Pd (111) with Doped

Apr 3, 2017 - Au is a chemically inert metal, while Os is quite active to react with oxygen. Although Au and Os are in the two extremes in chemical pr...
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Origin of the Enhanced Catalytic Activity of PtM/Pd (111) with Doped Atoms Changing from Chemically Inert Au to Active Os Jun Wang, Dingfang Liu, Li Li,* Xueqiang Qi, Kun Xiong, Wei Ding, Siguo Chen, and Zidong Wei* College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China S Supporting Information *

ABSTRACT: Au is a chemically inert metal, while Os is quite active to react with oxygen. Although Au and Os are in the two extremes in chemical properties, unexpectedly, both of them can enhance the oxygen reduction reaction (ORR) activity of Pt-based alloys. In this work, a systematical density functional theory calculation was used to elucidate the mechanisms of enhanced activity in PtM/Pd with doped atoms changing from chemically inert Au to active Os. The calculations show that different sites on the PtAu/Pd and the PtOs/Pd surface adopt different ORR mechanisms due to the heterogeneous electronic structures, such as uneven surface charge and unequal d-band center. More importantly, all of the ORR steps on the sites far away from the doped atoms in the PtAu/Pd and PtOs/Pd display similar activation energy corresponding to better catalytic activity than the other sites. The catalytic activity is mainly affected by the ligand effect, and a proper distance between the doped atoms and the Pt atoms should induce the catalysts to possess the highest catalytic activity. These results also uncover why the different content of doped atom can lead to the different activity of catalysts. based alloy obviously.19−22 Ma et al.20 reported that the total noble-metal mass-specific activity of Pt@Au/C is 3.1 to 4.9 times higher than that of commercial Pt/C. Zhang et al.21 found that the kinetic current density of Pt0.8Os0.2 monolayer catalyst was more than three times larger than that of pure Pt. Tsai et al.22 also demonstrated that Pt2ML/Os shows 3.5 to 5 times better catalytic activity than Pt/C. Such enhanced ORR activity in Pt-based alloys with different types of heteroatoms, from chemically inert atom Au to active atom Os, is an interesting phenomenon. What roles do the inert and active atoms play in enhancing the activity of Pt-based alloy catalysts? What is the ORR mechanism corresponding to the doped metal in Pt alloy catalysts changing from inert atom to active atom? To tackle the above questions, in this work, Au and Os are chosen as typical inert and active atoms to clarify the origin behind this phenomenon. Herein, density functional theory (DFT) calculations are applied to explore the origin of the enhanced catalytic activity of PtM/Pd ternary catalysts with doped atoms changing from chemically inert Au to active Os. Through a careful comparison of geometric and electronic structure on PtM/Pd surface, adsorption of intermediates, such as O2, O, and OH, and ORR mechanisms, we successfully explore the essential reason for

1. INTRODUCTION Proton exchange membrane fuel cell (PEMFC) is considered to be the most competitive candidate to replace traditional energy conversion devices because of its high efficiency, environmental friendliness, and wide applications.1−3 The cathode catalyst represented by Pt/C, as a key component in PEMFC technology, still suffers from many challenges, such as high cost, unsatisfied activity, and low stability under the corrosive operating conditions.2,4−7 These issues seriously hinder commercial application of fuel cell. Alloying with other transition metals (such as Pd, Au, Os, Fe, Co, Ni, Cu, etc.), Pt-based bimetallic catalysts have shown significantly enhanced ORR activity and stability compared with pure Pt catalyst.8−11 In particular, PtPd alloy with special Pt@Pd core− shell is considered to be a very promising catalyst for ORR.12 Although PtM bimetallic catalysts have possessed excellent ORR activity from the view of previous experimental and theoretical studies, they still dissatisfy the demand of commercial usage of catalysts. To further enhance the ORR activity and reduce the cost of catalysts, Pt-based ternary catalysts with various transition metals were employed due to additional synergistic effects.13−17 Metals in the VIII and the IB are good candidates as the third metal to Pt-based ternary catalysts, such as Co, Ni, Os, and Au.18−22 Au and Os are completely different in chemical properties: Au is chemically inert and highly resistant to react with oxygen, while Os is active in chemical property and most readily reacts with oxygen. Interestingly, both of them can enhance the ORR activity of Pt © XXXX American Chemical Society

Received: February 20, 2017 Revised: March 31, 2017 Published: April 3, 2017 A

DOI: 10.1021/acs.jpcc.7b01624 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Pd (Figure S1) surface were chosen based on surface energies, which are listed in Table S1. The results show that different configurations of PtOs/Pd or PtAu/Pd surface have similar surface energies, regardless of slight 0.002 J/m2 higher for configuration c. It indicates that these three configurations are stable and may exist simultaneously in practical situation. Thus we chose one of the configurations (a) to calculate and explore the effect of the doped atoms on the catalytic activity. Figure 1 also shows the schematic view of the adsorption sites on the (111) surfaces of Pt, Pt/Pd, PtAu/Pd, and PtOs/ Pd. In the work, only bridge adsorption of O2 is considered because the bridge site is the preferential adsorption site to O2 at the low adsorbed coverage.34 In addition, four kinds of adsorption sites to intermediates O and OH, including top site, bridge site, face-centered cubic (fcc) site, and hexagonal close packed (hcp) site, were calculated, and the optimized geometric structures are shown in Figures S2−S4. To assess the adsorption strength of O2 and OH on the catalysts, the adsorption energy was given as Eads = Ecatalyst + Eadsorbate − Etotal, in which Ecatalyst, Eadsorbate, and Etotal correspond to the total energy of bare catalysts, gaseous O2 (O or OH) in vacuum, and catalyst with species adsorbed, respectively. Here the positive Eads represents a stable adsorption state. The dband centers were calculated from the projected electronic density of states (DOS), and the values were referenced to the Fermi level.

enhanced activity for ORR with the doped element varying from chemically inert Au to active Os.

2. COMPUTATIONAL DETAILS AND MODELS In this study, the DFT calculations were performed using the DMol3 software package.23,24 A generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE) was employed to describe electronic exchange and correlation effects.25 DFT semicore pseudopots approximation was utilized with the double numerical plus polarization (DNP) basis set. Monkhorst−Pack meshes with 3 × 3 × 1 k-grid sampling in the surface Brillouin zone were used for structural optimization. The convergence tolerance of energy was taken as 1 × 10−5 Hartree, and the maximum allowed force and displacements were 0.004 hartree per Å and 0.005 Å, respectively. The transition states (TS) were searched with complete linear and quadratic synchronous transit (LST/QST) process.26−28 As shown in Figure 1, the (111) surfaces of Pt, Pt/Pd, PtAu/ Pd, and PtOs/Pd were modeled using a periodic three-layer

3. RESULTS AND DISCUSSION 3.1. Geometric and Electronic Structure. The structures of (111) plane of PtAu/Pd and PtOs/Pd ternary alloy were constructed based on Pt/Pd surface. Geometric structure of these catalyst vary largely depending on types of doped atoms. As shown in Table 1, the Pt/Pd shows a shorter average Pt−Pt bond length than the Pt. Compared with the Pt/Pd, the PtAu/ Pd has a shorter average Pt−Pt bond length, while the PtOs/Pd has a longer average Pt−Pt bond length. The change of the bond length is attributed to the different atomic radius of the doped metal. The order of the atomic radius of doped metal is Au > Pt > Pd > Os. The doped metal Au with larger atomic radius pushes the surface Pt atoms closely, leading to the surface contraction, while the doped metal Os with smaller atomic radius pulls the bonding of Pt−Pt, resulting in the surface expansion. The former is so-called compressive strain effect, and the latter is named tensile strain effect. Figure 2 displays the distributions of Hirshfeld charge of the Pt, Pt/Pd, PtAu/Pd, and the PtOs/Pd surface, which implies the ligand effect on the surface. Compared with the Pt, the toplayer Pt atoms of the Pt/Pd have more negative charge, which is attributed to the electrons transfer from the substrate Pd atoms to the top-layer Pt atoms. After doping, the negative charge of the top-layer Pt atoms on the PtAu/Pd is more than that on the PtOs/Pd. These changes result from the different electronic configurations of the Au and Os atoms. The d orbital of Au ([Xe] 4f145d106s1) is filled with electrons, which makes it

Figure 1. Optimized geometric structures of (a) Pt, (b) Pt/Pd, (c) PtAu/Pd, and (d) PtOs/Pd (dark blue: Pt; light blue: Pd; yellow: Au; pink: Os) and (a) the schematic view of adsorption sites on Pt and Pt/ Pd, (c) the fcc (f-I, II, III), hcp (h-I, II, III), and top (t-I, II, III) adsorption sites on PtAu/Pd and PtOs/Pd, and (d) the bridge (b-I, II, III, IV, V) adsorption sites on PtAu/Pd and PtOs/Pd.

slab with a p(3 × 3) unit cell, which is a reasonable compromise between accuracy and computational efficiency.29−31 The vacuum region between the slabs is 12 Å, which is sufficiently large to ensure that the interactions between repeated slabs in the direct normal to the surface are negligible. In the calculations, the atoms in the top two layers and all adsorbates were allowed to be fully relaxed, whereas those in the bottom layer were fixed at their bulk-truncated structure. The lattice constant for bulk platinum is optimized to 3.92 Å, and that for bulk palladium is optimized to 3.89 Å. The Pt/M (M = Au, Os) ratio on the surface is 7/2 ML, which was selected according to the experimental data.32,33 The specific configurations of PtM/

Table 1. Average Bond Length, Hirshfeld Charge and d Band Center of Pt, Pt/Pd, PtAu/Pd, and PtOs/Pd, Respectively

a

parameters

Pt

Pt/Pd

PtAu/Pd

PtOs/Pd

PtAu/Pd (Pt)a

PtOs/Pd (Pt)a

average bond length (Å) Hirshfeld charge (e) d band center (eV)

2.775 −0.013 −2.371

2.751 −0.072 −2.311

2.774 −0.066 −2.560

2.730 −0.082 −2.362

2.741 −0.083 −2.308

2.771 −0.063 −2.444

PtAu/Pd (Pt) and PtOs/Pd (Pt) represent the surface Pt atoms on the PtAu/Pd and PtOs/Pd. B

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binding energy of adsorbates,38 we can speculate that the toplayer Pt atoms on the PtAu/Pd bind adsorbates a bit stronger than those on the PtOs/Pd. However, the Os atoms with a more positive εd are the strongest binding sites, and the Au atoms with a more negative εd are the weakest binding sites. The two opposite properties of the top-layer Pt atoms and the doped atoms would play different roles in the catalysis of ORR. 3.2. Adsorption of ORR Intermediates. Figure 4 displays the adsorption strength of O2 on the four catalysts (details are

Figure 2. Distributions of Hirshfeld charge on (a) Pt, (b) Pt/Pd, (c) PtAu/Pd, and (d) PtOs/Pd, respectively.

difficult to receive any other electrons. Then, the top-layer Pt atoms on the PtAu/Pd act as the acceptor to obtain the excess electrons from the substrate Pd atoms. Conversely, the d orbital of Os ([Xe] 4f145d66s2) is not filled and can receive extra electrons from the neighbor atoms, inducing the less negative charge of the neighbor Pt atoms. The results are also proved by the deformation charge densities (Figure S5), in which the Os atoms have more electron accumulation (blue). The d-band center (εd), an important parameter determining the surface electronic properties, depends on both the geometric effect (surface strain) and the surface electron (ligand effect).35,36 In general, compressive strain and less surface electron tend to downshift εd in energy, whereas tensile strain and more surface electron have the opposite effect.37 Figure 3a,b shows that the εd of Pt/Pd shifts up slightly

Figure 4. Distributions of the adsorption energy of O2(EO2) on (a) Pt, (b) Pt/Pd, (c) PtAu/Pd, and (d) PtOs/Pd, respectively.

shown in Table S1). For the PtAu/Pd, Au atoms (b-V) have no interaction with O2, while the Pt atoms (b-I) far away from the Au atoms bind O2 much stronger. For the PtOs/Pd, Os atoms (b-V) bind O2 so strong that O−O bond dissociate directly, while the Pt atoms (b-I) far away from the Os atoms exhibit a weaker adsorption energy. The adsorption trend of O2 on these sites agrees well with the electronic structure of catalysts, even considering the effect of the O coverage (Figure S6). However, the cases are different on sites near doped atoms. For instance, O2 bound to b-IV site on PtAu/Pd extremely weakly, with adsorption energy as low as 0.191 eV; in contrast, O2 bound to b-III site on PtOs/Pd tightly with adsorption energy as high as 1.249 eV. These results mean that the Pt atoms near the doped atoms have similar properties to the doped atoms, while the Pt atoms far away from the doped atoms exhibit the opposite properties. The heterogeneous electronic structures on the PtAu/Pd and the PtOs/Pd surface induce the different interaction with the O2. The adsorption of O and OH on these catalysts also shows the same trend as the adsorption of O2. As shown in Figure S7 and Table S3, although the Os atoms bind the OH extremely strong, the Pt atoms on the PtOs/Pd have the lowest interaction with OH compared with the other Pt atoms on the PtAu/Pd, Pt/Pd, and Pt. Additionally, the average OH adsorption energy of the Pt atoms on both the PtOs/Pd and the PtAu/Pd is slightly lower than that on the pure Pt, indicating an easier desorption of OH and a higher catalytic activity.39 3.3. Mechanism of ORR. For the most active catalysts, Pt and PtM alloy, 4e− ORR is generally believed. One type of the active site on the Pt and Pt/Pd is calculated owing to the homogeneous electronic structure, while two types of active sites are considered on the PtAu/Pd and the PtOs/Pd due to the heterogeneous electronic structures, including the weakest

Figure 3. Partial densities of state (PDOS) of Pt and doped atoms on (a) Pt, (b) Pt/Pd, (c) PtAu/Pd, and (d) PtOs/Pd, respectively.

compared with Pt. Although the Pt/Pd exhibits the surface contraction, the more negative charge of Pt on the Pt/Pd dominates the up-shifting of εd. Similarly, the surface electron induces the variation of the εd of the PtAu/Pd and the PtOs/ Pd. Compared with the Pt/Pd, the top-layer Pt atoms of the PtAu/Pd (Figure 3c) have an upshifting εd due to the more negative charge, while the top-layer Pt atoms of the PtOs/Pd (Figure 3d) possess a downshifting εd owing to the less negative charge. This means that the electronic structures of the PtAu/Pd and the PtOs/Pd are mainly affected by the ligand effect. According to the linear relationship between εd and C

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Table 2. Energy Barrier of Elementary Reactions Involved in ORR on Pt, Pt/Pd, PtAu/Pd, and PtOs/Pd, Respectively energy barrier (kcal/mol)

Pt

Pt/Pd

PtAu/Pd−W

PtAu/Pd−S

PtOs/Pd−W

PtOs/Pd−S

O2g → 2Oad O2ad + Had → OOHad OOHad → Oad + OHad Oad + Had → OHad OHad + Had → H2Oad

26.54 9.08 8.65 21.88 10.86

27.16 10.41 10.08 18.90 9.16

33.39 7.70 23.60 10.65 5.81

27.43 9.04 9.03 13.22 10.44

26.12 8.32 10.71 10.58 10.83

4.44

O breaking has the highest barrier energy (26 to 27 kcal/mol), indicating it is a rate-determining step (RDS). After doping, the PtAu/Pd and the PtOs/Pd−W promote the Oad protonation distinctly but cannot improve the O−O breaking, and the PtAu/Pd−W even increase the barrier energy of O−O breaking (33.4 kcal/mol). On the contrary, the PtOs/Pd−S accelerates the O−O breaking significantly, which shows a surprisingly low barrier of 4.44 kcal/mol, and the RDS is changed to the OHad formation step (eqs 3) with barrier of 20 kcal/mol. The comparison among the barrier energies of RDS on the all sites displays that the PtOs/Pd has the lowest barrier energy and shows the highest catalytic activity for dissociative pathway. The associative case is shown in the following steps, and the adsorption of O2 and the formation of OHad and H2Oad go through the same steps in the dissociative pathway.

adsorption site (PtAu/Pd−W and PtOs/Pd−W) and the strongest adsorption site (PtAu/Pd−S and PtOs/Pd−S). The 4e− ORR mechanism includes two possible pathways, that is, dissociative pathway and associative pathway.40−42 The dissociative pathway is listed in the following steps O2,g → O2,ad

(1)

O2,ad → 2Oad

(2)

Oad + Had → OHad

(3)

OHad + Had → H 2Oad

(4)

20.44 11.47

For the metal catalysts, the O2 adsorption (eqs 1) is a spontaneous step without barrier energy. Then, the steps of eqs 2−4 were investigated as described in detail in Table 2 and Figure 5, where the energy variation for the various

O2,g → O2,ad

(1)

O2,ad + Had → OOHad

(2a)

OOHad → Oad + OHad

(2b)

Oad + Had → OHad

(3)

OHad + Had → H 2Oad

(4)

As shown in Figure 5b, the O−OH breaking (eqs 2b) shows a relatively low barrier of 10 kcal/mol, which suggests that the dissociation of O−O bond becomes much easier when reactions go through the associative pathway. Accordingly, the protonation of Oad (eqs 3) with a barrier of 20 kcal/mol becomes the RDS in an associative pathway. Because the PtOs/ Pd−S dissociates the O−OHad bond directly, the associative pathway cannot be occurred on this type of sites. The PtAu/ Pd−W benefits the protonation of O significantly but inhibits the O−OH breaking with a barrier of 23 kcal/mol. Then, the O−OH breaking becomes the RDS on the PtAu/Pd−W. Unlike the above sites, the PtAu/Pd−S and the PtOs/Pd−W not only promote the protonation of O (RDS) but also benefit the O−O breaking. Remarkably, all of the steps on these two type of sites show similar barrier energy of ∼10 kcal/mol, which means that the reaction rates of all steps match each other closely and no RDSs exist. Thus the PtAu/Pd−S and the PtOs/ Pd−W exhibit better ORR performance than other sites under associative pathway. 3.4. Discussion. In general, the PtOs/Pd−S adopts the dissociative pathway as the most favorable pathway for ORR, whereas the Pt, Pt/Pd, PtAu/Pd, and PtOs/Pd−W approve that the associative pathway is the most favorable one. The PtAu/Pd−S and PtOs/Pd−W show a better catalytic activity than other sites. Interestingly, both the PtAu/Pd−S and the PtOs/Pd−W are far away from the doped atoms, and their electronic structure are mainly changed by the ligand effect. No doubt, if the distance between the Pt atoms and the doped atoms is too far, then the ligand effect on the Pt atoms becomes weak or even disappears; then, the catalytic activity of these

Figure 5. Reaction energy diagram for (a) dissociative mechanism and (b) associative mechanism on Pt, Pt/Pd, PtAu/Pd, and PtOs/Pd, respectively.

intermediates and transition structures is reported along the reaction coordinate, with the reference energy set at the value of the starting reagents. As shown in Figure 5a, the O−O breaking and Oad protonation on the Pt and the Pt/Pd exhibit relatively higher activation barrier energy than other steps; in particular, the O− D

DOI: 10.1021/acs.jpcc.7b01624 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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sites cannot be enhanced. Conversely, the sites close to the doped atoms exhibit similar properties as the doped atoms; then, these sites show too weak or too strong adsorption ability for O2, respectively. Thus the change trend of the catalytic activity of the top-layer Pt atoms on the PtAu/Pd and the PtOs/Pd indicates that the effect of the doped atoms on the catalytic activity can be modified by modulating the distance between the Pt atoms and the doped atoms, and a proper distance should induce the catalysts to possess the highest catalytic activity. Therefore, the fact that different concentration of the doped atoms on the noble-metal surface always lead to different activity can be elucidated explicitly.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b01624. Optimized geometric structures of O2, O, and OH adsorption, deformation charge densities, adsorption energies of O2, O, and OH, and surface energies of different configurations of PtOs/Pd and PtAu/Pd. (PDF)

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4. CONCLUSIONS DFT calculations were performed to explore the effect of surface doping on the enhanced catalytic activity of PtM/Pd (111) as the doped atom changed from chemically inert Au to active Os. DFT results indicate that the PtAu/Pd and the PtOs/Pd have heterogeneous electronic structure, such as uneven surface charge and unequal d-band center, which results in different roles of surface Pt atoms in the process of ORR. The calculations of the adsorption of ORR intermediates and the ORR mechanism demonstrate that the PtOs/Pd−S adopts the dissociative pathway as the most favorable pathway for ORR, whereas the Pt, Pt/Pd, PtAu/Pd, and the PtOs/Pd−W approve the associative pathway. More importantly, the sites far away from the doped atoms, such as the PtAu/Pd−S and the PtOs/Pd−W, show the best catalytic activity compared with the other sites. It indicates that the catalytic activity of catalysts is mainly affected by the ligand effect and can be modulated by varying the distance between the Pt atoms and the doped atoms. These findings provide elucidation for the enhanced activity brought by doped atoms with opposite nature as well as for the fact that Pt-based alloy shows activity depending on the concentration of dopant.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 2365678945. (Z.W.) *E-mail: [email protected]. (L.L.) ORCID

Zidong Wei: 0000-0001-8001-9729 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work was financially sponsored by National Natural Science Foundation of China (grant nos.: 21376284, 21376283, and 21576032). E

DOI: 10.1021/acs.jpcc.7b01624 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.7b01624 J. Phys. Chem. C XXXX, XXX, XXX−XXX