Stability of Carbon on Cobalt Surfaces in FischerâTropsch Reaction

Article pubs.acs.org/JPCC

Stability of Carbon on Cobalt Surfaces in Fischer−Tropsch Reaction Conditions: A DFT Study Manuel Corral Valero* and Pascal Raybaud IFP Energies nouvelles, Rond-point de l’échangeur de Solaize, BP 3, 69360 Solaize, France S Supporting Information *

ABSTRACT: We establish a thermodynamic model correlating the carbon deposition and desorption energies on cobalt surfaces with the temperature, the CO/H2 ratio, and the Anderson−Schulz−Flory (ASF) distribution in the Fischer−Tropsch (FT) synthesis reaction. Density functional theory (DFT) calculations predict that the surface sites of Co particles, whether located on terraces or steps, are covered by carbon species in FT reaction conditions unless extremely high H2/CO ratios are used. The graphene-covered Co(111) surface is the most stable system. At low ASF coefficient, the Co(111) surface undergoes a strong reconstruction associated with the insertion of C in subsurface sites. At high ASF coefficient, the Co(111) surface is covered by oligomeric C species which may be seen as either long chain alkane or graphene precursors. This theoretical work gives crucial insights into the link between the composition and structures of Co surfaces in FT reaction conditions. Furthermore, it highlights that the existence of purely metallic sites, whether located on terraces or steps, is questioned in long-run FT synthesis.

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Flory (ASF) distribution of products, α, which describes the relative ratios of hydrocarbon chain lengths obtained from their probabilities of occurrence.8,15 Hence, a thorough evaluation of the interplay between these conditions and the nature of C adsorption on Co surface sites is crucial to propose a clearer picture of the deactivation process of Co-based FT catalysts. In this paper, we will present a thermodynamic model that predicts the Gibbs free energy of formation of the Co(111), Co(112), and Co(221) surfaces with different carbon coverages and in the presence of the FT reagents and products. The discussion of our thermodynamic results with kinetic data published in the literature will help to understand the long activation decay of Co-based FT catalysts and will point out that the evolution of the active sites in Co-based catalysts is still an open question. The methodology section will explain how the reacting environment, imposed by the reaction T, PCO/PH2, PH2O, and the ASF coefficient, affects the carbon deposition and hence the nature of the available active sites.

INTRODUCTION Although the Fischer−Tropsch (FT) synthesis has been known for almost a century, many questions remain open about fundamental issues such as the involved reaction mechanisms and the deactivation process.1 These two issues are intimately linked since there is evidence that the reaction mechanism is site sensitive2,3 (that is to say, that it strongly depends on the structure of the catalyst active phase) and that the presence of carbon produced by the dissociation of CO leads to a rearrangement of the catalyst structure4,5 concomitant to its activity decay.6 Hence, the interaction of C with Co-based FT catalysts is a key parameter impacting the FT reaction mechanism, and it is at the origin of the deactivation process. It is well-known that the interaction of C with Co has undesirable effects on the activity of the FT catalysts.6,7 Experimental results show that there is an important buildup of carbon on the top of a used Co catalyst surface at the early stages of the FT synthesis reaction.8 Moreover, experimental and theoretical results reveal that an amorphous C overlayer and a Co2C-like phase form during the FT reaction on Cobased catalysts.9,10 Density functional theory (DFT) studies suggest that this carbon overlayer could have a graphene-like structure.11,12 Other theoretical studies propose that C also interacts predominantly with subsurface Co sites leading to the formation of a carbide-like surface.13 Besides, it has been shown that carbon atoms induce a structure rearrangement of the Co surface4,5 similar to the (111) to (100) surface transformation seen on Ni.14 In spite of these numerous studies, there is still a need to better quantify the stability domains of carbon species on the possible exposed sites of Co particles at the FT reaction conditions: T, PCO/PH2, PH2O, and also the Anderson−Schulz− © 2014 American Chemical Society

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METHODS 2.1. Structures and Parameters for Total Energy Calculations. We performed periodic slab calculations at the DFT level using the VASP package.16,17 We used the PAW pseudopotentials parametrized for the PBE functional18 according to the Blö c hl formalism.19 Electron energy computations were performed with a constant energy cutoff Received: January 14, 2014 Revised: September 3, 2014 Published: September 4, 2014 22479

dx.doi.org/10.1021/jp5004177 | J. Phys. Chem. C 2014, 118, 22479−22490

The Journal of Physical Chemistry C

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of 400 eV, a Gaussian smearing width of σ = 0.01 eV, a reciprocal space sampling of 0.04 Å−1 fineness, and an accuracy of 10−5 eV/slab. Geometry optimizations were performed until all interatomic forces were below 0.02 eV/Å. All wave function calculations were performed in the spin-polarized mode in order to account for cobalt magnetic properties. In this work, we consider the fcc-Co phase which is present in a non-negligible amount in fresh supported Co catalysts.20,21 Although bulk Co undergoes a phase transition from hcp to fcc at about 673 K, support effects are known to change this phase transition temperature. In addition, the particle size of supported Co clusters may influence this trend since on the basis of thermodynamic calculations the fcc structure was shown to be more stable for particles of size below 100 nm (as used in FT catalysts).22 The fcc-Co(111) surface was represented by a symmetric slab model of seven layers. We used a p(2 × 2) unit cell and placed a 15 Å long empty region perpendicular to the surface plane in order to describe the vacuum and to minimize lateral interactions. Carbon coverages are expressed with respect to the number of surface Co atoms (i.e., 1 ML corresponds to 4 C atoms on top of the p(2 × 2) unit cell). We studied the different C coverages in the range between 1/4 ML up to 2 ML in the p(2 × 2). This supercell was a good compromise to screen the carbide-like states and also the graphene-like states (see below) and their corresponding energetics. For each C coverage, we investigated various types of structural configurations: C atoms can be located either on the outer surface or on the octahedral subsurface sites (below the first Co atomic layer). For carbon coverages beyond 1 ML, we considered two initial configurations: those inherited from the structure at θC= 1 ML with an increasing number of C atoms on the top of the metal layer and those inherited from a graphene overlayer with a decreasing number of C atoms. The slab model containing a graphene overlayer consists of a graphene C(0001) sheet, extracted from a previously optimized P6_3/mmc graphite bulk structure,23 optimized in an empty periodic box using the above parameters and placed over the Co(111) surface model (Figure 1). The lattice mismatch

effects on the evaluation of the supported graphite structure and the graphene sheet. Lastly, in order to take into account the step surfaces often invoked in the literature as exposing key sites for CO dissociation,2,24 we considered the adsorption of up to four C atoms at the steps of the (112) and (221) surfaces (Figure 2).

Figure 2. Ball and stick representations of the (a) Co(112) and (b) Co(221) stepped surfaces (left: front view, right: side view). Red balls are cobalt atoms located on steps; green balls are located on terraces; and small blue balls are in the subsurface.

The two latter surfaces were used as model systems of step sites in Co-based catalysts.24 On these surfaces, all C atoms were located at the lower 4-fold (112) and 3-fold (221) sites of the steps, which are the sites where Co atoms have the lowest coordination numbers and are expected to be the most reactive.2,24 2.2. Thermodynamic Models. The global chemical equation of the Fischer−Tropsch synthesis reaction yielding an alkane can be written as follows nCO + (2n + 1)H 2 → CnH 2n + 2 + nH 2O

(1)

As it is often the case in heterogeneous catalysis, the catalyst does not appear explicitly in this equation since it is considered to accelerate the rate of the reaction without suffering any permanent chemical change. However, this is an ideal situation, especially for Co-based FT catalysts, as both experimental and theoretical data mentioned in the Introduction suggest that the metallic state of cobalt is not stable. Therefore, in order to analyze the true chemical state of the catalyst, it is mandatory to quantify the interaction of C with the catalyst during the FTS reaction. For that purpose, we propose to decompose eq 1 into the two following chemical processes

Figure 1. p(2 × 2) ball and stick representation of the graphene overlayer on the Co(111) surface. Blue balls: Co atoms, black balls: C atoms (large on the surface, small in the first subsurface).

Co(hkl) + NCCO + NCH 2 → Co(hkl) − C NC + NCH 2O (2a)

⎛1 ⎞ ⎜ + 1⎟NCH 2 ⎝i ⎠ N → Co(hkl) + C CiH 2i + 2 i

Co(hkl) − C NC +

between the Co(111) and C(0001) surfaces is rather small ( −6.65 eV at the considered T, P, and H2/CO ratio), the ΔGdes line for θC = 7/4 ML remains the lowest one, which means that the formation energy of alkanes Table 3. Relevant ΔGtrans Energies Computed from Equation 22a ΔGtrans (eV/surface Co atom) Δμ2 (eV) a nd α −6.44, α = 0.95 −6.65, α = 0.89 −6.85, α = 0.71 −7.1, α = 0.2 −7.2, α = 0

from θC = 7/4 to θC = 3/2 ML

from θC = 7/4 to θC = 5/4 ML

from θC = 7/4 to θC = 1 ML

0.06

0.39

0.38

∼0.00

0.28

0.15

−0.06

0.16

∼0.00

−0.11 −0.13

0.06 0.05

−0.19 −0.26

Correlation between Δμ2 and α is established at T = 500 K, PT = 20 bar, and H2/CO = 2 (see Figure 4). a

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∼ −6.71 and −5.76 eV at T = 450 and 850 K, respectively. At this Δμ2 values ΔGdes are positive, and hence the system has a tendency to adsorb more C atoms and increase its C coverage prior to the formation of any hydrocarbons. As intuitively expected, if one virtually keeps constant the ASF coefficient and decreases the H2/CO ratio to 0.001, the stability of C adsorbed species increases, since Δμ2 increases too (Figure 4). In order to reach the value Δμ2 = −7.4 eV at T = 500 K and P = 20 bar, at which the metallic surface is more stable than the one with θC = 1 ML, the H2/CO should be around 200. In addition, if one keeps the H2/CO ratio constant and increases the ASF coefficient, it also induces an increase of the chemical potential of Δμ2 which results in higher C coverages on the Co(111) surface or stronger stability of C species on-top of the surface (and not in the subsurface). As aforementioned, this means that the higher the selectivity in long chain hydrocarbons, the higher the stability of carbon on the cobalt surface. This trend is chemically intuitive although the risk of forming graphene-like species increases. Lastly, we remind the reader that our computed Δμ1 value for the Boudouard reaction at P = 20 bar and T = 500 K is −4.9 eV. Again, this value is much higher than the range for the FTS reaction, and according to the predictions in our model, there should be a heavy buildup of C on the surfaces of Co. 3.3.2. Cobalt Stepped (221) and (112) Surfaces. Considering now stepped surfaces, Figure 11 shows that step sites are also stabilized by carbon deposition.

accompanied by the desorption of C atoms is an endothermic process. In particular, the complete removal of C species at α values between 0.89 and 0.99 corresponds to ΔGdes values in a range between 1 and 2 eV/surface Co atoms which is far from being accessible. Although intermediate coverages cannot be excluded as being thermodynamically stable (such as θC = 3/2 ML whose transition energy from the 7/4 ML coverage is below 0.4 eV/surface Co atom; Table 3), this means that for such high values of the ASF coefficient the working state of the cobalt surface should be covered by a high amount of C species leading to a competition in the formation of either polymers or graphene species. For 0.71 < α < 0.89, the 3/2 ML C coverage is preferred. This means that the C desorption process starting from θC = 7/ 4 ML is now exothermic (Table 3). For α ≤ 0.71 a different structural configuration for the surface exists as the most stable coverage, corresponding to the reconstructed surface at θC = 1 ML described in Section 3.2.1. We remind the reader that this surface also contains 50% of C atoms in subsurface positions and hence in a “carbide-like” environment. Moreover, the results in Table 3 are also consistent with recent chemical transient kinetic studies by Schweicher et al. according to which at low α (∼0.2) there is an important C build-up at the catalyst surface.36 This point will be further discussed by considering also stepped surfaces. Therefore, the present thermodynamic analysis reveals that the Co(111) surface may be covered by two kinds of C species during the FTS reaction: either surface carbons for α > 0.71 or C atoms in a carbide-like surface for α < 0.71. Of course, we should also bear in mind the impact of ZPE, thermal, and van der Waals corrections on these α crossing points, so we invite the reader to carefully consider our analysis and discussion provided in the Supporting Information. If we take into account the hydrogenating environment, the hydrogenated derivatives of C species in on-top positions can also be seen as representative of the precursors of the long chain hydrocarbons. Therefore, considering the fact that for α ≤ 0.71 (T = 500 K, P = 20 bar, and H2/CO = 2) the reconstructed surface becomes more stable than the one at θC = 7/4 ML, we may consider this surface reconstruction to be intimately linked to the loss of catalyst selectivity in alkane formation. This insight is thus of significant impact for the optimization of the catalytic performance and selectivity. At this stage, it is interesting to evaluate the impact of the ASF distribution on the characterization of the surface state. For that purpose, we compare our results with previous DFT calculations,13 investigating the deposition of carbon on various metallic surfaces, including the Co(111) surface, from eq 4 only, e.g., in the absence of any hydrocarbon species. According to that work, at θC = 1/3 ML cobalt would form either a surface carbide with C atoms in subsurface sites at T = 450 K and standard pressure or a structure with C atoms on-top of the surface at T = 850 K. Using the same (T, P) conditions, our calculated Δμ1 value is −6.01 eV (for both temperatures; i.e., the temperature effect is negligible), and considering carbon coverages θC = 1/4 and 1/2 ML, which are the closest to the one used in ref 13, we find that 50% C atom would be located in subsurface sites for θC = 1/2 ML (Figure 6). For θC = 1/4 ML, C atoms occupy preferentially subsurface sites in the absence of any produced hydrocarbons which is consistent with ref 13. According to Figure 10, at this Δμ1 value the surface at θC = 7/4 ML should be preferred. Moreover, if one considers now the FTS reaction at a typical α value such as 0.9,26−28 Δμ2

Figure 11. Carbon deposition and desorption Gibbs free energies for the Co(112) (red), Co(221) (blue), and Co(111) (black and dashed) surfaces at different carbon coverages and as a function of Δμ1 and Δμ2. The bottom X-axis represents the link between the ASF coefficient and Δμ2 for a H2/CO ratio of 2 at T = 500 K and P = 20 bar.

The slab models used for Co(112) and Co(221) orientations contain 12 and 16 surface Co atoms, respectively, and are thus larger than the flat Co(111) surface, which implies that the carbon coverages investigated in Figure 11 are smaller. Moreover, the ratio between cobalt atoms at steps and terrace sites is rather large in our slab models: 2/3 on the Co(112) orientation and 1/2 on the Co(221) orientation. So carbon coverages reported in Figure 11 correspond to this initial distribution of step and terrace sites although we do not expect the main trends described below to change for other step/ terrace atomic ratios. Figure 11 shows that the carbon deposition energies at θC = 1/4 ML for the two stepped 22487



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competing with C hydrogenation steps and CH 2 −CH 2 coupling. Since the Co reconstructed surface involves the diffusion of C on the inner layers of Co, its formation should compete with the FTS reaction, which again could be seen as leading to a loss in the selectivity of paraffin formation. By contrast, the coupling of C atoms should be slower than either the hydrogenation of C atoms, its diffusion to the inner Co layers, or the C−C coupling between hydrogenated CHx fragments. Therefore, according to the data published so far in the literature, two processes may kinetically compete: the chain growth mechanism and the formation of reconstructed Co surfaces. The graphene sheet formation through direct C−C coupling may be kinetically hindered and may deactivate the catalyst in the long run. This possibly explains why both C atoms in an environment similar to the Co2C phase and an amorphous polyaromatic C structure were found simultaneously on a spent Co-based FT catalyst by Saeys and coworkers.9 In our view, what the authors proposed as a surface carbide phase could be the outcome of a reconstructed Co structure, since in both cases C atoms are in the inner layers of the Co structure. With respect to the balance between the formation of Co2Clike phases and Co-supported graphene islands, it is interesting to note that in ref 40 the authors found similar results for Pd. According to the latter reference, the formation of graphene Pd-supported phases is delayed by the formation of a Pd carbide phase. According to Seriani et al.,40 a critical radius of C-supported particles is needed for the nucleation of a stable supported graphitic phase. Although these results concern a different metal, our results seem in line with their conclusions. Therefore, it seems that as the FT reaction proceeds (as we increase α) the metallic active catalyst sites change and sites in interaction with C are formed. Further work needs to be done in this field, in particular by revisiting some of the activation barriers computed in refs 35 and 38 in the presence of spectator C atoms in order to clearly find the balance between the FTS reactions and the formation of species leading to selectivity loss and catalyst deactivation. Perspectives in this work will certainly help to understand the long activation decay of Co-based FT catalysts as well as to clarify the balance between the different initiation steps for the FT synthesis proposed in the literature (direct versus H-assisted CO dissociation).

models considered in this work are more favorable than the deposition of this species on the flat Co(111) terrace for the same carbon coverage. Hence, the deposition of C on a cobalt particle should be initiated at stepped sites, and once these sites are covered by C atoms, the deposition of C on the flat terraces would begin.9 In particular, at low ASF coefficients (∼0.2 corresponding to the one by Schweicher et al.37), it may be suggested that part of the observed carbon species could be located at the steps of the Co(112) structure. According to our results, the (112) stepped surface remains metallic for Δμ2 < −8 eV. In order to reach the necessary reaction conditions to achieve this chemical potential, we would need extremely high H2/CO ratios (1011 at P = 20 bar, T = 500 K, considering eq 6 for the formation of methane only) or low temperatures (T ∼ 50 K for P = 20 bar and H2/CO = 2), which are not representative of the FT process. According to this thermodynamic analysis, we demonstrated that metal sites on both steps and flat terraces can be carburized at different ASF coefficients and H2/CO ratios fixing the C deposition and desorption free energies on the catalyst surface. Hence, both Co terraces and step sites have a strong tendency to capture C single atoms produced by the CO dissociation step during the FT synthesis and, as far as the Co(111) surface is concerned, to be covered by a graphene overlayer. As for the availability of metallic sites, they are stable only at extremely high H2/CO ratios or low ASF values corresponding to methanizing conditions. 3.4. Discussion on Kinetic Effects. The previous investigation is the outcome of a thermodynamic analysis that does not take into account the rates of the many chemical reactions that take place during the FT synthesis. A careful analysis of the activation barriers of different FT reactions steps published in the literature35,38 gives a complementary view to our present thermodynamic analysis. However, it should be borne in mind that the works published so far did not take into account C coverage effects, and thus in our view they are more likely to be representative of the catalyst state at the very beginning of the FT synthesis reaction (very low α). Nevertheless, there is truly an interplay between thermodynamic and kinetic effects which may explain why different carbonaceous species are detected at experimental conditions. Cheng and co-workers report the activation energies of all possible coupling reactions between C1Hx (x = 0...3) species on the flat Co(0001) surface at the DFT level using the PBE (GGA) functional.38 The coupling of bare C single atoms (C− C) exhibits an activation energy of 1.22 eV. By contrast, the coupling between monomers containing one or more H atoms presents lower activation energies, the lowest one being that involving two CH2 fragments (0.70 eV). Besides, the hydrogenation barriers of C1Hx (x = 0, 1, 2, and 3) fragments published in the same reference are in a range between 0.60 and 0.96 eV. Moreover, Li and co-workers found that activation energies (using the PW91 functional) for C diffusion in the Co(111) and Co(0001) subsurfaces are 0.74 and 0.65 eV, respectively.35 Note that the effects of the PBE and PW91 functionals are not significant here according to refs 38 and 39. As a consequence, although these activation barriers published so far in the literature might be representative of the catalyst at the beginning of the FT synthesis reaction (according to our thermodynamic analysis), there seems to be a competition between the diffusion of C atoms to the inner layers of the Co catalyst and their participation to the FT reaction. For instance, the C diffusion leading to the subsurface carbide is kinetically

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CONCLUSION We achieved a thermodynamic model depicting the stability of Co sites at terraces and steps in FT reaction conditions based on a thermodynamic analysis of the carbon deposition and desorption reactions at the catalyst surface. We show that Co sites have a strong tendency to be covered by carbon during the FTS reaction. By contrast, the metallic sites are stable for extremely low ASF values, high H2/CO ratios, or low temperatures. According to our results, the graphene-covered Co(111) structure is the most stable at standard FT reaction conditions (T = 500 K, P = 20 bar, H2/CO = 2) and should be considered as a thermodynamic well for the surface state of the catalyst. The formation of the reconstructed Co(111) surface at θC = 1 ML is less stable than the graphene-covered Co(111) surface but significantly more stable than the metallic Co(111) surface for α ≤ 0.71 and at typical FT operating conditions. Therefore, it strongly suggests that the formation of reconstructed Co(111) surfaces is intimately associated with a selectivity loss of the catalyst. For α > 0.71 we found that carbon coverage 22488



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(2) van Santen, R. A.; Ciobîcă, I. M.; van Steen, E.; Ghouri, M. M. Mechanistic Issues in Fischer−Tropsch Catalysis. In Advances in Catalysis; Elsevier Academic Press Inc: San Diego, USA, 2011; Vol. 54, p 127−187. (3) van Santen, R. A.; Ghouri, M. M.; Shetty, S.; Hensen, E. M. H. Structure Sensitivity of the Fischer−Tropsch Reaction; Molecular Kinetics Simulations. Catal. Sci. Technol. 2011, 1, 891−911. (4) Weststrate, C. J.; Kizilkaya, A. C.; Rossen, E. T. R.; Verhoeven, M. W. G. M.; Ciobîcă, I. M.; Saib, A. M.; Niemantsverdriet, J. W. Atomic and Polymeric Carbon on Co(0001): Surface Reconstruction, Graphene Formation, and Catalyst Poisoning. J. Phys. Chem. C 2012, 116, 11575−11583. (5) Ciobîcă, I. M.; van Santen, R. A.; van Berge, P. J.; van de Loosdrecht, J. Adsorbate Induced Reconstruction of Cobalt Surfaces. Surf. Sci. 2008, 602, 17−27. (6) Saib, A. M.; Moodley, D. J.; Ciobîcă, I. M.; Hauman, M. M.; Sigwebela, B. H.; Weststrate, C. J.; Niemantsverdriet, J. W.; van de Loosdrecht, J. Fundamental Understanding of Deactivation and Regeneration of Cobalt Fischer−Tropsch Synthesis Catalysts. Catal. Today 2010, 154, 271−282. (7) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. Density Functional Theory Study of Iron and Cobalt Carbides for Fischer−Tropsch Synthesis. J. Phys. Chem. C 2010, 114, 1085−1093. (8) Peña, D.; Griboval-Constant, A.; Lecocq, V.; Diehl, F.; Khodakov, A. Y. Influence of Operating Conditions in a Continuously Stirred Tank Reactor on the Formation of Carbon Species on Alumina Supported Cobalt Fischer−Tropsch Catalysts. Catal. Today 2013, 215, 43−51. (9) Tan, K. F.; Xu, J.; Chang, J.; Borgna, A.; Saeys, M. Carbon Deposition on Co Catalysts During Fischer−Tropsch Synthesis: A Computational and Experimental Study. J. Catal. 2010, 274, 121−129. (10) Moodley, D. J.; van de Loosdrecht, J.; Saib, A. M.; Overett, M. J.; Datye, A. K.; Niemantsverdriet, J. W. Carbon Deposition as a Deactivation Mechanism of Cobalt-Based Fischer−Tropsch Synthesis Catalysts under Realistic Conditions. Appl. Catal., A 2009, 354, 102− 110. (11) Swart, J. C. W.; Ciobîcă, L. M.; van Santen, R. A.; van Steen, E. Intermediates in the Formation of Graphitic Carbon on a Flat FccCo(111) Surface. J. Phys. Chem. C 2008, 112, 12899−12904. (12) Swart, J. C. W.; van Steen, E.; Ciobîcă, I. M.; van Santen, R. A. Interaction of Graphene with Fcc-Co(111). Phys. Chem. Chem. Phys. 2009, 11, 803−807. (13) Sautet, P.; Cinquini, F. Surface of Metallic Catalysts under a Pressure of Hydrocarbon Molecules: Metal or Carbide? ChemCatChem. 2010, 2, 636−639. (14) Klink, C.; Stensgaard, I.; Besenbacher, F.; Lagsgaard, E. An Stm Study of Carbon-Induced Structures on Ni(111): Evidence for a Carbidic-Phase Clock Reconstruction. Surf. Sci. 1995, 342, 250−260. (15) Peña, D.; Griboval-Constant, A.; Lancelot, C.; Quijada, M.; Visez, N.; Stéphan, O.; Lecocq, V.; Diehl, F.; Khodakov, A. Y. Molecular Structure and Localization of Carbon Species in Alumina Supported Cobalt Fischer−Tropsch Catalysts in a Slurry Reactor. Catal. Today 2013, 228, 65−76. (16) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15. (17) 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. (18) Kresse, G.; Joubert, J. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. (19) Blö chl, P. E.; Jepsen, O.; Andersen, O. K. Improved Tetrahedron Method for Brilloin-Zone Integrations. Phys. Rev. B 1994, 49, 16223−16233. (20) Enache, D. I.; Rebours, B.; Roy-Auberger, M.; Revel, R. In Situ Xrd Study of the Influence of Thermal Treatment on the Characteristics and the Catalytic Properties of Cobalt-Based Fischer−Tropsch Catalysts. J. Catal. 2002, 205, 346−353.

of 7/4 ML and 3/2 ML become predominant. These structures consist of polyaromatic C species ontop of the Co(111) surface that could be precursors of the graphene overlayer. They could also be considered as representative of oligomeric C species leading to long-chain alkanes. Moreover, the activation barriers published in the literature for the elementary steps involved in FT synthesis suggest that the formation of the graphene carbon overlayer is a slow process compared to the single C atom diffusion processes involved in the formation of the reconstructed Co(111) surface, which implies that, contrary to the catalyst deactivation, its selectivity loss is not kinetically hindered. Therefore, considering our thermodynamic results and kinetic considerations reported in the literature, we believe that several types of carbons may strongly modify the nature of the active sites present on Co particles: • the formation of a graphene overlayer that may be the cause of the slow deactivation of Co-based catalysts observed after several catalytic cycles; • a Co-reconstructed (111) surface leading to a carbidelike surface corresponding to α ≤ 0.71 (H2/CO = 2, P = 20 bar, PH2 = 10 bar, PCO = 5 bar, and T = 500 K) which may be considered as a cause of selectivity loss; • carbon deposition from the beginning of the FTS reaction on both terraces and steps, particularly on the (112) surface sites which may rapidly inhibit the direct CO dissociation at steps. The evolution of carbon species on the Co surfaces reveals that the high selectivity regime (high ASF coefficient) of the cobalt catalyst should result from an optimal thermodynamic balance imposed by FT conditions which must favor the formation of long chain-carbon oligomers, while avoiding the formation of the carbide surface structure and of the graphene species (precursor of coke). In particular, the formation of the Co-reconstructed (111) surface is the fingerprint of the catalyst loss of selectivity in paraffin formation. Our work has also brought relevant insights into the possible evolution of active sites involved in the FT reaction on Cobased catalysts. According to our results, one should consider with great caution purely metallic cobalt sites (either on terraces or on steps) as being representative of active sites present in real FT reaction conditions. Therefore, we hope that our work will encourage the research community in this area to investigate more closely the impact of carbon deposition on the mechanisms and elementary steps involved in the FT synthesis.

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ASSOCIATED CONTENT

S Supporting Information *

Brief discussion on the effects of ZPE, entropy, and Grimme corrections for dispersive forces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Valero, M. C.; Raybaud, P. Cobalt Catalyzed Fischer−Tropsch Synthesis: Perspectives Opened by First Principles Calculations. Catal. Lett. 2012, 143, 1−17. 22489



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Stability of Carbon on Cobalt Surfaces in FischerâTropsch Reaction

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