CO Activation on Realistic Cobalt Surfaces: Kinetic Role of Hydrogen

Jul 7, 2017 - Using density functional theory, we evaluate several CO activation pathways at the edges of these nanoislands and propose a cycle involv...
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CO Activation on Realistic Cobalt Surfaces: Kinetic Role of Hydrogen Arghya Banerjee, Alexander P. van Bavel, Herman P. C. E. Kuipers, and Mark Saeys ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00846 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 9, 2017

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CO Activation on Realistic Cobalt Surfaces: Kinetic Role of Hydrogen Arghya Banerjee‡, Alexander P. van Bavel§, Herman P.C.E. Kuipers§, Mark Saeys† ‡

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576

§

Shell Technology Centre Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands

†

Laboratory for Chemical Technology, Ghent University, Technologiepark 914, 9052 Gent, Belgium ABSTRACT: CO activation to generate CH groups is a key reaction step in Fischer-Tropsch synthesis. While several mechanistic proposals can be found in literature, they often do not account for the high coverage and the cobalt surface structure under reaction conditions. Thermodynamic arguments and in situ Scanning Tunneling Microscope images demonstrate that the cobalt surface undergoes a reconstruction to form triangular nano-islands terminated by carbon and CO-covered step sites. Using Density Functional Theory, we evaluate several CO activation pathways at the edges of these nano-islands, and propose a cycle involving the creation of a free site through C hydrogenation and CH migration to terraces, followed by CO dissociation at the step sites of the nano-islands. Hydrogen is hence required to create a vacancy site where CO can dissociate, providing an alternative explanation for the kinetic role of hydrogen. The Arrhenius activation energy of 135 kJ/mol and CO TOF of 10-2 s-1at 500 K are consistent with experimental data. KEYWORDS. Fischer-Tropsch, DFT, CO activation, Cobalt, Nano-islands The Fischer-Tropsch (FT) process converts synthesis gas, a mixture of CO and H2, to long-chain hydrocarbons and water.1,2Due to its high activity and selectivity, cobalt is often the preferred catalyst for FT synthesis. Although FT synthesis has been practiced on an industrial scale for decades, the detailed reaction mechanism remains intensely debated.3-7 One of the critical steps in the FT mechanism is the activation of CO to generate CHx groups. Chain growth occurs either via the coupling of CHx monomers (carbide mechanism2) or via the direct insertion of CO into a growing RCHx chain, followed by CO bond scission (CO insertion mechanism8,9). In either mechanism, the formation of CHx species is an essential step. In the carbide mechanism, a high rate of CO dissociation is required to ensure a sufficiently high CHx monomer coverage to form long chain products. In the CO insertion mechanism, a slower rate of about 1/20th of the CO turn over frequency (TOF) is sufficient since only 1 CHx species is required to initiate the growth of a long-chain hydrocarbon, assuming C20 as the average product. CHx species have been proposed to form by direct CO dissociation followed by carbon hydrogenation10-14, by a hydrogen-assisted route3,11,13,14 and by a hydroxyl-assisted route15.In most cases, those mechanistic studies did not account for the cobalt surface structure and for the coverage under reaction conditions. Direct CO dissociation is very difficult on cobalt terraces with activation barriers of 250 kJ/mol and higher10-14 (Figure S1). Also for the hydrogen-assisted route, barriers are typically close to 200 kJ/mol, and the kinetically dominant route forms CH2O,15 which readily desorbs (Figure S1). In the hydroxyl-assisted route, CHOH is formed via transfer of a proton from a surface OH group to the O-

atom of CO. CH-OH undergoes a rate-limiting C-O scission step to regenerate the OH group and a CH species.15 The corresponding free energy barrier of 159 kJ/mol and TOF of 2 x 10-3 s-1 provide a plausible route for C-O activation on cobalt terraces. In contrast to cobalt terraces, barriers of less than 100 kJ/mol have been calculated for direct CO scission at special step and defect sites.10,16-20 For example, Ge and Neurock10 studied CO dissociation on various corrugated cobalt surfaces and reported a CO scission barrier of 89 kJ/mol on Co(1124). Shetty and van Santen17 reported an even lower barrier of 68 kJ/mol on Co(10-10). These special sites have a very high affinity for carbon,21 which stabilizes the transition state for C-O dissociation. The high affinity for carbon however also leads to high carbon coverages at these sites, making them unavailable for CO dissociation. The high surface energy of highly corrugated surfaces further limits the availability of such sites. The nature of the active sites in FT synthesis has long been debated. Several studies have shown that the FT TOF decreases with particle size for cobalt particles below 6-10 nm, but is independent of particle size above 6-10 nm,22-25 suggesting that the active sites are located on or created on cobalt terraces. The kinetics of the FT reaction is hence identical on particles above 10 nm and on extended cobalt surfaces. Early ex situ STM studies by Wilson and De Groot showed that cobalt terraces undergo a massive surface reconstruction under FT conditions to form cobalt nano-islands of about 2 nm diameter and

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Table 1. Gibbs free binding energy of CO at the transition state# (∆Gads‡),reaction free energies for CO dissociation#(∆Grxn) and CO dissociation transition state structures for various coverages at the B5 edge sites of the cobalt nano-islands.

Transition State

Figure 1. Shape, size and coverage of cobalt nano-islands formed on cobalt terraces under FT conditions. The most stable islands contain 45 Co atoms and are terminated by B5 edges covered by 50% carbon and 100% CO. The sigmaaromaticity of the Co4C unit limits the carbon coverage, leaving B5 vacancies between carbon atoms. (a) top view, (b) side view, indicating the model that was used in this study (white box), as well as the B5 vacancies between carbon atoms (black squares).

with a height of one atomic step26. The edges of these islands consist of step sites and the reconstruction hence introduces a significant concentration of step sites on previously perfect cobalt terraces. The stability, shape and size of such islands27,28 was analyzed, and it was found that this surface reconstruction is driven by the high stability of carbon and CO at four-fold B5 step sites. A balance between the stability gained from the creation of carbon and CO-covered B5 edge sites and the stability penalty from the creation of the corners of the triangular islands determines their size.28 Under FT conditions, the most stable islands are triangular, contain 45 Co atoms and have edge sites with a carbon coverage of 50% and a CO coverage of 100% (Figure 1). In situ STM studies under FT conditions also observed the formation of triangular nano-islands during reaction, however, with a somewhat wider size range than predicted by the thermodynamic calculations, which can be expected based on entropic arguments.28 Both experimental and theoretical results hence show that there is a substantial concentration of B5 step edge sites on cobalt terraces under FT conditions. However, these B5 step sites are covered with strongly bound carbon and CO. Although several DFT studies have evaluated CO activation at various cobalt facets10-14,16-20, the effect of realistic surface CO and C coverages and of the experimentally observed reconstruction of cobalt terraces have not been reported. Since the FT kinetics on relevant particles is identical to the kinetics on reconstructed cobalt surfaces, we evaluate the kinetics on a realistic surface model27,28. In this study, we evaluate several possible pathways for CO dissociation at these carbon- and CO-covered B5 step sites created by the surface reconstruction of the extended cobalt terraces, and propose a plausible CO activation mechanism at the C/COcovered B5 edges of the Co45 nano-islands formed on the cobalt terraces. Similar B5 sites have been proposed for 510 nm Co particles18,29-30, and our model might also apply to the B5 sites on Co particles. Various possible direct CO dissociation transition states are shown in Table 1. The full reaction paths and activation energies can be found in Table S1. To allow a direct

Clean step, Up

Clean step, Down

50%C Up

92 / -41

106 / -31

139 / -1

50%C Down

50%C/100%CO, Up

50%C/100%CO, Down

159 / 11

131 / -10

158 / -12

50%C/100%CO, additional CO, Down

VacancyStep, Down

VacancyStep, additional CO

230/58

138/-22

∆Gads‡/ ∆Grxn (kJ/mol)

Transition State

∆Gads‡/ ∆Grxn (kJ/mol)

Transition State

∆Gads‡/ ∆Grxn (kJ/mol)

113/-45

#

Gibbs free energy for the adsorption of the C--O TS at the B5 step from a gas phase CO reservoir at 4.4 bar, 500 K, and Gibbs free reaction energy for CO dissociation, relative to a CO reservoir. Reaction profiles (reactant and TS) can be found in Table S1.

comparison between the reaction paths, Gibbs free bindings energies for the transition states relative to gas phase CO are reported in Table 1. The C-O dissociation transition states are much more stable at the clean B5 sites (+97 and +111 kJ/mol, Table 1) than on cobalt terraces (+211 kJ/mol, Figure S1). The lower barriers are caused by the high carbon affinity of the empty B5 sites. A hydrogenassisted mechanism is also possible at the clean B5 sites, with a stability of 92 kJ/mol for the CH-O dissociation transition state (Table S2 and S3). Under reaction conditions, the B5 sites are however occupied by both carbon and CO (Figure 1). Carbon binds strongly at the four-fold B5 sites with a binding energy of -678 kJ/mol (Table S4). This value can be compared with a carbon binding energy of only -618 kJ/mol at terraces and of -710 kJ/mol in graphite. The unusual stability of carbon at the B5 sites was analyzed in detail by Nandula et al.21 and attributed to the sigma-aromaticity of the Co4C motif. The carbon binding energy at the B5 edge sites decreases drastically from -674 kJ/mol at 50% coverage to -628 kJ/mol for

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Figure 2. Gibbs free energy profile for CO dissociation at the carbon and CO-covered B5 edge sites of the cobalt nanoisland in Figure 1, and for a CO saturation coverage on the terraces of 7/12 ML. Carbon at the B5 sites (CStep) hydrogenates to CHStep which diffuses to CO-covered terraces displacing a CO, and generating a vacancy at the B5 site without neighboring carbon atoms (vacancystep). CO readsorbs at this vacancystep and dissociates to C and O (CO_TS) or undergoes Hassisted CO dissociation via COH (COH_TS and C+OH_TS). OTerrace and OHTerrace are removed as water to close the catalytic cycle. CH_TS, CO_TS, COH_TS, C+OH_TS refer to the transition states for carbon hydrogenation, CO dissociation, CO hydrogenation to COH and COH dissociation, respectively. Enlarged images of the TS structures are provided in Table S8. coverages above 50% (Table S4).27 The reduced binding energy next to B5 carbon atoms was rationalized by electron count arguments2– the number of electrons in cobalt is not sufficient to achieve sigma-aromaticity in adjacent B5 sites. Adsorption of carbon at a B5 site next to a B5 site occupied by carbon is hence unfavorable under FT conditions, leaving B5 vacancies between carbon atoms and limiting the step coverage to 50% (Figure 1). These B5 vacancies are potential active sites for CO dissociation, as discussed below. CO adsorbs strongly at the B5 edge sites, and even stronger at C-covered B5 edge sites, with an adsorption free energy of -105 kJ/mol (Table S1). This value is significantly higher than the CO adsorption free energy on cobalt terraces, -65 kJ/mol. CO molecules avoid through-space repulsion at the B5 sites by tilting, allowing a 100% CO coverage at the steps under FT conditions (Figure 1). Beyond 100%, CO has a positive Gibbs free adsorption energy at the step edges, and CO coverages above 100% are not thermodynamically favorable under FT conditions.25 Hence, while clean B5 sites are very active for CO dissociation, B5 sites are saturated with both carbon and CO during FT synthesis. The presence of carbon at the B5 sites destabilizes the transition for C-O scission (CO_TS) that is adsorbed between two carbon atoms by 40 and 46 kJ/mol (Table 1(c) and (d), respectively). The lower stability of the transition states can be directly related to the 46 kJ/mol decrease in the carbon affinity at the vacancy next to carbon. The presence of CO near the edge sites further affects their activity. Since the cobalt terraces have a CO coverages of ~50% under FT condi-

tions,5,23,31 we included CO molecules next to the step sites in our calculations. In a first pathway, a CO molecule moves from the bridge site of the B5 edge to the B5 vacancy between two carbon atoms and dissociates. The stability for these transition states (131 and 158 kJ/mol, Table 1 (e) and (f)) is similar to transition states for the reaction path with 50% carbon. Adsorption and dissociation of an additional CO molecule from the gas phase at the B5 vacancy is also very difficult with an effective Gibbs free energy barrier of 230 kJ/mol (Table 1(g)). The low reactivity at B5 vacancies next to carbon can be attributed to the low carbon affinity of such a B5 vacancy. Hydrogen-assisted CO dissociation at a vacancy does not offer an alternative pathway, with hydrogenation barriers of 209 and 216 kJ/mol (Table S2). A carbon coverage of 50% hence completely passivates the B5 sites for CO activation. To create an active site at the B5 edges of the nanoislands, a carbon atom therefore needs to be removed to create a vacancy that is not next to carbon. The energy profile for this reaction path is shown in Figure 2. Diffusion of a carbon atom from the B5 edge to the cobalt terraces is highly unfavorable by 58 kJ/mol. The creation of a vacancy can be made much more favorable by hydrogenation to CH. C* hydrogenation by surface hydrogen adsorbed next to the step site has a barrier of 88 kJ/mol (CH_TS in Fig. 2), and a Gibbs free energy barrier of 90 kJ/mol relative to gas phase H2. This barrier includes the entropy penalty to reach the transition state from the gas phase H2 reference state. Because of the high stability of

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B5 carbon atoms, hydrogenation is thermodynamically unfavorable by 35 kJ/mol. This is very different from the cobalt terraces, where CH species are significantly more stable than carbon under FT conditions. The CH group at the B5 edge can then diffuse to the CO-covered terraces. CO adsorbs at cobalt terraces in two stable configurations: a √3x√3 structure (1/3 ML) and a 2√3x2√3 structure (7/12 ML).31 The calculated difference in stability between both structures is only 5 kJ/mol under FT conditions. We therefore consider a 7/12 ML CO coverage at the terraces as the saturation coverage. Calculations for a lower 1/3 ML saturation coverage can be found in Figure S2. Near CH, the saturation CO coverage on the terraces is reduced to 6/12 ML (Table S5). In other words, CH displaces a CO molecule when it moves to the terrace (Figure 2). The diffusion of CH from the step to the terrace is slightly favorable by 3 kJ/mol, even though 1 CO molecule needs to desorb from the terrace (Figure 2). The CH group on the cobalt terrace can initiate or participate in chain growth, suggesting a dual site model for FT synthesis. Dual site models have recently been proposed for FT synthesis.32 The high carbon affinity of the B5 vacancy not next to carbon dramatically increases the stability of the C-O scission transition state. Either a CO molecule from the edge can move to the B5 vacancy and dissociate (Table 1 (h)), or an additional CO can adsorb at the vacancy and dissociate (Table 1(i) and CO_TS in Figure 2). The attraction between B5 carbon and CO at the edge27, makes the latter option more favorable. A competitive pathway involving CO hydrogenation to COH, followed by C-OH scission at the vacancy site is also shown in Figure 2. Removal of O and OH from the terrace via hydrogenation closes the catalytic cycle (Figure 2). Oxygen hydrogenation has a substantial barrier of 115 kJ/mol on COcovered terraces, and has been suggested to be potentially rate controlling.33-34 Taking CO dissociation at a B5 vacancy as the rate-determining step, and using the most stable B5 step edge covered with 50% carbon and 100% CO and terraces saturated with 7/12 ML CO as the starting point for the catalytic cycle, we calculate a CO TOF at the edge sites of the cobalt nano-islands of 0.9 x 10-2 s-1 under FT conditions (Eq. S7) with an effective Arrhenius activation energy of 135 kJ/mol. Note that the corresponding rate equation (Eq. S7) is zero order in CO, in agreement with experimental observations,35 but depends on the CO and CH coverage on the terraces. The CO TOF was also calculated for a single-site microkinetic model (SI.9) using the elementary steps from Figure 2 and assuming typical CO and CH coverages on the terraces. The TOF for the microkinetic model (Eq. S6) is identical to the model with a rate limiting step. The TOF for the single-site microkinetic model depends parametrically on the coverages on the terraces (in particular the ratio between the CO and CH coverage), but changes by less than an order of magnitude for typical ranges of CO and CH coverages on the terraces.23 For the reaction path via COH, the TOF is 2.1 x 10-2 s-1, with an Arrhenius activation energy of 91 kJ/mol. Note that the calculated TOF is a strong function of the activation barriers, and a 10 kJ/mol increase in the activation barrier decreases the TOF by a factor 10. The surface con-

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centration of B5 sites depends on the size and the concentration of the nano-islands under FT conditions, but can be estimated between 1 and 10%. It should be noted that the proposed reaction mechanism involves hydrogen to go from the reference state, i.e., B5 step edges covered with 50% carbon, to the TS of the rate determining step (CO_TS, Eq. S7), leading to a positive order in hydrogen. The experimentally observed kinetic role of hydrogen3, 29 is hence related to the removal of carbon and the creation of a vacancy at the island edge sites where CO dissociates. To confirm the DFT barriers, we also computed the Gibbs free binding energy of the CO dissociation transition state (CO_TS) using the recent BEEF-vdW functional.36 The BEEF-vdW value, 115 kJ/mol, agrees very well with the vdW-DF value, 113 kJ/mol. In conclusion, we propose a reaction mechanism for CO dissociation to create CH groups accounting for the structure and coverage of the cobalt catalyst under FT reaction conditions. Experimental and theoretical studies indicate that cobalt terraces undergo a thermodynamically driven reconstruction to form nano-islands. The reconstruction is driven by the high stability of carbon and CO at the B5 edge sites of the nano-islands. The edge sites are hence saturated with carbon and CO and not available for reaction. The first step of the proposed mechanism removes a carbon atom from the B5 step to create a vacancy where CO subsequently adsorbs and dissociates, either directly or via COH. In this dual-site mechanism, the CH groups initiate chain growth on the cobalt terraces. The experimentally observed positive order in hydrogen3, 29 therefore stems from the creation of a vacancy. The overall Arrhenius activation energy, 135 kJ/mol, and the turn-over frequency, 0.9 x 10-2 s-1, are consistent with experimental kinetic data23-26 reported for supported cobalt catalysts. This study illustrates the importance of accounting for both the catalyst structure and the coverage of the active sites under reaction conditions in computational catalysis.

ASSOCIATED CONTENT Supporting Information. (1) Computational details, (2) Gibbs free energy profile for the direct and hydrogen assisted CO activation on CO covered terraces, (3) Reactant and transition state structures, Gibbs free energy barriers (ΔGa) and Gibbs free reaction energies (ΔGrxn) for the CO dissociation pathways in Table 1, (4) Reactant and transition state structures, Gibbs free energy barriers (ΔGa), reaction energies for the formation of HCO and COH (ΔGrxn) at clean and C/CO-covered B5 step sites, (5) Gibbs free energy barriers (ΔGa), reaction energies (ΔGrxn) and transition state structures for the dissociation of HCO and COH at clean and C/CO-covered B5 step sites, (6) Average and differential binding energies for carbon at B5 site for different coverages, (7) Gibbs free energy for CO adsorption on Co terraces near a CH group for a 2√3x2√3 unit cell, (8) Gibbs free energy profile for CO dissociation at a carbon and CO-saturated B5 edge site of a cobalt nano-island and (9) Microkinetic Model. (10) Transition state structures for direct and H assisted CO dissociation at C covered B5 edge sites This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by Shell Global Solutions and by an Odysseus grant (FWO G0E5714N). We thank G.T. Kasun Kalhara Gunasooriya for his help on the microkinetic model.

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