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J. Phys. Chem. C 2007, 111, 16934-16940
Factors Controlling the Interaction of CO2 with Transition Metal Surfaces Sheng-Guang Wang,† Xiao-Yuan Liao,† Dong-Bo Cao,† 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, Shanxi 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: June 13, 2007; In Final Form: August 28, 2007
On the basis of density functional theory calculations, the chemisorption of CO2 on the transition metal surfaces was investigated to find out the key factors controlling its adsorption strength and activation degree. The interaction mechanism of CO2 with the metal surfaces was discussed by analyzing the density of states. The adsorption strength of CO2 is controlled by the d-band center of the metal surfaces and also affected by the charge transfer from the metal surfaces to the chemisorbed CO2. The degree of CdO bond activation depends on the transferred charge. Therefore, both d-band center of the metal surfaces and the charge transfer should control the chemisorption of CO2.
Introduction CO2 has been considered as one of the molecules that might enhance the green house effect and contribute to global warming. There are two counter options available: either reduce the CO2 emission into the atmosphere or transfer CO2 into useful chemicals through catalytic control.1 Although CO2 is both very abundant and a cheap C1 feedstock, at present it is only used in few industrially relevant technical processes. The main reason for the limited use of CO2 as a chemical reactant is its low-energy content. However, when reactive hydrogen is involved, the thermodynamics become much more favorable.2 Therefore, the thermodynamic argument alone is not sufficient to explain the limited use of CO2; the reasons are more likely to be kinetic in origin.1 One possibility to overcome the kinetic barrier is to activate CO2 by a catalyst. Supported transition metals were used as important catalysts for C1 industrial chemistry. The interaction between CO2 and transition metal surfaces has been experimentally studied extensively.3-47 Two adsorption states were found on transition metal surfaces: the physisorbed linear CO2 and the chemisorbed bent CO2δ-. Physisorption was found in most cases when the temperature approaches 80 K, whereas the chemisorbed species were only observed under very specific conditions; that is, the surfaces usually are either atomically rough or alkali metal modified. The observation of CO2δ- in certain cases, of course, does not exclude its intermediate formation in other cases, where only reaction products, i.e., the dissociation products (CO + O) and/or carbonate formation, were observed.1 Despite many experimental studies, it is still not clear about the factors controlling CO2 adsorption and the subsequent reaction pathways on transition metal surfaces. Although theoretical calculations are a useful tool, very few theoretical investigations were focused on CO2/metal system.48-52 On the basis of previous theoretical calculations,49 the transfer of * To whom correspondence should be addressed. E-mail: haijun.jiao@ catalysis.de. † Chinese Academy of Sciences. ‡ Universita ¨ t Rostock.
electrons into the antibonding orbital of the chemisorbed CO2δinduces the activation of its CdO bonds. In the present paper, we perform density functional theory (DFT) calculations on the chemisorption of CO2 on the VIIIA and IB transition metal surfaces (Figure 1), which were usually used as catalysts in C1 chemistry. The studies on Fe, Co, Ni, and Cu are interesting to industrial processes because of their low price, whereas those on the relatively expensive Rh, Pd, Ag, Pt, and Au have also attracted theoretical interests. Our goal is to find out the key factors controlling the chemisorption and activation of CO2 on these transition metal surfaces. Methods and Models The chemisorption of CO2 on transition metal surfaces was computed at the DFT level by using the Cambridge Sequential Total Energy Package (CASTEP).53 The exchange correlation energy was described by the Perdew-Burke-Ernzerhof functional within the generalized gradient approximation (GGAPBE54). Ionic cores were described by ultrasoft pseudopotential,55 and the Kohn-Sham one-electron states were expanded in a plane wave basis set up to 300 eV. A Fermi smearing of 0.1 eV was utilized. Brillouin zone integration was approximated by a sum over special k-points (4 × 4 ×1) chosen using the Monhorst-Pack scheme.56 Spin polarization was also used to calculate the energies and structural parameters of all models. Without counting the adsorbates, the vacuum between the slabs was set to span the range of 10 Å for the slabs without significant interaction. The convergence criteria for structure optimization and energy calculation were 1.0 × 10-6 eV/atom for SCF, 2.0 × 10-5 eV/atom for energy, 0.05 eV/Å for maximum force, and 2.0 × 10-3 Å for maximum displacement. The 5-layer models were employed as shown in Figure 2. In our calculations, the metal atoms in the bottom three layers were fixed in their bulk positions, whereas those in the top two layers were allowed to relax. As shown in Figure 2, the p(2 × 2) unit cells were used, which were relative to the coverage of 0.25 monolayer. In our previous paper,49 it was found that there was no apparent lateral interaction between the molecules at this coverage. In this paper, we analyze the interaction mech-
10.1021/jp074570y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
Interaction of CO2 with Transition Metal Surfaces
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16935
Figure 1. Metals computed in the present paper.
Figure 3. Binding modes of CO2 on the Fe(110) surface.
Figure 2. Surface models.
anism of CO2 with the metal surface without considering the lateral interaction between the adsorbates. Therefore, we did not calculate the adsorption at the higher coverage. Fe has the body-centered cubic crystal structure, and we chose the most close-packed Fe(110) surface as the sample, as shown in Figure 2, panels a and b. Other metals computed in the present paper have the face-centered cubic crystal structures; and the most close-packed (111) surfaces were chosen, as shown by Figure 2, panels c and d. The binding energy of the chemisorbed COδ2 was defined as ∆Eb ) E(CO2/slab) - [E(CO2) + E(slab)], where E(CO2/ slab) is the total energy of the slab with the chemisorbed COδ2 on the surface, E(CO2) is the total energy of free CO2, and E(slab) is the total energy of the bare slab of the surface. Therefore, a negative ∆Eb value means an exothermic chemisorption. The Mulliken atomic charges were used for the discussion of the effects of charge transfer. Results and Discussion (a) CO2/Fe(110). The structures of the chemisorbed CO2 on the Fe(110) surface were shown in Figure 3, and the computed binding energies and the structure parameters were listed in Table 1. Five stable structures were found with the binding modes of B-C2O2-Fe (Figure 3a), H-C3O2-Fe (Figure 3b), H-C3O3-Fe-a (Figure 3c), H-C2O3-Fe (Figure 3d), and H-C3O3Fe-b (Figure 3e), respectively.
Figure 4. Binding modes of CO2 on the other metal surfaces.
As shown in Figure 3a, the chemisorbed B-C2O2-Fe adsorbs on the bridge site of the Fe(110) surface, and two C-Fe bonds and two O-Fe bonds are formed. The computed C-O bond lengths of the chemisorbed CO2 are 1.240 and 1.239 Å. The computed ∆Eb of B-C2O2-Fe is -0.24 eV. The structure H-C3O2-Fe (Figure 3b) adsorbs on a hollow site, three C-Fe bonds and two O-Fe bonds are formed. The computed C-O bond lengths of the chemisorbed CO2 are both 1.291 Å. The computed ∆Eb of H-C3O2-Fe is -0.35 eV. H-C3O3-Fe-a (Figure 3c) adsorbs on a couple of hollow site of the Fe(110) surface, and three C-Fe bonds and three O-Fe bonds are formed. The computed C-O bond lengths of the chemisorbed CO2 are 1.276 and 1.326 Å, respectively. H-C3O3Fe-a has the ∆Eb value of -0.56 eV. H-C2O3-Fe (Figure 3d) adsorbs on another couple of hollow site of the Fe(110) surface, and two C-Fe bonds and three O-Fe bonds are formed. The computed C-O bond lengths of
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TABLE 1: Computed Binding Energies (∆Eb, eV), Bond Lengths (d, Å), on the Fe(110) Surface and Bond Angles (r, Degree) as Well as the Charge (q) of the Chemisorbed COδ2 binding mode
∆Eb
dC)O1
dC)O2
dC-Fe
dO1-Fe
dO2-Fe
ROCO
q(CO2)
free CO2 B-C2O2-Fe H-C3O2-Fe H-C3O3-Fe-a H-C2O3-Fe H-C3O3-Fe-b
-0.24 -0.35 -0.56 -0.43 -0.68
1.171 1.240 1.291 1.276 1.294 1.353
1.171 1.239 1.291 1.326 1.321 1.224
2.073/2.082 1.904/2.255/2.259 1.922/2.200/2.272 1.866/2.101 1.958/2.231/2.228
2.122 1.961 2.024 2.009
2.126 1.965 2.024/2.115 2.078/2.173 2.008/2.194/2.007
180 138 125 123 124 128
0 -0.60 -0.75 -0.81 -0.82 -0.83
TABLE 2: Computed Binding Energies (∆Eb, eV), Bond Lengths (d, Å), on the Transition Metal Surfaces and Bond Angles (r, Degree) as Well as the Charge (q) of the Chemisorbed COδ2 surface Co(111) Rh(111) Ni(111)
Pd(111) Pt(111) Cu(111)
binding mode free CO2 B-C2O2 H-C2O2 H-C3O2 B-C1O1 H-C2O2 H-C3O2 B-C1O1 B-C2O2 H-C2O2 H-C3O2 B-C1O1 B-C2 T-C1 B-C1O1 B-C2
∆Eb
dC)O1
dC)O2
0.32 0.42 0.47 0.31 0.45 0.54 0.30 0.43 0.36 0.44 0.30 0.46 0.47 1.01 0.82
1.171 1.230 1.265 1.262 1.216 1.253 1.240 1.203 1.224 1.262 1.254 1.231 1.237 1.227 1.275 1.240
1.171 1.227 1.263 1.258 1.235 1.249 1.243 1.261 1.224 1.264 1.255 1.200 1.234 1.228 1.202 1.238
the chemisorbed CO2 are 1.294 and 1.321 Å, respectively. H-C2O3-Fe has the ∆Eb value of -0.43 eV. As the most stable structure, H-C3O3-Fe-b (Figure 3e), adsorbs on a couple of hollow site of the Fe(110) surface, and one of the O atoms does not bind with the surface. The computed C-O bond lengths of the chemisorbed CO2 are 1.224 and 1.353 Å, respectively. H-C3O3-Fe-b has the strongest ∆Eb value of -0.68 eV. The OdCdO bond angles of the chemisorbed CO2 on the Fe(110) surface are in the range of 123∼138°. (b) CO2 on the Other Metal Surfaces. The structures of the chemisorbed CO2 on other metal surfaces were shown in Figure 4. The computed binding energies and the structure parameters were listed in Table 2. On the Co(111) surface, three stable structures were found with the binding modes of B-C2O2 (Figure 4g), H-C2O2 (Figure 4i), and H-C3O2 (Figure 4j), respectively. The chemisorbed CO2 with the binding mode of B-C2O2 adsorbs on the bridge site of the Co(111) surface, and two C-Co bonds and two O-Co bonds are formed. The computed C-O bond lengths of the chemisorbed CO2 are 1.230 and 1.227 Å. The computed ∆Eb of CO2/Co(111) with the B-C2O2 binding mode is 0.32 eV. The CO2/Co(111) with the H-C2O2 binding mode adsorbs on the hollow site formed by a face-centered cubic (fcc) site and a neighboring hexagonal close-packed (hcp) site, and two C-Co bonds and two O-Co bonds are formed. The computed C-O bond lengths of the chemisorbed CO2 are 1.265 and 1.263 Å. The computed ∆Eb of CO2/Co(111) with the H-C2O2 binding mode is 0.42 eV. The CO2/Co(111) with the H-C3O2 binding mode adsorbs on the fcc site, and three C-Co bonds and two O-Co bonds are formed. The computed C-O bond lengths are 1.262 and 1.258 Å. The computed ∆Eb of CO2/Co(111) with the H-C3O2 binding mode is 0.47 eV. On the Rh(111) surface, three stable structures were found with the binding modes of B-C1O1 (Figure 4f), H-C2O2 (Figure 4i), and H-C3O2 (Figure 4j), respectively. For the B-C1O1
dC-M 2.064/2.275 1.945/2.219 1.998/2.232/2.244 2.096 2.131/2.285 2.168/2.425/2.589 1.988 2.141/2.143 1.961/2.056 1.966/2.232/2.238 2.137 2.209/2.230 2.091 2.093 2.146/2.171
dO1-M 2.141 2.139 2.136 2.376 2.435 2.123 2.108 2.131
dO2-M 2.198 2.154 2.109 2.250 2.460 2.319 1.964 2.121 2.134 2.069 2.247 2.135
ROCO
q(CO2)
180 141 132 132 142 135 137 138 143 134 134 146 141 143 134 139
0 -0.51 -0.65 -0.67 -0.46 -0.62 -0.59 -0.44 -0.44 -0.60 -0.59 -0.36 -0.49 -0.45 -0.37 -0.52
binding mode, one C-Rh bond and one O-Rh bond are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.216 and 1.235 Å. The computed ∆Eb of CO2/Rh(111) with the B-C1O1 binding mode is 0.31 eV. For the H-C2O2 binding mode, two C-Rh bonds and two O-Rh bonds are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.253 and 1.249 Å. The computed ∆Eb of CO2/Rh(111) with the H-C2O2 binding mode is 0.45 eV. For the H-C3O2 binding mode, three C-Rh bonds and two O-Rh bonds are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.240 and 1.243 Å. The computed ∆Eb of CO2/Rh(111) with the H-C3O2 binding mode is 0.54 eV. On the Ni(111) surface, four stable structures were found with the binding modes of B-C1O1 (Figure 4f), B-C2O2 (Figure 4 g), H-C2O2 (Figure 4i), and H-C3O2 (Figure 4j), respectively. This system has been calculated by using 3-layered models.49 CO2/Ni(111) in the B-C1O1 binding mode is the most stable structure with the ∆Eb of 0.30 eV. The C-O bond lengths of the chemisorbed CO2 in this binding mode are 1.203 and 1.261 Å. Choe et al. modeled the adsorption and dissociation reaction of CO2 on Ni(111) by a 25-atom cluster by using atomsuperposition and electron-delocalization molecular orbital method.48 The most stable state of CO2 on Ni(111) was reported on the σ(1) site, i.e., the bridge site with two oxygen bonding with two surface nickel atoms, respectively. The binding energy was reported to be -2.31 eV. However, our previous49 and present theoretical investigation found that, on the Ni(111) surface, the most stable structure is the 2-fold B-C1O1 state with the binding energy of 0.3 eV. Recently, Ding et al. computed the CO2/Ni(110) system,50 the most stable structure and the binding enery (-0.22 eV) agrees with our previous results (-0.39 eV49). On the Pd(111) surface, three stable structures were found with the binding modes of B-C1O1 (Figure 4f), B-C2 (Figure 4h), and T-C1 (Figure 4k), respectively. For the B-C1O1 binding
Interaction of CO2 with Transition Metal Surfaces
Figure 5. Reactivity of the metals to CO2 chemisorption.
mode, one C-Pd bond and one O-Pd bond are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.231 and 1.200 Å. The computed ∆Eb of CO2/Pd(111) with the B-C1O1 binding mode is 0.30 eV. For the B-C2 binding mode, two C-Pd bonds are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.237 and 1.234 Å. The computed ∆Eb of CO2/Pd(111) with the B-C2 binding mode is 0.46 eV. For the T-C1 binding mode, one C-Pd bond is formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.227 and 1.228 Å. The computed ∆Eb of CO2/ Pd(111) with the T-C1 binding mode is 0.47 eV. On the Pt(111) surface, only one stable structure was found, B-C1O1 (Figure 4f), in which one C-Pt bond and one O-Pt bond are formed. The computed C-O bond lengths of the chemisorbed CO2 are 1.275 and 1.202 Å. The computed ∆Eb of CO2/Pt(111) with the B-C1O1 binding mode is 1.01 eV. On the Cu(111) surface, only one stable structure was found, B-C2 (Figure 4h). In this structure, two C-Cu bonds are formed, and the computed C-O bond lengths of the chemisorbed CO2 are 1.240 and 1.238 Å. The computed ∆Eb of CO2/Cu(111) with the B-C2 binding mode is 0.82 eV. Furthermore, we have calculated the possibility of the chemisorption of CO2 on the Ag(111) and Au(111) surfaces, but no stable structures could be obtained. It is of note that the chemisorbed states of CO2 on these transition metal surfaces differ strongly in binding energies and structures. The most stable structures of the chemisorbed CO2 on the (111) surfaces of Ni, Pd, Pt, and Rh are in B-C1O1 mode. On the Co(111) surface, the most stable one is B-C2O2, and that of Cu(111) is B-C2. Therefore, the bridge sites of these surfaces are favorable for the chemisorption of CO2. The C-O bonds of the chemisorbed CO2 are elongated to 1.200∼1.302 Å, compared to those (1.171 Å) of free CO2 molecule, indicating the activation of C-O bonds by the interaction of CO2 with the metals. On the basis of the computed binding energies, the reactivity of metal to CO2 chemisorption relative to their sites in the periodic table is shown in Figure 5. In our computed region, the activities of the metals in the top left side are higher than those in the lower right side of the periodic table. CO2 strongly chemisorbs on the Fe(110) surface with negative binding energies and moderately on the Co(111) surface with slight positive binding energies on Ni, Rh, and Pd. On the Pt(111) and Cu(111) surfaces, the binding energies of the chemisorbed CO2 are highly positive, indicating very unstable chemisorption. For the Au(111) and Ag(111) surfaces, no stable structures of chemisorbed CO2 were obtained during our calculations.
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16937
Figure 6. Relationship of the binding energies (∆Eb) with the d-band centers (Ed) of the metal surfaces; the values of Ed were taken from ref 62.
(c) Factor Controlling the Binding Energy. Generally, chemisorption and activation of CO2 on metal surfaces might be affected by many factors, such as binding modes of the chemisorbed CO2, charge transfers between CO2 molecule and the metals, as well as surface structures and electronic properties of the metals. It is important to find out the key factors affecting the interaction of CO2 with the metal surfaces. It is noteworthy that there is no direct correlation between the binding energies and the activation degrees of the chemisorbed CO2 molecules. The binding energy and the activation degree are controlled by different factors. In this subsection, we are focusing on the factors controlling the binding energies of CO2 on the metal surfaces. It was found that the binding energy increases with the decreasing of the d-band center of the metals, as shown in Figure 6. This means that the chemisorption stability can be modified by changing the d-band center. A linear fit was put on to the relationship of binding energy with the d-band center. The correlation coefficient of R ) -0.83 is low, since linear relationships between the binding energies of the adsorbates and the d-band centers of the metal surfaces were found in many adsorption systems with high R values,57-60 which could be well predicted by using the Hammer-Nørskov model.61,62 Since the d-band center is a local electronic property, the adsorption form is usually assumed to be the same when compared. However, there is no similar adsorption state found for comparison during our calculation. This might be one of the reasons for the relatively low linearity. We chose the CO2/Ni(111) structure (B-C1O1 mode) to analyze the interaction of CO2 with the surfaces. Figure 7 shows the interaction mechanism of CO2/Ni(111), and Figure 8 shows the interaction of CO2 with other metal surfaces. Two main changes take place during the chemisorption of CO2 on the metal surfaces, i.e., the bending of the OdCdO skeleton and the binding with the surfaces. To describe the effects of the two changes, we separated the chemisorption into two steps for analyzing the electronic properties. The first step is the bending of the CO2 molecule. In Figure 7, the structural parameters of the bent CO2 were the same as those in CO2/Ni(111) structure (B-C1O1 mode). In free CO2 molecule, the highest occupied molecular orbital (HOMO) is the degenerate 1πg bonding orbital, while the lowest unoccupied molecular orbital (LUMO) is the degenerate 2πu antibonding orbital. When the OdCdO skeleton was bent, the 1πg split into 1a and 1b, and the 2πu split into 2a and 2b. It is noteworthy that, after the split of the ortibals, the 2b orbital from the split
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Figure 7. Interaction mechanism of CO2 with the Ni(111) surface.
Figure 9. Relationship of the binding energies (∆Eb) and the transferred charge (q). Figure 8. LDOS of CO2 on the metal surfaces.
of 2πu becomes the LUMO and the 1a from the split of 1πg becomes the HOMO. The split of orbitals by the bending of the CO2 molecule has been reported previously;63,64 however, the 2b from the split of 2πu was considered to have the lowest energy of the four new molecular orbitals. Before analyzing the binding between the bent CO2 and the Ni(111) surface, we calculated the local density of states (LDOS) of the distorted Ni(111) surface induced by the adsorption of CO2 by directly removing the chemisorbed CO2 from the surface model and making single-point energy calculation. No apparent change was found for the d band, by comparison with the optimized Ni(111) surface. The second step is the binding of the bent CO2 with the Ni(111) surface. As shown in Figure 7, the carbon centers in 2b and 2a have σ-orbital and π-orbital character, respectively, and they can interact effectively with the d-orbitals for charge transfer from metal to carbon. Since the C-O orbital in 2b and 2a has antibonding character, the stronger the electron transfer, the stronger the antibonding and therefore the longer the C-O bond. Indeed, all of the orbital energies of the bent CO2 shift below the Fermi level, indicating strong interaction between the chemisorbed CO2 and the Ni(111) surface. Similar behaviors have been found for other surface as shown in Figure 8. On the
other hand, the DOS of the Ni(111) surface does not change significantly. On the basis of this analysis, we have plotted the relationship between the binding energies and the transferred charges (q) on the chemisorbed CO2 (Tables 1 and 2). As shown in Figure 9, there is a liner relationship with a low correlation coefficient of R ) 0.79, indicating a qualitative relationship. Both Figures 6 and 9 reveal that the strength of CO2 chemisorption depends on the energy level of the d-band center of the catalysts and also on the charge transfer from catalyst to CO2. Both parameters will give a reasonable measure for the activity of a catalyst. (d) Factor Controlling the C-O Bond Activation. On the point of view of catalysis, it is important to find out the key factors controlling the activation degree of CO2 on the metal surfaces, since the activation degree is affected by the factors different from binding strength. The CO2 molecule was found to become chemisorbed ionic 49 In our calculaCOδ2 after chemisorbing on metal surfaces. tion, the computed net charge of the chemisorbed CO2δ- is in the range of -0.36 to -0.83 e. For all of the computed metal surfaces, the C-O bond lengths (d) of the chemisorbed CO2 have a linear relationship with their net charges (q), no matter what their binding energies, as shown in Figure 10 (R ) -0.97). It reveals that the activation of the CdO bonds correlates with
Interaction of CO2 with Transition Metal Surfaces
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16939 Thus, both parameters should be considered when forecasting the catalyst activity. The activation degree of CdO bonds in CO2 has a linear relationship with the transferred charges from the metal surfaces, as explained by the electronic analyses. The interaction mechanism of CO2 with the metal surfaces was discussed by analyzing the density of states. This investigation implies that we could adjust the activation degree and the adsorption strength of CO2 by the modification of the Lewis basicity and the d-band center of a catalyst, respectively. We expect that these results could provide impetus for future experiments and the development of methods for controlling the interaction of small molecules with metal surfaces.
Figure 10. Linear relationship of the net charge (q) and the CdO bond lengths of CO2.
the charge transfer from the metal surface to CO2. This is in line with the orbital interaction in Figure 7. It is now interesting to compare the most activated adsorption on each metal surface. The Fe(110) surface elongates the sum of the C-O bond lengths to 2.615 Å, and this is the most activated structure in our study. The Co(111), Rh(111), and Ni(111) surfaces elongate the C-O bond length to the sum value of about 2.50 Å, whereas the Pd(111), Pt(111), and Cu(111) surfaces only activate them to about 2.48 Å. The difference between the metals also could be explained by orbital occupation in Figure 1; that is, the stable electron structures of an atom are those with its atomic orbital fully filled, half-filled, or empty. For Fe, it prefers to lose one electron to become the 3d5 halffilled state, and therefore, Fe profitably donates an electron to the chemisorbed CO2 and strongly activates its CdO bonds. For Co (3d7), Rh (4d8), and Ni (3d8), the donation of electrons to CO2 is flexible. However, CO2 obtaining electrons from Cu (3d10) or Pd (4d10) is very difficult because these metals are stable with their fully filled electron states. Pt (5d9) inclines to obtain an electron to form 5d10 but not donate them. Therefore, this metal is also inert for CO2. In summary, the activation degree of C-O bonds in CO2 has a linear relationship with the charges received. Therefore, we could modify its activation degree by choosing the metals or alloys with proper Lewis basicity or by electrochemical methods. Conclusion The chemisorption of CO2 on transition metal surfaces was computed at the level of density functional theory. CO2 strongly chemisorbs on the Fe(110) surface with the strongest binding energies, whereas it has moderate strengths on the (111) surfaces of Co, Ni, Rh, and Pd with slightly positive binding energies. On the Pt(111) and Cu(111) surfaces, the binding energies of the chemisorbed CO2 are highly positive, indicating very unstable chemisorption. On the Au(111) and Ag(111) surfaces, no stable structures of chemisorbed CO2 were found during our calculation. The aim of the present investigation is to find out the key factors controlling the interaction of CO2 with the surfaces. It was found that there is an approximate linear relationship between the binding energy of CO2 with the energy levels of the d-band centers of the metal surfaces. In addition, the binding energy has also an approximate linear correlation with the transferred charge from the metal surface into chemisorbed CO2.
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