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Article

Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface

by
Yanlin Yu
1,
Zhiming Liu
1,
Wenxian Huang
2,
Shan Zhou
1,
Zuofu Hu
1 and
Ligen Wang
3,*
1
School of Mathematics and Physics, Jinggangshan University, Ji’an 343009, China
2
School of Architectural Engineering, Jinggangshan University, Ji’an 343009, China
3
General Research Institute for Nonferrous Metals, Beijing 100088, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1407; https://doi.org/10.3390/catal12111407
Submission received: 29 September 2022 / Revised: 31 October 2022 / Accepted: 1 November 2022 / Published: 10 November 2022
(This article belongs to the Special Issue Synthesis and Applications of Copper-Based Catalysts)
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Abstract

:
Surface adsorption and dissociation processes can have a decisive impact on the catalytic properties of metal alloys. We have used density functional theory to investigate the adsorption and dissociation of O2 on Cu-skin Cu3Au(111) surface. The calculated results show that the b-f(h)-b adsorption configuration is the most energetically favorable on the Cu-skin Cu3Au(111) surface. For O2 dissociation, there are two thermodynamically favorable dissociation paths. One path is from b-f-b to two O atoms in hcp sites, and the other path is from b-h-b to two O atoms in fcc sites. Moreover, the stability of O2 adsorption is higher and the dissociation energy barrier of the adsorbed O2 is lower as compared to those on the Cu(111) surface. This theoretical work provides valuable guidance for the practical application of Cu-Au alloys as highly efficient CO oxidation catalysts.

1. Introduction

Bimetal alloys exhibit promising applications in various technologies related to surface science and heterogeneous catalysis [1,2,3]. In particular, they are significant in dominating catalytic reactions, such as selective hydrogenation [4], oxygen reduction [5], synthesis gas reaction [6], and synthesis of vinyl acetate [7]. This is attributed to their specific surface electronic properties affecting the surface adsorption and dissociation processes. Many theoretical studies have shown that the reaction process can have a decisive impact on catalytic performance. For example, Feng et al. [8] surveyed the electronic structures of cobalt-molybdenum bimetallic, arguing that H2 adsorption/dissociation on the alloy surface was a key process determining the catalytic activity. Shin et al. [9] investigated the catalytic properties of Ag-Cu binary alloy in oxygen reduction reactions, and further confirmed that the activation energy of O2 dissociation was a rate-determining process in the catalytic reaction. The study by Liu et al. [10] demonstrated that O2 adsorption and dissociation on the surface of Cu-Au alloys were considered to be key processes to limit the rate of CO oxidation. Therefore, studying the reaction process will provide a significant support for understanding the catalytic reaction mechanism of binary catalysts.
Among the bimetallic systems, copper alloys have received much attention because of their wide application in a large number of chemical reactions including heterogeneous methanol reforming [11], methanol synthesis [12], the synthesis of diamond [13], and the reduction of 4-nitrophenol [14]. In the context of the CO oxidation reaction, the experiments have demonstrated that the Cu alloying with Au element could improve the efficiency of the CO oxidation reaction and promote the reaction toward desired products [15]. This is attributed to the synergistic interaction between Cu and Au, which improves the catalytic performance of the alloy. Although some studies [10,16,17] have indicated that the adsorption and dissociation of O2 were the key processes to limit the rate of CO oxidation on the surface of noble metal catalysts, few studies have been carried out on the adsorption and dissociation of O2 on the surface of Cu-Au catalysts. Obviously, the theoretical calculations can provide necessary help for understanding the reaction processes.
In this work, we perform first-principles calculations to investigate the adsorption and dissociation of O2 on the Cu-skin Cu3Au(111) surface. The calculated results show that the b-f(h)-b adsorption configuration is the most energetically favorable on the Cu-skin Cu3Au(111) surface. For O2 dissociation, there are two thermodynamically favorable dissociation paths. One path is from b-f-b to two O atoms in hcp sites, and the other path is from b-h-b to two O atoms in fcc sites. Moreover, the stability of O2 adsorption is higher and the dissociation energy barrier of the adsorbed O2 is lower as compared to those on the Cu(111) surface. The remainder of the paper is organized as follows. In Section 2, the computational details are described. Section 3 presents the calculated results and discussion. A brief summary is given in Section 4.

2. Computational Methods

The spin-polarized DFT calculations were performed employing the Vienna Ab-initio Simulation Program (VASP) [18,19,20]. We used the projector augmented-wave method (PAW) [21,22] and the Perdew–Burke–Ernzerhof formulation of the generalized-gradient approximation (GGA-PBE) for the exchange correlation functional [23]. The plane-wave cutoff energy was set at 400 eV. The convergence criterion for the electronic self-consistent cycle was fixed to 10−5 eV per supercell and the ionic relaxation loop were set to 0.02 eV/Å. The electric dipole was neglected, and the Brillouin zone was sampled with a 5 × 5 × 1 Monkhorst-Pack [24] k-point mesh for the slab calculations.
In this work, we chose low index (111) surface to simulate the surfaces of pure Cu and CuAu alloy system, mainly because the (111) surface may be the dominant facet [25]. The previous studies have shown that Cu3Au alloys with L12 crystal structure can be widely used in various chemical reactions [26,27,28]. In addition, the catalyst inevitably contacts with air in the process of preparation and practical application, which will lead to oxygen adsorption-induced segregation, forming a copper segregation structure with 100% copper in the topmost surface layer and 50% copper in the second surface layer, which is the so-called Cu-skin structure [28]. Therefore, we employed the Cu-skin Cu3Au(111) alloys to investigate the adsorption and dissociation of O2 on the surface of Cu-Au catalysts. The DFT-lattice constants for Cu3Au and pure Cu were 3.79 and 3.63 Å, respectively, which are consistent with previously reported results [26,29]. Figure 1 shows the slab models for pure Cu(111) and Cu3Au(111) surfaces. We employed a 2 × 2 (111) unit cell for both slab models, which consisted of four atomic layers and six equivalent layers of vacuum. In all slab models, atoms in the top two layers were allowed to relax to the most stable configuration, while atoms in the bottom two layers were fixed to their bulk positions. Adsorbates were adsorbed on the surface of the model.
The adsorption energies ( E ads ) of molecular O2 adsorbed on the pure Cu(111) and Cu-skin Cu3Au(111) surfaces were calculated as following:
E ads , O 2 = E O 2 − slab − E slab − E O 2
In the equation, the first and second terms on the right-hand side correspond to the energies of substrates with and without O2 adsorption, respectively, and the remaining term corresponds to the energy of the O2 molecule. A negative value of E ads indicates that it favors the adsorption of O2 on the metal surface.
The transition states and minimum energy path (MEP) for O2 dissociation on the Cu(111) and Cu-skin Cu3Au(111) surfaces was studied by using the climbing-image nudged elastic band (CI-NEB) method [30,31]. The CI-NEB is a small modification of the NEB method, where the highest energy image is driven to the saddle point (transition state) in an attempt to maximize its energy along the band and minimize it in all other directions. When the algorithm converges, the highest energy images will be at an exact saddle point, so the CI-NEB method is currently the predominant method for finding the transition state and the minimum energy path between known reactants and products. Once a minimum energy path is determined, the transition state is located and the activation energy will be calculated as following:
E a = E b T S − E b P S
In the equation, the E b T S corresponds to the total energy of the transition state and the E b P S corresponds to the total energy of the precursor state. The basis set superposition errors correction for the precursor state in our calculation was ignored because it had little effect on the calculated results.

3. Theoretical Results

3.1. The Adsorption of O2

For Cu-skin Cu3Au(111) surface, there are two kinds of nonequivalent Cu atoms in the topmost layer of the alloy because of the uneven distribution of Cu and Au atoms in the second layer. These two Cu atoms are denoted as Cu1 and Cu2, respectively, as shown in Figure 2a. For the Cu1 atom, it is located at the center of one Au atom and two Cu atoms in the second layer of the surface; while for the Cu2 atom, it is located at the center of one Cu atom and two Au atoms in the second layer of the surface. We can find that the Cu1 atom is about 0.082 Å lower than the Cu2 atom in the vertical position after the surface structure is relaxed. In addition, the Bader [32] analysis shows that Au in the alloy obtains fewer electrons from Cu1 than from Cu2, and the number of electrons is 0.04 and 0.07, respectively. This may be because the amount of Au close to Cu1 is smaller than that of Au close to Cu2. The similar phenomena can also be found in the Pt-skin Pt3Fe [33] and Pt-skin Pt3Cr [34] systems.
Many studies have shown that five types of stable adsorption sites for O2 existed on the low-index (111) surface of transition metals, namely, b-f-b, t-f-b, b-h-b, t-h-b, and t-b-t sites [35]. For Cu-skin CuAu(111) surface, due to the presence of two nonequivalent Cu atoms in the topmost layer of the alloy, there are mainly two different adsorption configurations for each type of adsorption site. We calculated the adsorption energies of O2 on these adsorption sites of Cu(111) and Cu-skin Cu3Au(111), and the stable adsorption configurations and calculated results are shown in Figure 2 and Table 1, respectively. For the Cu(111) surface, we can find that the adsorption energies of O2 on the b-h-b and b-f-b sites are the highest, both of which are −0.76 eV. When O2 is adsorbed at the t-h-b, t-f-b, and t-b-t sites, their adsorption energies are −0.71, −0.71, and −0.50 eV, respectively. This calculation result is consistent with the previous theoretical calculation result [25]. For the Cu-skin Cu3Au(111) surface, the adsorption energies at each adsorption site are very close, and they range from −1.07 to −1.15 eV. Similar to the Cu(111) surface, b-f-b1 and b-h-b1 are the two most stable adsorption configurations with values of −1.14 and −1.15 eV, respectively. In contrast to the Cu(111) surface, the t-b-t adsorption configuration is not stable on the Cu-skin Cu3Au(111) surface. This may be due to the weak interaction between O2 and the substrate [36]. The Bader analysis shows that the adsorption of O2 will cause the Cu on the surface to lose more electrons. A part of these electrons is acquired by Au atoms in the second layer, and the other part is acquired by O2 adsorbed on the surface. Furthermore, O2 gains more electrons from the Cu-skin Cu3Au(111) surface than from the Cu(111) surface, suggesting that the modification of the surface electronic structure by alloying promotes the formation of strong adsorption bonds between O2 and Cu atoms on the alloy surface.

3.2. The Electronic Structure of Cu-Skin Cu3Au Surface

The density of states (DOS) can provide key information for understanding the electronic structure and bonding properties of alloy compounds, especially when it comes to investigating the adsorption properties of metal alloy surfaces, the d-band DOS is an extremely significant reference [37,38]. Figure 3 shows the d-band DOS of the Cu atoms in the outermost layer of alloys and pure metals in the absence of adsorption. From the figure, we can find that compared with the d-band center for Cu on the Cu(111) surface, the d-band center for Cu on the Cu-skin Cu3Au (111) alloy surface is closer to the Fermi level. This similar phenomenon can also be seen in Pt and Pd alloys [39,40]. This phenomenon is attributed to the overlapping of the d-electron density of states for Cu and Au atoms in the alloy, which leads to the re-hybridization of the d-band DOS for Cu, thus promoting the d-band center of copper in the alloy to move closer to the Fermi level [41]. On the basis of the adsorption model established by Hammer and Nørskov [37,38], it is shown that the adsorption strength of the adsorbate on the metal surface is closely related to the d-band center. When the center of the d-band moves to the high-energy region, the adsorption strength is larger. Therefore, the adsorption strength of O2 on the Cu-skin Cu3Au(111) surface should be greater than that on the Cu(111) surface. This result is consistent with our calculation above.

3.3. The Dissociation of O2

Many studies have shown that studying the dissociation process of the adsorbate on the catalyst surface by using DFT calculations required knowledge of the initial and final states of the reaction [35,36,42,43,44]. According to the above calculation results, the oxygen molecule tends to be adsorbed on the b-f(h)-b sites. Therefore, we chose the b-f(h)-b adsorption configuration as the initial state of the reaction. Many studies [35,36,42,43] have also shown that it is impossible for two O atoms to simultaneously adsorb on a set of adjacent hcp and fcc sites after the O-O bond of the precursor is broken; this is because they will be very strongly repulsed, and the two O atoms are more inclined to adsorb on two adjacent hcp or two adjacent fcc sites. Therefore, there are two thermodynamically favorable dissociation paths for O2 dissociation. One path is from b-f-b to 2×hcp (two O atoms adsorb on a set of adjacent hcp sites), and the other path is from b-h-b to 2×fcc (two O atoms adsorb on a set of adjacent fcc sites). For the O2/Cu3Au(111) system, we only studied the dissociation process of the adsorption configurations of b-f-b1 and b-h-b1 with higher adsorption strength, and their dissociation processes are shown in Figure 4a,b, respectively. From the figure, we can find that the dissociation process of the adsorption configuration of b-f-b1 and b-h-b1 is very similar. Early in the dissociation process, they both form a transition state with a low energy barrier (less than 0.1 eV) by stretching the O-O bond. When the O-O bond is stretched to break, the two separated O atoms will be adsorbed on two adjacent stable adsorption sites. This shows that O2 easily dissociates on the Cu-skin Cu3Au(111) surface and forms a stable adsorption structure, which is consistent with other theoretical studies [28,45].
To further understand the dissociation process of O2 on the Cu-skin Cu3Au(111) surface, we also investigated the dissociation process of the O2/Cu(111) system, and the results are shown in Figure 4c,d. It can be seen from the figure that although the dissociation process of the O2/Cu(111) system is very similar to that of the O2/Cu3Au(111) system, the dissociation energy barrier of the O2/Cu(111) system is higher. For example, for the b-h-b and b-f-b states, their dissociation energy barriers are 0.14 and 0.18 eV, respectively, which are consistent with the estimate of 0.08–0.18 eV/O2 by Habraken et al. [46]. This also explains why Cu alloying with the Au element can improve the efficiency of the CO oxidation reaction and promote the reaction toward desired products [15,47].

4. Conclusions

We have used density functional theory to investigate the adsorption and dissociation of O2 on the Cu-skin Cu3Au(111) surface. The calculated results show that the b-f(h)-b adsorption configuration is the most energetically favorable on the Cu-skin Cu3Au(111) surface. For the Cu-skin Cu3Au(111) surface, the adsorption energies at each adsorption site are very close, and they range from −1.07 to −1.15 eV. In contrast to the Cu(111) surface, the t-b-t adsorption configuration is not stable on the Cu-skin Cu3Au(111) surface. This may be due to the weak interaction between O2 and the substrate. The Bader analysis shows that O2 gains more electrons from the Cu-skin Cu3Au(111) surface than from the Cu(111) surface, suggesting that the modification of the surface electronic structure by alloying promotes the formation of strong adsorption bonds between O2 and Cu atoms on the alloy surface. For O2 dissociation, there are two thermodynamically favorable dissociation paths. One path is from b-f-b to 2 × hcp (two O atoms adsorb on a set of adjacent hcp sites), and the other path is from b-h-b to 2 × fcc (two O atoms adsorb on a set of adjacent fcc sites). The dissociation energy barrier of O2 on the Cu-skin Cu3Au(111) surface is lower as compared to those on the Cu(111) surface. This theoretical work provides valuable guidance for the practical application of Cu-Au alloys as highly efficient CO oxidation catalysts.

Author Contributions

Conceptualization, Y.Y. and L.W.; methodology, Y.Y.; software, Z.L.; validation, Z.L., S.Z. and Z.H.; formal analysis, S.Z.; investigation, W.H.; resources, Z.H.; data curation, W.H.; writing—original draft preparation, Y.Y.; writing—review and editing, Y.Y.; visualization, Z.L.; supervision, L.W.; project administration, L.W.; funding acquisition, Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and technology planning project of Jiangxi Provincial Department of Education, grant numbers GJJ180572 and GJJ160733, and the doctoral research project of Jinggangshan university, grant number JZB1815.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01407 g001 550
Figure 1. The slab models consist of (a) Cu-skin Cu3Au(111) surface, (b) pure Cu(111) surface. In the figure, brick-red and gold balls represent Cu and Au atoms, respectively.
Figure 1. The slab models consist of (a) Cu-skin Cu3Au(111) surface, (b) pure Cu(111) surface. In the figure, brick-red and gold balls represent Cu and Au atoms, respectively.
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Figure 2. The top view for optimized configurations of O2 adsorption on Cu-skin Cu3Au(111) surface. The two kinds of nonequivalent Cu atoms (Cu1 and Cu2) are marked in Figure 2a. The brick-red, gold, and red balls represent copper, gold, and oxygen atoms, respectively. For clarity, the topmost atoms on the surface are displayed with the biggest balls and only the top two layers are displayed.
Figure 2. The top view for optimized configurations of O2 adsorption on Cu-skin Cu3Au(111) surface. The two kinds of nonequivalent Cu atoms (Cu1 and Cu2) are marked in Figure 2a. The brick-red, gold, and red balls represent copper, gold, and oxygen atoms, respectively. For clarity, the topmost atoms on the surface are displayed with the biggest balls and only the top two layers are displayed.
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Figure 3. The calculated densities of states (DOS) of the Cu atoms in the Cu-skin Cu3Au(111) and Cu(111) surfaces. The d-band DOS of surface Cu atoms on Cu(111) is presented for comparison.
Figure 3. The calculated densities of states (DOS) of the Cu atoms in the Cu-skin Cu3Au(111) and Cu(111) surfaces. The d-band DOS of surface Cu atoms on Cu(111) is presented for comparison.
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Figure 4. The minimum energy path (MEP) of the dissociation process for: (a) b-f-b1 adsorption state on Cu-skin Cu3Au(111) surface, (b) b-h-b1 adsorption state on Cu-skin Cu3Au(111) surface, (c) b-f-b adsorption state on Cu(111) surface, (d) b-h-b adsorption state on Cu(111) surface. The 2 × fcc (two O occupying a set of neighboring fcc sites) and 2 × hcp (two O occupying a set of neighboring hcp sites) are designated as the dissociation state of b-h-b and b-f-b adsorption configuration, respectively. TS represents the transitions state and the numbers in the brackets represent the energy of each state relative to initial state.
Figure 4. The minimum energy path (MEP) of the dissociation process for: (a) b-f-b1 adsorption state on Cu-skin Cu3Au(111) surface, (b) b-h-b1 adsorption state on Cu-skin Cu3Au(111) surface, (c) b-f-b adsorption state on Cu(111) surface, (d) b-h-b adsorption state on Cu(111) surface. The 2 × fcc (two O occupying a set of neighboring fcc sites) and 2 × hcp (two O occupying a set of neighboring hcp sites) are designated as the dissociation state of b-h-b and b-f-b adsorption configuration, respectively. TS represents the transitions state and the numbers in the brackets represent the energy of each state relative to initial state.
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Table
Table 1. The adsorption energy of O2 (Eads in eV) and the number of electrons obtained by O2 from the surfaces of Cu-skin Cu3Au(111) and Cu(111).
Table 1. The adsorption energy of O2 (Eads in eV) and the number of electrons obtained by O2 from the surfaces of Cu-skin Cu3Au(111) and Cu(111).
Cu3Au(111)Cu(111)
SiteEadsNchgSiteEadsNchg
t-f-b1−1.080.92t-f-b−0.710.87
t-f-b2−1.080.93t-h-b−0.710.87
t-h-b1−1.090.91t-b-t−0.500.70
t-h-b2−1.070.92b-f-b−0.760.94
b-f-b1−1.141.01b-h-b−0.760.94
b-f-b2−1.121.00
b-h-b1−1.151.01
b-h-b2−1.111.00
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Yu, Y.; Liu, Z.; Huang, W.; Zhou, S.; Hu, Z.; Wang, L. Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface. Catalysts 2022, 12, 1407. https://doi.org/10.3390/catal12111407

AMA Style

Yu Y, Liu Z, Huang W, Zhou S, Hu Z, Wang L. Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface. Catalysts. 2022; 12(11):1407. https://doi.org/10.3390/catal12111407

Chicago/Turabian Style

Yu, Yanlin, Zhiming Liu, Wenxian Huang, Shan Zhou, Zuofu Hu, and Ligen Wang. 2022. "Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface" Catalysts 12, no. 11: 1407. https://doi.org/10.3390/catal12111407

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