First-Principles Study of Water Dissociation on PdZn near Surface

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First-Principles Study of Water Dissociation on PdZn near Surface Alloys Yucheng Huang†,‡ and Zhao-Xu Chen*,† †

Institute of Theoretical and Computational Chemistry, Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ School of Chemistry and Material Science, Anhui Normal University, Wuhu 241000, People’s Republic of China

bS Supporting Information ABSTRACT:

Recently, it was proposed that the different behavior of water on PdZn multilayer and monolayer is responsible for the different selectivity toward CO2 in methanol steam reforming process on the Zn deposited Pd surface annealed at different temperatures. [Angew. Chem. 2010, 49 (18), 3224
3227.] To explore this interesting and important phenomenon, we investigate water adsorption and dissociation in various aggregation forms on PdZn multi- and monolayer model surface alloys of both flat and stepped surfaces, using density functional theory and slab models. Our calculations show that the water is more stable on the multilayer. Surface defects and aggregation favor H2O dissociation. Contrary to the point of view that PdZn monolayer cannot activate H2O whereas multilayer can, our first-principles results clearly demonstrate that the multilayer is less active for water dissociation than the monolayer. This discrepancy calls for further studies on this system both experimentally and theoretically.

1. INTRODUCTION As an effective means to produce hydrogen, methanol steam reforming (MSR) has received intensive attention in recent years.1
22 Pd/ZnO exhibits good catalytic performance for this reaction.1,2 Although many endeavors have been devoted to the PdZn alloy formation,3
6 the effect of Zn,7
11 and the MSR reaction mechanism,12
14 there are few studies on water, a participant of MSR, over PdZn alloy. Undoubtedly, a thorough understanding of the behavior of water on PdZn alloy is a prerequisite for obtaining comprehensive knowledge of MSR. Previously, Lim. et al.15 calculated an adsorption energy of 23 kJ/mol for the water monomer on the PdZn(111) surface. Such a small adsorption energy implies that water dissociation may be difficult on the PdZn(111) or that the PdZn(111) is not the active site for water activation. A recent in situ ambient pressure X-ray photoemission spectroscopy (AP-XPS) experiment2 demonstrated that while it stays intact on PdZn monolayer alloy, water dissociates on multilayer of PdZn alloy. This conclusion seems to contradict the above-mentioned theoretical calculations. In particular, the different chemistry of water on multi- and monolayer PdZn alloy, though understandable if one accepts the point of view that the surface chemistry of a catalyst may be mainly controlled by the structure and composition of the subsurface,2 is nevertheless doubtful. r 2011 American Chemical Society

In this contribution, we performed a series of first-principles calculations to investigate the adsorption and dissociation of H2O on both flat (111) and stepped (221) surfaces of multi- and monolayer PdZn alloy. We examined the dissociation of water in the form of monomer, dimer, tetramer, and hexamer on flat (111) surfaces to explore the effect of aggregation on water dissociation. Moreover, dissociation of water monomer and one-dimensional (1D) water chain on stepped surfaces was also addressed to check the influence of surface defect on the process. The paper is structured as follows: In section 2, the models and computational details are described; in section 3, the results and discussions of H2O adsorption and dissociation on flat and stepped surfaces of PdZn multi- and monolayer are presented; the conclusions are given in section 4.

2. MODELS AND COMPUTATIONAL DETAILS Flat multi- and monolayer PdZn alloy (Figure 1a, b) and stepped multi- and monolayer PdZn alloy models (Figure 1c, d) were constructed to mimic the catalyst surfaces. The multilayer Received: June 28, 2011 Revised: August 13, 2011 Published: August 18, 2011 18752

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Figure 1. Models adopted in the study. (a) Multilayer of (4  4) PdZn(111); (b) monolayer of (4  4) PdZn(111) over Pd(111); (c) multilayer (221) from refs 17 and 18; (d) monolayer (221). (e) Pd (221Pd) and Zn exposed (221Zn) (221) surfaces. Blue balls denote Zn atoms.

alloy models were built on the basis of the tetragonal PdZn bulk alloy with the theoretical lattice parameters of 4.148 and 3.385 Å (expt 4.11 and 3.35 Å); the monolayer models were constructed from the fcc metal Pd bulk structure at the optimized lattice length of 3.954 Å compared to the experimental value of 3.8907 Å.16 We chose the (111) and (221) surfaces for the flat and stepped surface models, respectively. Four layer slabs of (4  4) and (2  2) surface unit cells were adopted for the flat (111) and stepped (221) surfaces, respectively. There are two types of step edge for the (221) surface:17,18 in one, the step edge is composed of Pd (denoted as 221Pd), and in the other, the edge is composed of Zn (221Zn) (Figure 1e). For the multilayer alloy models, each layer is composed of both Pd and Zn, while in the monolayer models, only the top layer comprises both Pd and Zn (Figure 1a
d). A vacuum spacing of 11 Å was used to separate the adjacent slabs in all the models. During the structural optimization, the bottom two (stepped) layers were fixed while the top two (stepped) layers and adsorbates were allowed to relax. All the first-principles calculations were performed with the VASP (Vienna ab initio simulation package) code.19
21 The projector augmented wave method22,23 with a frozen-core approximation was used to describe the ion
electron interactions. The generalized gradient approximation (GGA) with the Perdew
Wang 91 (PW91) functional24 was employed to describe electron exchange and correlation. The GGA extension is crucial for the accurate treatment of the hydrogen bonds and water structures,25 and the PW91 form has been tested extensively for a variety of intermolecular interactions including H bonding.26 Monkhorst
Pack scheme with 2  2  1 and 5  5  1 k-point sampling in the surface Brillouin zone27 was used for the flat and stepped PdZn models, respectively. A plane-wave cutoff of 400 eV was adopted in all the calculations. The Fermi level was smeared by the Methfessel and Paxton approach28 with a Gaussian width of 0.1 eV. The free energy was extrapolated to zero Kelvin to yield total energies of the systems. The geometries of all stationary points were located with the conjugate-gradient algorithm and were considered converged when the force on each atom was smaller than 0.02 eV/Å. Transition states were located using the climbing nudged elastic band method (CNEB)29,30 or the quasi-Newton algorithm. All the transition states were verified by vibrational mode analysis. The binding energy per adsorbed water molecule, Eb, is defined as Eb = (Esub + nEH2O
E(H2O)n/sub)/n. Here, E(H2O)n/sub is the total energy of the adsorption system, Esub and EH2O are those for the relaxed clean slab and free molecules, respectively, and n is the number of water molecules per unit cell.

3. RESULTS AND DISCUSSION 3.1. Water Adsorption and Dissociation on Flat (111) Surfaces. 3.1.1. Water Adsorption. Figure 2 illustrates adsorption

of monomer, dimer, tetramer, and hexamer of water on the flat

Figure 2. Illustrations of adsorption of water monomer, dimer, tetramer, and hexamer on flat surfaces. dnm denotes the H-bond distance. hnZn/Pd is the height of the oxygen of the nth water from the surface Zn/ Pd atom. αn represents the angle formed by the bisector of H
O
H of nth water and the O-metal bond; θn is the H
O
H angle of the nth water. Blue balls denote Zn atoms.

surfaces. Table 1 lists the geometrical parameters describing the adsorption structures. The binding energies are listed in Table 2. Discussion will be focused on comparing the results on the multilayer and monolayer. The water monomer interacts with metal surfaces via its lone electron pair of the O atom. On PdZn substrate, the lone pair of electrons may interact with either Pd or Zn atom. Our calculated results show that the binding energy on the top Zn site is nearly twice as large as that on the top Pd position over the multilayer surface alloy; the O
Zn distance, 2.31 Å, is 0.31 Å shorter than the corresponding O
Pd contact (Table 1). At variance, on the monolayer surface, the binding energies on top Zn and top Pd positions are rather close (Table 2). Correspondingly, the O
Zn distance, 2.39 Å, is comparable to the O
Pd contact, 2.42 Å. The above energetic and geometrical results indicate that while water interaction with multi-Zn (meaning the Zn site on the multilayer alloy surface) is notably stronger than with multi-Pd, such interaction difference diminishes significantly on the monolayer as evidenced by the close binding energy on monolayer atop Zn and atop Pd sites (Table 1). Structurally, the adsorbed water on multi-Zn resembles that on mono-Zn. The molecular plane of water forms an angle of 23
24
(= α
90
) with the substrate surface. The values of the H
O
H angle ( — HOH), θ in Table 1, 106.0
(multi-Zn) and 105.3
(mono-Zn), are larger than 104.9
for free H2O molecule, indicating a charge transfer from the O to the substrate.31 The above results show that water monomer prefers adsorption at top sites and a flat orientation on the alloy surfaces. Detailed analyses reveal that this preference is owing to the maximizing soft
soft interactions in terms of the reactivity index.32 Water dimer adsorption may have three possibilities in terms of the combination of the substrate atoms bonding to the water 18753

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Table 1. Structural Parameters of Water Adsorbed on Flat PdZn Multilayer and Monolayer Surfacesa multilayer monomer

monolayer

h1Zn = 2.31; α = 114; θ = 106.0

h1Zn = 2.39; α = 113; θ = 105.3

h1Pd = 2.62; α = 112; θ = 105.5

h1Pd = 2.42; α = 116; θ = 105.8

dimer

h1Pd = 3.30; h2Zn = 2.17; d12 = 1.67; α1 = 51; α2 = 125;

h1Pd = 3.16; h2Zn = 2.22; d12 = 1.71; α1 = 54; α2 = 136;

tetramer

h1Pd = 3.25; h2Zn = 2.33; h3Pd = 3.24; h4Zn = 2.35;

θ1 = 104.6; θ2 = 107.8

hexamer

θ1 = 104.8; θ2 = 107.8 h1Pd = 3.25; h2Zn = 2.31; h3Pd = 3.16; h4Zn = 2.73; d12 = 1.58;

d12 = 1.63; d23 = 1.86; d34 = 1.63; d41 = 1.90; α1 = 43;

d23 = 1.86; d34 = 1.67; d41 = 1.76; α1 = 43; α2 = 124; α3 = 53;

α2 = 124; α3 = 48; α4 = 124; θ1 = 104.5; θ2 = 108.1;

α4 = 127; θ1 = 104.9; θ2 = 107.4; θ3 = 105.0; θ4 = 107.4

θ3 = 104.4; θ4 = 107.7 h1Pd = 3.29; h2Zn = 2.41; h3Zn = 2.44; h4Pd = 3.26; h5Zn = 2.40;

h1Pd = 3.03; h2Zn = 2.63; h3Zn = 2.50; h4Pd = 3.03;

h6Zn = 2.46; d12 = 1.57; d23 = 1.69; d34 = 1.81; d45 = 1.58;

h5Zn = 2.60; h6Zn = 2.52;d12 = 1.64; d23 = 1.68; d34 = 1.81; d45 = 1.63;

d56 = 1.69; d61 = 1.78; α1 = 45; α2 = 116; α3 = 107; α4 = 48;

d56 = 1.68; d61 = 1.80; α1 = 51; α2 = 110; α3 = 117; α4 = 51; α5 = 117;

α5 = 116; α6 = 104; θ1 = 104.0; θ2 = 107.7;

α6 = 117; θ1 = 104.8; θ2 = 107.3; θ3 = 106.8; θ4 = 104.8;

θ3 = 106.7; θ4 = 103.9; θ5 = 107.6; θ6 = 106.7

θ5 = 107.2; θ6 = 106.9

a

The unit of distance and height is Å, and angle is in degree. dnm denotes the H-bond distance. hnZn/Pd is the height of the oxygen of the nth water from the surface Zn/Pd atom. αn represents the angle formed by the bisector of H
O
H of nth water and the O-metal bond; θn is the H
O
H angle of the nth water. The molecular numbering is shown in Figure 1.

molecule, that is, Pd + Pd, Pd + Zn, and Zn + Zn. Our calculations reveal that the dimer favors the Pd + Zn combination in which each of the two H2O molecules resides on Pd and Zn separately. A notable feature of the dimer adsorption is that the two water molecules orientate differently with respect to the substrate surface. The H2O atop Zn atom is roughly parallel to the substrate surface while that atop Pd is perpendicular to the surface with one OH bond normal to the surface and the other OH bond parallel to the substrate plane (Figure 2). For the convenience of the following presentation, the water whose molecular plane is roughly parallel to the substrate surface (always atop Zn) is denoted as H2Op, and the one whose molecular plane is approximately perpendicular to the substrate surface (always atop Pd) is denoted as H2Ov. Because of the different orientation of water, the dimer exhibits a bilayer structure with the vertical O
O distance of 0.7 and 0.9 Å on the multi- and monolayer, respectively. Similar structures were also obtained on Pt(111) and Pd(111)31 and Cu(111)33,34 surfaces. The low-lying H2Op acts as a proton donor to the high-lying H2O and binds more tightly to the Zn (h2Zn = 2.17 and 2.22 Å on the multi- and monolayer, respectively, Table 1), and its molecular plane is titled at an angle of 35
46
with respect to the surface normal (Table 1). Compared with free H2O, — HOH of H2Ov decreases slightly while that of H2Op increases (Table 1). The values of the — HOH of H2Op in the dimer are larger than the results for the monomer, indicating more charge transfer from the H2Op to the substrate, in line with the shorter O
Zn distance (h2Zn in Table 1) compared to h1Zn for the monomer. The calculated binding energy per water molecule in the dimer increases from less than 0.3 eV for the monomer to 0.41 (multilayer) and 0.36 eV (monolayer) (Table 2). The distance between Ov (Ov, Hv, and OHv denote the O, H, and OH groups from H2Ov, respectively, and similar notations are used for H2Op, see below) and Hp is 1.67 (multi-) and 1.71 (mono-) Å, indicating that there is H-bonding between Ov and Hp, which accounts for the increased binding energy (see below). H2O tetramer consists of H2Ov (atop Pd) and H2Op (on Zn site) positioned alternately, and the adsorption structure can be regarded as being composed of two sets of water dimers (Figure 2). The molecules connect to each other through H-bonds, forming a

Table 2. The Binding Energies (in eV) of H2O on Flat PdZn(111) Surfacea multilayer

monolayer

monomer

0.27 (0.13)

0.21 (0.20)

dimer

0.41 (0.10)

0.36 (0.09)

tetramer

0.48 (0.32)

0.44 (0.31)

hexamer

0.48 (0.34)

0.45 (0.34)

a

Values in parentheses are the H-bonding contribution for dimer, tetramer, and hexamer. For monomer, the parenthesized results refer to the binding energies atop Pd.

quadrangle structure. The averaged O
Zn height is 2.34 Å on the multilayer and 2.52 Å on the monolayer. The hydrogen bond length (O 3 3 3 H distance) in the H-bond network ranges from 1.58 to 1.90 Å. The — HOH in the tetramer shows a similar regularity as in the dimer: it is larger on Zn sites than on Pd positions. The binding energy per water molecule is calculated to be 0.48 (multilayer) and 0.44 eV (monolayer), consistent with the ordering of their O
Zn distance (Table 2). These binding energies are slightly larger than those of the dimers. H2O hexamer is widely observed in scanning tunneling microscopy studies of low-temperature adsorption of water on different surfaces.35
38 Generally, water hexamer belongs to one of the three types of structures. The first, as seen on Pt(111), Ag(111), and Cu(111) surfaces, is the so-called puckered honeycomb structure in which one-half of the H2O binds to the substrate through the O atoms in H2Op mode and the other one-half in H2Ov configuration with one H pointing toward the vacuum and the O atoms elevated approximately 1 Å with respect to the O atoms in H2Op.38
40 The second case is a quasi-planar monolayer structure with one-half of H2O dissociated into H and OH as on Ru.41
45 In the third one, as reported recently,37,46,47 H2O molecules remain intact and form quasi-coplanar hexagonal structures. In such a structure, the molecular planes of H2O molecules are alternatively parallel and vertical to the substrate surface. On PdZn flat surface alloy, we find that the most stable configuration for H2O hexamer exhibits a puckered honeycomb structure with the largest O
O vertical distance of 0.6 Å. The O 3 3 3 H distance is between 1.57 and 1.81 Å. Because H2O 18754

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Figure 3. (a) Structures of initial state (IS), transition state (TS), and final state (FS) for water monomer, dimer, tetramer, and hexamer dissociation on the PdZn multilayer. The bond length is in Å. Insets are top view. Blue balls denote Zn atoms. (b) Energy profile for water dissociation on the PdZn multilayer. The zero-point energy (ZPE) corrected values are displayed in parentheses.

interacts with Zn slightly more strongly than with Pd, H2O hexamer occupies four Zn and two Pd atoms. Similar to the dimer and tetramer, H2O displays a H2Op structure on each top Zn site and a H2Ov structure on the top Pd position. The — HOH in H2Op/H2Ov structure is also wider/narrower than in free H2O as seen in the dimer. The calculated binding energies are 0.48 (multilayer) and 0.45 eV (monolayer), which are close to the values of the tetramer. Table 2 shows that the binding energy on the multilayer is larger than the corresponding value on the monolayer. Furthermore, the calculated binding energy increases with the extent of clustering and arrives at the limit around 0.5 eV. To understand the variation of binding energy, we estimate the H-bonding strength per H2O with EH
bond = (nEH2O
E(H2O)n)/n because H-bonding is vital for clustered H2O adsorption.44 In the formula, E(H2O)n is the total energy of the H2O cluster calculated with the geometry in the adsorption complex. EH2O is the total energy of an isolated water molecule. Our computed results reveal that the H-bonding contributes 0.1 eV per H2O to the stability of H2O in the dimer. Since the binding energy of the dimer is larger than 0.36 eV, the main contribution is due to H2O
metal interaction. With the aggregation increase, the H-bonding dominates and accounts for 67
75% of the binding energy in the tetramer and hexamer, which agrees well with the results obtained on various metal surfaces.45,46 On the other hand, H2O
metal interaction contributes only 0.11
0.16 eV to the calculated binding energy, which is about half of the contribution from the H-bonding. The H2O
metal interaction in the dimer is 3 times as large as that of H-bonding (Table 2). The weaker H2O
metal interaction in the tetramer and hexamer than in the dimer is also reflected in the Zn
O distance which is more than 0.1 Å longer in the former than in the latter (Table 1). Not unexpected, our calculated H-bonding energy clearly shows that the thickness of the alloy layer (four layers for the multilayer

versus one layer with the monolayer) has negligible influence on H-bonding. Thus, the stability difference of H2O on multi- and monolayer surface alloy is determined by the H2O
metal interaction which depends on the composition of the subsurface as well as on the surface geometry induced by the subsurface. Since water favors Zn and more Zn atoms in the substrate enhances water adsorption, the binding energy on multi-Zn is larger than that on mono-Zn, which is also consistent with the average O
Zn distance that is shorter on the multilayer than the corresponding one on the monolayer (Table 1). 3.1.2. Water Dissociation. Structures of initial states (ISs), transition states (TSs), final states (FSs), and the energy profiles are illustrated in Figures 3 and 4 for water dissociation on the PdZn multi- and monolayer surface alloy. The FSs were chosen on the basis of the finding that H/OH prefers Pd/Zn site. The reaction heats and energy barriers are given in Table 3. a. Water Monomer. For the water monomer, we only consider the dissociation of water atop Zn site because water atop Zn site is more stable than that atop Pd site. The dissociation begins from the movement of the dissociating H atom (Hd) toward the Pd neighbor. When O
Hd distance is 1.67 (multi-)/ 1.59 Å (mono-), the TS is reached (Figures 3a and 4a). In the FSs, Hd locates on a nearby Pd2 bridge site and the OH situates on a Zn2 bridge position. This process needs to overcome an energy barrier of 1.15 (multi-)/0.63 eV (mono-) with a reaction heat of 0.22 (multi-)/
0.32 eV (mono-) (Figures 3b and 4b). b. Water Dimer. For dissociation of aggregated water, we always assume that the H atom of H2Ov nearest to the substrate surface dissociates favorably. The dimer dissociation begins with the Hd atom gradually approaching the surface Pd atom, which enhances the Hd
Pd interaction and weakens the O
Hd bonding. In the TS, O
Hd distance is elongated from 0.99 to 1.59(multi-)/1.54(mono-) Å (Figures 3a and 4a). In the TSs, the Ov atom is 1.41 and 1.57 Å away from the closest Hp atom of the 18755

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Figure 4. (a) Structures of initial state (IS), transition state (TS), and final state (FS) for water monomer, dimer, tetramer, and hexamer dissociation on the PdZn monolayer. The bond length is in Å. Insets are top view. Blue balls denote Zn atoms. (b) Energy profile for water dissociation on the PdZn multilayer. The ZPE corrected values are displayed in parentheses.

H2Op compared to 1.67 and 1.71 Å in the ISs for the multi- and monolayer, respectively, indicating an enhanced H-bonding between the Ov and the Hp atoms. In the FSs, the produced OHv captures the Hp atom, leaving the OHp residing on the top Zn site and the newly formed H2O (HOvHp) on the top Pd site which was previously occupied by H2Ov. The Hd atom sits on the Pd2 bridge site (Figures 3a and 4a). The calculated energy barrier is 1.11(multi)/0.76(mono) eV (Figures 3b and 4b). Similar to the monomer, the barrier on the monolayer is lower than that of the multilayer, demonstrating that the monolayer film is more active for water activation than the multilayer surface alloy. c. Water Tetramer. The dissociation process of the H2O tetramer is similar to that of the dimer. The hydrogen dissociation also begins with the approaching of the Hd atom in H2Ov toward a nearby Pd atom. With the elongation of the Ov
Hd distance, the interaction between the Ov and the neighboring Hp of the H2Op that is located on the nearby Zn atom increases. Formation of the Ov
Hp bond facilitates the dissociation. Our calculations indicate that the TS is formed after the Ov
Hp bond formation is completed on the multilayer whereas on the monolayer the Ov
Hp bond is formed after the TS as in the dimer (Figures 3a and 4a). In the TS, the formed HOvHp is located atop Pd site over the multilayer. On the monolayer surface, the produced OHv is on the Pd atom. The Ov
Hd distance is 1.60 and 1.50 Å over multi- and monolayer surface alloy, respectively. The Hd atom occupies an off-top Pd position (multi-) and a bridge site (mono-). After the TS, the Hp is snatched by the OHv on the monolayer, while on the multilayer, the main change is the diffusion of Hd toward the bridge site. In the FS, the Hd is at the bridge site and HOvHp is on the top Pd site over both the multi- and monolayer. The FS looks as if the Hd was from the H2Op that is on the top Zn site. The process is calculated to be thermodynamically favorable with an exothermicity of 0.53 and 0.09 eV on the multi- and monolayer, respectively; the corresponding energy barrier is 0.90 and 0.96 eV

Table 3. Reaction Heat and Energy Barrier (in eV) for Water Dissociation on the Flat Multilayer and Monolayer Surfacesa multilayer ΔH

a

monolayer Ea

ΔH

Ea 0.63 (0.44)

monomer

0.22 (0.06)

1.15 (0.94)


0.32 (
0.46)

dimer

0.42 (0.28)

1.11 (0.87)

0.07 (0.07)

0.76 (0.55)

tetramer

0.53 (0.40)

0.90 (0.70)

0.09 (
0.03)

0.96 (0.72)

hexamer

0.14 (
0.02)

0.75 (0.54)


0.10 (
0.20)

0.73 (0.53)

Zero-point correction values are in parentheses.

(Figures 3b and 4b). Compared with the monomer and dimer, the dissociation of the water tetramer becomes easier on the multilayer surface while it tends to be slightly hard on the monolayer alloy film. d. Water Hexamer. As described above, the dissociation of the water tetramer finally results in an OHp group derived from the H2Op molecule adjacent to the initially dissociating H2Ov. With the water hexamer, such peculiarity is magnified and displays some variations. As described in section 3.1.1, the hexamer consists of two H2Ov and four H2Op molecules (Figures 3a and 4a). In the course of dissociation, the Hd atom gets closer to the substrate surface, which weakens the Ov
Hd bond. The Ov
Hd bond weakening is accompanied by the enhancement of the Ov
Hp interaction as well as the Op
Hp0 bonding (Hp0 is the H atom of the H2Op molecule next to H2Ov but one). Prior to the TS formation, the Op0 H hydroxyl group is formed on both surfaces. To explore whether there is an intermediate structure between the ISs and TSs, we used eight intermediate structures between IS4/IS8 and TS4/TS8. This exploration reveals the detailed process as follows. The approaching of Hd to the substrate Pd atom enhances the Ov
Hp interaction and triggers the transfer of Hp to Ov. Before Hd is fully detached, the transfer of Hp to Ov is completed, forming a H3Ov species, which is a 18756

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Table 5. Reaction Heat and Energy Barrier (in eV) for Water Dissociation on 221Zn Step of PdZn(221) Surfacea multilayer ΔH

a

Figure 5. Water adsorption on the (221) stepped surfaces. (a) Wps and Wpt configuration of H2O monomer. (b) 1D-Wps, 1D-Wpt, and 1D-Wps zigzag structures. Black dashed line denotes the step. The surface unit cells are shown with yellow rectangles. Blue balls denote Zn atoms.

Table 4. Binding Energies (in eV) of H2O Adsorbed on Stepped (221) Surfaces multilayer

monomer 1D-chain

Pd

configuration

221

monolayer Zn

221

Pd

221

221Zn

Wpt

0.29

0.34

0.29

0.30

Wps

0.49

0.36

0.43

0.35

Wpt

0.42

0.43

0.42

0.42

Wps

0.60

0.46

0.60

0.46

zigzag

0.55

0.44

0.56

0.44

protonated H2Ov with a weak Ov
Hd bond. Then, the OHp snatches the Hp0 atom from the H2Op0 , accompanied by the departure of the Hd atom from the Ov atom. The mapped energy profile monotonously increases and reaches the maximum at TS4 and TS8, indicating that there is no local minimum between the ISs and TSs. The process detailed above demonstrates the high mobility of the proton in the H-bond network, which has been revealed on Pt(111), Ru(0001), and Ni(111) surfaces.48 In the formed TS (TS4 and TS8), the Hd is 1.64 (multi-) and 1.62 Å (mono-) away from the Ov atom, and the Op0 H group is anchored on the top site of the Zn atom (Figures 3a and 4a). In the FS, the Hd is positioned at the Pd2 bridge site and the produced OHp0 is on the top site of Zn. The calculated reaction energy and energy barrier on the multilayer surface are 0.14 and 0.75 eV, respectively. A look at Table 3 reveals that aggregation reduces the dissociation barrier monotonously, indicating that clustering favors water dissociation kinetically on multilayer surface alloy. On the monolayer surface, the calculated barrier, 0.73 eV, is slightly lower than the one on the multilayer, and the reaction energy is
0.10 eV. Both results indicate that the monolayer is more active for water hexamer activation than the multilayer. 3.2. Water Adsorption and Dissociation on Stepped (221) Surfaces. Defects play an important role in catalyzing various surface reactions.49
51 We used (221) stepped surfaces to model water dissociation on surface defects. As pointed out in section 2, there are two types of step edges: 221Pd and 221Zn. We only

monolayer Ea

ΔH

Ea

monomer


0.34 (
0.47)

0.82 (0.62)


0.39 (
0.51)

0.77 (0.58)

1D-chain


0.45 (
0.64)

0.66 (0.45)


0.44 (
0.56)

0.56 (0.37)

Zero-point correction values are in parentheses.

investigated the adsorption and dissociation of the water monomer and 1D-H2O chain on step edges as the atoms on the edge are more active because of low coordination (Figure 5). 3.2.1. Water Adsorption. As shown in Figure 5a, the adsorption of the H2O monomer has two possible configurations: molecular plane parallel to the terrace (Wpt) and to the step (Wps). Our calculated results show that the binding energy of water on the step edge is larger than on the flat surfaces (Tables 2 and 4) because of low coordination of the step atoms. The binding energy of the Wpt configuration follows the finding on the flat surface that water on Zn site tends to be more stable than on Pd position. For example, the binding energy of water monomer on the multi-221Zn, 0.34 eV, is larger than 0.29 eV on the 221Pd. However, for Wps modes, water on 221Pd is calculated to be more stable than that on 221Zn (Table 4). This phenomenon is due to the additional interaction of the H atom with the terrace Pd atom (the nearest H
Pd distance in Wps is ∼2.8 Å compared to 3.3 Å in Wpt). The H
Pd interaction also makes Wps more stable than Wpt as evidenced by the calculated binding energy (Table 4). To examine aggregated water adsorption on the stepped surfaces, we doubled the coverage of water on the step edge, leading to 1D-chain structure of water. The 1D-chain structures examined here are 1D-Wpt chain and 1D-Wps chain which are derived from Wpt and Wps configurations, respectively (Figure 5b). In addition, we also studied 1D zigzag chain derived from Wps (Figure 5b). As shown on the flat surfaces, water molecules in aggregated state are more stable because of H-bonding. Similarly, the calculated binding energies of water in 1D-H2O chains are also larger than the corresponding ones of the monomer. For example, the binding energies of Wps, 0.46
0.60 eV, is >0.1 eV larger than that of the monomer (Table 4). It is notable from Table 4 that for the Wpt derived water chain, the binding energies on 221Pd and 221Zn are essentially the same, indicating that the composition of step edge does not affect the stability of water in 1D-Wpt chain. However, for the 1D-Wps chain and the 1D- zigzag chain, water always prefers 221Pd to 221Zn, which can also be rationalized by the H
Pd interaction as for the water monomer. 3.2.2. Water Dissociation. On the basis of the adsorption, we scrutinized the dissociation of water on the 221Zn and 221Pd surfaces. On 221Zn, we selected the Wpt configurations as the initial states for the dissociation of water monomer and 1D-H2O chain. This choice is based on the following facts: Wpt is only slightly less stable than Wps; however, our calculated dissociation barrier of Wpt monomer on 221Zn, 0.82 eV, is much lower than 1.5 eV for Wps. On 221Pd, we considered Wps configurations as initial states for the dissociation of water monomer and 1D-H2O chain because evidently Wps is more stable than Wpt. Dissociation of water in the zigzag structure is not considered. Our calculations reveal that water dissociation on 221Pd is kinetically 18757

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Figure 6. (a) Structures of initial state (IS), transition state (TS), and final state (FS) for water monomer (1) and 1D-chain (2) dissociation on the multilayer 221Zn surface. The upper/lower panel is the side/top view. The bond lengths are in Å. Blue balls denote Zn atoms. (b) Energy profile for water dissociation on the PdZn multilayer. The ZPE corrected values are displayed in parentheses.

and thermodynamically much unfavorable compared to that on 221Zn (Tables 5 and S1 and S2 of the Supporting Information). To be concise, we only discuss water dissociation on 221Zn surfaces and deposit the structures on 221Pd in the Supporting Information. a. Monomer. Water monomer dissociation on 221Zn is illustrated in Figures 6 and 7 for multi- and monolayer models, respectively. In the ISs, the O
Hd distance is 0.99 Å and the O atom is atop Zn with the O
Zn distance of 2.27 and 2.28 Å on the multilayer and monolayer, respectively. The reaction starts from the displacement of the Hd atom toward a Pd neighbor. In the TS, the O
Hd bond is elongated to 1.66/1.97 Å in TS9/ TS11. The produced OH and Hd finally situate on the Zn2 bridge site on the step edge and on the Pd2 bridge site on the terrace, respectively. This process is exothermic by 0.34 and 0.39 eV, and the energy barriers are 0.82 and 0.77 eV on the multi- and monolayer surfaces, respectively, compared to the corresponding barriers of 1.15 and 0.63 eV on the flat surfaces (Figures 6b and 7b). b. Chained Water. The dissociation of 1D-water chain resembles that of water monomer. Starting from the ISs, the Hd atom approaches the Pd2 bridge site on terrace. The O
Hd bond extends from 0.98 Å in the ISs to 1.42/1.44 Å in the TS10/TS12 (Figures 6a and 7a). Unlike the FSs of the monomer where the produced OH group sits at a Zn2 bridge site, the formed OH group in the chain occupies a top Zn position with the O
Zn distance of 1.96 and 1.95 Å, and the hydrogen-bonding length decreases from 2.03 and 1.90 Å to 1.39 and 1.52 Å on the multiand monolayer, respectively (Figures 6a and 7a). Compared with the monomer dissociation, the energy barriers decrease by >0.15 eV to 0.66 and 0.56 eV on multi- and monolayer, respectively, further indicating that aggregation makes the dissociation more feasible (Table 5). Thermodynamically, dissociation of 1Dwater chain is also more favorable than that of water monomer. 3.3. Discussion. Above, we have described the adsorption and dissociation of H2O on the flat and stepped surfaces of PdZn

multi- and monolayer in various aggregation forms. Our calculation results demonstrate that water interacts more strongly with the flat PdZn multilayer. This can be rationalized by the finding that species adsorbing via oxygen atom prefers a site with more Zn atoms on the PdZn alloy surface.8 The dissociation barriers on the monolayer surface alloy are lower than those on the multilayer surfaces (only the barrier for tetramer on the monolayer of PdZn flat surface is slightly higher than the one for the multilayer). This phenomenon is in line with the Brønsted
Evans
Polanyi (BEP) relationship52
54 since water dissociation is usually more exothermic on the monolayer surfaces than on the multilayer ones. The more exothermicity on the monolayer can be traced back to the fact that the interaction of OH group with the monolayer is much stronger than with the multilayer. Thus, in TSs which feature late transition state, OH is more stable on the monolayer than on the multilayer, which makes the dissociation barrier on the monolayer lower than the one on the multilayer. As demonstrated previously on Ru surfaces,45 aggregation, in most cases, reduces the dissociation barriers. This may be due to the enhanced stability of the produced OH group (owing to formation of H-bonding in TS). In fact, our calculations show that aggregation not only reduces the dissociation barrier but also stabilizes water on surfaces. Without aggregation water will, instead of dissociation, desorb from the surface favorably as indicated by the binding energy of monomer on the flat surfaces, 0.60 eV. Surface defects enhance water adsorption and reduce the dissociation barrier. According to our results, the barriers of 1D-H2O chains on 221Zn are lowest among all the models studied in the present paper. Since water dissociation does occur (at least on PdZn multilayer2), according to our present calculations, the dissociation most likely takes place on surface defects. Whether the dissociation happens on flat surfaces or defects, our results clearly show that monolayer surface alloy is more 18758

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Figure 7. (a) Structures of initial state (IS), transition state (TS), and final state (FS) for water monomer (1) and 1D-chain (2) dissociation on the monolayer 221Zn surface. The upper/lower panel is the side/top view. The bond lengths are in Å. Blue balls denote Zn atoms. (b) Energy profile for water dissociation on the PdZn multilayer. The ZPE corrected values are displayed in parentheses.

active for water activation than the multilayer surfaces. This conclusion contradicts the point of view2 that PdZn multilayer can activate H2O while monolayer cannot. If the interpretation of the experiment results is correct, the discrepancy between our theoretical prediction and the experiment may be due to the models adopted in the present study. First, the surface alloy was constructed on the basis of the 1:1 PdZn alloy structure. However, our recent Monte Carlo simulation10 indicates that at the experimental temperature PdZn surface exhibits a zigzag morphology. Different surface structure may lead to different chemical reactivity toward water activation. Another possibility is the composition of monolayer surface alloy. In the model we used, the atomic ratio of Pd to Zn is 1:1 for the top layer, and the subsurface has no Zn atoms at all. However, the low energy ion scattering experiments2 indicate the existence of subsurface Zn atoms, and the ratio of Pd to Zn of the top layer is larger than 1:1. Clearly, the monolayer alloy model employed in our present paper cannot appropriately represent the experimental monolayer surface alloy. Our previous studies show that H2O is difficult to dissociate on Pd(111),55 indicating that surfaces with more Pd content inhibit water dissociation. Therefore, models with more surface Pd atoms are expected to be more inactive for water dissociation. In a word, to understand why PdZn multilayer and monolayer behave so differently toward water dissociation, more studies should be performed; building a reasonable model for the monolayer is a prerequisite for theoretical investigations, which is under way. Finally, the functional PW91 used in this paper does not account for van der Waals dispersion forces properly. According to a very recent study,56 considering the dispersion forces by using nonlocal van der Waals density functionals will not affect the adsorption structures of water much. However, owing to the much larger polarizability of the metal atoms compared to oxygen and hydrogen, the water
metal bonding will increase, leading to larger binding energies of water. If the energetics of the transition states is not affected so significantly as that of water, the dissociation barriers are expected to be higher than the present results because the initial states become more stable.

4. CONCLUSIONS It was concluded that water remains intact on monolayer PdZn surface alloy while the PdZn multilayer surface alloy can activate water. To examine this interesting phenomenon, in this paper, we performed periodic density functional calculations, using 1:1 PdZn multi- and monolayer surface alloy models. Our investigations show that aggregation not only makes water stable but also reduces the dissociation barrier because of H-bonding that can stabilize the dissociation product OH group. Multilayer surface alloy interacts more strongly with water than the monolayer. Surface defects interact more strongly with water molecules and are favorable for the dissociation of aggregated water. Finally, our results demonstrate that the monolayer surface alloy is more active for H2O dissociation than the multilayer, which contradicts the conclusion based on the experimental observation and indicates the necessity of more work on this system. ’ ASSOCIATED CONTENT

bS

Supporting Information. Structures of IS, TS, and FS for water monomer and 1D-chain dissociation on the monolayer surface. The reaction heat and energy barrier for water dissociation on 221Pd step of PdZn(221) surface. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from NSFC No. 20973090, 973 Program 2009CB623504 and 2011CB808604 is greatly acknowledged. 18759

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