Density functional study of H2S binding on small cationic AgnAum+ (n + m ⩽ 5) clusters

Computational and Theoretical Chemistry 997 (2012) 70–76

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Density functional study of H2S binding on small cationic Agn Auþ m (n + m 6 5) clusters Shuang Zhao a, YunLai Ren a, WeiWei Lu a, JianJi Wang a,b,⇑, WeiPing Yin a a b

School of Chemical Engineering, Henan University of Science and Technology, Luoyang, Henan 471003, PR China School of Chemical and Environmental Sciences, Henan Key Laboratory of Environmental Pollution Control, Henan Normal University, Xinxiang, Henan 453007, PR China

a r t i c l e

i n f o

Article history: Received 12 July 2012 Received in revised form 28 July 2012 Accepted 28 July 2012 Available online 10 August 2012 Keywords: Bimetallic clusters Density functional theory H2S adsorption

a b s t r a c t Density functional theory calculations were performed to investigate the structural and energetic properties of H2S binding on small cationic Agn Auþ m clusters (n + m 6 5). The adsorbates prefer binding to Au atoms when both Au and Ag co-exist in the cluster with the exceptions of Ag4Au+ and Ag3 Auþ 2 . The binding energy decreases as the cluster size grows and is further correlated with the LUMO energy of bare þ Agn Auþ m clusters. Our calculation suggests that the Agn Aum cations may react with H2S dissociatively by way of a H2 molecule loss. The SAH and MAS vibrational frequencies are highly related to whether S atom is attached to Au or Ag atoms. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The bimetallic Ag/Au clusters and nanoparticles have attracted considerable interest both experimentally and theoretically since they play an important role in colloidal chemistry, medical science and catalysis [1–9]. A new class of highly efficient optical materials based on Ag/Au clusters, showing vastly enhanced optical nonlinearity over the bulk metals, has been synthesized [1]. The photoelectron spectroscopy of Agm Au (m + n 6 4) indicated that n electron affinities of those cluster anions tend to increase with increasing gold composition [2]. Pyridine prefers binding to Ag when both Au and Ag atoms co-exist at active sites of a mixed Ag/Au cluster [3]. Our previous theoretical study indicated that the adiabatic ionization potentials, electron affinities and hydrogen binding energies of bimetallic Ag/Au cluster hydrides increase as the Au content increases [4]. H2S is one of the most useful gas sensors and yet one of the most common surfactants in hydrodesulfurization of hydrocarbons [10,11]. The interaction of H2S with gold and silver surface is widely used as a model system to understand the various reasons for the poisoning of metals by sulfur compounds [12,13]. The Fourier-transform ion-cyclotron resonance (FT-ICR) mass spectrometry has been performed to investigated the reactions of gold cluster cations Auþ n (n = 1–12) with H2S at room temperature + [14]. Most Auþ n clusters are reactive towards H2S except for Au ⇑ Corresponding author at: School of Chemical Engineering, Henan University of Science and Technology, Luoyang, Henan 471003, PR China. Tel./fax: +86 379 64212567. E-mail address: [email protected] (J. Wang). 2210-271X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2012.07.040

+ and Auþ 3 . Sequential sulfuration reactions of AunS produced Aunþ + Sm and finally stopped at Aun Smþx H2 when H2 release did not occur. While unambiguous determination of the geometric and energetic properties of a small atomic cluster is an almost impossible task from experiments alone, quantum-chemical studies may shed light on the understanding of the catalytic activity of small metal clusters. The theoretical studies of H2S adsorption on coinage metal clusters have also been appeared [15–19]. The H2S adsorption energies on cationic Auþ n clusters (n = 1–8) are generally greater than those on the corresponding neutral Aun clusters [15]. The small gold cluster would like to bond with sulfur in the same plane and the H2S molecule prefers to occupy the on-top and single fold coordination site in the AunH2S (n = 1–13) clusters [16]. Hamilton et al. have performed a theoretical calculation on the decomposition channels of MnH2S+ complex (M = Cu, Ag, Au, n = 1–4) involving loss of an H atom, H2 molecule, M atom or M2 dimer [17]. Alloying neutral Auk with copper and silver decreases the attraction of Au towards H2S, while alloying Agk and Cuk by gold increases the attracting of Ag and Cu towards H2S with k = 1–3 [18]. Compared to neutral clusters, experimentally, it is much more straightforward to study the properties of ionic clusters. To our knowledge, there is no theoretical report about the interaction between H2S and the cationic Agn Auþ m clusters available. In this contribution, we systematically investigate the interaction between H2S and small Agn Auþ m (n + m 6 5) cluster cations using the first principles methods on the basis of density functional theory (DFT). The goal of this work is to have a better understanding of the correlation between the energetic properties of the cationic Ag/Au clusters and their structure, composition and size. Moreover, to fully understand the catalytic activity of bimetallic clusters for a

S. Zhao et al. / Computational and Theoretical Chemistry 997 (2012) 70–76

specific reaction, it is important to study the interaction between bimetallic clusters and the reactant and product molecules. The results of geometry, binding energy, dissociation channel and vibrational frequency for bimetallic and monometallic clusters were analyzed and discussed in Section 3. Before presenting the results, we summarize the computational details in Section 2. Finally, the conclusion is drawn in Section 4. 2. Computational details The calculations were carried out using Gaussian 03 package [20]. The PW91PW91 [21] exchange and correlation functional was employed for all the calculations in this study. The Los Alamos relativistic effective core potential (RECP) plus DZ basis set [22] was used for Ag and Au atoms. The 6-311++G(d, p) basis set was used on H and S atoms. All calculations were performed with (99, 590) pruned grid (ultrafine grid as defined in Gaussian 03). Natural bond orbital (NBO) [23] analysis was used to provide the natural charge distribution. Vibrational frequency calculations including thermochemical analysis were carried out at 298.15 K and 1 atmosphere of pressure. These frequency calculations also guarantee the optimized structures locating the minima, not as transition structures. To have the results of bare clusters and complex clusters for comparison, we also studied the geometries and properties of bare Ag/Au clusters in cationic charge state up to five atoms. The most stable Agn Auþ m (n + m 6 5) clusters taken from our previous work [4] are displayed in Fig. 1. Small cationic Agn Auþ m clusters tend to be planar structures up to four atoms. The lowest energy isomer of Agþ 5 cation has three-dimensional D2d structure while the most stable Auþ 5 is a twisted X-shaped geometry with D2h symmetry. þ The most stable Ag4Au+, Ag2 Auþ 3 and AgAu4 isomers can be viewed as the replace of Ag with Au atoms in the Ag2 unit of the D2d structure, while the lowest energy form of Ag3 Auþ 2 is trigonal bipyramid with D3h symmetry. Those structures of Agn Auþ m cations are in good agreement with previous theoretical reports [24,25]. 3. Results and discussion

71

slightly perturbed despite the strong MAS bond. The AuAS distances range from 2.37 to 2.50 Å and the AgAS distances range from 2.52 to 2.59 Å. The lanthanide contraction makes the gold atom has a radius very close to that of silver atom. If the difference of AuAS and AgAS distance is due to the bonding interaction, it is expected that in all cases the AuAS bond is stronger than the AgAS bond. However, there are two exceptions of Ag4AuS+ and Ag3Au2S+ clusters whereby the Ag-binding is more favorable than Au-binding. We also note that the lowest-energy structures of AgnAumH2S+ complex are related to the ground-state isomers of bare Agn Auþ m clusters except for Ag3Au+ and Ag2 Auþ 2 . The competition of the stability here is between two processes: larger electrostatic interaction between Au and S atom and the tendency for the metal framework in complex to keep the similar shape to the ground states of the bare clusters. To further understand the interaction between the metal cluster and adsorbate, Table 1 tabulates the electron populations of þ + + + the atomic orbitals for Agþ 2 , Ag2H2S , AgAu , AgAuH2S , Au2 and + Au2H2S clusters. It is observed that irrespective of binding sites, the adsorbate bares a certain amount of positive charge. Higher charge transfer is observed when the S atom is attached to the Au atom (which usually have higher binding energies) than when the S atom is attached to the Ag atom. This can be simply understood by the larger electronegativity of Au than Ag (2.54 versus 1.93). After binding, the p population of the S atoms shows the most dramatic decrease, while the s and d populations almost remain intact. The decrease of the s orbital of H is less pronounced but still noticeable. Regarding the metal part, the s populations of both the two metal atoms increase, whereas the d population decreases. The p populations of the metal atom connected to S atom are also higher in comparison with that in bare dimers. Thus, on the one hand, electrons are transferred from the d level of S and s level of H to the s and p level of metal atoms. On the other hand, electrons are back transferred from the d level of metal atoms to the adsorbate. This cooperative donation and back-donation may be responsible for the bending of the MAMAS axis in the complex clusters. Previous theoretical study of neutral AunH2S (n = 1–8) systems also indicated that the bending of AuAAuAS axis results in a better overlap of the p orbital of S and the sd-hybridized orbitals of Au [15].

3.1. Structures and population analysis 3.2. Binding energies Previous theoretical studies indicated that small metal clusters prefer to bond with the S atom of H2S rather than to bond with hydrogen or to bond with both S and H atoms [15–19]. Thus, the H2S was constrained to S atom binding in this calculation. The most stable Au-binding structures are listed in Fig. 2 and the most stable Ag-binding structures are listed in Fig. 3 (more structures can be seen in the supporting information for the conciseness of the text). Consistent with previous studies, the S atom is bonded to only one metal atom of the clusters and H2S molecule is only

The H2S binding energy (BE) is defined by the follow equation:

BE ¼ EðAgn Auþm Þ þ EðH2 SÞ  EðAgn Aum H2 Sþ Þ þ where EðAgn Auþ m Þ and EðAgn Aum H2 S Þ are the total energies of the bare cluster and the complex cluster, respectively. The more positive the BE is, the stronger the bond is. The calculated binding energies for all the complex clusters studied above are given in Figs. 2 and 3, together with the amount of the charge transfer. One can

Fig. 1. Optimized geometries of the most stable cationic Agn Auþ m clusters, with n + m 6 5. The structures are arranged in the order of increasing Au composition.

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S. Zhao et al. / Computational and Theoretical Chemistry 997 (2012) 70–76

BE=2.16 q= +0.39

BE=1.62 q= +0.35

BE=1.87

BE=1.62

BE=1.25

q= +0.34

q= +0.33

q= +0.29

BE=1.62

BE=1.41

BE=1.25

BE=1.01

q= +0.35

q= +0.32

q= +0.34

q= +0.30

BE=1.59

BE=1.49

BE=1.08

BE=0.87

BE=0.72

q= +0.31

q= +0.32

q= +0.28

q= +0.27

q= +0.26

Fig. 2. The most stable Au-binding structures for AgnAumH2S+ clusters, with n + m 6 5. The structures are arranged in the order of increasing Ag composition. Binding energies in eV and NBO charges in e on H2S are also given.

BE=1.13

BE=1.32

q= +0.20

q= +0.22

BE=0.95

BE=1.12

BE=1.18

q= +0.20

q= +0.21

q= +0.22

BE=0.87

BE=0.98

BE=1.10

BE=1.08

q= +0.21

q= +0.23

q= +0.22

q= +0.23

BE=0.78

BE=0.82

BE=0.99

BE=1.03

BE=1.07

q= +0.18

q= +0.19

q= +0.23

q= +0.20

q= +0.20

+

Fig. 3. The most stable Ag-binding structures for AgnAumH2S clusters, with n + m 6 5. The structures are arranged in the order of increasing Au composition. Binding energies in eV and NBO charges in e on H2S are also given.

observe the following trends from the data of Figs. 2 and 3: (1) for both Au-binding and Ag-binding, the BEs decrease as the cluster size grows. This is not surprising: as the cluster size grows the same

charge is distributed within a larger volume, which implies that the effect of the charge, at a given binding site, will be smaller; (2) since H2S binds to metal cluster by donating electrons to them, it is

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S. Zhao et al. / Computational and Theoretical Chemistry 997 (2012) 70–76 Table 1 + + + + + Net atomic charges (in e) and atomic orbital populations from NBO analyses for Agþ 2 , Ag2H2S , AgAu , AgAuH2S , Au2 and Au2H2S . Species H2S

Species Ag2+

Species Ag2H2S+

Species AgAu+

Species AgAuH2S+

Species Au2+

Species Au2H2S+

H

H

S

Charges

+0.144

+0.144

0.288

s p d

0.85

0.85

5.77 10.50 0.01

Charges

Ag1 +0.500

Ag2 +0.500

s p d

2.50 6.02 9.98

2.50 6.02 9.98

Charges

Ag1 +0.320

Ag2 +0.476

H +0.196

H +0.196

S 0.187

s p d

2.59 6.13 9.96

2.55 6.01 9.97

0.80

0.80

5.77 10.40 0.01

Charges

Ag +0.715

Au +0.285

s p d

2.29 6.02 9.98

2.79 6.02 9.91

Charges

Ag +0.619

Au +0.028

H +0.202

H +0.202

S 0.051

s p d

2.43 6.01 9.95

2.96 6.13 9.88

0.79

0.79

5.76 10.27 0.02

Charges

Au1 +0.500

Au2 +0.500

s p d

2.59 6.01 9.90

2.59 6.01 9.90

Charges

Au1 +0.149

Au2 +0.457

H +0.206

H +0.206

S 0.019

s p d

2.90 6.12 9.83

2.73 6.01 9.81

0.79

0.79

5.76 10.24 0.02

expected that the bond strength is correlated with the tendency of the cluster to accept electrons. Because Au is more electronegative than Ag, a gold atom in a mixed Ag/Au cluster will gain some electronic charge density at the expense of Ag. The electron density on each Au atom will correspondingly increase when Ag composition increases while the electron density on Ag atom will correspondingly decrease when Au composition increases. This should lower the ability of the Au atoms to bind H2S and make the Ag atom more active towards H2S binding in a mixed Ag/Au cluster. As seen from Figs. 2 and 3, for the given cluster size, for top-Au binding the BEs decrease as the number of Ag atoms increases, while for top-Ag binding the BEs increase as the number of Au atoms increases. When the adsorbate binds to metal cluster by donating electrons, there may be a good correlation between the binding energy and the LUMO energy of the naked clusters [7,26]. In this study we have plotted the absolute value of the orbital energy of LUMO of Agn Auþ m as a function of cluster size. As displayed Fig. 4, the plot of the binding energy of the most stable AgnAumH2S+ is very similar to the plot of the LUMO energy of Agn Auþ m clusters: the lower the energy of the LUMO, the higher the binding energy. However, this trend failed at m = 1 for bimetallic tetramer and m = 1 and 2 for pentamer. Nevertheless, it is likely that multiple interrelated factors, the cooperative donation and back-donation, extent of overlaps of orbitals contribute simultaneously to the observed subtle BE trends, and quantitative analysis of these complex interactions is not easy.

3.3. Dissociation channels To explore the possible dissociation channels of the AgnAumH2S+ systems, we examine the energetics of two possible fragmentation pathways, as defined in Eqs. (1) and (2), respectively. The fragmentation energy D0e of AgnAumH2S+ into AgnAumS+ and H2 is defined by:

D0e ¼ EAgn Aum Sþ þ EH2  EAgn Aum H2 Sþ

ð1Þ

The structures of the AgnAumS+ complex are shown in Fig. 5. Generally the S atom is located at the hollow site of Agn Auþ m clusters when n + m P 3[27]. The dissociation energy D1e of AgnAumH2S+ system to eject metal monomer or dimer as products is defined by:

D1e ¼ EAgna Aumb H2 Sþ þ EAga Aub  EAgn Aum H2 Sþ

ð2Þ

where a 6 n, b 6 m. The calculated data of dissociation energies D0e and D1e are shown in Table 2 as a function of the number of gold atoms in AgnAumH2S+ system. The most favorable dissociation channel corresponds to the minimum dissociation energy. From the data of Table 2, it can be seen that the calculated dissociation channel for Eq. (2) is a single metal atom ejection for the clusters with odd number of electrons while a metal dimer ejection (Ag2, AgAu or Au2) for the clusters with even number of electrons.

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S. Zhao et al. / Computational and Theoretical Chemistry 997 (2012) 70–76 Table 2 The energetically preferred dissociation channels and the corresponding dissociation energies (D0e and D1e in eV) of the most stable AgnAumH2S+ clusters with n + m 6 5.

2.2

Binding energy (in eV)

2.0

Dissociation channel

1.8

+

1.6 1.4 1.2

n+m=2 n+m=3 n+m=4 n+m=5

1.0 0.8 0

1

2

3

4

5

The number of Au atoms

Absolute orbital energy (in eV)

10.4

+

Ag2H2S ? Ag2S + H2 AgAuH2S+ ? AgAuS+ + H2 Au2H2S+ ? Au2S+ + H2 Ag3H2S+ ? Ag3S+ + H2 Ag2AuH2S+ ? Ag2AuS+ + H2 AgAu2H2S+ ? AgAu2S+ + H2 Au3H2S+ ? Au3S+ + H2 Ag4H2S+ ? Ag4S+ + H2 Ag3AuH2S+ ? Ag3AuS+ + H2 Ag2Au2H2S+ ? Ag2Au2S+ + H2 AgAu3H2S+ ? AgAu3S+ + H2 Au4H2S+ ? Au4S+ + H2 Ag5H2S+ ? Ag5S+ + H2 Ag4AuH2S+ ? Ag4AuS+ + H2 Ag3Au2H2S+ ? Ag3Au2S+ + H2 Ag2Au3H2S+ ? Ag2Au3S+ + H2 AgAu4H2S+ ? AgAu4S+ + H2 Au5H2S+ ? Au5S+ + H2

D0e 1.19 1.28 1.13 0.97 1.12 1.64 1.51 0.75 0.79 1.03 1.10 1.31 0.49 0.68 1.17 1.02 1.31 1.37

Dissociation channel +

D1e

+

Ag2H2S ? AgH2S + Ag AgAuH2S+ ? AgH2S+ + Au Au2H2S+ ? AuH2S+ + Au Ag3H2S+ ? AgH2S+ + Ag2 Ag2AuH2S+ ? AgH2S+ + AgAu AgAu2H2S+ ? AgH2S+ + Au2 Au3H2S+ ? AuH2S+ + Au2 Ag4H2S+ ? Ag3H2S+ + Ag Ag3AuH2S+ ? Ag2AuH2S+ + Ag Ag2Au2H2S+ ? AgAu2H2S+ + Ag AgAu3H2S+ ? AgAu2H2S+ + Au Au4H2S+ ? Au3H2S+ + Au Ag5H2S+ ? Ag3H2S+ + Ag2 Ag4AuH2S+ ? Ag3H2S+ + AgAu Ag3Au2H2S+ ? Ag2AuH2S+ + AgAu Ag2Au3H2S+ ? AgAu2H2S+ + AgAu AgAu4H2S+ ? AgAu2H2S+ + Au2 Au5H2S+ ? Au3H2S+ + Au2

1.27 1.32 1.54 2.21 2.12 2.26 2.48 1.13 1.14 1.26 1.30 1.40 1.49 1.60 1.77 1.73 1.90 1.97

10.0 9.6 Table 3 The energy difference between the products and the reactants give by DE ¼ D0e  BE (in eV) and Gibbs free energy changes (in kJ/mol) for reaction (3).

9.2 8.8 8.4 8.0

n+m=2 n+m=3 n+m=4 n+m=5

7.6 7.2 0

1

2

3

4

5

The number of Au atoms Fig. 4. Binding energies of the most stable AgnAumH2S+ clusters and absolute values of the LUMO energy of the most stable Agn Auþ m clusters versus the content of Au atoms, with n + m 6 5.

In all the cases the values of D1e are greater than D0e , indicating that AgnAumH2S+ clusters prefer the ejection of H2 than the ejection of metal monomer or dimer. Previous theoretical study also indicated that the dissociation of cationic AunH2S+ into AunS+ and H2 is preferred over the dissociation into AumH2S+ and Aunm [15]. Regarding the dissociation energies, for both Eqs. (1) and (2) the AunH2S+ complexes have larger dissociation energies than the AgnH2S+ complexes. Besides, the dissociation energies generally increase

Reaction (3)

DE

DG

Ag2+ + H2S ? Ag2S+ + H2 AgAu+ + H2S ? AgAuS+ + H2 Au2+ + H2S ? Au2S+ + H2 Ag3+ + H2S ? Ag3S+ + H2 Ag2Au+ + H2S ? Ag2AuS+ + H2 AgAu2+ + H2S ? AgAu2S+ + H2 Au3+ + H2S ? Au3S+ + H2 Ag4+ + H2S ? Ag4S+ + H2 Ag3Au+ + H2S ? Ag3AuS+ + H2 Ag2Au2+ + H2S ? Ag2Au2S+ + H2 AgAu3+ + H2S ? AgAu3S+ + H2 Au4+ + H2S ? Au4S+ + H2 Ag5+ + H2S ? Ag5S+ + H2 Ag4Au+ + H2S ? Ag4AuS+ + H2 Ag3Au2+ + H2S ? Ag3Au2S+ + H2 Ag2Au3+ + H2S ? Ag2Au3S+ + H2 AgAu4+ + H2S ? AgAu4S+ + H2 Au5+ + H2S ? Au5S+ + H2

0.05 0.33 1.03 0.01 0.05 0.02 0.35 0.12 0.23 0.23 0.31 0.31 0.09 0.13 0.18 0.07 0.18 0.21

10.6 26.8 94.3 7.1 12.0 15.7 26.0 0.80 0.10 14.1 21.8 21.3 5.5 6.9 24.1 1.8 8.4 17.9

as the number of Au atoms increases for the given cluster size, indicating the dissociation becomes more and more difficult with the increasing Au composition. This also reflects the increasing

Fig. 5. The optimized structures for AgnAumS+ clusters, with n + m 6 5. The structures are arranged in the order of increasing Au composition.

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S. Zhao et al. / Computational and Theoretical Chemistry 997 (2012) 70–76 Table 4 SAH and MAH vibrational frequencies in cm1 of AgnAumH2S+ clusters. Species +

Ag2H2S AgAuH2S+ Ag3H2S+ Ag2AuH2S+ AgAu2H2S+ Ag4H2S+ Ag3AuH2S+ Ag2Au2H2S+ AgAu3H2S+ Ag5H2S+ Ag4AuH2S+ Ag3Au2H2S+ Ag2Au3H2S+ AgAu4H2S+

Freq (SAH for Ag-binding) 2605, 2602, 2612, 2608, 2607, 2614, 2610, 2608, 2611, 2616, 2615, 2614, 2610, 2609,

2588, 2585, 2595, 2591, 2590, 2596, 2592, 2592, 2594, 2599, 2597, 2596, 2593, 2593,

1155 1150 1154 1153 1152 1155 1154 1156 1152 1154 1154 1157 1153 1156

R (AgAS) 2.55 2.52 2.57 2.53 2.53 2.59 2.53 2.53 2.55 2.59 2.58 2.55 2.53 2.53

Freq (AgAS)

+

208 222 210 224 222 204 224 220 214 209 209 210 222 222

Au2H2S AgAuH2S+ Au3H2S+ Ag2AuH2S+ AgAu2H2S+ Au4H2S+ Ag3AuH2S+ Ag2Au2H2S+ AgAu3H2S+ Au5H2S+ Ag4AuH2S+ Ag3Au2H2S+ Ag2Au3H2S+ AgAu4H2S+

stability of AgnAumH2S+ complex clusters with increasing Au composition. Although the adsorption of H2S on bimetallic Agn Auþ m clusters to form stable complex clusters is energetically favorable according to the calculations of binding energies, it is possible for metal clusters to dissociate during reactive collision with the adsorbate. Sugawara et al. found that the initial products of the reaction of Auþ n clusters with H2S were mainly AuSH+ for n = 2, and AunS+ for n = 4–8 [14]. However, no reactions of Au+ and Au3+ with H2S were observed. In this study, for the following reaction

Agn Auþm þ H2 S ! Agn Aum Sþ þ H2

Species

ð3Þ

The energy difference between the products and the reactants is given by DE ¼ D0e  BE. In most cases the H2S binding energies are larger than the dissociation energies of AgnAumH2S+ complex. We also calculate the Gibbs free energy change DG for reaction (3). If DG is negative, then reaction (3) is thermodynamically favorable. As shown in Table 3, the calculated DG has negative values for þ all the Auþ n clusters. The reaction of Agn clusters is much less exoþ þ thermic than Aun . With the exception of Ag2Au+, AgAuþ 2 , Ag3 Au2 þ and Ag2 Auþ , the bimetallic Ag Au also has a negative D G. Theren 3 m fore, Agn Auþ m may prefer H2 molecular loss upon H2S reactive collision, leading to the product AgnAumS+.

Freq (SAH for Au-binding)

R (AuAS)

Freq (AuAS)

2568, 2577, 2583, 2594, 2583, 2589, 2599, 2587, 2587, 2586, 2600, 2598, 2598, 2585,

2.37 2.42 2.41 2.42 2.47 2.44 2.50 2.43 2.45 2.42 2.50 2.48 2.48 2.41

274 245 259 228 255 244 217 252 239 254 213 221 226 255

2555, 2563, 2568, 2579, 2569, 2573, 2582, 2571, 2571, 2572, 2584, 2583, 2582, 2570,

1148 1148 1150 1152 1150 1152 1152 1151 1151 1151 1152 1153 1154 1149

to Au atoms. The symmetric and bend vibrations are also shifted to lower wavenumbers if change the binding site from Ag to Au. 4. Summary A theoretical study was carried out on the interaction between H2S and small cationic Agn Auþ m (n + m 6 5) clusters using density functional method. H2S prefers binding to Au atom when both Ag and Au site co-exist in the mixed cluster except for Ag4AuS+ and Ag3Au2S+. The interaction between the H2S and metal dimers can viewed as a large electron transfer from the p orbitals of the S atom and s orbitals of H atom to s and p orbitals of the metal atoms in addition to the back donation from the d orbitals of metal to S. The alloying makes the Ag atoms more reactive with H2S and the Au ones less reactive. The binding energy of the most stable AgnAumH2S+ complex is related to the LUMO energy of bare Agn Auþ m clusters: generally the lower the energy of the LUMO, the higher the binding energy. For the given cluster size the dissociation of AgnAumH2S+ becomes more and more difficult with the increasing Au content. Further investigation indicated that the Agn Auþ m reacting with H2S may lead to the dissociative loss of H2 molecule. There are two distinguished adsorption bands of SAH and MAS vibrational frequencies correlated to whether the S atom is attached to Ag or Au atoms.

3.4. Frequency analysis Since many experiments on H2S adsorption system were based on FTIR method and focused on the vibrational frequency [13,28– 30], in this adsorption system, we have calculated the vibrational frequencies for all the complex clusters studied above. The frequencies listed in Table 4 can be divided into two classes: vibrations of H2S (SAH mode) and vibrations involving both metal and S atoms (MAS mode). For MAS mode, the AuAS stretching frequencies range from 217 to 274 cm1 while the AgAS stretching frequencies range from 204 to 224 cm1, indicating a weaker bond between Ag and S than that between Au and S. This is also supported by the smaller AuAS distances (2.37–2.50 Å) than the AgAS distances (2.52–2.59 Å). For H2S in gas-phase, the wavenumbers for the asymmetric and symmetric SAH stretching vibrations, together with the bend mode, are calculated to be 2642, 2624 and 1164 cm1, respectively. The corresponding experimental values are 2626, 2615 and 1183 cm1 [31]. Substantial red shifts of SAH mode can be observed in H2S binding on Agn Auþ m clusters. The results in Table 4 show that there are two distinguished adsorption bands correlated to whether the S atom is attached to Ag or Au atoms. The asymmetric SAH stretch frequencies above 2600 cm1 correspond to the S atom attaching to Ag atoms while the asymmetric SAH stretch frequencies below 2600 cm1 correspond to the S atom attaching

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