Revisiting CO Activation on Co Catalysts: Impact of Step and Kink

Jan 30, 2017 - Fischer–Tropsch synthesis of hydrocarbons from CO and H2 is an established industrial process, during which the C–O bond must break...
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Revisiting CO Activation on Co Catalysts: Impact of Step and Kink Sites from DFT Melissa A. Petersen, Jan-Albert Van Den Berg, Ionel Mugurel Ciobîc#, and Pieter van Helden ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02843 • Publication Date (Web): 30 Jan 2017 Downloaded from http://pubs.acs.org on January 30, 2017

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Revisiting CO Activation on Co Catalysts: Impact of Step and Kink Sites from DFT Melissa A. Petersen,*† Jan-Albert van den Berg,† Ionel M. Ciobîc㇠and Pieter van Helden† †

Sasol, Group Technology, 1 Klasie Havenga Road, Sasolburg 1947, South Africa

‡

Sasol Technology Netherlands B.V., Vlierstraat 111, 7544 GG Enschede, The Netherlands

ABSTRACT: Fischer-Tropsch synthesis of hydrocarbons from CO and H2 is an established industrial process, during which the C-O bond must break. The preferred mechanism and sites at which CO is activated and hydrocarbon products are formed remains under debate. Density functional theory calculations are used to investigate direct and H-assisted dissociation of CO at kink and step sites on FCC Co(321) and Co(221), at which direct dissociation is demonstrated to be intrinsically preferred at low coverage. The CO dissociation rate is predicted to be higher at defect sites than on extended facets on FCC Co, with dissociation at the kink site having the highest relative rate of all FCC Co site types predicted to be exposed on FCC Co nanocrystals. Nevertheless, the increase in activity is not sufficient to exceed that of highly active HCP Co sites. However, the intrinsically preferred mechanism of CO activation at low coverage, direct rather than H-assisted, is concluded to be the same on FCC and HCP Co. KEYWORDS: cobalt, catalysis, Fischer-Tropsch synthesis, DFT, CO activation, defect sites

INTRODUCTION The Fischer-Tropsch synthesis (FTS) process can be used to generate transportation fuels and chemicals from synthesis gas (CO and H2) derived from coal, natural gas or biomass.1,2 The mechanism of FTS has been debated over many decades, but no consensus has yet emerged.3 Currently, two main schools of thought persist:4-7 (1) that CO first undergoes dissociation and hydrogenation to give CHx surface species, that act as chain initiators and propagators in the formation of long chain hydrocarbon products, in variations of the so-called carbide mechanism;8 or (2) that CO is inserted into a growing hydrocarbon chain prior to breaking of the C–O bond, in the socalled CO insertion mechanism.9 Even in the CO insertion mechanism, the initial formation of a surface CHx species is required to initiate chain propagation in FTS,10 which is generally described as a polymerization reaction.11 The question therefore arises as to how the CHx species form on the catalyst surface. An intuitively appealing idea is that CO adsorbs and dissociates on the catalyst surface to yield C adatoms, which undergo sequential hydrogenation to yield CHx surface species. In the case of Co-based catalysts, early surface science experiments demonstrated that CO adsorbs molecularly on the close-packed Co(0001) surface in the temperature range of 100–450 K,12 and studies employing density functional theory (DFT) calculations report high activation energies for CO dissociation on this surface termination at moderate coverage,3,13-17 that are inconsistent with direct CO dissociation at Fischer-Tropsch reaction temperatures of ca. 500 K employed with Co-

based catalysts.1 These observations led investigators to consider defect sites, such as steps and kinks, as the catalytically relevant sites for CO activation and hydrocarbon formation in FTS.14,18,19 Recent surface science experiments have indeed demonstrated that CO dissociation can already occur at 330 K on Co(0001), in which the sites for dissociation were associated with step defect sites present on the imperfect single crystal surface,20 a possibility that has also been raised in independent work.21 However, it has been argued that under the conditions of high CO pressure typical of FTS, such defect sites may be unavailable for surface reaction, both due to blocking by adsorbed CO and stabilization of reaction intermediates at these sites.22,23 In contrast to invoking defect sites, Inderwildi et al.16 put forward an alternative mechanism for CO activation on Co(0001), previously proposed by Holmen and coworkers,24 by demonstrating that partial hydrogenation of adsorbed CO to formyl (HCO) and formaldehyde (H2CO), significantly lowers the activation energy for subsequent C–O bond scission, compared to the direct dissociation of adsorbed CO. Such H-assisted CO dissociation routes have also been demonstrated to lower the overall activation energy for C-O bond breakage on surfaces of the FTS-active metals Fe and Ru using DFT calculations.25-27 However, hydrogenation of CO is typically reported to be endothermic at moderate coverage,15-17,28 so that the overall activation energy to break the bond in CO remains high on the close-packed Co(0001) plane.10 The alternative hydrogenation of CO at the O-atom to form COH has also been considered; the activation energy for H-addition to

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the O-atom of adsorbed CO is significantly higher than to the C-atom on Co(0001) at moderate coverage.28 However the stability of COH has been demonstrated to be sensitive to the CO coverage using DFT calculations, and its potential role in the activation of CO in FTS has not been discounted.29,30 More recently, a mechanism in which surface hydroxyl groups facilitate hydrogenation of CO to COH has been presented,10 demonstrating that coadsorbed intermediates can influence the kinetic and thermodynamic feasibility of alternative paths for CO activation on the Co(0001) surface. Indeed, DFT studies at higher CO coverage have argued in favor of a mechanism for CHx formation involving a HCOH intermediate,22,29-31 quite distinct from the hydroxymethylene pathway originally proposed by Anderson and Emmett.3,32,33 A separate issue in FTS using Co catalysts is the role of crystal structure in determining activity and selectivity.34,35 The FCC crystal structure has been reported to be the thermodynamically preferred phase for Co particle diameters below ca. 20 nm.36 Wulff constructions of FCC Co particles predict exposure of Co(111), Co(100), Co(110) and Co(311) surface terminations for ideal equilibrium particles.15 However, the number of atoms in a particle need not necessarily match that required to achieve an ideal particle shape, which can give rise to surface defects such as step and kink sites.37,38 Recent results of simulated FCC Co nanocrystals up to 8 nm diameter have revealed the contribution of (111) and (100) facets, B5 step sites and kink sites to the site composition as a function of particle size.39 CO activation at B5 sites was investigated previously on the Co(211) surface,40 which exposes a square and triangular step site geometry (B5-A in ref. 39). A second type of B5 site also exists38 (B5-B in ref. 39) with 3-fold symmetry, which has been stated as being less reactive.41 Indeed, experimental studies of CO dissociation on a stepped Ru surface exposing both types of step sites demonstrated preferential dissociation at B5-A sites,42 although it should be noted that B5 sites of HCP and FCC metals differ. Site composition analysis of simulated FCC Co nanocrystals predict a slightly higher contribution from B5-B sites than B5-A for larger diameter particles (ca. 8 nm),39 so that their propensity for CO dissociation is of interest. Kink sites (i.e. B6 sites in ref. 39) are predicted to occur at similar concentrations as both types of B5 sites on the FCC Co nanoparticle. However, these B6 sites do not arise in Wulff constructions of FCC Co nanoparticles.15,39 The transition state (TS) for CO dissociation at a kink site exposed on HCP Co(11-24) has a significantly lower energy than at step or terrace sites,14 suggesting that kink sites may similarly display enhanced activity for CO dissociation on FCC Co particles. Liu et al15 recently compared the mechanism of CO dissociation on HCP and FCC Co, based on particle morphologies predicted using Wulff constructions. They concluded that a greater proportion of the HCP Co particle surface area exposes sites that are more active for CO activation than for FCC Co, and that there is a preference for the direct CO dissocia-

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tion route on HCP Co, and an H-assisted route via the formyl intermediate for FCC Co.15 Their analysis did not include the possible contribution of kink sites, which may be more active for CO dissociation. In this work, we investigate direct and H-assisted CO dissociation at step and kink sites of FCC Co using DFT calculations. We demonstrate that while CO dissociation is indeed more facile at FCC Co kink sites, the predicted increase in activity does not exceed that of highly active HCP Co sites reported by Liu et al.15 However, the preferred mechanism of CO activation, direct or H-assisted, is concluded to be the same on FCC and HCP Co, in contrast to previous reports.15 Furthermore, the implications of the sustained availability of different site types are discussed.

COMPUTATIONAL METHOD Spin-polarized periodic DFT calculations were performed using VASP43,44 with the PW91 exchangecorrelation energy functional,45 ultrasoft pseudopotentials46 and a plane wave basis set with a kinetic energy cutoff of 400 eV. Methfessel-Paxton smearing47 to first order was employed with a smearing width of 0.2 eV. The Co(321) surface was used to investigate CO activation at kink sites in a p(1×1) surface unit cell; adsorption of one CO per unit cell results in a coverage of one adsorbate per two edge Co atoms (i.e. 0.5 ML). A slab consisting of 20 atomic layers in the [321] direction (i.e. five closepacked (111) layers) was used to model the Co(321) surface, with an inter-slab vacuum spacing of 13 Å. In geometry optimizations, the bottom 10 atomic layers were fixed at the bulk-optimized positions, based on the calculated bulk lattice parameter for FCC Co of 3.538 Å (cf. 3.55 Å).48 Brillouin zone integration was achieved by summation over a 7×6×1 Monkhorst-Pack mesh49 of k-points. CO activation at B5-B sites was investigated with the Co(221) surface within a p(2×1) surface unit cell, which also results in a coverage of one CO per two Co edge atoms when one adsorbate is added to the unit cell. A slab consisting of 16 atomic layers in the [221] direction (i.e. five close-packed (111) layers) was used to model the Co(221) surface, with an inter-slab vacuum spacing of 13 Å. In geometry optimizations, the bottom 8 atomic layers were fixed at the bulk-optimized positions, and a 6×4×1 Monkhorst-Pack mesh49 of special k-points was used. Vibrational frequencies were calculated to verify stable geometries and transition states using finite differences to calculate the Hessian matrix with a step size of 0.02 Å. TSs were located using the climbing image nudged elastic band technique.50 TSs located in this way were further refined using a quasi-Newton algorithm in VASP until the forces converged to less than 0.02 eV/ Å. In all calculations, adsorption was only investigated on one side of the slab, and the energies adjusted using the dipole correction method implemented in VASP. For both Co(221) and Co(321), a thorough investigation of the respective poten2

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tial energy surfaces (PES) for adsorption of intermediates at all high symmetry sites on each surface was performed. Multiple paths for each of the five surface reactions i.e. CO→C+O, CO+H→HCO, HCO→CH+O, CO+H→COH and COH→C+OH were also investigated; only the lowest energy geometries of intermediates and transition states are reported, unless otherwise stated. Further details of the extent of the calculations are given in the Supporting Information. The energy convergence with respect to coverage was verified by reoptimizing lowest energy intermediates and TSs in a surface unit cell doubled along the length of the step-edge i.e. a p(2×1) cell for Co(321) and a p(4×1) cell for Co(221) (see Table S2 and S3 in section 5 of the Supporting Information for further details). CO adsorption energies Ea are defined as Ea = ECO-surf ECO -Esurf where ECO-surf is the energy per unit cell with CO adsorbed on the respective surface, ECO is the energy of gas-phase CO and Esurf the energy per unit cell of the clean surface. Overall activation (Eact) and reaction (ΔErxn) energies are defined as Eact = ETS – EIS and ΔErxn = EFS – EIS, where ETS is the energy of the TS, and EIS (EFS) is the energy of the initial (final) state, defined in terms of the respective intermediates separately adsorbed in their lowest energy geometries (shown in Figure S1 and Figure S3 in the Supporting Information for the Co(221) and Co(321) surfaces, respectively). For intrinsic activation energies, the initial state is the respective pre-dissociation state. Adsorption and activation energies are reported with zero point energies (ZPE) added unless otherwise stated.

RESULTS AND DISCUSSION As discussed in the introduction, a site composition analysis of simulated FCC Co nanocrystals predicts the presence of B5-B sites, such as those exposed on Co(221), with a higher fractional contribution than B5-A sites (as exposed on Co(211)) for larger diameter particles (ca. 8 nm),39 so that investigation of their propensity for CO dissociation is of interest. Adsorption of CO on the Co(221) surface was investigated at all available high symmetry sites. The lowest energy adsorption site is the fcc 3-fold site at the step edge, as shown in Figure 1a. CO adsorbs in this site with a calculated adsorption energy of –1.77 eV, a C–O bond length of 1.20 Å and a stretch frequency of 1744 cm-1 (cf. 1.15 Å and 2106 cm-1 calculated for gas-phase CO). Adsorption in the B5-B site (Figure 1b) is energetically less stable by 0.31 eV and is characterized by a lengthening of the C–O bond to 1.28 Å and a substantial red-shift of the stretch frequency to 1294 cm-1. In the lowest energy TS located for CO dissociation in the B5-B site (Figure 1c), the C–O bond is stretched further to 1.87 Å. The O-atom moves over the step edge towards the upper (111) terrace and at the TS, the O-atom and C-atom only share one Co substrate atom. The activation energy is 1.24 eV, considerably lower than reported activation energies in excess of 2.2 eV for direct dissociation on the close-packed Co(0001) and Co(111) planes.13-17 Since CO is destabilized in the (221) B5-B site relative to

adsorption in the fcc 3-fold site adjacent to the step-edge (Figure 1), the overall activation energy for C–O bond breakage is 1.55 eV at this site. The B5-B site exposed on Co(221) is equivalent to a single unit cell of the Co(110) surface. Liu et al15 investigated CO adsorption and dissociation on Co(110) using DFT, for which an equivalent TS geometry was reported, together with an activation energy of 1.47 eV (using the PBE51 functional). On Co(211), an activation energy of 1.47 eV for CO dissociation in a B5-A site has been calculated using DFT.40 (a)

(b)

(c)

TS

Figure 1. Adsorption geometries for: (a) CO in the lowest energy site, (b) CO in the B5-B site and (c) the TS for CO dissociation on the Co(221) surface. Co atoms are blue, carbon atoms green and O atoms red. The surface is viewed perpendicular to the (111) terraces.

Hydrogen addition to adsorbed CO prior to C–O bond scission was also investigated, through formation of either a formyl (HCO) or a carbon-hydroxyl (COH) intermediate. It has been argued that under FTS conditions high CO coverages may be expected, and that coadsorbed species may influence the relative stability of hydrogenated intermediates involved in C–O bond breakage, and may facilitate further hydrogenation to HCOH.10,22,29-31 However, the purpose of the current study is to investigate the intrinsic propensity of different site types to activate and dissociate CO. Therefore, only low coverage results are reported here such that the influence of coadsorbed intermediates is minimized. (a)

(b)

TS

(c)

TS

(d)

(e)

TS

(f)

TS

Figure 2. Intermediates and TS geometries for H-assisted CO dissociation in the B5-B site on Co(221): (a) HCO; (b) TS for CO+H→HCO; (c) TS for HCO→CH+O; (d) COH; (e) TS for CO+H→COH; (f) TS for COH→C+OH. Atom coloring as before, with H atoms in white.

TSs located for the formation and dissociation of HCO in the B5-B site on Co(221) are shown in Figure 2. These TSs are structurally equivalent to those reported on the Co(110) surface.15 As found for CO, HCO is less stable in the B5-B site (Figure 2a) than when adsorbed at the stepedge (Figure S1 in the Supporting Information) by 0.23 eV. 3

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ACS Catalysis Nevertheless, the C–O bond is lengthened further from 1.28 Å for CO to 1.37 Å for HCO in the B5-B site, with a stretch frequency of only 942 cm-1. The energy barrier for breaking the C–O bond in HCO is lowered to 0.49 eV. However, as previously noted,15-17,28 formation of HCO is an endothermic reaction, so that the overall activation energy for C–O bond scission via the formyl intermediate is 1.38 eV at the B5-B site, higher than for direct dissociation of CO (1.24 eV), and similar to the overall barrier of 1.30 eV reported15 for Co(110) using the PBE51 functional. The overall potential energy profile for the dissociation reactions on Co(221) is shown in Figure 3 with corresponding energy and structural parameters summarized in Table 1. An alternative path for formation and dissociation of HCO along the step-edge also exists (shown in Figure S2 in the Supporting Information), with an overall activation energy for C–O bond scission only 0.04 eV higher than in the B5-B site. 2.4

CO(g)+½H2(g)

C–O scission H addition 1.97

2.0

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.72

1.6

(1.41)

(0.63) 1.69

1.55 COH

1.2

HCO (1.24)

0.8

(0.81)

C+OH

0.4 0.0

(0.49)

1.12

CO (B5) +H

C+O+H

CH+O

CO+H

Figure 3. Potential energy profile for direct and H-assisted CO dissociation on the B5-B site of Co(221). Intrinsic activation energies are in italics and overall energy barriers in bold. The reference energy zero is CO and H adsorbed in their respective lowest energy sites on Co(221).

COH exhibits the same energetic preference as HCO of at least 0.23 eV for the fcc 3-fold site at the step-edge on Co(221) (Figure S1) relative to adsorption in the B5-B site (Figure 2d). COH is also less stable than the HCO isomer in the B5-B site by 0.15 eV. The C–O bond is 1.34 Å and the stretch frequency is 1234 cm-1. Considering the lowest energy TSs for formation (Figure 2e) and decomposition (Figure 2f) of COH in the B5-B site, the activation energy of 0.63 eV for C–O bond cleavage is higher in COH compared to HCO (0.49 eV), and the H-assisted path involving the formation of a COH intermediate is energetically less favorable than via the HCO intermediate at the coverage of 0.5 ML along the step-edge (Figure 3). Nevertheless, both H-assisted paths are less favorable energetically than direct dissociation at the B5-B site (Figure 3, Table 1). Thus in the case of the B5-B step site, the results are in line with the general conjecture for step sites based on results for corrugated Ru surfaces.52

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Table 1. Energy and structural parameters for CO dissociation paths at the B5-B site on Co(221). a

Eact (eV)

ΔErxn (eV)

d (Å)

1.24

–0.16

1.87

CO+H→HCO

0.81

0.89

b

1.44

HCO→CH+O

0.49

–0.96

1.84

CO+H→COH

1.41

1.04

1.26

COH→C+OH

0.63

–0.99

1.97

CO→C+O

a

Distance at the TS between atoms undergoing bond forb mation (hydrogenation) or breakage (dissociation). Excluding the ZPE contribution, the HCO intermediate is 0.05 eV lower in energy than the TS for its formation.

As discussed in the introduction, kink sites such as those exposed on the Co(321) surface are predicted to occur at similar concentrations as step sites on FCC Co nanocrystals,39 but do not arise in Wulff constructions of FCC Co nanoparticles.15 Furthermore, the TS for CO dissociation at a kink site exposed on HCP Co(11-24) has a significantly lower energy than at step or terrace sites.14 The presence of kink sites on FCC Co nanocrystals at comparable concentrations as step defect sites, together with the possibility of enhanced activity for CO dissociation at such sites, justifies their further investigation. Adsorption of CO in the vicinity of kink sites was therefore investigated on Co(321). CO preferentially adsorbs in an fcc 3-fold site at the step-edge on Co(321) (Figure 4a) with an adsorption energy of –1.69 eV, a C–O bond length of 1.20 Å and a stretch frequency of 1757 cm-1. The energetic preference for this site is not particularly strong, as adsorption in adjacent bridge and atop sites at the edge differ by 0.10 Å; ΔνC-O > 600 cm-1 compared to gas-phase CO) has not yet been established. We therefore base the remainder of the discussion on CO 4

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adsorption energies and relative stabilities that have not been adjusted with this empirical technique. (a)

(b)

(c)

TS

Figure 4. Adsorption geometries for: (a) CO in the lowest energy site, (b) CO in the kink site and (c) the TS for CO dissociation on the Co(321) surface. Co atoms are blue, carbon atoms green and O atoms red. The surface is viewed perpendicular to the (111) terraces. (a)

(b)

TS

(c)

TS

(d)

(e)

TS

(f)

TS

Figure 5. Intermediates and TS geometries for H-assisted CO dissociation in the kink site on Co(321): (a) HCO; (b) TS for CO+H→HCO; (c) TS for HCO→CH+O; (d) COH; (e) TS for CO+H→COH; (f) TS for COH→C+OH. Atom coloring as before.

In the pre-dissociation state of CO (Figure 4b) that leads to the lowest energy TS (Figure 4c) located for direct dissociation at the kink site, the C–O distance is 1.28 Å and the stretch frequency is 1284 cm-1. CO in this state is higher in energy by 0.30 eV than in the fcc 3-fold site at the step-edge (Figure 4a), as similarly noted for CO in the B5-B site on Co(221). However, the intrinsic activation energy for direct dissociation at the kink site is only 0.91 eV (cf. 1.24 eV at the B5 site on Co(221)). Ge and Neurock14 investigated direct CO dissociation at a kink site on the HCP Co(11-24) surface, for which an activation energy of 0.92 eV (89 kJ/mol) was reported using the PW91 functional. It is of interest to note that, despite the crystal phase being HCP and the degree of corrugation being greater on Co(11-24) than on Co(321) (i.e. the close-packed terrace of HCP Co(11-24) is one atomic row narrower), the same intrinsic activation energy is obtained, which suggests that the effect of the kink site geometry is quite localized. A slightly higher activation energy of 0.96 eV (92.4 kJ/mol) has been reported on Co(11-21).56 Of further significance is the observation that the activation energy for the kink site is the lowest reported for CO dissociation at FCC-Co defect sites,3,15,40 including C-induced B5 sites on Co(221),57 as well as on Co surfaces in general.3,14,15,40 The only exception so far (0.70 eV; 68 kJ/mol) is the high energy termination of Co(10-10) (the so-called Btermination),58 which is not predicted to be exposed on

HCP Co nanoparticles.15 Single crystal Co(10-10) surfaces also do not favor this surface termination, as verified experimentally.59,60 Table 2. Energy and structural parameters for CO dissociation paths at the kink site on Co(321). a

Eact (eV)

ΔErxn (eV)

d (Å)

0.91

–0.17

1.89

CO+H→HCO

0.71

0.51

1.57

HCO→CH+O

0.56

–0.78

1.91

CO→C+O b

CO+H→COH

1.22

0.80

1.35

COH→C+OH

0.40

–0.91

1.93

a

Distance at the TS between atoms undergoing bond forb mation (hydrogenation) or breakage (dissociation). The TS is accessed via H-addition to CO in a higher energy initial state in the kink site, by 0.13 eV compared to direct dissociation in the kink site (see Table S1).

Considering H-assisted CO dissociation in the kink site of Co(321), the formyl (HCO) intermediate that results in the lowest energy TS located for HCO decomposition is shown in Figure 5a. Compared to the pre-dissociation state for CO (Figure 4b), the C–O bond in HCO is lengthened further to 1.38 Å and the C–O stretch frequency redshifted to 950 cm-1. Although the intrinsic activation energy for C–O bond scission in HCO (Figure 5c) is considerably lower at 0.56 eV compared to direct CO dissociation, the overall activation energy remains high at 1.50 eV (Figure 6), owing to the endothermic formation of HCO (Table 2), that involves H-addition (Figure 5b) to CO in the kink site that is already 0.43 eV higher in energy than CO in the fcc 3-fold site at the step-edge (refer to Table S1). Thus, compared to direct dissociation of CO, this Hassisted route is kinetically less favorable at this coverage. 2.4

CO(g)+½H2(g)

C–O scission H addition

2.0

Energy (eV)

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1.50 1.50

1.51

1.6 (1.22)

(0.40) 1.21

1.2 (0.91)

0.8

1.14

COH

(0.71)

0.4 0.0

(0.56)

HCO

CO (kink) +H

C+O+H

C+OH CH+O

CO+H

Figure 6. Potential energy profile for direct and H-assisted CO dissociation on the kink site of Co(321). Intrinsic activation energies are in italics and overall energy barriers in bold. The reference energy zero is CO and H adsorbed in their respective lowest energy sites on Co(321).

5

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ACS Catalysis Similarly for COH in the pre-dissociation state (Figure 5d) that leads to the lowest energy TS (Figure 5f) located for C–O cleavage on Co(321), the C–O distance is increased to 1.40 Å and the stretch frequency red-shifted to 973 cm-1. The C–O bond is more activated than at the B5 site of Co(221) (cf. 1.34 Å and 1234 cm-1 for COH on Co(221), Figure 2d); the intrinsic activation energy to break the C–O bond is also lower at 0.40 eV (cf. 0.63 eV at the B5 site on Co(221)). Considering the TSs for COH formation (Figure 5e) and decomposition (Figure 5f) in the kink site, the overall activation energy for C–O bond scission of 1.50 eV is again higher than for direct dissociation of CO in the kink site (Figure 6, Table 2). In fact, both H-assisted paths have the same overall activation energy to break the C–O bond in this kink site, as COH is 0.15 eV higher in energy than the HCO isomer (Figure 5a) at the kink site (Table 2, Figure 6). Liu et al15 presented a comparative analysis of the CO direct dissociation rates at low coverage on HCP and FCC Co sites predicted to be exposed on Co nanoparticles based on Wulff constructions of the particle morphology. Using their approach15 to calculate the relative rates of direct CO dissociation on FCC Co defect sites, together with literature data15 for dissociation on FCC and HCP Co facet sites (i.e. (10-10), (10-11), (10-12), (0001), (11-20) and (11-21) for HCP Co; (111), (110), (100) for FCC Co), the relative increase in the direct CO dissociation rate at FCC Co defect sites can be assessed. Specifically, CO dissociation is considered at B5-A sites exposed on Co(211) or Co(311),15,40 B5-B sites on Co(221) and kink sites on Co(321). Since the results reported by Liu et al15 were obtained with the PBE functional and results for Co(211),40 Co(221) and Co(321) (this work) were calculated using the PW91 functional, the geometries of TSs and intermediates at these defect sites were reoptimized using the PBE functional, to eliminate systematic differences between the results due to the choice of functional. The TS for CO dissociation on Co(311), which exposes as B5-A site, was also recalculated, for which a lower energy TS was found (see Figure S4) than reported by Liu et al.15 Full details of the calculation of the relative rates of CO dissociation are given in section 6 of the Supporting Information. Although extended (321), (211) and (221) facets were not predicted to be exposed on an FCC Co nanoparticle obtained using the Wulff construction,15 the relevance of the B5-A [(211), (311)], B5-B [(221)] and kink sites [(321)] has been highlighted in recent simulations of FCC Co nanocrystal morphology and analysis of site composition.39 In particular, these B5 and kink sites readily arise at the boundaries of (111) islands supported on the close-packed facets of the nanoparticles,38,39 and their importance has also been highlighted in independent studies.37,61 Furthermore, kink sites are predicted to contribute a similar fraction to the site composition as both type of step site (B5-A, B5-B),39 so that their reactivity is of particular interest.

16 14

reaction rate (log(r/r(111))

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HCP Co

(11-21) (10-11)

(321)

(10-12)

12

(211)

(11-20)

10 8

FCC Co defects

(311)

FCC Co facets

(221) (100)

(110)

(10-10)

6 4 2 0

(0001)

Figure 7. Calculated low coverage CO dissociation rates at -1 -1 500 K (in site ·s ) on HCP Co facets (blue), FCC Co facets (green) and FCC Co defect sites (red). Rates have been normalized to the CO dissociation rate calculated for FCC Co(111).

The relative CO dissociation rates at low coverage, normalized to the rate on the FCC Co(111) facet, are shown in Figure 7. The results for the HCP Co sites and for FCC Co(100) and Co(110) are equivalent to those reported by Liu et al.15 The only significant difference is for the Co(311) site, for which a lower energy TS was located (refer to Figure S4 in the Supporting Information) with an activation energy of 1.38 eV (cf. 1.56 eV reported by Liu et al15). As a result, the reactivity of the Co(100) and Co(311) sites for direct CO dissociation is reversed in Figure 7 compared to ref. 15, with the predicted rate at the Co(311) site, which exposes a B5-A type site, being higher. Considering the FCC Co terrace and defect sites as a whole, three features are worth noting. Firstly, the relative rate of CO dissociation on FCC Co is higher at all the defect sites than on the extended facets. Secondly, as anticipated based on the energy analysis presented above for kink sites on Co(321), the CO dissociation rate at the kink site is the highest of all FCC Co site types predicted to be exposed on FCC Co nanocrystals.39 Additionally, the B5-A sites, exposed on the Co(211) and (311) surfaces, are marginally more active than the B5-B sites, Co(221), owing to a higher activation energy and similar adsorption energy (Table S4), as also concluded for stepped Ru.42 Thirdly, despite the increased rate at the (321) kink site, the rate still lies below that predicted for HCP Co sites on the (1011) and (11-21) facets.15 Therefore, although the FCC Co kink and B5 sites are predicted to be more active for CO dissociation, the increased relative rate at low coverage is not sufficient to exceed that of the highly active HCP Co(10-11) and Co(11-21) facets identified by Liu et al.15 It should also be noted that if close-packed islands can be supported on the Co(0001) facets of the HCP Co nanoparticle, equivalent B5 and kink sites for the HCP phase are expected to arise along the boundaries of such islands, although their relative contribution to the site composition for HCP Co may be different from that of FCC Co.39 It is therefore concluded that while CO dissociation is indeed more facile at FCC Co kink and B5 sites, the predict6

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ed increase in activity does not exceed that of highly active HCP Co sites reported by Liu et al.15 As noted above for the kink site (as exposed on Co(321)) and the B5-B site (as exposed on Co(221)), the direct CO dissociation route has a lower overall activation energy than either H-assisted path via an HCO or COH intermediate (see Figure 3 and Figure 6). In contrast, Liu et al15 argued that the preferred mechanism for CO activation on FCC Co is H-assisted and different from that on HCP Co, which favors the direct dissociation route. To assess the impact of defect sites, the relative stability of the TSs for C–O bond scission in CO and HCO referenced to gas phase CO and ½H2 is shown in Figure 8 for all FCC Co sites. To eliminate systematic differences due to the choice of exchange-correlation energy functional, and to ensure that ZPE contributions are included, all TSs were reoptimized with the PBE functional and PAW potentials (see Table S5 in the Supporting Information). The ZPE contribution to the stability of the TSs leads to a further destabilization of the TS for C–O bond breakage in HCO relative to that in CO i.e. by ~0.13 eV for HCO dissociation compared to ~0.04 eV for CO dissociation (refer to Table S6 and Table S7). The importance of including the effect of ZPE contributions in systems containing C–H bonds (such as in HCO) has been highlighted before.62 Considering Figure 8a, it is seen that TSs for direct CO dissociation on FCC Co lie at lower energies in general than TSs for H-assisted CO dissociation, consistent with the results reported for HCP Co facets15 and opposite to the trend reported for a subset of FCC Co sites.15 Thus, at least within the energy analysis reported here, it is concluded that the intrinsically preferred mechanism of CO activation is the same on HCP Co and FCC Co, via the direct dissociation path. The origin of the different conclusion for FCC Co sites based on Figure 8a is three-fold: (i) CO dissociation is more facile at the defect sites and the direct dissociation path is energetically preferred at these sites; (ii) a lower energy TS for direct CO dissociation at Co(311) exists than previously reported,15 and on inclusion of ZPE contributions H-assisted and direct CO dissociation become comparable in energy at this site; (iii) inclusion of ZPE contributions generally favors the direct path relative to the H-assisted route. In particular, for Co(100) the TS energy for the H-assisted route changes from being comparable to direct dissociation without ZPE effects, to being 0.17 eV less favorable energetically when ZPE contributions are added. An alternative representation of the results is shown in Figure 8b, in which the relative stability of the TSs for C–O bond breakage in CO and HCO are aligned for each FCC Co site type. Again it is seen that the TS for direct dissociation lies below that of H-assisted dissociation. The only exception63 is for Co(110), which had the lowest contribution to the surface area in a Wulff construction of the FCC Co particle,15 and is not predicted to be exposed as extended facets on nonideal FCC Co nanoparticles up to a diameter of 8 nm.39 Thus the intrinsically preferred mechanism of CO activa-

tion is concluded to be the direct dissociation path on FCC Co. (a)

-0.4 (100)

-0.5

(221) (110)

-0.6

relative energy eV

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(321)

(110)

(311)

-0.7

(100) (311)

(211)

-0.8

(221) (211)

-0.9 -1.0

FCC Co H-assisted

(321)

FCC Co Direct

-1.1

(b)

-0.4 (110)

(100)

-0.5 (221)

-0.6

relative energy eV

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(100)

(321)

(311)

-0.7 -0.8

(221)

(311)

(211)

-0.9 -1.0

(211)

(110)

(321)

-1.1

FCC Co H-assisted FCC Co Direct

Figure 8. Calculated TS energies for C–O bond dissociation in CO (direct path) and HCO (H-assisted path) referenced to gas-phase CO and ½H2 and the respective clean surface. TS energies for direct and H-assisted dissociation (a) ordered from lowest to highest; (b) aligned for each FCC Co site type. ZPE contributions are included. Lines are to guide the eye.

Experimental verification of the mechanism of CO dissociation in Co-catalyzed FTS is challenging, as is evident by the on-going research in this area. Experimentally, steady state isotopic transient kinetic analysis (SSITKA) has been used to investigate elementary steps in Co-based FTS (see for example ref. 64) and spectroscopic techniques have been used to indirectly infer the mechanism of CO activation on Co nanoparticles65 and single crystal surfaces,20 through the detection of dissociation products. Nevertheless, whether the mechanism is the same for HCP Co versus FCC Co is difficult to verify experimentally, as pure-phase size-controlled samples would be needed. Experimental results are also affected by adsorbate coverage, and the overall mechanism may change under different reaction conditions. DFT calculations, such as those reported in the current work, are therefore useful to probe the intrinsic propensity of sites to activate CO, as coverage and phase effects can be accounted for. Nevertheless, it is important to emphasize that the mechanism that dominates (i.e. direct versus H-assisted) under the conditions of FTS will be influenced by the steady state coverages of adsorbed CO and H, the effect of coadsorb7

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ate coverage on the stability of the intermediates and TSs,3 and the availability of free active sites on the catalyst surface. Experimental evidence in support of H-assisted CO activation in FTS and methanation on Co has been put forward.64,66 However, the possibility of the FTS rate being limited by removal of surface oxygen through hydrogenation to water has also been raised,20 while the consequences of the mechanism of CO activation in a regime in which CO dissociation is not rate limiting in FTS has recently been discussed, based on a detailed microkinetic analysis using DFT-calculated results.67 The point made in the current study is that the intrinsic propensity of sites to dissociate CO via different routes favors direct dissociation on both HCP Co15 and FCC Co, with direct dissociation being more facile on HCP Co(11-21) and Co(10-11) sites than the most active FCC Co sites, the step and kink sites. Finally, the origin of the lowering of the activation energy for CO dissociation at step defect sites has been rationalized in terms of a reduction in bond competition in the TS geometry, relief of Pauli repulsion and stabilization of the constituent C and O atoms.68,69 Furthermore, an up-shift of the d-band center for step sties compared to terrace sites is expected to be associated with a stronger interaction of adsorbates and stabilization of TSs at defect sites.70 Generally, CO dissociation is characterized by a late transition state, and linear relations have been successful in correlating the energy of the TS for diatomic dissociation with the adsorption strength of the dissociated products.71 Given the diversity of TS geometries for direct CO dissociation on HCP Co facets13,15,57,58,72 and FCC Co (reported in ref. 15 and 40 and in the current work, see Table S5) no single linear relationship is to be expected between the energies of the TSs for direct CO dissociation and the adsorption strength of the C and O adatom products.73 However, it is worth highlighting that the removal of strongly adsorbed products may be kinetically slow, as embodied in the Sabatier principle and the resulting volanco plots.74 This point is particularly relevant if the removal of products such as oxygen limits the overall rate of FTS.20,67 Considering the adsorption strength of the C and O adatom products on the HCP Co facets15 and FCC Co sites (Table 3), it is evident that both C and O bind more strongly to the HCP Co(10-11) surface than any other FCC or HCP Co site. This raises the interesting question as to whether the Co(10-11) sites, which show a high activity for CO dissociation (Figure 7), and have the highest proportional contribution to the surface area (cf. the Co(11-21) sites, which have been predicted to contribute the least, only 1% based on Wulff construction),15 become blocked under steady state FTS conditions. In contrast, C and O bind much less strongly at Co(321) kink sites (Table 3), for which the predicted fractional contribution to the site composition is up to 5% for a 8 nm FCC Co nanocrystal.39 Nevertheless, the possibility that step and kink sites are blocked under conditions of FTS cannot be excluded a priori. Further studies incorporating microkinetic simula-

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tions are needed to address the interplay between site availability, adsorbate concentration and site activity for CO activation, and to help elucidate the origins of the absolute and relative behavior of HCP and FCC Co nanoparticles under the operating conditions of Co-catalyzed FTS. Table 3. Calculated adsorption energies (Eads) for atomic C and O on HCP and FCC Co sites. FCC a Co

Eads(C) eV

Eads(O) eV

HCP b Co

Eads(C) eV

Eads(O) eV

(100)

–8.01

–5.99

(10-11)

–8.15

–6.06

(311)

–7.69

–5.74

(10-12)

–7.85

–6.04

(211)

–7.63

–5.94

(11-21)

–7.55

–5.85

(321)

–7.42

–5.74

(11-20)

–7.22

–5.63

(221)

–7.28

–5.92

(10-10)

–7.07

–5.70

(110)

–7.25

–5.50

(0001)

–6.83

–5.65

(111)

–6.80

–5.61

a

Results for FCC Co (100), (110), (111) and (311) from ref. 15. Results for (211), (221) and (321) are recalculated in the current study with the PBE functional and the PAW method. b Results for HCP Co are from ref. 15.

CONCLUSIONS CO dissociation at step and kink sites on FCC Co has been investigated with DFT using the Co(221) and Co(321) surfaces. In both cases, the direct CO dissociation path yields the lowest overall activation energy for CO dissociation, with H-assisted routes via a formyl (HCO) or COH intermediate being higher in energy. Comparison of the relative rates of direct CO dissociation on FCC Co sites confirms that the step and kink defect sites are more active than the facets for direct CO dissociation at low coverage, with the kink site exhibiting the highest activity overall. However, the predicted increase in activity of the FCC Co defect sites does not exceed that of highly active HCP Co (10-11) and (11-21) sites. The intrinsically preferred mechanism of CO dissociation on FCC Co is concluded to be the same as for HCP Co, and corresponds to direct CO dissociation.

AUTHOR INFORMATION Corresponding Author * Sasol, Group Technology, P.O. Box 1, Sasolburg 1947, South Africa. Email: [email protected].

ASSOCIATED CONTENT Supporting Information. Lowest energy geometries of intermediates on Co(321) and Co(221), analysis of CO adsorption energies on Co(321), analysis of coverage effects on intermediates and TS energies, kinetic analysis of direct CO dissociation, TS geometries for C-O scission on FCC Co sites. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENT We thank Prof. Rutger van Santen, Prof. Hans Niemantsverdriet and Dr Kees-Jan Weststrate for helpful discussions, and Dr Werner Janse van Rensburg for his support of this project. Ivan Bester is thanked for maintaining the high performance computing facilities at Sasol.

ABBREVIATIONS DFT, density functional theory; FTS, Fischer-Tropsch synthesis; HCP, hexagonal close packed phase; FCC, face centered cubic phase.

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(57) Ciobîcă, I.M.; van Helden, P.; van Santen, R.A. Surf. Sci. 2016, 653, 82-87. (58) Shetty, S.; van Santen, R.A.; Phys. Chem. Chem. Phys. 2010, 12, 6330-6332. (59) Lindroos, M.; Barnes, C.J.; Hu, P.; King, D.A. Chem. Phys. Lett. 1990, 173, 92-96. (60) Over, H.; Kleinle, G.; Ertl, G.; Moritz, W.; Ernst, K.-H.; Wohlgemurth, H.; Christmann, K.; Schwarz, E. Surf. Sci. Lett. 1991, 254, L469-L474. (61) Honkala, K.; Hellman, A.; Remediakis, I.N.; Logadottir, A.; Carlsson, A.; Dahl, S.; Christensen, C.H.; Nørskov, J.K. Science 2005, 307, 555-558. (62) Govender, A.; Curulla Ferré, D.; Niemantsverdriet, J.W. ChemPhysChem 2012, 13, 1591-1596. (63) The H-assisted path is energetically preferred on HCP Co(0001), however the overall activation energy for C–O bond breakage has been reported16,17,72 to be in excess of 1.8 eV and the contribution of this close-packed plane to CO activation is therefore expected to be kinetically insignificant at FTS temperatures of ca. 500 K in the low coverage regime. The energetics of CO activation on FCC Co(111) is anticipated to be similar, due to the close structural relationship between these close-packed surfaces in HCP and FCC Co. (64) Yang, J.; Qi, Y.; Zhu, J.; Zhu, Y.-A.; Chen, D.; Holmen, A. J. Catal. 2013, 308, 37-49.

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(65) Tuxen, A.; Carenco, S.; Chintapalli, M.; Chuang, C.-H.; Escudero, C.; Pach, E.; Jiang, P.; Borondics, F.; Beberwyck, B.; Alivisatos, A.P.; Thornton, G.; Pong, W.-F.; Gui, J.; Perez, R.; Besebacher, F.; Salmeron, M. J. Am. Chem. Soc. 2013, 135, 22732278. (66) Ojeda, M.; Li, A.; Nabar, R.; Nilekar, A.U.; Mavrikakis, M.; Iglesia, E. J. Phys. Chem. C 2010, 114, 19761-19770. (67) van Helden, P.; van den Berg, J.-A.; Petersen, M.A.; Janse van Rensburg, W.; Ciobîcă, I.M.; van de Loosdrecht, J. Faraday Discuss., in press: http://dx.doi.org/10.1039/C6FD00197A. (68) Liu, Z.-P.; Hu, P. J. Am. Chem. Soc. 2003, 125, 1958-1967. (69) Liu, Z.-P.; Hu, P. J. Chem. Phys. 2001, 114, 8244-8247. (70) Jiang, T.; Mowbray, D.J.; Dobrin, S.; Falsig, H.; Hvolbæk, B.; Bligaard, T.; Nørskov, J.K. J. Phys. Chem. C 2009, 113, 1054810553. (71) Wang, S.; Temel, B.; Shen, J.; Jones, G.; Grabow, L.C.; Studt, F.; Bligaard, T.; Abild-Pedersen, F.; Christensen, C.H.; Nørskov, J.K. Catal. Lett. 2011, 141, 370-373. (72) Liu, J.-X.; Su, H.-Y.; Li, W.-X. Catal. Today 2013, 215, 3642. (73) Dahl, S.; Logadottir, A.; Jacobsen, C.J.H.; Nørskov, J.K. Appl. Catal. A: Gen.2001, 222, 19-29. (74) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C.M. J. Phys. Chem. C. 2008, 112, 1308-1311.

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Insert Table of Contents artwork here FCC Cobalt

kinks

increased rate

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CO+H

steps terraces

CH+O C+O C+OH CO dissociation



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