Density Functional Theory Study on Surface CxHy Formation from CO

Nov 22, 2010 - Jónsson , H. ; Mills , G. ; Jacobsen , K. W. Classical and Quantum Dynamics in Condensed Phase Simulations; World Scientific: Singapor...
1 downloads 0 Views 6MB Size
Download PDF
J. Phys. Chem. C 2010, 114, 21585–21592

21585

Density Functional Theory Study on Surface CxHy Formation from CO Activation on Fe3C(100) Li-Juan Deng,† Chun-Fang Huo,† Xing-Wu Liu,† Xun-Hua Zhao,† Yong-Wang Li,† Jianguo Wang,† and Haijun Jiao*,†,‡ State Key Laboratory of Coal ConVersion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001 China, and Leibniz-Institut fu¨r Katalyse eV. an der UniVersita¨t Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany ReceiVed: September 6, 2010; ReVised Manuscript ReceiVed: NoVember 11, 2010

Spin-polarized density functional theory calculations have been performed to investigate the mechanisms for CxHy formation on Fe3C(100). It is found that H-assisted CO dissociation (CO + H f CHO; CHO f CH + O) has lower barrier than CO direct dissociation (CO f C + O), but surface Cs atom hydrogenation to form surface CsH is the most favored pathway. As the first C2 surface species, surface ketenylidene CsCO rising from CO adsorption is an important intermediate for C2Hx formation. Initial surface C2Hx forms from CsCO hydrogenation instead of direct dissociation. The formation of CsCH, CsHCH and CsH2CH has close effective barriers and depends on the CO/H2 ratio. In addition, surface vacancy can activate CO strongly and lower the CO dissociation barrier considerably, and this regenerates the carburized active surface and closes a catalytic cycle. 1. Introduction Fischer-Tropsch synthesis (FTS) is a complex catalytic process, which can convert synthesis gas (CO + H2) into high molecular weight hydrocarbons.1-5 This technology becomes increasingly attractive and important because of the rapid resource depletion, the unpredictable price of crude oil, and the drastically increased fuel demands worldwide. Because of their low prices and high activity, iron-based catalysts are widely used in industrial FTS.6-9 Many experiments have shown that iron carbides are the active phases in FTS,10,11 and several carbide phases as ε-Fe2C, χ-Fe5C2, θ-Fe3C, and Fe7C3 have been detected.12 Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Mo¨ssbauer spectroscopy studies indicated Fe3C being one of the active phases in FTS.13,14 Although iron carbides are considered as the active phases, the reaction mechanisms remain to be fully understood. FTS involves a complex reaction scheme composed of myriad reactive intermediates and elementary reaction steps; and the general reaction paths include CO activation, CxHy formation and coupling, and termination pathways. As an important step in FTS, CO activation has been extensively studied for decades.15-27 On the other hand, various CxHy surface species were detected in FTS process,28 and therefore understanding the CO activation pathway to form CxHy is important for catalyst design. Many studies indicated that the adsorbed CO first dissociates into surface C and O, which react with the dissociatively adsorbed H to form surface CHx. Van Santen et al.29 showed that direct CO dissociation on the corrugated Ru(1121) surface has lower barrier than H-assisted CO dissociation. Storch et al.30 suggested a mechanism in which an H atom is added directly to the adsorbed CO to form CHO, which dissociates subsequently to CH and O. On a double-stepped Co(0001) * To whom correspondence should be addressed. E-mail address: [email protected]. † Chinese Academy of Sciences. ‡ Leibniz-Institut fu¨r Katalyse eV. an der Universita¨t Rostock.

surface, Huo et al.31 found that H-assisted CO dissociation has lower barrier than direct CO dissociation. Inderwildi et al.32 showed that H-assisted CO dissociation is thermodynamically and kinetically favored. Recently, Cheng and Hu33 investigated CO dissociation on stepped Fe5C2(100) and found that CO dissociation barrier on iron carbide surface is higher than that on the corresponding iron surface, iron carbide surface is more active for CO hydrogenation than iron surface, and iron carbide possesses similar CH4 selectivity to that on iron surface. Although several theoretical studies34-40 have been performed on Fe carbides during recent years, only few investigations about the structure and stability of cementie (Fe3C)41 and the surface reactions about CO42 and H243 adsorption were performed. In this work, we have performed a detailed density functional theory (DFT) study on CO activation on Fe3C(100) surface, aiming to show the pathways of CO activation and CxHy formation. 2. Method and Surface Model All calculations were performed using plane-wave periodic density functional method as implemented in the Vienna ab initio simulation package (VASP).44 The exchange and correlation energies were calculated using the Perdew, Burke, and Ernzerhof (PBE) functional.45 The electron-ion interaction was described by the projector augmented wave (PAW) method,46 TABLE 1: Cell Parameters (Å) and Average Magnetic Moment (µB) of Fe3C with Different Method method

a

b

c

µB (Feg, Fes)a

PAW-PW91 PAW-PBE PAW-RPBE exptb

5.02 5.02 5.07 5.09

6.71 6.73 6.77 6.74

4.47 4.47 4.52 4.52

1.83, 1.91 1.87, 1.95 2.01, 2.08 1.78c

a Magnetic moments per atom, including Fe in general (g) and special (s) positions. b The available experimental values. c Reference 48.

10.1021/jp108480e  2010 American Chemical Society Published on Web 11/22/2010

21586

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Deng et al.

Figure 1. Top (a) and side (b) views of the Fe3C(100) surface (first layer Fe atoms in blue; second layer Fe atoms in green; third layer Fe atoms in brown and surface C atoms in gray).

TABLE 2: Computed Bond Lengths (d, Å), Adsorption Energies (Eads, eV) and Stretching Frequencies (ν, cm-1) of CO on Fe3C (100) 1F-1 1F-2 2F-1 2F-2 3F 4F-1 4F-2

Eads

d(C-O)

-1.78 -1.78 -1.79 -1.81 -1.83 -1.72 -1.72

1.799 1.799 1.189 1.194 1.214 1.295 1.295

d(Fe-C) 1.765 1.762 1.792, 1.802, 1.946, 2.017, 2.013,

2.233 2.188 2.002, 2.102 2.095, 2.159 2.094, 2.164

d(C-C)

ν

1.412 1.413

1885 1910 1820 1801 1648 1347 1342

and the Kohn-Sham one-electron states were expanded in a plane wave basis set up to 400 eV. Because of its large effect on magnetic system, spin polarization was included for correct description of magnetic properties. Without counting the adsorbates, slab vacuum was set to span a range of 10 Å to exclude the interactions between the periodic slabs. For evaluating the energy barriers, all transitional states were located using the Nudged Elastic Band (NEB) method.47 In this

approach, the reaction path is discretized with the discrete configurations or images between minima connected by elastic springs to prevent the images from sliding to the minima in optimization. The vibrational frequencies of adsorption species and the transitional states were calculated. The Hessian matrix was determined based on a finite difference approach with a step size of 0.024 Å for the displacements for individual atoms of the adsorbate along each Cartesian coordinate. The cell parameters of Fe3C were calculated with projector augmented wave (PAW) in VASP. As given in Table 1, the calculated cell parameters and magnetic moments are very close to each other, and they are in reasonable agreements with the experimental values.48 In this work, the periodic slab model was employed. For modeling the Fe3C(100) surface (Figure 1), the slab consisting of seven Fe layers and three C layers (7Fe + 3C) was used. The first layer is composed of Fe atoms; the second layer is composed of one exposed C and two exposed Fe atoms, and the third layer is composed of two Fe atoms and is also exposed to the surface. In all calculations, the bottom four Fe layers and one C layer (4Fe + 1C) were fixed in their bulk position, while the top three Fe layers and two C layers (3Fe + 2C) were allowed to relax. A 5 × 5 × 1 k-point sampling within the Brillouin zone was used in the p(1 × 1) unit cell. 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 energy of the bare slab, and E(adsorbates) is the total energy of the free adsorbates. Thus, the more negative the Eads, the stronger the adsorption. The reaction energy and barrier are calculated by ∆Er ) E(FS) - E(IS) and Ea ) E(TS) - E(IS), where E(IS), E(FS), and E(TS) are the energies of the corresponding initial state (IS), final state (FS), and transition state (TS), respectively. The present work of building the reaction network in each part is carried out by iterating the following procedures: searching all possible paths for a given adsorbed species obtained from the previous step and continuing only those with the lowest reaction barriers while discarding those with obviously higher barriers. 3. Results and Discussion 3.1. CO Adsorption. To investigate the CO dissociation process, we first calculated CO adsorption on Fe3C(100). All

Figure 2. CO adsorption on Fe3C(100) at 1/5 ML (Fe atoms in purple, O atom in red, adsorbed C atom in black, and surface C atoms in gray).

Surface CxHy Formation from CO Activation on Fe3C(100)

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21587 CO adsorption and 4F has the most activated C-O bond (1.295 Å), in agreement with the previous data.42 In addition, there is no direct correlation between the calculated CO adsorption energy and the vibrational frequency; instead the vibrational frequency is related to the number of CO coordination and the C-O distance, that is, the higher the coordination number, the longer the C-O distance and the lower the vibrational frequency. 3.2. CO Dissociation. On the basis of the above results, we computed CO dissociation from the most stable 3F configuration and the most activated 4F configuration. The optimized structures of the transition states and products are illustrated in Figure 3.

Figure 3. Transition state structures and final states of CO dissociation on Fe3C(100) (Fe atoms in purple, O atom in red, adsorbed C atom in black, and surface C atoms in gray).

possible adsorption sites were considered at 1/5 ML, and seven stable configurations (1F-4F) were located. The corresponding adsorption energies, key bond parameters, and CO stretching frequencies are given in Table 2. As shown in Figure 2, there are two one-fold (1F), two 2-fold (2F), one 3-fold (3F) CO adsorption configurations, and two 4-fold (4F) configurations with surface ketenylidene (CsCO) formation. Table 2 shows that all coordination sites have very close adsorption energies and can coexist in equilibrium. Among them, 3F has the strongest

In 3F, CO upright adsorbs on the 3-fold site with the C-O bond length of 1.214 Å. During the early stage of CO dissociation (IS f TS), CO tilts toward to the surface until new Fe- bonds are formed. In TS(3F-dis), the O atom resides at bridge site with the Fe-O distances of 1.816 and 1.906 Å, and the C-O distance elongates to 2.002 Å. After that, the O atom further moves away to a 3-fold site forming FS. This CO dissociation reaction is endothermic by 0.83 eV and has barrier of 1.71 eV. It is interesting to note that CO adsorbed at the 4-fold site (4F) forms surface ketenylidene (CsCO) species, which has been observed by 13C NMR spectroscopy,49 and was suggested as an important intermediate in FTS.37,39 In 4F-1, the C-O bond is highly activated (1.295 Å); CsCO dissociation leads to surface CsC and O atom. In TS(4F-dis), the C-O bond is elongated to 1.761 Å with the O atom close to a bridge site. Following the transition state, the O atom moves away to a 3-fold site while the C atom remains bonded to the surface Cs atom and three Fe atoms in corresponding FS. The overall reaction is endothermic by 0.15 eV and has a barrier of 1.60 eV. The result shows that

Figure 4. CO and H coadsorption on Fe3C(100) (Fe atoms in purple, O atom in red, H atom in white, adsorbed C atom in black, and surface C atoms in gray).

TABLE 3: Computed Bond Lengths (d, Å) and Adsorption Energies (Eads, eV) for CO and H Co-adsorption on Fe3C(100) 3F-H-1 3F-H-2 3F-H-3 4F-H-1 4F-H-2 4F-H-3 4F-H-4

Eads

d(C-O)

-4.52 -4.69 -4.21 -4.33 -4.36 -4.31 -4.11

1.185 1.205 1.212 1.283 1.272 1.282 1.290

d(O-Fe)

2.064 2.150 2.074 2.060

d(C-Fe) 1.777 1.922, 1.937, 2.061, 2.075, 2.002, 1.965,

2.041, 1.991, 2.091, 2.116, 2.101, 2.175,

2.116 2.116 2.091 2.177 2.204 2.241

d(C-C)

d(H-Fe)

1.400 1.410 1.423 1.434

1.713,1.868,1.738 1.766, 1.762, 1.762 1.635, 1.742 1.724,1.777,1.851 1.842,1.713, 1.801 1.858, 1.768, 1.849 1.599, 1.773

21588

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Deng et al.

Figure 5. Structures of key stationary points for CHx formation on Fe3C(100) (Fe atoms in purple, O atom in red, H atom in white, adsorbed C atom in black, and surface C atoms in gray).

TABLE 4: Calculated Activation Barrier (Ea, eV) and Reaction Energies (∆Er, eV, in Parentheses) for CHx Formation on Fe3C (100) and Bond Parameters (d, Å) for Key Stationary Points Ea (∆Er)

d(C-O)

d(C-H)

TS(CsH) CsH

0.53 (-0.37)

1.202 1.213

TS-CHO CHO TS(CHO-dis) CH+O

0.98 (0.47) 0.79 (-0.17)

CO + H f CHO f CH + O 1.258 1.225 2.068, 1.888, 2.160 1.355 1.119 2.166, 2.181, 1.918 1.946 1.100 1.824, 2.035, 1.848 2.758 1.103 1.863, 1.870, 1.952

C S+ H f C SH 1.483 1.107

although CO dissociation in 4F-1 is slightly easier than in 3F, it is still difficult kinetically. 3.3. CO and H Co-adsorption. Since H is a key component in FTS, it is interesting to know the role of H on surface carbon species formation; for that the coadsorption of CO and H was investigated at first. For studying CO and H coadsorption, it is necessary to consider the H adsorption sites on the surface. Since CO adsorption on Fe3C(100) surface is stronger than H,43 it is reasonable to consider that CO should adsorb prior to H under a syngas environment. Therefore, H adsorption was built up on the model of preadsorbed CO system. For CO preadsorbed on the 3-fold site (3F) and the 4-fold site (4F-1), all possible H adsorption sites were considered, and only seven stable CO and H coadsorption modes were found (Figure 4). The calculated adsorption energies and bond parameters are given in Table 3. For CO preoccupied in the 3-fold site (3F), we consider H adsorption around CO in top sites, 2-fold sites and 3-fold sites, and only three coadsorption modes were found. In 3F-H-1, CO shifts from the 3-fold site to the top site and H is in the 3-fold site, and the adsorption energy is -4.52 eV. In 3F-H-2, CO still occupies the 3-fold site and H shares one metal atom with CO, and this mode has the highest adsorption energy of -4.69 eV. In 3F-H-3, CO occupies the 3-fold site and H is in the 2-fold site, and the adsorption energy of -4.21 eV is slightly lower. For CO preoccupied on the 4-fold site (4F-1), 10 coadsorption configurations were considered, but only four stable configura-

d(C-Fe)

d(O-Fe)

2.149, 2.252, 1.849 2.072, 1.972, 1.992

2.128, 2.002 1.764, 1.838 1.857, 1.847, 1.899

tions were found. The H atoms of 4F-H-1 and 4F-H-3 are all in the 3-fold sites and share one Fe atom with CsCO. In 4FH-2, the H atom shares two Fe atoms with CsCO. In 4F-H-4, the H atom is in the bridge site. It shows that the adsorption energies of 4F-H-1, 4F-H-2 and 4F-H-3 are very close (-4.33, -4.36, and -4.31 eV, respectively), and higher than that of 4F-H-4 (-4.11 eV). Because of their close adsorption energies, we also investigated H diffusion on the surface. However, it is found that H diffusion on the surface is practically no barriers. On the basis of these results, the following discussion about the pathway for CxHy formation is based on 3F-H-2 and 4FH-2. 3.4. Pathway for CxHy Formation. 3.4.1. C1 Species Formation. Since surface CHx species are important intermediates in FTS, it is desired to know their formation mechanism on Fe3C(100). Since CO direct dissociation is very difficult as discussed above, the question about the CHx formation paths raises immediately. To answer this question, we analyzed the Fe3C(100) surface carefully and found two potential paths by exposing iron carbides to syngas. One is surface reaction between the coadsorbed H and CO to surface formyl CHO intermediate, followed by the C-O bond cleavage into surface CH and O; another is surface Cs hydrogenation to form surface CsH. Here, both reactions were considered. The optimized structures are given in Figure 5, and the calculated reaction barriers as well as bond parameters are given in Table 4. The corresponding energy profiles are shown in Figure 6.

Surface CxHy Formation from CO Activation on Fe3C(100)

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21589 endothermic by 0.47 eV and has a barrier of 0.98 eV. The succeeding surface formyl dissociation takes place via TS(CHOdis), in which the C-O distance is 1.946 Å, and the surface O atom occupies the bridge site. This step has barrier of 0.79 eV and is exothermic by 0.17 eV.

Figure 6. Energy profiles (eV) for CHx formation on Fe3C(100) surface.

For the surface CHO formation path, 3F-H-2 was taken as the starting point. During this course, the coadsorbed H and CO approaches gradually. In the transition state TS(CHO), the C-H distance is 1.225 Å, and the C-O distance is 1.258 Å. In surface CHO, the C-H and C-O distances are 1.119 and 1.355 Å, respectively. It is found that the surface CHO formation is

For surface CsH formation from surface C hydrogenation, surface H migrates to surface Cs directly with CO as the spectator. In the transition state TS(CsH), H atom adsorbs on the top of Fe atom, and the C-H distance is 1.483 Å. This reaction is exothermic by 0.37 eV and has a barrier of 0.53 eV. Comparison of these paths along with the direct CO dissociation path in Figure 6, we can clearly see that the pathway of surface C hydrogenation to form surface CsH has the lowest barrier and the largest exothermicity and is therefore most preferable both thermodynamically and kinetically. 3.4.2. C2 Species Formation. For C2Hx formation, many studies50-53 focused on C1 + C1 coupling on pure metallic surfaces, but on Fe3C(100) surface CsCO can form directly by CO adsorption. Since H is available in FTS, when exposing CsCO to H, CsCO not only has the possibility of direct dissociation, but also is likely to be hydrogenated first forming oxygenated intermediates, followed by the C-O bond cleavage leading to surface C2Hx species. The optimized

Figure 7. Structures of key stationary points for C2 species on Fe3C(100) (Fe atoms in purple, O atom in red, H atom in white, adsorbed C atom in black, and surface C atoms in gray).

21590

J. Phys. Chem. C, Vol. 114, No. 49, 2010

Deng et al.

TABLE 5: Calculated Activation Barriers and Reaction Energies (Ea and ∆rE, eV) for C2Hx Formation on Fe3C(100) and the Bond Parameters (d, Å) for Key Stationary Points Ea (∆rE) TS(1a) 1a TS(1a′) 1a′ TS(1a′′) 1a′′

0.40 (-0.11) 0.53 (0.31) 1.30 (0.43)

2a TS(1adis) 1b

(0.53) 1.19 (-0.04)

TS(3a) 3a TS(2adis) 2b

0.56 (0.23) 0.66 (-0.71)

TS(4a) 4a TS(3adis) 3b

2.10 (-0.22) 0.37 (-0.89)

a

d(C-H)

d(Cs-H)

d(C-O)

CSCO + H f CSCHO (1a)/CSHCO 1.453 1.099 1.681 1.143 1.090a 0.978a

(1a′)/CSCOH 1.312 1.354 1.275 1.293 1.421 1.381

d(C-Fe) (1a′′) 2.185/2.016/2.247 2.097 2.109/1.992/2.160 2.031/1.972/2.072 1.978/1.935 1.979/2.178

CSCHO + H f CSHCHO; CSCHO f CSCH + O 1.095 1.217 1.366 2.045 1.097 1.765 2.247/2.000 1.105 2.780 2.132, 1.912 CSHCHO + H f CSH2CHO; CSHCHO f CSHCH + O 1.096 1.186/1.617 1.358 2.028 1.100 1.120/1.139 1.375 2.020 1.096 1.148 1.895 1.934 1.105 1.105 2.780 2.112, 1.926 CSH2CHO + H f CSH3CHO; CSH2CHO f CSH2CH + O 1.098 1.108/1.122/1.558 1.338 2.173 1.102 1.106/1.102/1.102 1.372 2.026 1.105 1.137/1.105 2.176 1.923 1.106 1.132/1.100 3.312 2.124/2.250/2.048

d(O-Fe) 1.995 1.947/2.227 2.019 2.045 2.231

1.993/2.098 1.876/1.972 1.854/1.876/1.964 2.043/2.018 1.969, 2.032 1.828/1.926 1.872/1.955/1.885 2.183/1.985 2.055/1.914 1.894/1.785 1.869/1.954/1.880

The O-H bond length, d(O-H).

Figure 8. Energy profiles (eV) for C2Hx formation on Fe3C(100).

structures are shown in Figure 7; the calculated reaction barriers and bond parameters are given in Table 5. To study the formation of oxygenated intermediates, 4F-H-2 was taken as the starting point. The energy profiles of all pathways for C2Hx formation are shown in Figure 8. Starting from 4F-H-2, the coadsorbed H can bind to either carbon or oxygen to form CsCHO (1a), CsHCO (1a′), and CsCOH (1a′′), and the corresponding barriers are predicted to be 0.40, 0.53, and 1.30 eV, respectively. Furthermore, the formation of CsCHO (1a) is exothermic by 0.11 eV, while those of CsHCO (1a′) and CsCOH (1a′′) are endothermic by 0.31 and 0.43 eV, respectively. This indicates that the formation of CsCHO (1a) is more favorable than CsHCO (1a′) and CsCOH (1a′′) both thermodynamically and kinetically. Once CsCHO (1a) is formed, it can either dissociate into CsCH and O or be further hydrogenated. Since adding H to O atom of CsCHO (1a) is difficult according to the first hydrogenation step, we mainly focus on Cs hydrogenation of CsCHO (1a). As shown in Figure 8, three pathways were examined: (i) CsCHO f CsCH + O; (ii) CsCHO + H f CsHCHO and

CsHCHO f CsHCH + O; and (iii) CsCHO + 2H f CsH2CHO and CsH2CHO f CsH2CH + O. For CsCHO (1a) dissociation into CsCH + O, the reaction has barrier of 1.19 eV and is exothermic by 0.04 eV. For CsCHO (1a) hydrogenation into CsHCHO, it can occur by elevating energy in 0.53 eV under high hydrogen coverage, because the reverse process of CsHCHO to CsCHO is no barrier. CsHCHO dissociation into CsHCH and O needs to overcome a barrier of 0.66 eV and is exothermic by 0.71 kcal/mol. For CsHCHO hydrogenation into CsH2CHO, it needs a barrier of 0.56 eV and is endothermic by 0.23 eV, and the dissociation of CsH2CHO into CsH2CH and O has a barrier of 0.37 eV and is exothermic by 0.89 eV. A detailed comparison of these reaction paths starting from CsCHO (1a) in Figure 8 shows that the formation of these three C2Hx hydrocarbons has very close effective barriers and is also exothermic in close extent. Under hydrogen rich condition, CsHCH can either be formed from CsCHO (1a) dissociation and hydrogenation or hydrogenation and dissociation pathways; and CsH2CH formation can also be either from CsHCHO hydrogenation and dissociation or from CsHCH

Surface CxHy Formation from CO Activation on Fe3C(100) hydrogenation. Therefore, the formation of C2Hx depends on the CO/H2 ratio, which will determine the reaction sequence of the chain growth process. For comparison we have computed the hydrogenation step into surface acetaldehyde formation (CsCHO + 3H f CsH3CHO), it has very high effective barrier of 2.86 eV and is endothermic of 0.54 eV. 3.5. Role of Surface Vacancy in CO Dissociation. As discussed above, surface Cs atom participates in the formation of surface C1 and C2 species; which will produce hydrocarbons via chain growth and generate vacancy site on the surface. Here we investigated CO adsorption on a vacancy site. On Fe3C(100) there is one vacancy site per p(1 × 1) surface unit cell, on which the C atom of CO bonds with four Fe atoms, and the O atom tilts to bond with another Fe atom. The C-O bond length is 1.317 Å, and the calculated CO stretching frequency is around 1152 cm-1, indicating the strongest activation of adsorbed CO. In addition, the adsorption energy of CO on vacancy site is -2.19 eV, much larger than those on the perfect Fe3C(100) (Table 2). Due to its strong activation, the adsorbed CO on the vacancy site should also dissociate easily. Indeed, the calculated dissociation barrier is 0.77 eV, and the dissociation is exothermic by 0.76 eV. In the transition state, the C and O atoms are separated by 1.921 Å and share bonding with one of surface Fe atoms. This shows that vacancy site lowers the CO dissociation barrier strongly; once a vacancy site is formed, CO will adsorb and dissociate on the vacancy site prior to other sites and the newly formed surface C atom further takes part in C1 and C2 species formation. All these processes constitute a catalytic cycle. 4. Conclusion In this article, the mechanisms of CO adsorption, CO and H coadsorption, and CO dissociation as well as CxHy formation on Fe3C(100) have been investigated by using spin-polarized density functional theory method. It is found that all CO adsorption sites have very close adsorption energies (-1.72 to -1.83 eV), indicating the possible coexistence of these sites. The largest CO adsorption energy is found at the 3-fold site (3F) with highly activated C-O bond; the strongest CO activation is found at the 4-fold site (4F) with the formation of surface ketenylidene (CsCO). Direct CO dissociation is very difficult due to its high barrier of 1.71 eV; and H-assisted CO dissociation via surface CHO intermediate has lower effective barrier (1.26 eV); both reactions are endothermic. In contrast, surface Cs hydrogenation to form surface CsH has the lowest barrier (0.53 eV) and is also exothermic (-0.37 eV) and thus represents the most favorable pathway for surface CH formation. As an important intermediate for C2Hx formation, surface CsCO can either dissociate or be hydrogenated. Direct surface CsCO dissociation into CsC and O has barrier of 1.60 eV, while CsCO hydrogenation into CsCHO has the lowest barrier of 0.40 eV, and is exothermic by 0.11 eV. Starting from the oxygenated CsCHO intermediate, it is noteworthy that the surface C2Hx species, CsCH, CsHCH, and CsH2CH, can come from either the hydrogenation and dissociation pathway or the dissociation and hydrogenation pathway. On the basis of the comparable effective barriers and reaction energy, the formation of C2Hx depends on the CO/H2 ratio. Since surface C atom participates in the formation of C1 and C2 species for chain growths, surface vacancy formation becomes obvious. It is found that surface vacancy can activate CO strongly and lower the CO dissociation barrier considerably. As a consequence, the vacancy is reoccupied and the carburized surface is regenerated for the next catalytic cycle.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21591 Acknowledgment. We thank the National Natural Science Foundation of China (No. 20873173) and the National Outstanding Young Scientists Foundation of China (No. 20625620) for financial supports. References and Notes (1) Davis, B. H. Technology DeVelopment for Iron and Cobalt Fischer-Tropsch Catalysts, quarterly Report: Germany, 1998. http:// www.fischer-tropsch.org/DOE/DOE_reports/. (2) Dry, M. E. Catal. Today 2002, 71, 227. (3) Dry, M. E. Appl. Catal., A 2004, 276, 1. (4) Davis, B. H. Fuel Process. Technol. 2001, 71, 157. (5) Cheng, J.; Hu, P.; Ellis, P.; French, S. Top Catal. 2010, 53, 326. (6) Govender, N. S.; Schouten, J. C. Appl. Catal., A 2010, 373, 81. (7) Li, S.; Ding, W.; Meitzner, G. D.; Iglesia, E. J. Phys. Chem. B 2002, 106, 85. (8) Gaube, J.; Klein, H.-F. J. Mol. Catal. A 2008, 283, 60. (9) Davis, B. H. Catal. Today 2009, 141, 25. (10) Li, S.; O’Brien, R. J.; Meitzner, G. D.; Hamdeh, H.; Davis, B. H.; Iglesia, E. Appl. Catal., A 2001, 219, 215. (11) de Smit, E.; Beale, A. M.; Nikitenko, S.; Weckhuysen, B. M. J. Catal. 2009, 262, 244. (12) Crystallite Phase and Their Relationship to Fischer-Tropsch Catalysis Emmett, P. H., Ed.; Reinhold: New York, 1956; p 407. (13) Herranz, T.; Rojas, S.; Pe´rez-Alonso, F. J.; Ojeda, M.; Terreros, P.; Fierro, J. L. G. J. Catal. 2006, 243, 199. (14) (a) Mirzaei, A. A.; Habipour, R.; Faizi, M.; Kashi, E. Appl. Catal., A 2006, 301, 272. (b) de Smit, E.; Cinquini, F.; Beale, A. M.; Safonova, O. V.; van Beek, W.; Sautet, P.; Weckhuysen, B. M. J. Am. Chem. Soc. 2010, 132, 14928. (15) Huo, C.-F.; Ren, J.; Li, Y.-W.; Wang, J.; Jiao, H. J. Catal. 2007, 249, 174. (16) Jiang, D. E.; Carter, E. A. Surf. Sci. 2004, 570, 167. (17) Sorescu, D. C.; Thompson, D. L.; Hurley, M. M. Phy. ReV. B 2002, 66, 031456. (18) Ojeda, M.; Nabar, R.; Nilekar, A. U.; Mavrikakis, M.; Iglesia, E. J. Catal. 2010, 272, 287. (19) Ferre, D. C.; Govender, A.; Bromfield, T. C.; Niemantsverdriet, J. W. J. Phys. Chem. B 2006, 110, 13897. (20) Lo, J. M. H.; Ziegler, T. J. Phys. Chem. C 2008, 112, 3692. (21) Sorescu, D. C. J. Phys. Chem. C 2008, 112, 10472. (22) Bromfield, T. C.; Ferre, D. C.; Niemantsverdriet, J. W. Chem. Phys. Chem. 2005, 6, 254. (23) Chen, J.; Liu, Z.-P. J. Am. Chem. Soc. 2008, 130, 7929. (24) Liu, Z.-P.; Hu, P. J. Am. Chem. Soc. 2003, 125, 1958. (25) Ciobica, I. M.; van Santen, R. A. J. Phys. Chem. B 2003, 107, 3808. (26) Shetty, S.; Jansen, A. P. J.; van Santen, R. A. J. Phys. Chem. C 2008, 112, 14027. (27) Ge, Q.; Neurock, M. J. Phys. Chem. B 2006, 110, 15368. (28) Brady, R. C.; Pettit, R. J. Am. Chem. Soc. 1980, 102, 6181. (29) Shetty, S.; Jansen, A. P. J.; van Santen, R. A. J. Am. Chem. Soc. 2009, 131, 12874. (30) Storch, H. H.; Goulombic, N.; Anderson, R. B. The FischerTropsch and Related Syntheses; Wiley: New York, 1951. (31) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2008, 112, 14108. (32) Inderwildi, O. R.; Jenkins, S. J.; King, D. A. J. Phys. Chem. C 2008, 112, 1305. (33) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. J. Phys. Chem. C 2010, 114, 1085. (34) Petersen, M. A. J. Phys. Chem. C 2010, 114, 7863. (35) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. J. Am. Chem. Soc. 2009, 131, 14713. (36) Cao, D.-B.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2008, 112, 14883. (37) Deng, C.-M.; Huo, C.-F.; Bao, L.-L.; Feng, G.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2008, 112, 19018. (38) Sorescu, D. C. J. Phys. Chem. C 2009, 113, 9256. (39) Cao, D.-B.; Zhang, F.-Q.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. B 2005, 109, 10922. (40) Cao, D.-B.; Wang, S.-G.; Li, Y.-W.; Wang, J.; Jiao, H. J. Mol. Catal. A 2007, 272, 275. (41) Chiou, W. C., Jr.; Carter, E. A. Surf. Sci. 2003, 530, 87. (42) Liao, X.-Y.; Cao, D.-B.; Wang, S.-G.; Ma, Z.-Y.; Li, Y.-W.; Wang, J.; Jiao, H. J. Mol. Catal. A 2007, 269, 169. (43) Liao, X.-Y.; Wang, S.-G.; Ma, Z.-Y.; Wang, J.; Li, Y.-W.; Jiao, H. J. Mol. Catal. A 2008, 292, 14. (44) (a) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 48, 13115. (b) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (c) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169.

21592

J. Phys. Chem. C, Vol. 114, No. 49, 2010

(45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (46) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (47) Jo´nsson, H.; Mills, G.; Jacobsen, K. W. Classical and Quantum Dynamics in Condensed Phase Simulations; World Scientific: Singapore, 1998, p 385. (48) Shabanova, I. N.; Trapeznikov, V. A. JETP Lett. 1973, 18, 339. (49) Kolis, J. W.; Holt, E. M.; Shriver, D. F. J. Am. Chem. Soc. 1983, 105, 7307.

Deng et al. (50) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, M. C. J. Phys. Chem. C 2008, 112, 6082. (51) Cheng, J.; Gong, X.-Q.; Hu, P.; Lok, M. C.; Ellis, P.; French, S. J. Catal. 2008, 254, 285. (52) Ge, Q.; Neurock, M.; Wright, H. A.; Srinivasan, N. J. Phys. Chem. B 2002, 106, 2826. (53) Lo, J. M. H.; Ziegler, T. J. Phys. Chem. C 2007, 111, 13149.

JP108480E