SAPO-11 catalysts and

Journal of Computational Methods in Sciences and Engineering 12 (2012) 391–396 DOI 10.3233/JCM-2012-0427 IOS Press

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Theoretical study of small clusters Au3−4 on Au/SAPO-11 catalysts and their interactions with CO Beulah Griffe∗ , Joaquín L. Brito and Anibal Sierraalta

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Labs. Química Computacional y Fisicoquímica de Superficies, Centro de Química, Caracas, Venezuela

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Received 24 May 2011 Accepted /Revision 10 October 2011

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Abstract. Quantum chemistry calculations were done, using the ONIOM2 methodology at two different levels of calculation, B3LYP for the high level and UFF for the low level. These calculations were performed on Au3 /SAPO-11, Au4 /SAPO-11, CO-Au3 /SAPO-11 and CO-Au4 /SAPO-11 aggregates to analyze the geometries of small clusters of Au3 and Au4 on SAPO-11 support. Au3 cluster present a triangle structure in Au3 /SAPO-11. Au4 cluster shows a “Y shaped” structure in Au4 /SAPO-11. Au4 as a rhombus structure is also studied but it is an unstable intermediate to the “Y shaped” structure. The CO interaction with Au3 and Au4 /SAPO-11 is studied, this CO adsorption is different from reported in the literature. The formation energy ΔEF of the aggregates and the CO adsorption energy ΔEads on them are presented. Keywords: ONIOM, quantum chemistry, theoretical calculations, Au-aggregates

1. Introduction

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Nowadays, many theoretical and experimental studies of structural and electronic properties of gold clusters have been carried out. This fact is due to the interest of using gold clusters in electronic devices, as nanomaterials and catalysts among others [1]. Some authors [1–3] have published the most stable structures of gold clusters containing 2 to 20 Au atoms. Dong and Springborg [1] found that for small gold clusters up to 6 atoms the structures of the lowest total energy was planar. For larger clusters, truly three-dimensional structures are found to be more stable but for clusters with up to 15 atoms, planar structures were found to lie very close in energy to those found for three-dimensional ones. Wang et al. [2] investigated the lowest-energy structures and electronic properties of the Aun (n = 2–20) clusters. They found that the small Aun clusters adopt planar structures up to n = 6. Flat cage structures are preferred in the range of n = 10–14 and a structural transition from flat-cage-like structure to compact near-spherical structure is found around n = 15. Yang et al. [3] reported that the polarizabilities of the 2D clusters are strongly anisotropic. It is found that the polarizabilities of the 2D cluster in the x and y directions (the lateral directions) are much bigger than the one in the z direction (the normal direction) and display metallic dielectric properties. In ∗ Corresponding author: Beulah Griffe, Labs. Química Computacional y Fisicoquímica de Superficies, Centro de Química, IVIC, Apdo Postal 20632 Caracas 1020-A, Venezuela. E-mail: [email protected].

c 2012 – IOS Press and the authors. All rights reserved 1472-7978/12/$27.50 

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392 B. Griffe et al. / Theoretical study of small clusters Au3−4 on Au/SAPO-11 catalysts and their interactions with CO

Fig. 2. CO-Au3 /SAPO-11.

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Fig. 1. Au3 /SAPO-11.

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the normal direction, the polarizability has the same magnitude as that of a semiconductor, displaying nonmetallic properties. A fitting formula is proposed to estimate the polarizability of a 2D Au cluster with arbitrary size and shape in the normal direction. Häkkinen and Landman [4] studied neutral and anionic Au clusters and published that the ground state optimal geometries of the neutral and anionic clusters are found to be planar up to n = 7 and 6, respectively, with the Au− 6 cluster predicted to have a planar triangular D3h geometry. Wang et al. [5] studied Aun clusters with n < 14. The energetic and electronic properties of the small gold clusters are strongly dependent on sizes and structures, which are in good agreement with experiment and other theoretical calculations. In the past decade, many experimental and theoretical studies for the CO oxidation have been committed to unveiling the origin of the catalytic activity of Au nanoparticles. Ding et al. [6] have performed a detailed theoretical study for the CO oxidation promoted by cationic, neutral, and anionic Au trimers. The theoretical results support the experimental observations that all Au species with different charge states show the catalytic activity toward CO oxidation, as pointed out by calculated low energy barriers and high exorthemicities. This fact indicates that the charge state of Au has substantial effect on the elementary mechanisms but plays a less important role for its catalytic activity toward CO oxidation. Previously to the oxidation reaction, the adsorption of CO on Au clusters should be studied. The objective of this work is to present a preliminary theoretical study of small clusters on Au3−4 supported on silico-aluminophospates, SAPO-11 employing ONIOM2 methodology and the CO interaction with them. 2. Computational details and models

All geometry optimizations and energy calculations were performed using the Gaussian-03 program [7]. The lower energy structures were obtained using the two-layer ONIOM2 methodology. For the high level, (represented by spheres in Fig. 1), DFT approach (B3LYP) with the full-electron 3-21G* basis set for H, C and O and LANL2DZ with its pseudo potentials for the rest of atoms of the SAPO11 ring, Si, Al, P, composed by 6 tetrahedral was employed. All adsorbed and reactive molecules were included in the high level. Universal force field approach (UFF) was employed for the low level (represented by sticks in Fig. 1). H atoms were used as boundary atoms in the ONIOM2 calculations. The

Fig. 3. CO-Au3 /SAPO-11, CO adsorbed on Au3 triangle moiety.

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Fig. 4. Au4 /SAPO-11 with a “Y-shaped” Au4 structure.

3. Results and discussion

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silicoalumino-phospates molecular sieves (SAPO) model comprises a total of 506 atoms and was already described in previous works [8–10].

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Figure 1 displays the Au3 /SAPO-11 minimum energy structure. The [Au3 ] moiety presents triangle geometry in agreement with previous published results [1,2,5,11]. An acute triangle of Au3 was obtained, with bond lengths of 2.81, 2.65 and 2.61 Å and angles of 64.6, 58.5 and 57.0◦. This result is in agreement with the results reported by Wang et al. [2] that show that both the obtuse and acute triangles of Au3 are more stable than equilateral triangle. The formation energy ΔEF (Au3 ) was calculated according to the following reaction:

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Au2 /SAPO-11 + Au −→ Au3 /SAPO-11

(1)

ΔEF (Au3 ) = E Au3/SAPO-11 −(EAu2-SAPO-11 + EAu ) where E Au3/SAPO-11 , EAu2-SAPO-11 , and EAu are the total ONIOM energies of Au3 /SAPO-11 aggregate, Au2 /SAPO-11 aggregate and the Au atom [10]. Our results show a ΔE value of −71.9 kcal/mol. This fact indicates that the Au3 /SAPO-11 formation process from Au2 /SAPO-11 is exothermic and energetically favored. All three Au atoms interact with the support, having bond distances of 2.21, 2.30 and 2.31 Å from the oxygens of the support. Figure 2 displays the CO interaction with Au3 /SAPO-11. The CO adsorption reaction presents an energy difference of ΔEads (CO-Au3 ) = −23.8 kcal/mol. The ΔEads was calculated according to the following reaction:

Au3 /SAPO-11 + CO −→ CO-Au3 /SAPO-11

(2)

Fig. 5. Au4 /SAPO-11 with a rhombus Au4 structure.

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Fig. 6. CO-Au4 /SAPO-11 (Au4 /SAPO-11 with “Y-shaped” Au4 ).

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The CO-Au3 /SAPO-11 exhibits a 3D trigonal pyramidal distorted conformation, being the CO at an apex on top and the Au atoms in the base (see Fig. 2), the CO-Au bond distances being 2.23, 2.22 and 2.06 Å. It has been reported that CO molecule prefers to adsorb in an “atop” manner (on a single Au atom of Au3 cluster) and that the ground state CO-Au3 cluster geometries are planar [11,13]. Although in Au3 /SAPO-11 the Au3 moiety exhibits a planar triangular geometry, the CO adsorption is produced in a non planar way to give CO-Au3 /SAPO-11. Another CO-Au3 /SAPO-11 geometry with the Au3 triangle moiety is obtained as an unstable intermediate structure as shown in Fig. 3 (with an energy difference of ΔEads (CO-Au3 ) = − 3.6 kcal/mol). This fact could be explained as to be due to the influence of the pore structure of the SAPO-11 and that the Au3 trimer attaches to the support. This Au3 bonding to the support could obstruct the CO interaction with an Au at an apex in a planar position and leads to the 3D structure (see Fig. 2). Fernandez et al. [14,15] pointed out that the adsorption of CO occurs on top of the least coordinated Au atom, except for Au5 and Au7 where the bridge position is preferred, in our case all three Au atoms are interacting with the substrate. Figure 4 illustrates the Au4 /SAPO-11 aggregate showing a non-planar “Y-shaped” structure [2] as the lowest-energy structure. Other Au4 cluster structures have been published such as a planar rhombus [1, 2,5,11] and a three-dimensional 3D tetrahedron [2,5]. A rhombus structure was also studied by us (see Fig. 5), but as a less stable structure intermediate to the Y-shaped. The Y-shaped Au4 cluster bond lengths are, 2.70, 2.73, 2.75 and 2.77 Å. The angles are 59.1, 60.1, 60.6 and 64.8◦ showing a distorted geometry. The formation of Au4 cluster on SAPO-11 is obtained with an energy difference of ΔEF (Au4 ) = −61.5 kcal/mol and was calculated according to the following reaction: Au3 /SAPO-11 + Au −→ Au4 /SAPO-11

(3)

Three of Au atoms interact with the support, at Au-O bond length of 2.13; 2.24 and 2.17 Å, being the fourth Au-O bond distance of 3.41 Å. Although for Au neutral cluster, a planar rhombus structure has been reported by several authors as the lowest-energy structure [1,2,5,11], in the case of Au4 /SAPO-11

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Fig. 7. CO-Au4 /SAPO-11 (Au4 /SAPO-11 with rhombus Au4 structure).

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the Au4 presents a degenerate Y-shaped structure due probably to the SAPO-11 pore structure and the interaction of three of four Au atoms with the support as already mentioned above with CO-Au3 /SAPO11. This Y-shaped structure has been reported as neutral species [1,2,5] as well as anionic species [4]. In Fig. 6 the CO interaction with Au4 /SAPO-11 is depicted. The CO adsorption energy in Au4 /SAPO11 is ΔEads (CO-Au4 ) = − 29.9 kcal/mol. This ΔE was calculated according to the reaction 4: Au4 /SAPO-11 + CO −→ CO-Au4 /SAPO-11

(4)

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In the CO-Au4 /SAPO-11 (Fig. 6) an Au3 triangle structure and Au-CO (1.90 Å) are observed. In fact the Y shaped form is kept but with an enlarged distance Au-Au to 3.34◦. The Au3 triangle bond lengths are 2.54, 2.70 and 2.77 Å and the angles are 64.0, 54.0 and 61.0◦. All four Au atoms interact with the support at Au-O bond distances of 2.13, 2.12, 2.12, and 2.42 Å. The Au-CO species interact with the support (2.12 Å) but the Au bond length with the Au triangle is larger 3.34 Å. The CO interaction with Au splits it from Au4 . In Fig. 7 it is shown CO adsorbed on Au4 /SAPO-11 with a rhombus Au4 structure obtaining the CO absorbed on a rhombus distorted structure (with an energy difference of ΔEads (CO-Au3 ) = −26.1 kcal/mol). This is possible approaching CO to the Au4 /SAPO-11 in a longitudinal way of the pore, avoiding being too close to the walls of the pore. The Au-Au bond lengths in the Au3 triangle are 2.65, 2.76 and 2.76 Å, and the angles are 61.3, 57.4 and 61.4◦. Au-C is 2.00 Å. The fourth Au forms a rhombus structure but with elongated distances. Thomson et al. [16] performed two-layer QM/MM calculations to investigate CO adsorption on Au1−5 clusters adsorbed on nondefect and metal-vacancy defect versions of T6-Ti and T6-Si sites inside TS-1 pore. They reported that the CO molecule prefers to adsorb in atop manner on Au1−4 clusters inside the TS-1 pores, while it prefers to adsorb in a bridge manner on Au5 (except on Au5 /Ti-defect). All the cluster-CO geometries are planar. The adsorbed CO is significantly anionic, and the corresponding elongation of the C-O bond suggests that this bond is activated upon adsorption. Thomson et al. [16] states that the CO adsorption on Au3 and Au4 are attained on an apex of triangle and planar rhombus respectively. The same is reported by Wu [11] for neutral pure clusters. Nevertheless

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in this study of Au3 and Au4 clusters supported on SAPO-11, different results are achieved. Non-planar geometries are obtained probably due to the influence of the pore structure, that doesn’t permit planar structures, and the interaction of the cluster with the support. Wu et al. [11] use small non supported clusters and Thomson et al. [16] employ Au1−4 clusters inside the TS-1 pores, with different Au interactions with Ti and Si. These results are preliminary and this study has to be completed and deepened. 4. Conclusions

References

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Y. Dong and M. Springborg, Eur Phys J 43 (2007), 15. J.L. Wang, G.H. Wang and J.J. Zhao, Phys Rev B 66 (2002), 035418. J. Zhao, J.Yang and J.G. Hou, Phys Rev B 67 (2003), 085404. H. Häkkinen and U. Landman, Phys Rev B 62 (2000), R2287. X.B. Li, H.Y. Wang, X.D. Yang, Z.H. Zhu and Y.J. Tang, J Chem Phys 126 (2007), 084505. F. Wang, D. Zhang, X. Xu and Y. Ding, J Phys Chem C 113 (2009), 18032. Gaussian 03, Revisión B.04, Gaussian Inc., Pittsburg, PA, 2003. A. Sierraalta, R. Añez and E. Ehrmann, J Mol Catal A Chem 271 (2007), 185. B. Griffe, A. Sierraalta and J.L. Brito, J Comput Met Sci Eng 9 (2009), 281. B. Griffe, J.L. Brito and A. Sierraalta, J Mol Catal A Chemical 315 (2010), 28–34. X. Wu, L. Senapati, S.K. Nayak, A. Selloni and M. Hajaligol, J Chem Phys 117 (2002), 4010. B. Griffe, A. Sierralta and J. Brito, International Journal of Quantum Chemistry 110 (2010), 2573. A.M. Joshi, M.H. Tucker, W.N. Delgass and K.T. Thomson, J Chem Phys 125 (2006), 194707. E.M. Fernandez, P. Ordejón and L.C. Balbás, Chem Phys Lett 408 (2005), 252. E.M. Fernandez and L.C. Balbás, J Phys Chem B 110 (2006), 10449. A.M. Joshi, W.N. Delgass and K.T. Thomson, J Phys Chem C 111 (2007), 11424.

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[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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– For the Au3 /SAPO-11 aggregate it is obtained ΔEF = − 71.9 kcal/mol, suggesting that this is a stable structure. – The [Au3 ] moiety on the SAPO-11 support has a triangle structure. – The Au4 /SAPO-11 aggregate is also formed as a stable structure with a ΔEF = − 61.5 kcal/mol. – The [Au4 ] moiety on the SAPO-11 shows a “Y shaped” structure. – The CO molecule interacts with the Au3 /SAPO-11 aggregate with a ΔEads (CO-Au3 ) = −23.8 kcal/mol exhibiting a distorted 3D trigonal pyramidal conformation, being the CO at an apex on top and the Au atoms in the triangle base. – CO adsorption is also obtained on the Au4 /SAPO-11 “Y-shaped” aggregate with a ΔEads (CO-Au4 ) = −29.9 kcal/mol. – CO adsorption on the less stable aggregate of rhombus distorted structure Au4 /SAPO-11 is achieved with a ΔEads (CO-Au4 ) = −26.1 kcal/mol and maintaining the same structure.