J. Phys. Chem. B 2001, 105, 9533-9536
9533
CO Adsorption on Pt-Ru Surface Alloys and on the Surface of Pt-Ru Bulk Alloy Q. Ge,† S. Desai,† M. Neurock,*,† and K. Kourtakis‡ Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22903, and DuPont Central Research and DeVelopment Experimental Station, Wilmington, Delaware 19880 ReceiVed: March 27, 2001; In Final Form: July 2, 2001
We present results of first-principle studies for energetics and structural properties of CO adsorbed on the Pt-Ru surface alloys and on the surface of a bulk Pt-Ru alloy. The results showed that the Ru-CO bond strength was increased from 2.0 to 2.1 eV, whereas the Pt-CO bond was weakened from 1.54 to 1.36 eV after Ru alloying with Pt for the bulk Pt-Ru alloy. On the other hand, both Ru-CO and Pt-CO bonds are strengthened on the surface alloys.
I. Introduction It is well established that the catalytic properties of metal can be markedly changed by alloying with a second metal.1 Many metal-catalyzed industrial processes are carried out with supported bimetallic particles that provide optimal activity and selectivity. Bimetallics provide a lever by which we can manipulate both the electronic and geometric features of the active metal ensemble. In general, the catalytic performance is governed by a sequence of bond-breaking and bond-making processes that are carried out over the active metal surface. Individual bond-breaking and forming steps are largely controlled by a balance between the metal-adsorbate bond strength and the intramolecular bond strengths. Alloys provide a means by which we can modify the strength of the metal-adsorbate bonds thus opening a mechanism by which we can begin to control catalytic chemistry. Pt-Ru alloys are known to substantially improve the catalytic performance in the electrochemical oxidation of carbon monoxide from CO contaminated hydrogen fuels. The presence of small amounts of CO that are carried over in the H2 feed from the reformer act to poison Pt surface sites. The addition of Ru to Pt helps to prevent CO poisoning of the Pt surface sites that carry out hydrogen oxidation. Ru promotes the catalytic oxidation of CO to form CO2 and may also weaken the Pt-CO interaction. Pt-Ru alloys also offer the best performance for the direct methanol fuel cell and play a similar role to that for the H2 fuel cell. Platinum is thought to carryout adsorption and subsequent dehydrogenation reactions to form CO. 2 The accumulation of CO, however, impedes further methanol decomposition. Introducing a second metal such as Sn or Ru has been shown to significantly improve the tolerance of the Pt electrode toward CO poisoning.2-5 A simple bifunctional mechanism whereby atoms of different metals act independently was originally proposed.6,7 The introduction of Ru to Pt is thought to promote H2O splitting and therefore to provide an oxidant for CO oxidation. This mechanism, however, ignores CO adsorption on Ru as well as the modification of CO adsorption on Pt because of the alloying interaction with Ru. * To whom correspondence should be addressed. E-mail: mn4n@ virginia.edu. Fax: (434) 982-2658. † University of Virginia. ‡ DuPont Central Research and Development Experimental Station.
In general, alloying two metals can alter their electronic structure as well as their geometric structure. It has been shown that the metal-adsorbate bond strength on the alloy surface can be very different from that on the pure metal surfaces and can therefore alter the mechanism of a reaction.8 CO adsorption on metal surfaces has been extensively studied, both experimentally and theoretically.9,10 The nature of the chemisorption bond between CO and the metal surfaces is well documented. On the basis of the orbital mixing model proposed by Hu et al.,11 the upward shift of d-band energy level across the periodic table from right to left, that is, from Cu to Fe, leads to an increased overlap in the population of the mixed 2p-d states, and hence the increase in the CO-metal bond strength. Further shifting to the left will result in C-O bond dissociation. This agrees with the general principle of the “d-band” model that was proposed by Hammer et al.12 Despite its critical significance for the direct methanol fuel cell, there has been only one theoretical study aimed at examining the chemisorption of CO on Pt-Ru alloys. Using clusters consisting of 10 metal atoms, Liao and co-workers studied CO adsorption on Pt-Ru and focused on the adsorption on the atop Pt site.13,14 They found that the presence of another metal atom weakens the Pt-C bond and slightly lowers the C-O stretching frequency of CO. However, the cluster models used to simulate surface are quite small and cannot fully represent the electronic properties of an extended alloy surface properly. In particular, the cluster model cannot account for the relaxation and restructuring of the solid surface accompanying the adsorption. In the present paper, we perform periodic DFT calculations of CO chemisorption on extended Pt, Ru, and alloyed Pt-Ru slabs. The influences of alloying Ru and Pt, both in bulk as well as on surfaces, on CO adsorption is examined. II. Computational Details Our total energy slab calculations were performed within the framework of density functional theory using a basis set consisting of plane waves, as implemented in the VASP code.15-17 The electron-ion interactions were described by ultrasoft pseudopotentials,18 and the exchange and correlation energies were calculated with the Perdew-Wang form of the generalized gradient approximation (GGA).19
10.1021/jp011144i CCC: $20.00 © 2001 American Chemical Society Published on Web 09/06/2001
9534 J. Phys. Chem. B, Vol. 105, No. 39, 2001
Ge et al.
TABLE 1: The GGA Calculated Lattice Constants and Formation Enthalpies of Pt1-xRuxa x
d (Å)
∆Hf (meV)
0.00 0.25 0.33 0.50 0.75 1.00
3.991 (1.7%) 3.949 (1.5%) 3.932 3.904 (1.0%) 3.860 3.823
0.0 38.9 -1.7 30.9 -12.1 0.0
a The percentage deviations from the experimental results, when available,22 are given in the parentheses after the lattice constants.
Figure 1. Calculated Pt-Ru bulk lattice constants for a fcc structure with different Ru compositions.
The surface was modeled by a four-layer slab with a (x3 × x3)R30° surface unit cell, separated by a vacuum region equivalent to six bulk metal layers, for both surface and bulk alloys. A plane wave cutoff energy of 320 eV was used in the calculation and the Brillouin zone of the surface unit cell was sampled with a 7 × 7 Monkhorst-Pack mesh.20 The metal atoms in the bottom two layers were fixed at their bulk positions while the atoms in the top two layers and the CO molecule were allowed to relax. Conjugate-gradient and quasi-Newton minimization schemes were used to optimize the geometric structure of the ions. III. Results and Discussion A. Pt-Ru Bulk Alloys. Ruthenium is known to harden platinum very effectively upon alloying. The solubility of Ru in Pt is rather high. At the temperature around 1000 °C, the Pt-Ru system will show a cubic platinum structure below 60 atomic percent of ruthenium.21 We calculated the equilibrium structural parameters of Pt-Ru alloy at a series of Ru compositions in a face-centered cubic structure. For 25%, 50%, 75%, and 100% Ru, conventional cubic unit cells are used while for 33% a three-layer slab with a (x3 × x3)R30° structure in each layer is adopted. The calculated bulk lattice parameters for each of these compositions are shown in Table 1. In the latter case, each layer which consists of two Pt atoms and one Ru atom in the unit cell are stacked along direction. The calculated bulk lattice constants show a linear decrease with the increasing Ru composition (Figure 1). This agrees with the Pt-Ru alloys prepared by high-energy ball-milling,22 which also showed a decrease of lattice parameter with increasing Ru contents. The calculated lattice parameters are bigger than the experimental values across the whole composition range. However, the deviations are all within 2%. The Pt atoms are relaxed by ∼0.03 Å from their ideal fcc positions with respect to the Ru atoms occupying the ideal fcc lattice sites. The fcc structures for 75% and 100% Ru are hypothetical, as phase
Figure 2. Schematics of the surface of Pt0.66Ru0.33 bulk alloy. (a) Top view showing the surface unit cell; (b) Side view showing the relaxations of Pt and Ru atoms in the top two layers.
separation occurs at Ru compositions greater than ∼60% atomically.21 The thermodynamically stable structure of Ru is a hexagonal, close-packed structure. The bulk Ru in a hcp structure is 0.12 eV/atom more stable than Ru in an fcc structure. The formation enthalpy ∆Hf of the PtmRun alloys is defined as the energy gain or loss with respect to the bulk constituents at their equilibrium lattice constants23 without changing the symmetry:
Etot(PtmRun) - mEtotPt - nEtotRu ∆Hf ) m+n
(1)
where Etot is the total energy of Pt-Ru alloy, Pt, and Ru, respectively. The calculated formation energies for various compositions are shown in Table 1. The formation enthalpies of the alloys are mostly positive, indicating a tendency of phase separation. Similar results were found for Al-Zn alloys when a fcc metal alloyed with a hcp metal.23 Herein we examine the Pt0.66Ru0.33 alloys along with pure Pt and Ru surfaces. B. CO Adsorption on the {111} Surface of Pt0.66Ru0.33 Bulk Alloy. The lattice constant of the Pt0.66Ru0.33 bulk alloy was calculated to be 3.932 Å. This value was therefore used to construct the slab for surface calculations. A schematic of the close-packed surface of the alloy is shown in Figure 2. Similar to the pure metal surface calculations, the bottom two layers of the slab were fixed at the relaxed bulk positions while the top two layers were allowed to relax. In the relaxed surface structure of the Pt0.66Ru0.33 alloy, the top layer Ru atoms relaxed inward by 4.8% of the calculated bulk layer spacing in the same direction while the Pt atoms relaxed outward by 1.7%. The
CO Adsorption
J. Phys. Chem. B, Vol. 105, No. 39, 2001 9535
TABLE 2: The GGA Adsorption Energies and Structures for CO on Pt0.66Ru0.33 Alloy Surface at 0.33 ML CO Coveragea Atop (Ru) Atop (Pt) Bridge (Pt-Pt) Bridge (Pt-Ru) HCP (Ru) HCP (Pt) FCC (Ru) FCC (Pt)
Eb (eV)
dC-Me (Å)
dC-O (Å)
θ°
2.13 1.36 1.45 1.68 1.75 1.62 1.62 1.63
1.86 1.86 2.04 2.16, 1.97 2.24, 2.04 2.26, 2.00 2.27, 1.98 2.30, 1.97
1.16 1.16 1.18 1.18 1.19 1.18 1.18 1.18
0 0 0 11. 5 8.8 9.5 9.4 9.6
a All adsorption energies, Eb, are referred to the relaxed clean surface. dC-Me: C-metal bond length; dC-O: C-O bond length; θ: CO tilt angle from surface normal.
TABLE 3: The GGA Adsorption Energies and Structures for CO on Pt{111} and Ru{0001} at the Same Coverage, for Comparisona Pt{111}
Ru{0001}
Eb (eV) dC-Pt (Å) dC-O (Å) Eb (eV) dC-Ru (Å) dC-O (Å) Atop Bridge HCP FCC
1.54 1.67 1.69 1.71
1.86 2.03 2.11 2.11
1.16 1.18 1.19 1.19
2.00 1.85 1.93 1.71
1.90 2.07 2.12 2.11
1.16 1.19 1.20 1.19
a All adsorption energies, E , are referred to the relaxed clean b surface.
atoms in the second layer show opposite trends but to a lesser extent: Ru relaxes outward by 2.5% while Pt relaxes inward by 1.0%. These relaxations result in corrugations of 0.08 Å in the top layer and 0.04 Å in the second layer. The lateral relaxations in the unit cell are very small and of the order of 6 thousandth of an Angstrom in the top two layers. The calculated structural and energetic results for CO on a number of pseudo high-symmetry sites are listed in Table 2. The adsorption structures and energetics of CO on pure Pt{111} and Ru{0001} surfaces are also calculated and shown in Table 3 for comparison. The chemisorption of CO favors the threefold fcc site (1.71 eV) on Pt{111} and the atop site (2.00 eV) on Ru{0001}. This is consistent with previous theoretical results.24,25 In both cases, the energy differences between the most stable and the least stable sites are less than 0.2 eV. The calculated energies agree with the reported values in the literature.24,25 In general, we can see that alloying between Ru and Pt weakens the Pt-CO bond, whereas it strengthens the Ru-CO bond. This is manifested by the increase in the Ru-CO binding energy of 2.00 eV for CO on pure Ru surface to 2.15 eV for CO at the atop site of Ru on the alloy surface. In contrast, the Pt-CO binding energy at both atop and bridge sites decreased with respect to their values on the pure Pt{111} surface. We also note that although the binding energies of CO on atop Ru and Pt site differ significantly, both the C-metal and C-O bond lengths on these two atop sites are essentially the same. Although the nature of the chemisorption bond formed between CO and a transition-metal surface is best described by the mixing of the CO molecular orbitals with the metal d states,11 the major features are captured in the Blyholder model.26 In this model, the CO-metal bond is described in terms of electron donation from the 5σ molecular orbital to the metal and backdonation from the metal to the CO 2π* antibonding orbitals. The results clearly show that mixing Pt and Ru in the bulk alloy strengthens the Ru-CO bond but weakens the Pt-CO bond on the surface of bulk alloy. To understand this, we performed a charge analysis for the bulk alloys on the basis of
TABLE 4: The GGA Adsorption Energies and Structures for CO on the Surface Alloy of Pt-Ru on a Pt{111} Substrate with Ru:Pt ) 1:2 at 0.33 ML CO Coveragea Atop(Ru) Atop(Pt) Bridge(PtPt) Bridge(PtRu) HCP FCC
Eb (eV)
dC-Me (Å)
dC-O (Å)
θ°
2.11 1.58 1.71 1.85 1.84 1.84
1.86 1.85 2.04 2.15, 1.99 2.19, 2.06 2.17, 2.05
1.16 1.16 1.18 1.18 1.19 1.19
0 0 0 2 5 6
a All adsorption energies, Eb, are referred to the relaxed clean surface. dC-Me: C-metal bond length; dC-O: C-O bond length; θ: CO tilt angle from surface normal.
TABLE 5: The GGA Adsorption Energies and Structures for CO on the Surface Alloy of Pt-Ru on a Pt{111} Substrate with Ru:Pt ) 2:1 at 0.33 ML CO Coveragea Atop (Ru) Atop (Pt) Bridge (Ru-Ru) Bridge (Pt-Ru) HCP FCC
Eb (eV)
dC-Me (Å)
dC-O (Å)
θ°
2.10 1.71 1.91 1.98 2.01 1.93
1.88 1.84 2.06 2.09, 2.02 2.13, 2.13 2.13, 2.14
1.16 1.16 1.18 1.18 1.19 1.19
0 0 0 2 2 2
a All adsorption energies, E , are referred to the relaxed clean b surface. dC-Me: C-metal bond length; dC-O: C-O bond length; θ: CO tilt angle from surface normal.
TABLE 6: The GGA Adsorption Energies and Structures for CO on a Pseudomorphic Ru Layer on a Pt{111} Substrate at 0.33 ML CO Coveragea Atop (Ru) Bridge (Ru-Ru) HCP FCC
Eb (eV)
dC-Me (Å)
dC-O (Å)
θ°
2.18 2.04 2.21 2.07
1.87 2.14 2.12 2.07
1.16 1.19 1.20 1.19
0 0 0 0
a All adsorption energies, E , are referred to the relaxed clean b surface. dC-Me: C-metal bond length; dC-O: C-O bond length; θ: CO tilt angle from surface normal.
the topological theory of Bader27 as well as Mulliken population. The results of this analysis show that there is a slight charge transfer, about 0.2 electron/atom, from Ru to Pt. As a consequence, losing electronic charge from Ru will enhance the ability of Ru to accept charge from CO, thereby reducing the repulsive interaction between the CO 5σ orbital and the metal d states. In contrast, accumulating charge on Pt atoms will have the opposite effect: an increase in repulsive interaction between the CO 5σ orbital and metal d states. The above analysis also agrees with Hammer and Nørskov’s d-band model.12 Losing charge from the Ru atom shifts the local d-band center closer to Fermi level, EF. This should strengthen the chemisorption bond between CO and surface Ru atoms. On the contrary, the charge buildup on Pt atoms pushes the local d-band center downward with respect to EF and has the opposite effect on Pt-CO bond as to Ru-CO bond. In addition to the chargetransfer effect in the alloy, the alloying interaction between Pt and Ru will induce strain in the Pt lattice. The strain due to the lattice contraction was shown to cause the d states shift down in energy.9 C. CO Adsorption on the Pt-Ru Surface Alloys and Pseudomorphic Ru Layer. Tables 4-6 list the adsorption structures and energetics of CO on the Pt-Ru surface alloys with different compositions. The surface alloys are constructed by substituting various Pt atoms with Ru in the top layer of a Pt{111} surface with a (x3 × x3)R30° surface unit cell. The underlying bulk substrate remains Pt{111}. A substitution of
9536 J. Phys. Chem. B, Vol. 105, No. 39, 2001 one of the Pt atoms in the top layer with Ru will give a composition of 33% Ru and 66% Pt on a Pt{111} substrate. With all three Pt atoms substituted with Ru, we get a pseudomorphic Ru layer on top of a Pt{111} substrate. The surface alloy differs from the surface of the bulk alloy with the same composition by the underlying layers as well as the lattice parameters defining the surfaces. The average spacing between atoms in the top layer of the surface alloy which adopts the Pt lattice constant is larger than that in the bulk alloy. We note that the binding energies of CO at all the highsymmetry sites on the surface alloy are higher than those on the pure metal surfaces. The binding energy of CO on the atop Ru site shows little change as we increase the Ru composition from 33% to 66%. The binding energies at all other sites, however, increase significantly as we increase from 33% Ru to 66% Ru. At the composition of 33% Ru, the binding energy for CO on the atop Pt site for the surface alloy shows an increase of 0.08 eV as compared with the pure Pt metal surface. This is in direct contrast with that on the surface of the bulk alloy with the same Ru/Pt ratio in the surface layer. We recall that the Pt-CO bond is weakened to 1.36 eV on the surface of the bulk alloy from 1.54 eV on a Pt{111} surface. Comparing the results of the Ru pseudomorphic layer with those of the pure Ru metal surface, that is, Table 3 vs Table 6, we find that the binding energies of CO to the surface in all the high-symmetry sites on the pseudomorphic layer are higher than those on the pure Ru surface. The increases in the binding energies from the pure Ru metal surface to the Ru pseudomorphic layer on Pt are larger in the three-fold sites than at the bridge and atop sites. We also note that the CO structure is almost identical for the corresponding site on both surfaces. The increased binding energies of CO on surface alloys can be attributed to the strain induced by substituting Pt with Ru. In the bulk alloy, Pt and Ru forms a stronger bond than Pt-Pt and Ru-Ru. The bonding interaction between Pt and Ru and the accompanying charge-transfer result in the weakened PtCO bond and the strengthened Ru-CO bond. The scenario is different for CO adsorption on surface alloys. While Pt and Ru still tend to form bonds, the larger substrate lattice counterinteracts with this effect. The Ru-Ru distance in bulk Ru is almost 3% less than the Pt-Pt distance in bulk Pt. Replacing Pt atoms in the surface layer of a Pt slab will induce strain in the surface. It has been shown that a Ru-CO bond is strengthened by increasing the surface lattice constant of a Ru{0001} surface.28 IV. Concluding Remarks From our calculations of CO adsorption on the surface of bulk Pt-Ru alloys and the surface alloys, we conclude that (a) the Ru-CO bond is strengthened while the Pt-CO bond is weakened on the surface of the Pt-Ru bulk alloy and (b) both Ru-CO and Pt-CO bonds are strengthened on the surface PtRu alloys supported on a Pt{111} substrate. CO is clearly adsorbed and can be oxidized on Ru sites on the basis of both experimental and theoretical results. Therefore, bifunctionality of the Pt-Ru electrode, or bimetallic alloys in general, is an oversimplified picture of CO electro-oxidation process. In fact, the bifunctional mechanism has been questioned
Ge et al. in an experimental study of CO adsorption and oxidation on bimetallic Pt/Ru{0001} surfaces.29 Both Ru-CO and Pt-CO bonds are expected to be weakened on the surface Pt-Ru alloys supported on a Ru{0001} substrate because of the strain caused by lattice misfit. A logical mechanism would be that CO is adsorbed on both Ru and Pt sites and that these adsorbed CO are in equilibrium with CO in liquid and gas phases. Displacive desorption of CO from Ru may take place when OH produced from water splitting starts to compete with CO to occupy Ru sites. Of course, the changes in metal-OH and metal-O bonds because of alloying also need to be taken into account to understand the mechanism of CO electro-oxidation on bimetallic surfaces. Acknowledgment. We gratefully acknowledge the financial support from the DuPont Chemical Company. A portion of the calculation has been carried out on the centurion nodes of the Legion group at the University of Virginia. References and Notes (1) Ponec, V.; Bond, G. C. Catalysis by Metals and Alloys; Studies in Surface Science and Catalysis 95; Elsevier: Amsterdam, 1995. (2) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (3) Friedrich, K. A.; Geyzers, K.-P.; Linke, U.; Stimming, U.; Stumper, J. J. Electroanal. Chem. 1996, 402, 123. (4) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41. (5) Lin, W. F.; Zei, M. S.; Eiswirth, M.; Ertl, G.; Iwasita, T.; Vielsitch, W. J. Phys. Chem. B 1999, 103, 6968. (6) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (7) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (8) Besenbacher, F.; Chorkendorff, I.; Clausen, B.; Hammer, B.; Molenbroek, A.; Norskov, J. K.; Stensgaard, I. Science 1998, 279, 1913. (9) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis-Calculations and Concepts; Advances in Catalysis 45; Academic Press: San Diego, 2000; pp 71-130. (10) Ge, Q.; Kose, R.; King, D. A. Adsorption Energetics and Bonding from Femotomole Calorimetry and from First Principles Theory; Advances in Catalysis 45; Academic Press: San Diego, 2000; pp 207-260. (11) Hu, P.; King, D. A.; Lee, M. H.; Payne, M. C. Chem. Phys. Lett. 1995, 246, 73. (12) Hammer, B.; Morikawa, Y.; Nørskov, J. K. Phys. ReV. Lett. 1996, 76, 2141. (13) Liao, M.-S.; Cabrera, C. R.; Ishikawa, Y. Surf. Sci. 2000, 445, 267. (14) Binning, R. C.; Liao, M.-S.; Cabrera, C. R.; Ishikawa, Y.; Iddir, H.; Liu, R.; Smotkin, E. S.; Aldykiewicz, A.; Myers, D. J. Int. J. Quantum Chem. 2000, 77, 589. (15) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (16) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (17) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (18) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (19) Perdew, J.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (20) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (21) Hutchinson, J. M. Platinum Metal ReV. 1972, 16, 88. (22) Lalande, G.; Denis, M. C.; Guay, D.; Dodelet, J. P.; Schulz, R. J. Alloys Compd. 1999, 292, 301. (23) Mu¨ller, S.; Wang, L.-W.; Zunger, A. Phys. ReV. B 1999, 60, 16448. (24) Eichler, A.; Hafner, J. Phys. ReV. B 1999, 59, 5960. (25) Zhang, C. J.; Hu, P.; Alavi, A. J. Chem. Phys. 2000, 112, 10564. (26) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (27) Bader, R. F. W. Atom in molecules: a quantum theory; Oxford University Press: Oxford, 1990. (28) Mavrikakis, M.; Hammer, B.; Norskov, J. K. Phys. ReV. Lett. 1998, 81, 2819. (29) Buatier de Mongeot, F.; Scherer, M.; Gleich, B.; Kopatzki, E.; Behm, R. J. Surf. Sci. 1998, 411, 249.
