QIU Mei TAO Hui-Lin LI Yi HUANG Xin ZHANG Yong-Fn

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The First-principle Investigations on the Activation for CO2on TM (TM = Fe, Co, Ni, Cu) Supported Cu Surface①

QIU Meia②TAO Hui-LinbLI YibHUANG XinbZHANG Yong-Fanb

a(330045)b(350116)

Periodic density functional theory calculations have been carried out to investigate the effect of TM atom supported on different Cu surfaces towards the activation for CO2molecules. The most stable configuration of CO2on various TM/Cu (TM = Fe, Co, Ni, Cu) surfaces is determined and the results show that the cobalt is potentially excellent admetal to enhance the chemisorption of CO2on copper surfaces among the late 3-metals. To deep understand the adsorption property, the bond characteristics of the adsorption bonds are carefully examined by the crystal orbital Hamilton population technique and charge density difference analysis. The result reveals that the interaction between the CO2molecule and TM/Cu surface primarily derive from the TM–C bond. Moreover, the defined adsorption bond strength () between CO2and substrate could be a descriptor for TM-supported surface.

carbon dioxide, transition metal surfaces, chemisorption, DFT;

1 INTRODUCTION

It is well known that carbon dioxide is the most important greenhouse gas that arises from human activities. The increase in CO2concentrations contri- butes over half of the enhanced greenhouse effect[1]. With continued industrial development, the problem of global warming is becoming more and more serious. Therefore, carbon dioxide chemistry is currently of great technological interest[2-4]. However, the main problem is the large thermodynamic stability involved in the use of CO2, requiring a proper modification of molecular or catalyst structure before any useful application can be produced.

Many efforts have been made on CO2adsorption of various materials, such as photocatalyst[5, 6], elec- trocatalyst[7, 8], porous material[9-11]and metallic sur- face[12-16]. However, Cu[17]was found to be able to activate CO2. Especially, the copper surface shows favorable catalytic properties for CO2activation. To improve the catalytic performance of the current catalyst, recently some efforts have been devoted to understanding the adsorption behavior of CO2on Cu-based bimetallic surface and the results indicate that the reactivity of Cu surface toward CO2can be modified by introducing other TM atoms into the surface[18-21]. The addition of the second metal to the surface of the host may exhibit novel properties that are not present on their parent metal surfaces. Therefore, it is worthwhile to investigate the adsorp-tion behaviors of CO2on the bimetallic surfaces, which may offer an effective way to develop new metal catalysts to activate CO2molecule.

Our previous results indicated that among the late 3-metals, cobalt is the potentially best dopant to activate CO2on three copper surfaces[22, 23]. However, these previous theoretical studies have focused exclusively on the case where the foreign atoms are dispersed into the Cu surfaces. Although the corresponding calculations may provide some useful information to understand the interactions between CO2and the substrate, they are not good represent- tative of a true bimetallic Cu surface which often does not have such idealized structure. Since the composition and geometry can have a large influence on the surface chemistry, it is necessary to see how the TM atom supported on different Cu surfaces affects towards the activation for CO2molecules.

In this work, density functional theory (DFT) was employed to perform a systematic investigation of the adsorption behavior of CO2molecule on a series of TM/Cu surface alloys that are constructed by supporting isolated late 3TM atoms (TM = Fe, Co, Ni, Cu) on Cu(100), Cu(110) and Cu(111) surfaces. Firstly, the most stable configurations of CO2chemisorbed on TM-supported Cu surfaces were investigated. Next, the bonding characteristics are examined by the crystal orbital Hamilton population (COHP) technique.

2 COMPUTATIONAL DETAILS

All DFT calculations were carried out utilizing the Viennasimulation package (VASP)[24-26]and the projected augmented wave (PAW) me- thod[27-29].The generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) exchange-correla- tion functional was employed[30]. The kinetic energy cutoff for the plane-wave expansion was set to 400 eV and the Brillouin-zone integrations were sampled using a (5′5′1) Monkhorst-Pack mesh[31]. The effects of spin polarization were considered and the dipole corrections in the direction of the surface normal were applied. The convergence thresholds of the energy change and the maximum force for the geometry optimizations were set to 10-6eV and 0.02 eV/?, respectively.

The Cu(100) and Cu(111) surfaces were modeled by periodic slabs consisting of five atomic layers, whereas a seven-layer slab was used for the Cu(110) surface with a more open structure. During the structural optimization, the atoms at the top three layers of the slabs for Cu(100) and Cu(111) surfaces were allowed to relax, while the bottom two layers were fixed to their bulk positions. However, for the Cu(110) surface, the outermost four layers were fully relaxed in all directions and other Cu atoms at the remaining layers were fixed. The spacing between the adjacent slabs was set to about 15 ?. To simulate the individual TM atoms supported on the Cu surfaces and to avoid the obvious interactions between neighboring CO2adsorbates, a supercell consisting of a (3 × 3) surface unit cell was adopted, in which one TM atom as admetal was supported. For the adsorption of CO2molecules on different TM/Cu surface alloys, many possible adsorption configurations were explored to compare the thermodynamic stability of different structure, with the adsorption energy defined as:

3 RESULTS AND DISCUSSION

In the present study, the TM-supported Cu surfaces that one TM atom (TM = Fe Co Ni and Cu) is supported on the top layer of different Cu surfaces are investigated. However, the results show that the configurations of CO2on the corresponding surfaces are all energetically unfavorable for Cu/Cu(100) and Cu/Cu(111) surfaces. Therefore, we mainly focus on the results that TM atom (TM = Fe, Co and Ni) supported on the top layer of three Cu surfaces and Cu/Cu(110) surface in thefollowing sections.

3. 1 Adsorption of CO2 on the TM/Cu(100) surfaces

Firstly, the possible sites for TM atom supported on the Cu(100), Cu(110) and Cu(111) surfaces were considered. As displayed in Fig. 1, there are three possible sites for Cu(100) surface:-site,- site and-site (Fig. 1(a)). For Cu(110) and Cu(111) surfaces, there are four supported sites:-site,-site,--site and-site shown as Fig. 1(b) and-site,- site,-site and-site in Fig. 1(c), respectively. According to the calculated results, the most stable supported site is the-site for Cu(100) surface,-site for Cu(110) surface and-site for Cu(111) surface. Therefore, we now focus mainly on the adsorption behavior for CO2on the most stable TM/Cu surface.

Fig. 1. Possible sites for TM supported on different Cu surfaces

The most stable adsorption structures of CO2on the different TM/Cu(100) (TM = Fe, Co and Ni) surfaces are displayed in Fig. 2(a). For Fe/Cu(100) surface, selected geometrical parameters are listed in Table 1. All three atoms of the CO2molecule involved the interaction with the substrate. It is interesting that CO2prefers to bind above the solute Fe atom by the formation of two adsorption bonds (namely, Fe–C and Fe–O(1) bonds), which corresponds to the η2(C, O) linkage. The calculated lengths of Fe–C and Fe–O(1) bonds are 1.919 and 1.949 ?, respectively. Furthermore, another oxygen atom of CO2is attached to one neighboring Cu atom on the substrate. However, the corresponding Cu–O(2) distance is 2.202 ?, indicating that the interaction between Cu and O(2) atoms is weak. The CO2moiety is displayed as a bent structure with the O(1)–C–O(2) angle of 136.2°. Due to the strong interaction between the CO2and Fe atoms, a positive adsorption energy of 11.65 kcal/mol is predicted for the Fe/Cu(100) surface. Thus, the chemisorption of CO2becomes an exothermic process by supporting Fe atoms on the Cu(100) surface.

Fig. 2. Most stable adsorption structure for CO2on TM/Cu(100), TM/Cu(110), Cu/Cu(110) and TM/Cu(111) (TM = Fe Co and Ni) surfaces

Table 1. Some Optimized Bond Lengths (?) and O–C–O Bond Angles (o) for the Chemisorption of CO2 Molecule on the TM/Cu Surface Alloys

Compared to the Fe/Cu(100) surface alloy, similar stable configurations are obtained for the Co/Cu(100) and Ni/Cu(100) systems (Fig. 2(a) and Table 1), in which the CO2molecule still binds to the surface by the formation of TM–C, TM–O(1) and Cu–O(2) multiple bonds. The calculated adsorption energies are 22.94 (TM = Co) and 10.69 kcal/mol (TM = Ni), respectively, which imply that the adsorption energies follow an order of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100). Therefore, the Co/Cu(100) surface shows the strongest interaction with CO2. Moreover, the stability for chemisorption of the CO2molecule could be improved by TM atoms (especially Co) supported on the Cu(100) surface. Compared with the case of the parent TM(100) surface[13], the CO2molecule exhibits a distinct chemisorption behavior on TM/Cu(100) surface alloys for both the adsorption geometry and the relative stability.

To explain the electronic structures of CO2adsorption on TM/Cu(100) surface alloys, the local density of states (LDOS) is investigated. As shown in Fig. 3(a), the highest occupied molecular orbital (HOMO) of free CO2(bottom) is 1, which is located on oxygen atoms. The lowest unoccupied molecular orbital (LUMO) is 2which is the antibonding orbital of CO2and situates on the carbon and two oxygen atoms. After the chemisorbed CO2, the bands shift downward to the metal surface Fermi level, which is consistent with charge transfer from the substrate to the CO2moiety.

Fig. 3. Local density of states of chemisorbed CO2on TM/Cu(100) surfaces, projected TM partial-orbital and alpha and beta orbital of Fe atom, representing a, b and c, respectively, absorbed CO2orbital for CO2on substrate (upper, middle) and that of the free CO2(bottom)

Additionally, in order to analyze the composition of the most important active state for active sites on the surface, the DOS of the total-orbital of the dopant atom is split into the LDOS of five-orbitals as shown in Fig. 3(b). It is clearly indicated that the main active states for TMs ared,d2andd2–2- orbitals. However, for dopant Fe atoms, the interaction between the Fe atom and the CO2molecule is formed mainly via thestate and partialstate, as shown in Fig. 3(c).

To deeper understand the adsorption behavior of CO2on the TM/Cu(100) surface alloys, the -COHP of TM–C, TM–O(1) and Cu–O(2) adsorption bonds are calculated, respectively.

Fig. 4. -COHP for TM–C, TM–O(1) and Cu–O(2) bonds of CO2adsorbed on Fe/Cu(100), Co/Cu(100) and Ni/Cu(100) surfaces, respectively

For the TM–C bond, as shown in Fig. 4(a), there are three sharp bonding peaks at –9.35, –8.10 and –7.05 eV, respectively. Moreover, an additional broad peak is observed from –4.0 to 0 eV. Further analyses suggest that these peaks correspond to the-bonding interactions between the 4/3d2–y2/d2/dorbitals of the TM atom and the 2/2porbitals of the C atom. Compared with the-bonding interactions between TM 3d2– y2/3d2and C 2at –1.7 eV, the-bonding interactions between the TM 4/3dand C 2/2porbitals at –2.5 eV are weaker. Therefore, the TM–C adsorption bonding is mainly formed by 3d2–y2/3d2For Fe-C bonding, the-bonding interactions mainly depend on the beta state for the Fe atom. Further- more, the Fermi level is located in the COHP curve between the bonding and antibonding regions.

In Fig. 4(b), the -COHP curve of the TM–O(1) adsorption bond for different surfaces is displayed. Three sharp bonding peaks and one broad bonding peak are found at –9.5, –8.1, –7.2 and –4.5 eV, indicating that the TM atom mainly overlaps with O 2, 2pand 2porbitals by the 4/3d/3dorbitals. The contributions of TM 3d/3dorbitals are also observed from –4.0 eV to the Fermi level. However, it is noted that the Fermi level falls in a strong TM–O(1) antibonding region, implying that the bonding interactions between the TM and O(1) atoms are heavily weakened. In other words, the strength of TM–O(1) adsorption bonding can be enhanced by removing some electrons from the system.

For the Cu–O(2) adsorption bond, the Fermi level also falls in a strong Cu–O(2) antibonding region, as shown in Fig. 4(c). The Cu–O-bonding peaks are mainly derived from the interactions between the Cu 4and O 2/2porbitals. The antibonding peaks in the energetic region from –3.0 eV to the Fermi level contain noticeable 3d/d2contributions of Cu atoms.

Thus, the TM–C adsorption bond plays an impor- tant role for the stability of the system. The Fermi level lies in the clear antibonding region, indicating that the TM–O(1) and Cu–O(2) adsorptions are weak.

Table 2. Adsorption Bond Strength and Adsorption Energy of CO2 on Different TM/Cu Surfaces

Furthermore, to describe the interactions between CO2moiety and TM/Cu surfaces, the adsorption bond strength is defined by the following equation:

whereIrepresents the adsorption strength of bonding or antibonding andrepresents the total adsorption bond strength that is the difference between the strength of bonding orbitals and antibonding orbitals,Cis the -COHP value. The value ofis more positive and the interaction between CO2and the surface is stronger.

Fig. 5. Adsorption energies of CO2on (a) TM/Cu(100), (b) TM/Cu(110) and (c) TM/Cu(111) surface alloys as a function of the adsorption bond strength ().: correlation coefficient

The adsorption bond strength for CO2on TM/Cu(100) surfaces is calculated and shown in Table 2. It indicates that the interaction between the CO2moiety and the TM/Cu(100) surface is mainly contributed by the TM–C bond. The maximum value of adsorption strength is 1.88 for the Co/Cu(100) surface, 1.79 for Fe/Cu(100) and 1.78 for Ni/Cu(100). Therefore, the adsorption bond strength follows a sequence of Co/Cu(100) > Fe/Cu(100) > Ni/Cu(100). Thus, the stability for chemisorption of CO2could be improved through TM atoms (especially Co) supported on the Cu(100) surface. Additionally, the excellent linear relationship between total adsorption strength and the adsorption energies is observed (Fig. 5(a)). The linear regression fit is characterized by a correlation coefficient of= 0.9998.

Charge transfers at interface can be obtained via analyzing the Bader charge using the Bader code[35-37](Table 3). For TM/Cu(100) surfaces, the charge of the interface shift from the substrate to the CO2moiety. The charge (0.67 e (Fe), 0.63 e (Co) and 0.51 e (Ni), respectively) transfer from the TM/Cu(100) surface to the CO2moiety according to Bader Charge analysis. It indicates that the strong interaction is formed via the CO2moiety bonding with the TM/Cu(100) surface, which coincides with the most stable adsorption configuration in Fig. 2(a). From Table 3, it is obviously observed that the partial electrons are mainly transferred from the surfaces to the C atom in the CO2moiety to form CO2-. Therefore, compared with the gas phase CO2mole- cule, the bond lengths of C–O in the CO2moiety are weakened, resulting in CO2distortion. Furthermore, the spin difference charge density is also calculated and plotted in Fig. 6. In Fig. 6(a), the interaction between CO2and TM/Cu(100) forms via the TM–C, TM–O(1) and Cu–O(2) bonds. The TM–C bond reveals the bond character, while the Cu–O(2) and TM–O(1) reveal the antibonding characters.

Table 3. Partial Charges for CO2 Adsorption on the TM/Cu Surfaces by Bader Analysis (|e|)

Fig. 6. Spin difference charge density for CO2on TM/Cu surfaces

3. 2 Adsorption of CO2 on the TM/Cu(110) surfaces

Figs. 2(b) and 2(c) display the most stable con- figurations for the chemisorption of CO2on different TM/Cu(110) surfaces (TM = Fe, Co, Ni and Cu) and partial structural parameters are shown in Table 1. It is clear that the CO2molecule is adsorbed above the fourfold hollow site of the TM/Cu(110) surfaces by all atoms in the CO2moiety. Taking the Fe/Cu(110) system as an example, five adsorption bonds (including one Fe–C bond and four Cu–O bonds) are formed between the CO2moiety and the substrate. Comparing with the Fe/Cu(100) and Fe/Cu(111) (see later section) systems, there are more adsorption bonds formed between the CO2moiety and the Fe/Cu(110) surfaces. The calculated Fe–C and Cu–O bond lengths are 1.959 and 2.160 × (4) ?, respectively. After CO2adsorption, two C–O bonds of CO2are stretched from 1.176 to 1.297 and 1.298 ?. In addition, the CO2moiety on the Fe/Cu(110) surface also exhibits a bent structure. The O–C–O angle is 123.3° which is close to that expected for CO2-. Compared with the configuration on Fe/Cu(100) and Fe/Cu(111) surfaces, the smaller O–C–O angle (123.3°. 136.2° and 140.2°) suggests that the CO2molecule obtains more electrons when it is bound to the Fe/Cu(110) surface. The calculated adsorption energy is –0.93 kcal/mol, which implies that the CO2adsorption is endothermic on the Fe/Cu(110) surface. For Co/Cu(110) and Ni/Cu(110) surface alloys (see Fig. 2(b)), the same configurations are yielded for CO2on the Co/Cu(110) and Ni/Cu(110) surfaces. The CO2molecule still binds to the substrate by the formation of TM–C and Cu–O multiple bonds. The adsorption energies are 15.88 (TM = Co) and 7.66 kcal/mol (TM = Ni), respectively. Therefore, the adsorption energies follow a sequence of Co/Cu(110) > Ni/Cu(110) > Fe/Cu(110).

For the Cu/Cu(110) surface, three adsorption bonds are found in Fig. 2(c), including Cu (as admetal)–O(1), Cu–C and Cu–O(2) bonds, i.e. all atoms of CO2are bound to the substrate. The lengths of Cu–O(1), Cu–C and Cu–O(2) are 2.084, 1.986 and 2.173 ?, respectively. Correspondingly, the calcula- ted adsorption energy is –6.11 kcal/mol, which indicates that the interaction between the CO2moiety and the Cu/Cu(110) surface is weak. After CO2adsorption, the C–O(1) and C–O(2) bonds in the CO2moiety are changed from 1.176 to 1.275 and 1.252?, respectively. Furthermore, compared to the CO2molecule in the gas phase, the CO2moiety exhibits a nonlinear configuration with the O–C–O angle of 129.8°. In summary, the final adsorption energies follow an order of Co/Cu(110) > Ni/Cu(110) > Fe/Cu(110) > Cu/Cu(110), which confirms that introduction of other TM atoms on the Cu(110) surface can improve stability for the chemisorption of CO2, especially introducing of the Co atom.

For CO2adsorption on TM/Cu(110) surfaces, the local density of states (LDOS) is calculated and shown in Fig. 7. It is obvious that the bands shift downward to the metal surface Fermi level. The antibonding 2orbital also falls below the Fermi level for the adsorbed case and the intensity is also lowered compared to the free CO2molecule, which is ultimately responsible for CO2activation. The 1+ 3orbitals split into one 3sharp peak and 1broad peak at –7.5 and –8.0 eV, respectively, confirming that bending of CO2will activate CO2.

In order to analyze the composition of the most important active state for active sites on surfaces, the DOS of the total-orbital of the dopant atom is split into the LDOS of the five-orbitals, as shown in Fig. 7(b). The main active states for TMs ared,dandd22orbitals obtained. However, for dopant Fe atoms, the interaction between the Fe atom and the CO2molecule is formed via the beta state and partial alpha state (Fig. 7(c)).

Fig. 7. Local density of states of chemisorbed CO2on the TM/Cu(110) surfaces, projected TM partial-orbital and alpha and beta orbital of Fe atom, representing a, b and c, respectively, absorbed CO2orbital for CO2on the substrate (upper, middle) and that of the free CO2(bottom)

Fig. 8. -COHP for TM–C, Cu(1)–O(1) and Cu(2)–O(1) bonds of CO2adsorbed on the Fe/Cu(110), Co/Cu(110), Ni/Cu(110) and Cu/Cu(110) surfaces, respectively

According to the above analyses, the interactions between the TM/Cu(110) surface alloy and the CO2molecule are formed through one TM–C adsorption bond and four Cu–O adsorption bonds, expect for the Cu/Cu(110) surface. The -COHP value for different surfaces was calculated. The -COHP curves of TM–C, Cu(1)–O(1) and Cu(2)–O(1) bonds are displayed in Fig. 8. For the TM–C bond (TM = Fe Co and Ni, Fig. 8(a)), there is one sharp peak and one broad peak at –9.8 eV and from –8.0 to –7.0 eV, which correspond to the-bonding interactions between TM 3d22/dand C 2/2p/2p. From –4.0 to –1.0 eV, the peaks observed mainly originate from the strong-bonding interactions between the TM 3d22/d(also containing some components of the 3dstate) and the C 2/2p/2porbitals. For the Cu–C adsorption bond, the major contributions are from TM 4and C 2/2porbitals at –9.2 and –8.0 eV. A few antibonding components are also observed at –1.3 and –4.0 eV, indicating that there is a negligible contribution to the formation of the Cu-C bond. For the Cu/Cu(110) surface, the bond peaks of Cu-C are less pronounced. Consequently, a weak Cu–C adsorption bond can be expected, which is in accor- dance with the fact that the chemisorption of CO2on the Cu/Cu(110) surface is energetically unfavorable. Furthermore, it is noted that the Fermi level is located on the -COHP curve between the bonding and antibonding regions.

The O atom in CO2bonds with two Cu atoms on the surface, forming a2linkage (named the Cu(1)–O(1) and Cu(2)–O(1) bonds). Therefore, the -COHP curves of Cu(1)–O(1) and Cu(2)–O(1) bonds are also shown in Figs. 8(b) and 8(c). For the Cu(1)–O(1) adsorption bond, three bonding features are found in the region between –10.0 and –3.0 eV and they are mainly derived from the-bonding interactions between the Cu 4and O 2orbitals in the TM/Cu(110) systems (TM = Fe Co and Ni). From –3.0 eV to the Fermi level, several antibonding peaks are also found and these peaks contain partial contributions of 3d2, 3dand 3dorbitals of the Cu atom. For CO2on the Cu/Cu(110) system, the bonding features are observed at –9.2, –7.0 and –4.0 eV. Compared with other TM/Cu(110) systems, the components of the bonding peaks are different. These components are contributed of Cu 3d/3dand C 2p/2p, which matches with the adsorption structure of CO2on Cu/Cu(110) surface. In addition, the Fermi level is located in the antibonding region, implying that the Cu–O adsorption bond is not strong.

In Fig. 8(c), the -COHP curve of the Cu(2)–O(1) adsorption bond is displayed. Clearly, three bonding peaks are observed from –10.0 to –3.0 eV and they are mainly derived from the Cu 4and O 2/2porbitals. From –3.0 to 0 eV, the antibonding peaks are below the Fermi level. The peaks are constituted not only Cu 4state, but also Cu 3d/3d2/3dorbital.

The Cu–O bond is weakened because the Fermi level is located in the antibonding region for the Cu(1)–O(1) and Cu(2)–O(1) adsorption bonds. This is the main reason why the binding strength between CO2and different Cu(110) surfaces is weak, although four Cu–O adsorption bonds are formed.

In addition, the adsorption strength of CO2on the TM/Cu(110) surface is computed and displayed in Table 2. The major contribution of the interaction between CO2and the substrate is similar to the case of CO2on the TM/Cu(100) surface. The adsorption bond strength is 3.85, 4.54, 4.35 and 0.55 for Fe/Cu, Co/Cu, Ni/Cu and Cu/Cu(110) surfaces, respectively. The sequence for total adsorption bond strength is similar to the order of adsorption energy. The adsorption energies as a function of the total adsorption bond strength () are presented in Fig. 5(b). The linear relationship between adsorption energy and the total adsorption bond strength () can be established. It seems that the adsorption bond strength can describe the chemisorption behaviours of CO2on different TM/Cu(110) surfaces.

The Bader charge is calculated of CO2adsorption on TM/Cu(110) surfaces (Table 3). It is obvious that the partial electrons are transferred from the surface to the CO2moiety, forming a CO2-structure. There is a total of 1.03 e (Fe), 0.97 e (Co), 0.92 e (Ni) and 0.72 e (Cu) respectively transferred from the TM/Cu(110) surface to the CO2moiety according to Bader Charge analysis. The strong interaction should be formed via the CO2moiety bonding with the TM/Cu(110) surface. However, the TM–C bond is weakened by forming four Cu–O antibonds. Therefore, the adsorption energy for CO2on TM/Cu(110) is no larger than the case of CO2on TM/Cu(100) surfaces. In addition, obviously in Table 3 the partial electrons transferred to the CO2moiety are mainly transferred from surface to the C atom in the CO2moiety, which is in agreement with the above mention. Therefore, the bond lengths of C–O in the CO2moiety are weakened compared to the gas phase CO2molecule, resulting in CO2distortion. Furthermore, the charge density difference is calcu- lated (Fig. 6). The interaction between CO2and TM/Cu(110) forms via TM–C, Cu(1)–O(1) and Cu(2)–O(1) bonds. The TM–C bond reveals the bond character, while Cu(1)–O(1) and Cu(2)–O(1) bonds shows the antibond character, which is in accord with the results of COHP analysis.

3. 3 Adsorption of CO2 on the TM/Cu(111) surfaces

The most stable configurations for the CO2chemi- sorption on different TM/Cu(111) (TM = Fe Co and Ni) surfaces are displayed in Fig. 2(d) and the corresponding structural parameters are shown in Table 1. It is clear that the adsorption configurations of CO2on TM/Cu(111) surfaces are similar to TM/Cu(100) (TM = Fe, Co and Ni) surface alloys. Three adsorption bonds, including TM–C, TM–O(1) and Cu–O(2) bonds, are formed between the CO2moiety and the substrate. Firstly, CO2adsorption on the Fe/Cu(111) surface alloy is investigated. CO2is also attached to a TM center by forming TM–C and TM–O(1) bonds (namely,2(C, O) linkage). The lengths of the TM–C and TM–O(1) bonds are 1.902 and 1.952 ?, respectively. Meanwhile, the other oxygen atom of CO2is bound with one neighboring Cu atom around the TM atom. However, the corresponding Cu–O(2) distance is 2.296 ?, indicating that the interaction between Cu and O(2) atoms is weak. Furthermore, the CO2moiety also displays a bent configuration with the O(1)–C–O(2) angle of 140.2°, which is smaller than that of CO2on the Fe/Cu(100) surface. The adsorption energy of 9.36 kcal/mol is predicted for the Fe/Cu(111) surface alloy, suggesting that the chemisorption for CO2is an exothermic process.

Compared to the Fe/Cu(111) surface, the most stable adsorption configuration for CO2on Co/Cu(111) and Ni/Cu(111) surfaces is similar (Fig. 2(d) and Table 1). The CO2molecule still binds to the surface by forming TM–C, TM–O(1) and Cu–O(2) bonds. The adsorption energies of CO2on Co/Cu(111) and Ni/Cu(111) surfaces are 12.60 and 11.99 kcal/mol, respectively. The average lengths of Co-Cu and Ni-Co bonds are obviously stretched 0.039 and 0.080 ?, respectively.

The sequence of adsorption energies is Co/Cu(111) > Ni/Cu(111) > Fe/Cu(111) > Cu/Cu(111), which also indicates that the introduce- tion of other TM atoms (especially the Co atom) as an admetal can improve the stability for CO2chemi- sorption.

Meanwhile, a comparison between CO2on TM/Cu(100) and TM/Cu(111) surfaces was made. Table 1 shows that the adsorption energies and O(1)–C–O(2) angle of CO2on the TM/Cu(111) sur- face are smaller than that of CO2on the TM/Cu(100) surface after CO2molecule is adsorbed, which means that the activity of the more openly structured Cu(100) surface is higher than the Cu(111) surface.

Fig. 9. Local density of states of CO2chemisorbed on TM/Cu(111) surfaces. Projected TM partial-orbital and alpha and beta orbital of Fe atom, representing a, b and c, respectively. Absorbed CO2orbital for CO2on substrate (upper, middle) and that of the free CO2(bottom)

For TM/Cu(111) surfaces, all configurations of CO2adsorption on these surfaces are similar to each other. Hence, the LDOS of CO2chemisorbed on Co/Cu(111) is considered as an example (Fig. 9). The bands shift downward and fall below the Fermi level after CO2adsorption on the Co/Cu(111) surface (Fig. 9(a)), which confirms that the transferred charge is located at the 2orbital. Meanwhile, the intensity of the 2orbital is also lowered as compared to the free CO2molecule. Further calculations on bent CO2show that the bending of CO2moiety will lower the intensity and the energy level of the LUMO. Therefore, the synergistic effect of both charge transfer and bending of the CO2moiety will promote the activation of CO2molecule.

On the other hand, to prove the primary com- position of the active state on the Co/Cu(111) surface, the LDOS of the-orbital for the Co atom is calculated (Fig. 9(b)). All-orbitals take part in the interaction between the CO2molecule and the TMs atom. Therefore, the active states are very complex, that is, the interaction between the CO2moiety and the substrate is dominated by the 2orbital in the CO2moiety and all of the-orbitals.

Fig. 10. COHP for TM–C, TM–O(1) and Cu–O(2) bonds of CO2adsorbed on Fe/Cu(111), Co/Cu(111) and Ni/Cu(111) surfaces, respectively

Fig. 10(a) displays the -COHP curves of TM–C, TM–O(1) and Cu–O(2) adsorption bonds on the TM/Cu(111) surface alloys. For the TM–C bond, three sharp peaks at –7.0, –8.0, –9.5 eV and one broad peak in the region from –3.0 to –1.0 eV are related to the-bonding interactions between the TMs 4/3d2- y2/d/dand C 2/2p/2porbitals. On the other hand, the Fermi level is falling into the antibonding region, indicating that partial electrons are filling to the antibonding. Therefore, the bonding interactions between the TM and C atoms are weakened. In addition, for the Fe/Cu(111) system, it is clear that the Fe-C bonding interaction is formed via the beta state of the Fe atom.

For the TM–O(1) adsorption bond, the -COHP curve is shown in Fig. 10(b). It is clearly noted that the Fermi level falls in a strong TM–O(1) antibon- ding region, indicating that removing partial electrons from the system can enhance the strength of the TM–O(1) adsorption bond. The bonding peaks are found below –4.0 eV, suggesting the TM atom mainly employs 4, 3d2, 3dand 3dorbitals to bind with the O 2, 2pand 2porbitals, while in the antibonding region from –4.0 to 0 eV, the contributions of TMs 4, 3dand 3dorbitals are also observed.

As shown in Fig. 10(c), for the Cu–O(2) adsorp- tion bond, the Fermi level also falls in a strong Cu–O antibonding region. In this case, the Cu–O-bonding peaks are mainly derived from the interactions between the 4, 3d2and 3dorbitals of the Cu atom and the 2, 2pand 2porbitals of the C atom. Furthermore, the antibonding peaks in the energetic region from –3.5 eV to the Fermi level contain noticeable 3d2contributions of the Cu atom. Hence, similar to the case of TM/Cu(100), it can be expected that the strengths of TM–O(1) and Cu–O(2) adsorption bonds in the TM/Cu(111) surface are also not strong.

The adsorption bond strength for CO2adsorption on the TM/Cu(111) surface is calculated in Table 2. It reveals that the TM–C bond is also mainly devoted to the interaction between the CO2moiety and the substrate for CO2adsorption on the TM/Cu(111) systems. In addition, the sequence of total adsorption bond strength is Co/Cu(111)(3.05) > Ni/Cu(111)(1.87) > Fe/Cu(111)(0.68), which is the familiar order of adsorption energy. Similarly, the active ability of the Co/Cu(111) surface is the strongest among TM/Cu(111) systems. Furthermore, Fig. 5(c) presents the variation of adsorption energies on the TM/Cu(111) surface alloys as a function of the total adsorption bond strength and a preferable linear relationship is established between theadsorption energies.

For CO2on the TM/Cu(111) surfaces, the Bader Charge is also computed and shown in Table 3. It is found that partial electrons are transferred from the substrate to the CO2moiety and 0.60 e (Fe), 0.58 e (Co) and 0.49 e (Ni), respectively, transferred from the TM/Cu(111) surface to the CO2moiety to form CO2-ion as expected. Furthermore, the deeper analysis indicates that the C atom is the major acceptor for electrons, instead of the O atom, that is, these electrons are transferred from the substrate to the C atom, resulting in weakened C–O bond in the CO2molecule. The charge density difference (Fig. 6(d)) clearly reveals that the interaction between CO2and TM/Cu(111) is formed by TM–C, Cu–O(1) and Cu–O(2) bonds. The TM–C bond reveals a bond character, while TM–O(1) and Cu–O(2) bonds reveal an antibond character, which is in accord with the results of COHP analysis.

4 CONCLUSION

In this work, the first-principles DFT calculations combined with a slab model have been carried out to study the adsorption behaviors of CO2on various TM/Cu(100), TM/Cu(110) and TM/Cu(111) (TM = Fe, Co, Ni and Cu) surfaces. The results indicate that chemisorptions of the CO2molecule on the Cu/Cu surfaces are obviously unfavorable from a thermo- dynamic point of view. However, the introduction of adsorbed TM at the top layer can improve the stability for chemisorption of CO2on the TM/Cu surfaces. It is noteworthy that the most energetically favorable chemisorption structures on three Cu surfaces all correspond to the cases of TM = Co. Therefore, cobalt may be a potential admetal to enhance the chemisorption of CO2on copper surfaces among the late 3-metals. The obvious charge transfer from the surface to the CO2moiety is observed after binding to the substrate. The CO2exhibits a bent configuration that is similar to CO2–anion, indicating the activation of carbon dioxide. In addition, the deeper charge analysis demonstrates that these transferred electrons mainly focus on the C atom.

To further understand the adsorption behaviors of CO2on the surface alloys, chemical bonding analy- ses were performed to study the characteristics of adsorption bonds by using the -COHP technique and charge density difference. It was found that the interaction between the CO2molecule and the TM/Cu surface is contributed via a TM–C bond. For the TM–O or Cu–O bonds, the Fermi level falls into the antibonding orbital. Thus, TM–O or Cu–O exerts the character of antibonding, which weakens the interaction between CO2and the substrate. Furthermore, the calculated results of total adsorption bond strength () confirm that the TM–C bond is primarily devoted to the adsorption energy. Further systematic work on other bimetallic systems is in progress in order to obtain more information about the performance of the adsorption intensity proposed in the work.

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4 September 2017;

15 December 2017

① The project was supported by the National Natural Science Foundation of China (21773030, 21203027 and 21371034)

. E-mail: qium@jxau.edu.cn

10.14102/j.cnki.0254-5861.2011-1816