ARTICLE pubs.acs.org/JPCC
Hydrogen Interaction in PdPt Alloy Nanoparticles A. Lebon,*,† A. García-Fuente,*,‡ A. Vega,†,‡ and F. Aguilera-Granja§ †
Laboratoire de Magnetisme de Bretagne, UEB/UBO EA 4522, CNRS, 6 avenue Victor Le Gorgeu, 29238 Brest Cedex, France Departamento de Física Teorica, Atomica, y Optica, Universidad de Valladolid, E-47011 Valladolid, Spain § Instituto de Física “Manuel Sandoval Vallarta”, Universidad Autonoma de San Luis Potosí, SLP 78000, Mexico ‡
ABSTRACT: A theoretical study of hydrogen deposition and insertion in bimetallic PdPt nanoalloys has been carried out in the framework of the density functional theory. Our model systems are 147-atom clusters of cuboctahedral shape and stoichiometries of Pd1.00, Pd0.91Pt0.09, Pd0.82Pt0.18, and Pd0.54Pt0.46, resembling those recently produced and characterized in the context of H insertion (Kobayashi, H. et al. J. Am. Chem. Soc. 2010, 132, 5576.). Adsorption and absorption energies have been computed for H in all stable positions for the different compositions. Absorption is found to improve for nanoalloys with a small Pt concentration of 820%. However, when the Pt content approaches 50%, the absorption capability worsens, even as compared with the pure Pd nanoparticles. These trends are fully consistent with the experimental data. The local geometrical and electronic environments of hydrogen in these alloy nanoparticles are explored in detail so as to understand the observed behavior. Concerning the H deposition, our results indicate that PdPt nanoalloys with high Pt content might be efficient in regard to preferential oxidation reaction mediated by hydrogen.
1. INTRODUCTION Physics and chemistry of hydrogen insertion in alloyed compounds is currently a matter of intense research because of its technological relevance13 Systems based on transition metals with different geometries and sizes have been envisaged to improve H absorption and desorption. Among them, FenVm superlattices are known to absorb H in a reversible fashion within the V layers.4 The combination of a porous carbon surface and Pd nanoparticles has also been investigated.5 H adsorption on metal surfaces is not only relevant for H uptake but also is a key factor to control the catalytic properties of a material. The insertion (absorption) or deposition (adsorption) of hydrogen in transition-metal-based materials is one of the most active research lines in this context. Ni, Pd, and Pt are three isoelectronic transition-metal elements with nearly full D-valence states, which crystallize in a facecentered cubic structure. Despite these common features, Pt exhibits a markedly different behavior compared with Ni and Pd regarding H insertion. Ni and Pd in their bulk arrangement are known to capture hydrogen. Therefore, for instance, capping layers of Pd are used in Mg/Ti multilayers to favor H insertion.6 Moreover, Ni and Pd form numerous hydrides and are introduced in alloys to improve H storage as in Mg6Ni0.5Pd0.5, where Pd stabilizes the structure.7 On the contrary, bulk Pt is a poor hydrogen absorber. A thorough first-principles study by Smithson et al.8 provided a framework to understand these different behaviors. Three contributions were proposed to explain the capability to insert H and to form hydrides. The first is the energy to convert the crystal structure of the metal in the structure r 2011 American Chemical Society
formed by the metal ions within the hydride. This contribution is expected to be null because of the same fcc structure of Ni, Pd and Pt as well as of their hydrides. The second contribution is the loss of cohesive energy upon expansion of the metal structure to form the hydride. The last contribution to the hydride formation energy is the chemical bonding between the hydrogen and the metal host, this being the only exothermic contribution and, hence, the sole contribution which favors the hydride formation. When the size decreases and reaches the nanoscale, the properties of the systems depart, in many cases, from those of their bulk counterparts. Downscaling opens new prospects for technological applications but demands, at the same time, a correct understanding from the fundamental point of view. This is the case for Pd and Pt nanoparticles in the context of hydrogen insertion. Pt clusters are more able to store H than Pd clusters.911 In other words, size reduction reverses the trends observed in the bulk regime. An interesting question arises in view of the above fact. What is the behavior of binary PdPt nanoparticles? Recent experimental investigations by Kobayashi et al.12,13 have shed light on this issue. In a first experiment, these authors built up Pd core/Pt shell nanoparticles and exposed them to hydrogen. Their hydrogen pressurecompositions isotherms and NMR measurements showed that those nanoparticles are efficient in absorbing hydrogen and that the loaded hydrogen lies preferentially around the interface. We note that this opens a route to Received: August 1, 2011 Revised: November 17, 2011 Published: November 22, 2011 126
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control the H loading. In a more general context, these systems are also very interesting in catalysis because they are used, for example, in catalytic converters in automobiles.14 These bimetallic core/shell nanoparticles belong to a very promising family of compounds that are gaining a growing interest because of their activity as catalysts. In fact, in the process of extracting hydrogen
from hydrocarbons, the reaction products contain H2 and a significant amount of CO; the latter is eliminated in a preferential oxidation (PROX). It was recently demonstrated that M core/Pt shell nanoparticles with M = Ru, Rh, Ir, and Pd enhance the PROX reaction of CO mediated by hydrogen.15 The decomposition of H2 into 2 H atoms and its further adsorption on surface or absorption on interface sites is an intermediate reaction that enters in the mechanism of the PROX reaction. Despite their reduced Pt content, these nanoparticles afford the design of more active PROX catalysts. Other core/shell nanoparticles, like Pd/FePt, are active and durable catalysts for the oxygen reduction reaction.16 Coming back to the experimental studies of Kobayashi et al., in a second experimental setup,13 they demonstrated also that hydrogenation of Pd core/Pt shell nanoparticles under a mild thermal treatment (temperature ∼400 K) serves as a mechanism to modify the chemical order of the PdPt nanoparticle. In fact, it gives way to a mixing of the chemical species, and the initial core/ shell nanoparticle ends up as a particle nanoalloy, that is, with a chemical order approaching the uniform mixing of both species. Repeating the same type of measurements for a set of Pd/Pt particles with different relative composition, Kobayashi et al. observed that a small amount of Pt improves the amount of H storage, with a maximum H storage capability for a low amount of Pt, of ∼8%. Pt content exceeding 50% worsens the absorption capability. They further showed that within these alloy nanoparticles the hydrogen (deuterium) is preferentially absorbed in the
Figure 1. Number of heterogeneous PdPt bonds for the 29 = 512 different PdPt nanoalloys of 147 atoms with Oh symmetry. Red points indicate the homotops that we have used for our calculations, which maximize the number of heterogeneous bonds for the different relative compositions: (a) pure Pd, (b) Pd0.91Pt0.09, (c) Pd0.82Pt0.18, and (d) Pd0.46Pt0.54.
Figure 2. View of one-eighth of the investigated cuboctahedral alloy nanoparticles of 147 atoms. Red balls represent Pt atoms and white balls represent Pd atoms. The different structures correspond to the selected homotop for each relative composition, as indicated in Figure 1. The index l indicates the available layers for H location: l = 0 is for H adsorption on the surface, whereas the rest layers are for H absorption in the first layer below the surface (l = 1), in the second layer below the surface (l = 2), and in the most internal layer (l = 3). 127
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in comparison with the experimental findings. In Section 4, we summarize our main conclusions.
2. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS Our calculations have been performed using the DFT code SIESTA2224 with the generalized gradient approximation to exchange and correlation as parametrized by Perdew, Burke, and Ernzerhof.25 We replaced the atomic core by a nonlocal normconserving TroullierMartins pseudopotential26 that was factorized in the KleinmanBylander form.27 The ionic pseudopotentials were generated using the following atomic configurations: 4d9, 5s1, and 5p0 for Pd and 5d9, 6s1, and 6p0 for Pt. Cutoff radii are reported elsewhere.28 For Pd and Pt, we have included nonlinear core corrections29 to account for the significant overlap between the valence d states and the semicore states. In the case of hydrogen, the pseudopotential is trivial. Within the same DFT approximation, these pseudopotentials have been tested through the comparison of the eigenvalues of different electronic configurations of the respective isolated atoms against the corresponding all electron eigenvalues. Concerning the basis sets, we have described the valence states using double ζ-polarized30 pseudoatomic orbitals for Pd, Pt, and H. A 200 Ry energy cutoff is used to integrate in real space. We also smoothed the Fermi distribution with an electronic temperature of 15 meV.22 Further lowering of the electronic temperature to 5 meV for particular cases did not change the results. In the calculations, the clusters were placed in the center of a supercell with enough empty space to prevent interactions with their replica in adjacent cells. Only the Γ point was used when integrating over the Brillouin zone. Structural relaxation is performed with a conjugate gradient algorithm and is stopped when each force component at each atom is smaller than 0.01 eV/Å. The interatomic distance found in the 147-atom Pd cluster (2.79 Å) is very close to the calculated bulk value (2.83 Å), which further supports the bulk-like character of these particles and the fact that this cluster size can be considered to be a good approximation of the larger clusters experimentally produced13,15 (diameters comprised between 3 to 6 nm). Going beyond this size would require a huge computational cost at the DFT level of accuracy. We have studied hydrogen adsorption (deposition) and absorption (insertion) in cuboctahedral PdPt nanoalloys with 147 atoms and for different relative compositions, modeling the experimental samples.13,15 This structure presents nine nonequivalent atomic sites, which lead to 29 = 512 different ways to combine Pd and Pt in this structure retaining the Oh point-group symmetry. According to the experimental data, the nanoalloys exhibit an homogeneous distribution of Pd and Pt atoms. To model this chemical order properly, we have chosen, for different Pt concentrations, the homotop that maximizes the number of heterogeneous bonds. This ensures the most homogeneous distribution for the Pt atoms within the Pd matrix. We plot in Figure 1 the number of heterogeneous PdPt bonds as a function of the Pt concentration for the 512 different homotops. Red points in this Figure correspond to the selected homotops of four different compositions ranging from pure Pd to Pt concentrations of 9, 18, and 46%. In Figure 2, we show oneeighth of the cuboctahedral structure of these homotops. A cuboctahedral cluster of 147 atoms is formed with four concentric layers (including the central atom) having 1, 12, 42, and 92 atoms, respectively. These layers are labeled with the letter l, where
Figure 3. All nonequivalent positions for adsorption or absorption of H (light blue hexagon) in the PdPt nanoparticles at the different interstitial levels within the cluster as well as at the surface. For the squared faces of the cluster, a shift along the (001) direction has to be considered, unless stated in the Figure, because the plot shows only the projection. For the triangular faces, a shift along the (111) direction has to be considered.
space where Pd and Pt are in direct contact with each other, especially in environments where the absorbed deuterium atom experiences homogeneous potentials. Important efforts have been devoted so far to investigate different sizes and chemical orders of bimetallic PdPt nanoparticles.1720 In the present work, we theoretically investigate the hydrogen deposition and insertion in alloy PdPt nanoparticles that simulate the experimentally produced ones. Our aim is to understand the physics of hydrogen adsorption and absorption in these binary systems, to complement our previous study devoted to the Pd core/Pt shell nanoparticles,21 and to extract general trends that may serve to design other nanostructures of these elements with optimal local geometrical and chemical environments in regard to hydrogen loading. For this purpose, we have conducted calculations within the density functional theory (DFT) as implemented in the SIESTA code,2224 the essential details of which are given in the next section. We consider Pd/Pt nanoparticles of 147 atoms with cuboctahedral symmetry and different relative amounts of Pd and Pt with chemical orders that maximize the number of heteroatomic bonds, thus approaching the uniform distribution of the atoms in the alloy particle. We have studied H deposition and absorption in all nonequivalent sites. In Section 3, we present and discuss the results 128
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Table 1. Characteristics of the Stable Positions for H Inside or at the Surface of the 147 Atom PdPt Nanoalloysa H sites
Pd1.00
Pd0.91Pt0.09
i
l
n
Ef
Ω
1
3
6
2.210
2.55
3
3
4
2.092
1
2
6
2.208
Pd0.82Pt0.18
Ef
H neig.
Ω
2.52
2.146
3Pd-1Pt
2.54
2.55
2.251
5Pd-1Pt
2.54
Pd0.54Pt0.46
Ef
H neig.
Ω
2.159
4Pd-2Pt
2.59
2.117
3Pd-1Pt
2.54
1.822
1Pd-3Pt
2.59
2.175
4Pd-2Pt
2.59
1.914
2Pd-4Pt
2.59
Ef
H neig.
Ω
2
2
4
2.122
2.54
2.250
4Pd
2.55
2.155
3Pd-1Pt
2.56
2.001
2Pd-2Pt
2.57
5
2
6
2.169
2.57
2.293
6Pd
2.58
2.269
6Pd
2.60
2.025
3Pd-3Pt
2.59
1.985
2Pd-2Pt
2.52
2.246 2.301
3Pd-3Pt 2Pd-2Pt
2.56 2.59 2.60
7
2
4
2.124
2.50
2.163
3Pd-1Pt
2.55
2.128
3Pd-1Pt
2.56
2
1
4
2.348
2.57
2.486
4Pd
2.58
2.380
3Pd-1Pt
2.59
3 5
1 1
6 4
2.336 2.388
2.54 2.55
2.393 2.441
5Pd-1Pt 3Pd-1Pt
2.55 2.57
2.292 2.425
4Pd-2Pt 3Pd-1Pt
2.55 2.58
8
1
4
2.271
2.61
2.427
4Pd
2.64
2.284
3Pd-1Pt
2.65
2.356
3Pd-1Pt
1
0
4
2.721
2.837
4Pd
2.701
4Pd
2.642
4Pt
2.658
2.776
2Pd
2.741
2Pd
2.924
2Pt
2.666
1Pt
2
0
2
3
0
1
4
0
4
2.693
5
0
2
2.610
6 7
0 0
4 2
9
0
2
10
0
1
2.820
4Pd
2.801
4Pd
2.673 2.694
2.708 2.839
4Pd 2Pd
2.707 2.803
4Pd 2Pd
2.711
2Pd
2.704
2Pd
2.774
1Pd-1Pt
2.016
2.142
1Pd
2.130*
1Pd
2.617
1Pt
2.827
1Pd-1Pt
2.664 2.780
2Pd-2Pt 2Pd
11
0
1
2.087
2.250
1Pd
2.641
1Pt
2.841
1Pt
12
0
3
2.724
2.861
3Pd
2.800
2Pd-1Pt
2.912
2Pd-1Pt
13
0
3
2.641
2.779
3Pd
2.731
2Pd-1Pt
2.832
2Pd-1Pt
18
0
3
2.643
2.749
3Pd
2.736
3Pd
2.717
2Pd-1Pt
The first columns gather the notation and type of the H sites, that is, the index (i) and the level (l) of the H position as defined in Figure 3 and also its nearest neighbor coordination (n). In the next blocks of columns. we report, for each cluster, the insertion energy (in electronvolts) of H sitting in the different stable positions, the chemical nature of the H neighbors, and the volume (in cubic angstroms) available for H insertion in the H-free clusters. All volumes are referred to a tetrahedral volume. a
l = 3 is the central layer, l = 0 is the surface layer, and l = 1 and 2 are the two intermediate inner layers. In this study, the hydrogen atom is interstitially inserted in different positions between the different layers. We have also calculated the H adsorption at the surface positions. In Figure 3, the different levels between layers, at which H is interstitially inserted, are defined. The level l = 0 corresponds to adsorption at the surface. Figure 3 also illustrates the nonequivalent sites that H can occupy at each level within the system. For the sake of simplicity, Pd and Pt atoms are not distinguished in the Figure. All possible sites for H adsorption and absorption have been considered as input of our self-consistent calculation. However, the converged solution in certain cases corresponded to a structural arrangement different from the one proposed as input. In what follows, we will identify each H position by means of both the level l and the corresponding number given in Figure 3.
columns for each composition. The first column gives the insertion energy Ef computed according to the following expression Ef ¼ EðPdx Pt1x þ HÞ ½EðPdx Pt1x Þ þ EðHÞ
ð1Þ
where E(PdxPt1x + H) stands for the total energy of the relaxed nanoparticle with the H atom inside, E(PdxPt1x) is the energy of the H-free nanoparticle, and E(H) is the energy of the isolated H atom, with x = 1.00, 0.91, 0.82, and 0.54. The next two columns describe the local environment of H at a given site, that is, the number of its nearest Pd and Pt neighbors and the available volume for H insertion before it enters this site. Within an fcc lattice, we can define tetrahedral (n = 4) or octahedral (n = 6) interstitial atomic environments. We can define these same environments in our particles because a cuboctahedral cluster can be obtained by cutting edges of an fcc structure. Because an octahedron is made of four tetrahedrons, all volumes are referred to a tetrahedral volume, which is easily calculated from the length of its edges, according to the CayleyMenger determinant.31 3.1. H Adsorption. On the surface, H can occupy top, bridge, or hollow sites. H is on top of one atom when it forms a single bond with either a Pd or a Pt atom. When the H atom bonds with two atoms, it sits on a bridge site. In hollow sites, H bonds with three or four surface atoms. Table 1 shows that the preferential sites for H adsorption on the surface are, in most cases, are of hollow type (triangular and the square faces sketched on the right and left side of Figure 3).
3. RESULTS Table 1 summarizes the data obtained for the stable H sites in the selected homotops of the PdPt nanoalloys of different stoichiometry: Pd1.00, Pd0.91Pt0.09, Pd0.82Pt0.18 and Pd0.54Pt0.46. The adsorption or absorption sites are identified according to their number following the notation of Figure 3 (column i), the layer to which they belong (column l), and their nearest neighbors coordination (column n). There is also a specific block of 129
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Figure 4. Adsorption energy of H on the different stable sites at the surface of the nanoparticles as a function of the Pt concentration. Different symbols are used to distinguish the local atomic coordination of H at each site, with squares for hollow sites of the square faces at which H has four nearest neighbors, triangles for hollow sites of the triangular faces (three nearest neighbors), diamonds for bridge sites (two nearest neighbors), and circles for top sites (one nearest neighbor). The different adsorption sites are identified in the inset, using the notation l.i, where l corresponds to the level and i corresponds to the inequivalent sites within this level, as illustrated in Figure 3.
Figure 5. Absorption energy of H in the different stable sites inside the nanoparticles as a function of the Pt concentration. Different colors (lines) are used to distinguish the different absorption layers, with blue (dotted) lines for l = 1, green (dashed) lines for l = 2, and red (dotted-dashed) lines for l = 3. Different symbols distinguish between different environments, with diamonds and squares for octahedral sites (six neighbors) and different triangles for tetrahedral sites (four neighbors). The different absorption sites are identified in the inset, using the notation l.i, where l corresponds to the level and i corresponds to the inequivalent sites within this level, as illustrated in Figure 3.
Therefore, for PdxPt1x with x = 1.00, 0.91, and 0.82, H atoms are bound with three or four Pd atoms. This result contrasts with what is obtained for x = 0.54 as well as with previous results for Pd core/Pt shell nanoparticles reported by Lebon et al.,21 where the most stable sites for H adsorption are of bridge-type. Among the bridge sites, bonding with two Pt neighbors gives the lowest energy. In general, for a cuboctahedral structure with n layers, there are 24n(n 1) bridge sites and 14(n 1)2 hollow sites. Therefore, the number of bridge sites is always larger than the number of hollow sites, and thus the richer content of Pt in the nanoalloy increases the number of favorable adsorption sites at the surface. Therefore, more hydrogen can be adsorbed on Ptrich outerlayers than on Pd-rich ones. The decomposition of H2 into 2 H atoms and its further adsorption on the surface is an intermediate reaction that enters the mechanism of the PROX reaction to eliminate CO. These alloy nanoparticles with high Pt content approaching 50%, might afford the design of more active PROX catalysts because a rich content of Pt in the nanoalloy increases the number of favorable adsorption sites for H at the surface. However, a higher content of Pt is a major drawback because it is an expensive material. In this context, a way to reduce the Pt content stabilizing Pt layers at the surface of Pd core/Pt shell nanoparticles is the doping with iron, as stated by Mazumder et al.16
The adsorption energies (Ef), listed in Table 1 for all investigated compositions, are also plotted in Figure 4 for the sake of clarity. Analyzing the values obtained for sites with l = 0 and a pure a Pd or Pt local environment, one notices that bonding with a Pt atom is more favorable for the H adsorption than bonding with a Pd atom (by an energy difference of ∼0.5 eV). However, the adsorption properties depend on the Pt content in a nontrivial manner. Figure 4 shows that the adsorption energy follows a nonmonotonous behavior as a function of the Pt content for most of the adsorption sites. A local minimum is obtained for the Pd0.91Pt0.09 composition, indicating an improved H uptake on these surface sites for low Pt content. However, the lowest energy of H adsorption is obtained when there is a high Pt content at the surface. (See Figure 4.) Only for a few sites (top sites) does the adsorption energy decrease in a monotonous way as increasing the Pt content. This feature could indicate that the inner part of the nanoparticle plays a role in H adsorption. When H sits on hollow or bridge sites, inner atoms of l = 1 belong to the local neighborhood of H (second nearest neighbors). Therefore, the local neighborhood of H is formed by surface and subsurface atoms. 3.2. H Absorption. The number of stable absorption sites in these alloy nanoparticles is larger than that found in the Pd core/Pt shell nanoparticles of the same size recently investigated.21 130
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Figure 6. Available volume for H absorption in the different stable sites inside the nanoparticles as a function of the Pt concentration. Colors (lines) and symbols as in Figure 5. The different absorption sites are identified in the inset using the notation l.i, where l corresponds to the level and i corresponds to the inequivalent sites within this level, as illustrated in Figure 3.
Figure 7. Values obtained for the Fukui function at the different stable sites for H absorption inside the nanoparticles, as a function of the Pt concentration. A color scale indicates the insertion energy of each site in units of electronvolts.
insertion in the inner region of the cluster is critical for its insertion energy. In Figure 6, we plot the available volume for H insertion in all possible sites as a function of the Pt concentration. The available volume increases for the low Pt content, that is, for Pd0.91Pt0.09 and Pd0.82Pt0.18, but it decreases when the Pt content approaches 50% in Pd0.54Pt0.46. Therefore, a low Pt content serves to increase the available volume yet preserve a large amount of Pd, which is known to have a high H absorption capacity when forming nanoparticles. In other words, for this small amount of Pt, both the capability of Pd to absorb H and the increased available volume work together in favor of improving the capacity of the nanoparticle as an H absorber. For a higher Pt concentration approaching 50%, however, the available volumes decrease as well as the Pd content. Electronic effects should also play a role on the stability of H inside the nanoparticles. Electronic bonding of H with its nearest neighborhood is expected to be more favorable in particular environments. To determine the most favorable interstitial sites according to electronic arguments, we have computed the Fukui functions f 0 in the H-free nanoalloys for the different stable H absorption site.3234 The Fukui function is defined as
In Figure 5, we plot the insertion energy for the different absorption sites as well as for the different compositions of the nanoalloy. Symbols and colors distinguish different type of sites and levels of insertion, respectively. The most relevant result is the decrease in insertion energy for low Pt content, with a minimum obtained around Pd0.91Pt0.09 and Pd0.82Pt0.18. When the Pt content approaches 50%, the insertion energy generally increases to values exceeding even those of the pure Pd cluster. (See values for Pd0.54Pt0.46 in Figure 5.) This trend is in complete agreement with the hydrogen pressurecomposition isotherms data of Kobayashi et al.13 on larger nanoalloyed cuboctahedral Pd/Pt particles. They observed that a small amount of Pt improves the H storage capacity, specifically for a Pt content of ∼8%, whereas a Pt content approaching 50% worsens the capacity even with respect to the pure Pd nanoparticles. Another important feature is that the type of absorption site depends on its depth within the cluster: it is tetrahedral close to the surface (l = 1) and octahedral in the inner layers (l = 2, 3). We note that at the vicinity of the surface (l = 1, blue symbols in Figure 5) H inserts preferentially in the tetrahedral sites despite the smaller available volume than in the octahedral sites, a result that can be traced back to the proximity of the H atom to the surface when it sits in a tetrahedral site for l = 1. Indeed, a general trend is that the deeper the H inserts, the higher the insertion energy, as it can be verified by comparing the average insertion energies for l = 0, 1, 2, and 3. For the inner layers, the octahedral sites become the most favorable ones regardless of the composition. The available volume for H
f 0ð B rÞ ¼
∂Fð B rÞ ∂N
ð2Þ
where F(r B) is the spatial charge density and N is the number of electrons. f 0 gives a measure of the local fluctuations of electronic charge, and it can be used as an index of electronic reactivity of the different spatial regions of the system. As we are using a localized basis set, we can define fi 0 as the variation in the Mulliken 131
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Table 2. Characteristics of Two Octahedral Sites Lying at Layer l = 2 for the Different Pt Concentrationsa site
Pd1.00
Pd0.91Pt0.09
Pd0.82Pt0.18
Pd0.54Pt0.46
l,i
dHX dHX
% %
1st nn 2nd nn
dHX dHX
% %
1st nn 2nd nn
dHX dHX
% %
1st nn 2nd nn
dHX dHX
% %
1st nn 2nd nn
2,1
1.99
1.0
6Pd
1.97
0.1
5Pd
2.01
2.0
4Pd
2.01
1.7
2Pd
2.22
8.7
1Pt
2.05
2.5
2Pd
2.02
1.9
4Pt
3.35
0.9
4Pd
2,5
3.40
0.3
8Pd
3.42
0.4
8Pd
3.42
0.5
8Pd
2.00
1.4
6Pd
2.00
1.5
6Pd
2.00
1.2
6Pd
3.41
0.4
8Pd
3.44
0.4
4Pd
3.39
0.4
4Pt
3.42
0.4
8Pt
3.53
1.7
4Pt
1.79
10.5
3Pd
2.33 3.63
18.5 6.4
3Pt 4Pd
3.17
6.4
4Pt
The first column identifies the site. The following blocks are divided in three columns. The first column gives the average distance of H with its first and second nearest neighbors in the relaxed structure. The cutoff radii considered for 1st and 2nd neighbors are 2.4 and 3.7 Å, respectively. The second column indicates the variation of these distances with respect to the initial distance before relaxation. The last column yields the number and the species of the neighbors. a
atoms of H are repelled, whereas the first Pd neighboring atoms are attracted. Conversely, the second Pt neighboring atoms are attracted toward H, whereas the second Pd neighboring atoms are repelled. Therefore, the induced distortion upon H insertion appears to be isotropic in the low Pt content regime but becomes anisotropic for Pt content approaching 50% in the nanoalloy. The respective displacements of Pd and Pt are not only in opposite directions but also become larger as the Pt content increases. For a low concentration of Pt atoms, the effect of distortion is only noticeable up to 2.00 Å and vanishes beyond. (The percentage of variation obtained for the characteristic second neighbor distance of 3.40 Å is negligible.) This is no longer the case for the richer Pt nanoparticles. Such a behavior at high Pt concentration indicates that strong internal distortions take place upon H insertion, which worsens the capacity for H absorption and may even generate failure in the compounds. The trends exhibited by the two aforementioned sites are common to all stable sites belonging to the l = 1, 2, and 3 layers.
population for the atom i.32 To calculate this derivative, we have converged the electronic structure of the different nanostructures with a global charge ranging from 5 to +5 electrons, and we have fitted our results with a linear regression. The Fukui function of a given site for H absorption is approximated by the sum of the Fukui functions of its nearest neighbors in the H-free nanoparticle. A larger value should correlate with more reactivity of the corresponding site within the nanoparticle. The values of the Fukui function obtained for the different stable sites are plotted in Figure 7 using a color scale. As a general trend, for a given Pt concentration, larger values of the Fukui function correlate with lower insertion energies of the corresponding absorption sites and, therefore, with higher stability of H when inserted in those more reactive sites. We note that the number of sites with a noticeable stability (green points in Figure 7, with large values of f0 also) in Pd0.54Pt0.46 is reduced as compared with the other compositions. This can be traced back to the lower reactivity of Pt with H as compared with that of Pd with H, which is a known fact in bulk-like systems of both elements. However, no significant change is obtained in the values of the Fukui functions when the Pt concentration is modified. In other words, the Fukui functions do not reflect, or they cannot explain, why a small amount of Pt improves the H storage capacity, with a maximum H storage capability for a Pt content of ∼8%. Therefore, our overall results indicate that the structural effects are the main origin of this trend rather than the electronic effects. To gain further insight into the structural effects, we have computed the HX interatomic distances (with X = Pd or Pt) within the H neighborhood up to second nearest neighbors, from which we can analyze the local distortion of our systems upon H insertion. In Table 2, we report the distances obtained for two selected octahedral sites, which can be referred to as volume sites because they belong to the l = 2 layer. (See Figure 3.) We also report the percentage of change of the HPd and HPt distances with respect to the atomic position of insertion before the structural relaxation. Interestingly, the first and second nearest neighbor distances of H with Pd or Pt are overall around 2.00 Å and 3.40 Å, respectively, for the nanoparticles with Pt concentration below 20%. The results for the higher Pt content show a strong variation with respect to the reference values indicated above. Besides, the distortion induced by H insertion is more and more important insofar the Pt content increases. For instance, for site 5 of layer l = 2 inside the Pd0.54Pt0.46 nanoalloy, the first Pt neighboring
4. CONCLUSIONS In summary, we have investigated, within the DFT, the adsorption and absorption of hydrogen in cuboctahedral alloy PdPt nanoparticles of 147 atoms. Different relative compositions have been considered as those characterized in recent experiments: Pd1.00, Pd0.91Pt0.09, Pd0.82Pt0.18, and Pd0.54Pt0.46. The preferential sites for H adsorption on the surface are of hollow type except when the Pt content approaches 50%, in which case H adsorbs preferentially in bridge sites like in the Pd core/Pt shell nanoparticles previously investigated. Therefore, more hydrogen can be adsorbed on the Pt-rich surface than on the Pd-rich one. The adsorption energy follows a nonmonotonous behavior as a function of the Pt content for all adsorption sites except for the top sites. Although a local minimum is obtained at Pd0.91Pt0.09, indicating an improved H uptake on the surface for low Pt content, the lowest adsorption energy corresponds to the Pt-rich surface. Because the decomposition of H2 into 2 H atoms and its further adsorption on the surface is an intermediate reaction that enters the mechanism of the PROX reaction to eliminate CO, these alloy nanoparticles with high content of Pt approaching 50% might afford the design of more active PROX catalysts. Outer layers rich in Pt can be obtained by increasing the Pt concentration, but this is obviously an expensive 132
dx.doi.org/10.1021/jp207329q |J. Phys. Chem. C 2012, 116, 126–133
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way. Other ways are being pursued at present, such as stabilizing Pt at the surface of M core/Pt shell nanoparticles by inserting a dopant at the Pd/Pt interface16 or by modifying the composition of the core.15 Understanding the role of different core composition on the adsorption and stability of M core/Pt shell nanoparticles would be of great importance. As regards H absorption inside the nanoparticles, the most relevant result is the decrease in insertion energy for low Pt content, with a minimum obtained around Pd0.91Pt0.09 and Pd0.82Pt0.18. When the Pt content approaches 50%, the insertion energy generally increases to values exceeding even those of the pure Pd cluster. This trend, which is in complete agreement with the experimental data of Kobayashi et al.,13 is mainly due to structural reasons. The available volumes for H insertion increase for the low Pt content, that is, for Pd0.91Pt0.09 and Pd0.82Pt0.18, but decrease when the Pt content approaches 50% in Pd0.54Pt0.46. Therefore, a low Pt content serves to increase the available volume yet preserve a large amount of Pd, which is known to have a high H absorption capacity when forming nanoparticles. Moreover, the slight addition of Pt not only expands the available volume for H insertion but also preserves the structural homogeneity of the system upon H loading. By contrast, with higher Pt content, namely, for the Pd0.54Pt0.46 composition, the H insertion generates strong anisotropic internal distortions. We have also determined the most favorable environments from the electronic point of view, for H inside the PdPt nanoalloys, by means of the Fukui functions. These give a measure of the local fluctuations of electronic charge, and they can be used as an index of electronic reactivity of the different spatial regions of the system. As a general trend, for a given Pt concentration, larger values of the Fukui function correlate with lower insertion energies of the corresponding absorption sites and, therefore, with higher stability of H when inserted in those more reactive sites.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (A.L.);
[email protected] (A.G.-F.).
’ ACKNOWLEDGMENT We acknowledge the financial support from the Spanish Ministry of Science and Innovation in conjunction with the European Regional Development Fund (project FIS2011-22957) and the Junta de Castilla y Leon (project VA104A11-2). We also acknowledge the numerical support of the Pole de Calcul Intensif pour la Mer in Brest. A.V. acknowledges the financial support and the kind hospitality from the Universite de Brest (UBO), France. F.A.G. acknowledges the financial support from PROMEP-SEP-CA230, CONACyT (Mexico) grant 2005-50650. A.G.-F acknowledges the Spanish Ministry of Science and Innovation for a FPU grant. F.A.-G. acknowledges the economical support of CONACyT proposal number 162651, Mexico. ’ REFERENCES (1) Filinchuk, Y.; Chernyshov, D.; Nevidomskyy, A.; Dmitriev, V. Angew. Chem., Int. Ed. 2008, 47, 529–532. (2) Filinchuk, Y.; Czerny, R.; Hagemann, H. Chem. Mater. 2009, 21, 925–933. (3) Arashima, H.; Takahashi, F.; Ebisawa, T.; Itoh, H.; Kabutomori, T. J. Alloys Compd. 2003, 356357, 405–408. 133
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