WC(0001) multilayer alloys under

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The enhanced stability of Pt /M /WC(0001) multilayer alloys under electrochemical conditions: a first principle study Xiaoming Zhang, Tianwei Gu, Shaodong Shi, Leyi Li, and Shansheng Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02360 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Materials & Interfaces

The enhanced stability of PtML/MML/WC(0001) multilayer alloys under electrochemical conditions: a first principle study Xiaoming Zhang1, 2, Tianwei Gu1, Shaodong Shi1, Leyi Li1, Shansheng Yu1* 1

State Key Laboratory of Automotive Simulation and Control, Department of Materials Science, Jilin

University, Changchun 130012, China 2

Division of Fuel Cell & Battery, Dalian National Laboratory for Clean Energy, Dalian Institution of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, China

KEYWORDS: Pt monolayer, Tungsten Carbide, Acid/Alkaline solution, ORR, Stability, DFT

ACS Paragon Plus Environment

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ABSTRACT:

Pt monolayer catalyst could greatly reduce the use of platinum. However, the core or subsurface transition metal could easily segregate to the surface and eventually selectively dissolve in the working conditions. In this work, a type of PtML/MML/WC multi-layer structure catalyst has been designed using DFT method. The results show that the stability of W-terminated core catalysts is lower than that of the corresponding C-terminated one. More importantly, C-terminated PtML/CoML/WC alloy could also maintain a stable platinum monolayer structure in various electrolyte solutions (eg, perchloric acid, phosphoric acid, alkaline solutions) and via different ORR pathways, in addition to the previously well-known precious alloy elements, such as Ru, Ir, etc. The segregation of M metal will be suppressed by introducing a high electronegativity nonmetal element in the core such as C through enhanced interaction between C and M interlayer. The improved ORR activity as well as stability of C-terminated PtML/Co(Ru)ML/WC(0001) multilayer structures highlight the importance of surface chemistry of substrate in rational design Pt monolayer catalysts.


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1. Introduction The need for clean and sustainable energy has inspired research in polymer electrolyte membrane fuel cells (PEMFCs) over recent years. The slow kinetics of oxygen reduction reaction (ORR) occurring at the cathode greatly limits its commercial application, especially in the transportation sector1-3. For this problem, Pt-based binary elctrocatalysts Pt3M, near surface alloys (NSAs), pseudomorphic surface alloys (PSAs) and so on have been extensively investigated with the aim of improving the performance and decreasing the Pt loading. Because the catalytic reaction mainly occurs on the surface, the Pt monolayer (PtML) catalysts are widely concerned. The properties of PtML are mainly determined by the subsurface and the core. At present, the common used “core” in PtML alloy includes single metal, alloy, metal compounds and so on. However, the low lattice matching degree between widely used base metal and PtML will lead to surface reconstruction. What's more, the inner layer (subsurface or core) base metals are tended to segregate to the surface and dissolve into the electrolyte. By increasing the stability of the base metal core, cheap, efficient and stable PtML catalyst can be obtained. For example, it is preferred to design a PtML coat on metal-nonmetal compounds (carbides, nitrides and so on). There have been a lot of studies on tungsten carbide and related metallic carbides before, mainly due to their “Pt-like” properties and could be used as alternatives for platinum group metals4-6. From this point of view, we pay close attention to WC based materials using as a support or core for PtML shell. Although it does not completely replace Pt, the load of Pt in PtML/WC electrocatalyst is really low and close to the minimum limit. The preparation of Pt/TMC catalyst has made great progress. Chen's research group7-9 has synthesized PtML/WC catalyst under ultrahigh vacuum (UHV) conditions using thermal evaporation and atomic layer deposition (ALD) method. The binding energy of oxygen atom on


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PtML/WC could approximate the case of Pt(111). WC@PtML particles have also been prepared by the same group

10

with the aim of enlarging the PtML/WC thin film system to industrially

related powder catalyst. Hunt et al.11-12 successfully prepared metal carbide nanoparticles coated with monolayer noble metal. They developed a self-assembly method in which a mixture of metal oxides and metal salts confined within a removable silicon template would be carburized. The Pt/TMC catalysts are practically feasible for experimental preparation. Furthermore, Yates et al.13 have studied Pt overlayers on various carbides (TiC, NbC, TaC, WC and SiC) by density functional theory (DFT). Zhang and his coworkers14 conducted an DFT experiment on Pt(Pd, Au)ML/WC(0001) slabs. It has been found that the strong interactions between the metal monolayers and the WC(0001) substrate are beneficial to enhance the stability of the catalysts. Humber et al.15 demonstrated the feasibility of PtML/NiML/WCbulk(0001) to maintain its desirable catalytic activity and enhanced thermal stability using a combination of experimental and theoretical methods. Vasicanicijevic et al.16 have studied the electronic property and surface segregation of

(Pd3Au)bilayer/WC with DFT method. Compared with bulk Pd3Au, the

electrocatalytic activity of WC-supported bilayer is higher, mainly because Au could remain segregated state under both H and CO adsorption conditions. Fako et al.17 have analyzed the electronic properties and durability of the WC supported bimetallic thin films with the composition A3B (A and B are transition metals) using DFT method. They find a strong influence of the support in the case of mono- and bilayers, while the surface strain seems to be the predominant factor in determining the surface properties of supported trilayers and thicker films. According to our previous predications, the NSAs18 and PSAs19 catalyst show increased activity and stability. However, the finite core elements being able to maintain a stable platinum


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monolayer structure in various electrolyte solutions (eg, perchloric acid, phosphoric acid, alkaline solutions) and via different ORR pathways limit their application. For example, only PtML/Rh(Pd)ML/Pt(111) and PtML/Ir(Pd)(111) could maintain a stable PtML in all studied alloys. The core elements are still precious metals. Inspired by Balbuena's work20 on multilayer metal alloy (Pt/Fe2C/Ir) electrocatalyst, in which case different amounts of carbon atoms are introduced into the interstitial vacancy of two subsurface layers of Fe and the oxygen reduction activity and stability have been improved or enhanced, we have design a PtML/MML/WC(0001) multilayer structure alloy with an extra transition metal interlayer to adjust and control the electronic properties of PtML, with WCbulk(0001) as core to replace the precious Ptbulk(111) or Mbulk(111) to reduce the cost of the whole catalyst. The C atoms in Fe2C and WC(0001) playing the same role, which will lock the transition metal atoms in their positions, preventing their segregation and subsequent dissolution. To the best of our knowledge, systematic studies on the activity and stability of PtML/MML/WC(0001) catalysts have not been conducted. In this paper, the activity and stability will be studied in various environmental conditions using DFT method. It is hoped that through this series of work, the design and preparation of ORR catalyst with high stability and activity could be enhanced. 2. Methods and models Calculation methods and parameters are similar to our previous work 18 and will not be described here. In DFT calculations, there are two kinds of WC (0001) surface structures, C-terminated and W-terminated substrate surfaces, as shown in Figure 1(a)(b). The corresponding two kinds of multilayer alloy structures, C-PtML/MML/WC(0001) and W-PtML/MML/WC(0001) (M = Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au) are modeled with a three-layer WC slab using a 2×2 super


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cell (Figure 1 (c) (d) ), the monolayer M and Pt are placed epitaxial on the WC(0001) slab, respectively.

Figure 1. Side view of the (a) W-terminated WC(0001), (b) C-terminated WC(0001), (c) Wterminated PtML/MML/WC(0001) and (d) C-terminated PtML/MML/WC(0001). Gray: C; Dark blue: Pt; Light blue: W; Yellow: M. We have calculated four-layer and five-layer WC slab and the results indicate that three layer is enough. In addition, the stepwise segregation of interlayer MML from the inner layer to the shell layer by swapping positions with adjacent Pt atoms in the surface, via PtML/MML/WC(0001) to

Pt0.75MLM0.25ML/Pt0.25MLM0.75ML/WC(0001)

,

Pt0.5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001),

Pt0.25MLM0.75ML/Pt0.75MLM0.25ML/WC(0001) and eventually MML/PtML/WC(0001), is considered for both surfaces with and without adsorbates (as shown in Figure 2).


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Figure 2. The optimal geometries of Pt-M-WC slab: (a) PtML/MML/WC(0001), (b) Pt0.75MLM0.25ML/Pt0.25MLM0.75ML/WC(0001),

(c)


(d)

Pt0.25MLM0.75ML/Pt0.75MLM0.25ML/WC(0001), (e) MML/PtML/WC(0001). Gray: C; Dark blue: Pt; Light blue: W; Yellow: M. The different adsorption sites of ORR intermediates and electrolyte anion, such as O, OH, -

-

OOH, ClO4 and PO4 , have been well studied at the of 0.25 ML coverage. The adsorption energy of atomic oxygen on PtML/MML/WC(0001) surface is defined as: ∆ O O-PtML/MML/WC(0001) PtML/MML/WC(0001) O

(eq. 1)

In vacuum, the segregation energy can be defined as: ∆ Pt(1-)ML MML/PtMLM(1-)ML /WC(0001) PtML/MML/WC(0001)

(eq. 2)

In ORR conditions, the segregation energy ∆ can be defined as： ∆ -Pt

(1-)ML MML /PtML M(1-)ML /WC(0001)

-Pt

ML /MML /WC(0001)

(eq. 3)

where E represents the total energy, and “ads” stands for an adsorbed species on the surface. “i” is the percent of M atom in the outmost layer (0≤i≤1). The binding strength (∆ ) between MML shell and PtML/WC(0001) substrate is defined as:


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∆ MML /PtML/WC(0001) PtML/WC(0001) M

(eq. 4)

where MML /PtML/WC(0001) , PtML/WC(0001) and EM are defined as the total energy of MML/PtML/WC(0001), PtML/WC(0001), and a free M atom, respectively. n represents the number of M atoms in shell. Our definition about ∆ is similar to energy of dissolution (∆ )17, so the dissolution behavior could be analyzed based on bonding energy data. 3. Results and Discussion 3.1 The Geometry Structures

The strain (θ) of interlayer MML is defined as θ

dWC-WC -dTM-TM dTM-TM

× 100%. We find that the strain

decreases as the number of d electrons in the MML increases. Through detailed analysis of the strain, three main conclusions are drawn. Firstly, the larger atomic radius would lead to wider layer spacing. Secondly, the stronger interaction between C-terminated core and active TM interlayer could generate larger layer spacing than W-terminated catalyst in which the interaction is weaker than C-terminated one. Finally, the transition metal interlayer with lateral stress will show an opposite strain trend at the interlayer direction, which will reduce the overall strain and energy of the whole system (For detailed analysis see Table S1and Supporting Information). 3.2 The ORR Activity of Pt Monolayer Catalyst Norskov et al. 21 believe that ∆ O can be used to evaluate the ORR activity of a catalyst. According to the results of Greeley et al 22, ∆ O of the most active ORR catalyst should be weaker ~0.2eV than Pt(111). We have done calculation on adsorption energies of O atoms within various PtML/MML/WC(0001) structures. The results indicate that PtML/Co(Os, Ru)ML/C-


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WC(0001) and PtML/RuML/W-WC(0001) are active for ORR due to the their weaker ∆ O

than Pt(111) surface (in the region (0, 0.2)). In addition, PtML/Ni(Ir, Cu)ML/C-WC(0001), PtML/Co(Os)ML/W-WC(0001) and PtML/W-WC(0001) should also be paid attention because their comparable ∆ O to Pt(111). As we can see, the ∆ O of PtML/W-WC(0001) is close to that on Pt(111), which agrees well with previous work8. This comparable adsorption energy suggests that PtML/WC(0001) could also be as an effective ORR catalyst, as shown in Table S2. In order to analyze the influencing factors of adsorption behavior, the surface d-electronic properties (for example, band center (εd), d band width (Wd), density of states at Fermi level (EF),) , geometric properties (interlayer distance) and number of charges of the catalyst have been calculated. The linear relationship between the εd and Wd indicates the same change trend, as shown in Table S2 and Figure S1. We have compared the εd and Mulliken charge of PtML/MML/WC(0001) with previous NSAs18 and PSAs19 in detail, as shown in Table S3. As for the d band center, the εd shifts positively with the increase in the number of d-electrons in the alloying element for all kinds of PtML studied. The only difference between PtML/MML/CWC(0001) and PtML/MML/W-WC(0001) is the surface atomic species of the core. Due to differences in the electronegativity and atomic radius between C (xp: 2.55, a: 0.77) and W (xp: 2.38, a: 1.41), shorter and stronger bonds will form between C and M layers, as shown in Table S4. Also, there will be a more obvious charge transfer between the two. So even though the coordination atom is the same, the charged state of the coordination atom changes greatly, this is the main reason for the difference in the electronic structure of the PtML. As for the Mulliken charge, there kinds of PtML are all negatively charged. For NSAs and PSAs, the charge transfer occurs mainly between Ptsurface and Msubsurface layers. However, obvious electrons transfer from Msubsurface to CWcore or WCcore in PtML/MML/WC(0001) structure has also been observed.


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Moreover, the number of charges on the Msubsurface layer in PtML/MML/C-WC(0001) is twice than that of NSAs, as shown in Table S4. The electronegativity and the lattice constant of the substrate will change the number of charge (Msubsurface) and bond lengths (Ptsurface and Msubsurface), respectively, which will change the electronic structures and adsorption properties of the platinum ML. The adsorption energies of oxygen atom, ∆ O , on PtML/WC(0001) and various PtML/MML/WC(0001) as a function of εd have been calculated and shown in Table S2 and Figure 3 (a), (c). The catalysts with higher εd have stronger atomic O binding strength, which is consist with previous results23. It is also found that there are so many scattering points and the linear relationship is difficult to draw. The scattering points are mainly PtML/WC and PtML/MML(3d)/WC structures. In PtML/WC catalyst, the PtML is directly bonded to C or W. What’s more, the number of charge transfer and interlayer spacing vary greatly compared to other systems, which will induce scattering. For the PtML/MML(3d)/WC structures, the smaller atomic radius of the 3d metal will lead to more significant surface reconstitution than 4d and 5d metal. In addition, the electronegativity difference is also relatively large. This may explain the cause of the scattering. Furthermore, the different slope of the fitted line could also be explained by the difference in electronegativity of substrate atoms. We also give the relationship between EF and ∆ O , as shown in Figure 3 (b), (d). The poor linearity or even the opposite trend is mainly due to the difference in the charge transfer direction caused by the difference in electronegativity, which has different effects on EF. This result reflects the influence of substrate surface chemistry on the adsorption property.


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Figure 3. Calculated adsorption energies of oxygen atom, ∆Eads(O), as a function of d-band center energies (εd) for (a) W-terminated and (c) C-terminated PtML/MML/WC(0001) as well as density of states at Fermi level (EF) for both for (b) W-terminated and (d) C-terminated PtML/MML/WC(0001). The dashed lines correspond to the properties of the Pt(111). 3.3 Stability under Vacuum Conditions Stability is one of the important parameters in evaluating catalyst performance. The segregation energies under vacuum conditions, ∆ , were calculated for stability evaluation of the multilayer structure alloys. From the results of the calculations, the observed variations

of

segregation

PtML/MML/M(111) or (001)

energy 19

are

similar

to

previous

PtML/MML/Pt(111)18

and

results. For most cases except for Ag and Au, the

PtML/MML/WC(0001) configuration is the most stable structure and the segregation increased in the order, ∆ 0.25ML ∆ 0.5ML ∆ 0.75ML ∆ 1ML , as shown in Figure 4 and Table S6. Furthermore, it is noted that ∆ depends on the subsurface M, that is, the interlayer M (e.g. Ru ) being far part from Pt in the periodic table has a more prominent


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effect on the stability or ∆ of the PtML shell (Figure 4). Along the same column, the multilayer alloys PtML/MML/C-WC(0001) with 3d metal (Co, Ni, Cu) show more pronounced change in ∆ than those with 4d M (Rh, Pd, Ag) and 5d M (Os, Ir, Au). As shown in Figure 4, PtML/MML/WC(0001) (M = Co, Ni, Cu; Ru, Rh, Pd; Os, Ir for C-terminated and M=Co, Ni; Ru, Rh; Os, Ir for W-terminated) surfaces could be considered as potential alternatives under vacuum, which means that if you want to build a stable PtML the segregation energy is either positive (exothermic, M = Co, Ni,; Ru, Rh; Os, Ir for C-terminated and M=Co, Ru, Os and Ir for W-terminated) or at least close to zero (thermoneutral, M = Cu and Pd for C-terminated and M=Ni and Rh for W-terminated) Upon alloying with Ag and Au, MML/PtML/WC(0001) is more favorable than PtML/MML/WC(0001). In addition to Ag and Au, MML/PtML/WC(0001) (M=Cu and Pd) is also the most stable structure.

Figure 4. The segregation energy, ∆Esegr(i), of PtML/MML/WC(0001) catalyst with different number of M on the surface: (a) C-terminated and (b) W-terminated PtML/MML/WC(0001). Previous studies18-19 have well illustrated the reason of this phenomenon from the surface energy, atomic radius and adsorption behavior. The results have shown that the positive-shift εd means more active of M and the higher surface energy of corresponding M is in Figure 5 and Table S7, which are consistent with other studies

24

. However, we also found some differences

changes in this case. For the PtML/CoML/Pt(111)18 alloy systems, the segregation energies


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∆ (i=1, 0.75, 0.5, 0.25ML ) are 3.86, 2.21, 1.68 and 0.65 eV, which are very small compared to PtML/PdML/C-WC(0001) surfaces (6.67, 4.71, 3.45 and 1.68eV). This is attributed to the high affinity between C and Co atoms in PtML/CoML/C-WC(0001) surfaces compared with that between Pt and Co atoms in PtML/CoML/Pt(0001), as shown in Figure 5. However, the segregation energies ∆ (i=1, 0.75, 0.5, 0.25ML) of PtML/CoML/W-WC(0001) alloy system are 0.56, 0.56, 0.38 and 0.24 eV, which are much smaller than PtML/CoML/Pt(111) and PtML/CoML/C-WC(0001) surfaces. This is attributed to the much lower affinity between W and Co atoms in W-PtML/PdML/WC(0001) surfaces than Pt atoms and Co in PtML/CoML/Pt(0001) near surface alloy as well as C atoms and Co atoms in C-PtML/CoML/WC(0001). The strength of the bonding energy not only reflects the affinity of MML with the substrate, but also indicates the dissolution potential of the MML. The more positive the bonding energy, the easier it dissolves. The dissolution potential indicates stability of the catalyst after segregation.

Figure 5. The correlation curves between the segregation energies as well as PtML binding energies and ∆ of MML/PtML/WC(0001) with respect to Pt2ML/WC(0001): segregation energies of (a) C-terminated, (b) W-terminated multilayer alloy structures; binding energies of (c) C-


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terminated and (d) W-terminated multilayer alloy structures. The dashed lines correspond to the properties of the Pt(111). In vacuum condition, the segregation energy of PtML/MML/C-WC(0001) was more positive than near surface alloy18 and pseudomorphic surface alloy19 so as to the stability. However, the segregation energy of PtML/MML/W-WC(0001) was much lower than the corresponding PtML/MML/C-WC(0001) structure. The difference of stability between PtML/MML/C-WC(0001) and PtML/MML/W-WC(0001) surfaces was attributed to the different chemistry property of the substrates. The strong binding between C atom and M atom in PtML/MML/C-WC(0001) enhance the stability of PtML. However, the weak binding between W atom and Pt atom in PtML/MML/WWC(0001) enhance the segregation energy of MML and destroy the stability of Pt monolayer. This enhancement or weaken effect is common and it can be applicable to the most metals. 3.4 Stability under Acidic and Alkaline Conditions The electrochemical reactions usually take place at the surface of the electrode, and the electrolyte solution is typically acidic or alkaline. According to our previous results of NSAs and PSAs

18-19

, we evaluated the effect of the solvation effect on the stability of the catalyst by

calculating the segregation energy ∆ of the catalyst under anions adsorption condition in Figure 6. Our previous calculations

18-19

have confirmed that if the Pt-skin surface can keep

stable in H3PO4, it will be stable in H2SO4 and HClO4 solution. Therefore, in this paper, we only consider acidic solution HClO4 (weak adsorption) and H3PO4 (strong adsorption). The adsorption of anions on the surface of the catalyst has a great influence on its stability25-26, and it becomes one of the most important topics of surface electrochemistry. In our calculations, we will consider the most stable structure and analyze the effect of ion adsorption on M segregation and also the final state of segregation.


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Figure 6. The optimal geometries of *ClO4 on the multilayer alloy surfaces: (a) PtML/MML/WC(0001),

(b)


(c)

Pt0.5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001), (d) Pt0.25MLM0.75ML/Pt0.75MLM0.25ML/ WC(0001). Gray: C; Dark blue: Pt; Light blue: W; Yellow: M; Red: O; Purple: Cl

Figure 7. The segregation energy, ∆Esegr(i), of PtML/MML/WC(0001) catalyst with different number of M on the surface in various solutions: (a) HClO4, (b) H3PO4 and (c) alkaline solution.


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As we have mentioned before18-19, the adsorption structures of tetrahedral *ClO4 (η3) were shown in Figure 6. The values of ∆ ML can be reduced in *ClO4 comparing to the one under vacuum, as shown in Figure 7 (a) and Table S8. However, the large changes observed in ∆ are different from our previous near surface alloy results18. In PtML/MML/C-WC(0001) alloys, the strong affinity between the substrate surface C atoms and interlayer M atoms in the multilayer alloy PtML/MML/C-WC(0001) will restrain the segregation of M atom. Oppositely, the O-M strong interaction in the multilayer alloy PtML/MML/C-WC(0001) will promote the segregation of M atom to the surface. Such two opposite acts will weaken the effect of ClO4 adsorption on stability. These results are different from those in the near surface alloy. For example, under ClO4 adsorption condition, the segregation energy ∆ 0.75ML of PtML/CoML/Pt(111) decrease from 1.97eV to 0.67eV; however, the segregation energy ∆ 0.75ML of PtML/CoML/C-WC(0001) decrease from 4.71eV to 3.68eV. The results show that the change of segregation energy in PtML/CoML/C-WC(0001) is small, and its stability under the perchloric acid condition is much higher than that in PtML/CoML/Pt(111). Therefore, more PtML/MML/C-WC(0001) (M=Co, Ni, Ru, Rh, Pd, Os, Ir) alloy could keep stable Pt monolayer structure in ClO4 adsorption condition. In contrast, for PtML/MML/W-WC(0001) alloys, in one hand, the weak affinity between the substrate W atoms and interlayer M atoms in the multilayer alloy PtML/MML/W-WC(0001) will promote the segregation of M atom to the surface. In another hand, the O-M strong interaction in the PtML/MML/W-WC(0001) multilayer alloy will also promote the segregation of M atom to the surface. The two same effects will enhance the segregation of interlayer M atoms. For example, under ClO4 adsorption condition, the segregation energy ∆ 0.75ML of PtML/CoML/W-WC(0001) decrease from 0.56eV to 0.80eV. The PtML shell structure loses its stability. In ClO4 adsorption condition, only


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PtML/MML/W-WC(0001) (M=Ru, Os, Ir) can keep stable Pt monolayer. In a word, the surface chemistry of substrate has a significant effect on the segregation phenomenon under electrochemical environments. Similar to ClO4, the same configuration is also adopted for the adsorption of PO4. The calculated results of PO4 adsorption are exhibited in Figure 7 (b) and Table S9. When the adsorption species change from ClO4 to PO4, the segregation energy is further more reduced. For PtML/CoML/C-WC(0001) alloy, the segregation energy ∆ 0.75ML decrease from 4.71 to 3.68 and final 2.50eV when the environment change from vacuum, ClO4 to PO4. In PO4 adsorption condition, PtML/MML/C-WC(0001) (M=Co, Ni, Ru, Rh, Pd, Os, Ir) can still keep stable Pt monolayer. For PtML/CoML/W-WC(0001) multilayer alloy, the segregation energy ∆ 0.75ML decrease from 0.56 to -0.80 and final -1.64eV when the environment change from vacuum, ClO4 to PO4. In PO4 adsorption condition, only PtML/IrML/W-WC(0001) can still keep stable Pt monolayer. Through the surface chemistry change of the core (from M to WC), we could obtain more kinds of stable Pt monolayer catalyst in phosphoric acid conditions. Alkaline environment has a weaker effect on catalyst stability, as shown in Figure 7 (c) and Table S10. Again, various adsorption structures have been considered. Figure 8 shows that the ∆ decrease under *OH species adsorption compared with the cases in a vacuum; yet they still stay as positive value. For PtML/CoML/C-WC(0001) alloy, the segregation energy ∆ 0.75ML decrease from 6.67 to 3.21eV when the environment change from vacuum to alkaline solutions. In alkaline solutions, PtML/MML/C-WC(0001)( M= Co，Ni，Ru，Rh，Pd， Os，Ir) can still keep stable Pt monolayer. As for PtML/CoML/W-WC(0001) multilayer alloy, the segregation energy ∆ 0.75ML decrease from 0.56 to -1.39eV when the environment


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change from vacuum to alkaline solutions. In alkaline solutions, only PtML/IrML/W-WC(0001) can still keep stable Pt monolayer.

Figure 8. The optimal geometries of *OH on the multilayer alloy surfaces: (a) top site on PtML/MML/WC(0001),

(b)

bridge

site

on

PtML/MML/WC(0001),

(c)

top

site

on


(d)

bridge

site

on


(e)

bridge

site

on

Pt0.5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001), (f) Pt0.25MLM0.75ML/Pt0.75MLM0.25ML/ WC(0001). Gray: C; Dark blue: Pt; Light blue: W; Yellow: M; Red: O; White: H. Combining with our previous studies of NSAs and PSAs 18-19, we predict that Co, Ni, Ru, Rh, Pd, Os and Ir can be considered as alloying metals with Pt to stabilize the PtML shell in HClO4, H2SO4,and alkaline solutions when the PtML/MML/C-WC(0001) conformations are adopted as shown in Figure 7. The number of stable PtM alloys has been greatly improved compared with previous studies. However, the Ir is the only element can be selected for alloying with Pt for all W-terminated PtML/MML/ WC(0001) catalyst. These results highlight the significant effect of surface chemistry of substrate or core on the segregation phenomenon. 3.5 Stability under ORR Environments


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It has been known that the slow kinetics of the ORR on the cathode hinders the overall performance of low-temperature fuel cell. Therefore, we have also studied the catalyst stability during the ORR. Following our previous studies of the NSAs and PSA

18-19

, we estimated the

effect of ORR on the segregation phenomenon by considering the adsorption of reaction intermediates involved in the ORR, where only the adsorptions of O, OH and OOH were considered as the key intermediates involved in direct and/or associative pathways of the ORR. Since the ORR occurs by exposing to the air, the stability under ORR can also be applied to that under air exposure. We consider top, bridge and hollow sites for *O adsorption and *O interacts strongly with the surfaces by occupying the three-fold hollow sites, as shown in Figure 9. Similar to PO4 adsorption condition, the surface segregation is decreased significantly with *O adsorption in comparison with the case in vacuum in Figure 10 (a) and Table S11. For example, under atomic O adsorption condition, the segregation energy ∆ 0.75ML of PtML/CoML/C-WC(0001) decrease from 4.71eV to 3.08eV. In atomic O adsorption condition, all studied multilayer alloy PtML/MML/C-WC(0001) (M=Co, Ni, Ru, Rh, Pd, Os, Ir) can keep stable Pt monolayer. However, for PtML/CoML/W-WC(0001) alloys, the segregation energy ∆ 0.75ML decreases from 0.56eV to -1.92eV. The PtML shell structure loses its stability. In atomic O adsorption condition, only Pt1ML/Ir1ML/W-WC(0001) can keep stable Pt monolayer. The results also demonstrate the significant effect of surface chemistry of substrate on the segregation phenomenon.


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Figure 9. The optimal geometries of *O on the hollow site of multilayer alloy surfaces: (a) PtML/MML/WC(0001),

(b)


5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001),

(c)

Pt0.

(d) Pt0.25MLM0.75ML/Pt0.75MLM0.25ML/WC(0001). Gray: C;

Dark blue: Pt; Light blue: W; Yellow: M; Red: O.

Figure 10. Calculated segregation energy, ∆Esegr(i), as a function of number of interlayer M segregated to the surface of PtML/MML/WC(0001) under ORR conditions: (a) *O; (b) *OOH. In the case of *OOH adsorption, the situation is complicated in Figure 10 (b) and Table S11. Depending on the surface composition, it can be adsorbed either molecularly on atop position or dissociative, where the *OH and *O fragments sit atop and hollow sites respectively, as shown in Figure 11. Our results show that the dissociative adsorption is energetically much more favorable


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than the molecular adsorption, which drives ∆ to be highly negative value in Figure 10. As the case for PtML/MML/C-WC(0001) (M=Co, Ni, Ru, Os and Ir) and PtML/MML/W-WC(0001) (M= Co, Ru, Os and Ir), the strong interaction with OOH after M atoms migrate to the surface, resulting in cleavage of O-O bond. For PtML/RuML/C-WC(0001), ∆ is highly positive, but *OOH does not stay in a molecular form and dissociate to *OH and *O fragments. In the cases of PtML/PdML/C-WC(0001), *OOH stays molecule and ∆ remains close to zero. PtML/MML/C-WC(0001) (Co, Rh, Ir) are the only systems with ∆ highly positive.

Figure 11. The optimal geometries of *OOH on the multilayer alloy surfaces: (a) PtML/MML/WC(0001), (b) Pt0.75MLM0.25ML/Pt0.25MLM0.75ML/WC(0001), (c) OOH break on Pt0.75MLM0.25ML/Pt0.25MLM0.75ML/WC(0001), (d) Pt0. 5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001) (e) OOH break on Pt0. 5MLM0.5ML/Pt0.5MLM0.5ML/WC(0001), Gray: C; Dark blue: Pt; Light blue: W; Yellow: M; Red: O; White: H. When the ORR is carried out a direct four-electron path, which meaning that the intermediate products include *O and *OH, Co, Ni, Ru, Rh, Pd, Os and Ir in C-terminated and Ir in Wterminated multilayer structures can be used as appropriate interlayer metals. However, when the ORR is carried through an associative four-electron path with *O, *OH and *OOH as


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intermediate products, only PtML/MML/C-WC(0001) (Co, Ru, Rh, Pd and Ir) can work well. When the NSA18 conformation are adopted, Co and Ni should not been considered as an ORR catalyst component neither via the direct or associative pathway. The enhancement of the stability is significant.

Figure 12. The stability of various PtML/MML/WC(0001) surfaces under different conditions. Color and depth represent the diverse stability of the catalyst according to the lowest ∆Esegr(i). Accordingly, we scaled the stability of PtML on a PtML/MML/WC(0001) surface with the lowest ∆Esegr(i) in Figure 12. ∆Esegr(i) represents the minimum energy required to allow M to segregate to the surface and destroy the stability of PtML. Based on the calculation results, different alloying metals should be considered if a stable PtML structure is to be obtained under different ORR reaction mechanisms for PtML/MML/WC(0001) catalysts. If the ORR follows the direct four-electron pathway via *O, all the alloy elements studied can be considered for C-terminated PtML but only Ir for W-terminated. PtML/OsML/C-WC(0001) and PtML/IrML/W-WC(0001) will be excluded if the reaction intermediate includes both O and OOH, in which case, the reaction take place in associated with a four-electron path. For those without *O but *OOH involved in the operating pathway, alloying with Co, Ni, Ru, Rh, Pd and Ir should be able to hold the surface PtML only for C-terminated ones. As for the influence of electrolyte solution, the above predictions should also be valid. Perchloric acid solution has little effect on the stability of the


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catalyst. However, in alkaline and H3PO4 solutions, Co, Ru and Os for W-terminated structures should be excluded. 4. Conclusion We have investigated the activity and stability of multilayer alloys PtML/MML/WC(0001) under different conditions with DFT method. Through the calculation of O atom binding energy, we predict that the PtML/MML/WC(0001) (M =Co, Ru, Os, for C-terminated and M = Ru for Wterminated WC(0001) substrate) would be active ORR catalysts due to their slightly weaker adsorption energies compared to Pt(111). As for the stability, the Co, Ru, Rh, Pd, and Ir in Cterminated multilayer structures are the only interlayer metals, being able to maintain a stable platinum monolayer structure in various electrolyte solutions (eg, perchloric acid, phosphoric acid, alkaline solutions) and via different ORR pathways. We have considered the relative stability of the two surfaces (C-WC(0001) and W-WC(0001)) because our purpose is to study the effect of the physical and chemical properties of the substrate surface on the outer shell. The improved ORR activity as well as stability of PtML/Co(Ru)ML/C-WC(0001) multilayer structures highlight the importance of surface chemistry of substrate in rational design Pt monolayer catalysts.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge. The segregation energies for some structures are listed. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected] (Shansheng Yu) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank the financial supports from Innovation Foundation of the 46th Research Institute of China Electronics Technology Group Corporation Program (Grant No. CJ20160902) and National Natural Science Foundation of China (Grant No. 51602305). The DFT calculations utilized resources at the High Performance Computing Center, Jilin University


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Table of Content


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WC(0001) multilayer alloys under

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