Adsorption of CO on Surfaces of 4d and 5d Elements in Group VIII

J. Phys. Chem. C 2007, 111, 1005-1009

1005

Adsorption of CO on Surfaces of 4d and 5d Elements in Group VIII W. Liu, Y. F. Zhu, J. S. Lian, and Q. Jiang* Key Laboratory of Automobile Materials (Jilin UniVersity), Ministry of Education, and Department of Materials Science and Engineering, Jilin UniVersity, Changchun 130022, China ReceiVed: September 19, 2006; In Final Form: October 31, 2006

Ab initio density functional theory (DFT) calculations with the all electron relativistic (AER) core treatment method were used to determine adsorption of CO on close-packed surfaces of Ru, Rh, Pd, Os, Ir, and Pt. The adsorption energy and work function orders are obtained, which are Pd > Pt > Rh > Ru > Os > Ir and Os > Ir > Ru > Rh > Pt > Pd, where the latter was interpreted through analyzing their densities of state (DOS). The CO/Ir(111) system with the most negative adsorption energy was considered in detail as a model system. In terms of the plot of electron density difference and the values of Mulliken analysis, it is found that charges transfer from metallic surfaces to CO molecules. The results correspond to available experimental and theoretical data.

1. Introduction It is important to understand the mechanism and catalytic processes of interaction of CO adsorption on metallic surfaces because of their potential use in methanol reforming, fuel cell technology, and especially in purification of automotive exhaust gases.1,2 With the development of calculation ability, many computational studies of CO adsorption on different kinds of surfaces have been reported over the years, especially for surfaces of monometallics,3-7 alloys,8-10 clusters,10-14 and metals supported by oxides.14-16 The catalytic oxidation on a metallic catalyst surface of 4d and 5d elements in group VIII is an efficient way for converting CO to CO2 at low temperature.17 In the reaction process of CO/Pt(111),18 CO/Ru(0001),19 CO/ Rh(111),20,21 and CO/Pd(111) systems,22 the transition state and reaction barrier are two crucial requirements.18 The former determines the amount of products, and the latter decides the catalytic rate. It is known that the amount of CO2 formation reversely depends on the adsorption energy, E, of CO molecules on metallic surfaces,20,21 which describes the chemical bond strength between the gas-phase molecule CO and metallic surfaces. When experimental errors are considered, experimental results show EPd > EPt ≈ ERh ≈ ERu > EIr (EOs is absent in author’s knowledge).23-27 However, the obtained E values are hard to compare because the known simulation results come from different functionals. In addition, because relativistic effects into the core are neglected in the most previous calculations for CO adsorption on the above elements, the state-of-the-art DFT fails to predict the correct site preferences for CO/Pt(111) and CO/Rh(111) systems1,28,29 because the relativistic effects lead to sizable modifications on bonding properties of these elements. Because catalysts are used as a “bridge” to transfer electrons between reactants, the electron transfer ability or catalytic reaction rate of a catalyst is another important factor,30 which is inversely proportional to the work function Φ of a metal surface after adsorption.31 Φ ) Evac - Ef is defined as the * Author to whom any correspondence should be addressed. Fax: +86 431 5095876. E-mail: [email protected].

required lowest energy to take an electron from the Fermi level Ef to the vacuum level Evac for a surface after the CO adsorption. When the vacuum region from the metal surface is infinitely large, Evac ≈ 0. Thus, there is

Φ ≈ -Ef

(1)

In fact, for the CO/Pt(111) system, Ef ) -5.4 eV obtained from both PW91 and B3LYP functionals is very close to Φ ) 5.7 eV from experiments,32 which confirms eq 1. In this contribution, E and Φ values and geometry on the above surfaces are simulated by DFT calculations performed in Dmol3 code.33,34 A uniform generalized gradient approximation (GGA) with the revised Perdew-Burke-Ernzerhof (RPBE) functional is selected as the exchange-correlation functional35 where all electron relativistic (AER)36 is introduced for relativistic effects. The simulation results correspond to available experimental and theoretical results where orders of E and Φ for the above elements are established. The simulated data are related with distributions of the density of states (DOS) of the systems. 2. Calculations To make our results comparable among different elements, we keep the parameter settings consistent with those in ref 1 for the CO/Pt(111) system because the calculation techniques used there are proven to be valid: k points are set to 5 × 5 × 1 for all slabs, which brings out the convergence tolerance of energy of 2.0 × 10-5 Ha (1 Ha ) 27.2114 eV), maximum force of 0.004 Ha/Å, and maximum displacement of 0.005 Å. Because different coverage amounts lead to different E values, a uniform (2 × 2) unit cell is set. The experimental C-O bond length of dC-O ) 1.13 Å has been set.37 A four-layer slab is used to represent the metallic substrate with a vacuum width of 12 Å, which ensures that the interaction between repeated slabs in a direction normal to the surface is small enough. Note that increasing the vacuum width from 12 to 15 Å does not change the energetic results within 0.1 eV. The above two layers are

10.1021/jp0661488 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006

1006 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Liu et al.

TABLE 1: Comparison among Calculated E1 Values in Electronvolts on the Most Favorable Site, Other Simulation Results E2,29 E3,40,41 E4,42 and Experimental Results E5 23-27 a Rhb Rh Rh Rh Osb Os Os Ru Pd Ir Pt

es

site

-E1

-E2c

4d85s1 4d85s1 4d85s1 4d85s1 5d66s2 5d66s2 5d66s2 4d75s1 4d105s0 5d76s2 5d96s1

atop bridge FCC HCP atop bridge HCP atop FCC atop atop

1.71 1.56 1.57 1.62 2.02 1.45 1.48 1.80 1.35 2.12 1.68

1.55 1.48 1.50 1.59

-E3d 1.56 1.41 1.38 1.47

1.69 1.68 1.64 1.34

-E4e

-E5

dC-M

dC-O

1.67 1.58

1.65 23

1.86 2.04 2.13 2.11 1.92 2.11 2.17 1.90 2.12 1.85 1.84

1.17 1.19 1.20 1.20 1.17 1.19 1.21 1.17 1.19 1.17 1.16

1.64

1.60 1.72, 1.39

1.56

1.66 26 1.30 24 >1.96 27 1.66 25

a

es denotes the electronic structure, d is the bond length in angstroms, and the subscript M indicates metallic element. b For Rh and Os, simulated E values at different adsorption sites are shown. c E2 is calculated with the usual GGA-RPBE method without AER.29 d E3 is determined by the molecular GGA+U method.40,41 e E4 is determined by extrapolating the values of the chemisorption energies obtained first from the usual GGAPBE method.42

allowed to relax for all energy calculations. E values are computed by

E ) Et - (Eslab + ECO)

(2)

where the subscripts t, slab, and CO denote the total amount of the considered system and the corresponding substances. It is known that the usual functionals overestimate the interaction of the LUMO of CO with the metal d orbital around the Fermi level. Thus, shifting the LUMO of CO to higher energies or shifting the Fermi level of metals to lower energies can solve the problem for the site prediction as it did for the CO/Pt(111) system.1 In this work, the simulated Ef values for the Rh(111) surface are -5.44 eV and -5.51 eV for all electron (a core treatment method without relativistic effects into the core) and AER, respectively. Thus, the AER method could realize the above goal and is employed for the CO/Rh(111) system here. Note that because spin-polarization contribution on the sitepreference energy has not been found up to now28 the spinpolarization is neglected. The slab of Rh(111) is established by the experimentally determined lattice constant of a ) b ) c ) 3.803 Å.38 A monolayer of CO is placed at various geometries on the topmost layer of the slab. Because Rh has a FCC structure with the stacking sequence of ABCABCABC, there are four adsorption sites of atop, bridge, FCC, and HCP. This is also made for a CO/Os(0001) system with a slab of Os(0001) where the experimentally determined lattice constants of a ) b ) 2.734 Å and c ) 4.317 Å are taken.38 Because Os has a HCP structure with the stacking sequence of ABABAB, the adsorption sites are atop, bridge, and HCP. For CO adsorption on other elements in group VIII, the most favorable sites are undoubted. Therefore, we only determine E values of the preferred sites to compare the adsorption sequence, where it is the atop site for Ru(0001), Ir(111), and Pt(111) but the FCC site for Pd(111).29 Φ values after the CO adsorption are obtained by eq 1 after the determinations of Ef. To identify the validity of our method and eq 1, we have checked Φ ) 5.82 eV for a clean Ir(111) surface as an example, which is merely 1.04% larger than Φ ) 5.76 eV of the experimental results.39 3. Results and Discussion All known experimental data23-27 and other theoretical data for E values29,40-42 denoted as E2-E4 are shown in Table 1. If experimental results E5 are taken as a reference, then our predictions E1 are merely 5.02% lower on average, where a uniform negative deviation can be found. E2 used the usual DFT method that lacks the relativistic effects into the core, ERh, EIr ,

and EPt are 6.06%, 16.33%, and 19.28% larger, whereas ERu and EPd are 1.81% and 29.23% lower than those of E5, respectively. The authors attempted to correct their results by altering exchange-correlation functionals but finally realized that “although the choice of the exchange-correlation functional leads only to a small difference in the calculated structural and vibrational properties, it influences very strongly the absolute value of the adsorption energy.”43 In addition, “all these changes do not correct the prediction of a wrong adsorption site for Cu, Rh and Pt.”29 E3 is determined by the molecular GGA+U method proposed by Kresse et al.,40,41 which is based on the fact that the interaction between the empty CO-2π* orbital and the metallic d band is overestimated.43 This method shifts the CO-2π* orbital to higher values by adding an additional Coulomb interaction, U, to the usual DFT Hamiltonian in order to solve the problem.41 On the contrary, our method solves the problem by shifting the metallic d band to lower energies. An obvious disadvantage for the GGA+U method is that the choice of U is quite empirical as what the authors admitted in their works.40,41 Take the CO/Pt(111) system as an example: five different U values of 0.25, 0.50, 0.75, 1.00, and 1.50 eV were used in their calculations, although for all U > 0.5 eV, the experimentally observed atop site can be yielded as the most stable adsorption site; U ) 0.75 eV is chosen for further calculations because its corresponding E value is in better agreement with the experimental results.41 Therefore, there are no priori known experimental results and there is no “correct” U value.42 Similar to the GGA+U method, E4 is also derived from an empirical correction scheme for usual DFT results, which extrapolates E values obtained first from the usual GGAPBE method.42 However, this method seems to be too complicated. To obtain an E value, at least three different sets of C and O pseudopotentials (sometimes, they even “expand the number of C and O pseudopotential sets to five”) should be used to do repeated calculations in order to get the slope. Good agreements can be found between E4 and E5, except Pd. It is known that the electronic structures of elements, which are the basis of adsorption ability, provide deep insight into the interaction between adsorbates and surfaces. Because the outer electron configuration for Pd is full-filled 4d,10 its adsorption ability should be weaker because of the extra stability from the d orbital. In fact, our result of EPd ) 1.35 eV is in better agreement with EPd ) 1.30 eV from experiments.24 Therefore, it is obvious that the “relativistic correction” method used in our paper is quite accurate, objective, and convenient. The ERh value of the most favorable atop adsorption site for the CO/Rh(111) system is similar to experimental results,23

Adsorption of CO on Surfaces

Figure 1. DOS plot for the free CO molecule, the clean Ir(111) surface, bulk Ir, and the CO/Ir(111) system at the atop site. For CO, the solid, dashed, and thin solid lines indicate the states of CO, C, and O, respectively. For the Ir slab, the solid, dashed, and thin solid lines indicate the states per orbital of 5d, 6s, and 6p, respectively. The Fermi level is located at 0 eV.

followed by the HCP, FCC, and bridge sites. For free gas, the determined dC-O ) 1.15 Å is 1.7% larger than experimental results.37 For the atop adsorption system after a full relaxation, dC-O ) 1.17 Å and dC-Rh ) 1.86 Å, which correspond to experimental data of dC-O ) 1.20 ( 0.05 Å and dC-Rh ) 1.87 ( 0.04 Å well.44 datop < dbridge < dhollow, namely, d is reversely proportional to E. Because CO molecules affect only substrate atoms around their local environments, dC-O and dM-C values increase with the coordination because of the larger occupation of electrons in the 2π* orbital.29 For the CO/Os(0001) system, the order of favorable adsorption sites is atop > HCP > bridge in terms of the determined EOs values with datop < dbridge < dHCP . The metallic surface atoms interacting with CO move outward from the surface, as hoped.20 In addition, dC-O is slightly elongated compared to that of the free gas due to backdonation of substrate electrons into a previously unoccupied antibonding CO-2π* orbital, which weakens the C-O bond.45 The calculated E values of the most favorable site of CO/M systems, as shown in Table 1, have an order of EPd > EPt > ERh > ERu > EOs > EIr , which is similar to the sequence of experimental results. The DOS plot for the atop site of the CO/ Ir(111) system is determined and shown in Figure 1, where the states of per orbital of 5d, 6s, and 6p are analyzed to shed light on the hybridization situation of Ir. As comparisons, DOS plots of a free CO molecule, a clean Ir(111) surface, and bulk Ir are also depicted there. For free CO, the orbitals of 4σ, 1π, 5σ, and 2π* (numerals represent appearance times of σ and π orbitals counted from the core) are localized at about -9, -6.5, -4, and 3.0 eV, respectively. In Figure 1, it can be seen that the accumulation of charge density for CO-4σ and CO-1π orbitals is around the O atom, whereas that for the CO-5σ orbital is around C atom. The same result has been achieved by analyzing charge density plots for free CO molecules.29 Generally, nonbonding lone pairs form when a pair of electrons of a specific atom occupies a directional bonding orbital, which happens to electronegative elements such as N, O, and F upon sp-orbital hybridization.46 In terms of a “4 - n” rule with n being the valence value of the adsorbate,47 O produces two lone pairs upon sp hybridization, while C cannot because its valence number is four. However, X-ray photoemission spectroscopy (XPS) experiments found that the CO molecular axis is normal to the surface, and it is C, not O, being bonded to metallic

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1007 surfaces.48,49 In fact, wide researches have proved the above adsorption configuration.16,28,41,50 A plausible explanation for this conflict is that a “coordination bond” is formed between C and O atoms by sharing two O electrons with C because one C-2p orbital is empty while one O-2p orbital is full in CO. The triple bond CtO (one σ and two π bonds) makes it possible for C atoms to interact with metallic surfaces.51 To further verify the validity of the above explanation, a CO/Ir(111) adsorption model with O being bonded to the Ir(111) surface is built and calculated. It is found that the system becomes unstable after adsorption (EIr ) +1.08 eV). This fact supports the above consideration. For the CO/Ir(111) system, both adsorbate and substrate states are moved left to that with lower energies, and the region above the Fermi level decreases largely after adsorption. In the CO side, both CO-5σ and CO-1π orbitals broaden and dominate the interaction, where the 5σ orbital shifts largely from -4 to -7.5 eV, which is even below the 1π orbital. In the Ir side, main peaks are moved left from the clean Ir(111) surface, atop Ir(111), to bulk Ir in turn. Thus, the clean Ir(111) surface is the most active because of its bond loss.47 A new peak of the Ir-dz2 orbital appears at -10 eV and interacts with the CO-I4σ orbital. For a higher energy range, both orbitals of Ir-6s and Ir-5d located at -6 eV split into two peaks at -7.5 eV and -6.5 eV. Ir-6s and Ir-dz2orbitals interact with the CO5σ orbital. Ir-6s, Ir-(5dxz + 5dyz), and even Ir-6p orbitals interact with the CO-1π orbital. It is widely accepted that Blyholder’s model gives a good picture of charge transfer for CO/M systems, which states that lone pair electrons donate from the nonbonding CO-5σ orbital into empty metal orbitals, and back-donate from occupied metal d orbitals to an empty CO-2π* orbital, simultaneously.50 Unfortunately, this model cannot judge the ultimate flow direction of charges; namely, what on earth do the charges move to after adsorption, from CO to metals or just the opposite? To make up this defect, as well as to further understand the bonding nature of the adsorption system, the plot of electron density difference ∆F for the CO/Ir(111) system is introduced, as shown in Figure 2. ∆F illustrates how the charge density changes during a chemical reaction or on binding of a molecule to a surface, which is expressed as

∆F ) Ft - (Fslab + FCO)

(3)

where F is the electron density, and the subscripts t, slab, and CO have the same meaning as those in eq 2. As shown in Figure 2, Ir and O atoms lose electrons while C atom gains electrons. Notably, some charges accumulate between C and Ir atoms, which confirm the binding between CO and metallic surfaces induced by adsorption. The above results derived from Figure 2 are supported by the data from Mulliken charge analysis, where C decreased from +0.41 to +0.22, O increased from -0.41 to -0.33, and the Ir(111) surface increased from 0 to +0.12 after adsorption. A similar charge-transfer conclusion has been achieved for a CO/Pt(111) cluster model.52 Therefore, both covalent bonds due to the overlap of orbitals and ionic bonds induced by the charge transfer influence the adsorption between CO molecular and metallic surfaces, where the latter is a dominated process. For comparison purpose, a DOS plot of six CO/M systems is determined and shown in Figure 3. Similar to the CO/Ir(111) system, the electronic states of free CO in the other five systems are lowered in energy after binding to substrates, and the electron states of substrates are also changed remarkably to hybridize with CO molecules except Pd(111). For CO/Pt(111) and CO/Pd(111) systems, the CO-2π* peak broadens and

1008 J. Phys. Chem. C, Vol. 111, No. 2, 2007

Liu et al. TABLE 2: Comparison among Calculated Work Functions of Φ1 and Φ2 with Other Theoretical Results Φ3 39 and Experimental Results Φ4 in Electronvolts,53 Where Φ1 Is the Φ Value of Surfaces Covered by CO and Φ2 to Φ4 Are the Φ Values of Clean Metallic Surfacesa Rh Os Ru Pd Ir Pt

Φ1

Φ2

∆Φ

Φ3b

Φ4c

5.61 6.17 5.75 5.53 5.86 5.56

5.52 6.10 5.60 5.28 5.82 5.53

0.09 0.07 0.15 0.25 0.04 0.03

5.91 6.42 5.84 5.90 6.63 6.74

(4.98) (4.83) (4.71) 5.60 5.76 5.70

a ∆Φ ) Φ - Φ , which indicates the change of Φ induced by 1 2 adsorption. b Φ3 is calculated with the tight-binding LMTO Green’s 39 c function. Φ4 in parentheses are polycrystalline values.53

Figure 2. Plot of the electron density difference for the CO/Ir(111) system taken along the (110) plane. The largest sphere shows a metallic atom, the middle size sphere is an O atom, and the smallest sphere is a C atom. In this plot a loss of electrons is indicated in blue, electron enrichment is indicated in red, and white indicates regions with very little change in the electron density.

Figure 3. DOS plot for CO/M systems at the preferred site. The solid and dashed lines indicate the states of CO and metals, respectively. The Fermi level is located at 0 eV.

overlaps with the 1π orbital, and the interaction between CO2π* and M-(5dxz + 5dyz) orbitals leads to a hybrid orbital at higher energetic level around 2.5 eV. Because the antibonding dipole CO-2π* orbital is formed by the production of a nonbonding band that polarizes electrons of the neighboring atoms, the antibonding band disappears correspondingly when lone pair electrons transfer to a metallic surface.39 This phenomenon can also be observed for Ir, Os, Ru, and Rh adsorption systems. Both calculated Φ values of before (Φ2) and after (Φ1) adsorption are listed in Table 2. Other available theoretical data Φ339 and experimental data Φ453 of clean metallic surfaces are also listed. We first compare Φ2 with Φ3 and Φ4. If experimental results Φ4 are taken as references, then Φ2 values are in good agreement with the single-crystal data, where ΦPd and ΦPt are 5.54% and 2.98% lower, ΦIr is 1.04% larger than those of Φ4. However, Φ2 values are generally above the corresponding polycrystalline data including ΦRh, ΦOs, and ΦRu; this is because only the single-crystal surfaces can be simulated in our calculations. The Φ values of polycrystals are lower than the corresponding single crystal because polycrystalline samples are

relatively open.39 Note that Φ3 values are 12.90% larger than Φ4 because the tight-binding LMTO Green’s function employed in Φ3 overestimates the cohesive energy, and thus hampered the movement of charges. Just the opposite, the GGA-RPBE functional used in this paper somewhat underestimates the cohesive energy and thus results in relatively lower values. From Table 2, all Φ1 values are larger than Φ2, where Φ1 is a litter larger than Φ2 for atop adsorption sites (the most ∆Φ values are within 0.10 eV), while much larger than Φ2 for FCC adsorption sites (∆Φ ) 0.25 eV for Pd), which are consistent with the conditions of CO/Cu(111) system.5,43 To explain the above phenomena, we employed sketch plots to look into how charge moves through a metallic surface in the process of catalytic oxidation. As shown in Figure 4a, if no CO molecule exists, then charge transfers directly from a clean metallic surface to an adsorbed O atom. When CO adsorbs, two aspects of charges from both CO and metallic atoms move into O, and thus the pathway to the O atom becomes “busier” as shown in Figure 4b. This is plausibly the reason that Φ values of surfaces covered by CO are larger than those of clean surfaces. Similarly, for FCC adsorption configuration, four aspects of charges are “crowded” to run through Pd(111) surfaces as shown in Figure 4c, which can be used to explain why the ∆Φ value of the highcoordination adsorption site is larger than that of the lowcoordination one. The order of the work function after adsorption ΦOs > ΦIr > ΦRu > ΦRh > ΦPt > ΦPd can be roughly determined by the concentration of electrons accumulated around the Fermi level in DOS plots. As shown in Figure 3, the concentration of Pd is the largest, while its Φ value is much lower than the rest. Against that, the concentrations of Os and Ir are the lowest but

Figure 4. Sketch plot of charges moving to O atoms: (a) from clean metallic surfaces; (b) from metallic surfaces covered by atop adsorbed CO; and (c) from metallic surface covered by hollow adsorbed CO. The arrow indicates the route of charge transfer. The largest sphere shows a metallic atom, the middle-size sphere is an O atom, and the smallest sphere is a C atom.

Adsorption of CO on Surfaces have the largest Φ values. The denser the electrons around Ef, the easier they could be emitted to vacuum. Thus, distribution of DOS is directly related to the kinetic factor of catalysts. 4. Conclusions In summary, DFT calculation with the AER core treatment method is employed to determine the adsorption abilities of CO on 4d and 5d metal surfaces in group VIII where the Pd(111) surface is the hardest and the Ir(111) surface the easiest because of different hybridizations of the s, p, and d orbitals. It is found that the size of the work function, which is directly related to the distribution of DOS, increases after adsorption, and the difference of the work function before and after adsorption increases with the coordination of adsorption sites. In addition, with the AER method, the most favorable atop site for the CO/ Os(0001) system is found for the first time. Acknowledgment. We acknowledge the financial support from the National Key Basic Research and Development Program of China (Grant No. 2004CB619301) and “985 Project” of Jilin University. References and Notes (1) Orita, H.; Itoh, N.; Inada, Y. Chem. Phys. Lett. 2004, 384, 271. (2) Rajasree, R.; Hoebink, J. H. B. J.; Schouten, J. C. J. Catal. 2004, 223, 36. (3) Curulla, D.; Govender, A.; Bromfield, T. C.; Niemantsverdriet, J. W. J. Phys. Chem. B 2006, 110, 13897. (4) Olsen, R. A.; Philipsen, P. H. T.; Baerends, E. J. J. Chem. Phys. 2003, 119, 4522. (5) Gajdosˇ, M.; Eichler, A.; Hafner, J.; Meyer, G.; Rieder, K. H. Phys. ReV. B 2005, 71, 035402. (6) Steckel, J. A.; Eichler, A.; Hafner, J. Phys. ReV. B 2003, 68, 085416. (7) Mavrikakis, M.; Rempel, J.; Greeley, J.; Hansen, L. B.; Nørskov, J. K. J. Chem. Phys. 2002, 117, 6737. (8) He, R.; Kusaka, H.; Mavrikakis, M.; Dumesic, J. A. J. Catal. 2003, 217, 209. (9) Gonzalez, S.; Illas, F. Surf. Sci. 2005, 598, 144. (10) Song, C.; Ge, Q.; Wang, L. J. Phys. Chem. B 2005, 109, 22341. (11) Gil, A.; Clotet, A.; Ricart, J. M.; Illas, F.; A Ä lvarez, B.; Rodes, A.; Feliu, J. M. J. Phys. Chem. B 2001, 105, 7263. (12) Eichler, A. Phys. ReV. B 2005, 71, 125418. (13) Curulla, D.; Clotet, A.; Ricart, J. M.; Illas, F. J. Phys. Chem. B 1999, 103, 5246. (14) Remediakis, I. N.; Lopez, N.; Nørskov, J. K. Appl. Catal., A 2005, 291, 13. (15) Molina, L. M.; Hammer, B. Phys. ReV. B 2004, 69, 155424. (16) Liu, L. M.; McAllister, B.; Ye, H. Q.; Hu, P. J. Am. Chem. Soc. 2006, 128, 4017. (17) Gandhi, H. S.; Graham, G. W.; McCabe, R. W. J. Catal. 2003, 216, 433.

J. Phys. Chem. C, Vol. 111, No. 2, 2007 1009 (18) Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. Phys. ReV. Lett. 1998, 80, 3650. (19) Narloch, B.; Held, G.; Menzel, D. Surf. Sci. 1995, 340, 159. (20) Krenn, G.; Bako, I.; Schennach, R. J. Chem. Phys. 2006, 124, 144703. (21) Schwegmann, S.; Over, H.; De Renzi, V.; Ertl, G. Surf. Sci. 1997, 375, 91. (22) Liu, D. J.; Evans, J. W. J. Chem. Phys. 2006, 124, 154705. (23) Hopstaken, M. J. P.; Niemantsverdriet, J. W. J. Chem. Phys. 2000, 113, 5457. (24) Guo, X.; Yates, J. T., Jr. J. Chem. Phys. 1989, 90, 6761. (25) Yeo, Y. Y.; Vattuone, L.; King, D. A. J. Chem. Phys. 1997, 106, 392. (26) Pfnu¨r, H.; Feulner, P.; Menzel, D. J. Chem. Phys. 1983, 79, 4613. (27) Comrie, C. M.; Weinberg, W. H. J. Chem. Phys. 1976, 64, 250. (28) Feibelman, P. J.; Hammer, B.; Nørskov, J. K.; Wagner, F.; Scheffler, M.; Stumpf, R.; Watwe, R.; Dumesic, J. J. Phys. Chem. B 2001, 105, 4018, and references therein. (29) Gajdosˇ, M.; Eichler, A.; Hafner, J. J. Phys.: Condens. Matter 2004, 16, 1141. (30) Vayenas, C. G.; Bebelis, S.; Ladas, S. Nature 1990, 343, 625. (31) Vayenas, C. G.; Bebelis, S.; Neophytides, S. J. Phys. Chem. 1988, 92, 5083. (32) Gil, A.; Clotet, A.; Ricart, J. M.; Kresse, G.; Garcı´a-Herna´ndez, M.; Ro¨sch, N.; Sautet, P. Surf. Sci. 2003, 530, 71. (33) Delley, B. J. Chem. Phys. 1990, 92, 508. (34) Delley, B. J. Chem. Phys. 2000, 113, 7756. (35) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. ReV. B 1999, 59, 7413. (36) Delley, B. Int. J. Quantum Chem. 1998, 69, 423. (37) CRC Handbook of Chemistry and Physics, 81st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2000. (38) www.webelements.com. (39) Skriver, H. L.; Rosengaard, N. M. Phys. ReV. B 1992, 46, 7157. (40) Ko¨hler, L.; Kresse, G. Phys. ReV. B 2004, 70, 165405. (41) Kresse, G.; Gil, A.; Sautet, P. Phys. ReV. B 2003, 68, 073401. (42) Mason, S. E.; Grinberg, I.; Rappe, A. M. Phys. ReV. B 2004, 69, 161401(R). (43) Gajdosˇ, M.; Hafner, J. Surf. Sci. 2005, 590, 117. (44) Gierer, M.; Barbieri, A.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1997, 391, 176. (45) Fo¨hlisch, A.; Nyberg, M.; Hasselstro¨m, J.; Karis, O.; Pettersson, L. G. M.; Nilsson, A. Phys. ReV. Lett. 2000, 85, 3309. (46) Keenan, C. W.; Kleinfelter, D. C.; Wood, J. H. General College Chemistry, 6th ed.; Harper and Row Publishers: San Francisco, CA, 1979. (47) Sun, C. Q. Prog. Mater. Sci. 2003, 48, 521. (48) Petersson, L.-G.; Kono, S.; Fadley, C. S.; Hall, N. F. T.; Pendry, J. B. Phys. ReV. Lett. 1979, 42, 1545. (49) Held, G.; Schuler, J.; Sklarek, W.; Steinru¨ck, H.-P. Surf. Sci. 1998, 398, 154. (50) Blyholder, G. J. Phys. Chem. 1964, 68, 2772. (51) Davlieva, M. G.; Lindeman, S. V.; Neretin, I. S.; Kochi, J. K. New J. Chem. 2004, 28, 1568. (52) Koper, M. T. M.; van Santen, R. A. J. Electroanal. Chem. 1999, 476, 64. (53) Michaelson, H. B. J. Appl. Phys. 1977, 48, 4729.