DFT calculation of oxygen adsorption on a core-single shell ZnNb catalyst

Abstract

Oxygen adsorption onto Pt, Nb, and core-single shell ZnNb particles is studied using ab initio molecular orbital (MO) calculations to realize their ability for the oxygen reduction reaction (ORR). The calculations demonstrate that the electronic state of oxygen adsorption on core-single shell ZnNb is very close to that on Pt, rather than that on Nb. The energy levels of the oxygen 2pσ* orbital and the antibonding orbital formed by the oxygen 2pπ* and metal d orbitals are different depending on the type of catalyst. In contrast, the energy level of the Nb–O π* (O₂ 2pπ* + Nb 4d antibonding) orbital of the core-single shell ZnNb is located at a position close to that of Pt. Our finding suggests that the oxygen adsorbed on the core-single shell ZnNb particle can easily be dissociated and desorbed owing to the disappearance of the interaction between the oxygen and Nb subsurface atoms through the replacement of Zn. We expect that core-single shell ZnNb can be utilized as an efficient and cost-effective catalyst for the ORR in a polymer electrolyte fuel cell.

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Introduction

The polymer electrolyte fuel cell (PEFC) is a promising power generation system, which utilizes the electrochemical reactions of hydrogen and oxygen at the two-side electrodes. Owing to its low operation temperature, high-energy conversion efficiency, and non-toxicity, the fuel cell is expected to be employed in vehicle systems and cutting-edge technological devices.^1,2 The oxygen reduction reaction (ORR) occurring at the cathode is one of the essential factors that significantly affects the overall performance and durability of PEFCs.³ Generally speaking, Pt-based materials have recently been recognized to be the most efficient and best commercial catalysts for the ORR in PEFCs. However, the high price and limited natural resources of Pt are the major technical bottlenecks that block the next step toward the wide commercialization of PEFCs.

To further implement PEFCs for practical applications, enormous research efforts have been intensively devoted to the development of an alternative ORR catalyst with high catalytic activity and durability to replace Pt catalysts. The recent developing trend in ORR catalysts has mainly relied on two strategies: (i) the reduction of the Pt amount through alloying with other metallic elements^4–6 and (ii) the use of non-Pt catalysts.^7–9 Considering the non-Pt catalysts, several families of materials have been explored and proposed as catalyst candidates for the ORR, such as metal oxides,^10–12 transition metal chalcogenides,^13–15 and heteroatom-doped carbons.^16–18 Metal oxides are stable and moderately active under acidic conditions; however, their low electric conductivity is still a crucial problem. In the case of transition metal chalcogenides, Ru-based chalcogenides have been proved to be the most active catalysts among other metal-based chalcogenides. Although Ru-based chalcogenides show excellent ORR activity, the rarity and high price of Ru are the major hurdles for its further development, like Pt. Heteroatom-doped carbon materials (e.g., N, B, P, S, and their mixtures) have been significantly developed over the past decade. However, their ORR performances are still much inferior to Pt and other candidate families. Therefore, the exploration and development of new ideal non-Pt catalysts for the ORR are still very challenging for both the experimental and theoretical aspects of modern PEFC technology.

Theoretically, the development of ORR catalysts has been conducted based on the relation between the adsorption energy of oxygen and the catalysts.¹⁹ It is known that an appropriate oxygen adsorption strength plays a significant role in promoting highly efficient ORR catalysis, because strong adsorption does not induce oxygen desorption, while weak adsorption does not cause oxygen dissociation. Therefore, inappropriate oxygen adsorption on catalysts can result in the undesired generation of hydrogen peroxide, which can decompose the electrolyte and deteriorate the catalysts.²⁰ From the viewpoint of molecular orbital (MO) interactions, the catalytic behavior of the Pt group and its alloys is explained based on the relationship between the adsorption energy and the d-band center. For this reason, the Pt group and its alloy catalysts are frequently tuned based on the d-band center.²¹ However, although the design guidelines for ORR catalysts by using energetics are shown in many calculation studies on non-platinum systems,^22–25 the design guidelines from the viewpoint of orbitals are not discussed in detail.

When oxygen adsorbs on a catalyst, both atoms in the first and second layers have significant influence on the adsorption and desorption of O₂ molecules from the viewpoint of electron transfer and bond formation, which thus affects the ORR process. It is reported that the second layer does not influence the adsorption of carbon monoxide,²⁶ while graphene which is located behind Pt influences the oxygen adsorption in the case of a Pt monoatomic wire on graphene nanoribbon.²⁷ Thus, the effect of the second layer on molecule adsorption is different when the combination of the first layer and second layer elements is different. In this paper, we report the first study of oxygen adsorption onto core-single shell ZnNb particles by focusing on the effect of the subsurface atoms on adsorbents through ab initio MO calculations. Nb was selected as a shell since it has a 4d orbital of a similar shape to the Pt 5d orbital in the vicinity of the Fermi level, while Zn was chosen as a core because of its stable 3d orbital at a low energy level and its 4s orbital around the Fermi level. The electron configuration of Pt is 5d⁹6s¹, which gives an unfilled d orbital in the vicinity of the Fermi level and the energy level is closer to that of the 6s orbital. To realize these features using other elements, we consider that an electronic state composed of Nb and Zn orbitals can be similar to that of Pt. Nb supplies the unfilled d orbital in the vicinity of the Fermi level, and Zn supplies the s orbital around the Fermi level. This combination is expected to lead to a similar catalytic mechanism as that of Pt. For comparison, analysis of oxygen adsorption was also performed on Pt and Nb.

Computational method

All the density functional theory (DFT) calculations were performed with the Gaussian 09 program package.²⁸ Wang and Perdew's function (PW91)²⁹ was applied as an exchange-correlation functional. The basis sets for Pt, Nb, and Zn were utilized using LanL2MB^30–32 and that for O was 6-31G. Clusters composed of 55 atoms were adopted as the catalyst model. After the structural optimization of the cluster, the oxygen molecule on the cluster system was optimized again. The thresholds for the maximum force, the RMS forces, the maximum atomic displacement, and the RMS atomic displacements on all the atoms were 0.000450, 0.000300, 0.001800, and 0.00120 in atomic units, respectively. The total energy difference between the O₂ molecules and the cluster system was used to determine the O₂ adsorption energy onto the selected clusters as follows:

ΔE = E(O₂/cluster) − E(O₂) − E(cluster)

Because of the high catalytic activity of the Pt (111) facet,^33,34 the adsorption on this facet was adopted as a reference. In the case of Nb₅₅ and Nb₄₂Zn₁₃, the same (111) facets were chosen in order to compare with Pt₅₅. For confirmation, we also calculated the oxygen adsorption on other sites, but the results relating to the energy levels of the orbitals for all the systems were largely unaltered compared to the (111) facet. This result indicates that the macroscopic properties of the facet are not important in the case of the cluster model. Thus, in this study, we picked and discussed oxygen adsorption states on the (111) facet. The full width at half maximum for density of states was set to 0.1 eV. Each molecular orbital is composed of the atomic orbitals of oxygen and metal in the oxygen adsorbed system. We selected the orbitals from oxygen and metal to be combined from the orbitals with a mixing contribution from oxygen of more than 2%. Moreover, these orbitals were categorized into metal–O π, metal–O π* and O₂ 2pσ* orbitals from the direction of the orbitals. Mulliken population analysis was also carried out to evaluate electron localization on the atoms and bonds in the catalyst systems.

Results and discussion

Icosahedron Pt₅₅, cuboctahedron Nb₅₅, and cuboctahedron Nb₄₂Zn₁₃ particles were optimized as the catalyst models. The core-single shell ZnNb particle is composed of a 13-atom Zn core covered with a 42-atom Nb single shell. The cluster diameters were obtained by averaging the length of the perpendicular lines through the centers of the clusters. The averaged cluster diameters of Pt₅₅, Nb₅₅, and Nb₄₂Zn₁₃ are 1.07 nm, 1.11 nm, and 1.09 nm, respectively. The oxygen adsorption calculations were performed on the catalyst (111)-surface. Fig. 1 illustrates the optimized structures and the bond distances between oxygen atoms on Pt₅₅, Nb₄₂Zn₁₃, and Nb₅₅. The Pt, Nb, Zn, and O atoms are represented as black, blue, orange and red spheres, respectively. The bond distance between oxygen atoms on Pt₅₅ is estimated to be 1.46 Å, which is nearly similar to that for Nb₄₂Zn₁₃ (1.47 Å). On the other hand, the bond distance between oxygen atoms on Nb₅₅ (1.59 Å) is quite a bit longer than that for Pt and Nb₄₂Zn₁₃. The adsorption energies of oxygen onto Pt₅₅, Nb₅₅, and Nb₄₂Zn₁₃ are −146.5, −296.2, and −284.3 kJ mol⁻¹, respectively. The adsorption energy onto Nb₄₂Zn₁₃ is smaller than that onto Nb₅₅. However, it is still larger than that onto Pt₅₅.

Fig. 1 The structure of O₂ adsorbed on (a) Pt₅₅, (b) Nb₄₂Zn₁₃, and (c) Nb₅₅. The corresponding bond distances for the O₂ molecules are shown in the lower panel.

Fig. 2 shows the density of states (DOS) and the projected density of states (PDOS) of oxygen adsorbed onto Pt₅₅, core-single shell Nb₄₂Zn₁₃, and Nb₅₅ particles. The Fermi energy is set to zero and indicated with a vertical black line. The DOS and PDOS are shown as black and red lines, respectively. The PDOS of oxygen is magnified twenty times to illustrate the differences easily. Oxygen adsorption is induced by the interaction between the 2pπ* unpaired electron in the oxygen molecule and the d orbital of the metal. Notably, the appearance of a metal–O π (O₂ 2pπ* + metal d bonding orbital) and metal–O π* (O₂ 2pπ* + metal d antibonding orbital) orbital can be seen. The area of the bonding orbital, the area of the antibonding orbital, and the O₂ 2pσ* band are displayed using green, red, and blue colors, respectively. In the case of Pt₅₅, it is found that Pt–O π exists in the occupied orbital and Pt–O π* exists above the Fermi energy in an unoccupied orbital. Moreover, the O₂ 2pσ* band lies at a higher energy level than the unoccupied orbital. On the other hand, in the case of Nb₅₅, although Nb–O π exists in the same occupied orbital as Pt₅₅, the O₂ 2pσ* band lies in the vicinity of the Fermi level and Nb–O π* is located at a higher energy level than the O₂ 2pσ* band. This is possibly due to the stabilization of the Nb–O π bonding orbital, leading to a longer distance between oxygen atoms (Fig. 1).

Fig. 2 Density of states (DOS) and projected density of states (PDOS) of oxygen for (a) O₂/Pt₅₅, (b) O₂/Nb₄₂Zn₁₃, and (c) O₂/Nb₅₅. The bonding orbital area formed by O₂ 2pπ* and metal, the antibonding orbital area formed by O₂ 2pπ* and metal, and the O₂ 2pσ* band area are colored green, red and blue, respectively. The DOS of oxygen is magnified in order to show the interactions.

The electronic state of the antibonding orbital is significant for oxygen dissociation and desorption. In ORR electrocatalysis in acidic media, oxygen receives four electrons after adsorption on the catalyst, to produce water (O₂ + 4H⁺ + 4e⁻ → 2H₂O). Subsequently, the oxygen molecule absorbed on the catalyst is dissociated, and the two oxygen atoms are desorbed from the surface. In fact, four electrons move to the oxygen adsorbed on the metal from the electrode when the electrode potential increases, and in this case the lowest energy orbital in the unoccupied orbital accepts these electrons. In the case of Pt₅₅, Pt–O π* (O₂ 2pπ* + Pt 5d antibonding orbital) just above the Fermi energy gets the electrons, so that both the O–O and Pt–O bonds are broken, and the reaction proceeds through these electron transfers. In contrast, in the case of Nb₅₅, the orbital above the Fermi energy is the O₂ 2pσ* band. Although the O₂ 2pσ* band receives the electrons, only the O–O bond is broken, and the oxygen does not desorb from Nb₅₅. Therefore, the energy level of the metal–O π* (O₂ 2pπ* + metal d antibonding orbital) and O₂ 2pσ* bands are very crucial factors in promoting the ORR. As can be seen from the PDOS of Nb₄₂Zn₁₃ in Fig. 2(b), the Nb–O π* band is located at a lower energy level than the O₂ 2pσ* band. Importantly, this electronic behavior is the same as for Pt₅₅ but is different from Nb₅₅. Another important difference that should be noted here is that the Nb–O π* band is located at a lower energy compared to Nb₅₅. The adsorption energies of Nb₅₅ and Nb₄₂Zn₁₃ are almost the same. By contrast, the molecular orbital formation is significantly different. The energy level of the metal–O π* orbital of Nb₄₂Zn₁₃ is lower than that of the O₂ 2pσ* band. The formation of the molecular orbitals in Nb₄₂Zn₁₃ resembles Pt₅₅, and the formation of the molecular orbitals in Nb₅₅ has no resemblance. From this result, it can be suggested that the oxygen adsorbed states on core-single shell Nb₄₂Zn₁₃ are very close to those on Pt₅₅, thereby leading to easy oxygen dissociation and desorption.

Furthermore, the number of electrons in oxygen–metal bonds can be calculated using Mulliken population analysis. Fig. 3(a)–(f) show the top and side views of the oxygen adsorption structures extracted from two-layer surfaces of Nb₅₅, Nb₄₂Zn₁₃, and Pt₅₅. Oxygen atoms, Nb atoms on the surface, Nb atoms on the subsurface, Zn atoms on the subsurface, and Pt atoms on the surface are represented as red, blue, black, orange, and gray spheres, respectively. Fig. 3(g)–(i) represent the main bonds with the corresponding allocated number of electrons. The electrons are assigned to the bonds between the atoms using Mulliken population analysis. Therefore, the large value indicates a stronger bond and the small value indicates a weaker bond and/or the larger effect of the repulsive force among nuclei. The allocated number of electrons shows that the oxygen atom does not only form a bond to the Nb surface, but also to the Nb subsurface. These molecular orbitals are composed of the 2p orbital of O and the 4d orbital of the Nb surface, and the 2s orbital of O and the 5p orbital of the Nb subsurface. The Nb–Nb bond consists of s–s and d–d bonds. Also, considering the location of the oxygen molecule, the oxygen molecule on Nb₅₅ slightly moves to a bridge site from a top site because of the interaction with the Nb subsurface. In contrast, in the case of the core-single shell Nb₄₂Zn₁₃, the allocated number of electrons between O and Zn exhibits negative values, indicating an increase in the repulsive force, meaning that the oxygen atom forms a bond to only the surface layer of Nb in the classical aspect. The Nb–Zn bond is formed through a σ bond composed of the s and d orbitals of Nb and the s and p orbitals of Zn.

Fig. 3 Oxygen adsorption structures for Nb₅₅, Nb₄₂Zn₁₃ and Pt₅₅: (a) top view of O₂ on Nb₅₅, (b) top view of O₂ on Nb₄₂Zn₁₃, (c) top view of O₂ on Pt₅₅, (d) side view of O₂ on Nb₅₅, (e) side view of O₂ on Nb₄₂Zn₁₃, (f) side view of O₂ on Pt₅₅, and (g)–(i) the number of electrons on each bond evaluated using Mulliken population analysis.

For comparison, the oxygen adsorption on Pt₅₅ was also investigated and discussed. When oxygen adsorbs onto the Pt surface, oxygen forms a bond only to the Pt₅₅ surface. That is, the Pt subsurface does not directly contribute to oxygen adsorption like in the case of the core-single shell Nb₄₂Zn₁₃ as discussed above. The Pt–Pt bond was mainly a σ bond composed of s orbitals, and the d orbital of Pt was left on each atom. These states agree with previously reported papers.³⁵

The difference mentioned above between Nb₅₅ and Nb₄₂Zn₁₃ relating to oxygen adsorption is caused by the electronic state difference between the elements in the second layer. The Nb atom has unfulfilled 4d, 5s, and 5p atomic orbitals around the Fermi energy, whereas the Zn atom has occupied 4s and unoccupied 4p atomic orbitals around the Fermi energy because its 3d orbital shell is closed, and consequently the energy levels become lower. When Nb exists in the subsurface, the unoccupied 5p orbital of Nb can interact with the occupied 2s orbital of O through the first layer. Oppositely, Zn cannot form bonds with O because the 4s and 4p orbitals of Zn contribute to the bonds with Nb. Therefore, when oxygen adsorbs on Nb₄₂Zn₁₃, the energy level of the bonding orbital between O and Nb becomes higher, resulting in the lower energy level of the antibonding orbital. In other words, the Zn subsurface avoids the formation of a Nb–O bond. On the other hand, the high energy level of the O₂ 2pπ* + metal d antibonding orbital results from the high energy level of the d orbital in Nb. Thus, if elements with a lower energy level d orbital comprise the single shell in a core-single shell structure, the energy level of the O₂ 2pπ* + metal d antibonding orbital will become lower.

Conclusions

Oxygen adsorption onto Pt₅₅, Nb₅₅, and core-single shell Nb₄₂Zn₁₃ particles was investigated and compared using ab initio MO calculations. It was found that oxygen adsorption on a core-single shell Nb₄₂Zn₁₃ particle has almost the same behavior as on Pt₅₅. In the case of oxygen adsorption on Nb₅₅, the Nb subsurface has a strong influence on the formation of an Nb–O bond, which results in a stronger Nb–O bond as compared to Pt₅₅ and core-single shell Nb₄₂Zn₁₃. However, in core-single shell Nb₄₂Zn₁₃, the energy levels of metal–O π (O₂ 2pπ* + metal d bonding orbital) and metal–O π* (O₂ 2pπ* + metal d antibonding orbital) become closer to the Fermi level, the same as for oxygen adsorption on Pt₅₅. The existence of a Zn subsurface can suppress the bond formation between O and the Nb surface in the Nb₄₂Zn₁₃ catalyst, resulting in more favorable O₂ dissociation and desorption. Our calculations have demonstrated that a core-single shell Zn₄₂Nb₁₃ particle can potentially be an efficient and cost-effective ORR catalyst in PEFCs because of its similar oxygen adsorption and desorption behavior as Pt catalysts. Furthermore, we also provide specific guidelines to design rationally non-Pt catalysts based on the core-single shell structure by choosing closed d orbital shell elements as subsurface atoms.

Acknowledgements

This research was supported by Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST), Grant-in-Aid for Challenging Exploratory Research of the Japan Society for the Promotion of Science (JSPS) and Grant-in-Aid for Research Fellows of the Japan Society for the Promotion of Science (JSPS Research Fellow).

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