pubs.acs.org/Langmuir © 2009 American Chemical Society
Density Functional Studies of the Adsorption and Dissociation of CO2 Molecule on Fe(111) Surface Hui-Lung Chen,*,† Hsin-Tsung Chen,‡ and Jia-Jen Ho*,§ † ‡
Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan, National Center for High-performance Computing, 28 Nan-Ke third Road, Hsinshi, Tainan 74147, Taiwan, and § Department of Chemistry, National Taiwan Normal University, 88, Section 4, Tingchow Road, Taipei 116, Taiwan Received June 16, 2009. Revised Manuscript Received August 11, 2009
Spin-polarized density functional theory calculation was carried out to characterize the adsorption and dissociation of CO2 molecule on the Fe(111) surface. It was shown that the barriers for the stepwise CO2 dissociation reaction, CO2(g) f C(a) + 2O(a), are 21.73 kcal/mol (for OC-O bond activation) and 23.87 kcal/mol (for C-O bond activation), and the entire process is 35.73 kcal/mol exothermic. The rate constants for the dissociative adsorption of CO2 have been predicted with variational RRKM theory, and the predicted rate constants, kCO2 (in units of cm3 molecule-1 s-1), can be represented by the equations 2.12 10-8T-0.842 exp(-0.258 kcal mol-1/RT) at T = 100-1000 K. To gain insights into high catalytic activity of the Fe(111) surface, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.
1. Introduction
2. Computational Details
Recently, the catalytic activation of carbon dioxide by metal centers has been extensively studied both experimentally and theoretically.1 Most studies have been performed on high-surface-area, body-centered-cubic (bcc), or single crystals.2 The Fe(111) surface is thought to have high catalytic activity from the very open surface structure,3 and this unique crystal face possess the highest turnover rate.4 Hess et al.5 examined the CO2 catalytic process at the Fe(111) surface and reported that the adsorption configurations of CO2 on Fe(111) exist as one linear state and two bent states. At higher temperature (T g 370 K), time-dependent HREELS (highresolution electron-energy-loss spectra) measurements reveal experimental evidence that CO2 dissociates completely, and the final surface contains only iron oxide and iron carbide species. To the best of our knowledge, however, no theoretical study regarding the CO2 decomposition mechanism on the Fe(111) surface was available. In the present study, we report our findings by applying periodic density functional theory (DFT) to study the adsorption and dissociation behaviors of CO2 molecule on the Fe(111) surface. We believe that this understanding is vital in the future study for rational design of surface catalytic model in decomposing the CO2 molecule.
All present calculations are performed with the DFT planewave method utilizing the Vienna ab initio simulation package (VASP).6-9 In these calculations we use the projector-augmented wave method (PAW)10,11 in conjunction with revised Perdew-Burke-Ernzerhof (rPBE)12,13 functional. The Brillouin zone is sampled with the Monkhorst-Pack grid.14 The calculations are carried out using the (4 4 4) and (4 4 1) Monkhorst-Pack mesh k-points for bulk and surface calculations, respectively. A 400 eV cutoff energy, which allows convergence to 1 10-4 eV in total energy, is used. All calculations are performed by using the spin-polarization method to properly describe the magnetic property of the Fe(111) surface model. It should be mentioned that we have also performed calculations for CO2 adsorption on (2 2) and (3 3) surfaces, corresponding to coverage of 1/4 ML and 1/9 ML, respectively, using the most stable FeCO2(S-μ3-C,O,O0 ) configuration. It was shown that the coverage effect is negligible due to the relative energy smaller than 1.0 kcal/mol. Therefore, in the present study we only use the computationally less expensive p(2 2) model of the Fe(111) surface. The p(2 2) lateral cell of Fe(111) surface is modeled as periodically repeated slabs with six layers. The bottom three atomic layers are kept frozen and set to the estimated bulk parameters, while the remaining layers are fully relaxed during the calculations. The lateral cell has dimensions of a = b = 8.02 A˚ and c = 20.01 A˚, which includes a vacuum region of thickness higher than 15 A˚ and guarantees no interactions between the slabs. In this study, we calculate adsorption energies according to
*Corresponding authors: e-mail
[email protected] (H.-L.C.),
[email protected] (J.-J.H.); Fax +886-2-28614212. (1) (a) Solymosi, F.; Klivenyi, G. Surf. Sci. 1994, 315, 255. (b) Hess, G.; Froitzheim, H.; Baumgartner, Ch. Surf. Sci. 1995, 331, 138. (c) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (d) Mebel, A. M.; Hwang, D.-Y. J. Phys. Chem. A 2000, 104, 11622. (e) Choe, S. J.; Park, D. H.; Huh, D. S. Bull. Korean Chem. Soc. 2000, 21, 779. (f) Mebel, A. M.; Hwang, D.-Y. J. Chem. Phys. 2002, 116, 5633. (g) Dobrogorskaya, Y.; Mascetti, J.; Papai, I.; Hannachi, Y. J. Phys. Chem. A 2005, 109, 7932. (h) Chen, H.-T.; Musaev, D. G.; Lin, M. C. J. Phys. Chem. C 2008, 112, 3341. (i) Poully, B.; Bergeat, A.; Hannachi, Y. J. Phys. Chem. A 2008, 112, 8148. (j) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. J. Am. Chem. Soc. 2008, 130, 2023. (k) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. J. Am. Chem. Soc. 2008, 130, 2902. (2) Ertl, G. Angew. Chem., Int. Ed. Engl. 1986, 6, 558. (3) Spence, N. D.; Schoonmaker, R. C.; Somorjai, G. A. J. Catal. 1982, 74, 129. (4) Strongin, D. R.; Somorjai, G. A. J. Catal. 1988, 109, 51. (5) (a) Hess, G.; Baumgartner, Ch.; Froitzheim, H. Phys. Rev. B 2001, 63, 165416. (b) Hess, G.; Baumgartner, Ch.; Petkova, A.; Froitzheim, H. Surf. Sci. 2004, 572, 355.
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(6) Kresse, G.; Hafner, J. Phys. Rev. B 1993, 47, 558. (7) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251. (8) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. 1996, 6, 15. (9) Kresse, G.; Hafner, J. Phys. Rev. B 1996, 54, 11169. (10) Bl€ochl, P. E. Phys. Rev. B 1994, 50, 17953. (11) Kresse, G.; Joubert, D. Phys. Rev. B 1999, 59, 1758. (12) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (13) Zhang, Y.; Yang, W. Phys. Rev. Lett. 1998, 80, 890. (14) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188.
Published on Web 08/28/2009
DOI: 10.1021/la9021646
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Table 1. Calculated and Experimental Lattice Parameters and Magnetic Moment of Bulk Fe methods PW91 (ISPIN = 1) PBE (ISPIN = 1) rPBE (ISPIN=1) PW91 (ISPIN = 2) PBE (ISPIN = 2) rPBE (ISPIN = 2) experiment a See ref 20.
lattice parameters/A˚
magnetic moment/μB
2.763 2.760 2.772 2.818 2.817 2.836 2.866a
2.20 2.23 2.23 2.22a
the following equation: ΔEads ¼ E½surface þ adsorbate -ðE½surface þ E½adsorbateÞ where E[surface + adsorbate], E[surface], and E[adsorbate] are the calculated electronic energies of adsorbed species on Fe(111) surface, a clean Fe(111) surface, and a gas-phase molecule, respectively. Vibrational frequencies of the adsorbed structures are analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged elastic band (NEB) method15-17 is applied to locate transition states, and minimum-energy pathways (MEP) are constructed accordingly. The rate constants for the reaction of CO2 molecule on the Fe(111) surface are calculated using the variational RRKM theory18 as implemented by the Variflex program.19
3. Results and Discussion 3.1. Computational Condition Tests. In Table 1, we present data for the lattice parameter and magnetic moment of bulk Fe calculated at various levels of theory, with pertinent experimental data from the literature. The lattice parameter of bulk Fe predicted with consideration of the spin-polarization at the rPBE level is 2.836 A˚, which approaches the experimental value (2.866 A˚)20 more closely than for the PW91 and PBE levels, 2.818 and 2.817 A˚, respectively. We calculated, at the same level (rPBE), the magnetic moment of bulk Fe; the result of 2.23 μB agrees satisfactorily with the experimental value, 2.22 μB.20 For the slab surface of Fe(111), the magnetism is reinforced; the magnetic moments of the first, second, and third layers are 2.90, 2.50, and 2.61 μB, respectively. The same tendencies are found for the Fe(100) and Fe(110) surfaces (3.05, 2.52, and 2.65 μB vs 2.74, 2.53, and 2.54 μB).21 As this agreement confirms the rPBE level of calculation to be suitable, in the following sections we use only rPBE energies for discussion. In addition, it has been shown previously22,23 that the predicted chemisorption energies of small molecules on metal surfaces at the rPBE functional are in better agreement with the experimental values than the PW91-calculated ones. For example, the PW91 functional gives too large chemisorption energies numerically by about 14 kcal/mol, while the rPBE functional proves to be rather accurate (less than 5 kcal/mol divergence) for CO on Ni(111) and Pd(111) surfaces.22 Moreover, (15) Ulitsky, A.; Elber, R. J. Chem. Phys. 1990, 92, 1510. (16) Mills, G.; Jonsson, H.; Schenter, G. K. Surf. Sci. 1995, 324, 305. (17) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. Chem. Phys. 2000, 113, 9901. (18) Baer, T.; Hase, W. L. Unimolecular Reaction Dynamics. Theory and Experiments; Oxford University Press: Oxford, 1996. (19) Klippenstein, S. J.; Wagner, A. F.; Dunbar, R. C.; Wardlaw, D. M.; Robertson, S. H. Variflex, 1999. (20) Kittel, C. Introduction to Solid State Physics, 7th ed.; John Wiley & Sons: New York, 1996. (21) Bzonski, P.; Kiejna, A.; Hafner, J. Surf. Sci. 2005, 590, 88. (22) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999, 59, 7413. (23) Schreiner, P. R. Angew. Chem., Int. Ed. 2007, 46, 4217.
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it is reported that the rPBE can give not only reliable geometries but also reasonable adsorption energies, and this behavior has been further examined and verified for the similar systems of hydrogen adsorption and CO and hydrogen coadsorption on the Fe(111) surface.24,25 3.2. Adsorption of CO2, CO, C, and O on the Fe(111) Surface. To locate possible stable intermediates, such as Fe(111)/ CO2, Fe(111)/CO, Fe(111)/C, and Fe(111)/O, we placed CO2, CO, C, and O species at various sites on the Fe(111) surface as shown in Figure 1. The four adsorption sites of Fe(111) considered are described as top (T), bridge (B), 3-fold-shallow (S), and 3-fold-deep (D). For the top site (T), the molecule is adsorbed on top of the first-layer Fe atom of Fe(111); at the (B) site, above the center of the Fe-Fe bond of the two first-layer Fe sites; at the (S) site, above the second-layer Fe atom; and at the (D) site, above the third-layer Fe atom. The coordination of CO2 on the Fe(111) surface leads to formation of a Fe(111)/CO2 intermediate that might exist in several isomeric forms, shown in Figure 2: (a) R states such as FeCO2(T-η1-C), FeCO2(T-η1-O), FeCO2(B-μ2-O), FeCO2(B-μ2O,O0 ), FeCO2(S-η1-O), and FeCO2(D-η1-O); (b) the β state, FeCO2(T-η2-C,O); and (c) γ states, FeCO2(S-μ3-C,O,O0 ), FeCO2(D-η1-C), and FeCO2(D-μ3-C,O,O0 ). Our data show that isomer FeCO2(S-μ3-C,O,O0 ) is energetically favored among all calculated structures of Fe(111)/CO2; the adsorption energy is -25.58 kcal/ mol (see Table 2). Isomer FeCO2(S-μ3-C,O,O0 ) has evidently the shortest Fe-C and Fe-O bonds and largest C-O distances, characteristic of a chemisorbed species. Structures FeCO2(T-η1-C), FeCO2(T-η1-O), FeCO2(B-μ2-O), FeCO2(B-μ2-O,O0 ), FeCO2(S-η1-O), and FeCO2(D-η1-O) are characterized as unbound with positive (unstable) adsorption energies. As expected, the coordination of CO on the Fe(111) surface yields a Fe(111)/CO intermediate that might also have several isomers (shown in Figure 3): FeCO(T-η1-C), FeCO(T-η1-O), FeCO(T, S-μ2-C)-a, FeCO(T, S-μ2-C)-b, FeCO(S-η1-C)-a, FeCO(S-η1-C)-b, FeCO(S-η1-C)-c, and FeCO(D-η1-C). Our calculations show (Table 3) that the linear isomer FeCO(T-η1-O) with CO coordinated to Fe through O is less stable than isomer FeCO(T-η1-C) with CO coordinated to Fe through C, indicating that an initial approach of CO to the Fe(111) surface would prefer the (C 3 3 3 Fe) construction. The three isomers FeCO(S-η1-C)-a, FeCO(S-η1-C)-b, and FeCO(S-η1-C)-c are energetically favored conformations among all calculated structures with adsorption energies -38.97, -36.39, and -37.43 kcal/mol, respectively. The coordination of C and O atoms on Fe(111) leads to Fe(111)/C and Fe(111)/O intermediates, respectively. The resulting species might involve six isomers (shown in Figure 4): FeX(Tη1-X), FeX(T, S-μ2-X)-a, FeX(T, S-μ2-X)-b, FeX(B-μ3-X), FeX(S-η1-X), and FeX(D-η1-X), with X = C and O. Table 4 shows that radical adsorbates atomic C and O adsorb strongly to the Fe(111) surface. Among the many adsorption sites, those between the top (T) and shallow (S) sites;FeX(T, S-μ2-X)-a, FeX(T, Sμ2-X)-b, and FeX(B-μ3-X);are favored (C, ca. -165.2 to -167.0 kcal/mol; O, ca. -128.4 to -129.2 kcal/mol) over the top site, FeX(T-η1-X) (C, ca. -107.5 kcal/mol; O, ca. -105.8 kcal/mol). 3.3. Dissociation of CO2 on a Fe(111) Surface; Calculation of the Rate Coefficient. As the FeCO2(S-μ3-C,O,O0 ) conformer is energetically the most stable among all tested conformers of Fe(111)/CO2, we chose it as an initial structure (denoted LM1) to study the dissociation of CO2 on Fe(111) at a (24) Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. B 2005, 109, 14160. (25) Ma, Z.-Y.; Huo, C.-F.; Liao, X.-Y.; Li, Y.-W.; Wang, J.; Jiao, H. J. Phys. Chem. C 2007, 111, 4305.
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Figure 1. Schematic presentation of Fe(111) surface used in the present studies: (a) side view and (b) top view. The T, D, and S represent top, deep, and shallow sites, while the middle of two top sites is considered as a bridge site and labeled as B.
Figure 2. Located isomers of adsorbed CO2 on Fe(111) surface and their important geometry parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms.
1/4 ML coverage. To perform these mechanistic calculations, we constructed potential energy profiles by mapping them with the NEB method depicted in Figure 5. The resulting profile indicates that the formation of LM1 occurs smoothly along the MEP on extending the two C-O bonds from 1.177 to 1.293 A˚ and shortening the Fe-C bond from 4.327 to 1.982 A˚, with no welldefined transition structure and exothermic by 25.58 kcal/mol. In the next transformation from LM1 to LM2 that involves the scission of one C-O bond from a CO2 adsorbate, we found a transition structure TS1, with a barrier 21.73 kcal/mol and exo thermic by 30.12 kcal/mol. The second deoxygenation might occur from LM2 with a slightly larger barrier, 23.87 kcal/mol for TS2, and forming the final product P, coadsorbed FeC(B-μ3C) plus two FeO(T, S-μ2-O)-a fragments. The overall reaction CO2(g) þ Fe(111) f CO2(a) (LM1) f CO(a) þ O(a) (LM2) f Langmuir 2010, 26(2), 775–781
C(a) þ 2O(a) (P) was thus calculated to be exothermic by 35.73 kcal/mol; the potential energies of the two transition structures (TS1 and TS2) are less than for the reactants CO2(g) þ Fe(111), indicating that no net activation energy is required for this dissociative adsorption. In summary, molecular CO2 is expected to proceed to its dissociative adsorption into atomic C and O on the Fe(111) surface, consistent with the experimental observation of Hess et al.5 On the basis of the PES for the dissociation of CO2 on Fe(111), we computed the rate coefficients for the reaction CO2(g) þ Fe(111) f CO2(a) (LM1) f CO(a) þ O(a) (LM2) f C(a) þ 2O(a) (P). For these calculations, the stretching potential energy surface representing the barrierless adsorption CO2(g)þ Fe(111) f CO2(a) (LM1), which is the rate-determining path, was calculated along the reaction coordinate Fe-C, which was stretched from its DOI: 10.1021/la9021646
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Table 2. Calculated Adsorption Energies (in kcal/mol), Geometrical Parameters (A˚), and Predicted Vibrational Frequencies (cm-1) of Adsorbed CO2 Species on Fe(111) Surface adsorption site
assignmenta
adsorption energy
d(Fe-CO2)b
d(C-O)c
top (T) T-η1-C T-η1-O T-η2-C,O
R R β
8.76 1.57 -2.40
2.194 2.347 1.988
1.212 (1.212) 1.172 (1.179) 1.214 (1.277)
bridge (B) B-μ2-O B-μ2-O,O0
R R
2.77 7.31
3.872 2.117
1.175 (1.178) 1.226 (1.226)
3-fold-shallow (S) S-η1-O S-μ3-C,O,O0
R γ
3.99 -25.58
3.052 1.982
1.175 (1.177) 1.293 (1.294)
vasym(CO)
vsym(CO)
vbend(OCO)
2368 1798
1323 1106
631 641
2380
1320
596
1375
1119
723
3-fold-deep (D) D-η1-C γ -6.55 1.997 1.335 (1.341) 1164 1011 657 R 3.26 3.860 1.175 (1.178) D-η1-O γ -6.60 2.834 1.305 (1.306) 1240 1081 675 D-μ3-C,O,O0 a The names of adsorbed species are consistent with those used by Hess et al. (see ref 5). b The shortest distance between the adsorbed atom (C or O) and the corresponding adsorption site of surface. c The values in parentheses are the second C-O bond length of CO2 molecule.
Figure 3. Located isomers of adsorbed CO on Fe(111) surface and their important geometry parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms. Table 3. Calculated Adsorption Energies (in kcal/mol), Geometrical Parameters (A˚), and Predicted Vibrational Frequencies (cm-1) of Adsorbed CO Species on Fe(111) Surface adsorption site top (T) T-η1-C T-η1-O T, S-μ2-C (a) T, S-μ2-C (b)
adsorption energy d(Fe-CO)a d(C-O) v(CO) -25.13 2.77 -30.57 -30.09
1.818 2.321 1.897 1.855
1.169 1.147 1.193 1.195
1762 1754
3-fold-shallow (S) S-η1-C (a) -38.97 -36.39 S-η1-C (b) -37.43 S-η1-C (c)
1.780 1.774 1.783
1.198 1.215 1.201
1775 1674 1753
3-fold-deep (D) -23.28 1.852 1.233 D-η1-C a The shortest distance between the adsorbed atom (C or O) and the corresponding adsorption site of surface.
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equilibrium value to 5.0 A˚ with step size 0.20 A˚. At each fixed Fe-C distance, the geometries of the bottom three atomic layers of Fe(111) surface were fixed while the atoms of remaining layers and CO2 were fully optimized at the rPBE level. On the basis of variational transition-state theory (VTST),26 the obtained stretching potential energy surface involving the Fe-C distance is approximated with a Morse potential, V(R) = De{1 - exp[-β(R - R0)]}2, in which R is the reaction coordinate, R0 is the equilibrium Fe-C bond distance, and De is the bond energy without zero-point energy correction. The parameters used for this Morse potential are R0 = 1.982 A˚, β = 3.309 A˚-1, and De = 25.58 kcal/mol for the reaction CO2(g) þ Fe(111). We performed (26) (a) Hase, W. L. J. Chem. Phys. 1972, 57, 730. (b) Wardlaw, D. M.; Marcus, R. A. Chem. Phys. Lett. 1984, 110, 230. (c) Wardlaw, D. M.; Marcus, R. A. J. Chem. Phys. 1985, 83, 3462. (d) Wardlaw, D. M.; Marcus, R. A. J. Phys. Chem. 1986, 90, 5383. (e) Wardlaw, D. M.; Marcus, R. A. Adv. Chem. Phys. 1988, 70, 231. (f) Song, K.; de Sainte Claire, P.; Hase, W. L.; Hass, K. C. Phys. Rev. B 1995, 52, 2949.
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Figure 4. Located isomers of adsorbed C and O (in parentheses) atoms on Fe(111) surface and their important geometry parameters calculated at the rPBE level of theory. The bond lengths are given in angstroms. Table 4. Calculated Adsorption Energies (in kcal/mol), Geometrical Parameters (A˚), and Predicted Vibrational Frequencies (cm-1) of Adsorbed C and O Atoms on Fe(111) Surface adsorption site for C atom T-η1-C T, S-μ2-C (a) T, S-μ2-C (b) B-μ3-C S-η1-C D-η1-C
adsorption energy
d(Fe-C or Fe-O)a
-107.53 -165.18 -165.16 -166.97 -145.45 -161.93
1.591 1.872 1.894 1.880 1.835 1.885
v(Fe-C or Fe-O)
546 535 543 571
for O atom -105.80 1.600 T-η1-O -129.21 1.799 544 T, S-μ2-O (a) -128.38 1.863 490 T, S-μ2-O (b) -128.36 1.858 500 B-μ3-O -123.59 1.887 537 S-η1-O -111.21 1.923 D-η1-O a The shortest distance between the adsorbed atom (C or O) and the corresponding adsorption site of surface.
Figure 6. Illustration of charge-density difference for CO2 dissociation on Fe(111) via the proposed minimun-energy pathway: (a) before interaction, (b) LM1, (c) LM2, and (d) P. ΔFdiff = F[surface þ adsorbate] - F[surface] - F[adsorbate], and the isosurfaces were calculated at 0.02 e A˚3. The values are effective charges which are calculated by the Bader analysis program.
these calculations for the temperature range 100-1000 K. The predicted rate coefficients (in units of cm3 molecule-1 s-1) in the broad temperature range are represented with kCO2 ¼ 2:12 10 -8 T -0:842 expð -0:258 kcal mol -1 =RTÞ
Figure 5. Calculated possible potential energy diagram for the dissociation of CO2(g) on the surface of Fe(111). For transition state, values of imaginary frequencies, vi, are also presented. Langmuir 2010, 26(2), 775–781
Nevertheless, for the other possible configurations of FeCO2(Dη1-C) and FeCO2(D-μ3-C,O,O0 ) which could be competitive with the process starting with FeCO2(S-μ3-C,O,O0 ), we also calculated the energy barriers of the transition states for their first step of CO2 dissociation (for OC-O bond activation). The predicted values, with respect to the reactant, are 17.85 kcal/mol (starting from FeCO2(D-η1-C)) and 16.69 kcal/mol (starting from DOI: 10.1021/la9021646
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Figure 7. Local density of states (LDOS) for CO2 dissociation on Fe(111) via the proposed minimun-energy pathway: (a) before interaction, (b) LM1, (c) LM2, and (d) P. The blue, gray, and red lines represent Fe(d), C(p), and O(p), respectively. The dashed line represents the Fermi level.
FeCO2(D-μ3-C,O,O0 )), which are much higher in energy than that of the FeCO2(S-μ3-C,O,O0 ) configuration obtained (-3.85 kcal/ mol). Therefore, from kinetic considerations, we can exclude these two channels. The rate coefficient for the dissociative adsorption of CO2 on Fe(111) is defined in27 d½Xsurf =dt ¼ kðθ=As Þ½Xg which has units of a flux, molecules cm-2 s-1; here θ represents the fraction of available surface sites, As the surface area, and [X]g the gaseous concentration of CO2 in molecules/cm3. 3.4. Vibrational Spectra Calculations. The vibrational wavenumbers of Fe(111)/CO2 were determined experimentally by Hess et al.5 in their measured HREEL spectra at 100-370 K. Three major signals at 2339, 1371, and 645 cm-1 (designated for R-state CO2) were detected at T = 100 K. The lines at 1597, 1169, and 766 cm-1 were assigned to a bent β-state CO2 after heating to 150 K. Upon annealing to 210 K, only γ-state CO2, with 1371, 1073, and 766 cm-1, remained. Our calculated (unscaled) wavenumbers for Fe(111)/CO2 are given in Table 2, in which we estimated vasym(CO), vsym(CO), and vbent(CO) of the most stable R-state CO2 (T-η1-O) to be 2368, 1323, and 631 cm-1, respectively, in agreement with experimental values 2339, 1371, and 645 cm-1. The calculated wavenumbers for the β-state (T-η2-C,O) are 1798, 1106, and 641 cm-1, respectively, which differ slightly from experimental values 1597, 1169, and 766 cm-1. For the most stable γ-state (S-μ3-C,O,O0 ), the calculated wavenumbers are 1375, 1119, and 723 cm-1, also consistent with experimental values 1371, 1073, and 766 cm-1. The large red shifts of the β and γ states (relative to the wavenumbers of gaseous CO2)28 reflect the weakening of the C-O bond, indicating that these two states are likely precursors for the dissociation. The predicted vibrational wavenumber v(CO) = 1775 cm-1 (see Table 3) of the most stable structure, FeCO(S-η1-C)-a, of adsorbed CO also agrees with experimental datum 1815 cm-1, and the computed v(Fe-C) (27) Rettner, C. T.; Ashfold, M. N. R. Dynamics of Gas-Surface Interaction; Springer-Verlag: Berlin Germany, 1991; Chapter 5. (28) Shimanouchi, T. Tables of Molecular Vibrational Frequencies; Consolidated Volume 1, NSRDS NBS-39; National Bureau of Standards: Washington, DC, 1972.
780 DOI: 10.1021/la9021646
and v(Fe-O) wavenumbers are within 535-571 and 490544 cm-1, respectively (see Table 4), also within the range 242-524 cm-1 of experimental values. 3.5. Electronic Structures of Intermediates and Products in CO2 Decomposition. Figure 6 shows plots of the contour surface of the electron-density difference, ΔFdiff = F[surface þ adsorbate] - F[surface] - F[adsorbate], for each adsorbate/ substrate system in the CO2 decomposition path: (a) before interaction, (b) LM1, (c) LM2, and (d) P, respectively (see the inset in Figure 6 for atom labeling). We recognize from these drawings whether the interaction reflected in this polarization is chiefly physical or whether it contains significant chemical contributions. Through charge transfer from the surface Fe atoms (first and second layers) to CO2 (π*u states; see the change in Figure 6b), the two C-O bonds weaken. Relative to the isolated reactant CO2 at 1.177 A˚, LM1 is elongated by 9.9% (see Figure 2). The effective charges calculated with Bader’s program29 also demonstrate clearly a transfer of charge between the adsorbates and the substrate; the charge becomes more negative as the adsorption or dissociation proceeds. Initially, only 0.11 electron of LM1 is transferred from Fe(111), but this value increases as adsorbed CO2 in LM1 dissociates (see Figure 6b,c). After the atomic carbon and oxygen species become adsorbed at their most stable sites (Figure 6d), the effective charge becomes more negative (from -1.22 to -1.24 |e|) through the increased interaction with Fe(111). In addition, to be more specific, we plotted the electronic local density of states (LDOS) of the system projected on the orbitals for the adsorbed constructs of carbon and oxygen species and the Fe(111) substrate (Figure 7). Figure 7a shows the LDOS before the CO2-Fe(111) interaction; Figures 7b to 7d, corresponding to the LDOS of LM1, LM2, and P configurations, respectively, clearly show stronger interaction (additional states emerge abruptly in the range -5.0 to -10.0 eV) between the C (or O) atom and the Fe(111) surface. As these interactions proceed (from Figures 7a to 7d), the electronic characteristics of the (29) (a) Bader, R. F. W.; Beddall, P. M. J. Chem. Phys. 1972, 56, 3320. (b) Bader, R. F. W. Atoms in Molecules-A Quantum Theory; Oxford University Press: Oxford, UK, 1990.
Langmuir 2010, 26(2), 775–781
Chen et al.
Fe(111) surface (blue line) at -0.35, -2.58, and -4.34 eV gradually decrease through distinct charge transfer from the Fe substrate to the adsorbed CO, O, and C species. As displayed in Figure 7d, pronounced broadening occurs at the final dissociation stage of an adsorbed CO species into the sites of the Fe substrate.
4. Summary Our calculations with spin-polarized density functional theory indicate that the Fe(111) surface exhibits a large catalytic activity to decompose CO2. Our data show that isomers FeCO2(S-μ3-C, O,O0 ), FeCO(S-η1-C), and FeX(T,S-μ2-X) or FeX(B-μ3-X), for X = C and O atoms, are energetically favored among all calculated structures of Fe(111)/CO2, Fe(111)/CO, and Fe(111)/X, respectively. The catalytic process is likely to proceed via a three-step mechanism. The overall reaction requires little thermal activation as all calculated transition structures have less energy than the reactants CO2(g) þ Fe(111). The rate coefficients for the dissociative adsorption of CO2 have been predicted with varia-
Langmuir 2010, 26(2), 775–781
Article
tional RRKM theory, and the interaction between adsorbate and substrate was subjected to a detailed electronic analysis. This information about the reaction mechanism, the catalytic activity of various surface sites, and the relevance of the surface structure would be laborious to obtain by experimental measurements, indicating that periodic DFT calculations might play a vital role in the rational design of improved catalytic surfaces for the dissociation of CO2. Acknowledgment. We are grateful to (1) National Science Council, Republic of China, under Grant Numbers NSC 98-2113M-034-002-MY2 and NSC 97-2113-M-492-001-MY2 for the financial support, (2) the financial support by Chinese Culture University, and (3) National Center for High-performance Computing, Taiwan, for the use of computer time. In addition, we are deeply indebted to Professor M. C. Lin (from NCTU in Taiwan and Emory University in the United States) for his persistent encouragement and instruction.
DOI: 10.1021/la9021646
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