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Comparative DFT study of electronic structure and geometry of copper and silver clusters: Interaction with NO molecule
Computational Materials Science 35 (2006) 268–271 www.elsevier.com/locate/commatsci
Comparative DFT study of electronic structure and geometry of copper and silver clusters: Interaction with NO molecule Vitaly E. Matulis *, Oleg A. Ivaskevich Research Institute for Physical Chemical Problems of the Belarusian State University, 14 Leningradskaya St., 220050, Minsk, Belarus Received 24 April 2004; accepted 12 August 2004
Abstract A comparative analysis of geometry and electronic characteristics, namely, vertical detachment energies (VDE) and excitation energies of neutral clusters with geometry of anions (Te) of anionic silver and copper clusters has been carried out within DFT model using a new functional developed recently [Vitaly E. Matulis, O.A. Ivashkevich, V. S. Gurin, J. Mol. Struct. (Theochem) 664–665 (2003) 291]. The obtained data show that the most stable anionic silver and copper clusters have very similar geometry. It has been shown, that properties defined by mainly s-electrons should be very similar for copper and silver clusters. However, the molecular orbitals with large contribution of d-atomic orbitals lie considerably closer in energy to the HOMO for copper clusters than for silver ones. Therefore, a substantial difference should be observed in such properties of silver and copper where d-electrons play an important role, for example, in their catalytic activity. The role of d-electrons along with VDE and ionization potential in catalytic activity is discussed in context of interaction of silver and copper tetramers with NO molecule. To study the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the reactivity of Cu2Au2 cluster towards NO molecule has been studied and compared with the data obtained for Cu2Ag2 cluster. Ó 2005 Elsevier B.V. All rights reserved. PACS: 82.65.Y; 71.24; 61.46; 31.15.E Keywords: Silver clusters; Copper clusters; DFT theory; Electronic structure; Geometry; Adsorption
1. Introduction In previous works [1,2] we have established the geom etries of anionic clusters Ag n (n = 7, 9, 10) and Cun (n = 6, 8–10) based on both total energy calculations and comparison of experimental and calculated photoelectron spectra (PHES) using a specially developed semi-empirical exchange-correlation functional denoted hereinafter as S2LYP. The suggested functional combines the contributions of Slater and HF exchange and VWN and LYP correlation with adjustable coefficients: *
Corresponding author. Tel.: +375 17 209 5254; fax: +375 17 226 4696. E-mail address: [email protected] (V.E. Matulis). 0927-0256/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2004.08.011
LYP a ESx þ ð1 aÞ EHF þ ð1 bÞ EVWN ; x þ b Ec c LYP where a = 0.7; b = 0.6; ESx ; EHF ; and EVWN are x ; Ec c the designations of exchange and correlation energy terms. This functional provides the results of calculations of electronic characteristics of both copper and silver clusters to a high accuracy. Therefore it can be used for investigation of binary copper–silver clusters. In the present paper, we carried out a comparison of geometries and electronic structure of silver and copper clusters in view of their catalytic activity. The interaction of NO molecule with anionic, neutral and cationic silver and copper tetramers has been studied. NO adsorption on copper and silver surface leads to formation of (NO)2 and N2O molecules containing N–N bonds [3]. This is important for catalytic reduction of NO
V.E. Matulis, O.A. Ivaskevich / Computational Materials Science 35 (2006) 268–271
molecule. To investigate the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the interaction of copper–gold binary clusters with NO molecule has been studied and compared with the data obtained for copper–silver clusters.
2. Computational details All calculations of silver and copper clusters have been carried out within density functional theory (DFT). The previously developed S2LYP functional [1] and two basis sets have been used including ECP double-zeta type basis set termed as LANL2DZ [4] for silver, and ECP Stuttgart–Dresden relativistic basis set (SDD) [5] for copper clusters. The evaluation of energetic characteristics of interaction of NO molecule with silver and copper clusters, S2LYP functional and D95V(d) basis set for N and O atoms were used, whereas for Cu, Ag and Au atoms LANL2DZ basis set was applied. All calculations were performed using NWChem 4.1 package [6].
3. Results The calculated VDE, Te and binding energies per atom ðE b =nÞ for the structures of Ag210 and Cu210
269
which should correspond to the experimental ones are summarized in Table 1. As it can be seen from Refs. [1,2], the geometries of the most stable anionic copper and silver clusters are similar. The calculated values of photoelectron spectra (PHES) characteristics and trends of VDE and Te against cluster nuclearity are also very close (Table 1). This can be explained by the fact, that the PHES of anionic copper and silver clusters give the picture of electronic structure of the ns derived electronic states, and they are very close for both metals [2]. Table 1 shows that the calculated values of E b =n of copper clusters are larger than those of silver ones. An analysis of Mulliken overlap populations shows that contribution of d-electrons in overlap populations for Ag 2 is 0.0549 corresponding to 7.5% of all-electron overlap population (0.7323). At the same time, this value for Cu 2 is 0.0740 and amounts to 17.1% of all-electron overlap population (0.4329). This result can be explained by the fact that the molecular orbitals with large contribution of d-atomic orbitals lie considerably closer in energy to the HOMO for copper clusters than for silver ones. Thus, the highest MO with the large contribution of d-atomic orbitals for anionic copper dimer lies 2.55 eV below the SOMO, whereas the corresponding value for silver dimer is 4.24 eV. Therefore, one can assume that properties in which d-electrons play an important role should be different for silver and copper clusters. We have examined the similarity and the
Table 1 Calculated E b =n, VDE and Te and experimental values of VDE and Te for anionic silver and copper clusters, which should be observed in experimenta Cluster Ag 2 Ag 3 Ag 4 Ag 5 Ag 6 Ag 7 Ag 8 b Ag 9 ðIÞ Ag9 ðIIÞb Ag 10 Cu 2 Cu 3 Cu 4 Cu 5 Cu 6 b Cu 7 ðIÞ Cu7 ðIIÞb Cu 8 Cu 9 Cu 10 a b
Calculations
Experiment [7,8]
E b =n, eV
VDE, eV
VDE + Te, eV
VDE, eV
VDE + Te, eV
0.89 1.36 1.45 1.63 1.70 1.86 1.87 1.94 1.94 1.94 0.91 1.45 1.62 1.83 1.99 2.17 2.17 2.20 2.27 2.30
1.06 2.34 1.73 2.10 2.17 2.68 1.63 2.79 2.46 2.20 0.92 2.32 1.57 1.97 2.07 2.57 2.09 1.53 2.37 2.07
2.82 3.59 2.44 3.39 2.40 3.69 3.15 3.24 3.25 2.83 2.93 3.48 2.35 3.31 2.31 3.53 3.24 3.09 3.07 2.73
1.1 2.4 1.63 2.12 2.08 2.60–2.73 1.61 2.77 2.45 2.13 0.96 2.37 1.49 1.98 1.96 2.49–2.61 2.19 1.60 2.40 2.05
2.8 3.59 2.40 3.32 2.36 3.60 3.06–3.18 3.19 – 2.80 2.82 3.49 2.27 3.14 2.25 3.32 3.1–3.2 2.92–3.06 2.99 2.70
For the details of geometry see Refs. [1,2]. The numbers in parentheses give the rank in increasing energy order.
270
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difference in reactivity of silver and copper clusters on a model of their interaction with NO molecule. NO molecule can be considered both as electron donor and acceptor. The p-accepting properties are dominant in NO in contrast to CO molecule [9], but the general trends can be changed depending on the nature of species which reacts with NO. When NO reveals p-accepting properties in interaction with metals, partial charge transfer from metal AO to the NO p-antibonding orbital (which is symmetry allowed only if MeNO is bent) results in a long NO bond and a decreased NO stretch frequency compared to gaseous NO. In this case metal appears as electron donor. Therefore, the metals that easily donate electrons can promote this process. In contrast, if NO molecule interacts with species with high electron affinity (e.g. Cu+), it reveals property of electron donor [10]. In this case partial charge transfer from antibonding MO of NO to AO of metal results in a short NO bond and an increased NO stretch frequency compared to gaseous NO. To study this phenomenon, we have calculated the geometry and energetic characteristics of complexes of NO with silver and copper tetramers. The calculations of NO complexes with anionic tetramers as a model of species, which are easily donating electrons, and cationic tetramers as a model of species with high electron affinity have also been done. Tetramers were chosen as model systems because of very close structures of neutral, anionic and cationic clusters. Besides, in this case different sites of adsorption (terminal and bridged) can be considered. The results of calculations are given in Table 2. The optimized geometries of three most stable complexes of Ag4 with NO, as well as the surfaces of HOMO of Ag4, NO and Ag4–NO complexes are shown in Fig. 1. The structures are numbered in the order of increasing energy. The atop structures I and II have Cs symmetry while the bridged structure III has C2v symmetry. The optimized geometries of complexes of Cu4 with NO, as well as in the case of anionic and cationic copper and silver clusters are very close to those in Fig. 1.
Fig. 1. S2LYP optimized geometries of complexes of Ag4 with NO and the surfaces of HOMO of Ag4, NO and Ag4–NO complexes.
Table 2 shows that adsorption energy (Eads) of NO on copper tetramer and their ions considerably stronger than for silver ones. For both metals binding of NO molecule is increased in the range cation–neutral–anion. Reaction of NO with neutral clusters and especially with anions leads to a elongated NO bond and a decreased NO stretch frequency compared to gaseous NO (Table 2). On the other hand, reaction with cations results in a short NO bond and almost unchanged NO stretch frequency compared to gaseous NO. In contrast to binding energy, changing of NO stretch frequency and bond length are nearly the same for copper and silver clusters. To study the interaction of silver–copper clusters with NO relative to that of silver and copper one-component systems, the geometries and the energetic characteristics of complex of Cu2Ag2 with NO have been calculated. In addition, to investigate the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the reactivity of Cu2Au2 cluster towards NO molecule has been studied and compared with the data obtained for copper and copper–silver clusters. The optimized geometries of Cu4, Cu2Ag2 and Cu2Au2 with atomic charges are shown
Table 2 S2LYP calculated properties of complexes of silver and copper tetramers with NO moleculea Silver
I II III I() II() III() I(+) II(+) III(+)
Copper
DE, eV
R(N–O)
0.00 0.40 0.49 0.00 0.45 0.68 0.00 0.22 0.11
1.163 1.170 1.166 1.219 1.208 1.198 1.145 1.142 1.160
˚ A
m(N–O), cm 1873.42 1827.67 1841.08 1630.71 1600.88 1637.35 2043.65 2062.14 1876.36
1
Eads, kJ/mol
DE, eV
˚ R(N–O) A
m(N–O), cm1
Eads, kJ/mol
85.88 47.53 38.36 122.56 79.77 57.86 77.36 56.02 66.91
0.00 0.38 0.60 0.00 0.42 0.76 0.00 0.28 0.022
1.168 1.180 1.203 1.222 1.211 1.215 1.150 1.149 1.171
1885.58 1656.55 1762.81 1668.38 1679.22 1626.50 2006.37 2021.53 1832.84
144.86 108.47 86.89 181.44 140.96 108.06 121.14 94.40 123.26
Anionic clusters are noted () and cationic clusters (+). a S2LYP/D95V(d) calculated for NO molecule v(N–O) and R(N–O) values are 2076.82 cm1 and 1.151, respectively.
V.E. Matulis, O.A. Ivaskevich / Computational Materials Science 35 (2006) 268–271
Fig. 2. S2LYP/LANL2DZ optimized geometries of Cu4, Cu2Ag2 and Cu2Au2 with atomic charges.
in Fig. 2. We have considered complexes of binary systems with NO like structure I in Fig. 1 only. Since VDEs of Ag and Cu are similar, the bonding in Cu2Ag2 does not contain ionic contribution. Therefore, it is expected that interaction of Cu2Ag2 cluster with NO molecule should be similar to that of onecomponent systems. Indeed, the calculated values of ˚ ) and m(N–O) Eads (137.25 kJ/mol), R(N–O) (1.166 A 1 (1901.00 cm ) for complex of Cu2Ag2 with NO are very close to corresponding values for structure I of copper (Table 2). Due to slightly larger electron affinity of silver, copper atom have larger positive charge in binary copper–silver cluster than in the case of copper tetramer (Fig. 2), which leads to slightly less charge transfer from copper to NO and, as a consequence, to less adsorption energy and increased NO stretch frequency. On the other hand, the bonding in Cu2Au2 is enhanced by an ionic contribution owing to considerably larger electron affinity of Au atom. Thus, copper has large positive charge in Cu2Au2 cluster (Fig. 2). The calculated values ˚ ) and m(N–O) of Eads (114.81 kJ/mol), R(N–O) (1.156 A 1 (1991.99 cm ) for complex of Cu2Au2 cluster with NO are very close to corresponding values for structure I(+) of copper (Table 2).
4. Conclusions According to our calculations, the most stable structures of anionic silver and copper clusters have very similar geometry. Calculated characteristics of PHES are also close for silver and copper clusters. At the same time, properties in which d-electrons play an important
271
role should be different for silver and copper clusters. Calculations of interaction of cationic, neutral and anionic silver and copper tetramers, Cu2Ag2 and Cu2Au2 with NO molecule show that the geometry and energetic characteristics of these systems should be determined mainly by two factors. On the one hand, it is the ability of metal to donate or add electrons, which should be depended on total atomic charge of interacting atom and the nature of metal. It is clearly followed from the comparison of the calculated characteristics of com plexes of Cuþ 4 , Cu4 , Cu4, Cu2Ag2 and Cu2Au2 with NO molecule. It has been shown that interaction of NO with silver and copper anions leads to a considerable lengthening of NO bond, that is important for the reaction of NO dissociation. On the other hand, d-electrons play an important role in the adsorption and sufficiently influence the binding energy. Thus, in spite of similar electron affinity, copper clusters interact with NO considerably stronger than silver ones. However, the geometrical characteristics, such as NO bond length changing, are determined by charge transfer and mainly depend on VDE, ionization potential and atomic charges.
References [1] Vitaly E. Matulis, O.A. Ivashkevich, V.S. Gurin, J. Mol. Struct. (Theochem) 664–665 (2003) 291. [2] Vitaly E. Matulis, O.A. Ivashkevich, V.S. Gurin, J. Mol. Struct. (Theochem) 681 (2004) 169. [3] W.A. Brown, D.A. King, J. Phys. Chem. B 104 (2000) 2578. [4] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. [5] D. Andrae, U. Haussermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim. Acta 77 (1990) 123. [6] R.J. Harrison et al., NWChem, A computational chemistry package for parallel computers, Version 4.1, Pacific Northwest National Laboratory, Richland, Washington 99352-0999, USA, 2002. [7] H. Handschuh, C.-Y. Cha, P.S. Bechthold, G. Gantefo¨r, W. Eberhardt, J. Chem. Phys. 102 (1995) 6406. [8] C.-Y. Cha, G. Gantefo¨r, W. Eberhardt, J. Chem. Phys. 99 (1993) 6308. [9] P. Treesukol, J. Limtrakul, T.N. Truong, J. Phys. Chem. B 105 (2001) 2421. [10] M. Zhough, L. Andrews, J. Phys. Chem. A 104 (2000) 2618.
Comparative DFT study of electronic structure and geometry of copper and silver clusters: Interaction with NO molecule Vitaly E. Matulis *, Oleg A. Ivaskevich Research Institute for Physical Chemical Problems of the Belarusian State University, 14 Leningradskaya St., 220050, Minsk, Belarus Received 24 April 2004; accepted 12 August 2004
Abstract A comparative analysis of geometry and electronic characteristics, namely, vertical detachment energies (VDE) and excitation energies of neutral clusters with geometry of anions (Te) of anionic silver and copper clusters has been carried out within DFT model using a new functional developed recently [Vitaly E. Matulis, O.A. Ivashkevich, V. S. Gurin, J. Mol. Struct. (Theochem) 664–665 (2003) 291]. The obtained data show that the most stable anionic silver and copper clusters have very similar geometry. It has been shown, that properties defined by mainly s-electrons should be very similar for copper and silver clusters. However, the molecular orbitals with large contribution of d-atomic orbitals lie considerably closer in energy to the HOMO for copper clusters than for silver ones. Therefore, a substantial difference should be observed in such properties of silver and copper where d-electrons play an important role, for example, in their catalytic activity. The role of d-electrons along with VDE and ionization potential in catalytic activity is discussed in context of interaction of silver and copper tetramers with NO molecule. To study the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the reactivity of Cu2Au2 cluster towards NO molecule has been studied and compared with the data obtained for Cu2Ag2 cluster. Ó 2005 Elsevier B.V. All rights reserved. PACS: 82.65.Y; 71.24; 61.46; 31.15.E Keywords: Silver clusters; Copper clusters; DFT theory; Electronic structure; Geometry; Adsorption
1. Introduction In previous works [1,2] we have established the geom etries of anionic clusters Ag n (n = 7, 9, 10) and Cun (n = 6, 8–10) based on both total energy calculations and comparison of experimental and calculated photoelectron spectra (PHES) using a specially developed semi-empirical exchange-correlation functional denoted hereinafter as S2LYP. The suggested functional combines the contributions of Slater and HF exchange and VWN and LYP correlation with adjustable coefficients: *
Corresponding author. Tel.: +375 17 209 5254; fax: +375 17 226 4696. E-mail address: [email protected] (V.E. Matulis). 0927-0256/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2004.08.011
LYP a ESx þ ð1 aÞ EHF þ ð1 bÞ EVWN ; x þ b Ec c LYP where a = 0.7; b = 0.6; ESx ; EHF ; and EVWN are x ; Ec c the designations of exchange and correlation energy terms. This functional provides the results of calculations of electronic characteristics of both copper and silver clusters to a high accuracy. Therefore it can be used for investigation of binary copper–silver clusters. In the present paper, we carried out a comparison of geometries and electronic structure of silver and copper clusters in view of their catalytic activity. The interaction of NO molecule with anionic, neutral and cationic silver and copper tetramers has been studied. NO adsorption on copper and silver surface leads to formation of (NO)2 and N2O molecules containing N–N bonds [3]. This is important for catalytic reduction of NO
V.E. Matulis, O.A. Ivaskevich / Computational Materials Science 35 (2006) 268–271
molecule. To investigate the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the interaction of copper–gold binary clusters with NO molecule has been studied and compared with the data obtained for copper–silver clusters.
2. Computational details All calculations of silver and copper clusters have been carried out within density functional theory (DFT). The previously developed S2LYP functional [1] and two basis sets have been used including ECP double-zeta type basis set termed as LANL2DZ [4] for silver, and ECP Stuttgart–Dresden relativistic basis set (SDD) [5] for copper clusters. The evaluation of energetic characteristics of interaction of NO molecule with silver and copper clusters, S2LYP functional and D95V(d) basis set for N and O atoms were used, whereas for Cu, Ag and Au atoms LANL2DZ basis set was applied. All calculations were performed using NWChem 4.1 package [6].
3. Results The calculated VDE, Te and binding energies per atom ðE b =nÞ for the structures of Ag210 and Cu210
269
which should correspond to the experimental ones are summarized in Table 1. As it can be seen from Refs. [1,2], the geometries of the most stable anionic copper and silver clusters are similar. The calculated values of photoelectron spectra (PHES) characteristics and trends of VDE and Te against cluster nuclearity are also very close (Table 1). This can be explained by the fact, that the PHES of anionic copper and silver clusters give the picture of electronic structure of the ns derived electronic states, and they are very close for both metals [2]. Table 1 shows that the calculated values of E b =n of copper clusters are larger than those of silver ones. An analysis of Mulliken overlap populations shows that contribution of d-electrons in overlap populations for Ag 2 is 0.0549 corresponding to 7.5% of all-electron overlap population (0.7323). At the same time, this value for Cu 2 is 0.0740 and amounts to 17.1% of all-electron overlap population (0.4329). This result can be explained by the fact that the molecular orbitals with large contribution of d-atomic orbitals lie considerably closer in energy to the HOMO for copper clusters than for silver ones. Thus, the highest MO with the large contribution of d-atomic orbitals for anionic copper dimer lies 2.55 eV below the SOMO, whereas the corresponding value for silver dimer is 4.24 eV. Therefore, one can assume that properties in which d-electrons play an important role should be different for silver and copper clusters. We have examined the similarity and the
Table 1 Calculated E b =n, VDE and Te and experimental values of VDE and Te for anionic silver and copper clusters, which should be observed in experimenta Cluster Ag 2 Ag 3 Ag 4 Ag 5 Ag 6 Ag 7 Ag 8 b Ag 9 ðIÞ Ag9 ðIIÞb Ag 10 Cu 2 Cu 3 Cu 4 Cu 5 Cu 6 b Cu 7 ðIÞ Cu7 ðIIÞb Cu 8 Cu 9 Cu 10 a b
Calculations
Experiment [7,8]
E b =n, eV
VDE, eV
VDE + Te, eV
VDE, eV
VDE + Te, eV
0.89 1.36 1.45 1.63 1.70 1.86 1.87 1.94 1.94 1.94 0.91 1.45 1.62 1.83 1.99 2.17 2.17 2.20 2.27 2.30
1.06 2.34 1.73 2.10 2.17 2.68 1.63 2.79 2.46 2.20 0.92 2.32 1.57 1.97 2.07 2.57 2.09 1.53 2.37 2.07
2.82 3.59 2.44 3.39 2.40 3.69 3.15 3.24 3.25 2.83 2.93 3.48 2.35 3.31 2.31 3.53 3.24 3.09 3.07 2.73
1.1 2.4 1.63 2.12 2.08 2.60–2.73 1.61 2.77 2.45 2.13 0.96 2.37 1.49 1.98 1.96 2.49–2.61 2.19 1.60 2.40 2.05
2.8 3.59 2.40 3.32 2.36 3.60 3.06–3.18 3.19 – 2.80 2.82 3.49 2.27 3.14 2.25 3.32 3.1–3.2 2.92–3.06 2.99 2.70
For the details of geometry see Refs. [1,2]. The numbers in parentheses give the rank in increasing energy order.
270
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difference in reactivity of silver and copper clusters on a model of their interaction with NO molecule. NO molecule can be considered both as electron donor and acceptor. The p-accepting properties are dominant in NO in contrast to CO molecule [9], but the general trends can be changed depending on the nature of species which reacts with NO. When NO reveals p-accepting properties in interaction with metals, partial charge transfer from metal AO to the NO p-antibonding orbital (which is symmetry allowed only if MeNO is bent) results in a long NO bond and a decreased NO stretch frequency compared to gaseous NO. In this case metal appears as electron donor. Therefore, the metals that easily donate electrons can promote this process. In contrast, if NO molecule interacts with species with high electron affinity (e.g. Cu+), it reveals property of electron donor [10]. In this case partial charge transfer from antibonding MO of NO to AO of metal results in a short NO bond and an increased NO stretch frequency compared to gaseous NO. To study this phenomenon, we have calculated the geometry and energetic characteristics of complexes of NO with silver and copper tetramers. The calculations of NO complexes with anionic tetramers as a model of species, which are easily donating electrons, and cationic tetramers as a model of species with high electron affinity have also been done. Tetramers were chosen as model systems because of very close structures of neutral, anionic and cationic clusters. Besides, in this case different sites of adsorption (terminal and bridged) can be considered. The results of calculations are given in Table 2. The optimized geometries of three most stable complexes of Ag4 with NO, as well as the surfaces of HOMO of Ag4, NO and Ag4–NO complexes are shown in Fig. 1. The structures are numbered in the order of increasing energy. The atop structures I and II have Cs symmetry while the bridged structure III has C2v symmetry. The optimized geometries of complexes of Cu4 with NO, as well as in the case of anionic and cationic copper and silver clusters are very close to those in Fig. 1.
Fig. 1. S2LYP optimized geometries of complexes of Ag4 with NO and the surfaces of HOMO of Ag4, NO and Ag4–NO complexes.
Table 2 shows that adsorption energy (Eads) of NO on copper tetramer and their ions considerably stronger than for silver ones. For both metals binding of NO molecule is increased in the range cation–neutral–anion. Reaction of NO with neutral clusters and especially with anions leads to a elongated NO bond and a decreased NO stretch frequency compared to gaseous NO (Table 2). On the other hand, reaction with cations results in a short NO bond and almost unchanged NO stretch frequency compared to gaseous NO. In contrast to binding energy, changing of NO stretch frequency and bond length are nearly the same for copper and silver clusters. To study the interaction of silver–copper clusters with NO relative to that of silver and copper one-component systems, the geometries and the energetic characteristics of complex of Cu2Ag2 with NO have been calculated. In addition, to investigate the influence of ‘‘ionic’’ versus ‘‘metallic’’ bonding on NO adsorption, the reactivity of Cu2Au2 cluster towards NO molecule has been studied and compared with the data obtained for copper and copper–silver clusters. The optimized geometries of Cu4, Cu2Ag2 and Cu2Au2 with atomic charges are shown
Table 2 S2LYP calculated properties of complexes of silver and copper tetramers with NO moleculea Silver
I II III I() II() III() I(+) II(+) III(+)
Copper
DE, eV
R(N–O)
0.00 0.40 0.49 0.00 0.45 0.68 0.00 0.22 0.11
1.163 1.170 1.166 1.219 1.208 1.198 1.145 1.142 1.160
˚ A
m(N–O), cm 1873.42 1827.67 1841.08 1630.71 1600.88 1637.35 2043.65 2062.14 1876.36
1
Eads, kJ/mol
DE, eV
˚ R(N–O) A
m(N–O), cm1
Eads, kJ/mol
85.88 47.53 38.36 122.56 79.77 57.86 77.36 56.02 66.91
0.00 0.38 0.60 0.00 0.42 0.76 0.00 0.28 0.022
1.168 1.180 1.203 1.222 1.211 1.215 1.150 1.149 1.171
1885.58 1656.55 1762.81 1668.38 1679.22 1626.50 2006.37 2021.53 1832.84
144.86 108.47 86.89 181.44 140.96 108.06 121.14 94.40 123.26
Anionic clusters are noted () and cationic clusters (+). a S2LYP/D95V(d) calculated for NO molecule v(N–O) and R(N–O) values are 2076.82 cm1 and 1.151, respectively.
V.E. Matulis, O.A. Ivaskevich / Computational Materials Science 35 (2006) 268–271
Fig. 2. S2LYP/LANL2DZ optimized geometries of Cu4, Cu2Ag2 and Cu2Au2 with atomic charges.
in Fig. 2. We have considered complexes of binary systems with NO like structure I in Fig. 1 only. Since VDEs of Ag and Cu are similar, the bonding in Cu2Ag2 does not contain ionic contribution. Therefore, it is expected that interaction of Cu2Ag2 cluster with NO molecule should be similar to that of onecomponent systems. Indeed, the calculated values of ˚ ) and m(N–O) Eads (137.25 kJ/mol), R(N–O) (1.166 A 1 (1901.00 cm ) for complex of Cu2Ag2 with NO are very close to corresponding values for structure I of copper (Table 2). Due to slightly larger electron affinity of silver, copper atom have larger positive charge in binary copper–silver cluster than in the case of copper tetramer (Fig. 2), which leads to slightly less charge transfer from copper to NO and, as a consequence, to less adsorption energy and increased NO stretch frequency. On the other hand, the bonding in Cu2Au2 is enhanced by an ionic contribution owing to considerably larger electron affinity of Au atom. Thus, copper has large positive charge in Cu2Au2 cluster (Fig. 2). The calculated values ˚ ) and m(N–O) of Eads (114.81 kJ/mol), R(N–O) (1.156 A 1 (1991.99 cm ) for complex of Cu2Au2 cluster with NO are very close to corresponding values for structure I(+) of copper (Table 2).
4. Conclusions According to our calculations, the most stable structures of anionic silver and copper clusters have very similar geometry. Calculated characteristics of PHES are also close for silver and copper clusters. At the same time, properties in which d-electrons play an important
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role should be different for silver and copper clusters. Calculations of interaction of cationic, neutral and anionic silver and copper tetramers, Cu2Ag2 and Cu2Au2 with NO molecule show that the geometry and energetic characteristics of these systems should be determined mainly by two factors. On the one hand, it is the ability of metal to donate or add electrons, which should be depended on total atomic charge of interacting atom and the nature of metal. It is clearly followed from the comparison of the calculated characteristics of com plexes of Cuþ 4 , Cu4 , Cu4, Cu2Ag2 and Cu2Au2 with NO molecule. It has been shown that interaction of NO with silver and copper anions leads to a considerable lengthening of NO bond, that is important for the reaction of NO dissociation. On the other hand, d-electrons play an important role in the adsorption and sufficiently influence the binding energy. Thus, in spite of similar electron affinity, copper clusters interact with NO considerably stronger than silver ones. However, the geometrical characteristics, such as NO bond length changing, are determined by charge transfer and mainly depend on VDE, ionization potential and atomic charges.
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