Density Functional Calculations on the Hydrogenation of Carbon

Dec 7, 2009 - Han-Jung Li and Jia-Jen Ho*. Department of Chemistry, National Taiwan Normal University, 88, Section 4, Tingchow Road, Taipei, Taiwan 11...
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J. Phys. Chem. C 2010, 114, 1194–1200

Density Functional Calculations on the Hydrogenation of Carbon Dioxide on Fe(111) and W(111) Surfaces Han-Jung Li and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal UniVersity, 88, Section 4, Tingchow Road, Taipei, Taiwan 116 ReceiVed: October 1, 2009; ReVised Manuscript ReceiVed: NoVember 17, 2009

With quantum-chemical calculations, we investigated the hydrogenation of a CO2 molecule on Fe(111) and W(111) surfaces using the density functional theory (DFT) with the projector-augmented wave (PAW) approach in periodic boundary condition. The structures and geometric parameters of the hydrogenation products, and the potential-energy surfaces, were calculated. It was shown that similar reaction paths for the hydrogenation of CO2 on Fe(111) and W(111) surfaces were found but with disparate energies. The rate-controlling energy barriers from M-CO2 (M ) Fe, W) plus H atom to form formate (HCOO) and carboxyl (COOH) on a Fe(111) surface are 0.37 and 1.69 eV, respectively, but 0.54 and 2.79 eV, respectively, on a W(111) surface. The most probable path for the hydrogenation of a CO2 molecule on either the Fe(111) or W(111) surface is the formation of a formate-vertical structure. To understand the interaction between adsorbates and surfaces, we calculated the Bader charges and analyzed the local densities of states. 1. Introduction The recovery of carbon dioxide for subsequent catalytic transformation to hydrocarbons is an important process to synthesize organic chemicals such as methanol, a promising energy carrier used in fuel cells or internal-combustion engines.1–3 As carbon dioxide is well-known to be a major greenhouse gas that increases global warming,4,5 the recycling technology for the released carbon dioxide becomes important; one process for this purpose is catalytic hydrogenation of carbon dioxide on metallic surfaces. Although much research has been focused on the interaction of CO2 with surfaces of transition metals (Pt, Pd, Cu, and Ag) both experimentally and theoretically,6–22 the hydrogenation of CO2 on Cu(111) was investigated theoretically as a possible step in connection with the water-gas shift reaction.23 The linear CO2 molecule was found to bind to Cu(111) and Pt(111)24 surfaces with energies of only -0.09 and -0.11 eV, respectively, indicating that CO2 maintains an almost linear conformation on these surfaces. Nevertheless, Hess et al. reported that the adsorption conformation of CO2 on a Fe(111) surface existed in one linear and two angular forms,25 with the latter having much larger binding energies. In our previous work,26 we also found a larger CO2 adsorption energy of -1.11 eV on the Fe(111) surface with an angular form. In addition, Chen et al.27 also investigated the CO2 adsorption and decomposition on the W(111) surface with DFT and found quite a large CO2 adsorption energy of -1.63 eV (-37.6 kcal/mol) on the surface. Therefore, the CO2 molecule adsorbed on both Fe(111) and W(111) metal surfaces might have a larger binding energy with respect to other metals. CO2 hydrogenation on a pure metal or a metal oxide has been investigated experimentally or theoretically,28–35 but no theoretical work regarding the mechanism of CO2 hydrogenation on Fe(111) and W(111) surfaces is available. Here, we present the mechanism of CO2 hydrogenation on both Fe(111) and W(111) surfaces using calculations with density functional theory and * Corresponding author. E-mail: [email protected]. Phone: (886)-229309085. Fax: (886)-2-29324249.

address the geometric structures of the adsorbed intermediates and the potential-energy surfaces for CO2 hydrogenation. 2. Computational Methods All present calculations were performed with the DFT planewave method utilizing the Vienna simulation package (VASP).36–39 In these calculations, we used the projector-augmentedwave method (PAW)40,41 in conjunction with the revised Perdew-Burke-Ernzerhof (rPBE)42,43 density functionals. The Brillouin zone is sampled with the Monkhorst-Pack grid.44 The calculations were performed using (4 × 4 × 4) and (4 × 4 × 1) Monkhorst-Pack mesh k-points for bulk and surface calculations, respectively, and a truncation energy of 400 eV. All calculations for the Fe(111) and W(111) surfaces were performed using the spin-polarization ferromagnetic and spinrestricted diamagnetic methods, respectively.45,27 Side- and topview models of the metal surface appear in parts a and b of Figure 1, respectively. We carefully evaluated the model used in the present work and demonstrated that the adsorption energies of CO2 on p(2 × 2) and p(3 × 3) lateral cells of Fe(111) and W(111) surfaces are consistent: the difference in the calculated adsorption energies is negligible (smaller than ca. 0.5 kcal/mol). In the present work, we therefore used only the computationally less expensive p(2 × 2) model of the Fe(111) and W(111) surfaces. The p(2 × 2) lateral cell of these two surfaces is modeled as periodically repeated slabs with six layers, shown in Figure 1a. The bottom three atomic layers are kept frozen and set to the estimated bulk parameters, whereas the remaining layers are fully relaxed during the calculations. We calculated adsorption energies according to the following equation:

∆Eads ) E[adsorbate + surface] - (E[adsorbate] + E[surface]) in which E[adsorbate + surface], E[adsorbate], and E[surface] are the calculated electronic energies of adsorbed species on the surface, a gas-phase molecule, and a clean surface,

10.1021/jp909428r  2010 American Chemical Society Published on Web 12/07/2009

Hydrogenation of CO2 on Fe(111) and W(111) Surfaces

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Figure 1. Schematic presentation of a Fe(111) surface: (a) side view; (b) top view. The T, SB, and DB represent top, shallow bridge, and deep bridge, respectively.

TABLE 1: Calculated Adsorption Energies (Eads) and Geometrical Parameters of Adsorbed H and CO2 and Coadsorbed H Plus CO2 Species on Fe(111) and W(111) Surfaces (in Parentheses) Fe(111) (W(111)) species

site

Eads/eV

dFe(W)-H/Å

H H H CO2 H + CO2

T SB DB µ3-C,O,O SB + µ3-C,O,O

-2.18 (-2.41) -2.78 (-2.87) -2.73 -1.11 (-1.65) -3.45 (-4.56)

1.59 (1.78) 1.73 (1.92)a 1.82a

a

1.72 (1.91)a

dFe(W)-C/Å

dFe(W)-O1/Å

dFe(W)-O2/Å

dC-O1/Å

dC-O2/Å

1.98 (2.16) 2.00 (2.17)

1.98 (2.07) 1.99 (2.06)

1.98 (2.07) 1.99 (2.07)

1.29 (1.32) 1.29 (1.32)

1.29 (1.32) 1.29 (1.33)

The values are average bond lengths.

respectively. Vibrational wavenumbers of adsorbed structures were analyzed by diagonalizing the Hessian matrix of selected atoms within the VASP approach. The nudged-elastic-band (NEB) method46–48 was applied to locate transition structures, and paths of minimum energy (MEPs) were constructed accordingly. 3. Results and Discussion 3.1. Adsorption of H, CO2, HCOO, and COOH on Fe(111) and W(111) Surfaces. To locate the H atom on both Fe(111) and W(111) surfaces, we placed a H atom at various sites on these surfaces. The three adsorption sites on the surfaces considered are described as a top site (T), which means atop the first-layer metal atom, a shallow bridge site (SB), bridging between the first-layer and the second-layer metal atoms, and a deep bridge site (DB), between the second- and third-layer metal atoms of the surface, shown in Figure 1b. The adsorption energies of hydrogen atom adsorbed on various sites with the corresponding geometrical parameters are listed in Table 1. On the Fe(111) surface, among three adsorption sites T, SB, and DB, with the adsorption energies -2.18, -2.78, and -2.73 eV, respectively, the shallow bridge site is preferable with an average Fe-H bond length of 1.73 Å. Nevertheless, on the W(111) surface, we found only two stable adsorption sites, T and SB, with adsorption energies of -2.41 and -2.87 eV, and W-H bond lengths of 1.78 and 1.92 Å, respectively, also preferable on the SB site. The adsorption energies of these two respective sites are both larger on the W(111) surface than on the Fe(111) counterparts. The carbon dioxide molecule adsorbed on a Fe(111) surface with µ3-C,O,O structure is energetically favored:26 the carbon

atom is on top of the second-layer Fe atom, and the two oxygen atoms are on top of two separate Fe atoms of the first layer (Figure 2a). The calculated adsorption energy of CO2 (µ3-C,O,O) on the Fe(111) surface is -1.11 eV, and the bond lengths of Fe-C, Fe-O1, Fe-O2, and C-O are 1.98, 1.98, 1.98, and 1.29 Å, respectively. A similar adsorbed configuration of carbon dioxide was found on the W(111) surface with an adsorption energy of -1.65 eV, larger than that on the Fe(111) counterpart by 0.54 eV, and the bond lengths of W-C, W-O1, W-O2, and C-O were 2.16, 2.07, 2.07, and 1.32 Å, respectively. Chen et al.26 also calculated that the adsorption energy of carbon dioxide on the W(111) surface was -1.63 eV (-37.6 kcal/mol), in agreement with our result. The coadsorption energy of both a hydrogen atom and carbon dioxide, located on a SB site and a µ3-C,O,O site, respectively, is -3.45 eV on the Fe(111) surface. The calculated bond lengths of Fe-H, Fe-C, Fe-O1, Fe-O2, and C-O are 1.72, 2.00, 1.99, 1.99, and 1.29 Å, respectively, whereas the coadsorption energy on the W(111) counterpart is -4.56 eV, much larger than that on the Fe(111) surface, because of the larger adsorption energies of both the hydrogen atom and carbon dioxide adsorbed on the W(111) surface. The calculated adsorption energies and bond lengths of formate (HCOO) and carboxyl (COOH) radicals adsorbed on both Fe(111) and W(111) surfaces are listed in Table 2; the corresponding structures of Fe(111) and W(111) are depicted schematically in Figure 2 and Figure S1 of the Supporting Information, respectively. Since the corresponding adsorbed structures (formate and carboxyl) on both surfaces are so similar, we also denote the respective parameters of W(111) counterparts in parentheses in Figure 2 just for convenient comparison.

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Figure 2. Located isomers of adsorbed CO2, HCOO, and COOH on metallic surfaces (M ) Fe and W) and their important geometric parameters calculated at the rPBE level of theory (the data shown are for the structures on Fe(111), while those on W(111) counterparts are in parentheses; the detailed drawings can also be found in Figure S1 of the Supporting Information). The bond lengths are given in Å. The C, O, and H atoms are represented by black, red, and white balls, respectively.

TABLE 2: Calculated Adsorption Energies (Eads) and Geometric Parameters of HCOO and COOH Species Adsorbed on Fe(111) and W(111) Surfaces (in Parentheses) Fe(111) (W(111))

a

species

Eads/eV

dFe(W)-O/Å

HCOO (unidentate) HCOO (vertical) HCOO (horizontal) cis-COOH trans-COOH

-2.75 (-3.51) -3.71 (-4.22) -3.28 (-4.03) -2.98 (-3.03) -2.79 (-2.76)

1.88 (1.95) 2.00 (2.12)a 1.98 (2.00)a 2.04 2.14)b 2.02 (2.09)b

dFe(W)-C/Å

dC-O1/Å

dC-O2/Å

2.79 (2.34) 1.92 (2.12) 1.93 (2.19)

1.22 (1.21) 1.27 (1.28) 1.28 (1.37) 1.44 (1.43) 1.39 (1.37)

1.32 (1.36) 1.27 (1.28) 1.29 (1.37) 1.28 (1.30) 1.27 (1.30)

The values are average bond lengths. b The values are the smallest bond lengths.

Formate (HCOO) shows three stable structures on a Fe(111) surface, namely, unidentate (Figure 2b), bidentate vertical (Figure 2c), and horizontal (Figure 2d). The unidentate formate has an adsorption energy of -2.75 eV, via its oxygen atom on the top site of an Fe atom of the first layer, with a Fe-O bond of length of 1.88 Å, and then its C-O bond elongates to 1.32 Å. However, the vertical formate (stands vertically to the Fe surface) is significantly more stable with an adsorption energy of -3.71 eV, with the two oxygen atoms attached on top of two Fe atoms of the first layer, respectively. Both Fe-O bond lengths are 2.00 Å, whereas the two C-O bonds of HCOO, due to its interaction of O atom with the surface Fe, become elongated (1.27 Å) as compared to the free formate (1.18 Å). In contrast, the horizontal analogue has an adsorption energy of -3.28 eV, with the Fe-O, Fe-C, C-O1, and C-O2 bonds of lengths 1.98, 2.79, 1.28, and 1.29 Å, respectively. The vertical form is clearly favored. Similar adsorption structures of formate were found on the W(111) surface, shown in Figure S1 of the Supporting Information, but with much larger adsorption energies of -3.51, -4.22, and -4.03 eV for the unidentate, vertical, and horizontal forms, respectively.

The adsorbed carboxyl radical (COOH) has two stable structures on the Fe(111) and W(111) surfaces: (1) cis-COOH, with the O-H bond pointing away from the surface, and (2) trans-COOH, with the O-H bond pointing toward the surface. The Fe-cis-COOH is adsorbed via its carbon atom on the top site of the second-layer Fe atom, with an adsorption energy of -2.98 eV and Fe-C, C-O1, and C-O2 bonds of lengths 1.92, 1.44, and 1.28 Å, respectively, whereas the Fe-trans-COOH has a smaller adsorption energy of -2.79 eV and smaller C-O bond lengths, with Fe-C, C-O1, and C-O2 of 1.93, 1.39, and 1.27 Å, respectively, indicating a stronger interaction between the Fe surface with the cis-COOH. Similar results were found on the W(111) counterparts, shown in Table 2 and Figure S1 of the Supporting Information. Calculations by Gokhale et al.23 show that the adsorption energies of H atom, CO2 molecule, HCOO(unidentate), HCOO (vertical), cis-COOH, and trans-COOH species on the Cu(111) surface are -2.55, -0.09, -2.32, -2.77, -1.68, and -1.88 eV, respectively. All adsorption energies of these species on the Cu(111) surface are smaller than those on the Fe(111) and W(111) surfaces. Density functional calculations by Grabow

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Figure 3. Potential-energy diagram for the hydrogenation of CO2 on metallic surfaces (M ) Fe and W). The energy data shown are for the Fe(111) surface, while those on W(111) counterparts are in parentheses; the completed diagram can also be found in Figure S2 of the Supporting Information.

et al.24 show adsorption energies of the H atom, CO2 molecule, HCOO (vertical), cis-COOH, and trans-COOH species on the Pt(111) surface to be -2.71, -0.11, -2.31, -2.28, and -2.44 eV, respectively. Although most of these species are more stable than on the Cu(111) surface (except HCOO(vertical)), they are still smaller, respectively, than those on a Fe(111) or W(111) surface. The reason is the closely packed first-layer metallic atoms on Cu(111) and Pt(111) surfaces leading to weak linear CO2 physisorption on these surfaces. On the contrary, the metallic atoms of Fe(111) and W(111) surfaces pile loosely, causing the adsorbed CO2 molecule to bind on these surfaces via its C atom on top of the second-layer metallic atom and the two oxygen atoms on top of two separate metallic atoms of the first layer, which produces a chemically adsorbed angular CO2 on these surfaces, so that energies for CO2 adsorption are much larger on these surfaces. 3.2. Mechanism of CO2 Hydrogenation on Fe(111) and W(111) Surfaces. The potential-energy surfaces of CO2 hydrogenation on Fe(111) are shown in Figure 3. The formate (HCOO) formation paths are drawn in the top panel and that of carboxyl (COOH) on the bottom. There are two possible reaction paths of formate formation on the Fe(111) surface: through the Fe-HCOO-unidentate and the Fe-HCOO-horizontal. The H atom and CO2 molecule first become coadsorbed on the Fe(111) surface with a coadsorption energy of -3.45 eV (Fe-IM1). The H atom then diffuses to a site near CO2 to form another intermediate, Fe-IM2, with an energy rising 0.35 eV. The next step of hydrogenation (rate-determining step) of this coadsorbate

might proceed through an association of the H atom and the C atom of CO2 on crossing the transition structure (Fe-TS1) with a reaction barrier of 0.57 eV, forming Fe-HCOO-unidentate (FeIM3) and endothermic by 0.41 eV. Successively, through a Fe-O bond rotation of a slight barrier of 0.02 eV (Fe-TS3), the Fe-IM3 forms the most stable adsorbed configuration of formate, namely, Fe-HCOO-vertical (Fe-IM5) and exothermic by 0.98 eV. An alternative is via a formation of Fe-HCOOhorizontal (Fe-IM4) on crossing the transition structure (FeTS2) with a barrier of 0.37 eV and exothermic by 0.13 eV. Then, Fe-IM4 immediately forms Fe-HCOO-vertical (Fe-IM5) with no barrier but continuously releasing heat by 0.42 eV. The latter pathway is kinetically preferable. For COOH formation on the Fe(111) surface, the H atom first diffuses from Fe-IM1 to Fe-IM6 with the energy rising 0.06 eV. From the coadsorbed configuration, to produce the carboxyl molecule (COOH), it is more likely first to form transCOOH, and then the cis-counterpart. Therefore, Fe-trans-COOH (Fe-IM7) might be formed on attaching the H atom to the nearer O atom of a CO2 molecule, which is endothermic by 0.68 eV and with a huge barrier, 1.69 eV (Fe-TS4). The Fe-trans-COOH might be trapped in the well or it might undergo a structural transformation to form the more stable Fe-cis-COOH (Fe-IM8) if it gains enough energy to overcome the barrier of 0.4 eV (Fe-TS5) and exothermic by 0.01 eV. Comparing the above hydrogenation paths, we found that the formation of Fe-HCOOvertical is both thermodynamically and kinetically favored. The formate (HCOO) formation has consequently the smallest energy

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TABLE 3: Bader Charge Analysis of Intermediates of Hydrogenation of Carbon Dioxide on Both Fe(111) and W(111) Surfaces (in Parentheses) Fe(111) (W(111))/e species

qC

qO

H (SB) CO2 H (SB) + CO2 HCOO (unidentate) HCOO (vertical) HCOO (horizontal) cis-COOH trans-COOH

2.47 (1.90) 2.49 (1.87) 2.90 (2.71) 2.81 (2.70) 2.69 (1.90) 1.71 (1.49) 1.80 (1.69)

-3.58 (-3.50) -3.57 (-3.48) -3.57 (-3.46) -3.60 (-3.66) -3.51 (-3.42) -3.57 (-3.62) -3.56 (-3.61)

barrier and is the preferable path during hydrogenation of CO2 on a Fe(111) surface. We calculated the potential-energy surfaces for CO2 hydrogenation on a W(111) surface with its completed diagram shown in Figure S2 of the Supporting Information, and the respective data are also shown in parentheses in Figure 3 just for comparison with those on the Fe(111) counterparts. Formate (HCOO) can form on a W(111) surface in two ways: through the W-HCOO-unidentate (W-IM3) and via the W-HCOOhorizontal (W-IM4). The H atom and CO2 molecule coadsorb on the W(111) surface on the most stable site with an adsorption energy of -4.56 eV (W-IM1), much larger than that for the Fe(111) analogue. To react with a CO2 molecule, the H atom must diffuse from W-IM1 to form W-IM2 with an increased energy of 0.25 eV. To form a W-HCOO-unidentate (W-IM3), it must cross a reaction barrier of 1.34 eV extent (W-TS1). It might transfer to a more stable structure, W-HCOO-vertical (WIM5), on climbing a small barrier of 0.03 eV (W-TS3) and exothermic by 0.70 eV. Alternatively, W-IM2 might form W-HCOO-horizontal (W-IM4) on passing another transition structure (W-TS2, barrier 0.54 eV) and exothermic by 0.33 eV. W-HCOO-horizontal transfers to W-HCOO-vertical (W-IM5) without barrier and exothermic by 0.18 eV. To form COOH on a W(111) surface, the H atom first diffuses from W-IM1 to form W-IM6, endothermic by 0.12 eV. Then, the H atom associates with the nearest O atom of the CO2 molecule, forming W-trans-COOH (W-IM7) with a much larger barrier, 2.79 eV (W-TS4) (with respect to the Fe(111) counterpart), and endothermic by 1.38 eV; it then undergoes a structural transformation to the more stable W-cis-COOH (WIM8) on crossing a barrier of 0.53 eV and exothermic by 0.18 eV (W-TS5). Like the Fe(111) surface, among all paths, the one leading to the formation of W-HCOO-vertical via WHCOO-horizontal is kinetically preferable. Comparing the hydrogenation of CO2 on the Fe(111) and W(111) surfaces, we found that all respective intermediates on the W(111) surface are more stable than those on the Fe(111) counterparts, but all respective reaction barriers on the W(111) surface are larger than those on the Fe(111) analogues. This is due to the much stronger interactions between adsorbates (H atom) and the W(111) surface (larger H adsorption energy), thus causing H and CO2 adsorbates to be more difficult to recombine, relative to Fe(111) analogues. Vesselli et al. investigated experimentally and theoretically the hydrogenation of CO2 on a Ni(110) surface,49 indicating that it is easier to form HCOO than COOH species, similar to our calculated results on the Fe(111) and W(111) surfaces. The measured barrier of formation of HCOO on the Ni(111) surface is 0.43 eV, slightly smaller than our calculated values both on the Fe(111) surface (0.46 eV) and on the W(111) surface (0.54 eV). DFT calculations indicated also that the reaction barriers

qH

qSum

-0.42 (-0.60)

-0.42 (-0.60) -1.10 (-1.60) -1.47 (-2.18) -0.70 (-0.80) -0.75 (-0.84) -0.84 (-1.57) -0.85 (-1.14) -0.76 (-0.93)

-0.40 (-0.57) -0.02 (-0.06) 0.03 (0.12) -0.02 (-0.05) 1.00 (1.00) 1.00 (1.00)

of HCOO formation are 1.02 and 1.39 eV on the Cu(111) and Pt(111) surfaces, respectively.18,19 In our previous work,26 it showed that without the presence of a H atom the barriers for the stepwise CO2 dissociation reaction on the Fe(111) surface were 0.94 eV (21.73 kcal/mol) for OC-O bond activation and 1.04 eV (23.87 kcal/mol) for further C-O bond scission. In this work, our calculations show that with the presence of a H atom on the surface the dissociation of CO2, however, would be unfavored and be replaced by the hydrogenation of CO2, forming HCOO (0.37 eV). 3.3. Charge and Electronic Structure Analysis. In Table 3, we present the results of a Bader-charge analysis50–54 of adsorbates on both the Fe(111) and W(111) surfaces. On the Fe(111) surface, the coadsorbate of the H (SB) atom withdraws electrons from the surface and becomes negatively charged (-0.40), whereas C and O atoms of CO2 are 2.49 and -3.57, respectively. From the attraction of the negatively charged H atom and positively charged C atom, formation of formate (HCOO) is preferable. In contrast, the small repulsion between the negatively charged O and H atoms makes carboxyl (COOH) formation unlikely, consistent with our calculated results that HCOO formation has a smaller reaction barrier than COOH. A similar Bader charge distribution is found on the W(111) surface; the H atom becomes negatively charged (-0.57), and C and O atoms of CO2 are 1.87 and -3.48, respectively, which favors formate (HCOO) formation over COOH on the W(111) surface. The sum of charges of adsorbates or coadsorbates on a W(111) surface is more negative than that on the Fe(111) surface, which means that more charge is transferred from the W(111) surface than from Fe(111), resulting in larger adsorption energies of these adsorbates on W(111) than on Fe(111) surfaces, respectively. Analysis of the detailed local densities of states (LDOS) of Fe-HCOO-vertical and Fe-cis-COOH adsorbed on the Fe(111) surface yields the results shown in Figure 4a and b, respectively, and those of W-HCOO-vertical and W-cis-COOH adsorbed on W(111) in Figure 5a and b. In Figure 4, the overlap of the density of states between the HCOO-vertical molecule and Fe atoms of the top three layers of the Fe(111) surface is larger than that of the cis-COOH counterpart, indicating a stronger interaction between HCOO-vertical and the Fe(111) surface that results in a larger adsorption energy of HCOO-vertical on the Fe(111) surface with respect to the cis-COOH analogue, in agreement with our calculated results. Similar LDOS structures are found on the W(111) surface, also indicating a stronger interaction and a larger adsorption energy between HCOOvertical and the W(111) surface than for the cis-COOH counterparts. Despite the similar overlap of LDOS structures of the HCOO-vertical molecule on Fe(111) and on W(111), the maxima of overlap LDOS of W(111) are located at a region of smaller energy (from -4 to -6 eV) than that of Fe(111) (from -3 to -5 eV), shown in the left inset of Figures 4a and 5a,

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Figure 4. Local density of states (LDOS) for Fe-HCOO-vertical (a) and Fe-cis-COOH (b) molecule adsorbed on a Fe(111) surface. The dashed, red, blue, and green lines represent Fe atoms of the top three layers, Fe-HCOO-vertical (Fe-cis-COOH), O, and C, respectively.

Figure 5. Local density of states (LDOS) for W-HCOO-vertical (a) and W-cis-COOH (b) molecule adsorbed on a W(111) surface. The dashed red, blue, and green lines represent Fe atoms of the top three layers, W-HCOO-vertical (W-cis-COOH), O, and C, respectively.

implying a stronger interaction of adsorbate on the W(111) surface, in agreement with our calculated results of a larger adsorption energy of HCOO-vertical on the W(111) surface than on the Fe(111) surface. Analogously, comparing the overlap region of the left inset in Figures 4b and 5b, we see a larger adsorption energy of cis-COOH on W(111) than on Fe(111), in agreement with our calculated results.

4. Conclusion The most probable path for the hydrogenation of a CO2 molecule on either the Fe(111) or the W(111) surface is the formation of a formate-vertical structure. The least reaction barrier is 0.37 eV on the Fe(111) surface on passing through a Fe-TS2 transition structure but 0.54 eV on the W(111) surface on climbing over the W-TS2 structure. The formation of

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carboxyl (cis-COOH) on these two surfaces is also possible, as all transition structures in the path have energies less than those of the reactants (reference point); nevertheless, these reaction barriers are much greater than the counterparts for formate formation. They are 1.69 and 2.79 eV on Fe(111) and W(111), respectively. The trans-COOH is likely to form first with a greater potential energy; then, it might convert to the more stable cis-COOH on crossing a barrier (ca. 0.5 eV) and releasing a little heat (ca. 0.01-0.18 eV). From a thermodynamic point of view, the cis-form would predominate eventually. Acknowledgment. National Science Council of Republic of China (NSC 96-2113-M-003-007-MY3) supported this work; National Center for High-performance Computing provided computer time. Supporting Information Available: Information on the located isomers of adsorbed CO2, HCOO, and COOH with their important geometric parameters and potential-energy diagram for the hydrogenation of CO2 calculated at the rPBE level on a W(111) surface. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (2) (3) (4)

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