Bonding Nature of Monomeric H2O on Pd - American Chemical Society

Jan 12, 2009 - Technology Owerri, PMB 1526, Owerri, Nigeria. ReceiVed: October 30, 2008; ReVised Manuscript ReceiVed: December 4, 2008. Using density ...
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J. Phys. Chem. C 2009, 113, 1931–1938

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Bonding Nature of Monomeric H2O on Pd: Orbital Cooperation and Competition Jibiao Li,† Shenglong Zhu,† Hong Li,‡ Emeka. E. Oguzie,§ Ying Li,† and Fuhui Wang*,† State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110015, China, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China, and Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal UniVersity of Technology Owerri, PMB 1526, Owerri, Nigeria ReceiVed: October 30, 2008; ReVised Manuscript ReceiVed: December 4, 2008

Using density functional theory (DFT) calculations, we provide a unified picture of the bonding nature of a H2O monomer on palladium: varying surface symmetry from hexagonal (Pd{111}) to square (Pd{100}) to rectangular (Pd{110}) lattices. Theoretical evidence shows that three distinct ranges of whole d-band energy exhibit cooperative roles in the H2O-Pd bonding mechanisms. Bonding states at low-energy resonances (LERs) are assisted by Pauli repulsion-induced electron rearrangement in nonbonding states around d-band centers (intermediate-energy resonances (IERs)) and by depopulation of partially filled antibonding states at highenergy resonances (HERs). Moreover, we have identified the symmetry-tuned lone-pair competitions. As the degree of surface symmetry is reduced, water adsorption consistently enhances nonsigma components of lp-d overlaps, which originate from distinct balance between electrostatic attraction and Pauli repulsion. Furthermore, we have found divergency effects of s-d hybridization within the first substrate layer. Introduction Water-metal interactions are essential to environmentally important surface processes such as atmospheric and electrochemical corrosion.1 Besides the scientific importance of such interactions in ice nucleation,2 water wetting,3 heterogeneous catalysis,4 and electrocatalysis,5 they are also technically relevant for such important applications as fuel cells6 and water splitting7 for hydrogen production. Previous studies8,9 revealed that the existence of supported monomeric water requires low water coverages under extremely low temperatures. These conditions post a huge challenge in characterizing isolated water monomers at higher temperatures because essentially of water diffusion and clustering which result from the comparable strength of water-metal interactions and hydrogen bondings. Obviously, monomeric water adsorption that is in absence of the hydrogen bonding serves as an archetype for studying the nature of fundamental water-metal interactions. Driven by a series of fundamental questions, the nature of such interactions has been a focus of water science at metal surfaces raising questions as to whether the adsorption of water monomers on close-packed fcc{111}surfaces shares the same geometric features with that on open fcc{100} and ridged fcc{110} surfaces and how geometric and electronic structures of the substrate surfaces control the binding nature of H2O onto the metal surfaces. In 1984, Andersson et al.10 reported the onset of H2O monomers on the open Pd{100} surface (10 K) on the basis of electron energy loss spectroscopy (EELS). To our knowledge, this is the only experimental observation of water monomers on open fcc{100} surfaces. On close-packed transition-metal * To whom correspondence should be addressed. E-mail: jibiaoli@ imr.ac.cn, [email protected]. † State Key Laboratory for Corrosion and Protection, Institute of Metal Research. ‡ Shenyang National Laboratory for Materials Science, Institute of Metal Research. § Department of Chemistry, Federal University of Technology.

surfaces, direct scanning tunneling microscopy (STM) evidence for supported water monomers has been provided by Morgenstern and co-workers11-13 and Salmeron and co-workers.14-16 Many earlier spectroscopic investigations, by Nakamura and Ito17-20 and Yamamoto et al.,21 also revealed their existence on metal surfaces. One common deduction from these experiments was that water monomers adsorbed at on-top sites of closepacked fcc{111} and open fcc{100} surfaces; however, detailed information including water orientations is still unclear. For ridged fcc{110} surfaces, both previous experimental and theoretical studies identifying surface water monomers are controversial22-24 because of limitations of spectroscopic techniques such as EELS and IR. The dilemma also appeared in the STM studies by Komeda et al.25 who reported that isolated water monomers on Pd{110} appeared at a cryogenic temperature (4.7 K). On the other hand, the smallest STM feature may not necessarily correspond to a water monomer, as pointed out by the authors, owing to the elongated protrusion size. Thus, the adsorption geometries of H2O/Pd{110}, such as adsorption sites and molecular orientations, remain mysterious at atomic levels. Quantum mechanical calculations based on density functional theory (DFT) play a unique role in elucidating detailed geometric structures of adsorbed water monomers. Among low-index transition-metal surfaces, monomeric water molecules on closepacked metal surfaces have been intensively studied by Michaelides and co-workers26-29 and by several other groups including Meng et al.,30 Cao and Chen,31,32 Pozzo et al.,33 and Sebastiani and Delle Site.34 These DFT calculations confirmed the on-top adsorption site of the earlier mentioned experimental deduction and predicted that the H2O monomers favor virtually flat-lying configurations. This geometric picture was also found to be applicable to monomeric water adsorption on open fcc{100} surfaces as demonstrated by Qin and Whitten,35 Ignaczak and Gomes,36 Tang and Chen,37 Wang et al.,38 and our group.39-41 The consensus between theoretical and experi-

10.1021/jp809595y CCC: $40.75  2009 American Chemical Society Published on Web 01/12/2009

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Figure 1. Local density of states (LDOS) of the on-top adsorptions of water monomer on Pd{111}, Pd{110}, and Pd{100}. In each system, the LDOS is projected onto H2O orbitals, individual 4d and 5s bands, as indicated in the right-most panels. Figure 3b in ref 40 is included to make a complete picture. The shaded and solid curves represent the LDOS of the clean surfaces with and without water adsorption, respectively. Energy scales are shifted with respect to EF taken in each case as the zero of energy. To compare the changes of 1b2-3a1 and 3a1-1b1 spacings, the LDOS of free water is shifted with respect to 1b2 orbital (not shown here; no occupation changes were found for 1b2 orbital). The electron occupation is shown in each panel; the data in parentheses represent the clean surfaces or a free water molecule. Subpeak positions are marked as red and black arrows for the clean and water-adsorbed surfaces, respectively, and a blue arrow is used when the two types of arrows coincide at an equivalent position.

mental results seems to have been arrived at via adsorption geometry irrespective of chemical species and the lattice symmetries of the substrates. Nonetheless, ridged fcc{110} surfaces are exceptions; the preference of an asymmetric water structure on Cu{110} has been reported by Tang and Chen,37 Ren and Meng,42 and Salli et al.,43 where one OH bond approaches more closely to the substrate surface, while the other one directs away from it. It is still unknown whether this geometric picture is applicable to other ridged fcc{110} surfaces. Apart from the adsorption geometries, the binding nature of a water monomer to metal surfaces has also been intensively discussed. Using electron density difference, Michaelides et al.27 reported that the main orbital interaction of H2O on Ru{1000} is through the H2O 1b1 orbital that combines with Ru dz2 states accounting for the on-top flat-lying configurations. A similar conclusion was also obtained for H2O/Ag{111}28 and H2O/ Pt{111}.30 A similar study26 showed that such interaction was appropriate for other close-packed surfaces. The question of how geometric and electronic structures of substrate surfaces determine water-metal bondings has yet to be answered. Recently,

we found that Ni 3p orbitals play an equally important role in the adsorption of a H2O on Ni{111}44 thus providing evidence on new bonding channels for water-metal interactions. On an open, Pd{100} surface, the role of Pd 5s state in the system of H2O/Pd{100}40 was also recognized. Nevertheless, a complete picture that elucidates the roles of both d- and sp-electrons has not been obtained so far. Recently, Schiros et al.45 proposed that metal-water affinity depends strongly on the degree of Pauli repulsion between the water lone pair and the metal dand sp-electrons as well as on the attractive electrostatic interaction. These in turn depend on the occupation and depopulation of involved orbitals, which are influenced by both the electronic and geometric structures of substrate surfaces. Issues relating to the energy scales at which depopulation occurs, including the influence of different geometric structures, in particular for a substrate metal with different surface planes as well as the effect of different electronic configurations of the same surface symmetry are yet to be satisfactorily resolved. In this study, we focus on addressing the basic question regarding the effect of lattice symmetry on dynamic roles of

Bonding Nature of Monomeric H2O on Pd whole d-bands and 5s states in orbital interactions. Three low Miller index surfaces, namely, Pd{111}, Pd{100}, and Pd{110}, have been selected as the model surfaces to vary the degree of the surface symmetry from hexagonal to square to rectangular lattices corresponding to the six-, four-, and twofold on-top surface symmetries, respectively, for a single sheet of the atomic layers. Since a free palladium atom in its ground state has a closed d-shell (4d105s0) electronic configuration that acts as the borderline for open d-shell and closed d-shell configurations, understanding the effect of such an in-between occupation of d-band on Pauli repulsion and orbital interactions is of scientific importance. By a careful comparative study, we demonstrate that the whole energy ranges of the d-bands are responsible for the orbital interactions involving occupied water orbitals and partially filled 4d and 5s states and that these orbitals in three distinct d-band energy ranges play rather different but cooperative roles in water-palladium interactions. In low resonances, a slight electron transfer between symmetry-allowed orbital overlaps (e.g., σ and π symmetries) governs water-palladium bondings that are balanced by attractive electrostatics and Pauli repulsions which give rise to electron rearrangement around d-band centers, where electrons transfer from the axial to the equatorial orbitals. Such rearrangement is simultaneously mediated by depopulation of the partially filled antibonding states near Fermi levels (high-energy resonances). By reducing the degree of surface symmetry, water adsorption consistently enhances nonsigma components of lp-d overlaps, which originate from distinct balance between attractive electrostatics and Pauli repulsions. Furthermore, we have reported the effect of s-d hybridization within the first substrate layer of 2D electron gas. Our results provide insight into water-bonding nature which in turn governs several surface processes. Computational Methods The calculations were performed in the framework of density functional theory as implemented by PWscf code.46 Electron-ion interactions were included through the use of ultrasoft pseudopotentials.47 Electron exchange and correlation effects were described by the generalized gradient approximation (GGA) in Perdew-Burke-Ernzerhof form (PBE).48 A plane wave cutoff of 38 Ry (300 Ry for the charge density) and k-point mesh of 3 × 3 × 1 and 4 × 4 × 1 were used. Brillouin zone integration was performed by Methfessel-Paxton49 special point techniques. The clean Pd{111}, Pd{110}, and Pd{100} surfaces were modeled by a five- or seven-layer slab separated by a vacuum region of 17 Å. Water was placed on one side of the slabs in (2 × 2) unit cells for the Pd{111} and Pd{100} surfaces and in a (2 × 3) unit cell for the Pd(110) surface. Structure optimizations were performed with the central layer fixed at their bulkdetermined positions. The rest of the Pd atoms and the adsorbed water were fully relaxed, for the latter until the Hellmann-Feynman forces were lower than 0.001 Ry/au. For reference, the structural optimization of an isolated H2O molecule in gas phase was performed using a box with the same size of the absorbed systems. A calculation was considered converged when the energy per atom was less than 1 × 10-5 Ry and the mean displacement of the atoms was less than 0.001 Å. Geometric and energetic results of the water adsorption on Pd surfaces are presented in Supporting Information (SI). In each adsorption system, a plot of charge density differences (CDD) was obtained by subtracting the densities of a bare Pd slab and a H2O molecule from the adsorption system: n(r) ) nwater/metal(r) - nmetal(r) - nwater(r). Atomic positions in the separated systems of the Pd slab and the H2O molecule were

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1933 identical to those of the relaxed system of water adsorption. The CDD plots are presented in both cross-sectional and inplane views cutting through the Pd atom bonding with the water monomer. The two perspectives are helpful in identifying roles of different atomic orbitals; dz2, dzx, and dzy orbitals can be easily visualized in cross-sectional views, and dx2-y2 and dxy orbitals can be easily visualized in in-plane views. To elucidate the underlying adsorption mechanisms, local density of states (LDOS) was projected onto individual 4d orbitals and 5s states as well as on the molecular H2O orbitals. Lowdin charges and occupation in the adsorption systems are presented to elucidate charge transfer and rearrangements. Local density of electronic entropy (LDEE) was calculated through the equations below50

S(σ) ) χi )

∫-∞+∞ D(ε, σ)φ(χ)dε, φ(χt) ) - ∫-∞χ tg(t)dt i

µ(σ) - εi σ

(1)

where D(ε, σ) and g(t) are density of states and broadening function, respectively, and σ, µ, and ε are related to broadening parameter, chemical potential, and electron energy, respectively. LDEE thus provides important information on surface electronic structures.51 A positive value of LDEE corresponds to filled surface resonances (e.g., low-energy resonance (LERs) and MERs in the following section) below Fermi level, and a negative value corresponds to partially filled (e.g., high-energy resonance (HERs)) and empty resonances around and above the Fermi level, respectively. Plots of LDEE differences are calculated in a way similar to that in CDD to reveal electron polarization and rearrangement around Fermi levels. Results and discussion are presented in five subsections. First (in section A), we try to understand the electronic structures of palladium with the six-, four-, and twofold on-top symmetries. In this section, we address fundamental questions on how surface symmetry and geometry affects s- and d-band structures of the clean Pd surfaces. Section B is focused on the direct water-palladium interactions to give insight into the surface electronic factors that govern the H2O-Pd bonding nature. Section C demonstrates the electronic rearrangement and perturbation within all the substrates followed by remarks on CDD in Section D. Results and Discussion A. Geometric Effect on Clean-Surface Electronic Structure. 1. Surface d-Band Resonances. A prominent effect of reducing the surface symmetry is related to increasing degree of d-band decoupling (Figure 1). For instance, for the sixfold on-top symmetry, the dz2 state of the surface atoms is in relatively strong couple with dx2-y2 and dxy states resulting in a triplet resonance with subpeak positions for the low-energy resonance (LER), intermediate-energy resonance (IER), and high-energy resonance (HER) at -3.8, -2.2, and -0.2 eV, respectively, which is denoted as dz2-dx2-y2@ {-3.8, -2.2, -0.2} eV. Reducing the surface symmetry to the square lattice leads to the absence of the triplet resonance, and the coupling exhibits either a single resonance involving three d-bands (e.g., MER: dzx-dzy-dxy@ -3 eV) or a doublet resonance involving two d-bands (e.g., LER: dz2-dx2-y2@ -3.8 eV, MER: dzx-dzy@ -1.6 eV, and HER: dzx-dzy@ -0.5 eV) or a singlet resonance involving only two d-bands (e.g., MER: dz2-dx2-y2@ -2.2 eV). On reducing the surface symmetry to the rectangular lattice, only the simplest resonances remain (LER: dzx-dzy-dx2-y2@ -4 eV and HER: dxy-dx2-y2@ -0.2 eV) in addition to the onset of a new resonance involving four d-bands (MER:

1934 J. Phys. Chem. C, Vol. 113, No. 5, 2009 SCHEME 1: Surface s-d Hybridizations with Different Degrees of Orbital Coupling Depending on the Diffusive Nature of 5s State which Is Influenced by Surface Geometric Structure

dz2-dx2-y2-dzx-dxy@ -2 eV). In summary, the three subresonances (LERs, MERs, and HER) are commonly observed in the whole d-band energy ranges irrespective of the surface lattice types that exert the effect of d-band decoupling on reducing the degree of surface symmetry. 2. Surface s-d Hybridization. Because of the diffusive nature of the 5s state, it can in principle hybridize with 4dbands at different energy levels (e.g., LERs and MERs) depending on geometric structures of the surface lattices. Scheme 1 shows only two resultant cases (low energy vs high energy) of such hybridizations. Our main finding is that 5s-4dz2 mixing is a common feature in the three surfaces. The LDEE plots provide direct evidence for the dominant 5s-4dz2 mixings which are demonstrated as the out-of-plane axial orbitals in the hexagonal (Figure 2b′), square (Figure 2f′), and rectangular (Figure 2d′) lattices. Although the patterns show a marked similarity between the hexagonal and square lattices, moderate s-d hybridization is reflected in the latter higher energy. In the rectangular lattice, however, we observe a unique hybridization pattern that is compressed along the surface normal because of the resultant surface electrostatics. This could result from removal of the 5s state from Pd top sites yielding the highest coupling energy. This is also applicable to the in-plane hybridization (Figure 2d): 5s-4dx2-y2@ {-3.8, -2} eV (Figure 2). Furthermore, these observations provide a basis of Smoluchowski effect52 in ridged transition metal surfaces. In addition to the hybridization patterns, the corresponding energy correlates well with those shown in Scheme 1. The latter observation is further supported by the PDOS results showing that the 5s-4dz2 mixing energy consistently shifts upward (e.g., LER: -4 eV, LER: -3.8 eV, and MERs: -3 to -1 eV) with a decrease in both 5s occupation (0.73, 0.69, and 0.67) and LER peak height when the degree of surface symmetry is reduced. In brief, we have shown that the mobile s-state hybridizes with the localized d-bands; the s-d hybridizations are determined by both the diffusive nature of the 5s orbital and the surface d-band resonances. Importantly, decreasing the degree of surface symmetry opens up a channel for the orbital decoupling effect in the surface d-bands as well as the s-d hybridization. B. Water-Pd Interactions. 1. d-Band Cooperation. An important general finding concerning the effect of electronic structures on bonding mechanisms is related to collective intraband cooperation in each d-band (Figure 1), which is evidenced by the fact that the d-band center exhibits either

Li et al. depopulation compensated by collective repopulation around LERs and HERs or repopulation assisted by depopulation around LERs and HERs. Such population changes are strong indications of two types of intraband cooperation; the first electronic process is denoted as d (LER) r d (MER) f d (HER), and the second one is denoted as d (LER) f d (MER) r d (HER). The cooperation in the former case (bonding d-band) results in a noticeable d-band splitting. The axial dz2 state is an example of such a d-band in H2O/Pd{111}, and it is the bonding d-band in H2O/Pd{100} indicating a similar role of the three surface resonances of dz2 state in the two systems. However, bonding d-bands and the splitting thereof are different for H2O/Pd{110} because of the involvement of equatorial dzx state, while in the second case, other d-bands (nonbonding) exhibit slight band narrowing keeping their d-band centers essentially unchanged. Therefore, we have actually indentified two counteracting ways of adsorption-induced d-band energy redistribution, which originate from Pauli exchanges and also constitute the interband cooperation. 2. Effect on s-d Hybridization. The PDOS shows that water adsorption strengthens the s-d hybridizations involving the bonding d-bands as evidenced by intraband electronic rearrangements: 5s (LERs) r 5s (MERs) and by the downward shifts of major 5s peaks. Therefore, hybridizations of 5s with the nonbonding d-bands are consistently weakened because of the deviations of the corresponding peaks. This general observation is firmly supported by the LDEE data. For instance, the hybridization of the bonding dzx with 5s is directly visualized in H2O/Pd{110} (Figure 3c′) although s-dz2 hybridization is also included. The observation is in excellent agreement with the proposed s-dzx hybridization (Scheme 1). Also clearly presented is the enhanced s-dz2 hybridization in H2O/Pd{111} (Figure 2a′) and H2O/Pd{100} (Figure 2e′) and the s-dx2-y2 character (Figure 2c) that is in fact related to the MERs in H2O/ Pd{110}. The results thus provide clear evidence supporting the proposed s-d hybridization in Scheme 1. 3. Bonding, Nonbonding, and Antibonding States. What are the roles of the three distinct energy ranges of surface resonances for water-palladium interactions? Another general feature from the PDOS (Figure 1) is that bonding d-bands overlap with water lone-pair orbitals resulting in the bonding characters at lower d-band edges (LERs) and antibonding states around Fermi levels (HERs). Specifically, we observe two channels of the bonding states (e.g., high and low) because of the two water orbitals (e.g., 1b1 and 3a1). H2O-Pd antibonding states coincide with energies of the HERs, which is evidenced by the partially filled surface states in the inset figures. The MERs act as nonbonding states around the d-band centers. Generally, all d-band centers that mark the nonbonding states shift more or less downwardly. We find that the antibonding states originate from different d-bands in different adsorption systems. As can be seen from the PDOS plot, dz2(HER) in H2O/Pd{111}and H2O/Pd{100} predominantly contributes to the antibonding states around the Fermi levels. In contrast, dzx (HER) and dz2 (HER) are found to account for the antibonding state in H2O/Pd{110}. These observations are quite similar to the case of the involved orbitals at bonding states. According to Nørskov and co-workers’ theory,53-55 the antibonding states below the Fermi levels lead to repulsive coupling to the d-bands at the HERs. The decreasing d-band occupation (9.32, 9.28, 9.28) that is used as a measure of filling of water-Pd antibonding states is also in accord with the decreasing energy position of the antibonding states. Thus, the three distinct ranges of the d-bands play their roles as the bonding, nonbonding, and antibonding states, respectively, in

Bonding Nature of Monomeric H2O on Pd

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Figure 2. Plots of local density of electronic entropy (LDEE): (a, a′) H2O/Pd{111}, (b, b′) Pd{111}, (c, c′) H2O/Pd{110}, (d, d′) Pd{110}, (e, e′) H2O/Pd{100}, and (f, f′) Pd{100}. The upper (lower) panels stand for top (side) views cutting through the bonding Pd atoms. For brevity and clarity, all values in the labels are multiplied by a factor of 100.

Figure 3. Plots of LDEE differences for the on-top adsorption of H2O on (a) Pd{111}, (b) Pd{110}, and (c) Pd{100}. The upper (lower) panels stand for top (side) views cutting through the bonding Pd atoms. For brevity and clarity, all values in the labels are multiplied by a factor of 100.

the water-palladium interactions. The physical origin can be understood by interband electron redistribution. 3.1. Symmetry-Tuned Lone-Pair Competition. In the higher bonding states, we find that 1b1 contribution to overall H2O-Pd interactions decreases with decreasing degree of surface symmetry. This conclusion is supported by the decreasing degree of 1b1 depopulation (e.g., 2.11/1.95, 2.11/1.96, 2.11/2.04), which is in line with the decreasing repopulation of 1b1-induced adsorption states at LERs (e.g., dz2, dz2, dzx, respectively) and thereby with the decreasing degree of s-d rehybridization at

the LERs (Figure 1). Such population changes, which are not observed in other d-bands around the LER, suggest that there are two channels for interband electronic transfer: 1b1 f dz2 (LER) and 1b1 f 5s (LER). In another channel of the bonding states, the reduction in the degree of surface symmetry increases the role of 3a1 in the orbital interactions as evidenced by the increasing population of 3a1-induced adsorption states of the dz2 state and by the increasing adsorption state of 5s-3a1 interactions. These observations also indicate two interband electron transfers: 3a1 f dz2 and 3a1 f 5s. Therefore, by varying

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Figure 4. Charge density differences for the on-top adsorption of H2O on (a) Pd{111}, (b) Pd{110}, and (c) Pd{100}. Upper and lower panels represent the top and cross-sectional views, respectively, which pass through the bonding Pd atom and the molecular axis. Negative values indicate loss of electron density upon binding, while the positive values correspond to electron density accumulation. For brevity and clarity, all values in the labels are multiplied by a factor of 100.

the geometric structure, in particular the lattice symmetry, we have revealed the competitive nature of the two lone-pair orbitals in their overlap with the metal s-d states. However, the second channel (3a1) has higher priority over the first one (1b1) in affecting the adsorption energy of weak physical interaction regimes, although 1b1 experiences a more significant downward shift (e.g., 0.77, 0.86, and 1.07 eV) than 3a1 (e.g., 0.12, 0.24, and 0.45 eV) in the decreasing degree of surface symmetry. It is the low-energy bonding channel that serves as an indicator for comparing strength of water-metal interactions. 3.2. Bonding Mechanisms. Generally, three major interactions control the process of chemical bonding:56,57 orbital interaction, Pauli exchange repulsion, and classical electrostatic attraction.58 There is no doubt that these principles govern water-metal interactions as well even though a single water molecule has a closed-shell electronic structure which is weakly coupled to metal surfaces. First, we discuss the involved orbital interactions that are essential to a complete bonding picture. We have obtained a common feature that the symmetry-allowed overlapping of occupied lone-pair (lp) orbitals with the hybridized s-d states (nearly filled metal d-states) leads to typical σ- and π-like mixings at HERs (Scheme 2). For water adsorption on the closepacked (Pd{111}) and open (Pd{100}) surfaces, involved orbital interactions are rather similar; they are associated with the overlap of hybridized 5s-4dz2 with both 1b1 and 3a1 orbitals leading to predominant σ mixing with a small amount of π-like contribution. However, lp-sd overlapping in water adsorption on the ridged surface (Pd{110}) is fundamentally different; both σ- and π-like mixings are mainly related to the overlaps of 1b1 and 3a1 orbitals with hybridized s-d states including 5s-4dz2 and 5s-4dzx/dzy. Such a mixed nature has never been reported previously and reveals that σ-like and π-like mixings play equally important roles in the bonding channels.

SCHEME 2: Orbital Interactions in Water Adsorption on Palladiuma

a

The left panel characterizes the bonding nature of a H2O/Pd{111} and H2O/Pd{100} and the right panel for H2O/Pd{110}. The arrows indicate the possible charge transfer and rearrangement.

How do these bonding channels respond to the Pauli repulsion? What factors influence the degree of Pauli repulsion? We have found that the answers are highly related to several properties such as occupation, types, and symmetries of involved orbitals as well as orbital depopulation. Conceptually, there are two channels of electrons (e.g., 5s and 4d) to respond to Pauli repulsion in the substrates; nonetheless, the s-band is more diffusive than the d-bands, and thus in principle the electrons occupying the former repel each other much less than those in the latter. Importantly, we have found that the two channels are correlated as highlighted by the s-d hybridizations; thus, Pauli repulsion response of the two channels can surely be affected by the s-d cooperation and thereby gives rise to different channels of charge redistributions. In H2O/Pd{111}, for instance, direct consequences of the Pauli repulsion are the inter- and intraband charge rearrangements involving the axial d-orbital and equatorial d-orbitals, for example, dz2 (MER) f dzx/dzy(MER), 1b1 f dz2 (LER), 1b1 f 5s (LER), as we have shown.

Bonding Nature of Monomeric H2O on Pd How occupation and overlap symmetry (σ- or π-like) affect the degree of Pauli repulsion is another issue that should be acceptably resolved. Intuitively, combinations of fully occupied states and σ overlapping thereof lead to the strongest Pauli repulsion among any others. For example, in H2O/Pd{111}, the same degree of 4dz2 depopulation (0.15) was observed, although 4dz2 occupation in clean Pd{111} (1.81) is smaller than that in the clean Pd{100} (1.90). The observation indicates that H2O/ Pd{111} exerts a stronger Pauli repulsion in the axial orbital. We notice that 4dz2 occupation is consistently smaller than 4dzx/ dzy occupation before (1.81 vs 1.89) and after (1.66 vs 1.92) the water adsorption on Pd{111} while 4dz2 versus 4dzx/dzy occupation is reversed before (1.90 vs 1.86) and after (1.75 vs 1.86) the water adsorption on Pd{100}. This is also due to the stronger Pauli repulsion and the reflection of enhanced role of non-σ contribution in H2O/Pd{111}. Interestingly, simultaneous depopulation in 4dz2 (from 1.85 to 1.80) and 4dzx/dzy (from 1.90 to 1.82) states is a unique feature of H2O/Pd{110}. Thus, Pauli repulsion resulting from 4dz2 is significantly reduced because of the presence of the new binding channel, 4dzx/dzy, which is notable evidence for the new bonding nature of a H2O on Pd{110}. C. Influence on Whole Substrate. 1. DiWergency Effect of s-d Hybridization. In H2O/Pd{111} (Figure 2), the bonding Pd atom and its neighboring Pd atoms exhibit two different s-d hybridizations which are obviously diverged from s-d hybridization of the clean Pd{111}. The s-dz2 hybridization in the former is strengthened while that in the latter is reduced with respect to the clean surface. This is more clearly presented in Figure 3. Interestingly, these hybridizations are also reflected in Scheme 1 which further confirms that they are also applicable to H2O/Pd{100}, although such an effect is weakened in the water adsorption for the reduced surface symmetry. However, this effect disappears in H2O/Pd{110}, and enhancement of s-d hybridization is found in both the bonding and exposed Pd atoms. Therefore, we suggest that the divergency effect is diminished with decreasing degree of surface symmetry. 2. 2D and 3D Charge Redistribution. Cross-sectional views are useful for characterizing the influence of interlayer spacing on out-of-plane charge redistribution in the substrates. In the lower panels of Figure 3, we find that the degree of electronic perturbation of the substrates upon water adsorption follows the sequence of Pd{111} < Pd{100} < Pd {110}, which coincides with our common intuition that the smaller the interlayer spacing, the deeper the perturbation. With the largest interlayer spacing (2.25 Å), a unique feature for H2O/Pd{111} is that the perturbation is confined within the first atomic layer resulting in a two-dimensional (2D) in-plane charge rearrangement. The adsorption of H2O on Pd{100} and Pd{110} shows a threedimensional (3D) perturbation in the substrate. We then closely examined the 2D in-plane rearrangement in H2O/Pd{111}. A small amount of electrons is transferred from the bonding Pd atom to its neighboring Pd atoms as evidenced by a simultaneous net charge reduction of both the 5s and 4d states with respect to the clean surfaces. Such transfers are dominated by 5s states as shown in Table 1. Figure 3a provides evidence of charge rearrangement between dxy and dx2-y2 states of the surrounding Pd atoms. In H2O/Pd{100}, repopulation occurs in the nextneighbor Pd atoms, while depopulation takes place in the nearest-neighbor atoms. It is obvious that surface symmetry determines the rearrangement patterns. D. Remarks on CDD. All the previous DFT studies concluded that dz2 hybridized with 1b1 orbital on the basis of simultaneous depopulation in both the dz2- with 1b1-like orbitals.

J. Phys. Chem. C, Vol. 113, No. 5, 2009 1937 TABLE 1: Lowdin Charges of the Water-Adsorbed Palladium Surfacesa Pdwater Pdneighbor Pdclean Pdbulk

H2O/Pd{111}

H2O/Pd{110}

H2O/Pd{100}

9.92, (0.69, 9.23) 10.05, (0.74, 9.31) 10.04, (0.725, 9.315) 9.87, (0.73, 9.14)

9.86, (0.64, 9.22) 9.95, (0.67, 9.28) 9.936, (0.66, 9.275) 9.87, (0.73, 9.14)

9.89, (0.66, 9.23) 10.01, (0.71, 9.30) 9.985, (0.70, 9.28) 9.87, (0.73, 9.14)

a For each adsorption system, Pdwater and Pdexposed, respectively, stand for the water-bonded Pd atom and its neighboring Pd atom that is exposed to vacuum. The charges for a palladium atom in the clean Pd surfaces and bulk Pd are also shown for reference.

However, the energy levels that correspond to the occurrence of the depopulation processes were ignored. As clearly provided in the current study, dz2 depopulation in fact originates from energy scale ranging from the d-band centers up to slightly below the Fermi level, whereas 1b1 depopulation occurs around the bottom energies of the d-band. Therefore, there is at least 2 eV energy difference in the two processes. In attempting to understand how the two orbitals hybridize through the concurrent depopulation, a minor setback arises from an inherent limitation of CDD analysis, which provides only the net charge change in an orbital. The repopulation of dz2 around the LERs which accounts for the direction of water-palladium bonding is obscured. Therefore, the current study unambiguously demonstrates that combined analysis involving PDOS and CDD as well as LDEE possesses the additional power for rigorous characterization of the underlying bonding nature. It is dangerous to separately use CDD analysis that is commonly accepted as a standard procedure and that actually ignores energy levels which characterize electronic processes such as electron transfer, rearrangement, and polarization. Conclusion In conclusion, DFT calculations have been used to elucidate the role of both the sp- and d-electrons in the bonding nature of a water monomer on palladium with three plane directions, namely, close-packed Pd{111}, open Pd{100}, and ridged Pd{110} surfaces. Theoretical evidence reveals that the whole energy range of the d-bands hybridized with the 5s state and that this determines the binding mechanisms of a H2O molecule on the Pd surfaces exhibiting cooperative roles in the H2O-Pd interactions in three different subranges of energy. In the low energy range of s- and d-bands, H2O-Pd interactions are dominated by symmetry-allowed orbital overlaps (e.g., σ- and π-like symmetries) that are balanced by attractive electrostatics and Pauli exclusion and that are assisted by Pauli repulsioninduced electron rearrangements in the midst of the energy range of d-bands. The water adsorption consistently enhances s-d hybridization of the surface Pd atoms. The reduction of the degree of surface symmetry (e.g., in the sequence of Pd(111), Pd(100), and Pd(110) surfaces) increases the extent of s-d hybridization of the bonding Pd atom and enhances the role of non-σ mixing in the water-metal interactions. Decoupling of d-bands facilitates overlapping of the lone-pair orbitals with hybridized s-d bands. The new bonding nature of a H2O on Pd{110} is beyond the simple mechanism pertaining to Pd{111} and Pd{100}. Acknowledgment. Jibiao Li acknowledges useful discussions with L. G. M. Pettersson, Sheng Meng, Shangyi Ma, and Ziming Wang. Supporting Information Available: Adsorption geometry and work function. The material is available free of charge via the Internet at http://pubs.acs.org.

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