Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

Thu Ha, Nguyen Thi; Minh Hue, Van Thi; Trinh, Bui Cong; Ha, Nguyen Ngoc; Cam, Le Minh

doi:https://doi.org/10.1155/2019/4341056

Journal of Chemistry

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Abstract Introduction Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments Supplementary Materials References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 4341056 | https://doi.org/10.1155/2019/4341056

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu₄) and Alumina-Supported Copper Catalyst (Cu₄/Al₂O₃) by means of Density Functional Theory

Nguyen Thi Thu Ha,¹Van Thi Minh Hue,¹Bui Cong Trinh,²Nguyen Ngoc Ha,¹and Le Minh Cam¹

Academic Editor: Pasquale Longo

Received04 Sept 2019

Accepted12 Dec 2019

Published30 Dec 2019

Abstract

The adsorption and activation of carbon dioxide over copper cluster (Cu₄) and copper doped on the alumina support (Cu₄/Al₂O₃) catalytic systems have been investigated by using density functional theory and climbing image nudged elastic band. The adsorption energies, geometrical configurations, and electronic properties are analysed. The results show the strong chemical interaction between the copper cluster and the alumina support. Both the Cu₄ cluster and Cu₄/Al₂O₃ systems have a high adsorption ability for CO₂, and the adsorption process is of chemical nature. The role of the alumina support in the adsorption and activation of CO₂ has been addressed. The calculated results show that the “synergistic effect” between Al₂O₃ and Cu₄ is the key factor in the activation of CO₂.

1. Introduction

The greenhouse effect causing global climate change is now a special concern for all scientists. Capture and utilization of carbon dioxide (CO₂) through its conversion into useful products such as methane, methanol, and hydrocarbon not only benefit economically but also reduce CO₂ emissions into the environment [1–3]. Because CO₂ is thermodynamically very stable, the C–O bond dissociation of CO₂ is not easy and it always requires novel catalytic processes. In CO₂ conversion, the adsorption and activation of CO₂ over catalytic surfaces are crucial steps.

Density functional theory (DFT) today has become an effective tool for studying adsorption and activation of molecular substances on metal oxide surfaces. Therefore, adsorption and activation of CO₂ have been extensively theoretically studied using the DFT method on many simple catalytic models such as flat transition metal surfaces [4–9] and oxide single crystals [10–16]. For example, in the work of Liu et al. [6], adsorption and decomposition of CO₂ on the face (100) of four metals (Fe, Co, Ni, and Cu) in the 3d group element were investigated to understand the intrinsic chemical properties of the 3d elements. Their calculations indicated that all four studied metals are capable of adsorbing and activating CO₂. On the metal’s surfaces, CO₂ chemisorption occurs spontaneously and is thermodynamically favourable. Ding et al. [8] studied the CO₂ adsorption on the Ni (110) surface, and they indicated that CO₂ is weakly adsorbed on Ni (110). Studies on comparing catalytic activities on different faces of a metal surface are also of interest. For example, O’Shea et al. [9] studied the adsorption of CO₂ on Co (100), Co (110), and Co (111) fcc surfaces. Their results showed that the (110) surface exposes the strongest interaction with CO₂, and the activation process involves charge transfer from the metal surface to the CO₂. Adsorption of CO₂ on the surfaces of several cesium oxides (Cs₂O, Cs₂O₂, and CsO₂) have been studied by Tai et al. [17]. By analysing the Mulliken charges, the authors reported there is a decrease on the basicity when going from Cs₂O > Cs₂O₂ > CsO₂.

Transitional metals supported on oxides or porous materials have been shown to be very effective in CO₂ conversion [18–23]. Aluminium oxides (Al₂O₃) are a class of advantaged materials with widespread applications as adsorbents and catalysts [24, 25]. Particularly, γ-Al₂O₃ is one of the most employed supports for heterogeneous industrial catalysts. The assets of alumina include low cost, relatively high surface area, large porosity, and good chemical and thermal stability [26–29]. Moreover, Al₂O₃ also possesses a special surface property, and the simultaneous presence of acid and base centers will certainly affect the ability of the catalytic system to interact with CO₂ [27, 28]. Although many works have been devoted to the study of interaction between CO₂ with different oxides, theoretical studies of using γ-Al₂O₃ as a substrate were not common due to the complexity in its structure [30–33]. Adsorption and protonation of CO₂ on the (100) and (110) partially hydroxylated surfaces of γ-Al₂O₃ have been investigated by Pan et al. [10] using the DFT slab calculations. Their calculated results indicated that, on the partially hydroxylated γ-Al₂O₃ (100) surface, the formation of the bicarbonate species through the reaction between CO₂ and the neighbouring surface hydroxyl is both thermodynamically and kinetically more favourable than that on the partially hydroxylated γ-Al₂O₃ (110) surface. Casarin et al. [30] studied CO₂ adsorption on α-Al₂O₃ using density functional molecular cluster calculations. They found that CO₂ interacts with the α-Al₂O₃ (0001) surface to form a bidentate-chelating carbonate species.

Recently, noble metal clusters have attracted much attention in scientific and technological fields because of their thermodynamic, electronic, optical, and catalytic properties in nanomaterials [34–38]. Among them, size-selected subnanometer Cu clusters have received considerable attention and copper-based catalysts are widely investigated for hydrocarbon productions from CO₂ due to their ability to catalyse CO₂ related reactions [39–42]. Especially, highly dispersed Cu cluster on γ-Al₂O₃ was reported to exhibit high catalytic activity and selectivity for methanol synthesis from CO₂ [43–48]. Therefore, many efforts have been devoted to discover the reaction mechanism and the nature of the active sites in Cu catalysts, including cluster size, oxidation state, and support effects [40–42, 48–50]. It is known that Cu nanoparticles show a better performance for CO₂ electroreduction than bulk Cu. However, the roles of the size and symmetry of the Cu clusters as well as the temperature in the CO₂‐reduction process remain elusive, which hinders the development of advanced catalysts. In the work of Zhao et al. [41], the density functional theory (DFT) method is applied to investigate these factors. They found that the decrease in icosahedron Cu cluster size but the increase in truncated octahedron Cu clusters’ size contributes to the selectivity of CO₂ reduction. However, it was found that, for oxide-supported metal catalysts, the metal/support interface plays an important role in the catalytic activity of the metal, and reaction mechanisms on a supported metal cluster can be quite different from the mechanisms on pure metal catalysts. For example, Padama et al. [42] studied the interaction of CO, O, and CO₂ on the Cu (111) surface and Cu₃ cluster/Cu (111) surface. Their obtained calculations revealed that the Cu atoms in the cluster are more reactive towards adsorption than the Cu atoms of the flat surface. Therefore, the performance of transition metals depends not only on the chemical properties of the metals themselves but also on other factors such as the size of the cluster and the role of the support, and it is necessary to study the characterization of oxide-supported transition metal clusters [47, 51–53]. However, the size effects in catalysis have been well addressed for Cu nanoparticle studies on the effects of the support and of the cluster’s size for supported ultrasmall Cu clusters with a few atoms are still rare [42, 54, 55].

Liu et al. [48] who studied on a subnanometer Cu₄ catalyst have revealed that ultrasmall copper clusters can exhibit extraordinary catalytic activity for methanol synthesis at near atmospheric pressure compared with other, larger size catalysts. Yang et al. [55] studied the catalytic performances of Cu_n (n = 3, 4, 20) supported on Al₂O₃ for CO₂ hydrogenation into CH₃OH both theoretically and experimentally. The authors found that the activity for the methanol product strongly depends on the size of cluster and on the charge transfer interaction between clusters and the support. They observed a strong size dependence in the activity of Cu_n clusters with similar oxidation states at elevated temperatures, and their results indicated that the Cu₄/Al₂O₃ possesses the highest activity for CH₃OH formation. However, their work does not pay attention to the effect of the cluster structure when interacting with the support.

In this paper, we report a study of CO₂ adsorption and activation on the Cu₄ cluster and Cu₄ cluster placed on Al₂O₃ using DFT-based calculations. The reason for choosing the Cu₄ cluster is that, based on the experimental results reported by Yang et al. [55], DFT simulation will provide insights into the geometrical structure of the Cu cluster and the effects of the support of the Cu cluster for CO₂ adsorption and activation. In order to better understand the role of Al₂O₃ in CO₂ conversion and find out the key factors affecting the activity and selectivity of catalysts, we perform the similar calculations of CO₂ interaction with Al₂O₃. The adsorption energies, geometrical configurations, and analysis of the electronic properties are presented.

2. Computational Details

According to Yang et al. [55], among the Cu₃, Cu₄, and Cu₂₀ supported on Al₂O₃ catalysts, the Cu₄/Al₂O₃ system shows the highest turnover rate for methanol production. Therefore, in this study, cluster of Cu₄ was selected to investigate the adsorption behaviors. Two configurations of Cu₄—tetrahedral and rhombus—are investigated to find out the most stable structure. Then, the selected stable cluster will be placed on the alumina carrier (Al₂O₃). The Al₂O₃ (104)-(3 × 3 × 1) was chosen based on the XRD experimental results by Yang et al. [56].

All the geometry and energy calculations were performed using the density functional theory (DFT) approach. The generalized gradient approximation (GGA) with the Perdew, Burke, and Ernzerhof (PBE) nonlocal gradient-corrected functional was employed to estimate the exchange correlation energy [57]. The double zeta basis plus polarization (DZP) orbitals was used for valence electrons, while the core electrons were treated using the norm-conserving pseudopotentials (NCPs) in its fully nonlocal (Kleinman–Bylander) form [58]. The Coulomb potential was expanded in a plane-wave basis with an energy cut-off of 150 Ryd. Spin-polarized calculations have been performed for all systems including metals. For all calculations with alumina supports, the periodic boundary conditions were applied. The sizes of the simulation box were 20.9891 × 14.277 × 16.7703 Å. All equilibrium structures were obtained using the Quasi Newton algorithm with the convergence criteria is that the forces acting on the dynamic atoms are smaller than 0.05 eV/Å. The DFT calculations were performed using the SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) code [59] due to its advantages in robust and accurate aspect.

The adsorption energy (E_ads) was calculated using the following equation:

The adsorption energy is considered as thermodynamic criterion to predict the possibility of the process. If the changes in the geometrical parameters are significant, the analysis will be done to get more inside the nature of the interaction between adsorbate and substrate. Besides, the atomic partial charges, estimated by means of the Voronoi deformation density (VDD) method, were reported. A remarkable charge transfer between atoms may indicate the chemisorption occurs. The VDD method avoids the problems inherent to basis set-based schemes and provides meaningful charges that conform to chemical experience [60]. The Mayer bond orders [61] were used in bonding analysis to describe the chemical interaction (if any) between molecules. The Mayer quantities are close to the corresponding classical values and are less dependent on the basis set choice. The Mayer bond order between two atoms A and B is defined as follows:where P^α and P^β are the density matrices for spin α and β [61].

The transition states were determined using a climbing image nudged elastic band (CI-NEB) method [62]. The total number of configurations that appeared in the reaction coordinates is seven. All forces acting on the dynamic atoms were <0.1 eV/Å. If the adsorption involves a transition state, the activation energy is considered as the difference between the energy of the transition state and the energy of the initial structure. The activation energy (if any) will be considered to estimate kinetics of the process.

3. Results and Discussion

3.1. Electronic Properties of Cu₄ and Cu₄/Al₂O₃ Systems

Copper cluster is considered as an active center, and thus, the stability and the electronic properties of cluster are important. To determine the most stable structure of Cu₄ clusters, the binding energies (E_b = [4E (Cu) − E (Cu₄)]/4) for the tetrahedral and rhombus structures (Figure 1) are calculated and presented in Table 1. The total spin polarization for the optimal structure (Q_up–Q_down, where Q_up and Q_down are the number of electrons with α-spin and β-spin, respectively) is defined using collinear spin-polarized option in the SIESTA code.

(a)

(b)

Since the binding energy of the rhombus structure is higher than that of the tetrahedral one, the former structure is more stable than the latter. The total spin polarization for the tetrahedral structure is two, whereas for the rhombus form, it is zero. This indicates the presence of two unpaired electrons in the tetrahedral cluster, while the rhombus structure has no unpaired electron. It is noted that the isolate copper atom has one unpaired electron. The reduction in the number of unpaired electrons in the clusters compared to that in the copper atoms alone suggested that the formation of copper clusters involves the electron pairings. In other words, the chemical bonds between copper atoms in the cluster have metallic covalent characterizations as well. This suggestion is confirmed by evaluating the geometrical parameters and the population analysis. The minimal distances between copper atoms in the tetrahedral and the rhombus clusters are determined to be 2.341 and 2.355 Å, respectively, which are significantly smaller than twice of the atomic radii of Cu (1.35 Å) [63]. The calculated total bond order in the Mayer scale between atoms for the rhombus structure is determined to be 1.819 which is higher than that for the tetrahedral structure (1.288). This result suggests that the covalent bonds (contributed to the electron pairing) in the rhombus are stronger than that in the tetrahedral one. This suggestion is in a good agreement with the calculated binding energies and spin polarization for Cu₄ clusters. Therefore, the rhombus structure is more stable than the tetrahedral one.

In the next step, to evaluate the interaction between the cluster and the support, the rhombus Cu₄ cluster is placed on the Al₂O₃ surface and the Cu₄/Al₂O₃ structure is optimized. The obtained results are quite interesting: in the most stable geometrical configuration, the initial rhombus Cu₄ cluster changed into the tetrahedral structure, as illustrated in Figure 2.

The reason for changing the geometrical structure may lay on the lack of unpaired electrons in the rhombus structure; therefore, in order to be able to interact with the surface oxygen atoms of the alumina support, rhombus has tended to convert into a tetrahedral structure. The total spin polarization (Q_up–Q_down) calculated for the Cu₄/Al₂O₃ structure equals to zero. This result confirms the above suggestion. The total bond order between copper atoms in Cu₄/Al₂O₃ is 1.272, which closes to that in the parent tetrahedral cluster (1.288). A total bond order of 0.659 indicates a strong interaction between copper atoms and oxygen atoms of the support. And this interaction is attributed to the partly electron transfer from the metal clusters to the alumina support. Due to the charge transfer, the VDD charge on the Cu₄ cluster increases from 0 to 1.045 in Cu₄/Al₂O₃. Significant changes in the atomic charges of the oxygen atoms in the alumina which directly bound with the copper atoms are also observed and presented in Table 2.

Analysis of the composition of the molecular orbitals of the Cu₄/Al₂O₃ structure reveals the positive overlap between 3d orbitals (AOs) of copper atoms and 2p orbitals of oxygen atoms, as demonstrated in Figure 3.

The expression of the HOMO-35 molecular orbital of the Cu₄/Al₂O₃ structure is as follows:where the last terms correspond to the AOs of the other atoms.

The positive overlap mainly between p AOs of oxygen in Al₂O₃ and 3d AOs of copper atoms leads to the chemical adsorption of Cu₄ on the alumina support. The calculated projected density of states (PDOSs) for 3d orbitals of copper atoms in Cu₄ clusters as well as in the Cu₄/Al₂O₃ structure is presented in Figure 4.

Figure 4 shows that there is an increase in the density of states which is higher than the Fermi level for 3d orbitals of Cu₄ in the Cu₄/Al₂O₃ system in comparison to that in the Cu₄ clusters. This result indicates the electron transfer from the d orbitals of copper atoms to the oxygen atoms of the alumina support. This finding is in a good agreement with the total bond order (0.659) between copper atoms and oxygen atoms discussed above. Due to the formation of chemical bonds, the adsorption and activation ability of Cu₄/Al₂O₃ for CO₂ are expected to differ from the pristine Cu₄ clusters. To clarify this hypothesis, in the next steps, we investigate the adsorption of CO₂ on the Al₂O₃ support, on the Cu₄ tetrahedral cluster and on the Cu₄/Al₂O₃ systems.

3.2. Adsorption of CO₂ on Al₂O₃ Support

Alumina support is considered as a fairly good adsorbent for CO₂ [64]. Our calculation results have shown that the adsorption of CO₂ on Al₂O₃ releases an amount of 79.31 kJ·mol⁻¹. The adsorption configuration of CO₂ on Al₂O₃ support is presented in Figure 5. When adsorbing on the alumina support, the angle OCO is slightly decreased from 180° to 177.89°. The C–O bond lengths are determined to be 1.168 and 1.192 Å, which are not significantly differ from the C–O bond in the gas phase (1.175 Å). The minimal distance from CO₂ molecule to the Al₂O₃ surface is found to be 2.245 Å, which is much longer than the sum of the covalent radii of O (0.60 Å) and Al (1.25 Å) [59]. Therefore, the adsorption of CO₂ over the alumina surface can be considered as physisorption.

3.3. Adsorption and Activation of CO₂ on Cu₄ Cluster and Cu_4/Al₂O₃ Systems

When carbon dioxide is adsorbed on the Cu₄ cluster or Cu₄/Al₂O₃, there are two possibilities: (i) CO₂ molecule is bound to one copper atom via O atom (d1-perpendicular orientation); (ii) CO₂ molecule is simultaneously bound to two copper atoms via C and O atoms (d2-parallel orientation). There will be four configurations, therefore, for the CO₂ adsorption on the Cu₄ tetrahedral cluster and on the Cu₄/Al₂O₃ (Figure 6). They are named as follows: for Cu₄ cluster: d1a (perpendicular orientation) and d2b (parallel orientation); for Cu₄/Al₂O₃, d1c (perpendicular orientation) and d2d (parallel orientation). The adsorption energies for d1 and d2 configurations are presented in Table 3.

(a)

(b)

(c)

(d)

From the calculated results, one can obtain the following: (i) the d2-configurations are thermodynamically more favourable compared to the d1-configuration due to the more negative adsorption energy; (ii) the CO₂ molecule is more strongly adsorbed by the tetrahedral cluster than by the cluster supported on alumina. The lowest adsorption energy (−150.49 kJ·mol⁻¹) is obtained for d2b configuration. However, to compare the adsorption ability for CO₂ between Cu₄ and Cu₄/Al₂O₃ structures, it is necessary to include the kinetic factor. Thus, in the next step, the CI-NEB calculations have been performed to investigate the kinetics of the CO₂ adsorption process over the Cu₄ cluster and Cu₄/Al₂O₃. The obtained results show that there is a continuous decrease in energy from the initial configuration over five intermediates to the final product on the reaction pathways. It means that the CO₂ adsorption process does not involve a transition state on both the Cu₄ cluster and the Cu₄/Al₂O₃ as well. Figure 7 demonstrates the relative energies of the initial, final, and five configurations during the process of CO₂ adsorption on the tetrahedral Cu₄ cluster.

Because the adsorption process of CO₂ is not governed by the kinetic factors, the adsorption energy (E_ads) can be considered as a criterion to estimate the adsorption ability. Due to the lower E_ads, the Cu₄ tetrahedral cluster is predicted to adsorb CO₂ more effectively than Cu₄/Al₂O₃. To figure out the nature of the interaction during the adsorption process, the changes in geometrical parameters and partial VDD atomic charges on atoms for the d2b and d2d configurations are determined. The results are presented in Table 4.

For CO₂ adsorbed configurations, the OCO angles are bent from 180 in the initial CO₂ gas molecule to 134.82 and 157.33° in the tetrahedral Cu₄ cluster and Cu₄/Al₂O₃, respectively. The C–O bond lengths are significantly elongated. The values of OCO angles and the total charges on the carbon dioxide molecule indicate that CO₂ is more strongly bound to the Cu₄ cluster than to Cu₄/Al₂O₃. In the d2b configuration, an amount of 0.349 e is transferred from the Cu₄ cluster to CO₂, while in the d2d configuration, CO₂ receives only 0.026 e. The total atomic charge on Cu₄ in d2d is determined to be 1.092, whereas the value on the Cu₄ cluster in Cu₄/Al₂O₃ is 1.045. This means the charge transfer from the Cu₄ cluster to CO₂ is more significant than from Cu₄ doped on the alumina support. We further investigate the molecular orbital compositions of the CO₂ gas molecule and CO₂–Cu₄ cluster (d2b) configurations. Figure 8 shows that electrons are transferred from the copper atoms to the LUMO-antibonding -MO of CO₂. Consequently, an elongation of C–O bond lengths is observed (from 1.175 Å to 1.273 Å) due to the decrease in the bond orders of CO₂. Obviously, copper plays an active site to weaken the C–O bonds.

(a)

(b)

When the Cu₄ cluster is placed on Al₂O₃, electrons from the Cu₄ cluster are strongly transferred to the support (the total charge of Cu₄ on Al₂O₃ is +1.045 e), and thus, in the CO₂–Cu₄/Al₂O₃ system (in d2d configuration), CO₂ receives less electrons from Cu₄ compared to the pristine Cu₄ cluster.

3.4. Type of Adsorption

The adsorption of CO₂ on the Cu₄ cluster as well as on the Cu₄/Al₂O₃ system can be considered as chemisorption due to the relative negative adsorption energies and remarked charge transfers from the Cu₄ cluster to the CO₂ molecule. The CO₂ molecule is activated via C and O atoms. The bond orders between the CO₂ molecule and the catalytic system are determined for d2 configurations and presented in Table 5. We also calculated the vertical binding energy C–O (E_bind), which is characterized for the following step:where the subscript (ads) denoted the adsorbed species on the catalytic surface. To calculate E_bind, all atoms are fixed except oxygen atom cleaved.

Clearly, from Table 5, it can be seen that CO₂ molecule is “rigid” over the Cu₄ cluster due to the bond formation with the total bond order of 1.275. Meanwhile, for CO₂–Cu₄/Al₂O₃ system (d2d configuration), the CO₂ molecule is hold by copper atoms more weakly. The total bond order between CO₂ and copper atoms is determined to be 0.34. The difference in the CO₂ adsorption ability of the Cu₄ cluster and Cu₄/Al₂O₃ systems will result in the difference of CO₂ activation efficiency. Obviously, two C–O bonds in the adsorbed CO₂ molecule are not equally activated on the catalytic systems. For the CO₂ activation on Cu₄ (d2b), the bond orders of two C–O bonds are determined to be 1.197 and 1.571, while for the CO₂ activation on Cu₄/Al₂O₃ (d2d), these values are 1.750 and 1.910. The sharp decrease in the C–O bond order of adsorbed CO₂ in d2b configuration compared to that of adsorbed CO₂ in d2d and isolated CO₂ gas molecule is expected to result in the significant decrease in the binding energies. In other words, the vertical C–O binding energies for the CO₂–Cu₄ system (d2b) are predicted to be the lowest. However, it occurs in the opposite manner: E_bind of two oxygen atoms of CO₂ in d2b configuration are calculated to be 669.56 and 696.31 kJ·mol⁻¹ which are although lower than that for the isolated CO₂ gas molecule, but they are higher than that of adsorbed CO₂ in Cu₄/Al₂O₃ (d2d) (648.11 and 632.58 kJ·mol⁻¹). Thus, it could be said that, on the Cu₄/Al₂O₃ system, the CO₂ molecule is more strongly activated and C–O bonds are more easily broken. The obtained calculations for E_bind (C–O) do not conflict with the calculated results for bond orders discussed above. It is noted that the bond orders characterize for the covalent properties of a bond, while the calculated E_bind are the sum of all interactions between atoms including electrostatic forces. Because the distances between adsorbed CO₂ molecule and adsorbents are close to each other, we suggest that the key factor in the electrostatic interaction will be the charge on atoms. The copper atoms in Cu₄/Al₂O₃ have a higher positive charge than that of Cu atoms in the Cu₄ cluster; therefore, the higher electrostatic forces between oxygen atoms of CO₂ and copper atoms in Cu₄/Al₂O₃ system may govern the effectiveness of the CO₂ activation process and result in the decrease of E_bind.

The calculated results suggest that when CO₂ is activated over Cu₄/Al₂O₃ catalyst, E_bind of two C–O bonds are close (648.11; 632.58); thus, the products will contain long carbon chain, whereas, over the pristine Cu₄ cluster, E_bind of two C–O are significantly different; therefore, if one C–O bond of CO₂ is activated, the products will contain one carbon atom such as CH₃OH and HCHO. If both C–O bonds are broken, the products with more carbon atoms will be dominant due to the insertion chain growth mechanism.

4. Conclusions

By using density functional theory (DFT) and CI-NEB methods, we have studied the adsorption and activation of carbon dioxide over copper cluster (Cu₄) and copper cluster doped on the alumina support (Cu₄/Al₂O₃) catalytic systems. Our results have showed that, for the Cu₄ cluster, the rhombus structure is more stable than the tetrahedral one. When being placed on the alumina support, the rhombus structure changed to the tetrahedral due to the strong chemical interaction with the support. The calculated results have also indicated that both the Cu₄ cluster and Cu₄/Al₂O₃ systems possess a high CO₂ adsorption ability. When adsorbing on the Cu₄ cluster as well as on Cu₄/Al₂O₃, the C–O bond orders significantly decreased due to the electron transfer from the copper atoms to the antibonding MO of the CO₂ molecule. During the adsorption and activation of CO₂, the alumina oxide plays not only role of a support but also as an electron acceptor. The later role results in the increasing electrostatic interaction between copper active centers and the negative charged oxygen atoms of CO₂, and thus, the C–O bonds are activated. This “synergistic effect” between Al₂O₃ support and Cu₄ cluster plays an essential role in the activation of CO₂.

Data Availability

The coordinate files (word files) of all optimized structures used to support the findings of this study are included within the supplementary information files.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This research was funded by the NAFOSTED (grant number 104.06-2014.84).

Supplementary Materials

Coordinates of all optimized structures. (Supplementary Materials)

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Copyright © 2019 Nguyen Thi Thu Ha et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Chemistry

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu4) and Alumina-Supported Copper Catalyst (Cu4/Al2O3) by means of Density Functional Theory

Abstract

1. Introduction

2. Computational Details

3. Results and Discussion

3.1. Electronic Properties of Cu4 and Cu4/Al2O3 Systems

3.2. Adsorption of CO2 on Al2O3 Support

3.3. Adsorption and Activation of CO2 on Cu4 Cluster and Cu4/Al2O3 Systems

3.4. Type of Adsorption

4. Conclusions

Data Availability

Conflicts of Interest

Acknowledgments

Supplementary Materials

References

Copyright

Study on the Adsorption and Activation Behaviours of Carbon Dioxide over Copper Cluster (Cu₄) and Alumina-Supported Copper Catalyst (Cu₄/Al₂O₃) by means of Density Functional Theory

3.1. Electronic Properties of Cu₄ and Cu₄/Al₂O₃ Systems

3.2. Adsorption of CO₂ on Al₂O₃ Support

3.3. Adsorption and Activation of CO₂ on Cu₄ Cluster and Cu_4/Al₂O₃ Systems