promotion of 15% Co/ Al2O3 Fischer−Tropsch Synthesis Catalysts. Appl. Catal., A 2009, 361, 137−151. (29) Mo, X.; Tsai, Y.-T.; Gao, J.; Mao, D.; Goodwin, J. G., Jr. Effect of Component Interaction on the Activity of Co/CuZnO for CO Hydrogenation. J. Catal. 2012, 285, 208−215. (30) Tsai, Y.-T.; Mo, X.; Goodwin, J. G., Jr. The Synthesis of Hydrocarbons and Oxygenates during CO Hydrogenation on CoCuZnO Catalysts: Analysis at the Site Level Using Multiproduct SSITKA. J. Catal. 2012, 285, 242−250. (31) Prieto, G.; Beijer, S.; Smith, M. L.; He, M.; Au, Y.; Wang, Z.; Bruce, D. A.; de Jong, K. P.; Spivey, J. J.; de Jongh, P. E. Design and Synthesis of Copper−Cobalt Catalysts for the Selective Conversion of Synthesis Gas to Ethanol and Higher Alcohols. Angew. Chem., Int. Ed. 2014, 53, 6397−6401. (32) Nørskov, J. K.; Abild-Pedersen, F.; Studt, F.; Bligaard, T. Density Functional Theory in Surface Chemistry and Catalysis. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 937−943. (33) Honkala, K.; Hellman, A.; Remediakis, I. N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C. H.; Nørskov, J. K. Ammonia Synthesis from First-Principles Calculations. Science 2005, 307, 555− 558. (34) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. A First-Principles Study of Oxygenates on Co Surfaces in Fischer− Tropsch Synthesis. J. Phys. Chem. C 2008, 112, 9464−9473. (35) van Santen, R. A.; Ciobîcă, I. M.; van Steen, E.; Ghouri, M. M. Mechanistic Issues in Fischer−Tropsch Catalysis. In Advances in Catalysis; Bruce, C. G., Helmut, K., Eds.; Academic Press: San Diego, CA, 2011; Vol. 54, pp 127−187. (36) Zhuo, M.; Borgna, A.; Saeys, M. Effect of the CO coverage on the Fischer−Tropsch synthesis mechanism on cobalt catalysts. J. Catal. 2013, 297, 217−226.

(37) van Santen, R. A.; Markvoort, A. J. Chain Growth by CO Insertion in the Fischer−Tropsch Reaction. ChemCatChem. 2013, 5, 3384−3397. (38) Schweicher, J.; Bundhoo, A.; Kruse, N. Hydrocarbon Chain Lengthening in Catalytic CO Hydrogenation: Evidence for a COInsertion Mechanism. J. Am. Chem. Soc. 2012, 134, 16135−8. (39) Inderwildi, O. R.; Jenkins, S. J.; King, D. A. When Adding an Unreactive Metal Enhances Catalytic Activity: NOx Decomposition over Silver−Rhodium Bimetallic Surfaces. Surf. Sci. 2007, 601, L103− L108. (40) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556. (41) Rocha, A. L.; Solórzano, I. G.; Vander Sande, J. B. Heterogeneous and Homogeneous Nanoscale Precipitation in Dilute Cu−Co Alloys. Mater. Sci. Eng., C 2007, 27, 1215−1221. (42) Chanenchuk, C. A.; Yates, I. C.; Satterfield, C. N. The Fischer− Tropsch Synthesis with a Mechanical Mixture of a Cobalt Catalyst and a Copper-Based Water Gas Shift Catalyst. Energy Fuel 1991, 5, 847− 855. (43) Schmid, A. K.; Hamilton, J. C.; Bartelt, N. C.; Hwang, R. Q. Surface Alloy Formation by Interdiffusion across a Linear Interface. Phys. Rev. Lett. 1996, 77, 2977−2980. (44) Ruban, A. V.; Skriver, H. L.; Nørskov, J. K. Surface Segregation Energies in Transition-Metal Alloys. Phys. Rev. B 1999, 59, 15990− 16000. (45) Carenco, S.; et al. Dealloying of Cobalt from CuCo Nanoparticles under Syngas Exposure. J. Phys. Chem. C 2013, 117, 6259−6266. (46) Kresse, G.; Hafner, J. Ab initio Molecular Dynamics for OpenShell Transition Metals. Phys. Rev. B 1993, 48, 13115−13118. (47) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for ab initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169−11186. (48) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (49) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (50) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids, and Surfaces: Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671−6687. (51) Wyckoff, R. W. Crystal Structures; Krieger Publishing: Malabar, FL, 1982; Vol. 1, p 11. (52) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (53) Henkelman, G.; Jónsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985. (54) Henkelman, G.; Uberuaga, B. P.; Jónsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (55) Henkelman, G.; Jónsson, H. A Dimer Method for Finding Saddle Points on High Dimensional Potential Surfaces Using Only First Derivatives. J. Chem. Phys. 1999, 111, 7010−7022. (56) Zhuo, M.; Tan, K. F.; Borgna, A.; Saeys, M. Density Functional Theory Study of the CO Insertion Mechanism for Fischer−Tropsch Synthesis over Co Catalysts. J. Phys. Chem. C 2009, 113, 8357−8365. (57) Liu, J.-X.; Su, H.-Y.; Li, W.-X. Structure Sensitivity of CO Methanation on Co(0001), (10−12), and (11−20) Surfaces: Density Functional Theory Calculations. Catal. Today 2013, 215, 36−42. (58) Gong, X.-Q.; Raval, R.; Hu, P. CHx Hydrogenation on Co(0001): A Density Functional Theory Study. J. Chem. Phys. 2005, 122, 024711. (59) Gong, X.-Q.; Raval, R.; Hu, P. CO Dissociation and O Removal on Co(0001): A Density Functional Theory Study. Surf. Sci. 2004, 562, 247−256. K



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(60) Liu, J.-X.; Su, H.-Y.; Sun, D.-P.; Zhang, B.-Y.; Li, W.-X. Crystallographic Dependence of CO Activation on Cobalt Catalysts: HCP versus FCC. J. Am. Chem. Soc. 2013, 135, 16284−16287. (61) Ciobica, I. M.; van Santen, R. A. Carbon Monoxide Dissociation on Planar and Stepped Ru(0001) Surfaces. J. Phys. Chem. B 2003, 107, 3808−3812. (62) Andersson, M. P.; et al. Structure Sensitivity of the Methanation Reaction: H2-Induced CO Dissociation on Nickel Surfaces. J. Catal. 2008, 255, 6−19. (63) Inderwildi, O. R.; Jenkins, S. J.; King, D. A. Fischer−Tropsch Mechanism Revisited: Alternative Pathways for the Production of Higher Hydrocarbons from Synthesis Gas. J. Phys. Chem. C 2008, 112, 1305−1307. (64) Maitlis, P. M.; Zanotti, V. The Role of Electrophilic Species in the Fischer−Tropsch Reaction. Chem. Commun. 2009, 1619−34. (65) Zhao, Y.-H.; Sun, K.; Ma, X.; Liu, J.; Sun, D.; Su, H.-Y.; Li, W.X. Carbon Chain Growth by Formyl Insertion on Rhodium and Cobalt Catalysts in Syngas Conversion. Angew. Chem., Int. Ed. 2011, 50, 5335−5338. (66) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. Chain Growth Mechanism in Fischer−Tropsch Synthesis: A DFT Study of C−C Coupling over Ru, Fe, Rh, and Re Surfaces. J. Phys. Chem. C 2008, 112, 6082−6086. (67) Ojeda, M.; Nabar, R.; Nilekar, A. U.; Ishikawa, A.; Mavrikakis, M.; Iglesia, E. CO Activation Pathways and the Mechanism of Fischer−Tropsch Synthesis. J. Catal. 2010, 272, 287−297. (68) den Breejen, J. P.; Bezemer, G. L.; Bitter, J. H.; Frøseth, V.; Holmen, A.; de Jong, K. P. On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis. J. Am. Chem. Soc. 2009, 131, 7197−7203.

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