Density Functional Study of Hydrogen Binding on Gold and Silver

Mar 22, 2010 - School of Chemical Engineering, Henan University of Science and Technology, Luoyang, Henan 471003, P. R. China and School of Chemical a...
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J. Phys. Chem. A 2010, 114, 4917–4923

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Density Functional Study of Hydrogen Binding on Gold and Silver-Gold Clusters Shuang Zhao,† YunLi Ren,† YunLai Ren,† JianJi Wang,*,†,‡ and WeiPing Yin† School of Chemical Engineering, Henan UniVersity of Science and Technology, Luoyang, Henan 471003, P. R. China and School of Chemical and EnVironmental Sciences, Henan Key Laboratory of EnVironmental Pollution Control, Henan Normal UniVersity, Xinxiang, Henan 453007, P. R. China ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: March 05, 2010

A theoretical study was carried out on the binding of hydrogen on small bimetallic AgmAun (m + n e 5) and pure Aun (n e 5) clusters with neutral, negative, and positive charge state. It is found that the composition and charge state of clusters have strong influence on the most favorable binding site. The adiabatic ionization potentials, electron affinities, and hydrogen binding energies of cluster hydrides increase with the Au content increasing for the given cluster size. The cationic silver-gold cluster hydrides prefer ejection of Au-containing products whereas the anionic silver-gold cluster hydrides prefer ejection of Ag-containing products. The magnitude of metal-H frequency in combination with the metal-H bond length indicates that, with the same type of the binding site, the Au-H interaction is stronger than the Ag-H interaction. I. Introduction The nanoparticles of Ag and Au are used extensively in catalysis, electrochemistry, and medical science.1,2 Silver has a similar electronic structure to gold: a completely filled d shell and a singly occupied s shell. However, gold has a significant relativistic effect, which decreases the energy difference between 6s and 5d orbitals and as a consequence the d electrons are expected to participate in the bonding, whereas the d and s shells in silver are energetically well separated and the d electrons do not participate directly in the bonding. Recently there has been a growing interest in the geometric and electronic structures of bimetallicsilver-goldclustersbothexperimentallyandtheoretically.3-8 Weis et al. performed density functional theory (DFT) calculations for the structures of small bimetallic silver-gold cluster cations (AgmAun+, m + n < 6).6 Negishi et al. studied the photoelectron spectroscopy of AgmAun- (m + n e 4) and found that electron affinities tend to increase with increasing gold content.7 Bonacˇic´-Koutecky´ et al. carried out a systematic DFT study of neutral and charged AgmAun clusters up to five atoms.8 They found the difference in electronegativity results in charge transfer from silver to gold atoms. Although many of the studies were focused on bare silver-gold clusters, much less is known about how the mixing of Ag and Au will influence the chemical reactivity of the metal clusters. A related task is that of using physical probes in combination with theoretical calculations to understand cluster chemisorption, namely to identify size and composition effects and especially the most probable chemisorption sites a bimetallic cluster offers to atoms and small molecules. It is found that pyridine prefers binding to silver when both silver and gold atoms coexist at active sites of a mixed silver-gold cluster.9 The binding interaction between propene and silver-gold clusters strongly depends on the shape and energy of the LUMO orbitals of the clusters.10 CO always binds in a head-on fashion to a gold atom, and the binding energy drops with increasing silver content in small AgmAun+ (4 < m + n < 7).11 * Corresponding author: E-mail: [email protected]; Phone and Fax: 86-379-64212567. † Henan University of Science and Technology. ‡ Henan Normal University.

Recently there have been a number of theoretical studies on the interaction between atomic hydrogen and coinage metal clusters.12-18 Zhao et al. carried out a systematic DFT calculation of the geometric, electronic, and bonding properties of small neutral and anionic and cationic AgnH (n e 7) clusters.15 The hydrogen binding energy shows a clear odd-even oscillation. The structures of gold cluster hydrides containing up to 13 Au atoms were also predicted using DFT.16 The photoelectron spectroscopy of AunH- clusters exhibit a surprising similarity with Aun+1- clusters.17 However, to our knowledge, there has been no detailed theoretical works for H interaction with bimetallic silver-gold clusters and with cationic gold clusters. In this paper, we systematically investigate the interaction between H and small AgmAun (m + n e 5) and Aun (n e 5) clusters with neutral, positive, and negative charge state using the first principles methods on the basis of DFT. The results of geometry, binding energy, dissociation channel, and frequency for bimetallic and monometallic clusters were analyzed and are discussed in Section III. II. Computational Details The calculations are carried out using DFT with the generalized gradient approximation (GGA) implemented in the GAUSSIAN 03 package.19 The PW91PW91 DFT method20 in conjunction with the LANL2DZ basis set and the corresponding Los Alamos relativistic effective core potential (RECP)21 on Ag and Au atoms and the 6-311++G(d, p) basis set on hydrogen atoms was used. To calibrate the accuracy of this approach, we first compared the calculation results of the smallest AgAu, Au2, AgH, and AuH clusters with previous experimental results. Table 1 lists the adiabatic ionization potentials (AIPs) and electron affinities (EAs) of Ag2 and Au2 and the bond length and the harmonic frequency of AgH and AuH. The PW91PW91 results in Table 1 show good agreement with experimental values. The hydrogen binding energy (BE) is defined by the follow equation:

BE ) E(AgmAun) + E(H) - E(AgmAunH) Where E(AgmAun) and E(AgmAunH) are the total energies of the bare cluster and the complex cluster, respectively. All

10.1021/jp910230p  2010 American Chemical Society Published on Web 03/22/2010

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TABLE 1: Comparison of Calculated and Experimental Adiabatic Ionization Potentials (AIPs) and Electron Affinities (EAs) of AgAu and Au2, Bond Lengths (R) and Frequencies (freq) of AgH and AuHa AgAu species calc exp a

Au2

AgH

AuH

AIP (eV)

EA (eV)

AIP (eV)

EA (eV)

R (Å)

freq (cm-1)

R (Å)

freq (cm-1)

8.75 50%, the most favorable binding site is a Au-Au bridge site. 5. Anionic Gold and SilWer-Gold Clusters. Results for the most stable structures of anionic AgmAun- are presented in Figure 3. The Ag-Au bond in AgAu- is 2.72 Å, which is 0.03 Å longer than the calculated Au2- and is almost the same value of Ag2-. Au3- has a linear structure with the Au-Au bond length of 2.62 Å. Bimetallic trimers Ag2Au- and AgAu2- are both linear with Au occupying peripheral positions. Au4- prefers a zigzag geometry, which is the same structure that Wu et al. found.31 Bonacˇic´-Koutecky´ et al. have studied the geometries of AgmAun- with m + n e 5 using DFT calculation.8 In this study, we get similar results to them except for AgAu3-. They assigned the rhombic geometry in which the Ag atom is located at the short diagonal by BLYP27,32 method. However, the PW91PW91/lanl2dz calculations indicated that a Y-shape with the silver atom in the center is the most favorable. All the anionic pentamers prefer trapezoidal structures in which one Ag atom remains four-coordinated independently from the composition. 6. H Binding on Anionic Gold and SilWer-Gold Clusters. The results in Figure 6 show that the top site is the most stable binding site of anionic silver-gold clusters. The similar feature can also be found in the studies of hydrogen binding on pure Agn- and Aun- clusters.15,17 For AgAuH-, H prefers binding to the Ag atom and has a Ag-H distance of 1.66 Å. This

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TABLE 2: Adiabatic Ionization Potentials (AIPs) and Electron Affinities (EAs) in eV of Bare Clusters and Their Corresponding Cluster Hydrides species AgAu Au2 Ag2Au AgAu2 Au3 Ag3Au Ag2Au2 AgAu3 Au4 Ag4Au Ag3Au2 Ag2Au3 AgAu4 Au5

AIP (eV) 8.75 9.59 6.44 6.97 7.37 6.56 7.91 7.97 8.11 6.18 6.46 6.76 7.13 7.43

species -

AgAu Au2Ag2AuAgAu2Au3Ag3AuAg2Au2AgAu3Au4Ag4AuAg3Au2Ag2Au3AgAu4Au5-

EA (eV) 1.48 2.12 2.64 3.30 3.77 2.39 1.80 2.40 2.92 2.32 2.61 2.90 3.17 3.26

species

AIP (eV)

AgAuH Au2H Ag2AuH AgAu2H Au3H Ag3AuH Ag2Au2H AgAu3H Au4H Ag4AuH Ag3Au2H Ag2Au3H AgAu4H Au5H

species AgAuH-

7.07 7.85 7.71 8.31 8.67 6.17 6.55 6.82 7.11 7.30 7.64 8.07 8.26 8.37

Au2HAg2AuHAgAu2HAu3HAg3AuHAg2Au2HAgAu3HAu4HAg4AuHAg3Au2HAg2Au3HAgAu4HAu5H-

EA (eV) 2.61 3.27 1.63 2.00 2.52 2.40 2.78 3.44 3.70 1.47 1.62 1.85 2.13 2.32

TABLE 3: Hydrogen Binding Energies (BEs) and NBO Atomic Charges of the H Atoms of the Most Stable Silver-Gold and Gold Cluster Hydrides species AgAuH Au2H Ag2AuH AgAu2H Au3H Ag3AuH Ag2Au2H AgAu3H Au4H Ag4AuH Ag3Au2H Ag2Au3H AgAu4H Au5H

BE (eV)

qH (e-)

species

1.59 1.76 2.68 2.85 3.02 1.74 1.61 1.87 2.07 2.71 2.82 2.96 2.94 2.91

-0.09 -0.20 -0.21 -0.18 -0.15 -0.47 -0.24 -0.24 -0.25 -0.49 -0.48 -0.33 -0.30 -0.30

AgAuH+

Au2H+ Ag2AuH+ AgAu2H+ Au3H+ Ag3AuH+ Ag2Au2H+ AgAu3H+ Au4H+ Ag4AuH+ Ag3Au2H+ Ag2Au3H+ AgAu4H+ Au5H+

structure is about 0.16 eV lower in energy than the linear isomer in which H is linked to the Au atom. Another top-Ag binding structure was found for Ag3Au-. For other cases of anionic complex clusters the top-Au sites were preferred. For trimers, H drives the transformation of metal framework from linear to triangle structure. Significant geometric differences of the metal parts between the bare clusters and hydride clusters can also be observed in tetramers. The most stable AgmAunH- (m + n ) 5) has similar structures in which H is connected to the Au atom at the low-coordinated exposition site. The most stable Au5H- has a different geometry form AgmAunH-, and the next stable Au5H-, which has similar shape with AgmAunH- (m + n ) 5), is only 0.02 eV higher in energy. Energetic Properties. 1. Adiabatic Ionization Potentials and Electron Affinities. From Table 2 it can be seen that the AIPs and EAs of metal clusters increase as the Au content increasing for the given cluster size. A dramatic decrease of EA occurs for Ag2Au2- due to its largest ionic character of the anionic tetramer.7,25 We also calculated the AIPs and EAs of the cluster hydrides. Similar to bare clusters, the AIPs and EAs of cluster hydrides also increase as the Au content increases. This indicates that substitution of Ag with Au atoms makes the cluster more difficult to lose electrons and easier to gain electrons. 2. Hydrogen Binding Energy. For the monomer AgH, the calculated binding energy (2.21 eV) is in excellent agreement with other theoretical studies,15,33 while the experimentally observed value34 is somewhat larger (2.39 eV). Similar to the case of AgH, our calculations underestimate the binding energy of AuH by about 0.40 eV compared with the experimental value of 3.39 eV.34 The 5s orbital of Ag is -0.17 au and the 6s orbital of Au is -0.23 au; it is clear that Au has a better match of the H-1s level of -0.28 au.

BE (eV)

qH (e-)

species

3.25 3.44 1.40 1.50 1.53 2.79 2.96 3.01 3.00 1.59 1.63 1.65 1.82 1.93

-0.22 -0.23 -0.28 -0.17 -0.18 -0.36 -0.34 -0.17 -0.19 -0.21 -0.17 -0.12 -0.09 -0.13

AgAuH-

Au2HAg2AuHAgAu2HAu3HAg3AuHAg2Au2H AgAu3HAu4HAg4AuHAg3Au2HAg2Au3HAgAu4HAu5H-

BE (eV)

qH (e-)

2.72 3.02 1.67 1.55 1.72 2.42 2.61 2.92 2.89 1.87 1.83 1.92 1.94 2.01

-0.49 -0.30 -0.30 -0.27 -0.26 -0.46 -0.30 -0.24 -0.24 -0.27 -0.26 -0.24 -0.22 -0.18

The hydrogen binding energies (H BE) for all the cluster hydrides studied above are listed in Table 3, together with the amount of the charge transfer. The H BEs show a clear odd-even oscillation as both Au and bimetallic clusters: the clusters with odd number of electrons bind H more strongly than the clusters with even number of electrons. This trend is similar to the cases of H binding on pure Ag clusters.15 Natural bond orbital (NBO) population analysis35 indicates that, irrespective of the charge state, electrons transfer from metal clusters to H. The negative charge of H in gold cluster hydrides is from -0.13 to -0.30 e-. This is about half of the cases of H binding on pure silver clusters15 due to the larger electronegativity of Au than Ag (2.54 vs 1.93). In silver-gold cluster hydrides, the top-Ag and Ag-Ag bridge binding structures have the largest charge transfer of -0.46 to -0.49 e-. For other cases, the negative charge of H generally changes from -0.17 to -0.36 e- and slightly decreases as the Au content increasing. Previous studies indicated that the larger charge transfer between metal clusters and adsorbates often leads to larger binding energy.36 However, in thepresent study, it seems that the electron donation is not the dominating factor of the H BEs. The BEs between H and gold clusters are larger than their silver counterparts.15 For bimetallic clusters, the binding energy increases as the number of gold atoms increasing for the given cluster size, which corresponds to a smaller charge transfer. As the gold content increases, the number of Au-H bonds increase in the neutral and cation forms. It can be argued that the larger electronegativity of gold results in stronger electrostatic attraction than silver in H-metal clusters. The shift of the HOMO of the metal cluster toward lower energy with the increasing Au content may also enhance the chemical bonding. We note that this trend is not well followed with several exceptions. For example, of neutral

Hydrogen Binding on Gold and Silver-Gold Clusters

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TABLE 4: Energetically Preferred Dissociation Channels and the Corresponding Dissociation Energies (in eV) of the Most Stable Gold and Silver-Gold Cluster Hydrides AunH+

AunH Au2H Au3H Au4H Au5H

f f f f

Au + AuH Au2 + AuH Au3 + AuH Au4 + AuH

1.03 1.19 1.19 2.10

f f f f

Au+ + AuH Au2H+ + Au Au3+ + AuH Au4H+ + Au

2.76 1.53 1.78 1.70

AgmAunH+

AgmAunH AgAuH f AuH + Ag Ag2AuH f AuH + Ag2 AgAu2H f AgAu + AuH Ag3AuH f Ag2AuH + Ag Ag2Au2H f AgAu2H + Ag AgAu3H f AgAu2 + AuH Ag4AuH f Ag2AuH + Ag2 Ag3Au2H f Ag2Au2 + AgH Ag2Au3H f Ag2Au2 + AuH AgAu4H f AgAu3 + AuH

Au2H+ Au3H+ Au4H+ Au5H+

AunH-

0.76 1.13 1.18 1.22 1.33 1.38 1.90 1.98 1.63 1.81

AgAuH+ f Ag+ + AuH Ag2AuH+ f Ag2+ + AuH AgAu2H+ f AgAuH+ + Au Ag3AuH+ f Ag3+ + AuH Ag2Au2H+ f Ag2Au+ + AuH AgAu3H+ f AgAu2+ + AuH Ag4AuH+ f Ag4+ + AuH Ag3Au2H+ f Ag3Au+ + AuH Ag2Au3H+ f Ag2Au2+ + AuH AgAu4H+ f AgAu3H+ + Au

tetramers the H BE shows a minimum at 50% Au composition. The neutral AgmAun cluster is the most stabilized when the compositions of Au and Ag are equal due to the maximum of hetero bonds and largest charge transfer from Ag to Au atoms,4,8 which may poison the reactivity of the cluster toward H. AgAu2shows the H BE is 0.12 eV less than Ag2Au-. It is likely that multiple interrelated factors, such as the cooperative electrostatic and covalent interaction, even the configuration, and charge redistribution of clusters, may contribute to the observed subtle BE trends, and quantitative analysis of these complex interactions is difficult. 3. Dissociation Channels. The most possible dissociation channels and the corresponding dissociation energies (DEs) of the most stable complex clusters are shown in Table 4. We define the DE as DE ) E(AgaAubH) + E(Agm-aAun-b) E(AgmAunH), a e m, b e n. The favorable dissociation channel corresponds to the minimum DE. There is no experimental or theoretical data of the dissociation channels of neutral and charged AunH clusters. From our DFT result, the calculated lowest-energy dissociation channel is an AuH molecule ejection for neutral and charged gold cluster hydrides. However, there are two cases of Au3H+ and Au5H+ whereby the ejection of an Au atom is the most preferred. This observation is very different with the neutral and charged AgnH in which the lowest-energy dissociation channel is an Ag atom ejection for the clusters with odd number of electrons and an AgH molecule ejection for the clusters with even number of electrons.15 Compared with silver cluster hydrides and gold cluster hydrides, the dissociation channel of the bimetallic clusters is more complicated because of the alloying and strongly depends on the charge state. For neutral AgmAunH, the preferred decay channel for the Au-rich clusters (n > m) is to eject an AuH molecule. Whereas when m g n, the most favorable dissociation channel of AgmAunH with odd number of electrons is to eject an Ag atom, and the most favorable dissociation channel of AgmAunH with even number of electrons is to eject Ag2 dimer or AgH molecule. For charged AgmAunH the most favorable fragmentation channels correspond to processes in which the charge is carried away by the larger fragment. A larger cluster has a larger surface area, which facilitates the distribution of the charge over the cluster and reduces the electrostatic energy.37 If we compare the products of AgmAunH+ and AgmAunH-, it is interesting to found that the caitons prefer ejection of the Au-containing products (AuH or Au), while the anions prefer ejection of the Ag-containing products (Ag, Ag2, or AgH). For anionic Ag-

Au2HAu3HAu4HAu5H-

f f f f

Au-+ AuH Au2- + AuH Au3- + AuH Au4- + AuH

1.99 1.59 1.45 1.55

AgmAunH1.54 1.32 1.38 1.19 1.35 1.53 1.29 1.47 1.47 1.51

AgAuH- f Au- + AgH Ag2AuH- f AgAuH- + Ag AgAu2H- f Au2H- + Ag Ag3AuH- f AgAuH- + Ag2 Ag2Au2H- f Au2H- + Ag2 AgAu3H- f AgAu2- + AuH Ag4AuH- f Ag3AuH- + Ag Ag3Au2H- f Ag2Au2H- + Ag Ag2Au3H- f AgAu3H- + Ag AgAu4H- f Au4H- + Ag

1.87 1.16 1.08 1.37 1.42 1.51 1.52 1.41 1.24 1.39

mAunH , the ejection of Ag-containing product will leave more Au atoms in larger fragment with negative charge, which involves a larger EA. For example, there are two competition dissociation channels for Ag3Au2H-: ejection of Ag atom to produce Ag2Au2H- and ejection of Au atom to produce Ag3AuH-. Ag2Au2H- has a larger EA than Ag3AuH-. The corresponding dissociation energy is 1.41 eV for the channel to produce Ag2Au2H- and 2.38 eV for the channel to produce Ag3AuH-, respectively. For AgmAunH+, the ejection of Aucontaining product will leave more Ag atoms in larger fragment with positive charge, which involves a lower AIP. The higher the EA is, the more difficult it should be to remove an electron from the anionic cluster. The lower the AIP is, the easier it should be to form the cationic cluster from the corresponding neutral cluster. This feature indicates that for the given cluster size, the cation with more Ag atoms will be relatively more stable than the cation with more Au atoms, whereas the anion with more Au atoms will be relatively more stable than the anion with more Ag atoms. It is argued that the relative stability of the larger fragment contributes a lot to the different dissociation patterns of anionic and cationic clusters. It can be seen from the data of Table 4 that the DEs of gold cluster hydrides are always larger than the silver-gold cluster hydrides except for Au4H. For bimetallic cations the DEs generally increase as gold content increasing, indicating the dissociation becomes more and more difficult with the increasing Au composition. The similar trend can also be found in neutrals up to tetramer, while for neutral pentamers the DEs oscillate as the number of Au atoms changes. There is also an increase of DEs as the cluster size grows for neutral. For anions, as the Au composition increases, the DEs increase for the trimer but decrease for the tetramer. Ag4AuH- has the largest dissociation energy of 1.52 eV for anionic pentamer. Frequency Analysis and M-H Bond Length. Many experiments on H adsorption system were based on the FTIR method and focused on the vibrational frequency of the M-H species.34,38 In the adsorption system, we have calculated the vibrational frequencies for all the clusters studied above. These vibrations can be divided into two classes: vibrations of metal atoms (M mode) and vibrations involving both metal and H atoms (MH mode). M modes give relatively small frequencies, which are usually less than 300 cm-1 in our results. M-H modes give frequencies between 480 and 2100 cm-1, strongly depending on the binding sites and the type of atoms to which H is bound. The M-H vibrational frequencies are listed in Table 5.

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TABLE 5: M-H Vibrational Frequencies of the Most Stable Silver-Gold and Gold Cluster Hydrides species

binding site

freq (cm-1)

species

binding site

freq (cm-1)

species

binding site

freq (cm-1)

AgAuH Au2H Ag2AuH AgAu2H Au3H Ag3AuH Ag2Au2H AgAu3H Au4H Ag4AuH Ag3Au2H Ag2Au3H AgAu4H Au5H

top-Au Au-Au top-Au top-Au Au-Au Ag-Ag top-Au Au-Au Au-Au Ag-Ag Ag-Ag Ag-Au Au-Au Au-Au

2043 1185, 1409 2031 2087 1189 (Au1-H), 1434 (Au2-H) 807 (Ag1-H), 1187 (Ag2-H) 1972 490 (Au1-H), 1523 (Au2-H) 734(Au1-H) 1340(Au2-H) 934 (Ag1-H), 1170 (Ag2-H) 997, 1164 545 (Ag-H), 1688 (Au-H) 843, 1298 800, 1282

AgAuH+ Au2H+ Ag2AuH+ AgAu2H+ Au3H+ Ag3AuH+ Ag2Au2H+ AgAu3H+ Au4H+ Ag4AuH+ Ag3Au2H+ Ag2Au3H+ AgAu4H+ Au5H+

Ag-Au Au-Au Ag-Au Au-Au Au-Au Ag-Au Ag-Au Au-Au Au-Au Ag-Au Ag-Au Au-Au Au-Au Au-Au

488 (Ag-H), 1960 (Au-H) 1070, 1416 877 (Ag-H), 1597 (Au-H) 1045 (Au1-H), 1444 (Au2-H) 1081 (Au1-H), 1437 (Au2-H) 1076 (Ag-H), 1419 (Au-H) 1010 (Ag-H), 1549 (Au-H) 926 (Au1-H), 1598 (Au2-H) 951(Au1-H) 1552(Au2-H) 802 (Ag-H), 1513 (Au-H) 530 (Ag-H), 1749 (Au-H) 891 (Au1-H), 1341 (Au2-H) 957 (Au1-H), 1365 (Au2-H) 1050, 1166

AgAuHAu2HAg2AuHAgAu2HAu3HAg3AuHAg2Au2HAgAu3HAu4HAg4AuHAg3Au2HAg2Au3HAgAu4HAu5H-

top-Ag top-Au top-Au top-Au top-Au top-Ag top-Au top-Au top-Au top-Au top-Au top-Au top-Au top-Au

1575 2002 1945 2027 2034 1558 1937 2045 2047 1847 1852 1911 1938 2008

The top-Au adsorption gives the highest stretching frequency in the range of 1847-2087 cm-1. The Ag-H stretching frequencies of the top-Ag adsorptions are 1575 cm-1 of AgAuH-and 1558 cm-1 of Ag3AuH-, respectively. The adsorption on the M-M bridge site gives two different M-H frequencies. For the C2V structures of Au2H, Ag3Au2H, AgAu4H, Au5H, Au2H+, and Au5H+ in which the two M-H bonds are equal, the larger frequency corresponds to the symmetric M-H stretching mode while the smaller frequency corresponds to the asymmetric M-H stretching mode. In other cases of the M-M adsorption, the larger frequency corresponds to the stretch of the shorter M-H bond (1166-1598 cm-1 for Au-Au and 1164-1187 cm-1 for Ag-Ag) while the smaller frequency correspond to stretch of the longer M-H bond (490-1189 cm-1 for Au-Au and 807- 997 cm-1 for Ag-Ag). In the bridge Ag-Au binding structures the M-H frequencies can be divided into two bands: the Au-H stretching frequencies from 1419 to 1960 cm-1 and the Ag-H stretching frequencies from 488 to 1076 cm-1. We note that the Au-H distance increases in the order: top-Au site (1.571-1.618 Å) < Ag-Au bridge site (1.580-1.694 Å) Ag-Au bridge site > Au-Au bridge site. On the other hand, the Ag-H distance increases in the order: top-Ag site (1.660-1.669 Å) < Ag-Ag bridge site (1.765-1.889 Å) < Ag-Au bridge site (1.792-2.075 Å), which also correlates well with the order of Ag-H frequency. The larger Au-H frequency and the shorter Au-H distance of the Ag-Au bridge adsorption than Au-Au bridge adsorption indicate that the Au-H interaction becomes stronger when the binding site changes from Au-Au to Ag-Au, while the smaller Ag-H frequency and the longer Ag-H distance of the Ag-Au bridge adsorption than that of the Ag-Ag bridge adsorption indicate that the Ag-H interaction becomes weaker when the binding site changes from Ag-Ag to Ag-Au. The lanthanide contraction makes the gold atom have a radius very close to that of a silver atom. If the difference of Au-H and Ag-H distance with the same type of binding site is due to the bonding interaction, we can see in all the cases the Au-H bond is stronger than the Ag-H bond, which is also supported by the magnitude of M-H vibrational frequencies. IV. Summary A theoretical study was carried out on the binding of atomic hydrogen on small neutral and charged Aun (n e 5) and AgmAun (m + n e 5) clusters using density functional methods. Hydrogen prefers the bridge site for neutral and cationic Aun

clusters but prefers the top site for anionic Aun clusters. For neutral bimetallic clusters, hydrogen prefers the top-Au site for dimer and trimer, whereas for tetramer and pentamer it is likely that hydrogen migrates from Ag to bonding with Au atoms as the Au content increases. For bimetallic cations the most favorable binding site is the Ag-Au bridge site when the Au composition is e50%, whereas it is the Au-Au bridge site when the Au composition is >50%. For bimetallic anions hydrogen prefers the top-Au site except for AgAuH- and Ag3AuH- (topAg binding structures). As the Au content increass, one can observe the following trends of the energetic properties: (1) the binding energy increases while the charge transfer decreases; (2) the AIPs and EAs increase, which indicates the replacement of Ag with Au and leads the cluster to be more difficult to lose electrons while easier to obtain electrons; (3) the dissociation becomes more and more difficult with the increasing dissociation energy in cations. The most preferred dissociation channel of neutral and charged AunH clusters is to eject an AuH molecule. AgmAunH+ prefers ejection of Au-containing product whereas AgmAunHprefers ejection of Ag-containing product, which is due to the relative stability of the larger fragment. The magnitude of M-H frequency is related with the M-H bond length. With the same type of the binding site, in all cases Au-H interaction is stronger than Ag-H interaction. Acknowledgment. The project was supported by the Outstanding Talent Program of Henan Province (084200510015) and the Fund for Doctorates of Henan University of Science and Technology. References and Notes (1) Jin, X.; Zhuang, L.; Lu, J. J. Electroanal. Chem. 2002, 519, 137. Busch, P. A.; Cheston, S. P.; Greywall, D. S. Cryogenics 1984, 24, 445. Padilla, A. P.; Rodrı´guez, J. A.; Saitu´a, H. A. Desalination 1997, 114, 203. (2) Heiz, U.; Sanchez, A.; Abbet, S.; Scheider, W. D. Eur. Phys. J. D 1999, 9, 35. Haruta, M. Catal. Today 1997, 36, 153. Grunwaldt, J.-D.; Baiker, A. J. Phys. Chem. B 1999, 103, 1002. Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (3) Lopez, N.; Norskov, J. K. J. Am. Chem. Soc. 2002, 124, 11262. Sanchez, A.; Abbet, S.; Heiz, U.; Scheider, W. D.; Hakkinen, H.; Barnett, R. N.; Landman, U. J. Phys, Chem. A 1999, 103, 9573. (4) Lee, H. M.; Ge, M.; Sahu, B. R.; Tarakeshwar, P.; Kim, K. S. J. Phys. Chem. B 2003, 107, 9994. (5) Zhao, G. F.; Zeng, Z. J. Chem. Phys. 2006, 125, 014303. (6) Weis, P.; Welz, O.; Vollmer, E.; Kappes, M. M. J. Chem. Phys. 2004, 120, 677. (7) Negishi, Y.; Nakamura, Y.; Nakajima, A.; Kaya, K. J. Chem. Phys. 2001, 115, 3657.

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