English

• 1.   Introduction

  Catalyst is the key to improving chemical reactions, which can be found in most industrial chemical processes. With the rapid development of the chemical and energy industry, finding the new catalysts with high activity^{[1, 2]}, selectivity^{[3, 4]}, and stability^{[5, 6]}becomes the focus of research. As one of the scarcest noble metals on Earth, Pt is the industrial catalyst with excellent performance, which has broad application prospect in fuel cell and electrocatalysis^[7~10]. However, due to its high cost, Pt metal cannot be heavily used in industrial production, and the Pt catalyst is easily poisoned by reacting with sulfide or carbon monoxide^{[11, 12]}.

  A number of methods have been explored to improve the Pt metal utilization and catalytic activities of Pt-based catalysts. Among them, doping heteroatom into Pt catalysts has been proved to be an effective way. The influence of heteroatoms seems to obey the Atom-Realm (AR) effect^{[13, 14]}, which reveals that the doped heteroatoms can produce an environmental effect and alter the electronic properties of surrounding atoms, resulting in the regulation of the catalytic activity. The AR effect is more remarkable on atomically dispersed 2D nanomaterials, which provides new strategies for designing catalysts with high activity and selectivity.

  Based on AR effect, doping transition metals in Pt to make binary alloy is a simple approach. Compared with the pure Pt metal catalyst, the Pt-M (M=Fe, Co, Ni, etc.) alloys not only reduce the cost of catalyst, but also increase the catalytic activity for some reactions. In addition, the doping of transition metals can adjust both the geometric and electronic structures of surface Pt atoms, thereby improving the selectivity of the catalyst with good anti-poisoning ability^[15~18]. In general, the distribution of both elements in a binary alloy is disordered (high-entropy). Using Pt and other transition metals to build the layer-by-layer structure (low-entropy) alloy in atomic-scale can bring more active sites and some unique natures to the catalyst. Using the atomic layer deposition, Steven et al. have successfully prepared a layer-by-layer structure of Ir-Pt alloy films, and grew the Ir-Pt alloy film on high-aspect-ratio trenches^[19]. Xie et al. reported a new catalyst towards oxygen reduction with high activity, stability and lower cost by depositing Pt on Pd nanocubes^[20].

  Compared with experiments, theoretical simulations can quickly analyze the interaction between catalysts and adsorbates, thus reveal the catalytic mechanism^{[21, 22]}, and even predict new feasible catalysts^{[23, 24]}. In this work, based on the face-centered cubic (FCC) Pt and L1₀-ordered FePt structures, we built the (001) surfaces of pure Pt and Pt-Fe alloy, which have the same uppermost layer. As the adsorption and dissociation of oxygen are two key steps in oxygen reduction reaction^[25], we studied the adsorption of O₂ molecules on both surfaces and found that the introduction of Fe layers obviously weakens the adsorption of O₂ molecules without affecting their dissociation.

  2.   Computational Methods

  All calculations were performed with Vienna Ab Initio Simulation Package (VASP) based on the density functional theory (DFT)^{[26, 27]}.We used the generalized gradient approximation (GGA) within Perdew-Burke- Ernzerhof (PBE) functional to describe the exchange-correlation energy and the projector augmented wave (PAW) method for the pseudopotentials^[28~30]. The plane-wave energy cutoff was fixed at 400 eV. The van der Waals (vdW) interactions were also involved by using the DFT-D3 scheme of Grimme^[31]. To calculate the energy and the projected density of states (PDOS) of an isolated O₂ molecule, we put one O₂ molecule in a cubic box of 10×10×10ų, and used a 1×1×1 k-point grid for the first Brillouin zone sampling. The 3×3 slabs of Pt(001) and L1₀-FePt(001) were modeled with 5-layer-thick. And we used a 15 Å vacuum to separate periodic images. For structural relaxation, the 3×3×1 k-point grids for the first Brillouin zone sampling was used, and the bottom three layers were fixed, thus the top two layers and the adsorbates were allowed to relax fully until reaching the force convergence (10^-2eV/Å). As shown in Fig. 1 (a, b), except that the Pt atoms in the second and fourth layers are replaced by Fe atoms, the others are the same as Pt slab. A much larger k-point grid (5×5×1) was used in the PDOS calculations. The details of the investigation will be discussed below.

  Figure 1

  

  图 1.  (a)和(b)分别表示纯铂和铂铁合金模型的俯视和侧视图；图(c)为氧气在铂铁合金表面的吸附构型；图中的蓝球、紫球、红球分别表示铂、铁、氧原子

  Figure 1.  Top and side views of the (a) pure Pt and (b) Pt-Fe alloy. (c) Top view of O₂ molecular configuration on Pt-Fe. Here, blue, violet, and red balls symbolize Pt, Fe, and O atoms, respectively

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  3.   Results and Discussion

  All the possible O₂ adsorption sites were studied as shown in Fig. 1 (c). In order to easily describe the O₂ adsorption site, we referred to some papers^{[32, 33]}, and indicated the top site, bridge site and the hollow site by "t, b, h", respectively. For example, the t-h-t means that two O atoms are located on the top of two Pt atoms, the center of mass (cm) of the oxygen is on the hollow site between two Pt atoms, and the O-O bond is parallel to the surface. In this work, we also considered the configurations of O-O bond perpendicular to the surface. After structural relaxation, we found that all the perpendicular configurations are not stable and will lie down quickly. For parallel configurations, only the t-b-t is stable in both slabs, in which O₂ holds its molecular configuration. Therefore, we only considered this configuration for further study.

  Tab. 1 lists some calculated data on the t-b-t adsorption site of those two systems. Firstly, we calculated the adsorption energy of O₂ on Pt(001) and L1₀-FePt(001) surfaces based on the following equation:

  Table 1

  

  表 1  d_Pt-O, d_O-O以及z分别表示铂氧、氧氧键的键长以及氧气分子距吸附表面的高度

  Table 1.  d_Pt-O, d_O-O, and z represent the Pt-O, O-O bond length, and the vertical distance of the O₂ molecule from two metal surfaces, respectively

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  Slab	dPt-O/Å	dO-O/Å	Z/Å	Ead/eV	ΔCPt/e	M/μB
  Pt	1.97	1.37	1.80	-1.41	0.14	0
  Pt-Fe alloy	2.06	1.37	1.94	-0.82	0.45	0.38
  Ead is the adsorption energy of O2 molecule on metal surfaces. ΔCPt and m are the charge transfer and magnetic moment of a surface Pt atom.

  $ {E_{{\rm{ad}}}} = {E_{{\rm{total}}}} - {E_{{\rm{slab}}}} - {E_{{\rm{adsorbate}}}} $

  (1)

  Where the E_total and E_slab is the energy of the system with and without adsorbate, respectively, and E_adsorbate is the energy of an isolated O₂ molecule. The calculated adsorption energies were shown in Tab. 1. Obviously, the adsorption of O₂ on Pt(001) surface is stronger than that on FePt(001) surface (-1.41 eV vs -0.82 eV). In accordance with the adsorption energy, the bond length of Pt-O on FePt(001) surface is longer than that on Pt(001) surface (2.06 Å vs 1.97 Å), and we also noted that the height of the O₂ molecule from the slab surface is higher for Pt-Fe alloy (1.94 Å vs 1.80 Å) meaning that the adsorption strength is markedly reduced on Pt-Fe alloy. However, the d_O-O (1.37 Å) has no difference between both systems. For gaseous O₂ molecule, the bond length is 1.23 Å in our calcul-ation, which is consistent with the experimental value (1.21 Å)^[34]. From simple geometry analysis, we can find out that although the adsorption of O₂ on Pt-Fe alloy is weakened, the catalytic activity for O₂ dissociation still remains. In order to reveal this phenomenon, we need to further analyze the electronic structure of the system and the effect of Fe on Pt.

  As shown in Fig. 2 (a), the 5d orbital of Pt is spin degenerate as the up-spin state is symmetry to the down-spin state. When Fe layers are inserted, the ferromagnetic state of Fe affects the spin state of Pt obviously due to the hybridization of Pt-5d states with Fe-3d states, making the whole system spin non-degenerate (see Fig. 2 (b)). Meanwhile, a net magnetic moment of 0.38 μB could be found on one surface Pt atom (Tab. 1). The d-band center can be used to describe the adsorption strength of transition metal and the adsorbates^[35]. The change of the atomic orbital can be expressed as:

  $ {\varepsilon _{\rm{d}}}{\rm{ = }}\frac{{\int_{ - \infty }^\infty {{{\rm{n}}_d}} \left( \varepsilon \right)\varepsilon d\varepsilon }}{{\int_{ - \infty }^\infty {{{\rm{n}}_d}} \left( \varepsilon \right)d\varepsilon }} $

  Figure 2

  

  图 2.  (a)纯铂体系表面一铂原子的5d轨道的投影态密度图；(b)铂铁合金体系表面一铂原子的5d以及其邻近一铁原子的3d轨道的投影态密度图(图中虚线标明了d带中心的位置)

  Figure 2.  (a) PDOS of the 5d orbital of a surface Pt atom in pure Pt; (b) PDOS of the 5d orbital of a surface Pt atom and the 3d orbital of an adjacent Fe atom in Pt-Fe alloy. The d-band center is also shown by dashed lines

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  Here, n_d(ε) represents the density of states at the energy of ε. In this study, the integral interval of this formula is from -∞ to +∞. With the transition metal ε_d closer to the Fermi level, the interaction between catalysts and adsorbates will become stronger. The d-band center of pure Pt is -1.81 eV, higher than that of Pt-Fe alloy, which is -2.21 eV. It is well consistent with the adsorption strength we obtained above. Actually, the downward shift of d-band center is due to the upward shift of Fermi level. From the Bader charge analysis shown in Tab. 1^[36], we can find out that more electrons are injected into Pt atoms in Pt-Fe alloy than in pure Pt (0.45 e vs 0.14 e), which leads to the relative downward shift of the d-band center.

  Fig. 3(a) shows the PDOS projected onto O atom orbitals (p_y, p_z) in the O₂ molecule. The p_z and p_y orbitals constitute the π and π^* orbitals, and the O-2p states are spin non-degenerate. The O₂-π^* orbitals split into two orbitals with opposite spin orientations near the Fermi level. To observe the overlapping of Pt-d and O-p orbitals before O₂ adsorption, we aligned the vacuum level of Pt and O₂, and showed the PDOS of O-p (actually O₂-π^*) and Pt-d states in Fig. 3 (b)(c). Around the spin up O₂-π^* orbitals (below Fermi level), the distribution of Pt-d orbitals in pure Pt and Pt-Fe alloy is similar. However, above Fermi level, due to the upward shift of the Pt-d orbital caused by Fe doping, more states of Pt-5d appear around the O₂-π^* in Pt-Fe alloy, but in pure Pt, Pt-5d states do not show the same character.

  Figure 3

  

  图 3.  (a)孤立氧气分子的投影态密度图(这里氧气轴线与X坐标轴相平行)；(b)和(c)分别表示纯铂和铂铁合金体系中铂原子的5d轨道以及氧原子的2p轨道的投影态密度图

  Figure 3.  (a) PDOS of O₂ molecule (Here O-O bond is parallel to the x-axis), and PDOS of Pt-5d and O-2p states in pure Pt (b) and Pt-Fe alloy (c)

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  Fig. 4 (a) and (b) show the PDOS of O-2p and Pt-5d states after O₂ adsorption. The apparent hybridization of both orbitals occurs around the Fermi level in pure Pt and Pt-Fe alloy, respectively. We noted that these hybridized states in Pt-Fe alloy are much stronger than in pure Pt, and these hybridized states are composed of p_y and p_zorbitals. In other words, they are composed of O₂-π^* orbitals.

  Figure 4

  

  图 4.  (a)和(b)分别为纯铂以及铂铁合金吸附模型中铂原子5d轨道以及氧原子2p轨道的投影态密度图；局域电荷密度被用来分析铂铁合金体系中表面铂原子与氧气分子所形成化学键的成键状态以及反键状态，蓝色和橙色的矩形分别标记出所选取的能量范围，图中黄色区域表示了电荷的分布，等值面选取了0.02 e/Bohr³

  Figure 4.  (a) and (b) is the Pt/O PDOS of the pure Pt slab and the Pt-Fe alloy slab. We used the local charge distribution to show the O₂-Pt bonding and anti-bonding in Pt-Fe alloy. The selected energy range is marked with the blue and orange rectangles, respectively, and the yellow area represents the charge distribution. The isosurface value is 0.02 e/Bohr³

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  Using the local charge distribution, we analyzed the bond character. First, focusing on the charge distribution near O-O bond, it indeed shows the anti-bonding character. Second, comparing the Pt-O bond, Fig. 4(b) shows the bonding character and anti-bonding character, respectively. Comparing the PDOS of pure Pt and Pt-Fe alloy, the anti-bonding state of O₂-π^* and Pt-5d in Pt-Fe alloy is much stronger than that in pure Pt, and filled by more electrons, so that the O₂ molecule adsorbed on Pt-Fe alloy is much weakened. Meanwhile, more electrons occupy the O₂-π^* orbital, making the O₂ dissociation easy (no change in O-O bond length).

  Till now, we can understand why doping Fe atoms can reduce the O₂ adsorption on Pt without affecting its dissociation. If we consider a complete reaction, such as oxygen reduction reaction (ORR), without changing the dissociation of O₂ molecules, the desorption of adsorbates becomes easier, resulting in faster release of active sites. Thus, we believe that such regulation improves the catalytic activity for O₂ related reactions, which has been confirmed experimentally^[37].

  In Fig. 5, we enlarged the region of afore-mentioned the O₂-π^* and Pt-5d hybridized regions and showed the PDOS of Pt-d_xy, d_yz, d_xz, d_z², and d_x²-y² in the pure Pt and Pt-Fe alloy, respectively. The Pt-dyz has the strongest hybridization with the O₂-π^*, while the remaining four Pt-d states show the weak hybridization. In terms of the spatial distribution of the electron cloud, it is easy to understand why the hybridization of Pt-d_yz and O₂-π^* is so strong, as shown in Fig. 6.

  Figure 5

  

  图 5.  (a)和(b)分别表示在纯铂以及铂铁合金体系中铂原子的d_xy、d_yz、d_xz、d_z2以及d_x2-y2轨道的态密度图

  Figure 5.  (a) and (b) show PDOS of the Pt-d_xy, d_yz, d_xz, d_z2 and d_x2-y2 in the Pt and Pt-Fe alloy, respectively

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  Figure 6

  

  图 6.  铂原子的d_yz以及氧气分子π^*的空间交叠示意图

  Figure 6.  The diagram of the overlap of Pt-dyz and O₂-π^* orbitals

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  4.   Conclusions

  Based on first-principles calculations, we investigated O₂ adsorption on the surfaces of pure Pt and low-entropy Pt-Fe alloy. Compared with pure Pt, the adsorption of O₂ on Pt-Fe alloy was weakened, but the dissociation of O₂ remained unchanged. Based on the PDOS analysis of Pt-5d states in both pure Pt and Pt-Fe alloy, we found that doping Fe atoms greatly changed the Pt-5d orbitals due to the hybridization of Fe-3d and Pt-5d. Some new states were induced above the Fermi level in Pt-Fe alloy, making the hybridization of Pt-d and O₂-π^* stronger. The regulation of Pt d-orbitals leads to increased catalytic activity of O₂. Therefore, this low-entropy alloying strategy provides a new strategy for advanced catalysts.

  
  
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• 

  Figure 1  Top and side views of the (a) pure Pt and (b) Pt-Fe alloy. (c) Top view of O₂ molecular configuration on Pt-Fe. Here, blue, violet, and red balls symbolize Pt, Fe, and O atoms, respectively

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  Figure 2  (a) PDOS of the 5d orbital of a surface Pt atom in pure Pt; (b) PDOS of the 5d orbital of a surface Pt atom and the 3d orbital of an adjacent Fe atom in Pt-Fe alloy. The d-band center is also shown by dashed lines

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  Figure 3  (a) PDOS of O₂ molecule (Here O-O bond is parallel to the x-axis), and PDOS of Pt-5d and O-2p states in pure Pt (b) and Pt-Fe alloy (c)

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  Figure 4  (a) and (b) is the Pt/O PDOS of the pure Pt slab and the Pt-Fe alloy slab. We used the local charge distribution to show the O₂-Pt bonding and anti-bonding in Pt-Fe alloy. The selected energy range is marked with the blue and orange rectangles, respectively, and the yellow area represents the charge distribution. The isosurface value is 0.02 e/Bohr³

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  Figure 5  (a) and (b) show PDOS of the Pt-d_xy, d_yz, d_xz, d_z2 and d_x2-y2 in the Pt and Pt-Fe alloy, respectively

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  Figure 6  The diagram of the overlap of Pt-dyz and O₂-π^* orbitals

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  Table 1.  d_Pt-O, d_O-O, and z represent the Pt-O, O-O bond length, and the vertical distance of the O₂ molecule from two metal surfaces, respectively

  Slab	dPt-O/Å	dO-O/Å	Z/Å	Ead/eV	ΔCPt/e	M/μB
  Pt	1.97	1.37	1.80	-1.41	0.14	0
  Pt-Fe alloy	2.06	1.37	1.94	-0.82	0.45	0.38
  Ead is the adsorption energy of O2 molecule on metal surfaces. ΔCPt and m are the charge transfer and magnetic moment of a surface Pt atom.

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•