Electrocatalytic Properties of PtBi and PtPb Intermetallic Line

In experiments,(10, 11) both PtPb and PtBi anodes are found to show no sign of CO .... basis is the following: Pt, (0,0,1/4) and (0,0,3/4) and p-metal...
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8266

J. Phys. Chem. C 2008, 112, 8266–8275

Electrocatalytic Properties of PtBi and PtPb Intermetallic Line Compounds via DFT: CO and H Adsorption L.-L. Wang* and D. D. Johnson Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, 1304 West Green Street, Urbana, Illinois 61801 ReceiVed: NoVember 21, 2007; ReVised Manuscript ReceiVed: February 28, 2008

Pt intermetallic line compounds, such as with Pb and Bi, have been observed to improve dramatically the anode carbon monoxide (CO) tolerance of fuel cells for oxidation of small organic molecules. We have used density functional theory to study the CO and H adsorption on different surfaces of these line compounds. Among different surface orientations of PtPb and PtBi, we find (100)B and (110) have much lower cleavage energies and CO adsorption energies than (100)A and (001) and also much lower CO adsorption energies than Pt(111). Thus, (100)B and (110) are the surfaces most relevant to experimental observations, and the increased CO tolerance is not attributable to the (001) surface of the line compounds as assumed experimentally, because it binds CO the strongest, even more strongly than Pt(111). We also find that CO is not likely to dissociate on these materials. Finally, we correlate d-band center and CO adsorption energy for these non close-packed systems by developing a more universal form of the original d-band center model that includes effects of symmetry of adsorption site and local relaxation. We find that the increased CO tolerance arises from a downward shift of Pt d-band center because of alloying, which also accounts for the difference between PtPb and PtBi. Introduction As an important part of pursuing a carbon neutral economy, improving fuel-cell technology has attracted intensive research in recent years.1 In fuel cells that convert chemical energy of small organic molecules (such as hydrogen, methanol, and formic acid) to electrical energy via electrocatlytic reactions, the best pure metal anode catalyst is Pt. However, Pt is poisoned easily by carbon monoxide (CO), which is present either as intermediates or in fuel gas. The strong adsorption of CO on Pt blocks active sites. Currently, PtRu-based materials are used to increase CO tolerance.2 Increased CO tolerance involves both a bifunctional effect,3–6 in which Ru helps activate water to form OH to oxidize effectively CO adsorbed on nearby Pt sites, and a ligand effect,4,7–9 in which CO adsorption is weakened by the downward shift of Pt d-band center (from shortened Pt-Pt distances induced by a smaller Ru lattice constant relative to fcc Pt) and also band hybridization. However, the disadvantage of PtRubased materials is that Pt and Ru phase segregate, and it is hard to control the number of active sites. In this regard, bimetallic line compounds (ordered phase with composition only existing at perfect stoichiometry), such as PtPb and PtBi,10,11 provide the advantage of uniform active sites, that is, more catalytic area, and an economic advantage because the cost of p-metal Pb and Bi is much lower than that of Ru. In experiments,10,11 both PtPb and PtBi anodes are found to show no sign of CO poisoning and also reduce the oxidation overpotential for formic acid. The increased CO tolerance has been attributed to the much larger Pt-Pt distance, that is, fewer Pt sites per surface area than Pt(111), such as (001) surface of PtBi.10,11 Understanding how Pt-based line compounds increase CO tolerance will help design better catalytic materials. Oana et al.12 recently used tight-binding extended Hückel calculations * Corresponding author.

to study the adsorption of CO on surfaces of PtBi and PtBi2 line compounds. They present a picture that Pt d bands are shifted down in energy because of alloying with p-metals and CO binds preferably on the bridge site between two neighboring Pt (i.e., Pt-brg site rather than Pt-atop site) to achieve maximum orbital overlap. They found that the CO antibonding 2π* orbitals are almost filled, and the CO bond is easily broken. However, their study did not include structural relaxation. Therefore, we address CO and H adsorption on various surfaces of PtPb and PtBi by using density functional theory13,14 (DFT) to provide a more accurate description of the electronic structure and adsorption energies and attempt to validate or correct previous theoretical results and experimental interpretations as well. In our DFT studies of PtPb and PtBi, we found, compared to (001) and (100)A, that the (100)B and (110) surfaces have the lowest cleavage energies. We studied the CO adsorption on different surfaces of PtPb and PtBi to understand the increased CO tolerance. On (100)B, (110), and (100)A surfaces of these materials, where Pt-brg site is available, the Pt-brg site is the most preferred adsorption site, and the Pt-atop site is not stable (relaxing to Pt-brg configuration) for CO. On (001) surface, where Pt-brg site is not available because of the cleaved Pt-Pt bond along the surface normal direction, the Pt-atop site is preferred, and it is the strongest binding site among all surfaces, althought the Pt site has the lowest density on (001) surface. We found that only (100)B and (110) bind CO more weakly than Pt(111) and, as a result, are the surfaces relevant to experimental observation of high CO tolerance, whereas (100)A and (001) actually bind CO more strongly than Pt(111). The increased CO tolerance can be explained by the much lower CO binding energy in comparison to that of pure Pt. This, in turn, is explained by the lower d-band center of PtBi and PtPb relative to Pt, in the order PtBi < PtPb < Pt. In addition, we find that the CO antibonding 2π* orbitals are not almost filled; therefore, CO dissociation is not expected.

10.1021/jp7110956 CCC: $40.75  2008 American Chemical Society Published on Web 05/02/2008

Electrocatalytic Properties of PtBi and PtPb Compounds via DFT

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Figure 1. Bulk and surface structures of PtPb and PtBi with top-most plane in view (a) (001), (b) (100)A, (c) (100)B, and (d) (110). In each panel, gray (red) spheres stand for Pt (Pb or Bi), respectively. The surface unit cells contain (a) four, (b) six, (c) six, and (d) two atomic layers in the surface normal direction. For a better view, larger supercells of 2 × 2 × 1 are shown in (a), (b), and (c) and 2 × 2 × 2 in (d). The boundaries of the supercells and Pt-Pt bonds are drawn to specify the coordination system and guide the eye. For (001) with hexagonal supercell, both the vertical boundary and Pt-Pt bond are in the z direction. For (100)A, (100)B, and (110) with orthorhombic supercells, the vertical boundary is the z direction, and Pt-Pt bond is in the x direction.

TABLE 1: Structural Parameters and Cohesive Energies of Bulk PtPb and PtBi PtPb PtBi

a (Å)

c/a

Ecoh (eV)

4.38 4.41

1.27 1.28

10.20 10.55

To correlate the catalytic behavior with electronic structures of the surfaces, we perform a detailed analysis of the d-band center model.15 We found that the Pt-atop site on (001) surface has a unique symmetry in that the Pt-Pt bond is cleaved, which enhances its ability to bind CO. To include these features, we have modified the original d-band center hybridization model,15 and only with these modifications did we find a good correlation for (001) surfaces of the line compounds. In addition, the linear relation between the change in Pt-Pt bond length and d-band center has been recently reported and explored very successfully for catalyst design.16–18 However, a universal form has not yet been given for this important relation. Here, we propose a simple formula, based on the Friedel’s19 uniform d-band distribution model and the universal d-band width from tight-binding theory given by Harrison,20 that estimates the shift in d-band center due to metal bond-length change. The paper begins with a section on theoretical and computational details, followed by the results and discussion, and ends with conclusions. Computational Details We used DFT13,14 with a generalized gradient approximation (GGA, given by PW9121) for exchange-correlation functional, a plane-wave basis set and projector-augmented-wave method22 with frozen core approximation as implemented in the Vienna atomic simulation package .23,24 The valence states are Pt 5d96s1,

Pb 6s26p2, and Bi 6s26p3. The (100)A and (100)B surfaces are modeled as slabs containing six atomic layers, with the bottom three layers fixed during ionic relaxation. The (110) surface contains four atomic layers with the bottom two layers fixed, and (001) surface has eight atomic layers with the bottom four layers fixed. The surface structures are explained in detail in the next section. We used a 400 eV kinetic energy cutoff and k-point meshes25 of 8 × 8 × 1 for (100)A, (100)B, and (110) surfaces and 12 × 12 × 1 for (001) surface in the first Brillouin zone. To study the correlation between d-band center and adsorption energies, CO and H adsorption on the fcc(111) surface of late transition metals Cu, Rh, Pd, Ag, Ir, Pt, and Au are also calculated. In these calculations, a four-atomic-layer 2 × 2 × 1 surface unit cell is used with a k-point mesh of 7 × 7 × 1 with the bottom two atomic layers kept fixed. All slabs have at least 12 Å of vacuum. Total energies were converged to 1 meV/atom with respect to size of vacuum and k-point mesh, whereas the magnitudes of the force on each atom that is allowed to move were reduced below 0.02 eV/Å by using conjugate gradient in ionic relaxations. Results and Discussion Bulk Structures of PtPb and PtBi. Figure 1a shows the bulk structures of PtPb and PtBi. They both have the NiAs structure,12 where Pt forms the simple hexagonal lattice and occupies the octahedral site of p-metals, and p-metals occupy the trigonal prismatic site of Pt. The basis is the following: Pt, (0,0,1/4) and (0,0,3/4) and p-metals, (1/3,1/3,0) and (2/3,2/3,1/2). The in-plane hexagonal lattice constant a and the ratio between the out- and in-plane lattice c/a are listed in Table 1. The data show that PtPb and PtBi have very close structural parameters. The last column of Table 1 lists the cohesive energy of the two

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Figure 2. (a) PDOS on each metal in bulk PtPb and PtBi and (b) DOS of bulk fcc Pt and Pb.

TABLE 2: Cleavage Energy for Different Surfaces of PtPb and PtBia Eclv (J/m2) (100)B

(110)

(100)A

(001)

0.59 0.53

0.69 0.61

0.96 0.91

1.09 0.99

PtPb PtBi

a The energy increases in the order of (100)B, (110), (100)A, and (001). From the bulk structure, interaction of Pb-Pb or Bi-Bi is the weakest, whereas Pt-Pt is the strongest, as indicated by the longest and shortest nearest neighboring inter-atomic distances (reported in the text).

TABLE 3: Structural Relaxation of Clean PtPb and PtBi Surfaces in angstrom with Respect to Bulk Terminationsa (100)B Pt

Pb/Bi

(110) Pt

Pb/Bi

(100)A Pt

Pb/Bi

(001) Pt

Pb/Bi

PtPb 1 -0.08 -0.18 -0.23 +0.14 -0.21 +0.14 -0.15 +0.14 x/y/2 (0.05 -0.03 +0.18 (0.09 (0.10 -0.06 -0.05 -0.01 PtBi 1 -0.04 -0.13 -0.29 +0.01 -0.31 +0.07 -0.17 +0.20 x/y/2 (0.01 +0.04 +0.08 (0.11 (0.11 -0.14 -0.08 +0.05 a Rows labeled with 1 are the position changes in the z direction of the first layer Pt, Pb, or Bi. Rows labeled with x/y/2 have different meanings for different metals on different surfaces. On (100)B and (100)A, they are the changes in Pt-Pt bond length in the x direction for Pt and the position changes in the z direction for Pb or Bi. On (110), they are the position changes in the z direction for Pt and the position changes in the y direction for Pb or Bi. On (001), they are the position changes in the z direction.

line compounds, which shows that PtBi has a slightly stronger bonding than PtPb. By using PtPb as an example, a closer inspection of the bulk structure shows that each Pt has two first nearest Pt neighbors at 2.78 Å and six first nearest Pb neighbors at 2.89 Å. Likewise, each Pb has six first nearest Pt neighbors at 2.89 Å, besides the six first nearest Pb neighbors at 3.76 Å. Within these structures, although the first nearest-neighbor Pt-Pt distance of 2.78 Å is very close to that of fcc Pt (2.79 Å), the number of first nearest neighbors of Pt has been reduced to eight (two Pt and six Pb) from 12 in fcc. The second nearest-neighbor Pt-Pt distance is 4.38 Å. Compared to the first nearest-neighbor Pt-Pt distance, Pt-Pb and Pb-Pb are longer, which means a much weaker interaction than for Pt-Pt. As such, it is easier to break Pt-Pb or Pb-Pb bonds to make a surface. Similar descriptions apply to PtBi, which just has a slightly larger lattice constant; the first nearest-neighbor Pt-Pt bond in PtBi is 2.82 Å.

Figure 2a shows the projected density of states (PDOS) of bulk PtPb and PtBi with Pt d-band center at -2.98 and -3.25 eV, respectively. The d-band center is calculated as an integrated average from the lower to the upper edges of Pt d-bands DOS. Compared to fcc Pt, which has a d-band center at -2.88 eV, as shown in Figure 2b, there are two major changes to Pt d bands in the line compounds. First, when alloying with Pb and Bi, the sp electrons from these p-metals shift the Fermi level up. Second, the width of Pt d bands (ranging from -7 to -1 eV) is now narrower than that of fcc Pt (from -7 to 1 eV). The decreasing d-band width is due to the smaller number of nearestneighbor Pt-Pt interactions in PtPb and PtBi than in fcc Pt. Usually, the d-band center moves higher in energy when the d-band width decreases. But in the current situation, the shift of the Fermi level to a higher energy is overwhelming; that is, the upper edge of the Pt d band is shifted below the Fermi level because of the filling of sp electrons from Pb and Bi. Thus, the d-band center is lower in energy than fcc Pt. These two findings from our DFT calculations confirm those of Oana et al.12 based upon a tight-binding extended Hückel model. Compared to Bi, Pb has one less p electron and also one less proton nuclear charge. Hence, the s- and p-bands of Pb when alloyed with Pt are higher than those of Bi. As a result, the center of Pt d band is 0.3 eV higher in PtPb than in PtBi. The s-band of Pb and its resonance with Pt is from -11 to -7 eV, whereas for PtBi, it is from -13 to -10 eV. Also, the difference in DOS between Pb and Bi from -7 to -5 eV accounts for the one less p electron. Surface Structures of PtPb and PtBi. There are four different ways to cleave PtPb and PtBi crystals to make surfaces as shown in Figure 1. The cleavage energies in J/m2 for these surfaces are listed in Table 2 in the order of the lowest to the highest. Among all surfaces, only (001), shown in Figure 1a, cleaves the short and strongly interacting Pt-Pt bond, along with the Pt-Pb (Pt-Bi) and Pb-Pb (Bi-Bi) bonds; thus, it has the highest energy. As shown in Figure 1b,c, the (100) orientation can be cleaved at two different places. (100)A cuts mostly through Pt-Pb (Pt-Bi) bonds, whereas (100)B cuts through the same number of Pb-Pb (Bi-Bi) bonds as Pt-Pb (Pt-Bi) bonds. For (110), the ratio between the number of cleaved Pt-Pb (Pt-Bi) bonds and Pb-Pb (Bi-Bi) bonds is larger than that for (100)B but smaller than that for (100)A. In summary, (100)B and (110) surfaces cost much less energy to make than (100)A and (001). Therefore, these two surfaces are most likely the ones exposed in experiments where either bulk materials10,11 or nanoparticles26 with diameters of 10-20 nm are used, and these two surfaces are also the ones having higher CO tolerance than Pt(111), as shown in the next section.

Electrocatalytic Properties of PtBi and PtPb Compounds via DFT

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Figure 3. PDOS of surface Pt d bands on the different surfaces of (a) PtPb and (b) PtBi.

TABLE 4: d-Band Center and Adsorption Energies of CO and H on Different Surfaces of PtPb and PtBia εd (eV) (100)B (110) (100)A (001)

EaCO (eV)

EaH (eV)

PtPb

PtBi

PtPb

PtBi

PtPb

PtBi

-2.76 -2.54 -2.16 -2.30

-2.91 -2.81 -2.29 -2.57

-1.47 -1.44 -1.88 -2.13

-1.12 -1.15 -1.66 -2.12

-0.54 -0.51 -0.71 -0.28

-0.40 -0.27 -0.59 -0.41

a The negative sign indicates that adsorption lowers the system energy. For comparison, the CO (H) adsorption energy is -1.76 (-0.50) eV on Pt(111), respectively.

To get the cleavage energy, we relaxed the slabs keeping the bottom two or three atomic layers fixed out of a total of four or six layers, respectively. During relaxations, there is no surface reconstruction, and metal atoms stay in the registry with bulk terminations, albeit there are some relaxations in the top two atomic layers, as shown by the data in Table 3. Atoms undergo relaxations mostly in the direction perpendicular to the surface, that is, the z direction in the coordination system specified in Figure 1. The first layer of Pt always relaxes inward on all surfaces. In contrast, the first layer of Pb (Bi) relaxes outward, except for the (100)B, where p-metals are in the very top layer. The outward relaxations of p-metals compensate the inward relaxation of the first Pt layer. There are also noticeable relaxations of first layer of p-metals on (110) in the y direction. The second layer of Pt that is allowed to relax relaxes inward on (001) and outward on (110). Most of the second layer of p-metals relaxes inward. Thus, the surface Pt-Pt bond length in the (001) of PtPb (PtBi) is contracted by 0.10 (0.09) Å, respectively. This 3% contraction of Pt-Pt bond length in (001) reflects the cleavage of the strong Pt-Pt bond in the z direction. On (110), the in-plane Pt-Pt bond length does not change. On (100)A and (100)B, the in-pane Pt-Pt bond length has an alternating contraction and expansion, respectively, of 0.10 and 0.05 Å for PtPb, and 0.11 and 0.01 Å for PtBi. In Figure 3, we show the PDOS of the surface Pt d bands for different surface orientations of PtPb and PtBi. They all have the two major features mentioned earlier for the bulk line compounds; that is, the upper edge of d bands is below the Fermi level, and the d-band width is narrower than that of fcc Pt. For the same surface orientation, the d-band center of PtPb is always higher than that of PtBi because of one less electron. For the same material, the d-band center changes significantly with different surface orientations. The Pt d band shifts and leans toward the Fermi level in the sequence of (100)B, (110), (001), and (100)A, because surface Pt loses more and more neighboring interactions. For (100)A, the upper edge of the Pt d bands even touches the Fermi level.

Table 4 lists the positions of d-band centers for different surfaces of PtPb and PtBi. The data of d-band center confirm the trends shown in Figure 3, that is, (100)B being the lowest, (100)A the highest, followed closely by (001) and (110). These features certainly affect the properties of the surfaces to bind molecules such as CO and H. CO Adsorption and H2 Dissociation. To study the increased CO tolerance on PtPb and PtBi surfaces and their (de)hydrogenation ability, we calculate the adsorption energies of CO and dissociation energies of H2 on various sites on different surfaces of PtPb and PtBi. The lowest adsorption energies of CO and H on each surface are listed in Table 4, and the lowestenergy configurations of CO adsorbed on PtPb surfaces are shown in Figure 4. The lowest-energy configurations for the adsorption of CO on PtBi surfaces and those of H on PtPb and PtBi surfaces (not shown) are on the same sites. The surfaces are presented in the order of increasing cleavage energy, with (100)B and (110) having a much lower cleavage energy than (100)A and (001). For both CO and H adsorption, we found that the Pt-brg is always the preferred one when it is available on (100)B, (110), and (100)A surfaces. On (001) surface, where the Pt-brg site is not available because of the cleavage of Pt-Pt bond in the z direction, the Pt-atop site is preferred. The preference of Pt-brg site agrees with the orbital analysis by Oana et al.12 As shown in Table 4, compared to the adsorption energy of -1.76 eV on Pt(111) surface, the (100)B and (110) surfaces of PtPb and PtBi bind CO more weakly, whereas (100)A and (001) surfaces bind CO more strongly or with almost the same intensity for PtBi(100)A. Therefore, the (100)B and (110) should be the surfaces that exhibit increased CO tolerance in the experiment,10,11 not (001), even though Pt sites are isolated and each is surrounded by p-metals on (001). The ability to strongly bind CO of (001) can be attributed to the fact that the Pt-Pt bond is cleaved, unlike the other three surface orientations. A detailed analysis of electronic structure and d-band center model is given in the next section. Compared to PtPb, PtBi has a much weaker binding of CO because of the lower d-band center. This also agrees with the experimental observation that PtBi shows virtual immunity to CO poisoning. As seen in the last two columns of Table 4, H2 dissociates on the surfaces of PtPb and PtBi. Compared to the dissociation energy of -0.50 eV on Pt(111), the (100)B and (110) surfaces of PtPb show little change, whereas those of PtBi have a weaker binding of H by about 0.2 eV. The relaxations of the surfaces caused by CO adsorption with respect to the relaxed clean surfaces are listed in Table 5, and the binding configurations of CO are listed in Table 6. Those for H adsorption are listed in Tables 7 and 8. When CO is

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Figure 4. The lowest-energy configurations of CO adsorbed on (a) (100)B, (b) (110), (c) (100)A, and (d) (001) surfaces of PtPb and PtBi. The large (small) gray spheres stand for Pt (C), respectively. The large (small) red spheres stand for Pb/Bi (O), respectively. In each panel, the left (right) subfigure is from the top (side) view in the xy (xz) plane, respectively, and a 2 × 2 surface supercell is shown for clarity.

TABLE 5: Structural Relaxation of CO-Adsorbed PtPb and PtBi Surfaces in angstrom with Respect to the Clean Surfaces Listed in Table 3a (100)B Pt

Pb/Bi

(110) Pt

Pb/Bi

(100)A Pt

Pb/Bi

(001) Pt

Pb/Bi

PtPb 1 +0.26 +0.51 +0.23 +0.02 +0.17 -0.09 +0.05 -0.10 x/y/2 (0.07 -0.04 (0.13 +0.36 (0.08 +0.01 +0.05 -0.02 PtBi 1 +0.18 +0.22 +0.24 +0.01 +0.19 -0.06 +0.04 -0.15 x/y/2 (0.04 -0.04 (0.07 +0.22 (0.09 +0.03 +0.07 -0.03 a Rows labeled with 1 are the position changes in the z direction of the first layer Pt, Pb, or Bi. Rows labeled with x/y/2 have different meanings for different metals on different surfaces. On (100)B and (100)A, they are the changes in Pt-Pt bond length in the x direction for Pt and the position changes in the z direction for Pb or Bi. On (110), they are the position changes in the z direction for Pt and the position changes in the y direction for Pb or Bi. On (001), they are the position changes in the z direction.

adsorbed on (100)B and (100)A surfaces, the Pt layer relaxes outward, and the position in the z direction increases a lot from that of the clean surface and comes close to the bulk-terminated value for (100)A and is even larger for the case of (100)B. Whereas the first layer of p-metal in (100)A moves in the opposite direction to Pt, the p-metals in (100)B move in the same direction as Pt, and the magnitude is as high as 0.51 Å for Pb on PtPb (100)B. This is because the first layer of p-metals in (100)B has the fewest neighboring interactions, and Pb is more prone to move than Bi because of its lower cohesive energy compared to that of Bi in PtBi. For the two in-plane Pt-Pt bonds on the (100)A and (100)B surfaces, the one underneath CO is longer than the other one. On the (110) surface, Pt also moves back to the bulkterminated value as the result of interactions with CO. An important aspect of the Pt-brg site on (110), compared to that

on (100)A and (100)B, is that it is asymmetric across the Pt-Pt bond along the y direction. There is a surface p-metal only on one side of the Pt-Pt bond, see Figure 1d. This does affect the relaxed configuration of CO and p-metals on (110). Even though the first layer of p-metals moves slightly in the z direction, p-metals move a lot in the y direction to be close to the Pt-Pt bond to get more surface-CO hybridization. As the result, CO is titled away from the nearby p-metals with an angle of 26.7° (18.9°) with respect to the surface normal direction on PtPb (PtBi), respectively. The increase of the z position of the first layer of Pt on the (001) surface is the smallest among all surfaces; the Pt-Pt bond length remains about the same as in the clean surface. As seen in Table 6, the Pt-C (C-O) bond lengths are around 2.02 (1.19) Å on the three Pt-brg sites, respectively, whereas they are shorter on the Pt-atop site on (001) with values of 1.88 (1.16) Å, respectively. The CO bond length upon adsorption only increases slightly from that of free CO at 1.14 Å. Therefore, in contrast to the earlier study,12 our study suggests that a dissociation of CO is very unlikely on the surfaces of the line compounds. The vibration frequencies of the adsorbed CO on different surfaces are also listed in Table 6. In comparison to the value of 2150 cm-1 for free CO, the vibration frequencies shift to lower values. The magnitude of the red shift is a good indicator of the change in CO bond length. The red shifts in CO vibration would be a useful measurement in experiments to distinguish different adsorption sites on different surfaces. For H adsorption (see Table 7), the (100)A, (100)B, and (110) surfaces relax in a similar way to those upon CO adsorption but with a smaller magnitude. Again, to accommodate the asymmetric nature of the Pt-brg site on (110), H adsorbs at 0.51 (0.52) Å away in the y direction from the middle point of the Pt-Pt bond. On (001), the relaxation of the first layer of Pt is actually larger than that upon CO adsorption. The Pt-Pt bond length returns close to the bulk-terminated value. Compared to CO adsorption, this different behavior of Pt-Pt bond on (001)

Electrocatalytic Properties of PtBi and PtPb Compounds via DFT

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TABLE 6: Pt-C and C-O Bond Length in angstrom on CO-Adsorbed PtPb and PtBi Surfacesa (100)B PtPb PtBi

(110)

(100)A

(001)

Pt-C

C-O

Pt-C

C-O

Pt-C

C-O

Pt-C

C-O

2.02 2.02

1.20(1860) 1.19(1860)

2.02 2.03

1.19 (1864) 1.19 (1863)

2.01 2.02

1.18 (1942) 1.18 (1942)

1.88 1.88

1.16(2075) 1.16(2075)

a On the Pt-brg site of (110) of PtPb (PtBi), CO tilts with an angle of 26.7° (18.9°), respectively, with respect to the z direction in the yz plane pointing away from the nearby Pb (Bi). On all other surfaces, CO sits straight upward in the z direction on the Pt-brg site of (100)A and (100)B and the Pt-atop site of (001). The vibration frequency of CO (in cm-1) is listed in parentheses.

TABLE 7: Structural Relaxation of H-Adsorbed PtPb and PtBi Surfaces in angstrom with Respect to the Clean Surfaces Listed in Table 3a (100)B Pt

(110)

Pb/Bi

Pt

Pb/Bi

(100)A Pt

Pb/Bi

(001) Pt

Pb/Bi

PtPb 1 +0.15 +0.26 +0.08 -0.01 +0.05 -0.07 +0.15 +0.18 x/y/2 (0.04 -0.01 (0.08 +0.13 (0.06 +0.02 +0.05 +0.08 PtBi 1 +0.11 +0.15 +0.10 -0.01 +0.09 -0.05 +0.12 +0.10 x/y/2 (0.06 -0.01 (0.06 +0.16 (0.07 +0.05 +0.06 +0.02 a Rows labeled with 1 are the position changes in the z direction of the first layer Pt, Pb, or Bi. Rows labeled with x/y/2 have different meanings for different metals on different surfaces. On (100)B and (100)A, they are the changes in Pt-Pt bond length in the x direction for Pt and the position changes in the z direction for Pb or Bi. On (110), they are the position changes in the z direction for Pt and the position changes in the y direction for Pb or Bi. On (001), they are the position changes in the z direction.

TABLE 8: Pt-H Bond Length in angstrom on H-Adsorbed PtPb and PtBi Surfacesa PtPb PtBi

(100)B

(110)

(100)A

(001)

1.76 1.76

1.76 1.77

1.73 1.74

1.60 1.60

a On (110) of PtPb (PtBi), H sits 0.51 (0.52) Å away in the y direction from the Pt-brg site.

upon H adsorption lies in the fact that, unlike CO’s multiplicity in hybridizing with Pt, H simply provides electrons to heal the cleaved Pt-Pt bond. As listed in Table 8, the Pt-H bond length on the Pt-brg site is about 1.76 Å on (100)B and (110) and 1.74 Å on (100)A, reflecting the strongest interaction on (100)A. On Pt-atop site of (001), the Pt-H bond length is 1.60 Å. Increased CO Tolerance and Electronic Structures. The increased CO tolerance on PtPb and PtBi surfaces compared to Pt(111) can be explained from the electronic structures. As an example, Figure 5a shows the electron density difference of CO adsorbed on the Pt-brg site of PtPb(100)B, and Figure 5c shows that for the Pt-atop site of PtPb(001). The difference is taken between the electron densities of the adsorbed system and that of an isolated CO layer and pure PtPb surface with the same atomic coordinates as in the adsorbed system. As seen in Figure 5, for both Pt-brg and Pt-atop sites, the interaction between CO and Pt still follows the well-known Blyholder model,27 in which there is a charge transfer from CO 5σ and, to a lesser extent, 4σ orbitals to Pt d bands, and at the same time, there is also a back-donation of charge from Pt d bands to the antibonding CO 2π* orbital. As seen in Figure 5c, even the second layer of Pt on (001) surface contributes to the interaction with CO. The PDOS of CO adsorbed on the Pt-brg site of (100)B surfaces and Pt-atop site of (001) surfaces of PtPb and PtBi are shown in Figure 5b and d, respectively. On the Pt-brg site of (100)B, the PDOS of CO from -5 to -1 eV, which overlap

with PDOS of Pt d bands, have contributions from both d-π and d-σ interactions. As explained earlier, the Pt d band narrows because of fewer Pt nearest neighbors compared to fcc Pt, and more importantly, the Pt d band center is lowered because of the upward shift of the Fermi level by sp electrons from Pb and Bi. Because Pb has one less electron than Bi, the Pb band is higher than that of Bi. As the result, the d-band center of PtBi is lower than that of PtPb, and both of them are lower than that of pure Pt. Hence, the binding of CO on the (100)B and (110) surfaces of PtPb and PtBi is much weaker than on Pt(111), with PtBi being weaker than PtPb. Compared to PtPb, the antibonding CO 2π* orbital does not broaden as much when adsorbed on PtBi. For the adsorption of CO on the Pt-atop site of (001), the d-π and d-σ interactions can be seen in the PDOS peaks in the energy range from -5 to -1 eV. Again, contrary to the approximate molecular orbital calculations,12 the CO 2π* orbital does not get almost filled, even on PtPb surfaces; therefore, CO dissociation is not likely. Improved d-Band Center Model and Electronic Structures. According to Norskov et al.,15,28 there are close correlations between the adsorption energies of small molecules on transition-metal surfaces and their d-band centers. Such correlations for H and CO adsorption on different surfaces of PtPb and PtBi are shown in Figure 6. To put these data in perspective, the adsorption energies of CO and H on the (111) surfaces of six fcc late transition metals are also included. The corresponding data are listed in Table 9 together with the most preferred adsorption sites. We are aware that current semilocal, exchangecorrelation functionals in DFT can give incorrect adsorption site preference compared to that observed in experiments29 because they overestimate the d-π interaction arising from an underestimation of the gap between the highest-occupied and lowestunoccupied molecular orbitals in CO.30 But the variations in the adsorption energies only slightly affect the correlation. The correlations for H and CO are quite clear, as shown by the linear fits in Figure 6. A higher d-band center means that the surface is more reactive and binds more strongly to both H and CO. According to the d-band center model,15 the d-π and d-σ interactions between CO and the transition metal surface can be written down explicitly in terms of both hybridization energy gain and orthogonalization energy cost, as in eq 1 in ref 15. The close correlations between the total hybridization energy in the d-band center model and the DFT-calculated energy have been shown before for different metal surface orientations31 and near surface alloys.32 The underlying close-packed lattice is either fcc or hcp. Here, we show the correlation for the metal line compound with very different structures from the close-packed lattice. The metal-CO hybridization energies calculated from the d-band center model are listed in Table 10, together with adsorption energies calculated in DFT-PW91 and the d-band centers for reference. The linear correlation is shown in Figure 7. The data show a better correlation than that in Figure 6a. The correlation for the

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Figure 5. Electron density difference of CO adsorbed on (a) the Pt-brg site of PtPb(100)B and (c) the Pt-atop site of PtPb(001). The violet (magenta) spheres stand for Pt (Pb), respectively. The green spheres with bond drawn in between are C and O with C adjunct to the surface. The red (yellow) iso-surface of (0.04 e/Å3 stands for electron depletion (accumulation), respectively. PDOS of surface Pt and CO on (b) the Pt-brg site of (100)B and (d) the Pt-atop site of (001) of PtPb and PtBi.

Figure 6. Correlation between d-band centers and adsorption energies of (a) H and (b) CO on the different surfaces of PtPb, PtBi, and fcc(111) surface of late transition metals. The corresponding data are listed in Tables 4 and 9. The red squares, green triangles, and blue circles stand for PtPb, PtBi, and fcc(111), respectively. The black lines are the linear fits.

TABLE 9: d-Band Center and Adsorption Energies of CO and H on Late Transition Metal fcc (111) Surfacesa εd (eV) Cu(111) Rh(111) Pd(111) Ag(111) Ir(111) Pt(111) Au(111)

-2.54 -1.99 -1.81 -4.06 -2.44 -2.41 -3.44

CO

Ea

(eV)

-0.82 (fcc) -1.94 (hcp) -1.96 (hcp) -0.19 (atop) -1.94 (atop) -1.76 (fcc) -0.23 (atop)

H

Ea (eV) -0.19 (fcc) -0.55 (fcc) -0.57 (fcc) 0.23 (fcc) -0.41 (fcc) -0.50 (fcc) 0.20 (fcc)

a The corresponding most preferred site is included in parentheses.

(100)A, (100)B, and (110) surfaces of the line compounds are similar to that for the close-packed (111) surface of fcc metals. The correlation is remarkable because we only use the parameters from ref 15, which are optimized only for the close-packed

(111) surface of fcc metals. Noting the location of the Pt(111) surface, it is easy to see that there are two groups of surfaces for PtPb and PtBi. One group, (100)B and (110), binds CO more weakly, and the other group, (100)A and (001), binds CO more strongly than Pt(111). Although Figure 7 shows a good correlation, two data points deviate significantly from the linear correlation, corresponding to the (001) surface of PtPb and PtBi. As mentioned earlier, the (001) surface is unique in two aspects. One is that the cleavage of Pt-Pt bond in the normal direction makes it have different symmetries when binding with CO in the form of the Pt-atop instead of the Pt-brg site on other surfaces; the other is that the cleavage causes a large contraction of the surface Pt-Pt bond (0.1 Å) in the normal direction during relaxation. The first aspect can be clearly seen by looking into the PDOS of the components of surface Pt d bands, shown in Figure 8 for

Electrocatalytic Properties of PtBi and PtPb Compounds via DFT

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8273

TABLE 10: d-Band Center, Hybridization Energy Calculated in the d-Band Model, and Adsorption Energy of CO Calculated in DFT-PW91 PtPb(100)B PtPb(110) PtPb(100)A PtPb(001) PtBi(100)B PtBi(110) PtBi(100)A PtBi(001) Cu(111) Rh(111) Pd(111) Ag(111) Ir(111) Pt(111) Au(111)

εd (eV)

Ehyb (eV)

EaCO (eV)

-2.76 -2.54 -2.16 -2.30 -2.91 -2.81 -2.29 -2.57 -2.54 -1.99 -1.81 -4.06 -2.44 -2.41 -3.44

-1.27 -1.42 -1.72 -1.60 -1.17 -1.23 -1.61 -1.40 -0.66 -1.80 -1.61 -0.26 -1.90 -1.52 -0.47

-1.47 -1.44 -1.88 -2.13 -1.12 -1.15 -1.66 -2.12 -0.82 -1.94 -1.96 -0.19 -1.94 -1.76 -0.23

PtPb(001) before and after CO adsorption. Because the Pt-Pt bond lies solely in the z direction, the dz2 orbital in Figure 8a has the largest broadening, which forms bonding and antibonding features, and the antibonding peak is pushed above the Fermi level. Among the less-broadened orbitals, compared to dxy and dx2-y2, the orbitals with dxz and dyz symmetries are skewed toward the Fermi level and form most of the upper edge of the total d bands. Unlike in the case when a Pt-brg site is present, when CO is adsorbed directly on top of the surface Pt on (001), dxy and dx2-y2 do not hybridize with the CO orbitals because of symmetry as seen clearly by the lack of broadening of these two orbitals in Figure 8b after CO adsorption. The CO 2π* interacts only with dxz and dyz, CO 5σ and 4σ interact with dz2, and, importantly, the different d components have very different energy centers, those of dxz and dyz being much higher than that of dz2. Therefore, to predict more accurately and universally the metal-CO hybridization energy and its correlation with adsorption energy and to reflect the different d-orbital bonding effects arising at different adsorption sites because of symmetry, we modify the d-band centers that enter the d-band center model as follows:

[

] [

Vπ2 Vσ2 Ehyb ) -4 f + fSπVπ - 2 (1 - f) + ε2π - εdxz,yz εdz2 - ε5σ

]

(1 + f)SσVσ (1) Here, the original d-band center, εd, is replaced with the d-band center for different components depending on whether they participate in the specific hybridization. All other symbols are the same as in ref 15. This modification is referred to as S1, and the data are listed in Table 11. In eq 1, because εdxz,yz is higher and εdz2 is lower in energy than the total εd, they move closer to ε2π and ε5σ, respectively, and make the two denominators smaller. Therefore, they both enhance the hybridization, and the resulting Ehyb is closer to the DFT-PW91-calculated adsorption energy by 0.2 eV, as shown by the half-filled triangle and square in Figure 7. The second critical aspect is that the contraction of Pt-Pt bond on (001) due to relaxation shifts the Pt d-band center to a lower energy. To separate the effect of relaxation from that solely due to the surface orientation (the symmetry of the

Figure 7. Linear correlation between the hybridization energies calculated in the d-band center model and the adsorption energies of CO calculated in DFT-PW91. The corresponding data are listed in Table 10. The legends are the same as in Figure 6. See text for the shift of the half-filled and hollow points for the (001) of PtPb and PtBi that show improved correlation accounting for critical effects.

adsorption site), we can restore the Pt-Pt bond length in the relaxed (001) surface to the bulk value. Without doing extra DFT calculations, the shift in the d-band center due to the variation in Pt-Pt bond length can be analytically calculated with the formula below by taking into account the universal nature of the d-band width formula presented by Harrison.20

( )(

3 p2 rd Wd ) 6.83 m r5 0

)

(2)

where Wd is the d-band width, p is Plank’s constant, m is the electron mass, r0 is the Wigner-Seitz radius, and rd is the d-state radius listed in the Solid State Table in Harrison’s book.20 To connect the d-band center εd to Wd, we use Friedel’s model;19 that is, we treat the DOS of the d band as an uniform distribution.

(

εd ) 0.5 -

)

nd W 10 d

(3)

where nd is the number of d electrons (e.g., nd is 9 for Pt, PtBi, and PtPb). By using the Wigner-Seitz radius r0 ) b[3⁄(4π√2)]1⁄3 for fcc, where b is the nearest-neighbor Pt-Pt bond length, we can get the change in d-band center with respect to the change in Pt-Pt bond length.

(

) ( )( )

nd p2 dεd 3 ) -34.15 0.5 db 10 m 4√2π

-5⁄3

() r3d

b6

(4)

As a result, DFT-calculated εrelax can be corrected back to the d bulk-terminated values εunrelax (with no further DFT calculations). d

εunrelax ) εrelax + d d

( ) dεd db

b0

∆b

(5)

where ∆b is the change in bond length during relaxation. This can be used to provide a very reliable correlation between Ehyb and Eads. We obtain (dεd/db)b0 of 4.92 (4.52) eV/Å for Pt at the bond length of 2.78 (2.82) Å for bulk PtPb (PtBi), respectively. As for (001) surface of PtPb and PtBi, the Pt-Pt bond only occurs in the z direction. The change of 0.1 Å in Pt-Pt bond length means a 0.49 (0.45) eV shift in the d-band center for the (001) surface of PtPb (PtBi), respectively. We refer to the shifts in d-band center from both adsorption-site symmetry and local relaxation effects as S2 in Table 11. The upshift in the d-band center for (001) surface gives a stronger hybridization and shifts the data points for (001) surfaces horizontally further to the left in Figure 7, as indicated by the hollow square and triangle. The

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Figure 8. PDOS on the d bands of surface Pt (a) before and (b) after CO adsorbed on the Pt-atop site of PtPb(001) surfaces.

TABLE 11: Centers of d-Band Components and Hybridization Energies in the Consecutively Modified d-Band Models (S1 and S2, See Text) for CO Adsorption on the (001) of PtPb and PtPb εdxz, yz S1 S2

-2.15 -1.65

S1 S2

-2.39 -1.99

εdz2 PtPb(001) -2.74 -2.25 PtBi(001) -2.92 -2.51

Ehyb -1.79 -2.28 -1.58 -1.93

data for the (001) surfaces now become linearly correlated with those of all other surface orientations of PtPb and PtBi and with the (111) surfaces of fcc metals. Importantly, we find excellent correlation between Ehyb and Eads for CO adsorption, even for the non close-packed systems, when the different d-orbital hybridization and local relaxation effects are directly included. Moreover, such effects are easy to include in a universal way. We hope that this analysis provides a better means for designing catalysts. These two effects are not important for all other surfaces of PtPb and PtBi and fcc(111) surfaces; because the preferred adsorption site of CO has a high coordination (being either 2-fold Pt-brg or 3-fold Pt-hollow sites), CO hybridizes more or less with all d-orbitals. The change in metal bond length on a clean surface is very small or has offsetting results for the (100)A and (100)B surfaces of PtPb and PtBi. As a last comment, the finding that the Pt-atop site on the (001) surface of PtPb and PtBi binds CO the strongest among all surfaces, and also more strongly than Pt(111) surface, can help correct conclusions from experimental results. On the (001) surface, the in-plane Pt-Pt distance of 4.38 Å is the second nearest-neighbor Pt-Pt distance, which is much larger than the first nearest-neighbor Pt-Pt distance of 2.78 Å that appears on all the other surfaces. The much larger Pt-Pt distance gives fewer Pt sites on the (001) surface, in the sense that Pt sites are surrounded by less reactive p-metal sites. Therefore, it has been suggested that the (001) surface is the reason for the increased CO resistance.10,11 But, from our analysis on electronic structures and d-band center model, the binding to CO is actually the strongest because of the symmetry change in adsorption site and modification of the electronic structures associated with cleaving the strongly interacting Pt-Pt bond; hence, the (001) surface is not the source of the increased CO tolerance. Conclusion In conclusion, we have used DFT-GGA calculations to study the electrocatalytic properties of intermetallic line compounds of PtPb and PtBi, a class of new anode catalysts for small

organic molecule fuel cells, exhibiting an increased CO tolerance. We found that the increased CO tolerance of PtPb and PtBi compared to that of fcc Pt is due to the much reduced adsorption of CO on (100)B and (110) surfaces, arising from the shift of Pt d-band center to a lower energy as a result of an upward shift of the Fermi level due to the presence of sp electrons from Pb and Bi. In addition to having the lowest cleavage energies compared to (100)A and (001) surfaces, we found that (100)B and (110) are the surfaces that are most relevant to the observation in experiments. In contrast, (100)A and (001) bind CO more strongly than Pt(111). The differences in CO adsorption and H dissociation among different materials and different surfaces all have their origins in electronic structures, mainly because of relative shifts of the d-band center. To correlate the d-band center and the CO adsorption energy, we formulated a more universal and improved d-band center model to include the effects of the symmetry of the adsorption site, that is, the shifts in each relevant d orbital, and the local relaxation. After these modifications, we find that the d-band center model works for all the surfaces of the line compounds, which have structures rather different from the close-packed structure originally derived from the model. Contrary to what is deduced from experiments, the increased CO tolerance is not due to the reduced density of Pt sites on the (001) surfaces of the line compounds, but the Pt-atop site on the (001) surface actually binds CO the strongest among all surfaces. The source of increased CO tolerance is from the much lower CO binding energy on (100)B and (110) surfaces. In contrast to the previous calculation with approximate molecular orbitals, we find that CO 2π* is not filled up, even on the strongly bonded surfaces, and thus does not support CO dissociation. Acknowledgment. This work was supported by the Department of Energy under Catalysis DE-FG02-03ER15476 and The Frederick Seitz Materials Research Laboratory at the University of Illinois under DEFG02-91ER45439. We also acknowledge critical computational support provided by the Materials Computation Center through NSF/ITR grant DMR-0325939 and DOE INCITE grant through collaboration with Oak Ridge National Laboratory. References and Notes (1) Whitesides, G. M.; Crabtree, G. W. Science 2007, 315, 796–798. (2) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U. J. Phys. Chem. B 2005, 109, 16230–16243. (3) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 275– 283. (4) Lu, C.; Masel, R. I. J. Phys. Chem. B 2001, 105, 9793–9797. (5) Desai, S.; Neurock, M. Electrochim. Acta 2003, 48, 3759–3773. (6) Saravanan, C.; Dunietz, B. D.; Markovic, N. M.; Somorjai, G. A.; Ross, P. N.; Head-Gordon, M. J. Electroanal. Chem. 2003, 554, 459–465.

Electrocatalytic Properties of PtBi and PtPb Compounds via DFT (7) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307– 314. (8) Koper, M. T. M.; Shubina, T. E.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 686–692. (9) Koper, M. T. M. J. Electroanal. Chem. 2005, 574, 375–386. (10) Casado-Rivera, E.; Gal, Z.; Angelo, A. C. D.; Lind, C.; DiSalvo, F. J.; Abruna, H. D. ChemPhysChem 2003, 4, 193–199. (11) Casado-Rivera, E.; Volpe, D. J.; Alden, L.; Lind, C.; Downie, C.; Vazquez-Alvarez, T.; Angelo, A. C. D.; DiSalvo, F. J.; Abruna, H. D. J. Am. Chem. Soc. 2004, 126, 4043–4049. (12) Oana, M.; Hoffmann, R.; Abruna, H. D.; DiSalvo, F. J. Surf. Sci. 2005, 574, 1–16. (13) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864–B871. (14) Kohn, W.; Sham, L. J. Phys. ReV. 1965, 140, A1133–A1138. (15) Hammer, B.; Morikawa, Y.; Norskov, J. K. Phys. ReV. Lett. 1996, 76, 2141–2144. (16) Mavrikakis, M.; Hammer, B.; Norskov, J. K. Phys. ReV. Lett. 1998, 81, 2819–2822. (17) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717–4725. (18) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493–497.

J. Phys. Chem. C, Vol. 112, No. 22, 2008 8275 (19) Friedel, J. In The Physics of Metals; Ziman, J. M., Ed.; Cambridge University Press: New York, 1969; pp 494–525. (20) Harrison, W. A. Electronic Structure and the Properties of Solids; W. H. Freeman and Company: San Francisco, 1980. (21) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244–13249. (22) Blöchl, P. E. Phys. ReV. B 1994, 50, 17953–17979. (23) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15–50. (24) Kresse, G.; Furthmüller, J. Phys. ReV. B 1996, 54, 11169–11186. (25) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188–5192. (26) Matsumoto, F.; Roychowdhury, C.; DiSalvo, F. J.; Abruna, H. D. J. Electrochem. Soc. 2008, 155, B148–B154. (27) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (28) Hammer, B.; Norskov, J. K. Nature 1995, 376, 238–240. (29) Feibelman, P. J.; Hammer, B.; Norskov, J. K.; Wagner, F.; Scheffler, M.; Stumpf, R.; Watwe, R.; Dumesic, J. J. Phys. Chem. B 2001, 105, 4018– 4025. (30) Kresse, G.; Gil, A.; Sautet, P. Phys. ReV. B 2003, 68, 073401. (31) Hammer, B.; Nielsen, O. H.; Norskov, J. K. Catal. Lett. 1997, 46, 31–35. (32) Kitchin, J. R.; Norskov, J. K.; Barteau, M. A.; Chen, J. G. Phys. ReV. Lett. 2004, 93, 156801.

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