Formation of CHx Species from CO Dissociation on Double-Stepped

Aug 15, 2008 - Wenping Ma , Gary Jacobs , Tapan K. Das , Cornelius Mduduzi Masuku , Jungshik Kang , Venkat Ramana Rao Pendyala , and Burtron H...
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J. Phys. Chem. C 2008, 112, 14108–14116

Formation of CHx Species from CO Dissociation on Double-Stepped Co(0001): Exploring Fischer-Tropsch Mechanism Chun-Fang Huo,† Yong-Wang Li,† Jianguo Wang,† and Haijun Jiao*,†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, P. R. China, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: May 6, 2008; ReVised Manuscript ReceiVed: June 27, 2008

Spin-polarized density functional theory calculations have been performed to investigate the formation of CHx species from CO dissociation on the double-stepped Co(0001) surface. It is found that the steps not only favor CO dissociation but also are an advantage over CO and C/O hydrogenation. Furthermore, the step with the hcp and fcc sites in alternate arrangement is more active than the other consisting of the 4-fold site. Under realistic Fischer-Tropsch reaction conditions, surface CHxO are the most favored intermediates; and surface CHx are formed from CHxO dissociation, while the direct CO dissociation with carbide intermediate is not competitive. Introduction Fischer-Tropsch synthesis (FTS) is a clean technology for the conversion of coal/natural gas into motor fuels and chemicals.1 Since its discovery in 1923, FTS has received considerable attention both academically and industrially.2-8 Although both iron- and cobalt-based catalysts have been utilized in fuel synthesis, the FTS mechanisms are still not fully understood,9-12 and this has become a bottleneck for the rational design and optimization of industrial catalysts as well as the development of detailed kinetics. Generally three basic mechanisms have been proposed: (a) carbide mechanism, (b) CO insertion mechanism, and (c) hydroxycarbene mechanism.12 In light of the available mechanistic data, the carbide mechanism is more appropriate for cobalt-catalyzed FTS and has been widely accepted.11 In this mechanism, chain growth proceeds via surface CH2 insertion. This monomeric building block is formed by CO dissociation (COads f Cads + Oads) and hydrogenation of surface C atom (Cads + Hads f CHads; CHads + Hads f CH2,ads), while surface O atom is removed by the formation and desorption H2O. Inspired by oxygen-containing organic compounds in FTS outputs, Storch et al. proposed an alternative mechanism in 1951 where hydrogen adds directly to the adsorbed CO to form an oxymethylene intermediate (HCOH).13 Although this mechanism received much attention and much evidence was obtained from combination and complex chemistry,14-19 it has faded from our view due to the lack of direct support. Recent studies have revisited the FTS mechanism and caused the renaissance of the CHO intermediate. Density functional theory (DFT) calculations indicated that the CHO species is likely an important intermediate in FTS. On the kinked Fe(111)20 and the flat Co(0001)21 surfaces, the carbide mechanism is not viable, and an alternative pathway with the hydrogenation of the adsorbed CO to oxymethylidyne species (CHO/CH2O) followed by C-O bond cleavage is preferred. Interestingly, the pathway with CHO as intermediate was also found in the related processes, i.e., CH * To whom correspondence should be addressed. E-mail: haijun.jiao@ catalysis.de. † State Key Laboratory of Coal Conversion. ‡ Leibniz-Institut fu ¨ r Katalyse e.V. an der Universita¨t Rostock.

oxidation on Rh(111)22 and CO2 reforming of CH4 on Ni(111).23 Furthermore, the CHO species was detected and characterized by means of high-resolution electron energy loss spectroscopy in CO hydrogenation on Ru(0001)24 and the in situ polarization modulation infrared reflection absorption spectroscopy in methanol dissociation on Pd(111).25 Watson et al. also presented indirect evidence from isothermal and temperature-programmed oxidation of CH over Pt(110).26 However, DFT calculations by Ciobica and van Santen showed that the CO dissociation pathway is highly associated with the properties and structures of the metals.27 The indirect route of CO dissociation with CHO as intermediate is favorable on Pd(111) and flat Ru(0001), but not preferred over stepped Ru(0001). Recently, a comprehensive work of ultrahigh-vacuum experiments and DFT calculations by Nørskov et al. investigated the structure sensitivity of the methanation reaction on nickel surfaces.28 It was found that, under methanation conditions, CO dissociation proceeds most favorably on low coordinated sites with COH as intermediate. As is well-known, supported cobalt particles are the active sites for cobalt-catalyzed FTS. Under syngas (CO + H2) atmosphere, the cobalt surface can restructure and form defects, especially steps.29 Meanwhile, considerable experimental and theoretical evidence suggested that steps play a critical role in the chemical activity of CO on metal surfaces.27,30-35 DFT calculations by Liu and Hu showed that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction and the particular metal.33 For CO T C + O reaction, both dissociation (>0.8 eV) and association (>0.6 eV) barriers are reduced on steps and kinks of Rh and Pd.33 On monatomic stepped Co(0001), the CO dissociation barrier decreases dramatically by 1.09 eV.34 Although steps and kinks of Rh and Pd can facilitate CH4 dissociation, there are essentially no differences in the barrier for the association reaction of CH3 + H on the flat surfaces and the defects.33 It is desired to know the mechanism of forming CHx species in cobalt-catalyzed FTS. Does the reaction have the carbide mechanism or the CHO intermediate mechanism or are there other possibilities? To answer this question, we performed a

10.1021/jp803976g CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

Formation of CHx Species from CO Dissociation

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detailed DFT study on the adsorption, diffusion, and dissociation of CO on the double-stepped Co(0001) surface. All possible paths for CO dissociation without and with hydrogen participation have been considered and discussed, and the role of steps is highlighted. Computational Details Methods. All calculations were performed at the DFT level with the CASTEP program36,37 in the Materials studio of Accelrys Inc. The exchange and correlation energies were calculated by using the Perdew-Wang generalized-gradient approximation (PW91).38 Ionic cores were described by ultrasoft pseudopotential (USPP),39 and the Kohn-Sham one-electron states were expanded in a plane wave basis set up to 340 eV. A Fermi smearing of 0.1 eV was utilized to evaluate the occupancy numbers. Because of its large effect on the adsorption energies for magnetic systems,40-42 spin polarization was included. The convergence criteria for structure optimization and energy calculation were set to medium quality with the tolerances for SCF, energy, maximum force, and maximum displacement of 2.0 × 10-6 eV/atom, 2.0 × 10-5 eV/atom, 0.05 eV/Å, and 2.0 × 10-3 Å, respectively. In addition, all transition states (TS) were located by using the complete linear synchronous transit/quadratic synchronous transit (LST/QST) method.43 It starts with LST maximization, followed by energy minimization in directions conjugate to the reaction pathway. These approximated TS are then used to perform QST maximization. From that point, another conjugate gradient minimization is performed. The cycle is repeated until a stationary point is located. It should be noteworthy that the PW91 functional can give reliable optimized geometry, but tends to overbind the adsorbates, and gives wrong site assignments for CO adsorption on many metals.44,45 In comparison, the RPBE46 can give a reasonable prediction on adsorption energies. Therefore, we further carried out RPBE single-point energy calculations on the PW91 optimized geometries, and our discussion is mainly based on the RPBE energies, while the PW91 values are provided for comparison. Models. Using a scanning tunneling microscopy technique, the atomic-level imaging of a model cobalt catalyst surface was detected by Wilson and de Groot.29 It was found that, under typical FTS conditions, the Co(0001) surface would restructure dramatically via an etch-regrowth process and form many triangular-shaped cobalt islands. These islands have an average diameter of 1.75 nm, and two distinct levels are separated by monatomic steps. Based on this image, a double-stepped Co(0001) model was set up (Figure 1). This model derives from a four-layer p(5 × 2) slab, in which three or two neighboring rows of cobalt atoms on the top and the second layers are removed, respectively. From the side views, the steps can be seen clearly. Step 1 consists of 4-fold site, while step 2 has the quasi-hexagonal close-packed (hcp) and face-centered cubic (fcc) sites in alternate arrangement. In all calculations, the vacuum region between the slabs was set to 10 Å, and the k-point mesh was chosen as (2 × 5 × 1). The adsorbates and the cobalt atoms of the top three layers were allowed to relax, while those in the bottom one layer were fixed in their bulk positions (3Co/1Co). Similar methods and models have been used extensively in the literature and have been validated and proven to be reasonable.34,47,48 The adsorption energy is defined as Eads ) E(adsorbates/slab) - [E(slab) + E(adsorbates)], where E(adsorbates/slab) is the total energy of the slab with adsorbates, E(slab) is the total

Figure 1. Side and top views of the double-stepped Co(0001) surface: edge-top site (A-B), edge-bridge site (C-D), terrace-hcp site (E-F), step-hcp site (G), terrace-fcc site (H-I), step-fcc site (J), and step-4fold site (K).

energy of the bare slab, and E(adsorbates) is the total energy of free adsorbates. Therefore, the more negative the Eads, the stronger the adsorption. Results and Discussion CO Adsorption. As starting points for the formation of CHx species, the bonding nature of CO adsorption on the doublestepped Co(0001) surface is examined first. To address the role of steps on CO dissociation, 7 types of adsorption sites around the steps were considered (Figure 1) and 11 configurations were located (1-11). The optimized structures are shown in Figure 2, and the adsorption energies and the key bond parameters are given in Table 1. In 1 (e1-t) and 2 (e2-t), CO adsorbs on the top sites of the step edges and tilts away from the surface normal by 38.5° and 32.1°, respectively. Since the cobalt atoms of the step edges are more unsaturated with the largest dangling bond of five, the stronger interaction between CO and the edge cobalt atoms can be expected. The most favored CO adsorption mode is 2 (e2-t) with the largest adsorption energy of -1.52 eV, and 1 (e1-t) is slightly less stable than 2 (e2-t) by 0.10 eV. Both 3 (e1-b) and 4 (e2-b) have CO adsorption on the bridge sites of the step edges. In 3 (e1-b), the adsorbed CO tilts away

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Figure 2. Structures of CO adsorption on the double-stepped Co(0001) surface (blue, Co atom; black, C atom; red, O atom).

TABLE 1: Calculated Adsorption Energies (Eads, eV) and Bond Parameters (d, Å) for CO Adsorption on the Double-Stepped and Flat Co(0001) Surfaces site

c

1 (e1-t) 2 (e2-t) 3 (e1-b) 4 (e2-b) 5 (t1-h) 6 (t2-h) 7 (s2-h) 8 (t1-f) 9 (t2-f) 10 (s2-f) 11 (s1-4f)

edge1-top edge2-top edge1-bridge edge2-bridge terrace1-hcp terrace2-hcp step2-hcp terrace1-fcc terrace2-fcc step2-fcc step1-4fold

F1 F2 F3 F4

top bridge hcp fcc

Eadsa

dC-O

∆dC-Ob

Double-Stepped Co(0001) -1.42 (-1.74) 1.167 0.022 -1.52 (-1.84) 1.168 0.023 -1.41 (-1.78) 1.181 0.036 -1.38 (-1.74) 1.185 0.040 -1.18 (-1.75) 1.280 0.135 -1.11 (-1.67) 1.276 0.131 -1.08 (-1.54) 1.231 0.086 -1.08 (-1.54) 1.199 0.054 -1.08 (-1.54) 1.231 0.086 -1.09 (-1.61) 1.268 0.123 -1.11 (-1.60) 1.222 0.077 Flat Co(0001)c (-1.74) 1.165 0.020 (-1.70) 1.183 0.038 (-1.77) 1.190 0.045 (-1.76) 1.189 0.043

a Values in parentheses are derived from the PW91 functional. Reference 49.

from the surface normal by 47.5°, while it is nearly vertical to the surface in 4 (e2-b). Despite these differences, the adsorption energies of 3 (e1-b) and 4 (e2-b) are very close (-1.41 and -1.38 eV) and also comparable to that for 1 (e1-t) (-1.42 eV), but the C-O bond in the bridge modes (1.181 Å for 3 and 1.185 Å for 4) is more activated than that in the top modes (1.167 Å for 1 and 1.168 Å for 2). Compared to the most stable 2 (e2-t), the high coordination binding sites (5-11) are lower in energy by 0.34-0.44 eV, and have stronger activated C-O bond. Noteworthily, whether O atom interacts with the surface or not, all high coordination modes (5-11) have comparable adsorption energies of -1.08 to -1.18 eV, but large differences in the C-O bond activation. In 5 (t1-h), 6 (t2-h), 7 (s2-h), 9 (t2-f), and 10 (s2-f), CO interacts with the double-stepped Co(0001) surface via both C and O atoms, which leads to a stronger C-O bond activation. Furthermore, the activation degree of the C-O bond depends highly on the coordination numbers of the O atom. 5 (t1-h), 6 (t2-h), and 10 (s2-f) with C atom at the terrace-hcp or step-fcc site and O atom at the edge-bridge site have the longest C-O

b

dC-Co

dO-Co

1.760 1.766 1.859/1.997 1.928/1.924 1.922/1.931/1.938 1.974/1.925/1.924 1.844/2.089/2.248/2.107 1.886/2.180/1.999 2.018/2.328/1.843/2.139 1.927/1.930/1.991 2.058/2.050/2.111/2.096

2.153/2.152 2.184/2.198 2.144 2.135 2.211/2.213

1.760 1.883/1.971 1.980/1.976/1.983 1.975/2.005/1.990

Elongation of the CO bond length with respect to free CO (1.145 Å).

Figure 3. Structures of transition states for CO diffusion on the doublestepped Co(0001) surface (blue, Co atom; black, C atom; red, O atom).

bond of 1.280, 1.276, and 1.268 Å, respectively. In 7 (s2-h) and 9 (t2-f), CO binds to surface with C atom in µ4-form and

Formation of CHx Species from CO Dissociation

Figure 4. Energy diagram (eV) for CO diffusion and dissociation on the double-stepped Co(0001) surface. (The corresponding energy barriers are marked on line).

O atom in µ1-form. Therefore, a moderate activation of the C-O bond (1.231 Å) is expected. As to 8 (t1-f) and 11 (s1-4f), CO ties with the surface only through a C atom, and thus, the shorter C-O bond lengths of 1.199 and 1.222 Å are predicted. In comparison, CO prefers adsorption on the low coordination sites, but the C-O bond becomes more activated as the number of CO coordination to the surface increases. At this stage, it is interesting to address the role of steps on CO adsorption. In comparison with the results of CO adsorption on the flat Co(0001) surface,49 no obvious effects of steps on the adsorption energies can be found (see PW91 values: -1.74 vs -1.74 and -1.84 eV for the top site, -1.70 vs -1.78 and -1.74 eV for the bridge site, -1.77 vs -1.75 and -1.67 eV for the hcp site, -1.76 vs -1.54 eV for the fcc site), but steps have a stronger influence on CO activation at the highcoordination binding sites (1.190 vs 1.280 and 1.276 Å for the hcp site, 1.189 vs 1.231 Å for the fcc site). Therefore, a higher reactivity of the C-O bond cleavage is expected on the steps. It is also worth noting that RPBE and PW91 differ in the preference of CO adsorption sites. The RPBE results show that CO favors mostly adsorption on the edge-top site, tightly followed by the edge-bridge site, while the high-coordination binding sites are lower in energy by ∼0.3 eV. This is in agreement with the experimental observations. By means of polarization modulation infrared reflection absorption spectroscopy, a model heterogeneous catalyst system, CO/Co(0001), was studied in the pressure range from 10-10 to 600 mbar at

J. Phys. Chem. C, Vol. 112, No. 36, 2008 14111 temperatures between 300 and 550 K by Beitel et al.50 It was shown that annealing at 450-490 K and at 100 mbar CO pressure leads to the creation of defects at the cobalt surface. On the defective surface, a high frequency of ∼2080 cm-1 is detected, and this is attributed to the linearly defect-bonded CO. However, the PW91 results suggest that there are no detectable differences for CO adsorption preference on the edge-top site (1 and 2), the edge-bridge site (3 and 4), and the highcoordination binding site (5). These verify again that PW91 not only overestimates the adsorption energies by 0.3-0.6 eV but also tends to the high-coordination binding sites for CO adsorption. CO Diffusion. Since CO prefers the edge-top and edge-bridge sites, while it is highly activated on the high-coordination binding sites, it is interesting to know the relationship between CO dissociation and diffusion. On the double-stepped Co(0001) surface (Figure 1), five possible paths for CO diffusion can be conceived: (A) 2 (e2-t) f 4 (e2-b) f 10 (s2-f), (B) 3 (e1-b) f 5 (t1-h), (C) 3 (e1-b) f 11 (s1-4f), (D) 3 (e1-b) f 6 (t2-h), and (E) 2 (e2-t) f 6 (t2-h). The optimized structures of the related transition states are shown in Figure 3, while the flowchart combining the relative energies and energy barriers is displayed in Figure 4. According to the diffusion barriers in Figure 4, we find that CO diffusion on the double-stepped Co(0001) surface can be divided into two types. One is CO diffusion from the edges down to the steps (e2 to s2 and e1 to s1) or up to the terraces (e1 to t2) with a small energy barrier at ∼0.2 eV; and the other is CO diffusion from the edges down to the terraces (e2 to t2 and e1 to t1) with a moderate energy barrier around 0.7 eV. These energy data reveal the high mobility of CO on defect surfaces. CO Dissociation. Taking 5 (t1-h), 6 (t2-h), 10 (s2-f), and 11 (s1-4f) as starting points, four possible paths for CO dissociation on the double-stepped Co(0001) surface; 5 (t1-h) f 5-P, 6 (t2-h) f 6-P, 10 (s2-f) f 10-P, and 11 (s1-4f) f 11-P, are examined. The optimized structures of all transition states and products are illustrated in Figure 5, while the energy data and the calculated bond parameters are given in Figure 4 and Table 2. From the energy data in Figure 4, we can see that four reaction paths have different thermodynamic properties. Path 10 (s2-f) f 10-P is slightly exothermic by -0.20 eV, while paths 5 (t1-h) f 5-P and 6 (t2-h) f 6-P are moderately exothermic by -0.61 and -0.69 eV. In contrast, path 11 (s1-4f) f 11-P is endothermic by 0.85 eV. From the thermodynamic

Figure 5. Structures of key stationary points for CO dissociation on the double-stepped Co(0001) surface (blue, Co atom; black, C atom; red, O atom)

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TABLE 2: Calculated Activation and Reaction Energies (Ea and ∆Ere, eV) for CO Dissociation on the Double-Stepped Co(0001) Surface, and Bond Parameters (d, Å) for Key Stationary Points Eads(C/O)a,b TS5dis 5-P TS6dis 6-P TS10dis 10-P TS11dis 11-P a

-12.72 (-13.70) -12.74 (-13.67) -12.21 (-13.20) -11.18 (-12.16)

Ea, ∆Ereb

dC-O

dC-Co

dO-Co

1.71 (1.67) -0.61 (-0.74) 1.60 (1.54) -0.69 (-0.79) 1.20 (1.03) -0.20 (-0.37) 1.79 (1.73) 0.85 (0.64)

2.123

1.815/1.814/1.845 1.852-2.048 1.868-2.222 1.842-2.058 1.849-2.122 1.849-2.042 1.816-1.983 1.835-1.978

1.841/1.840 1.901-2.196 1.747/1.934 1.891/1.905/1.903 1.886/1.935 1.987-2.022 2.026/1.948 1.926/1.896/1.860

2.185 1.612 1.753

Adsorption energies with respect to atomic C and O. b Values in parentheses are derived from the PW91 functional.

TABLE 3: Calculated Activation and Reaction Energies (Ea and ∆Ere, eV) for Hydrogen-Mediated CO Dissociation on the Double-Stepped Co(0001) Surface, and Bond Parameters (d, Å) for Key Stationary Points Ea, ∆Erea H-101 TS(10/I1) I1 TS(I1/P1) P1

0.09 (0.19) -0.65 (-0.62) 1.36 (1.30) 0.10 (-0.02) Path B: COads

H-I1 TS(I1/I2) I2 TS(I2/P2) P2

0.61 0.05 1.22 0.47

H-102 TS(10/I3) I3

2.29 (1.93) 0.73 (0.82)

a

(0.61) (0.14) (1.16) (0.36)

dC-O

dC-H [dO-H]

dC-Co

dO-Co

Path A: COads + Hads f CHOads; CHOads f CHads + Oads 1.267 1.955/1.965/1.926 2.205/2.215 1.243 1.798 1.923-2.049 2.307/2.315 1.350 1.102 1.951/1.950 2.019/2.016 2.069 1.091 1.873/1.880 1.861/1.855 1.101 1.922-2.021 1.990 ∼ 2.014 + Hads f CHOads; CHOads + H ads f CH2Oads; CH2Oads f CH2,ads + Oads 1.346 1.952/1.954 2.013/2.016 1.350 1.655 1.962/1.966 2.091/2.009 1.392 1.112 2.078/2.069 2.005/2.002 2.260 1.101 1.938/1.897 1.863/1.847 1.101 1.957-2.279 1.990-2.027 Path C: COads + Hads f COHads; COHads; f Cads + OHads 1.243 1.930/1.929/2.033 2.348/2.350 1.228 [1.598] 2.148/2.061/2.015 1.350 [0.980] 1.874/1.911/1.918

dH-Co 1.702/1.799/1.789 1.760/1.751

1.819/1.746/1.748 1.558

1.817/1.706/1.703 1.829

Values in parentheses are derived from the PW91 functional.

TABLE 4: Calculated Activation and Reaction Energies (Ea and ∆Ere, eV) for C/O Hydrogenation on the Double-Stepped Co(0001) Surface, and Bond Parameters (d, Å) for Key Stationary Points Ea, ∆Erea H-10P1 TS(P/P1) P1

dC-H, [dO-H]

0.75(0.76) -0.15(-0.07) Oads

H-10P2 TS(P/P3) P3 H-P3 TS(P3/P4) P4 a

0.81(0.90) -0.22(-0.12) 0.46(0.60) -0.60(-0.39)

dC-Co, [dO-Co]

Cads + Hads f CHads 1.850/1.851/1.878/1.880/2.040 1.441 1.867/1.864/1.920/1.913/2.151 1.101 1.922/1.925/2.021/2.017 + Hads f OHads; OHads + Hads f H2Oads [1.989/1.933/2.063/1.978] [1.457] [2.063/2.197/2.076/2.115] [0.980] [2.163/2.142/2.180/2.150] [2.127/2.109/2.213/2.192] [1.756] [1.948] [0.985] [2.181]

dH-Co 1.816/1.757/1.760 1.737/1.758 1.803/1.647/1.748 1.666 1.753/1.739/1.709 1.693

Values in parentheses are derived from the PW91 functional.

point of view, paths 5 (t1-h) f 5-P, 6 (t2-h) f 6-P and 10 (s2-f) f 10-P are feasible, while path 11 (s1-4f) f 11-P is at a disadvantage. It is noteworthy that the kinetics is another aspect for controlling the reaction paths. Comparing the dissociation energy barriers in Figure 4, we can find that path 10 (s2-f) f 10-P is more favored kinetically, while the other paths are not competitive (1.20 vs 1.71, 1.79, and 1.60 eV). In the dissociation product 10-P, two steps are filled with the C (in µ5-form) and O (in µ4-form) atoms. As expected, a large binding energy of the C and O atoms is found on the double-stepped surface with respect to that on the flat surface (-12.21 vs -9.99 eV). It is of note that CO dissociation on the flat Co(0001) surface is very difficult, as indicated by the high energy barrier of 2.79 eV at PW91.49 The step defects can effectively reduce the energy barrier by 1.0-1.6 eV and, thus, favor the CO dissociation.

Furthermore, step 2 with the hcp and fcc sites in alternate arrangement is more active toward the cleavage of the C-O bond. Unlike the flat surface, the CO dissociation on the steps can cause a large restructuring of the Co(0001) surface. H-Mediated CO Dissociation. Since hydrogen is a reaction component in FTS, it is interesting to know the role of hydrogen in CO dissociation. For that three hydrogen-mediated CO dissociation paths are conceived: (A) [COads + Hads f CHOads; CHOads f CHads + Oads], (B) [COads + Hads f CHOads; CHOads + Hadsf CH2Oads, CH2Oads f CH2,ads + Oads], and (C) [COads + Hads f COHads; COHads f Cads + OHads]. The corresponding structures of all transition states and intermediates are depicted in Figure 6, while the calculated bond parameters are given in Table 3, and the energy profiles are presented in Figure 7. First, the possibilities of the H atomʼs migration to the C atom of the adsorbed CO followed by the C-O bond

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TABLE 5: Adsorption Energies (Eads, eV) for Intermediates Involved in CO Dissociation and C/O Hydrogenation on the Double-Stepped and Flat Co(0001) Surfaces double-stepped Co(0001) adsorbate C O CHO CH2O COH CH CH2 OH H2O a

step2-5fold step1-4fold step2: C/bridge-O/bridge step2: C/bridge-O/bridge step2-fcc step2-4fold step2-4fold step1-4fold edge1-top

flat Co(0001) Eadsa

site

-7.84 -4.38 -2.96 -1.59 -3.37 -7.01 -4.37 -2.33 -0.10

(-8.32) (-4.88) (-3.53) (-2.17) (-3.84) (-7.52) (-4.85) (-2.84) (-0.41)

site

Eads

hcp hcp hcp: C/bridge-O/top hcp: C/bridge-O/top hcp hcp hcp hcp

-6.62b (PW91) -5.34c (PW91) -2.20d (PBE) -0.86d (PBE) -4.38d (PBE) -5.99b (PW91) -3.85b (PW91) -3.45c (PW91)

Values in parentheses are derived from the PW91 functional. b Reference 47. c Reference 34. d Reference 57.

Figure 6. Structures of key stationary points for CO dissociation on the double-stepped Co(0001) surface in the presence of hydrogen (blue, Co atom; black, C atom; red, O atom; white, H atom)

cleavage are considered (paths A and B). Path A involves CHO species (I1) as key intermediate. As illustrated in Figure 7, this stepwise path goes via the transition states TS(10/I1) and TS(I1/ P1) leading to surface CH and O species (P1). In I1, CHO attaches to the surface via C and O atoms occupying the edgebridge sites, and the C-O bond is further elongated to 1.350 Å. Noteworthily, the first step of H atom migration to C atom forming the CHO species (H-101 f TS(10/I1) f I1) is very facile with a negligible energy barrier of 0.09 eV, and it is exothermic by -0.65 eV, suggesting that, under the syngas (CO + H2) atmosphere, there is hardly any adsorbed CO on the double-stepped Co(0001) surface. Rising from the large stability of I1, the second step for C-O bond cleavage (I1 f TS(I1/P1) f P1) exhibits a moderate energy barrier of 1.36 eV and is slightly endothermic by 0.10 eV.

In path B, the first step is the formation of CH2O (I2) from CHO hydrogenation, and the second step is the cleavage of the C-O bond with the formation of the surface CH2 and O species (P2). The hydrogenation step of CHO species (H-I1 f TS(I1/ I2) f I2) is nearly thermoneutral (0.05 eV) with a low energy barrier of 0.61 eV. Furthermore, the C-O bond cleavage step (I2 f TS(I2/P2) f P2) is computed to be endothermic by 0.47 eV, with a moderate energy barrier of 1.22 eV. Comparing the energy profiles of path A and path B, we can find that the formation of CH2O is controlled kinetically, while the direct C-O cleavage of path A is controlled thermodynamically. Therefore, the formation of CH2 or CH depends on the reaction conditions. As an alternative case for CO hydrogenation, H atom migration to the O atom of the adsorbed CO forming the COH

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Figure 7. Energy profiles (eV, values in parentheses are derived from the PW91 functional) for CO dissociation on the double-stepped Co(0001) surface in the presence of hydrogen.

species (path C: H-102 f TS(10/I3) f I3) is also examined. As shown in Figure 7, this step is endothermic by 0.73 eV and has a very high barrier of 2.29 eV. Compared to paths A and B, path C is not favored both thermodynamically and kinetically, and thus should be ruled out. At this stage, it is interesting to compare the H-mediated CO dissociation mechanisms with the carbide mechanism (direct dissociation). On one hand, the CHO- and CH2O-containing mechanisms (path A or B) have energy barriers for the C-O bond cleavage comparable to the carbide mechanism (1.36 and 1.22 vs 1.20 eV). On the other hand, the adsorbed CO will be hydrogenated to CHO immediately, and thus, hardly any CO accumulates on the stepped surface when the ratio of H2 to CO is larger than 0.5. Hence, under realistic FTS reaction conditions, CHx species is formed mainly via the CHO or CH2O intermediates (path A or B), while the carbide mechanism is not competitive in C1 species formation on the double-stepped Co(0001) surface. This is in line with the conclusion by Inderwildi et al. on the flat Co(0001) surface.21 However, the steps significantly reduce the energy barrier for CO hydrogenation to CHO (0.09 vs 1.31 eV21) and largely stabilize the CHO and CH2O species (lower than (CO + H) in energy by -0.65 and -0.60 eV, respectively). Therefore, it becomes possible to spot these species on the stepped Co(0001) surface and provide further evidence for this CHO-intermediate mechanism. It has been reported that many dissociation reactions obey the Brønsted-Evans-Polanyi correlation, i.e., a linear relationship between the activation energy (transition-state potential energy) and reaction energy (dissociative chemisorption potential energy for the dissociation products). This correlation is independent of the reactant and of the metal but varies with the structure of the active site.51-56 As shown in Figure 8, however, there is no such correlation for CO dissociation on the doublestepped Co(0001) surface, and this is because of the difference in adsorption sites. C/O Hydrogenation. As a concomitant consequence for direct CO dissociation, the steps will be covered by the strongly adsorbed C and O atoms (Figure 5). Thus, their removal to refresh the steps is a key factor to maintain the high activity of a cobalt catalyst. Adsorbed C atom can be hydrogenated to CHx and then take part in the chain growth, while surface O atom

Figure 8. Calculated activation energies plotted as a function of the reaction energies for CO dissociation on the double-stepped Co(0001) surface.

can be removed by the formation and desorption of H2O. Here, both C hydrogenation to CH (H-10P1 f TS(P/P1) f P1) and O successive hydrogenation to water (H-10P2 f TS(P/P3) f P3, H-P3 f TS(P3/P4) f P4) are explored, as shown in Figure 9. The corresponding energy data and the calculated bond parameters are listed in Table 4. Although surface C atom binds strongly with the step in µ5form, the formation of CH on the double-stepped Co(0001) surface is facile both thermodynamically and kinetically, as indicated by the low energy barrier of 0.75 eV and the slightly exothermic character of -0.15 eV. Similar to surface C hydrogenation, two steps of surface O hydrogenation are also favored on the double-stepped Co(0001) surface. The first step (Oads + Hads f OHads) is predicted to be exothermic by -0.22 eV, with an energy barrier of 0.81 eV, and the second step (OHads + Hads f H2Oads) is exothermic (-0.60 eV) and has a lower energy barrier (0.46 eV). These indicate that CO dissociation would not deactivate the double-stepped Co(0001) surface by carburization or oxygenation. Comparing the adsorption energies on the double-stepped Co(0001) surface with those on the flat Co(0001) surface (Table 5), the adsorption of C,

Formation of CHx Species from CO Dissociation

J. Phys. Chem. C, Vol. 112, No. 36, 2008 14115

Figure 9. Structures of key stationary points for C/O hydrogenation on the double-stepped Co(0001) surface (blue, Co atom; black, C atom; red, O atom; white, H atom)

CHO, CH2O, CH, and CH2 species on the double-stepped surface is stronger than on the flat surface, while the adsorption of O, COH, and OH on the double-stepped surface is weaker than on the flat surface. Especially, the desorption of H2O is very easy on the double-stepped Co(0001) surface, as indicated by the low binding energy of -0.10 eV. Therefore, for cobaltcatalyzed FTS, the steps are the active sites for CO dissociation during the whole process. Conclusion In this work, CO adsorption, diffusion, and dissociation as well as the subsequent hydrogenation on the double-stepped Co(0001) surface are investigated at the level of density functional theory to explore the Fischer-Tropsch mechanism. It is found that CO prefers adsorption on the edge-top site (1.52 eV for 2(e2-t)), tightly followed by the edge-bridge site (1.41 and 1.38 eV for 3 (e1-b) and 4 (e2-b), respectively), while the high-coordination binding sites are lower in energy by ∼0.3 eV. Although the steps have no obvious effects on the intensity of CO adsorption, they are very helpful for CO activation and dissociation. In addition, the step with the hcp and fcc sites in alternate arrangement is more active than the other consisting of the 4-fold site. Without hydrogen, CO dissociation on the double-stepped Co(0001) surface proceeds stepwise. First, CO diffuses from the most stable edge-top site (2(e2-t)) to the highly activated step2-fcc site (10(s2-f)) by elevating 0.43 eV in energy and then dissociates into the C (µ5-form) and O (µ4-form) atoms filling at the steps. It is predicted that the dissociation step has a moderate energy barrier of 1.20 eV and is exothermic by -0.20 eV. In contrast, CO dissociation cannot occur on the flat surface due to the very high barrier of 2.79 eV.49 Under realistic FTS reaction conditions, the formation of CHxO as intermediates is more favored both kinetically and thermodynamically over direct CO dissociation with carbide as intermediate. As a consequence, surface CHx are formed mainly via the CHO (COads + Hads f CHOads f CHads + Oads) or CH2O (COads + 2Hads f CHOads + Hads f CH2Oads f CH2,ads + Oads) dissociation on the double-stepped Co(0001) surface, while the carbide mechanism is not competitive. Furthermore, the steps significantly reduce the energy barrier for CO hydrogenation to CHO (0.09 vs 1.31 eV21) and largely stabilize the CHO and CH2O species (lower than (CO + H) in energy by -0.65 and -0.60 eV, respectively). Therefore, it becomes possible to spot these species on the stepped Co(0001) surface and provide the further evidence for this CHO-intermediate mechanism.

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