A DFT study of the interaction of elemental mercury with small neutral and charged silver clusters

Chemical Physics Letters 517 (2011) 227–233

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A DFT study of the interaction of elemental mercury with small neutral and charged silver clusters Lushi Sun a, Anchao Zhang b,⇑, Sheng Su a, Hua Wang b, Junli Liu b, Jun Xiang a,⇑ a b

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo 454000, China

a r t i c l e

i n f o

Article history: Received 11 April 2011 In final form 18 October 2011 Available online 25 October 2011

a b s t r a c t Mercury adsorption on small neutral and charged Agn clusters has been investigated by using DFT method. The results show that frontier molecular orbital theory is a useful tool to predict the selectivity of Hg adsorption. The binding energies of Hg on the cations are generally greater than those on the corresponding neutral and anionic clusters. NBO analysis indicates the electron flow in the neutral and charged complexes is mainly from the s orbital of Ag to the s orbital of Hg. For neutral and anionic complexes, electron transfer also occurs from p orbital of Hg to s orbital of Ag. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Mercury, a high toxin, has adverse effects on the central nervous system and causes pulmonary and renal failure, severe respiratory damage, blindness, and chromosome damage [1]. Coal-fired utility boilers and municipal waste incineration plants are major anthropogenic sources of mercury emissions [2,3]. Among the three types of mercury species, elemental mercury (Hg) is very difficult to remove from the flue gases due to its low melting point, high equilibrium pressure and low solubility in water [4]. Efforts are underway to devise cost-effective methods to remove Hg from the flue gases [5]. Several sorbents, such as activated carbon [1,2], calcium-based sorbent [6], and coal-burned or oil-fired fly ash [2,3,7], can remove Hg from flue gases. However, there are some problems associated with the use of these sorbents for Hg removal from flue gases, such as a limited temperature range, poor utilization/selectivity for Hg, and regeneration. Recently, noble metals including silver [8–10], gold [11,12], platinum [13], and palladium [13,14] have been extensively investigated for incorporation in novel Hg removal sorbents, both for conventional coal-fired power plants and in the context of new integrated gasification combined-cycle (IGCC) power generation plants [15]. These noble sorbents, which exhibit excellent Hg removal performance, are regenerable, and stable for long-term operation. With the increase of the using noble metals on Hg removal, some basic information about the binding of Hg to noble metal surfaces, such as Ag, Au, and Pd, are still in need, especially for the research of smaller metal clusters. Theoretical studies [15–17] on the adsorption of Hg on noble metal clusters have been investigated in detail. However, to our ⇑ Corresponding authors. Fax: +86 27 87542417. E-mail addresses: [email protected] (A. Zhang), [email protected] (J. Xiang). 0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2011.10.033

knowledge, the literature published are all involved in the Hg adsorption on the larger cluster or the 0 0 1, and 1 1 1 surfaces of noble metals. In many cases, however, the active components of dispersed metal catalysis or metal supported sorbents are small clusters, and the cluster properties, rather than the bulk properties, are responsible for their particular characteristics [18,19]. Hence, a systematic study of Hg adsorption on small metal clusters, such as Agn, Aun, and Pdn, is needed for understanding electronic, chemical, and physical properties, such as adsorption to substrates, and catalysis effects [20]. The calculations of density functional theory (DFT) method have been evaluated extensively on the small neutral and/or charged Cu [21], Ag [22–27], Au [19,28,29], Pd [30], Pt, and Rh [20] clusters. In these Letters, many adsorbates, such as O2, H2, NO, CO, and propene, are investigated in detail. Unfortunately, little theoretical attention has been paid to the interactions of Hg with these small noble metal clusters. In this work, we present mainly a systematic theoretical study of Hg binding on the small neutral and charged Agn cluster (n = 1–6) by using DFT method, since both free and supported Ag clusters are often used as a regenerable sorbent in practical Hg removal experiments [8–11,14]. Binding energies (BEs), Mulliken charge, natural bond orbital (NBO) charge and frontier molecular orbitals (FMO) were used to provide insights on the binding mechanism of Hg to neutral and charged Agn clusters. 2. Computational methods As transition metal present a great number of electrons, the density functional theory (DFT) with generalized gradient approximation (GGA) was used to calculate Hg and Agn clusters. Considering that Ag and Hg have 47 and 80 electrons, respectively, a basis set with the inner electron substituted by effective core potential (ECP) was chosen. Recently, the performances of DFT

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method have been researched extensively on small neutral and charged Agn clusters [23–25]. The results show that DFT methods incorporating Perdew’s correlation functionals generally perform better than other DFT methods incorporating the LYP correlation functional, and PW91 is a functional that predicts fairly accurate energies and vibrational frequencies [24]. For the calculation of Hg and their halide species, Wilcox et al. [31], Peterson et al. [32,33], Tossel [34] and Khalizov et al. [35] found that the method of CCSD(T) and QCISD could provide much more accurate bond energies and somewhat better equilibrium geometries than B3LYP, PW91 and HF method. However, they are more demanding of computer time than B3LYP, PBE, PW91 and so on [34]. Kang et al. [36] found that for Hgn clusters, PBE1PBE (also known as PBE0) are much better than PW91 and PBE. Several studies have shown that the basis set of ECP60MWB for Hg result in significantly more accurate optimized bond lengths and frequencies than Lanl2dz in Hg with halogens or benzene system [5,35,37]. Therefore, to thoroughly investigate the effects of the functionals and basis sets for Hg on Agn clusters, test calculations have been carried out using B3LYP, mPW91PW91, PBEPBE, and PBE1PBE. The results are listed in Table S1 of the Supplementary data. One can find clearly that for the neutral and charged Ag dimer, the data obtained by mPW91PW91/SDD, PBE1PBE/SDD, PBE1PBE/aug-cc-pVDZ-PP, and PBE1PBE/ECP28MWB are in good agreement with the experimental values. While for Hg dimer, we obtain that at the PBE1PBE level, the basis sets of ECP60MWB and aug-cc-pVDZ-PP can give reasonable results compared with experimental data. In our present work, considering the match of different functionals and the saving of more expensive CPU time, the PBE1PBE/SDD, ECP60MWB (SDD for Ag and ECP60MWB for Hg) was employed for all the calculations. For each cluster size, a large number of possible isomers were optimized until the gradient forces vanished with respect to a threshold value. The analysis of vibrational frequencies was performed to make the optimized geometries as minima, not as transition structures. Throughout the Letter, we define the binding energy (BE) as follows [38]:

BE ¼ Ebare cluster þ EHg  Ecluster-Hg adduct

ð1Þ

where Ebare cluster represents the energy of the bare silver cluster, EHg is the energy of Hg molecule, and Ecluster-Hg adduct denotes the energy of the bare silver cluster and adsorbate Hg. Consequently, in the above definition, a more positive BE represents a stronger interaction. We also calculated the basis set superposition error (BSSE) by using the counterpoise correction method [18]. However, the BSSE correction was not taken in account for our present Hg/Agn systems, because the calculated values of BSSE correction were almost less than 0.01 eV (vide infra). All the calculations were carried out with DFT method and implemented in the GAUSSIAN 03 program package [39]. The threedimensional (3D) molecular structures were plotted using Molden package [40]. The isosurfaces of molecular orbitals were visualized using the Molekel 5.4 software [41]. 3. Results and discussion 3.1. Optimized geometries of bare Agn clusters and reactivity predictors Recently, many DFT investigations of the atomic and electronic structures of small neutral and charged silver clusters are already available in previous studies [19,22–27,42–44]. In this work, for each Agn cluster (3  n  6), several initial structures taken from Refs. [23–25,27,42] have been optimized. For comparison and the conciseness of text, the most stable geometries of neutral and

charged Agn clusters and their isomers with different spin multiplicities at the level of PBE1PBE/SDD are listed in Table S2 and Figure S1 of the Supplementary data. It can be seen clearly that our calculated geometry structures of all Agn clusters with lower spin multiplicities are more stable, and the calculated ground states are in good agreement with the previous DFT reports [19,22– 27,42]. For the naked metal clusters, it is important to predict the most favorable site for sorbate adsorption in order to save the calculation time and help the design of a better catalyst or adsorbent [28]. Fukui realized that a good approximation for reactivity could be found by looking at the frontier molecular orbitals (FMO). FMO theory simplifies reactivity to interactions between the highest occupied molecular orbital (HOMO) of one specie and the lowest unoccupied molecular orbital (LUMO) of the other. In many reports, the FMO theory was successfully applied. For instance, Wells et al. [45] reported that the strongest bonding of O2 with the Au 10 cluster is edge-on attachment to the HOMO region of the cluster. Joshi et al. [38] found that Frontier Orbital Picture (FOP) is useful in predicting the orientation of adsorbed O2. Poater et al. [21] used the LUMO theory to predict the adsorption site of CO on small Cun (n = 1–9) clusters. Furthermore, Chrétien et al. [26–28] used the LUMOs of Agn, Aun, or their alloy clusters as standards to predict the binding sites of propene. From the literature, it is clear that these rules coincide with the spirit of Fukui’s FMO theory [28]. Due to the dominant donor character of Hg when adsorption on neutral and cationic Agn clusters (vide infra), we prefer to use the rules of LUMOs reported by Chrétien et al. [26–28] and Poater et al. [21] to predict the reactivity of bare Agn clusters. While for anionic Agn clusters, we prefer to use HOMO of Ag n as a reactivity predictor because of receiver character of Hg [45]. Therefore, in this Letter, the most important orbitals to be considered are the LUMOs (for neutral and cationic) and HOMOs (for anionic) of the Agn clusters [21]. The representations of the HOMO and LUMO orbitals of the neutral, cationic and anionic Agn clusters are displayed in Figures S2–S4. For comparison with real molecular orbitals, the orbital parameters obtained by ab initio calculation (MP2/SDD) are also listed. It can be seen that the shapes and symmetry properties of the Kohn–Sham (KS) orbitals are very similar to those calculated by MP2/SDD method although the orbital energies have some differences. As have been reported by Stowasser et al. [46] and Zhang et al. [47], the energy order of the occupied orbitals is in most cases in agreement between the two methods and the KS orbitals are a good basis for qualitative interpretation of molecular orbitals. From inspection of LUMO (HOMO) orbitals (see Figures S2–S4), it is possible to predict the silver atoms in the given cluster that, in theory, will interact better with Hg, which are those having larger contributions to the LUMO (HOMO for anions) orbital [21]. For neutral Ag dimer, the a-LUMO of Ag2 extends into the vacuum at ends of the naked Ag2 cluster [28]. By looking at the shape and symmetry of a-LUMO of Ag2 and HOMO of Hg molecule, we predict that the side-on site will be the most stable structure. For the isosceles triangle Ag3 cluster, the central silver would be the most reactive atom of neutral Ag3, but the significant component of the aLUMO corresponding to the external atoms indicates that these atoms may also be adsorptive for Hg. Similarly, the silver atoms of the short diagonal of the rhombus of neutral Ag4 are expected to be more active [21]. When n = 5, because the b-LUMO has lower MO energy than a-LUMO, the two atoms of the short basis of the trapezium in Ag5 will be the most active site for Hg adsorption. The three vertices of the external triangle in Ag6 also are predicted to be the most suitable for the Hg attack according to the shape of the LUMO orbitals, which is similar to that of CO adsorption on Cu6 [21]. Similarly, for the even number Agþ n clusters, the b-LUMO has lower MO energy than a-LUMO. Therefore, in chemical reaction, the atoms, which contributed most for b-LUMO, will be the active

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center for Hg adsorption. While for even anionic Agn clusters, we find that the a-HOMO has higher MO energy than b-HOMO. It is  expected that the end-side atoms of Ag 2 and Ag3 are reactive,    and Ag4 , Ag5 and Ag6 will have similar adsorption patterns like neutral Agn clusters due to having the almost the similar shape of MO. 3.2. Hg adsorption on the neutral and charged Agn clusters

3.2.1. Adsorption of Hg on the neutral silver clusters Figure 1 presents the ground states of AgnHg complexes and their less stable isomers. The binding energies and charge on Hg of neutral and charged AgnHg complexes are listed in Table 1, in which the capital letters N, C, and A represent the neutral, cationic and anionic clusters, and the letters a, b, c, and d denote the most stable complex and their legs stable complexes, respectively. There is only one adsorption pattern for single Ag atom and Hg molecule. That is the head-to-head adsorption with C1v symmetry. For bare Ag2 and Ag3, only one stable structure is obtained. In Ag2Hg (2N-a), the Ag–Ag distance is elongated from 2.57 of naked Ag2 to 2.58 of Ag2Hg. The very smaller change of distance indicates that the adsorption between Ag2 and Hg is rather weak. Comparison of ground states of neutral Agn clusters and AgnHg complexes (see Figure 1) shows that the frameworks of the most stable and some low-lying isomers are similar to those of the ground states of their bare clusters. Among the given neutral AgnHg complexes, they all have planar or close-to-planar structures. It is found that the most and some low-lying stable neutral complexes all prefer top site adsorption except the 5-N-a and 5-N-c complexes. For the most stable complexes, the calculated Hg–Ag distances of ontop site adsorption range from 2.75 Å in 4-N-a to 2.86 Å in 6-N-a (see Table 1). The stronger interaction results in a shorter Hg–Ag bond distance. For example, for n = 4, the distance of 2.75 Å between Hg and Ag is the shortest one, and the BE of 4-N-a is the highest one (see Figure 2). Inspection of the data in Table 1 and Figure 2 shows that the BEs of the neutral systems increase first and then decrease. Besides the bond distance between the Hg and Ag closer to Hg, the charge transferred from the Hg is also a useful index to determine the intensity of Hg adsorption on neutral Agn clusters. One can find that the behavior of the adsorption energy

1-N-a, C∞v

2-N-a, Cs

3-N-a, C1

4-N-a, Cs

4-N-b, Cs

5-N-a, C1

5-N-b, C1

5-N-c, C1

5-N-d, C1

6-N-a, C1

6-N-b, C1

Figure 1. Optimized structures of neutral AgnHg complexes, n 6 6. The symmetry point group is indicated.

is closely related to the increase of Hg charge, especially for the top adsorption pattern. A similar phenomenon was found in the adsorption of O2 on small gold clusters [38]. For bridge adsorption, the rule is an exception. For instance, among all the obtained pentamer complexes, the 5-N-a exhibits the biggest BE, while its NBO charge is the smallest one. The BE of 5-N-c is a middle one, while its Mulliken charge is the highest one. The reason may be due to their complex charge transfer in bridge adsorption process. 3.2.2. Adsorption of Hg on the cationic silver clusters The most stable and some low-lying isomers of cationic AgnHg complexes are shown in Figure 3. We can easily find that the most stable clusters of 1-C-a, 2-C-a, 3-C-a, 4-C-a and 4-C-b, which prefer top site adsorption, have the similar structures as these of neutral AgnHg complexes. For n = 5 and 6, the ground states of AgnHg complexes present ‘two’ bridge structure. In addition, we also notice that for the only 3D structure of bare 5-C, the geometry is greatly changed after adsorption. The 5-C-a displays a planar structure, while its less stable isomer (5-C-b) still remains the structure of bare 5-C. Investigation of the data of the most stable AgnHg+ clusters in Table 1 and Figure 2 shows that with the increase of cluster size in AgnHg clusters, the BEs for Hg gradually decrease, which is similar to the adsorptions of CO [19], propene [28], and O2 [29] on the cationic gold clusters. Moreover, one also can observe that the system with shorter Hg–Ag length usually possesses larger BE. 3.2.3. Adsorption of Hg on the anionic silver clusters Figure 4 displays the lowest-energy structures of anionic AgnHg complexes as well as their isomers. We can clearly see that these complexes are all planar. For n = 5 and 6, the most stable anionic

Table 1 Calculated data for binding energies (BE, eV) bond distances (Å), BSSE (eV), and charges on Hg of neutral and charged AgnHg complexes. Species

Adsorption site

Distance of Hg–Ag

BE

BSSE

Mulliken charge

NBO charge

1-N-a 2-N-a 3-N-a 4-N-a 4-N-b 5-N-a 5-N-b 5-N-c 5-N-d 6-N-a 6-N-b 1-C-a 2-C-a 2-C-b 3-C-a 4-C-a 4-C-b 5-C-a 5-C-b 6-C-a 6-C-b 6-C-c 6-C-d 1-A-a 2-A-a 2-A-b 3-A-a 3-A-b 4-A-a 4-A-b 5-A-a 5-A-b 5-A-c 6-A-a 6-A-b

Top Top Top Top Top Bridge Top Bridge Top Top Top Top Top Bridge Top Top Top Bridge Top Bridge Bridge Top Top Top Top Top Top Top Top Top Bridge Top Top Bridge Top

2.85 2.82 2.77 2.75 2.89 3.00 2.83 2.86 2.85 2.86 2.88 2.68 2.72 2.94 2.75 2.75 2.77 2.93 2.77 2.94 2.89 2.77 2.78 3.01 2.89 3.13 2.92 2.98 2.85 2.89 2.92 2.90 2.90 2.91 2.91

0.248 0.288 0.345 0.377 0.201 0.269 0.255 0.248 0.239 0.219 0.204 1.226 0.815 0.725 0.656 0.595 0.541 0.590 0.505 0.519 0.514 0.489 0.460 0.369 0.393 0.340 0.267 0.267 0.346 0.270 0.328 0.275 0.247 0.310 0.281

0.002 0.007 0.009 0.010 0.008 0.010 0.009 0.009 0.007 0.006 0.009 0.004 0.011 0.008 0.010 0.011 0.010 0.010 0.007 0.011 0.012 0.008 0.009 0.001 0.007 0.004 0.005 0.004 0.008 0.008 0.008 0.008 0.006 0.008 0.005

0.075 0.052 0.104 0.054 0.182 0.302 0.133 0.071 0.173 0.168 0.105 0.432 0.152 0.372 0.034 0.047 0.013 0.155 0.051 0.063 0.151 0.07 0.021 0.091 0.356 0.038 0.304 0.111 0.303 0.315 0.296 0.284 0.309 0.282 0.37

0.062 0.106 0.135 0.144 0.063 0.086 0.111 0.102 0.099 0.096 0.085 0.259 0.249 0.259 0.235 0.229 0.211 0.206 0.202 0.223 0.222 0.207 0.209 0.096 0.121 0.076 0.013 0.034 0.025 0.001 0.017 0.005 0.004 0.038 0.042

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Binding energy of Hg (eV)

1.5 1.2

Neutral Cationic Anionic

0.9

1-A-a, Cv

2-A-a, Cs

2-A-b, Cs

3-A-a, C1

3-A-b, Cs

4-A-a, Cs

4-A-b, C1

5-A-a, C1

5-A- b, C1

5-A-c, C1

6-A-a, C1

6-A-b, C1

0.6 0.3 0.0 1

2

3

4

5

6

Cluster size Figure 2. Binding energies as a function of cluster size for AgnHg, AgnHg+, and AgnHg complexes.

Figure 4. Optimized structures of anionic AgnHg complexes, n 6 6. The symmetry point group is indicated.

1-C-a, C∞v

2-C-a, Cs

2-C-b, Cs

3-C-a, Cs

and anionic AgnHg clusters, they are a combination of chemisorption and physisorption. 3.3. Validation of reactivity predictors and investigation of some correlations

4-C-a, Cs

4-C-b, Cs

5-C-a, C1

5-C-b, C1

6-C-a, C1

6-C-b, C1

6-C-c, C1

6-C-d, C1

Figure 3. Optimized structures of cationic AgnHg complexes, n 6 6. The symmetry point group is indicated.

complexes prefer bridge adsorption, while the residual ground state complexes all prefer top sites. Interestingly, although the bond distance (3.01 Å) between Hg and Ag of in anionic AgHg complex is longer than that of neutral monomer (2.85 Å), the anionic AgHg complex displays higher BE. In addition, one thing should be noticed that for n = 3, we obtained two equal BEs. In view of the Mulliken and NBO charges of its adjacent complexes, we selected the side-on adsorption as the ground state. For the most stable AgnHg clusters, we find that the BEs for Hg adsorption have an odd–even pattern except for Ag 6 , and the BEs obtained range between 0.267 eV of 3-A-a to 0.393 eV of 2-A-a. The BEs of neutral and anionic complexes are relatively smaller than that of cationic Agn clusters. Normally, if the binding energy is below 0.31 eV, the attractive force between the adsorbent and adsorbate is weak and defined as very weak chemical adsorption or physical adsorption caused by van der force. Otherwise, if the BE is above 0.51 eV, the attractive force is strong and defined as chemical adsorption. From Figure 2, therefore, one can conclude that the adsorption of Hg on cationic Agn clusters is mainly chemical, while for neutral

Coming back to the results of the prediction rules of LUMO (HOMO for anions), it is observed that most of the predications done have been accomplished successfully. As have been discussed above, the LUMO of neutral Ag2 cluster extends into the vacuum at the ends of the naked Ag2, which means that the Hg will bind to those positions, and as shown in Figure 1 the DFT calculations find that it does [28]. Similarly, anionic Ag2 binds to Hg at the end sides. The Agþ 2 also adsorbs Hg with side-on type through the attraction of b-LUMO of Agþ 2 with HOMO of Hg, which is similar to the binding pattern of O2 on Au2 [38]. In brief, the rules successfully predicted the most stable cluster-Hg configuration for the following clusters: all the neutral Agn clusters, all the anionic Agn clusters, þ þ Agþ 1 , Ag2 and Ag3 . However, being semiempirical rules [26–28], there are some þ þ exceptions for reactivity predication, such as Agþ 4 , Ag5 and Ag6 . þ þ Examples of failure are presented by Ag4 and Ag5 . The b-LUMO of Agþ 4 has lower MO energy than a-LUMO, and according to the shape of b-LUMO of Agþ 4 , one can predict that the silver atoms of the long diagonal of the rhombus of Agþ 4 are the most reactive. However, the theoretical study shows that Ag4Hg+ complex with a short diagonal bond Hg is the most stable structure. The LUMO of the Agþ 5 , which protrudes most on the four corners of the Xshaped structure, infers that the most stable isomer will be obtained when Hg binds to the site with more contribution to LUMO. Unfortunately, in our calculation, we get the ground state of Ag5Hg+ complex with Hg binding to three Ag atoms of X-shaped Ag5 cluster. Similarly, when the adsorption of propene on Ag5 was studied, a failure also happened [28]. For these failures, Joshi et al. explained that it might be due to involvement of orbitals lower/higher in energy than the HOMO/LUMO, Coulomb forces, and back-donation of electron density [38]. Given the importance of LUMO (HOMO for anionic clusters) shape and energy order [21,26,38] for Hg adsorption, the relation between BEs and LUMO (HOMO) energies may also be close. For

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1.4

0.36

Binding energy of Hg (eV)

Binding energy of Hg (eV)

0.40

y = -0.2109x - 0.2257 2

R = 0.93

0.32

0.40 0.36

0.28

y = -0.0401x + 0.2048 2 R = 0.58

0.32 0.28

0.24

0.24 0.20

0.20 -3.0

1.2 y = -0.1828x - 0.6344 1.0

2

R =0.98

0.8 0.6

-3.0 -2.4 -1.8 -1.2 -0.6

0.4

-2.8

-2.6

-2.4

-2.2

-2.0

-10

-9

-8

-7

-6

LUMO energy (eV) of cationic Ag n clusters

LUMO energy (eV) of neutral Agn clusters

Binding energy of Hg (eV)

0.42 0.39 y = 0.0747x + 0.3494 2

0.36

R =0.84

0.33 0.30 0.27 -0.8

-0.4

0.0

0.4

HOMO energy (eV) of anionic Ag n clusters Figure 5. Relationships between BEs of the most stable complexes and LUMO (HOMO) energies of neutral and charged silver clusters.

NBO charge on adsorbed Hg

0.14 0.12 0.10 y = 0.3834x - 0.0018

0.08

2

R =0.83

0.06

0.26 0.25 0.24 0.23 y = 0.0775x + 0.1766

0.22

2

R =0.80

0.21 0.20

0.04 0.20

0.24

0.28

0.32

0.36

0.4

0.40

0.6

0.8

1.0

1.2 +

Binding energy of Hg for Ag nHg (eV)

NBO charge on adsorbed Hg

NBO charge on adsorbed Hg

0.16

Binding energy of Hg for Ag nHg (eV)

0.04 0.00 -0.04 -0.08

y = -0.6816x + 0.1810 2

R =0.67

-0.12 0.24

0.28

0.32

0.36

0.40 -

Binding energy of Hg for Ag nHg (eV) Figure 6. Relationships between binding energies of AgnHg, AgnHg+ and AgnHg complexes and NBO charge on adsorbed Hg.

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Table 2  +  NBO population analysis for Hg, Ag4, Ag4Hg, Agþ 4 , Ag4Hg , Ag4 , and Ag4Hg . Species Hg

Ag4

Ag4Hg

Agþ 4

Ag4Hg+

Ag 4

Ag4Hg

Ag1

Ag2

Ag3

Ag4

Hg

Charges s d p Charges s d p Charges s d p

0.302 3.30 9.98 6.02 0.286" 3.28; 9.98 6.02

0.302 3.30 9.98 6.02 0.279" 3.28; 9.98 6.02

0.302 2.67 9.97 6.05 0.147; 2.82" 9.98 6.05

0.302 2.67 9.97 6.05 0.274; 2.70" 9.99 6.04

0.144" 3.84; 10.00 6.02"

Charges s d p Charges s d p

0.250 2.75 9.98 6.02 0.258" 2.74; 9.98 6.02

0.250 2.75 9.98 6.02 0.258" 2.74; 9.98 6.02

0.250 2.72 9.98 6.05 0.057; 2.89" 9.99 6.06

0.250 2.72 9.98 6.05 0.197; 2.78" 9.98 6.05

0.229" 3.77; 9.99 6.00

Charges s d p Charges s d p

0.373 3.37 9.98 6.02 0.346; 3.34; 9.99 6.01

0.373 3.37 9.99 6.02 0.348; 3.34; 9.99 6.01

0.127 3.11 9.99 6.03 0.191; 3.15" 9.99 6.04

0.127 3.11 9.99 6.03 0.090" 3.08; 9.99 6.02

0.025; 3.95; 10.00 6.07"

e-Transfer

0.00 4.00 10.00 6.00

neutral Agn clusters, we get a poor relationship like the behavior of CO adsorption on small pure and binary-ally gold clusters [18]. However, for n = 2–6, an excellent linear relationship between the LUMO energy of the neutral clusters and BEs is found (see Figure 5). In addition, from Figure 5, we find that there is a linear relationship between the LUMO (HOMO) energy of the cationic (anionic) Agn clusters and the BEs, which implies that silver cations (anions) interact with Hg mainly via the LUMO (HOMO) of the silver clusters. Similar findings were also reported for propene [26– 28] adsorption on small Agn and Aun clusters. Since the charge transferring to cluster is also a descriptor of the BEs [38], we also investigated intuitively the corrections between BE and NBO or Mulliken charge. We prefer to report the NBO charges rather than more common Mulliken charges because we have a fairly large basis set on Ag and Hg atoms and hence Mulliken charges are likely to be less reliable [18]. For instance, although 3-A-a and 3-A-b give equal BEs, their Mulliken charges present a big difference (see Table 1) due to the very complicated electron density transfer processes in 3-A-b [18]. From Figure 6, one can observe an approximate linear correlation between BEs and the NBO charges on adsorbed Hg with a considerably higher correlation degree. It is found that the more the charges transfer to Hg, the higher the BE is. Interestingly, for anionic AgnHg clusters, the correlation between Hg BEs and NBO charges displays somewhat lower correlation degree due to a more complicated electron transfer (vide infra). For the scatter in this type of correlation, Joshi et al. gave a very detailed explanation: it is difficult to separate the BE contributions of Coulomb attraction, charge transfer to sorbate, back-dona-

tion, and extent of overlap of relevant orbitals, since all these effects are interrelated in a complex manner and operate simultaneously [38]. 3.4. NBO population analysis To investigate the tendency of electron transfers between Hg and Agn clusters after forming complexes, the NBO analysis was carried out and the results of the lowest-energy structures of neutral, charged Ag4 and Ag4Hg clusters are listed in Table 2, as representative of the study. For neutral Ag4Hg complex, the charge transfer is from Ag1, Ag2, and Hg to the Ag3 and Ag4, whereas Ag3 bonded to Hg obtains the most electrons (black arrow) and the charge transfer from Ag1 and Ag2 is less pronounced (gray arrow). In cationic Ag4Hg cluster, charge transfer occurs from Hg to the Ag3 and Ag4. The charge transfer in Ag4Hg complex is from Ag1, Ag2, and Ag4 to Hg and Ag3 connected to Hg. The results show that neutral and cationic Agn clusters are electron acceptor while anionic clusters are electron donator. For the Hg atom, irrespective of the charge state of the cluster, the s population significantly decreases with smaller increases in p orbital and almost no changes in d orbital. For the neutral and charged AgnHg complexes, the population of Ag atom closest to Hg shows an obvious increase in s orbital while that of the Ag atom far from Hg has smaller changes, which is accordance with that of Cl binding on Agn clusters [23]. It indicates that the electron flow in the neutral and charged Ag4Hg is mainly from the s orbital of Ag to the s orbital of Hg. Moreover, for neutral and anionic Ag4Hg complex, electron transfer also oc-

L. Sun et al. / Chemical Physics Letters 517 (2011) 227–233

curs from p orbital of Hg to s orbital of Ag. The reason may be that, as have been seen in Figures S2–S4, there is a better energy and MOs shape match for 1s (HOMO) or 2p (LUMO) orbital of Hg and FMOs of Ag atoms closest to adsorbate. 4. Conclusions In this Letter, we performed density functional study of Hg adsorption on a series of neutral and charged Agn (n = 1–6) clusters. The results indicate that the cluster size and the charged state affect the adsorption of Hg significantly. On the neutral clusters, the BEs increase first and then decrease. The BEs for cationic clusters decrease with increasing cluster size, while for the negatively charged Agn clusters, the BEs have an odd–even pattern except for Ag 6 . NBO analysis indicates that for neutral and anionic complexes, electron transfer also occurs from p orbital of Hg to s orbital of Ag. The rules of LUMO (HOMO for anions) introduced by Chrétien et al. are verified successfully on the predication of Hg binding orientation for neutral and charged Agn clusters. There is a linear relationship between the LUMO energies of the neutral and cationic Agn clusters and the BEs. The more the charge transfer, the higher the BE is. The effects of SO2, NO, O2, and H2O on small Agn clusters for Hg adsorption will be further evaluated for getting more understanding of competition adsorption and designing a type of promising and low-cost sorbent. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 50976038), the State Key Development Program for Basic Research of China (No. 2010CB227003), and Doctoral Fund of Henan Polytechnic University (No. B2011-089). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.cplett.2011.10.033. References [1] [2] [3] [4]

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