Au Bimetallic Clusters and Their ... - ACS Publications

J. Phys. Chem. B 2005, 109, 22341-22350

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DFT Studies of Pt/Au Bimetallic Clusters and Their Interactions with the CO Molecule Chunrong Song, Qingfeng Ge, and Lichang Wang* Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: August 18, 2005; In Final Form: September 23, 2005

Density functional theory (DFT) calculations were performed to study Pt/Au clusters of different size, structure, and composition as well as their interactions with a CO molecule. Among the Pt/Au isomers studied here, the planar structure is the most stable structure in many Pt compositions, although three-dimensional structures become more stable with increasing Pt composition. Furthermore, structures with the Pt atoms surrounded by Au atoms are more stable among homotops. However, these conclusions will be altered if ligands are attached to the Pt/Au bimetallic clusters, as evidenced from the results of CO adsorption. When both Au and Pt sites are exposed, CO adsorption at the Pt site is stronger. If only a Au site is available for CO adsorption, the strongest adsorption occurs at ∼25% Pt composition, which may correlate with the experimentally observed reactivity of the core-shell structured Pt/Au nanoparticles.

1. Introduction Platinum-gold bimetallic nanoparticles have received increasing attention due to their superior catalytic activities for a number of reactions. Platinum-gold bulk alloy and nanoparticles have been widely used as alkane conversion catalysts.1-4 In the early nineties, Pignolet’s group and collaborators found that Pt/Au cluster compounds made of one center Pt atom and 5-9 surrounding Au atoms5 are active catalysts for the H2-D2 equilibration reaction.6,7 Most recently, platinum-gold nanoparticles have been studied as catalysts for various reactions,8-12 such as NO reduction,8 CO oxidation,9 and CH3OH oxidation.10,11 Carbon monoxide adsorption on the Pt/Au nanoparticles was also investigated.9,13,14 The activity of a catalyst to a specific reaction is largely dictated by the local electronic environments of the catalyst. Pure transition metal nanoparticles or clusters generate significant charge localization compared to their bulk crystal surfaces.15 Alloying another element in the cluster further modifies the local electronic properties of pure metal clusters. For instance, partial charge transfer from a gold to silver atom takes place in Ag/ Au binary clusters.16 This makes bimetallic clusters more attractive catalyst candidates because extra degrees of freedom are provided and consequently can be manipulated to altering the local electronic environments of a catalyst. The charge localizations or transfers within a nanoparticle are strongly correlated with the structure, composition, and size of the nanoparticle. Therefore, it becomes crucial to have complete size, composition, and structural control over the synthesis of nanoparticles.17 New techniques have been developed to synthesize small Pt/ Au nanoparticles with sizes less than 10 nm.13,14,18-21 Recently developed synthesis techniques, such as those developed in Zhong’s group, include a better control of composition22 and increased water solubility21 of Pt/Au nanoparticles. The goal of these newly developed techniques with the assistance of characterization tools, such as TEM, XPS, AFM, UV-vis, and Raman spectra, is to engineer Pt/Au bimetallic nanoparticles in * To whom correspondence should be addressed. E-mail: lwang@ chem.siu.edu.

a completely controlled manner. Despite the progress made so far, preparing the Pt/Au nanoparticles according to the desired purposes remains to be a challenging task. The key to achieve this goal is to have a better understanding of the correlation between the properties of the bimetallic clusters and their structure, composition, and size. Furthermore, to fully understand the catalytic activities of Pt/Au bimetallic clusters for a specific reaction, we also need to study the interaction between the clusters and the reactant and product molecules. In a brief report of our recent work,23 62 Pt/Au clusters ranging from dimer up to 13-atom clusters were studied. Formation energies of these Pt/Au clusters were obtained and compared with those of the corresponding bulk alloy. Carbon monoxide adsorption on four selected Pt/Au clusters was also reported. The CO adsorption behaviors on these clusters, such as binding sites and adsorption energies, were examined and compared with those of the Pt/Au alloy surface. This previous report, however, left many important questions unanswered. For instance, what structure will the bimetallic cluster adopt when we alloy Pt and Au together? In other words, will it adopt the structure of the pure Au cluster of the same size or that of the pure Pt cluster? Furthermore, the most stable structure of the same sized binary clusters may be different at different compositions if two pure clusters of the same size have very distinctive stable structures. If it is so, then, what is the role of composition in determining the stable structure of Pt/Au clusters of the same size? How does CO adsorption change with the composition at a given cluster size and structure? To answer these questions, we carried out a more complete and systematic examination on the Pt/Au clusters as well as CO adsorption on these clusters. Specifically, 70 Pt/Au clusters and 59 CO-Pt/Au complexes were added to our previous study. These results are reported here with discussions in section 3. Before presenting our results, we summarize the simulation details in section 2. 2. Simulation Details Our simulations were carried out using the spin-polarized gradient-corrected DFT method that is implemented in the Vienna ab initio simulation package (VASP).24-26 The electron-

10.1021/jp0546709 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/04/2005

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ion interactions were described by the projector augmented waves (PAW) method.27,28 The exchange and correlation energies were calculated using the generalized gradient approximation (GGA) of Perdew and Wang (PW91).29 A planewave basis set was used with a cutoff energy of 400 eV. Only the Γ point is needed for the finite systems described in this work. Furthermore, the size of the unit cell was chosen such that the nearest distance between neighboring images is more than 10 Å. This choice is shown to be sufficient to eliminate the interaction between neighboring images. A full geometry relaxation was performed to all the systems that are described below. All the parameters, such as the cutoff energy and the size of unit cell, were tested for the convergence of results. We studied here 70 Pt/Au bimetallic clusters consisting of 2-13 atoms with different compositions and structures. Initial geometries of these clusters were chosen according to our experience on the studies of a small number of Pt/Au clusters23 as well as pure Au30 and Pt clusters.31 To probe the interactions between the Pt/Au cluster and the CO molecule, 59 CO-Pt/ Au complexes were investigated. The Pt/Au clusters used in the studies of CO adsorption were selected among the Pt/Au clusters we investigated mainly due to their high stability. For a bare Pt/Au cluster consisting of m Pt atoms and n Au atoms, denoted as PtmAun, its binding energy is calculated using

EB ) -(EPt/Au - mEPt - nEAu)/(m + n)

(1)

where EPt/Au, EPt, and EAu are the total energy of the binary cluster, PtmAun, the energy of an isolated Pt atom, and the energy of an isolated Au atom, respectively. A positive binding energy indicates a stable structure compared to their isolated or asymptotic states. As shown in eq 1, the calculated binding energies cannot be used to compare the relative stability of two clusters of the same size but different compositions. Although m + n is constant for these two clusters, different m and n values make the comparison meaningless. However, we can use EB to establish the relative stability of clusters of the same size and composition but with different structures. We use the cluster formation energy to describe whether Pt and Au atoms tend to form a binary cluster, PtmAun, and whether a homogeneous phase is preferred energetically. The cluster formation energy, ∆Ef,C, is calculated by

(

∆Ef,C ) EPt/Au -

m n E E / m + n Ptm+n m + n Aum+n

)

(m + n) (2) where EPtm+n and EAum+n represent the energy of the pure Pt and Au clusters that have the same number of atoms and the same structure as the binary cluster PtmAun. When m ) 0 or n ) 0, ∆Ef,C ) 0 because EPtm+n becomes the energy of the pure Au (m ) 0) or Pt (n ) 0) cluster. Negative formation energy indicates a tendency to form a stable bimetallic cluster while a positive value means that the formation of a binary cluster is less favorable, with the value of ∆Ef,C being the amount of energy required to form the bimetallic cluster. Moreover, to examine whether a homogeneous or segregated binary cluster is energetically preferred, that is, more stable, we can compare the cluster formation energies among homotops.32 The preferred state is the one that has smaller cluster formation energy. We note that in our previous work23 we also defined another formation energy, denoted as ∆Hf,B, where the energies of pure bulk Pt and Au were used instead of EPtm+n and EAum+n in eq 2. As we found out, ∆Ef,C can better describe whether Pt and Au atoms tend to form a binary cluster than ∆Hf,B.

The CO adsorption energy is defined to measure the strength of CO adsorption to the Pt/Au cluster, and it is calculated as

EAds ) -(ECO-Pt/Au - EPt/Au - ECO)

(3)

where ECO-Pt/Au, EPt/Au, and ECO are the total energy of the COPtmAun complex, the isolated PtmAun cluster, and the isolated CO molecule, respectively. To investigate whether CO adsorption induces changes in the electromagnetic properties of Pt/Au clusters, we calculated the HOMO-LUMO energy gap, which is taken as the energy difference between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We also calculated the magnetic moment by

µ ) (Mu - Md)/(m + n)

(4)

for both the bare PtmAun clusters as well as for the CO-PtmAun systems. In eq 4, Mu and Md are the number of electrons in one of the two spin states with the state having more electrons denoted as Mu. The magnetic moment defined in eq 4 is directly related to the spin multiplicity. For instance, our calculated magnetic moment is 0.67 µΒ/atom for the linear Pt1Au2 clusters. This means that both clusters have two unpaired electrons of the same spin and, therefore, are in a triplet state. The calculated magnetic moment of 0.25 µΒ/atom for three Pt1Au3 clusters indicates that only one electron is unpaired and therefore these clusters are doublet. 3. Results and Discussion 3.1. Pt/Au Bimetallic Clusters. To answer the questions listed in section 1, we have chosen to investigate a range of Pt/Au clusters with different sizes, compositions, and structures. The DFT results, including the relaxed structure, binding energy, cluster formation energy, HOMO-LUMO energy gap, and magnetic moment, for 70 PtmAun clusters are summarized in Table 1. The clusters in Table 1 are arranged in the order of increasing cluster size. For the isomers, we listed first the isomer with the smallest cluster formation energy, ∆Hf,C, followed by its homotops. Other isomers are presented after the homotops. We employed in this presentation the same notation for identifying the clusters as those used in the previous work, that is, using the superscripts to distinguish isomers. Some clusters from our previous work23 are also included in Table 1 for comparison purposes. A particular type of structures, referred to as core-shell structures, will also be discussed extensively below. Strictly speaking, the core-shell structures only apply to fPt1Au12 and hPt Au 1 12 shown in Table 1, in which one Pt atom (core) is surrounded by 12 Au atoms (shell). We here extended the definition of core-shell structures to include structures consisting of one or more Pt atoms bonded together and surrounded by Au atoms. For example, we refer to ePt1Au4 as a core-shell structure, in contrast to its homotops fPt1Au4 and gPt1Au4, bPt2Au4 as a core-shell structure with respect to cPt2Au4, and aPt1Au12 and bPt1Au12 as core-shell structures in a comparison with their homotops cPt1Au12-ePt1Au12. 3.1.A. Structures of the Bimetallic Clusters. In the studies of clusters, one of the most important issues is to find the structures of the most stable clusters. The most stable structures of pure Au and Pt clusters are different. Planar pure Au clusters up to 13 atoms were found to be the most stable clusters among the isomers.30 On the other hand, three-dimensional (3D) structures are the most stable structures among the isomers of pure Pt clusters.31 For instance, among the 4-atom isomers,

DFT Studies of Pt/Au Bimetallic Clusters

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TABLE 1: Structure, Binding Energy (EB), Cluster Formation Energy (∆Ef,C), HOMO-LUMO Energy Gap (EG), and Magnetic Moment (µ) of PtmAun Clustersa

rhombus is the most stable structure in Au4 clusters, while tetrahedron is the most stable structure for Pt4 clusters. Therefore, when alloying these two elements to form bimetallic clusters, we anticipate that the structure of the most stable bimetallic cluster can take the most stable structures of either

pure metal clusters. Furthermore, we expect that the most stable Pt/Au clusters are planar as the pure Au cluster would be when the Pt composition is small, that is, more Au atoms in the cluster, and vice versa. Therefore, the relative stability of the bimetallic clusters of different structures is also determined by composition


Song et al.

TABLE 1: Continued

in the case of bimetallic clusters in addition to the factors in pure metal clusters. It has been found that the structures of Ag/ Au clusters are strongly correlated to the structures of the pure Ag and Au clusters depending on the ratio of Au to Ag atoms in the cluster.16

The results in Table 1 show that the cluster stability as well as the HOMO-LUMO energy gap and magnetic moment are very sensitive to the structure. Furthermore, not all planar clusters are more stable than their 3D isomers. In what follows,



TABLE 1: Continued

a

Note: * means the structure was previously published.23

we will focus our discussion on the most stable structures of 4-, 5-, and 6-atom clusters at different Pt compositions. For the 4-atom Pt/Au clusters, the results show that the most stable structure is rhombus for the Pt1Au3 isomers. The tendency to form a planar structure is so strong that an initially designed tetrahedron structure is also relaxed to a quasi-planar structure, bPt Au , as shown in Table 1. A planar structure, aPt Au , is 1 3 2 2 also the most stable structure among the four Pt2Au2 isomers. The planar aPt2Au2 is more stable than the tetrahedron cPt2Au2 by 0.05 eV/atom. The most surprising result is perhaps the most stable structure of Pt3Au1 isomers. We expect that a 3D structure will become the most stable structure in this case. However, the results in Table 1 show that the planar structure, aPt3Au1, is still as stable as the 3D structure, bPt3Au1. The decreasing difference in binding energies between the most stable planar and the most stable 3D isomers studied here indicated that the tendency to form 3D clusters as the most stable structures increases with increasing Pt content. The planar structure was found to be the most stable structure in the 5-atom clusters in three of the four Pt compositions, as illustrated by the results in Table 1. Furthermore, the binding energy difference between the most stable planar and the most stable 3D clusters decreases with the increase of Pt contents. The planar ePt1Au4 is more stable than the 3D aPt1Au4 by 0.17 eV/atom. The planar ePt2Au3 is more stable than the 3D bPt2Au3 by 0.11 eV/atom. The planar gPt3Au2 is more stable than the 3D bPt3Au2 by 0.06 eV/atom. The 3D gPt4Au1 is more stable than the planar ePt4Au1 by 0.05 eV/atom. This demonstrated that the planar structure is more energetically favorable in small Pt compositions, but the relative stabilities of the 3D structures increase as the Pt content increases. More interestingly, the most stable structure of Pt/Au bimetallic isomers does not necessarily

Figure 1. The binding energy difference, defined as ∆EB ) EB(planar) - EB (3D), as a function of Pt content for the 4-atom clusters (b), the 5-atom clusters (9), and the 6-atom clusters (2), respectively. The data for the 4-atom as well as for the 5-atom clusters were taken from the clusters discussed explicitly in section 3.1.C. The data for the 6-atom clusters were taken as the binding energy difference between aPt1Au5 and hPt1Au5, dPt2Au4 and iPt2Au4, aPt3Au3 and ePt3Au3, aPt4Au2 and dPt4Au2, and aPt5Au1 and cPt5Au1, respectively.

adopt either of the most stable structures of the pure clusters with the same size, as shown by clusters aPt4Au1 and gPt4Au1 in Table 1. This demonstrates a further complication in searching for the most stable structure, that is, a global minimum, among possible isomers for alloy clusters. A planar structure was also the most stable structure in the 6-atom clusters in all compositions, as shown in Table 1. The correlation between the binding energy difference of the most stable planar and the most stable 3D isomers and composition discussed above is depicted in Figure 1 for the 4-, 5-, and 6-atom clusters. The trend of a decreasing binding energy difference with Pt content holds for the 4- and 5-atom clusters. The

22346 J. Phys. Chem. B, Vol. 109, No. 47, 2005 exception for the 6-atom clusters occurs at high Pt contents, that is, the last two points. This may be due to a limited number of isomers studied in the present work. As many possible 3D structures are feasible at a large cluster size; however, the most stable ones are difficult to locate. Combining DFT with another theoretical method is necessary for such a task. The reason for clusters forming the most stable planar structures at even high Pt composition is perhaps due to the fact that the binding energies of the pure planar Pt clusters are very close to those of the 3D isomers.31 For the Pt1Au12 clusters shown in Table 1, all the planar structures have larger binding energies than the cubo- and icosahedrons. Although no calculations were carried out for the 13-atom Pt/Au clusters at other Pt compositions, we expect the planar structure is more stable than the cubo- and icosahedron isomers due to the reasons mentioned above and because the planar structure of both pure Au and Pt clusters are more stable than their cubo- and icosahedron isomers. Finally, our results show that the core-shell structure is more stable than the other structures among the homotops. For instance, the initial structure of cPt1Au5 is the rectangular structure similar to dPt1Au5 but with the Pt atom placed in the middle instead of the left corner. The relaxed structure cPt1Au5, however, becomes a planar core-shell-like structure. Another example is the bPt2Au4 cluster, a core-shell-like structure compared to the cPt2Au4 cluster. The binding energies in Table 1 show that the core-shell-like bPt2Au4 is more stable than its homotop cPt2Au4. The third example is the isomers of Pt1Au12. The planar core-shell structures, aPt1Au12 and bPt1Au12, are more stable than the other homotops, that is, cPt1Au12-ePt1Au12. Although aPt1Au12 and bPt1Au12 are more stable than their 3D isomers; their formation energies are not as negative as those of the 3D core-shell structures, fPt1Au12 and hPt Au . This difference in formation energy can be understood 1 12 by the fact that the planar structures aPt1Au12 and bPt1Au12 are incomplete core-shell structures because the Pt atom in these planar structures is not completely surrounded by Au atoms. Therefore, the more negative formation energies in the 3D coreshell structures further demonstrate that it is favorable to form core-shell structures. 3.1.B. Homogeneous vs Segregated. It has been shown that the catalytic performance of Pt/Au clusters was also affected greatly by different synthesis routes.8 Detailed analysis indicates that these different catalytic behaviors come from the homogeneity of bimetallic clusters. A better performance was observed when two metals form homogeneous bimetallic clusters. Our results show that aPt4Au4 with isolation between Au and Pt atoms is less stable than the clusters with a certain degree of mixing of Au and Pt atoms, that is, bPt4Au4, cPt4Au4, and dPt4Au4. This indicates that the clusters with a certain degree of mixing between the Au and Pt atoms are more energetically favored with respect to those with a complete isolation between the two elements. As discussed at the end of section 3.1.A, among the more stable isomers, Pt atoms tend to form the core and Au atoms form the shell. In this regard, a segregated structure is more stable than a complete homogeneous structure. This can be explained by the significant difference between the binding energies of Pt2 and Au2 dimers. The binding energy of Pt2 is 1.76 eV/atom31 and that of Au2 is only 1.17 eV/atom.30 Therefore, a cluster forming the maximum number of Pt bonds will be more stable than the other isomers. The core-shell structure with Pt atoms as the core fulfills the above condition

Song et al.

Figure 2. Binding energy (EB) of PtmAun clusters as a function of cluster size at different Pt contents: 33%(b), 50%(9), and 67%(2). The data were used in the plot from aPt1Au2 and Pt2Au1 at the cluster size of three, from aPt2Au2 at the size of four, and from aPt2Au4, aPt3Au3, and aPt4Au2 at the size of six. Note that the Pt2Au1 is a triangular structure, and its information can be found in our previous work.23

and thus contributes positively to the overall stability of the clusters. Furthermore, cluster formation energies in Table 1 show that only a limited number of bimetallic clusters have negative formation energy. This indicates that the formation of Pt/Au binary clusters is less preferred energetically than the same structure and sized pure clusters. 3.1.C. Size Effect. To investigate how the stability of bimetallic clusters changes with cluster size, we need to compare the clusters of different size but with the same composition. However, at small cluster sizes, only three pairs of data can be used for this purpose, that is, two binding energy points for each of the Pt contents of 33%, 50%, and 67%. These data are plotted in Figure 2. The solid, dashed, and dotted lines are as an eye guide only. Nonetheless, two observations can be made from the plot. First, the binding energy increases with the cluster size at the same composition. This trend is essentially the same as those observed for the pure metal clusters. As we also expected, the binding energy increases with the Pt composition at the same size. This can be well understood as increasing Pt composition means more bonds involving Pt atoms, that is, PtAu or Pt-Pt bonds, which are stronger than the Au-Au bonds. A comparison is only made here on the cluster stability using cluster sizes of 3, 4, and 6. When cluster size becomes larger, more data should be available for comparison. However, the number of structures also grows exponentially. Locating the global minimum at each composition and size becomes a challenging task. Other methods such as molecular dynamics simulations combined with DFT may be a better way to find the global minimum. Finally, it is worth pointing out that the formation energy of Pt/Au binary clusters is independent of the cluster size. This is reflected by the fact that similar negative formation energies are observed in all sizes without an obvious trend of increasing or decreasing with the change of cluster size. 3.1.D. Composition Effect. We discussed above briefly the stability of Pt/Au bimetallic clusters as a function of Pt composition. We are also interested in learning whether the formation of binary clusters is energetically more favorable with increasing Pt composition. In our previous study of bulk alloy systems at different Pt compositions, positive formation energy is found for the bulk alloy in all compositions.23 This indicates that alloying Pt and Au is energetically less favorable. Furthermore, smaller formation energy is found for the bulk alloy at low Pt compositions. This suggests that the bulk alloy with low Pt compositions is more favorable with respect to the bulk alloy


Figure 3. Cluster formation energy (∆Ef,C) as a function of Pt content at different structures: cPt1Au3* (b), ePt1Au4 (9), and triangular (aPt1Au5) (2), respectively. All the data were taken from Table 1. In the case of multiple isomers, we chose the one with the smallest formation energy.

of high Pt compositions. It is, therefore, interesting to examine whether the above observation for the bulk alloy also holds for small Pt/Au binary clusters. In Figure 3, we plotted the formation energy of Pt/Au clusters at sizes 4, 5, and 6 as a function of Pt composition. For each size, we plotted the formation energies obtained for the most stable structure. We found negative formation energies at low Pt compositions for the 4- and 5-atom clusters. This indicates that a Pt/Au bimetallic cluster is favored energetically with respect to the same sized pure clusters. On the other hand, positive formation energies at high Pt compositions indicate that these clusters are energetically less favorable. Furthermore, the formation energy as a function of Pt composition does not show a simple relationship as those found in the bulk alloy, that is, a parabolic curve with the maximum positive formation energy at the 50% Pt composition and decreasing monotonically to zero toward pure Au (0% Pt) and pure Pt (100% Pt). The formation energy of bimetallic clusters as a function of Pt composition is more complex, as shown in Figure 3. 3.2. CO Adsorption on the Pt/Au Clusters. Fifty-nine COPtmAun complexes were studied in this work. The adsorption energies obtained from our DFT calculations were given in Table 2 together with the relaxed structures, the HOMOLUMO energy gaps, and magnetic moments. For comparison purposes, we also included nine adsorption complexes reported previously in Table 2 of ref 23. Our previous studies23 have shown that the CO adsorption energy to the Au site of the PtAu dimer is 1.81 eV. In contrast, the CO adsorption energy to a pure Au dimer is 1.73 eV. The CO adsorption to the Pt site of the PtAu dimer is 2.78 eV, which is much stronger than the adsorption energy to a pure Pt dimer, 2.25 eV. This indicates that the CO adsorption strength could be greatly enhanced when it was adsorbed to the PtAu dimer with respect to the pure Au2 or the pure Pt2. We note that the comparison is made between the adsorptions to the same metal site. The current results for the CO adsorption to larger binary clusters also show the same trend. For instance, the CO adsorption energy in the ePt2Au4CO complex is larger than the adsorption energy in the aPt6CO by 0.14 eV. Another general trend is that the CO adsorption changes the HOMO-LUMO energy gap and in most cases increases the energy gap. This implies that the optical properties of the Pt/Au clusters can be tuned by adsorbing different ligands. Experimentally, the change of optical property upon adsorption of different ligands was observed for pure 11-atom Au clusters.33 Furthermore, the energy gap is also sensitive to the orientation of the adsorbed

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22347 CO molecule, as shown by the results in Table 2. In contrast to the sensitivity of the energy gap, the CO adsorption changes the magnetic moment of the cluster in about one-third of the adsorption complexes studied here. In what follows, we will discuss in detail the other aspects of the DFT results shown in Table 2. 3.2.A. Composition Effect. To explore the correlation between the CO adsorption and the Pt composition in the Pt/ Au cluster, we investigated the CO adsorption in two Pt/Au clusters of different sizes at a series of Pt compositions. The results are shown in Table 2. As the adsorption energies of CO on the Au site and Pt site are very different, we plotted in Figure 4 two sets of data for each cluster size. The dashed lines and the solid lines are the adsorption energies of CO at a Pt site and at a Au site as a function of Pt composition, respectively. For both cluster sizes, the adsorption to the Pt site is much stronger than to the Au site. Furthermore, there is a maximum in the CO adsorption to the Au site at about 25% Pt composition. In the case of CO adsorptions to the Pt site, the adsorption is less composition dependent for the adsorption on the 4-atom cluster but the adsorption strength increases with the Pt composition in the 6-atom cluster. As the CO adsorption to the Pt site is much stronger, we would expect that CO adsorption predominantly occurs on the Pt sites of the Pt/Au cluster if both Pt and Au sites are available. In this case, the strongest adsorption would be observed at high Pt compositions as shown in Figure 4. On the other hand, as our calculations indicated, the core-shell structure with Au atoms being the shell is the most stable structure. This indicates that the Au sites will more likely be exposed at the surface of the Pt/Au clusters and consequently facilitate CO adsorption. When CO adsorption to the Au sites becomes dominant, the strongest CO adsorption would occur at ∼25% Pt content. Indeed, Zhong and co-workers reported that the core-shell structured Pt/Au nanoparticles of 2.5 nm at ∼27% Pt content show promising catalytic activity for CO and methanol oxidation.34 3.2.B. Adsorption Site Effect. Our previous study showed that the CO adsorption to the Au site is weaker than that to the Pt site.23 This is also true from the current work when more adsorption complexes were investigated. Analogous to the CO adsorption on surfaces, we define the adsorption sites such as those in the aPt2Au1CO and bPt2Au1CO complexes as atop sites and that in the cPt2Au1CO complex as the bridge site. Comparison between the results of atop site adsorption and bridge adsorption shows that CO adsorption is stronger on the atop site. The same conclusion can be drawn if we compare the adsorption energies between the cPt3Au1CO and the dPt3Au1CO complexes. The data in Table 2 also illustrated that CO adsorption on the Pt/Au cluster can be complicated by the extra degree of freedom which makes CO orientations possible. The CO orientation effect will be discussed below. 3.2.C. CO Orientation Effect. As we expected, the CO orientation plays important roles in the determination of adsorption strength. For instance, when the CO molecule in the bPt Au CO complex is reoriented into the position illustrated 1 2 in the cPt1Au2CO, the adsorption energy decreases by 0.17 eV and so does the HOMO-LUMO energy gap by 0.46 eV. To further explore the orientation effect, we designed two series of adsorption complexes: bPt3Au1CO-fPt3Au1CO and hPt3Au1CO-lPt3Au1CO. The results are shown in Table 2. Two types of orientation effects are explored through these complexes. The first is the orientation change of the CO


Song et al.

TABLE 2: Structure, Adsorption Energy (EAds), HOMO-LUMO Energy Gap (EG), and Magnetic Moment (µ) of CO-PtmAun Complexesa

molecule when it is adsorbed to the same Pt atom, such as bPt3Au1CO and cPt3Au1CO. The second type is that the CO molecule orientates itself and binds or even migrates to the neighboring Pt atoms. One example of the second type of orientation effect is shown in the complex cPt3Au1CO to dPt3Au1CO and further

to ePt3Au1CO. As shown in Table 2, with the changes of CO orientation, the adsorption energies obtained in these complexes change accordingly. The most significant change in adsorption energy comes from the second type of orientation effects, as we expected. We also note that the HOMO-LUMO energy gap



TABLE 2: Continued

a

Note: * means that the structure was reported previously.23

Figure 4. CO Adsorption energy (EAds) as a function of Pt content for the CO adsorbed on different structures: cPt1Au3* (b) and aPt1Au5 (2), at the Pt site (dashed lines) and at the Au site (solid lines). Data were taken from Table 2. In the case of multiple isomers of adsorption complexes, the one with the maximum adsorption energy was chosen.

changes as well, and in some cases, the change is rather significant. For instance, the HOMO-LUMO energy gap changes by 1.31 eV from cPt3Au1CO to dPt3Au1CO due to the second type of CO orientation. Furthermore, the magnetic moment also changes in some cases due to different CO orientations.

3.2.D. Ligand Effect. The results in Table 1 have shown that the planar structure is the most stable structure for Pt/Au clusters for most compositions. Furthermore, the core-shell structure is favored in the formation of Pt/Au clusters. However, these facts can be changed upon adsorption of CO molecules, that is, CO adsorption changes the relative stability of the clusters, as shown in Table 2. For instance, in the bPt2Au4CO complex, the planar Pt/Au cluster was transformed into 3D after CO adsorption. More supporting evidence can be found in the 3D adsorption complexes ePt2Au4CO and iPt2Au4CO. The results show clearly that they become more stable than the planar complexes aPt2Au4CO and nPt2Au4CO. The second effect of CO adsorption is that the none coreshell structure may also become as stable as the core-shell structure. For example, when the CO molecule is adsorbed to the none core-shell structure cPt2Au4, the adsorption complex ePt Au CO was found to be more stable than the hPt Au CO 2 4 2 4 complex that is formed by attaching the CO molecule to a more core-shell-like structure, that is, bPt2Au4. Carbon monoxide adsorption may also cause reconstructing of the cluster. For example, as a CO molecule is attached to a core-shell structure, the strong Pt-CO interaction may cause the Pt atoms from the core to migrate to the surface of the cluster. These reconstructions will be limited by kinetic factors. Adsorption-induced surface reconstructions have been observed in many systems.35,36 4. Conclusions DFT calculations were performed to study Pt/Au clusters consisting of 3-13 atoms with various structures and compositions. The binding energy, the cluster formation energy, as well as the HOMO-LUMO energy gap and the magnetic moment were obtained. The results show that planar structure is the most stable structure among many Pt/Au isomers studied in this work, although not all planar structures are more stable than their 3D isomers. Furthermore, the binding energy difference between

22350 J. Phys. Chem. B, Vol. 109, No. 47, 2005 the most stable planar and the most stable 3D clusters decreases with increasing Pt content, indicating that 3D clusters become more stable. Structures with Pt atoms surrounded by Au atoms are more stable structures among homotops. The relative stability between clusters discussed above will be changed upon CO adsorption. Fifty-nine CO-Pt/Au complexes were investigated in this work. Our results show that CO adsorptions to the Pt site are stronger than to the Au site. If only the Au site is available for CO adsorption, then the strongest adsorption occurs at ∼25% Pt composition, which may correlate with the experimentally observed reactivity of the Pt/Au nanoparticles for CO and methanol oxidations. In the studies of adsorption site and CO orientation effects, more complex behavior is observed for the CO adsorption on the Pt/Au clusters. Furthermore, the HOMOLUMO energy gap is changed, in some cases rather drastically, upon the CO adsorption. In about one-third of adsorption complexes, the magnetic property of the cluster is also changed. Acknowledgment. We acknowledge the start-up funds from Southern Illinois University Carbondale (SIUC) and the financial support from the Materials Technology Center at SIUC. References and Notes (1) O’Cinneide, A.; Gault, F. G. J. Catal. 1974, 37, 311. (2) Van Shaik, J. R. H.; Dessing, R. P.; Ponec, V. J. Catal. 1975, 38, 273. (3) Kane, A. F.; Clarke, J. K. A. J. Chem. Soc., Faraday Trans. 1980, 76, 1640. (4) Sachtler, J. W. A.; Somorjai, G. A. J. Catal. 1983, 81, 77; 1984, 89, 35. (5) Steggerda, J. J. Comments Inorg. Chem. 1990, 11, 113. (6) Aubart, M. A.; Pignolet, L. H. J. Am. Chem. Soc. 1992, 114, 7901. (7) Kappen, T. G. M. M.; et al. Inorg. Chem. 1993, 32, 1074. (8) Mihut, C.; Descorme, C.; Duprez, D.; Amiridis, M. D. J. Catal. 2002, 212, 125.

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