Pt–Pd Bimetallic Catalysts: Structural and ... - ACS Publications

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Pt−Pd Bimetallic Catalysts: Structural and Thermal Stabilities of Core−Shell and Alloyed Nanoparticles Rao Huang,† Yu-Hua Wen,*,† Zi-Zhong Zhu,† and Shi-Gang Sun*,‡ †

Institute of Theoretical Physics and Astrophysics, Department of Physics, Xiamen University, Xiamen 361005, China State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China

‡

S Supporting Information *

ABSTRACT: Atomic-level understanding of structural characteristics and thermal behaviors of nanocatalysts is important for their syntheses and applications. In this article, we present a systematic study on structural and thermal stabilities of Pt−Pd bimetallic nanoparticles with core−shell and alloyed structures by using atomistic simulations. It was revealed that the Pd-core/Pt-shell structures are the least structurally stable, while the inverted Pt-core/Pd-shell nanoparticles are more stable than the alloyed ones when the Pt percentage exceeds 42% or so. The origin for this order was clarified through analysis of atomic energy distribution in these structures. Furthermore, the core−shell structures exhibit enhanced thermal stability as compared to the alloyed ones for Pt composition more than about 30%. The diverse melting behaviors of bimetallic nanoparticles, associating with their thermally driven structural evolutions under the heating process, were characterized by the measurement of the Lindemann index. In addition, the analyses of diffusion behavior and atomic distribution suggest that the minimization of surface energy tends to form Pd surface segregation. This study is of considerable importance not only to experimental preparation of Pt−Pd nanocatalysts but also to design of bimetallic (even multimetallic) nanostructures of high catalytic activity and excellent stability.

1. INTRODUCTION Platinum is one of the best single metal catalysts and is used indispensably in the chemical industry, the petrochemical industry, automobile exhaust purification, and fuel cells because of its excellent reactivity and stability.1 The price of Pt, however, is extremely high due to its rare reserve on the earth and the continuously increasing demand in industry. Improving the activity and utilization efficiency of Pt catalysts is therefore the key issue in development of relevant catalysis fields. Normally, there are two types of target products to reduce Pt loading: Pt monometallic nanoparticles (NPs) with open surface structure and Pt-based bi- (or multi-) metallic NPs. As a representative of the former, tetrahexahedral Pt NPs, bounded by {730} and vicinal high-index facets, have been recently synthesized by Sun and co-workers.2 These NPs possess high density of active sites that are composed of low coordination atoms located in steps, kinks, and ledges of the high-index facets, exhibiting a significantly enhancing catalytic activity.2−4 Meanwhile, synthesis of Pt-based bimetallic NPs, as an alternative strategy, has received considerable attention as well because of their enhanced catalytic activities as compared to conventional Pt monometallic NPs.5−14 For example, Pt−Au bimetallic NPs exhibited higher electrocatalytic activity and durability for oxygen reduction reactions than Pt monometallic catalysts.5,6 Moreover, the superior activities of both Pt−Ru and Pt−Rh bimetallic NPs were also ascertained in preferential oxidation of CO in hydrogen (PROX).7,8 Among Pt-based nanocatalysts, Pt−Pd is one of the most attractive bimetallic systems due to its highly promising © 2012 American Chemical Society

application as a catalyst in formic acid oxidation and fuel cells.9−14 Furthermore, Pt−Pd nanocatalysts have been considered as preferred substitutes of Pt−Ru catalysts for low-temperature direct formic acid fuel cell (DFAFC) technology owing to the much cheaper price of Pd.11 Pt−Pd bimetallic NPs can be produced generally by either successive coreduction of Pt and Pd species or a seed-mediated growth method.12 In contrast to the former route, the latter normally synthesizes Pt−Pd bimetallic NPs with core−shell structures.12 The experimentally synthesized Pt−Pd bimetallic NPs can be classified into three configurations: Pt-core/Pd-shell, Pd-core/ Pt-shell, and Pt−Pd alloyed nanostructures.9,13,14 Owing to the extremely small difference of lattice parameter between Pt and Pd (about 0.77%),15 no defects and dislocations have been observed in the aforementioned core−shell NPs. It is well-known that the exceptional chemical (especially the catalytic) activity of these NPs depends strongly on not only the particle size but also the Pt/Pd ratio.16 Therefore, the effects of core sizes, shell thicknesses, and alloy compositions are urgent topics that should be addressed for further tailored design of highly active Pt−Pd nanocatalysts. Furthermore, considering that the catalysts are generally used at different temperatures, the structural and thermal stabilities of Pt−Pd bimetallic NPs are also important issues that need to be clarified.17 Although considerable experimental and theoretical studies have been dedicated to design, synthesis, characterReceived: February 16, 2012 Published: April 2, 2012 8664

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Figure 1. Schematic illustration of three types of Pt−Pd bimetallic NPs: (a) Pt-core/Pd-shell, (b) Pd-core/Pt-shell, and (c) Pt−Pd alloyed structures. Coloring denotes type of atom: olive, Pt atom; blue, Pd atom.

metals and their alloys.18−22 The total potential energy for a system of atoms can be written as

ization, and performance evaluation of Pt−Pd bimetallic NPs,9−14,17,18 there is still lacking a full atomic-level understanding of structural and thermal stabilities of Pt−Pd bimetallic NPs with different configurations and compositions. In this article, by using atomistic calculations, we present a systematic study on the structural and thermal stabilities of three types of Pt−Pd bimetallic NPs, namely, Pt-core/Pd-shell, Pd-core/Pt-shell, and Pt−Pd alloyed NPs. The shell thickness of core−shell structures was tailored from one atomic layer to several nanometers thick to trace the size effects of the core, while the Pt composition from 0 to 100% was considered in Pt−Pd alloyed NPs. We first investigated the dependence of structural stabilities of Pt−Pd bimetallic NPs on Pt percentage. Next, these bimetallic NPs were heated for examination of their thermal stabilities. Both the dynamics of thermally driven structural evolutions of NPs and the atomic diffusive behaviors were explored. In addition, the diverse melting behaviors associated with configurations and compositions of Pt−Pd bimetallic NPs were also reported.

U=

∑ Ui = i

⎡ 1 ε ∑ ⎢⎢ ⎣2 i

⎤ ⎥ V ( R ) − c ρ ∑ ij i⎥ ⎦ j≠i

(1)

in which V(Rij) is a pair interaction function defined by the following equation ⎛ a ⎞n V (R ij) = ⎜⎜ ⎟⎟ ⎝ R ij ⎠

(2)

accounting for the repulsion between the i and j atomic cores; ρi is a local electron density accounting for cohesion associated with atom i defined by ⎛ a ⎞m ρi = ∑ ⎜⎜ ⎟⎟ R j ≠ i ⎝ ij ⎠

(3)

In eqs 1−3, Rij is the distance between atoms i and j; a is a length parameter scaling all spacings (leading to dimensionless V and ρ); c is a dimensionless parameter scaling the attractive terms; ε sets the overall energy scale; n and m are integer parameters such that n > m. Given the exponents (n, m), c is determined by the equilibrium lattice parameter, and ε is determined by the total cohesive energy. The model parameters for Pt and Pd are listed in Table 1. To describe the atomic

2. COMPUTATIONAL DETAILS Pt−Pd bimetallic alloyed NPs (denoted as Pt−Pd) and core− shell NPs including Pt-core/Pd-shell (Pt@Pd) and the inverted Pd-core/Pt-shell (Pd@Pt) structures were constructed from a large cubic fcc single crystal; see Figure 1 for their detailed structures. Similarly, a series of bimetallic NPs were modeled for different compositions in alloys and core/shell ratios in core−shell structures. To facilitate a comparison study, the total number of atoms in each NP was set at 8247, corresponding to a radius of about 3.1 nm. To keep a complete shell layer in such NPs, the Pt percentage should be less than 66.4% for Pt@Pd structures and larger than 33.6% for Pd@Pt structures, while for the alloyed NPs, Pt and Pd atoms can be arbitrarily distributed by computer-produced random seeds. Thus, the Pt composition can be continuously varied from 0 to 100%. Note that the Pt−Pd bimetallic NP is evolved to the Pd monometallic one for Pt percentage of zero and the Pt monometallic one for Pt percentage of 100%. On the basis of our previous works,19−21 the quantum corrected Sutton−Chen (Q-SC) type potentials were adopted to describe the interatomic interactions. These potentials represent many-body interactions, and their parameters are optimized to describe the lattice parameter, cohesive energy, bulk modulus, elastic constants, phonon dispersion, vacancy formation energy, and surface energy, leading to an accurate description of thermodynamic and transport properties of

Table 1. Potential Parameters Used in Atomistic Simulations for Pd−Pt Bimetallic NPs23 component

n

m

ε (meV)

c

a (Å)

Pt Pd

11 12

7 6

9.7894 3.2864

71.336 148.205

3.9163 3.8813

interaction between Pt and Pd, the geometric mean was used to obtain the energy parameter ε, while the arithmetic mean was used for the remaining parameters.18 Upon starting the molecular dynamics (MD) simulations, all the NPs were first quasi-statically relaxed to a local minimum energy state through the conjugate gradient method (CGM).24 The cohesive energy could be obtained by summing the total energy of atoms in the NPs. After full relaxation, these NPs were subjected to a continuous heating. To make the simulations more reliable, we employed constant temperature and pressure molecular dynamics (NPT-MD) to allow energy and volume fluctuations, which may be critical to the resulting 8665

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verified that the results remained invariable when different random seeds were adopted in the construction of the alloyed NPs. It can be revealed from Figure 2 that there are pronounced differences aroused by structures. Evidently, the Pt@Pd structure is remarkably more stable than the inverted Pd@Pt one. In three types of bimetallic NPs, the alloyed NPs exhibit superior stability when Pt percentage is less than 42%. Beyond this range, their stabilities lay between the other two types. The reason for this order can be elucidated when one examines the characteristics of atomic energy distributions of Pt and Pd monometallic NPs at 0 K. In both monometallic NPs, the energies of interior atoms are basically equal to those of atoms in bulk, while the values of atoms at the outer layer are significantly increased (see Figure S2 in the Supporting Information), contributing to the surface energies of NPs. More detailedly, the overall increase is about 1066 eV for the Pt NP and 833 eV for the Pd NP (see Figure 2); namely, the energy increment of the Pt NP is larger than that of the Pd NP, in agreement with the fact that the experimental surface energy is 2489 mJ/m2 for Pt and 2003 mJ/m2 for Pd.29 This trend is still tenable in core−shell bimetallic structures. Considering that the energy contributed from Pt−Pd interatomic interactions at the core−shell interface is almost equal (see Figure S2a in the Supporting Information), the surface energy should be therefore higher for NPs with a Pt shell than those with the Pd shell. That is to say, that the Pd@Pt NPs possess higher energy and thus are less stable compared with the inverted structures. However, unlike the core−shell structures, the energy distributions of Pt and Pd atoms in the alloyed NPs exhibit completely different trends and present a little more complicacy. In the interior of alloyed NPs, owing to Pt−Pd interaction, the energies of Pt atoms tend to decrease, while those of Pd atoms increase (see Figure S2b in the Supporting Information). It is observed from Figure 2 that the alloyed NP with Pt percentage close to zero presents an ΔE smaller than the core−shell structures, demonstrating that the interactions between Pt and Pd atoms contribute to the lower total energy of the alloyed NP. Nevertheless, as the Pt percentage exceeds 20%, more and more Pt atoms start to appear in the outer layers. They gradually become dominant and make ΔE increase. When the Pt percentage reaches 42%, the influence of interior atoms becomes secondary. Afterward, owing to considerable Pt atoms in the outer layers, the alloyed NPs present a structural stability between the two types of core− shell structures. While the Pt percentage is beyond 66.4%, it is another range where the effect of interior atoms becomes primary because the Pd atoms are relatively less distributed in the outer layers. Hence, the alloyed structure is more stable than the Pd@Pt NP in this range. In the catalytic industry, especially the cracking of petroleum or petroleum products in the petrochemical industry, many catalytic reactions are usually carried out at high temperatures.30 Pt−Pd bimetallic NPs, as excellent nanocatalysts, play key roles in these catalytic reactions such as oxygen reduction reactions (ORRs), methanol oxidation reactions (MORs), and ethanol oxidation reactions (EORs).31 In addition, some procedures in synthesis or processing of these nanoparticles (e.g., annealing) are usually under high temperatures. This naturally motivates us to further investigate the thermal stabilities of all three types of bimetallic NPs under heating process or in a high-temperature environment. Subsequently,

dynamics. These NPs underwent a heating process consisting of a series of NPT-MD simulations from 0 to 2200 K with a temperature increment of 50 K. However, a smaller step of 10 K was adopted to investigate the melting behavior more accurately when the temperature went up to around the melting point. At each temperature, the MD simulations were carried out for 200 ps, during which atomic coordinates, velocities, and energies were extracted in the last 25 ps for calculation of the statistical quantities. The desired temperature and ambient pressure were maintained by a Nose−Hoover thermostat25 and Berendsen approach,26 respectively. The equations of atomic motion were integrated by the Verletvelocity algorithm27 with a 1 fs time step.

3. RESULTS AND DISCUSSION The cohesive energy is indicative of the stability of NPs since the structure with higher cohesive energy tends to be more stable. It is equal to the absolute value of the ground-state energy obtained from atomistic simulations. As is known, the experimental cohesive energy is 5.84 and 3.89 eV per atom for Pt and Pd bulk materials,28 respectively, indicating that Pt is superior to Pd in stability. To examine the structural stabilities of three types of Pt−Pd bimetallic NPs, the compositiondependent potential energies were calculated after full relaxation. Nevertheless, for all types of NPs, the total energy was decreased approximately linearly with increasing Pt percentage and did not show a significant discrepancy for the same composition (see Figure S1 in the Supporting Information). To make the energy differences more distinguishable, a common definition of ΔE has been introduced as N

ΔE =

∑ Ei − EPtNPt − EPdNPd i=1

(4)

where Ei represents the potential energy of atom i in the system; EPt = −5.84 eV and EPd = −3.89 eV are the potential energy per atom in Pt and Pd bulks, respectively; N, NPt, and NPd are the number of total, Pt, and Pd atoms in the NP, respectively. It should be noted that for all the NPs with the same composition, according to this equation, the lower total energy inevitably leads to the lower ΔE and thus implies better structural stability. The composition dependence of ΔE was illustrated in Figure 2, in which the data points were connected with smooth curves, while the dashed line signified the inexistence of the corresponding configuration. Also, it was

Figure 2. Composition dependence ΔE of the three types of Pt−Pd bimetallic NPs. 8666

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NPs, as mentioned before, the number of Pt atoms located in the outer layers will be unnegligible when the Pt percentage reaches 20% and hence is responsible for the significantly enhanced melting point (110 K above that of pure Pd NP). Moreover, the melting points of the alloyed NPs present an approximately linear rise with increasing proportion of Pt atoms. For the core−shell structures, however, the dependence of melting point on composition shows distinct nonlinearity. As the core radius is small, especially less than 5/8 of the NP radius (Pt percentage less than 24.1% in Pt@Pd or larger than 75.9% in Pd@Pt), no remarkable core effect on the melting point can be observed from Figure 3, consequently leading to the Tm close to that of the monometallic NP. Nevertheless, when the core radius is beyond the critical value, the core region progressively extends to the outer layer. Thereupon, the melting point of the core−shell structure gradually shifts toward 1380 or 1820 K, depending on the core element. It is importantly noted that, different from the Pd@Pt NPs, an obviously abrupt curve appears in the Pt@Pd NPs. This should be closely associated with the different melting mechanisms in the two types of core−shell structures, originating from the notable discrepancy of melting temperatures of core and shell, as discussed specifically in the following content. In general, these results above demonstrate that the melting points of NPs are strongly dependent on their configurations. Furthermore, the core−shell structures exhibit excellently enhanced thermal stability as compared to the alloyed ones for Pt composition larger than about 30%. It is known that under continuous heating surface atoms usually melt prior to interior atoms in monometallic NPs because of their fewer nearest neighbors and weaker bonding, resulting in the occurrence of surface premelting.20,21,32,33 Furthermore, both theoretical and experimental studies have verified that the melting behaviors of alloy were closely related to their compositions and atomic arrangements.34−36 Therefore, it is expected that three types of bimetallic NPs will exhibit diverse melting behaviors. To gain an in-depth understanding of the melting mechanism and accordingly make a comparison study, a simple but effective measurement, the Lindemann index, was adopted to characterize the thermal evolution of NP during the heating process. It provides an accurate description of the thermally driven disorder of a system.37 For a system of N atoms, the local Lindemann index for the ith atom is defined as the root-mean-squared (rms) bond length fluctuation as38

we made an investigation on temperature-dependent stability of the aforementioned NPs. Normally, the thermodynamic parameters such as potential energy and heat capacity can be explored to identify the firstorder transition from solid to liquid phase during heating.18−20,22,23 The temperature-dependent potential energies of the three types of bimetallic NPs were extracted from MD simulations (see Figure S3 in the Supporting Information). There emerged different tendencies in these NPs. Similar to the situation in monometallic NPs, the phase transition of the Pd@ Pt NPs can be clearly identified by an abrupt rise in potential energy, covering a temperature interval of about 10 K, as shown in Figure S3b (Supporting Information). For the Pt@Pd NPs, however, the potential energy corresponding to the phase transition presents a slow and steady increase, experiencing a temperature range of 350 K or so, although a sharp jump similar to that in the pure Pd NP reoccurred in small Pt core NPs (see Figure S3a, Supporting Information). In the case of the alloyed NPs, the results are analogous to those of Pd@Pt NPs, despite that the energy jumps all cover a temperature interval of about 20 K (see Figure S3c, Supporting Information). To determine the melting temperature, the heat capacity, which can be deduced from the temperature dependence of potential energy,32 was also calculated. The melting point, Tm, is generally defined as the temperature at which the heat capacity reaches its maximum. The composition-dependent melting points were depicted in Figure 3. In contrast to the experimental values of 2045 and 1825 K

Figure 3. Composition-dependent melting points of the three types of Pt−Pd bimetallic NPs. Inset: representative temperature-dependent potential energies around the melting points.

δi =

1 N−1

∑ j≠i

⟨R ij 2⟩ − ⟨R ij⟩2 ⟨R ij⟩

(5)

and the system-averaged Lindemann index is calculated by for Pt and Pd bulks,28 all the Tm of NPs were pronouncedly decreased by hundreds of Kelvins. Especially, the Tm was 1820 K for the Pt monometallic NP and 1380 K for the Pd monometallic NP. This reduction was frequently observed in previous studies of metallic NPs and was attributed to the presence of surface premelting since those atoms on the surface have lower coordination numbers and weaker bonding forces than bulk ones.18,20,21,32 Available research has verified that the melting of monometallic NPs generally proceeds from the surface to the interior.18,20,21,23 Therefore, the Tm is closely associated with the temperature at which the premelting initiates and thus is highly dependent on surface configuration. For the alloyed

δ=

1 N

∑ δi i

(6)

where Rij is the distance between the ith and jth atoms. The Lindemann index was originally developed to study the melting behavior of bulk crystals. The Lindemann criterion suggests that the melting occurs when the index is in the range of 0.1− 0.15, depending on materials.37 The temperature-dependent Lindemann indices during the heating process were calculated for the three types of NPs (see Figure S4 in the Supporting Information). They all increase slowly and linearly with rising temperature in the initial heating stage. Subsequently, they deviate from the linear increase and 8667

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Figure 4. Snapshots of (a) Pt@Pd, (b) Pd@Pt, and (c) alloyed NPs taken at four representative temperatures during the heating process and the corresponding distributions of Lindemann indices (right). Note that Pt percentage is 11.6%, 88.4%, and 50.2% for Pt@Pd, Pd@Pt, and Pt−Pd alloyed NPs, respectively, and the shell thickness is 4.0 a0 for the two core−shell structures. Coloring denotes type of atom: green, Pt nonLindemann atom; blue, Pd non-Lindemann atom; light green, Pt Lindemann atom; and light blue, Pd Lindemann atom. Dashed lines indicate the critical Lindemann index of 0.039.

present a sharp jump around the melting point, which exactly corresponds to the significant increases of potential energies (see Figure S3 in the Supporting Information) and also indicates the solid−liquid phase transition. When the jumps are accomplished, the Lindemann indices continue to increase linearly with temperature. However, compared with bulk, a much smaller critical index of 0.039 should be adopted for all three types of NPs. It is reasonable due to the relaxed constraint of surface atoms in the NPs, in accordance with the results of clusters and homopolymers during the melting process.38 To make a further exploration of the melting behavior, the snapshots of the three types of NPs taken at four representative temperatures were extracted from MD simulations of the heating process, as shown in Figure 4. The corresponding distributions of Lindemann indices are illustrated on its right side. In these snapshots, the concept of the Lindemann atom was introduced. For all the NPs, the atom whose Lindemann index is beyond the critical value of 0.039 was defined as the Lindemann atom. It is clear that at room temperature of 300 K the atoms in the three NPs are all orderly arranged, and the Lindemann indices are below 0.009. More accurately, the maximum value of the core is about 0.007, indicating that the interior atoms generally possess smaller Lindemann indices compared with the surface atoms, independent of atomic type. However, as the temperature rises to near the critical point, different melting modes can be identified although surface premelting all occurs in the three NPs. For the Pt@Pd NP, in which the Pt core has higher melting point than the Pd shell, the melting gradually expands from the surface to the interior, experiencing a very broad temperature range (about 200 K, from 1400 to 1600 K). During the heating process, the premelting of the Pt core can be clearly distinguishable after the complete melting of the Pd shell (see the snapshot at 1550 K in Figure 4a). By contrast, for the inverted Pd@Pt structure in which the Pt shell is thermally more stable, the temperature at which the shell was completely melted is obviously higher than

the critical point of the core, leading to an instantaneous accomplishment of the solid−liquid phase transition of the core (see the snapshot at 1810 K in Figure 4b). As a result, the melting of the Pd@Pt NPs experiences a quite narrow temperature interval (10 K, from 1800 to 1810 K), somewhat similar to what happens in monometallic NPs. In this case, the reason that the Pd core does not precurrently melt at a temperature around 1380 K should be attributed to the existence of the Pd/Pt interface which suppresses the lattice instabilities of the Pd core, resultantly elevating the melting temperature of the core. Finally, for the alloyed NP, the development of the melting process presents a typical characteristic of the monometallic NP; that is, the melting is homogeneously evolved from the surface into the interior (see Figure 4c), with a temperature range of about 20 K (from 1630 to 1650 K). Besides the thermal evolution of NPs in the heating process, as discussed above, the diffusive behavior of atoms in the bimetallic NPs is another important issue that needs to be addressed due to its technological importance for applications in catalysts.39 The thermally driven diffusion is generally evaluated by the diffusion coefficient D, which can be calculated from the mean-square displacement (MSD) of individual atoms by the equations40 D=

1 6Nt

N

∑ [ri(t ) − ri(0)]2 i=1

(7)

where N is the atomic number of the system; t is the time elapsed in the simulation; ri(0) is the original t = 0 position of atom i; and ri(t) is its current position. For the three representative NPs displayed in Figure 4, the calculated diffusion coefficients of Pt and Pd are shown in Figure 5. It can be found from this figure that at the initial heating stage all the values are close to zero, implying no distinct diffusion in the NPs. With rising temperature, the atomic diffusive activities 8668

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Figure 5. Temperature dependence of diffusion coefficients for Pt and Pd in those NPs presented in Figure 4. Dashed line denotes the diffusion coefficient of zero.

began to emerge. In the Pt@Pd NP, visible displacement of Pd atoms emerges at 1350 K, which is in agreement with the temperature at which the surface premelting happens (see the Lindemann indices in Figure 4a). At this time, those Pt atoms in the core are highly ordered. As the temperature reaches 1500 K, those Pd atoms in the shell have been fully disordered, and some of the Pt atoms in the outer core become active. Thereafter, the diffusion coefficients for both kinds of atoms similarly present a steady increase, suggesting an enhanced diffusivity. However, the other two NPs present some differences from the Pt@Pd one. In the Pd@Pt NP, Pt atoms start to diffuse at 1750 K, while Pd atoms diffuse at 1800 K. The diffusion coefficients of Pt and Pd show distinct jumps at the same critical temperature, implying the almost synchronous melting. A similar phenomenon can also be observed in the alloyed NP. These differences in the diffusive behaviors of the three types of NPs all originate from their structures and are consistent with the aforementioned melting behaviors. Furthermore, it can be discovered from Figure 5 that Pd atoms always diffuse faster than Pt ones after the overall melting, independent of structures, similar to the result that Au tends to possess higher diffusivity than Pt in Pt−Au bimetallic NPs.41 The activation energy is an important kinetic parameter to a catalytic reaction. For monometallic catalysts, it depends on the fraction of surface atoms on corners and edges, while for binary compounds it also depends on surface segregation.42,43 Therefore, thermally driven phase separation is crucial for application of bimetallic (even multimetallic) catalysts. An available study has verified that in Pt@Pd bimetallic NPs Pd atoms prefer to remain at the surface during the melting process because they can lower the surface energy.18 To estimate the atomic distributions of Pt and Pd, the concept of statistic radius Rg was introduced here Rg =

1 N

Figure 6. (a) Temperature-dependent statistical radii of both Pt and Pd atoms in the alloyed NP during heating and cooling processes. Atomic distribution functions of Pt and Pd in the alloyed NP at (b) 1350 K and (c) 1650 K for the heating process and at (d) 2200 K and (e) 300 K for the cooling process.

originated from temperature-induced lattice thermal expansion, while the crystalline structure is relatively stable. When the temperature is above 1650 K, the statistic radius of Pd is significantly larger than that of Pt, indicating that the difference in their diffusive coefficients tends to form a structure of Pdrich shell and Pt-rich core, different from the initial random dispersion in the alloyed NP. To quantify the distributions of Pt and Pd atoms in the NPs, the atomic distribution function N(r) for each element was calculated, where N(r)dr was the number of atoms within a shell of thickness dr at r from the center of mass. Figure 6b and 6c present the results at 1350 and 1650 K, corresponding to the temperature at which the segregation begins and ends (see Figure 6a), respectively. At 1350 K, two distribution curves almost overlap, demonstrating a uniform distribution (see Figure 6b). However, at 1650 K, the separated peaks clearly manifest the presence of phase segregation (see Figure 6c). This phase-separating behavior continues to proceed with rising temperature. Here, a doubt arises spontaneously: does the phase separation also take place when the NP is subjected to a cooling process? Accordingly, a spherical alloyed NP, in which Pt and Pd atoms were randomly distributed and numerically equal, was constructed at 2200 K. The statistic radii of both kinds of atoms were calculated during cooling, as also illustrated in Figure 6a. It is indicated that the phase separation occurs in the NP even at 2150 K due to the high atomic diffusivity. The N(r) at 2200 and 300 K are illustrated in Figure 6d and 6e, respectively. Note that the obviously lager radius (∼9.5 a0) of the NP at 2200 K arises from the lattice thermal expansion at high temperature. A distinct Pd-rich distribution in the outer layers can be clearly identified at 300 K (see Figure 6e). Furthermore, it could be

∑ (R i − R cm)2 i

(8)

where (Ri − Rcm) is the distance of atom i from the particle center of mass. A difference in statistic radii of Pt and Pd describes the extent of their inhomogeneous distributions. Figure 6a illustrates the temperature dependence of Rg for both Pt and Pd atoms in the Pt−Pd alloyed NP (Pt/Pd ratio ≈ 1) during the heating process. Evidently, the linear parts are 8669

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expected to achieve the more sufficient separation of Pt and Pd phases if the bimetallic NPs are able to heat/cool slowly enough. The atomic-level understanding of the phase separation in Pt−Pd bimetallic NPs, as discussed above, can be extremely useful for the design and engineering of bi- and multimetallic advanced nanocatalysts.

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS In summary, atomistic simulations were employed to examine the structural and thermal stabilities of Pt−Pd bimetallic nanoparticles with core−shell and alloyed structures, aiming at an in-depth understanding of their thermally driven structural evolutions and melting behaviors during continuous heating. The Lindemann index and diffusive coefficient were adopted to characterize thermal evolution of these NPs. First, the composition- and structure-dependent energies at 0 K were obtained by the conjugate gradient method. It was revealed that the Pd@Pt structures exhibited the worst structural stability. When the Pt percentage was above 42% or so, the Pt@Pd NPs were energetically more favorable than the alloyed ones. The origin of the energy discrepancy was elucidated according to atomic energy distribution of these NPs. Next, their thermal stabilities were systematically investigated through MD simulations of the continuous heating process. It was found that the composition-dependent melting points were different for these NPs, and the core−shell NPs possessed higher melting points as compared to the alloyed ones for Pt composition of more than 30%. Furthermore, diverse melting processes were discovered: a narrow melting temperature interval appeared in Pd@Pt and alloyed NPs, similar to monometallic NPs; for the Pt@Pd NPs, however, a slow melting from the Pd shell to the Pt core was found, experiencing a quite broad temperature range. Meanwhile, the diffusion coefficient and atomic distribution function were used to describe the migration of Pt and Pd atoms during both heating and cooling. It was demonstrated that Pd atoms preferred to be segregated in the outer layers, and phase separation may be achieved in the alloyed NPs. The present study provides insights into the structure and stability of Pt−Pd bimetallic NPs, motivating the further design of bimetallic (even multimetallic) nanostructures with both excellent catalytic performance and outstanding stability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. H. Wen); [email protected] (S. G. Sun). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 21021002 and 61036003), the Natural Science Foundation of Fujian Province of China (Grant No. 2011J05011), and the Fundamental Research Funds for the Central Universities (Grant No. 2012121010). 8670

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