Binding to CuâNi Bimetallic Clusters - American Chemical Society

Article pubs.acs.org/JPCC

Role of Composition and Geometric Relaxation in CO2 Binding to Cu−Ni Bimetallic Clusters Yang Yang and Daojian Cheng* Division of Molecular and Materials Simulation, State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: Adsorption and activation of CO2 can be considered as the first steps of the reaction mechanism of methanol synthesis via CO2 hydrogenation on Cu-based bimetallic clusters. In this work, the adsorption properties of CO2 on Cu55, Cu54Ni1, and Cu42Ni13 clusters with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures are studied by density functional theory calculations. It is found that icosahedral Cu42Ni13 cluster exhibits the strongest CO2 adsorption ability with all the −Coo, −Oco, and −OcO− adsorption models, compared to the icosahedral Cu55 and Cu54Ni1 clusters. In addition, the structural transformation from cuboctaheral and decahedral to icosahedral clusters upon CO2 adsorption is found, which is attributed to the fact that the stability of these clusters follows the order Cubo < Dec < Ico. Our results show that composition and geometric relaxation can modify the adsorption properties of CO2 on Cu−Ni bimetallic clusters, which can provide useful insights for the design and development of Cu−Ni bimetallic clusters used for methanol synthesis via CO2 hydrogenation.

1. INTRODUCTION In recent years, the chemical conversion of CO2 for its hydrogenation to fuels and chemicals has been considered to be an attractive route for controlling the atmospheric concentration of this greenhouse gas.1−4 Especially, the catalytic hydrogenation of CO2 to methanol has attracted great interest, since its product, methanol, is an excellent liquid fuel and also a primary raw material for the chemical industry.5,6 The grand challenge is that the catalysts for CO2 hydrogenation to methanol suffer from low activity and selectivity due to the high chemical stability of CO2.7−9 In general, Cu-based systems are used as efficient heterogeneous catalysts for methanol synthesis via CO2 hydrogenation. Commercially, Cu−ZnO−Al2O3 has been used as the catalyst to synthesize methanol from mixtures of CO2, CO, and H2.10 However, new catalysts still need to be developed to improve the yield of methanol. Bimetallic catalysts often show better catalytic performance compared to the corresponding elemental metal ones due to the composition and synergic effects on the catalytic properties.11,12 In previous work, Nerlov et al.13−15 showed that Cu− Ni bimetallic catalysts are 60 times more active for CO2 activation compared with pure Cu ones. In addition, Liu et al.16 found that doping of Pd, Rh, Pt, and Ni metals can promote the methanol production on Cu(111) surfaces, and the order for the yield of methanol is Au/Cu(111) < Cu(111) < Pd/Cu(111) < Rh/Cu(111) < Pt/Cu(111) < Ni/Cu(111). It means that adding the Ni to the Cu-based catalysts can improve the yield of methanol again. More interestingly, the heterogeneous catalysis by small metal clusters has attracted great attention for their fundamental interest and potential application due to the unique surface effect and the finite-size effect.17−19 In previous work, Yang et al.20 found that the catalytic activity on a Cu29 cluster for the synthesis of CH3OH © 2013 American Chemical Society

via CO2 hydrogenation is higher than that on the Cu(111) surface, indicating that Cu cluster could be a potential catalyst for CO2 hydrogenation to methanol. As mentioned above, Cu− Ni bimetallic clusters by the incorporation of Ni atoms into the Cu clusters could be more promising for the catalytic hydrogenation of CO2 into methanol than the pure Cu ones for methanol synthesis via CO2 hydrogenation. Adsorption and activation of CO2 can be considered as the first steps of the reaction mechanism of methanol synthesis via CO2 hydrogenation. However, a full understanding of CO2 adsorption on these complex catalysts is limited by the available experimental methods. Recent advances in theoretical methods, such as density functional theory (DFT) calculations,21,22 show that theoretical calculations can play an important role in understanding the adsorption processes of CO2 on metal catalysts. A lot of theoretical studies have been focused on the adsorption and activation of CO2 on metal surfaces,3,10,23,24 metal oxides,25−27 and elemental metal clusters.20,28,29 Only a little of the theoretical work attempted to understand the activation of CO2 on bimetallic clusters.30,31 Yin et al.30 studied the adsorption and activation of CO2 on γ-Al2O3-supported four-atom Cu−Co bimetallic clusters by DFT calculations. It is found that the introduction of Cu into the Co cluster can reduce the adsorption ability of CO2. In addition, Han et al.31 investigated the adsorption of CO2 on Ni-doping in Cun clusters (n = 1−12) and found that the Ni-doping can modify the adsorption properties of CO2 on pure Cun (n = 1−12) clusters. However, the reaction mechanism of methanol synthesis via CO2 hydrogenation on Cu−Ni bimetallic clusters Received: July 30, 2013 Revised: November 25, 2013 Published: December 10, 2013 250

dx.doi.org/10.1021/jp4075674 | J. Phys. Chem. C 2014, 118, 250−258

The Journal of Physical Chemistry C

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remains far from understood. Further studies are still necessary for the initial adsorption and activation of CO2 on Cu−Ni bimetallic clusters. In this work, we study the adsorption properties of CO2 on 55-atom Cu−Ni bimetallic clusters with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures by DFT calculations. The role of composition and geometric relaxation on the adsorption of CO2 on Cu−Ni bimetallic clusters is investigated. The paper is structured as follows. In the next section, we present the computational methodology. Section 3 describes our results and discussion, and section 4 offers our conclusions.

2. CALCULATION DETAILS DFT calculations were performed using the Quantum Espresso package,32 which is coded based on a plane wave basis set. The Perdew−Burke−Ernzerhof (PBE) xc-functional33 and ultrasoft pseudopotentials34 were used. Spin-polarized calculations were performed using values of 40 and 320 Ry as the energy cutoff for the description of the wave function and the electronic density, respectively. The first Brillouin zone was sampled at the gamma-point, and the electronic levels were broadened35 with a Gaussian smearing of about 0.002 Ry. The cluster was located in a 30 × 30 × 30 Å cubic supercell. The positions of the atoms in the complex were fully optimized until the forces were smaller than 0.01 eV/Å per atom. In this work, three compositions were selected for the 55atom bimetallic clusters, corresponding to Cu55, Cu54Ni1, and Cu42Ni13. For each composition, three kinds of morphologies were considered, which are highly symmetric Cubo, Dec, and Ico structures. Initially, the lowest-energy atomic ordering of Cu−Ni bimetallic clusters is calculated at the EAM empirical potential level36,37 and is then subjected to DFT relaxation. The resulting configurations are used to study adsorption properties of CO2.

Figure 1. Configurations and adsorption sites of Cu55, Cu54Ni1, and Cu42Ni13 with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures. The orange and blue atoms stand for Cu and Ni, respectively. For the meaning of the possible adsorption sites (T1, T2, T3, T4, T5, and T6), please see the text.

Table 1. Calculated Average Bond Lengths R (Å), Average Binding Energies Eb (eV/atom), and the Excess Energies Δ55 (eV)a cubo dec ico cubo dec ico cubo dec ico

Cu55

Cu54Ni1

Cu42Ni13

3. RESULTS AND DISCUSSION 3.1. Free Clusters. Figure 1 shows the equilibrium geometries of nine metal clusters used in this work. For the Cu54Ni1 cluster, one Ni atom tends to occupy the central site, forming the Cu-shell and Ni-core Cu54Ni1 structure. For Cu42Ni13, the 42 Cu atoms are located on the surface layer, and the 13 Ni atoms are located in the core, corresponding to the Cu-shell and Ni-core Cu42Ni13 structure. As shown in Figure 1, Cu atoms prefer to segregate at the surface of free Cu−Ni bimetallic clusters, which is in good agreement with previous studies.37,38 This phenomenon can be explained by considering surface energies and lattice parameters. First of all, surface energies of Cu and Ni are 114 and 149 meV Å−2, respectively; Cu has thus the lowest surface energy and therefore tends to segregate to the surface. On the other hand, the lattice parameters of bulk Cu and Ni are 1.28 and 1.25 Å, respectively.39 Consequently, the smaller Ni atoms tend to occupy the center of the cluster in order to minimize stress. The calculated average bond lengths (R), average binding energies (Eb), and excess energies (Δ55) for Cu55, Cu54Ni1, and Cu42Ni13 with Cubo, Dec, and Ico structures are represented in Table 1. It is found that the average bond length decreases with the increase of Ni concentration for the cluster with the same morphology, due to the fact that the Ni atom is smaller than that of Cu.39 In addition, the average bond length follows the order Cubo < Dec < Ico for the cluster with the same

a

R0 (Å)

Eb (eV/atom)

Δ55 (eV)

2.542 2.544 2.560 2.538 2.539 2.555 2.501 2.507 2.524

−2.619 −2.649 −2.681 −2.645 −2.669 −2.710 −2.920 −2.924 −2.963

N.A. N.A. N.A. −0.486 −0.150 −0.628 −4.553 −3.094 −3.298

N.A. stands for “not applicable.”

composition. The average binding energy corresponds to the stability of the cluster. As listed in Table 1, the absolute value of the average binding energy follows the order Cubo < Dec < Ico, meaning that the stability of the cluster follows the same order Cubo < Dec < Ico. To verify whether it is favorable to substitute Ni atoms in Cu clusters, the excess energy, Δ55, is defined as11 Δ55 = Etotal(Cu NCuNi NNi) − NCu − NNi

Etotal(NNi55) 55

Etotal(Cu55) 55 (1)

where Etotal(CuNCuNiNNi) is the DFT-based total energy for a given cluster, and Etotal(Cu55) and Etotal(Ni55) are the total energies for the pure Cu55 and Ni55 clusters at the DFT level. Clearly, negative values of the excess energy imply that mixing is favorable. Table 1 gives Δ55 for the 55-atom Cu−Ni clusters with Cubo, Dec, and Ico structures changing with Niconcentration at the DFT level. It is found that the values of Δ55 for the 55-atom Cu−Ni clusters are negative, and absolute values tend to gradually increase with the increasing Ni 251



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The initial and final adsorption sites, adsorption energies, and structural parameters of CO2 for the CO2 absorption on the icosahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters with the −Coo, −Oco, and −OcO− adsorption models are listed in Table 2. It is found that the initial adsorption site remains unchanged upon adsorption. For the structural parameters of CO2, the distances of C−O1 and C−O2 are about 1.18 Å, and the angle of O1−C−O2 is close to 180° for all the adsorption models. Figure 3a shows a plot of the adsorption energies of CO2 at the most favorable adsorption site changing with the Ni concentration for the icosahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters. It is found that the adsorption strength of CO2 follows the order Cu55 ≈ Cu54Ni1 < Cu42Ni13. Our results show that icosahedral Cu42Ni13 cluster exhibits the strongest CO2 adsorption ability with all the −Coo, −Oco, and −OcO− adsorption models, compared to the icosahedral Cu55 and Cu54Ni1 clusters. To understand the reason, we calculated the d-band center of icosahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters. Figure 3b shows the adsorption energy of CO2 versus the d-band center. As expected, the d-band center of icosahedral Cu42Ni13 cluster moves much closer to Fermi level, compared with icosahedral Cu55 and Cu54Ni1 clusters. In addition, the adsorption strength of CO2 on Cu42Ni13 cluster is stronger than that on Cu55 and Cu54Ni1 clusters. Accordingly, the more negative the d-band center, the weaker the adsorption strength of CO2, which is in qualitative agreement with the previous DFT results.41 3.3. Geometric Relaxation upon Adsorption. Table 3 gives the initial and final adsorption sites, the adsorption (or distortion for the case of the structural transformation) energies at different adsorption sites, and the structural parameters of CO2 for the CO2 absorption on the cuboctaheral Cu55, Cu54Ni1, and Cu42Ni13 clusters with the −Coo, −Oco, and −OcO− adsorption models. It is noted that the distortion energy is defined as the energy difference between the cluster

concentration. It means that the introduction of Ni atoms into Cu clusters is favorable. 3.2. Composition Effect on the Adsorption. In this work, only the adsorption of CO2 on top site of the clusters is focused on, which is consistent with the reference.40 As shown in Figure 1, three different top adsorption sites are found for a cuboctaheral structure: T1 (on the top of vertex), T2 (on the top of the edge atom), and T3 (on the top of the central atom on the (100) surface). For a decahedral structure, six possible top adsorption sites of T1 (on the top of the first vertex), T2 (on the top of the first edge atom), T3 (on the top of the second vertex), T4 (on the top of the second edge atom), T5 (on the top of the third edge atom), and T6 (on the top of the central atom on the (100) surface) are considered, as shown in Figure 1. In contrast, only two different top adsorption sites are found for an icosahedral structure: T1 (on the top of vertex) and T2 (on the top of the edge atom). Three kinds of adsorption models are studied based on the interaction of top Cu atom with C (−Coo) or one O atom (−Oco) or two O atoms (−OcO−), as shown in Figure 2. The

Figure 2. Different adsorption models for CO2 on Cu−Ni clusters.

adsorption energy of CO2 (Eads) at different adsorption sites is calculated by Eads = Ecluster + CO2 − Ecluster − ECO2

(2)

where Ecluster+CO2 is the total energy of cluster upon CO2 absorption, Ecluster is the energy of the free cluster, and ECO2 is the energy of an isolated CO2 in vacuum.

Table 2. Initial and Final Adsorption Sites, Adsorption Energies (in eV) at Different Adsorption Sites, and Structural Parameters (Distance of C−O1 and C−O2 in Å, Angle of O1−C−O2 in deg, and Distance of CO2 and Clusters in Å) of CO2 for the CO2 Absorption on the Icosahedral Cu55, Cu54Ni1, and Cu42Ni13 Clusters with the −Coo, −Oco, and −OcO− Adsorption Modelsa adsorption site

distance of CO2 and clusters

model

composition

initial

final

adsorption energy

C−O1

C−O2

∠O1−C−O2

C−Cu

O1−Cu

O2−Cu

−Coo

Cu55

T1 T2 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1−T2 T2−T2 T1−T2 T2−T2 T1−T2 T2−T2

T1 T2 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1−T2 T2−T2 T1−T2 T2−T2 T1−T2 T2−T2

−0.044 −0.013 −0.018 −0.425 −0.429 −0.076 −0.033 −0.066 −0.027 −0.463 −0.453 −0.044 −0.033 −0.064 −0.035 −0.437 −0.447

1.172 1.172 1.172 1.172 1.172 1.177 1.173 1.178 1.173 1.176 1.172 1.174 1.173 1.173 1.172 1.173 1.172

1.171 1.172 1.172 1.172 1.172 1.168 1.172 1.167 1.171 1.168 1.171 1.171 1.172 1.170 1.172 1.172 1.172

179.82 179.71 179.77 179.81 179.85 179.63 179.73 179.18 179.44 179.65 179.70 179.48 179.42 179.84 179.67 179.38 179.62

4.534 3.910 3.880 3.766 3.831 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.

N.A. N.A. N.A. N.A. N.A. 2.461 3.340 2.401 3.248 2.563 3.334 3.053 3.322 3.491 3.537 3.194 3.578

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 3.384 3.571 3.641 3.734 3.232 3.605

Cu54Ni1 Cu42Ni13 −Oco−

Cu55 Cu54Ni1 Cu42Ni13

−OcO−

Cu55 Cu54Ni1 Cu42Ni13

a

structural parameter of CO2

N.A. stands for “not applicable.” 252



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Figure 3. Adsorption energies at the most favorable adsorption site changing with the Ni concentration (a) and the d-band center of the cluster (b) for icosahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters.

Table 3. Initial and Final Adsorption Sites, Adsorption (or Distortion for the Case of the Structural Transformation) Energies (in eV) at Different Adsorption Sites, and Structural Parameters (Distance of C−O1 and C−O2 in Å, Angle of O1−C−O2 in deg, and Distance of CO2 and Clusters in Å) of CO2 for the CO2 Absorption on the Cuboctaheral Cu55, Cu54Ni1, and Cu42Ni13 Clusters with the −Coo, −Oco, and −OcO− Adsorption Modelsa adsorption site

energy



model

composition

initial

final

adsorption

distortion

C−O1

C−O2

∠O1−C−O2

C−Cu

O1−Cu

O2−Cu

−Coo

Cu55

T1 T2 T3 T2 T3 T2 T3 T1 T2 T3 T1 T2 T3 T1 T2 T3 T2−T3 T1−T2 T1−T3 T2−T3 T1−T2 T1−T3 T2−T3

T1 T2 T3 T2 T3 T2 T3 T1 T2 T3 T1 T1* T3 T1* T1* T3 T2−T3 T1*−T2* T1−T3 T2−T3 T1*−T2* T2*−T2* T2*−T2*

−0.046 −0.010 −0.039 −0.027 −0.041 −0.011 −0.038 −0.079 −0.029 −0.045 −0.061 N.A. −0.047 N.A. N.A. −0.034 −0.030 N.A. −0.037 −0.037 N.A. N.A. N.A.

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. −3.562 N.A. −2.780 −2.777 N.A. N.A. −3.562 N.A. N.A. −2.777 −2.434 −2.779

1.177 1.172 1.172 1.172 1.172 1.172 1.172 1.178 1.173 1.172 1.177 1.173 1.172 1.173 1.173 1.172 1.172 1.173 1.173 1.173 1.172 1.255 1.173

1.164 1.172 1.172 1.172 1.172 1.172 1.172 1.167 1.172 1.170 1.167 1.171 1.172 1.171 1.171 1.172 1.172 1.171 1.172 1.172 1.172 1.215 1.171

179.84 179.91 179.74 179.66 179.78 179.90 179.92 179.47 179.69 179.73 179.66 179.91 179.82 179.90 179.64 179.71 179.42 179.71 179.58 179.65 179.83 139.14 179.50

4.601 3.899 4.005 3.798 4.035 3.829 4.470 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.

N.A. N.A. N.A. N.A. N.A. N.A. N.A. 2.401 3.126 3.538 2.423 3.833 3.894 3.176 3.533 3.621 3.276 3.286 3.548 3.321 3.912 2.124 3.223

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 3.422 4.048 3.485 3.347 4.034 2.926 3.847

Cu54Ni1 Cu42Ni13 −Oco

Cu55

Cu54Ni1

Cu42Ni13

Cu55 Cu54Ni1

Cu42Ni13

a The italic and bold lines correspond to the case of the structural transformation. *means the new adsorption sites on the icosahedral cluster after the structural transformation. N.A. stands for “not applicable.”

icosahedral one. For the structural parameters of CO2, the distances of C−O1 and C−O2 are about 1.18 Å and the angle of O1−C−O2 is close to 180° for all the adsorption models except the adsorption of CO2 on the T1−T3 site of Cu42Ni13 with the −OcO− adsorption model (see Table 3). Table 4 gives the initial and final adsorption sites, adsorption (or distortion for the case of the structural transformation) energies at different adsorption sites, and the structural parameters of CO2 for the CO2 absorption on the decahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters with the −Coo, −Oco, and −OcO− adsorption models. In most cases without the structural transformation, the adsorption strength of CO2 is weak for the decahedral Cu55, Cu54Ni1, and Cu42Ni13 clusters

upon adsorption and without adsorption. It is found that the adsorption strength of CO2 is really small at different adsorption sites for the cuboctaheral Cu55, Cu54Ni1, and Cu42Ni13 clusters with the −Coo adsorption model. It means that the cuboctaheral Cu55, Cu54Ni1, and Cu42Ni13 clusters exhibit the weak CO2 adsorption ability with the −Coo adsorption model. For the −Oco and −OcO− adsorption models, the structural transformation from cuboctaheral to icosahedral clusters upon CO2 adsorption is found, due to the fact that the stability of the cuboctaheral cluster is lower than that of the icosahedral one. During the structural transformation, the initial adsorption sites of the cuboctaheral cluster are changed into the corresponding adsorption sites of the 253



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Table 4. Initial and Final Adsorption Sites, Adsorption (or Distortion for the Case of the Structural Transformation) Energies (in eV) at Different Adsorption Sites, and Structural Parameters (Distance of C−O1 and C−O2 in Å, Angle of O1−C−O2 in deg, and Distance of CO2 and Clusters in Å) of CO2 for the CO2 Absorption on the Decahedral Cu55, Cu54Ni1, and Cu42Ni13 Clusters with the −Coo, −Oco, and −OcO− Adsorption Modelsa adsorption site

.



model

composition

initial

final

adsorption

distortion

C−O1

C−O2

∠O1−C−O2

C−Cu

O1−Cu

O2−Cu

−Coo

Cu55

T2 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1−T2 T2−T3 T2−T4 T3−T6 T1−T2 T2−T3 T2−T4 T3−T4 T3−T5 T3−T6 T4−T5 T4−T6 T5−T6 T1−T2 T2−T4 T3−T4 T3−T5 T3−T6 T4−T5 T4−T6 T5−T6

T2 T4 T5 T5 T1 T2 T3 T4 T5 T6 T1 T2 T3 T4 T5 T6 T1 T1 T1* T4 T5 T6 T1 T2 T1* T4 T5 T6 T1−T2 T2*−T2* T2−T4 T3−T6 T1−T2 T2−T3 T2−T4 T3−T4 T3−T5 T3−T6 T1*−T2* T4−T6 T5−T6 T1−T2 T2*−T2* T1*−T2* T1*−T2* T1*−T2* T2*−T2* T4−T6 T2*−T2*

−0.026 −0.021 −0.029 −0.036 −0.012 −0.033 −0.076 −0.034 −0.026 −0.037 0.000 −0.018 −0.082 −0.010 −0.069 −0.029 −0.104 −0.072 N.A. −0.034 −0.041 −0.025 −0.045 −0.027 N.A. −0.015 −0.011 −0.045 −0.078 N.A. −0.042 −0.041 −0.076 −0.046 −0.039 −0.060 −0.034 −0.045 N.A. −0.033 −0.043 −0.049 N.A. N.A. N.A. N.A. N.A. −0.034 N.A.

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. −0.929 N.A. N.A. N.A. N.A. N.A. −1.421 N.A. N.A. N.A. N.A. −1.745 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. −0.823 N.A. N.A. N.A. −1.417 −1.419 −1.420 −1.423 −1.285 N.A. −0.644

1.172 1.173 1.173 1.172 1.174 1.172 1.173 1.172 1.172 1.172 1.175 1.172 1.179 1.172 1.172 1.172 1.179 1.174 1.175 1.173 1.173 1.173 1.176 1.173 1.175 1.174 1.172 1.172 1.174 1.173 1.172 1.172 1.174 1.174 1.173 1.176 1.172 1.173 1.276 1.172 1.172 1.173 1.172 1.172 1.175 1.173 1.172 1.172 1.174

1.172 1.171 1.171 1.171 1.167 1.172 1.168 1.172 1.171 1.172 1.175 1.172 1.173 1.172 1.172 1.172 1.167 1.170 1.169 1.172 1.172 1.172 1.169 1.172 1.170 1.171 1.172 1.172 1.170 1.171 1.171 1.172 1.170 1.171 1.172 1.169 1.172 1.172 1.222 1.172 1.172 1.171 1.172 1.172 1.170 1.172 1.172 1.172 1.170

179.73 179.89 179.96 179.71 177.85 179.74 179.16 179.63 179.89 179.71 179.75 179.86 178.22 179.62 179.88 179.61 179.18 179.91 179.92 179.65 179.77 179.63 179.64 179.48 179.86 179.97 179.66 179.60 179.62 179.43 179.36 179.38 179.71 179.34 179.29 179.94 179.56 179.64 134.14 179.49 179.60 179.66 179.80 179.70 179.84 179.61 179.91 179.63 177.50

3.854 3.731 3.690 3.881 3.150 3.856 3.901 3.745 3.955 3.876 2.920 3.854 4.684 3.738 4.200 3.878 N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A.

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 3.231 3.149 2.906 3.523 3.659 3.355 3.082 3.192 2.808 3.071 3.639 3.719 3.228 3.318 3.494 3.661 3.155 3.261 3.489 2.716 3.534 3.586 2.046 3.450 3.517 3.414 3.914 3.562 2.889 3.374 5.302 3.471 5.937

N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. N.A. 3.519 3.967 3.802 3.730 3.558 3.674 3.747 3.909 3.778 3.603 2.833 2.833 3.686 3.339 4.006 3.638 4.234 3.610 5.949 3.537 6.270

Cu54Ni1

Cu42Ni13

−Oco

Cu54Ni1

Cu42Ni13

−OcO−

Cu55

Cu54Ni1

Cu42Ni13

The italic and bold lines correspond to the case of the structural transformation. *means the new adsorption sites on the icosahedral cluster after the structural transformation. N.A. stands for “not applicable.” a

with the −Coo, −Oco, and −OcO− adsorption models. In some cases, the structural transformation from decahedral to icosahedral clusters upon CO2 adsorption is found, which is attributed to that the stability of the decahedral cluster is lower

than that of the icosahedral one. Accordingly, the distances of C−O1 and C−O2 are about 1.18 Å and the angle of O1−C− O2 is close to 180° for all the adsorption models, as listed in Table 4. 254



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Figure 4. Reduced pair correlation functions, g*cm(r), of (a) cuboctaheral, decahedral, and icosahedral Cu42Ni13 clusters without adsorption and (b) cuboctaheral Cu42Ni13 clusters at T2 site (−Oco), T1−T2 site (−OcO−), T2−T3 site (−OcO−), and decahedral Cu42Ni13 clusters at T3 site (−Oco), T2−T4 site (−OcO−), and T3−T6 site (−OcO−) upon CO2 adsorption.

< Ico, so the cuboctaheral and decahedral clusters are spontaneously transformed into the more stable icosahedral cluster upon CO2 adsorption. Figure 5 represents the atomic pathway on the (100) surface of a cuboctaheral Cu42Ni13 cluster during the structural

To understand the geometric relaxation upon CO 2 adsorption, the reduced pair correlation functions, gcm * (r), were calculated to analyze the structure evolution of the clusters upon adsorption. We define gcm(r) as the pair correlation function around the center of mass for the clusters and gcm * (r) = gcm(r)/(V/N2), and then the reduced pair correlation function n * (r) is given by gcm * (r) = ⟨∑i = 1 δ( ri ⃗ − rcm gcm ⃗ − r )⟩, where n is the atom number counted and rcm ⃗ is the coordinates of the center of mass. The reduced pair correlation functions were calculated from the configurations after the relaxation. Figure 4a shows the reduced pair correlation functions, gcm * (r), of cuboctaheral, decahedral, and icosahedral Cu42Ni13 without adsorption. It is found that six, five, and four sharp peaks are found in gcm * (r) for the cuboctaheral, decahedral, and icosahedral Cu42Ni13 without adsorption, respectively. Figure 4b shows the reduced pair correlation functions, * (r), of cuboctaheral Cu42Ni13 clusters at T2 site (−Oco), gcm T1−T2 site (−OcO−), T2−T3 site (−OcO−), and decahedral Cu42Ni13 clusters at T3 site (−Oco), T2−T4 site (−OcO−), and T3−T6 site (−OcO−) upon CO2 adsorption, respectively. It is * (r), corresponding found that four sharp peaks are found in gcm to the icosahedral structure for Cu42Ni13, which is also clearly seen in the icosahedral Cu42Ni13 cluster without adsorption. It means that these cuboctaheral and decahedral clusters upon CO 2 adsorption are completely transformed into the icosahedral cluster, as shown in Figure 4b. As mentioned before, the stability of the cluster follows the order Cubo < Dec

Figure 5. Atomic pathway on the (100) surface of a cuboctaheral Cu42Ni13 cluster during the structural transformation process from cuboctaheral to icosahedral clusters upon CO2 adsorption.

transformation process from cuboctaheral to icosahedral clusters upon CO2 adsorption. The red arrow corresponds to the pathway (radial movement) of the surface atoms. Accordingly, the green and yellow arrows indicate the pathways (tangential movement) of the vertex and edge atoms on the surface, respectively. Finally, the square (100) surface changes into two triangle (111) planes. In order to understand the structural transformation process upon CO2 adsorption, the snapshots and the energies during the geometric relaxation processes of cuboctaheral Cu42Ni13 clusters at T2−T3 site (−OcO−), decahedral Cu42Ni13 clusters 255



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Figure 6. Snapshots and energies during the structural transformation processes of (a) cuboctaheral Cu42Ni13 clusters at T2−T3 site (−OcO−), (b) decahedral Cu42Ni13 clusters at T2−T4 site (−OcO−), and (c) icosahedral Cu42Ni13 clusters at T1−T2 site (−OcO−) upon CO2 adsorption.

at T2−T4 site (−OcO−), and icosahedral Cu42Ni13 clusters at T1−T2 site (−OcO−) upon CO2 adsorption are shown in Figure 6. As shown in Figure 6a, the cuboctaheral configuration is formed by several triangle (111) and square (100) surfaces.

Upon CO2 adsorption, the square (100) surface transforms into the triangle (111) one through stretch and is not in the same plane. Finally, the cuboctaheral configuration is transformed into the icosahedral one upon CO2 adsorption. During the 256



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transformation process, the absolute value of the energy increases, meaning that the cuboctaheral structure becomes more stable icosahedral cluster during the geometric relaxation upon CO2 adsorption. As shown in Figure 6b, the middle row atoms in square surface of decahedral structure appear as dislocation, expanding outward continuously, and finally form an icosahedron-like structure. Although the absolute value of the energy increases gradually during the transformation process, the final absolute value is lower than that of the icosahedral cluster, indicating that that the decahedral structure is transformed into an icosahedron-like structure. As shown in Figure 6c, the icosahedral cluster remains unchanged upon CO2 adsorption. Accordingly, the absolute value of the energy remains almost unchanged. It can be concluded that the occurrence of the structure transformation from the cuboctaheral and decahedral clusters to the icosahedral cluster is attributed into the stability of the clusters, which follows the order Cubo < Dec < Ico.

4. CONCLUSIONS In summary, the adsorption properties of CO2 on Cu55, Cu54Ni1, and Cu42Ni13 clusters with highly symmetric cuboctaheral (Cubo), decahedral (Dec), and icosahedral (Ico) structures are investigated by density functional theory (DFT) calculations. It is found that composition can affect the adsorption energy of CO2, and icosahedral Cu42Ni13 cluster exhibits the strongest CO2 adsorption ability with all the −Coo, −Oco, and −OcO− adsorption models, compared to the Cu55 and Cu54Ni1 clusters. In addition, the cuboctaheral and decahedral clusters can be transformed into the more stable icosahedral cluster upon CO2 adsorption, which is due to the fact that the stability of the cluster follows the order Cubo < Dec < Ico. Our results can provide useful insights for the design and development of Cu−Ni bimetallic clusters that are used as catalysts for methanol synthesis via CO2 hydrogenation.

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AUTHOR INFORMATION

Corresponding Author

*Tel. +86-10-64453523; fax +86-10-64427616; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21106003, 91334203), Beijing Novel Program (Z12111000250000), “Chemical Grid Project” of BUCT, and Supercomputing Center of Chinese Academy of Sciences (SCCAS).

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