The stability and catalytic activity of W13@Pt42 core-shell structure

This paper reports a study of the electronic properties, structural stability and catalytic activity of the W13@Pt42 core-shell structure using the First-principles calculations. The degree of corrosion of W13@Pt42 core-shell structure is simulated in acid solutions and through molecular absorption. The absorption energy of OH for this structure is lower than that for Pt55, which inhibits the poison effect of O containing intermediate. Furthermore we present the optimal path of oxygen reduction reaction catalyzed by W13@Pt42. Corresponding to the process of O molecular decomposition, the rate-limiting step of oxygen reduction reaction catalyzed by W13@Pt42 is 0.386 eV, which is lower than that for Pt55 of 0.5 eV. In addition by alloying with W, the core-shell structure reduces the consumption of Pt and enhances the catalytic efficiency, so W13@Pt42 has a promising perspective of industrial application.

surface sites per Pt mass and enhance catalytic activity and durability for ORR. Because we used small model size, i.e., approximately 1 nm, containing 55 atoms, the computation time and cost is acceptable even though we adopt an entire ideal particle model throughout the calculations. In this article, comprehensively considering the catalytic activity and computation cost, we sampled the icosahedron W 13 @Pt 42 core-shell structure with a diameter of approximately 1 nm as an ORR catalyst, whose surface contains twelve vertex Pt v atoms and thirty edge Pt e atoms. It indicates that the icosahedron W 13 @Pt 42 core-shell structure is a promising nanocluster to replace the pure Pt NPs in ORR owing to lower Pt loading, stronger stability and higher catalytic activity.
It is important to emphasize that our work offers only a theoretical prediction of the structural effects on the catalysis properties of the W 13 @Pt 42 core-shell. The influence of ligands is not taken into consideration, which may affect the properties in real conditions. We hope the W 13 @Pt 42 cluster can be verified and developed by experimentalists.

Results and Discussion
The stability of W 13 @Pt 42 . The cubo-octahedron and icosahedron are observed in the nanocatalysts of PEMFCs with 55 atoms 35,36 . Two potential structures of W 13 @Pt 42 are shown in Fig. 1. As the result of our calculation shows, the icosahedron structure has more negative total energy than the cubo-octahedron structure (− 387.24 eV vs − 380. 49 eV). It is also demonstrated that the formation of the core− shell icosahedron configuration plays a decisive role in the stability of nanoalloys with 55 atoms because of the release of strain energy, which favors the formation of nanoalloys with only one species on the surface 16 42 40 and Rh 13 @Pt 42 41 . Thus, we here select an icosahedron core-shell W 13 @Pt 42 cluster as the ORR catalyst, whose surface is assembled with twelve vertex Pt v atoms and thirty edge Pt e atoms. Furthermore, using Equation (1), we calculated the binding stability of W 13 @Pt 42 ; it has a high stability in contrast to Pt 55 (E bind = − 5.45 eV/atom vs − 5.06 eV/atom). Thus, replacing the Pt 55 cluster with the W 13 @Pt 42 core-shell will not weaken the durability of the catalysts. To investigate the stability at room temperature, we carried out the molecular dynamics simulations at 300 K; the results indicate that the thermal stability of the structure is acceptable (the stable structures at T = 0 K and T = 300 K are shown in Figure S1).
The environmental conditions around the NPs, such as in contact with acidic solutions or adsorbing chemical species, will affect the stability and operation of the core-shell catalyst. We will investigate these effects in the following sections.
The dissolution resistance in acidic medium. To confirm the estimation of the stability of W 13 @Pt 42 , using Equations (2) and (4), we calculate the core-shell interaction energies E cs and the Pt 42 shell dissolution potentials U diss (TM 13 @Pt 42 ), as presented in Table 1. The results indicate that U diss and E cs are enhanced compared with TM 13 @Pt 42 (TM = Ni, Co, Fe, Al) and Pt 55 , which have been well studied [37][38][39][40][41] . The corresponding order is W 13 @Pt 42 > Al 13 @Pt 42 > Fe 13 @Pt 42 > Co 13 @Pt 42 > Ni 13 @Pt 42 > Pt 55 . Specifically, the Pt-skin layer that dissolves into the acidic solution is much weaker because there is a stronger binging and charge transfer between a W 13 core and Pt 42 shell. We conclude that the electrochemical stability of W 13 @Pt 42 is favorable to act as an ORR catalyst.  To identify the source of the stability of the core-shell W 13 @Pt 42 in an acid solution, the partial density of states (PDOS) of the W and Pt atoms in W 55 or W 13 @Pt 42 are shown in Fig. 2. From Fig. 2(a), it can be observed that the W-d electrons in the core-shell W 13 @Pt 42 structure distribute more discretely and occupy a larger energy scope compared with those in the W 55 structure. As Fig. 2 Both imply that a tight W-Pt bond has formed. Compared with the hybridization between W-d and Pt-d, the s-d interaction between W and Pt atoms is weak and can be ignored. Therefore, the excellent stability of W 13 @Pt 42 , especially in an acidic medium, is largely attributed to the hybridization between the Pt-d band and the W-d band.
To clarify the relationship between the structure stability and the charge transfer between the W core and Pt shell of W 13 @Pt 42 , the electron density difference is shown in Fig. 3. A sharp increase of the electron density mainly appears at the juncture of Pt and W atoms. It reveals an abundant charge transfer from W to Pt and verifies the existence of strong Pt-W bonds. As we have observed, the distribution of electron density difference is compatible with that of PDOS in Fig. 2.
Adsorbate-induced structure stability test. Existing research shows that one O atom adsorbed on the Co 13 @Pt 42 core-shell structure cannot raise Co to the surface but two O atoms would segregate a single Co atom from the surface 17 ; it is not clear whether that phenomenon also occurs in the W 13 @Pt 42 nanocluster. Once W  atoms are segregated, the structural integrity of the Pt-W nanocluster is seriously degraded because W is more easily dissolved in an acidic solution 42 than Pt within the electrode potential window of PEMFCs.
For the different adsorption sites, as displayed in Fig. 4, we investigate the structure change between the initial and segregated ones in Fig. 5. Using Equation (5), we calculated the segregation energy E seg energy of each adsorbate cluster. After comparing with the E seg of different structures, as shown in Table 2, we find that W atoms can not transfer to the surface if only one O atom is adsorbed. There is a change when the number of O atoms is two, however; E seg energy becomes a negative value, indicating that W atoms would rise to the surface. Moreover, the amount of W atoms that rise to the shell tends to increase when more O atoms are adsorbed. Because the W atoms are more easily dissolved in an acidic solution, the core-shell structure will be corroded. This result indicates that as the number of adsorbed O atoms increases, the stability of the catalyst decreases. Therefore, to prevent this phenomenon, the ORR should properly control the concentration of O atoms.
The kinetics of ORR mechanisms. To further confirm the adsorption energy of W 13 @Pt 42 lower than that for Pt 55 cluster, we consider the adsorption energy of Pt 55 for supplementary purposes. As is shown in Table S1, the adsorption energy of W 13 @Pt 42 for both O and OH is smaller than that for Pt 55 , indicating a better catalytic activity of the core-shell W 13 @Pt 42. Supported and unsupported cluster structures adsorption strength. In particular, anchoring nanocatalysts on C substrates or other supports adds an additional parameter to the electrocatalyst system, as it has a more suitable adsorption energy. Taking this into consideration, the adsorption energy of supported and unsupported core-shell structures on O or OH are listed in Table S1. The supported structures on the pristine graphene or single vacancy graphene are displayed in Figure S2. As a consequence, the adsorption ability of O or OH for supported and unsupported core-shell structures is less different. The stronger interaction appears between the core-shell structure and the support instead of that between the adsorbate and the cluster.
To minimize the computational cost but maintain the scientific accuracy, we focus on the unsupported core-shell W 13 @Pt 42 . We used the fact that the structures of icosahedral Pt-Co NPs are highly symmetric, i.e., all of the twenty (111) facets are symmetrically equivalent. Thus, we are able to only consider the symmetrically independent configurations of the adsorbed O atoms or OH on the surfaces.
The adsorption energies for the Pt atoms localized in the vertex (Pt v ) and edge (Pt e ) sites (Fig. 4) are presented in Table 3. At different sites, such as Pt v or Pt e , the adsorption energies of O and OH are not same. To clarify this phenomenon, the 5d state electronic density of states of Pt atoms in Pt 55 and W 13 @Pt 42 are plotted in Fig. 6. The d-band center of Pt atoms in W 13 @Pt 42 shifts away from E F compared with Pt 55 . Moreover, the d-band center moves towards the lower-energy range from − 2.306 eV of Pt e to − 2.075 eV of Pt v ; this evidence corresponds to the weaker adsorption ability for O and OH of Pt e . It is uncertain whether the ORR mechanism is changed from the presence of the low-coordinated atoms of nanometer size. As described by literatures 43,44 , we derive the effective coordination number (N eff ) to illustrate the effect of W 13 core. In Table 4, the N eff of atoms under different chemical conditions is displayed.
The larger effective coordination number of Pt e (10.5), than Pt v (9.5) and Pt(111) (9) atoms corresponds to a weaker adsorption function, suggesting an increase in coordination number with the decrease in adsorptive strength, as intuitively expected. This is consistent with the interrelation of Pt e and Pt v on the d-band center, as is depicted above. The fact that the d-band center is not entirely predictive of the O and OH adsorption energies suggests that a more careful analysis on the surface electronic structure is necessary to explain the binding of O or OH. Thus, an analysis of Bader charges is performed. Figure 7 displays the Bader charge analysis of adsorbed O and OH. When the O atoms are adsorbed on the H1 site, the electrons first transfer from W to Pt and then converge to O atoms. The calculation indicates that the     The possible elemental reaction steps involved in the ORR which is catalyzed by a W 13 @Pt 42 core-shell structure are shown in Figure S3; the optimal path is displayed in Fig. 8. As is shown in Fig. 8, the rate-limiting step (RDS) of the ORR mechanism is located in the O 2 diffusion into two O atoms, with E a = 0.386 eV, and is lower than that for cluster Pt55 of 0.5 eV 37 . Therefore, the path we present in this paper is more effective. It is well known that a magnitude of E a < 0.75 eV is regarded as a surmountable barrier for the surface reactions at room temperatures 46 . The potential barrier forming OH from H+ O is very low because the adsorption energy difference between O on the bridge and on H1 is small. However, the water and other solutions may play a considerable role in this process.

Conclusions
In summary, the stability of W 13 @Pt 42 core-shell structure and the ORR catalytic mechanism have been studied using first-principles calculations. Replacing the pure Pt cluster with a core-shell structure W 13 @Pt 42 as a cathode catalyst not only lowers the cost but also provides superior stability and catalytic performance. The dissolution resistance and core-shell interaction energies in an acidic medium are the primary parameters for evaluating the durability of a catalyst. Compared with other TM 13 @Pt 42 (TM= Ni, Co, Fe, Al), E cs and U diss of W 13 @Pt 42 are more negative, which indicates better stability. We have measured the structure stability under O atom adsorption; the evidence suggests that the structure could remain stable if the O atoms concentration is limited and suitable. Moreover, we plot the electron density difference and PDOS of the structure. The electron density difference reveals that the good stability of the W 13 @Pt 42 structure is attributed to the abundant charge transfer from core W 13 to shell Pt. Namely, the W 13 @Pt 42 core-shell structure is a good candidate for the ORR catalyst.
Furthermore, the reaction process and reaction barrier of ORR catalyzed by W 13 @Pt 42 have been presented. Better catalytic activity than for nanoclusters is due to the optimal OH formation energy. The weaker adsorption energy of OH prevents the poisoning of the O-containing intermediate. These conditions favor ORR activation at room temperature.
Computational details and method. Our calculations were performed within the density functional theory (DFT) 47,48 framework, in which the generalized gradient approximation (GGA) 49,50 to the exchange-correlation energy functional, as formulated by Perdew, Burke, and Ernzerhof (PBE) 49 , and the interaction potentials of the core electrons are replaced by the projector augmented wave (PAW) 51 pseudopotential, as   implemented in the Vienna ab initio Simulation Package (VASP) code 52,53 . We adopted the PAW method with 6s5d and s1d9 valence electrons for W and Pt atoms, respectively. Kohn-Sham orbitals were expanded by plane waves up to a cut-off energy of 400 eV; ionic and electronic relaxation converged within an error of 1 × 10 −3 eV/atom, and the convergence precision was set to a force of less than 5 × 10 −2 eV/Å. Only the gamma point was used to sample the Brillouin zone of each W 13 @Pt 42 NP and the core-shell structure adsorbed molecule. A smearing of 0.2 eV to the orbital occupation was applied to achieve accurate electronic convergence. All atoms in our model systems were fully relaxed to obtain optimized structures. The integration of the Brillouin-zone was performed using a 2 × 2 × 1 Monkhorst-Pack 54,55 grid with Γ points for the supported metal cluster. For free metal clusters, a rectangular supercell with a size of 30 × 30 × 30 Å 3 was employed in the calculations. For the W 13 @Pt 42 cluster supported on graphene, an orthorhombic supercell of 14.76 × 14.76 × 31.51 Å 3 with periodic boundary conditions was used. The choice of unit cell keeps the W 13 @Pt 42 -graphene system adsorbates approximately 10 Å apart laterally.
To analyze the structural stability of alloy clusters, the average binding energy (E bing ) of a cluster was calculated following: where E cluster , E Pt and E W are the total energies of Pt 55 or W 13 @Pt 42 clusters, Pt atoms, and Al atoms, respectively. N Pt and N W are the numbers of Pt and W atoms in the cluster, respectively. The pure Pt nanostructure or Pt-base core-shell structure is more likely to dissolve when exposed to acidic media; it is significant to the durability of the catalyst. Therefore, to explain the higher stability of W 13 @Pt 42 , the core-shell interaction energy (E cs ) and the dissolution potential of the Pt shell (U diss ) 15,56 were calculated. The interaction energy (E cs ) was given by Equation (2): cs 13 42 42 13 The dissolution potential of the Pt n-m @Pt m cluster shell was calculated using Equation (3) 56 where U diss (Pt bulk ) = 1.188 V 37 , m = m shell = 42, and n = 55.
To determine whether the adsorption of O atoms would transfer W atoms to the surface, we calculated the segregation energy E seg energy of the adsorbate cluster, which was defined as equation (5): where n is the number of O atoms and E seg energy is the segregation energy. The W atoms were located in the core will rise to the shell if the segregation energy (E seg energy ) was negative. And the more negative E seg energy is, the more likely it will appear.
The climbing image nudged elastic band (CINEB) method 57,58 , a tool in the VASP code, is an efficient method for finding the minimum energy paths (MEPs) between a given initial and final states of a transition. For an adsorption process of a molecule, the van der Waals interaction (vdW) has an important effect. In many circumstances, the current vdW-DFT 59,60 is sufficiently accurate and was used to correct the ORR energy barriers. The MEPs for ORR were obtained using NEB tools; we showed the optimized overall reaction path with the smallest potential barrier. For the rate-limiting step (RDS) of ORR, we also considered the influence of the water solvent, and the dielectric constant was set at 80 61 .