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The Effect of Gamma-Al2O3 Support on the NO Adsorption on Pd4 Cluster

Abstract

The effect of γ-Al2O3 support on the NO adsorption on Pd4 clusters was investigated by means of density functional theory (DFT) calculations. Pd4 adsorbed on γ-Al2O3 (represented by a Al14O24H6 cluster) changes its preferential geometry from tetrahedral to a distorted planar structure. The alumina support promotes a higher dispersion in the palladium catalyst and reduces the NO adsorption energy to -25.6 kcal mol-1 (computed at B3LYP/LANL2DZ/6-311+G(d)), in close agreement with the experimental value of -27.2 kcal mol-1. On the bare planar Pd4 cluster the NO molecule adsorbs in a bridge arrangement, with adsorption energy of -41.2 kcal mol-1. Adsorption on the tetrahedral Pd4 cluster occur preferentially in an atop mode, with adsorption energy of -30.6 kcal mol-1. Charge density analysis show that the electron flux between the NO molecule and Pd4 depends on the adsorption form, with back-donation being stronger in the bridge adsorption mode.

Keywords:
Pd clusters; DFT; supported-Pd clusters; alumina; NO adsorption; back-donation


Introduction

Palladium catalysts have found wide appllication in several areas of chemistry.11 Chinchilla, R.; Nájera, C.; Chem. Rev. 2014, 114, 1783.

2 Liao, F.; Lo, T. W. B.; Tsang, S. C. E.; ChemCatChem 2015, 7, 1998.
-33 Saldan, I.; Semenyuk, Y.; Marchuk, I.; Reshetnyak, O.; J. Mater. Sci. 2015, 50, 2337. One of the most common and established use of palladium catalyst is to control automobile exhaust gas emission.44 Bagot, P. A. J.; Mater. Sci. Technol. 2004, 20, 679.

5 Belton, D. N.; Taylor, K. C.; Curr. Opin. Solid State Mater. Sci. 1999, 4, 97.
-66 Heck, R. M.; Farrauto, R. J.; Appl. Catal., A 2001, 221, 443. Use of catalytic converter in automobile exhaust is a requirement to reach the recommended limits for emission of toxic byproducts. Although studies have been dedicated to the understanding of the catalytic conversion process in the exhaust emission,77 Ozensoy, E.; Hess, C.; Goodman, D. W.; Top. Catal. 2004, 28, 13. more fundamental studies are still necessary for a full comprehension of the catalytic process, particularly at the molecular level.88 Zaera, F.; Chem. Soc. Rev. 2014, 43, 7624.,99 Chen, X.; Cheng, Y.; Seo, C. Y.; Schwank, J. W.; McCabe, R. W.; Appl. Catal., B 2015, 163, 499.

Commonly, the platinum and palladium catalysts are supported on metal oxides, among them γ-Al2O3, although oxides of cerium, zirconium and titanium are also frequently employed.1010 Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M.; J. Mol. Catal. A: Chem. 2001, 173, 3.

11 Agostini, G.; Groppo, E.; Piovano, A.; Pellegrini, R.; Leofanti, G.; Lamberti, C.; Langmuir 2010, 26, 11204.
-1212 Lan, L.; Chen, S.; Cao, Y.; Gong, M.; Chen, Y.; Catal. Sci. Technol. 2015, 5, 4488. These supported catalysts are thus employed to remove carbon monoxide (CO), nitric oxide (NO), NOx and SOx contaminants, among others, from the exhaust engine.44 Bagot, P. A. J.; Mater. Sci. Technol. 2004, 20, 679.

5 Belton, D. N.; Taylor, K. C.; Curr. Opin. Solid State Mater. Sci. 1999, 4, 97.
-66 Heck, R. M.; Farrauto, R. J.; Appl. Catal., A 2001, 221, 443.

Nitric oxide has an odd number of electrons and is one of the most versatile molecules, making its chemistry and adsorption process of particular interest.1313 Koshland Jr., D. E.; Science 1992, 258, 1861. The adsorption of NO on the supported catalyst surface has been investigated by both experimental77 Ozensoy, E.; Hess, C.; Goodman, D. W.; Top. Catal. 2004, 28, 13.,1414 Weiss, B. M.; Iglesia, E.; J. Catal. 2010, 272, 74.

15 Hu, Y.; Griffiths, K.; Norton, P. R.; Surf. Sci. 2009, 603, 1740.

16 Hungría, A. B.; Fernández-García, M.; Anderson, J. A.; Martínez-Arias, A.; J. Catal. 2005, 235, 262.

17 Neyertz, M.; Volpe, M.; Perez, D.; Costilla, I.; Sanchez, M.; Gigola, C.; Appl. Catal., A 2009, 368, 146.

18 Wang, C.-B.; Yeh, T.-F.; Lin, H.-K.; J. Hazard. Mater. 2002, 92, 241.

19 Auvray, X.; Olsson, L.; Appl. Catal., B 2015, 168-169, 342.
-2020 Kaneeda, M.; Iizuka, H.; Hiratsuka, T.; Shinotsuka, N.; Arai, M.; Appl. Catal., B 2009, 90, 564. and theoretical procedures.2121 Begum, P.; Gogoi, P.; Mishra, B. K.; Deka, R. C.; Int. J. Quantum Chem. 2015, 115, 837.

22 Piotrowski, M. J.; Piquini, P.; Zeng, Z.; da Silva, J. L. F.; J. Phys. Chem. C 2012, 116, 20540.

23 Lacaze-Dufaure, C.; Roques, J.; Mijoule, C.; Sicilia, E.; Russo, N.; Alexiev, V.; Mineva, T.; J. Mol. Catal. A: Chem. 2011, 341, 28.

24 Grybos, R.; Benco, L.; Bučko, T.; Hafner, J.; J. Comput. Chem. 2009, 30, 1910.

25 Grybos, R.; Benco, L.; Bučko, T.; Hafner, J.; J. Chem. Phys. 2009, 130, 104503.

26 Geng, L.; Han, L.; Cen, W.; Wang, J.; Chang, L.; Kong, D.; Feng, G.; Appl. Surf. Sci. 2014, 321, 30.

27 Duarte, H. A.; Salahub, D. R.; Top. Catal. 1999, 9, 123.

28 Bertin, V.; delAngel, G.; Mora, M. A.; Poulain, E.; Olvera, O.; López-Rendón, R.; J. Mex. Chem. Soc. 2008, 52, 93.
-2929 Zeng, Z.-H.; da Silva, J. L. F.; Li, W.-X.; Phys. Chem. Chem. Phys. 2010, 12, 2459. NO adsorbs on small Pd clusters through its nitrogen atom in a tilted orientation, preferentially in the hcp threefold and bridge sites,2727 Duarte, H. A.; Salahub, D. R.; Top. Catal. 1999, 9, 123. although the relative binding energies are strongly dependent on the size and geometry of the cluster employed.2727 Duarte, H. A.; Salahub, D. R.; Top. Catal. 1999, 9, 123. For adsorption of NO on an extended surface at low coverage the interaction is determined by electron donation and back-donation involving the 5σ/2π* antibonding orbital of the NO molecule and the d-bands of the transition metal, with a net charge transfer from the transition metal to the adsorbed NO.2929 Zeng, Z.-H.; da Silva, J. L. F.; Li, W.-X.; Phys. Chem. Chem. Phys. 2010, 12, 2459. However, the extent of the interaction via the back-donation process is strongly dependent on the coordination mode of the NO molecule. On a flat surface, the preferential adsorption occurs on a hollow-site.2929 Zeng, Z.-H.; da Silva, J. L. F.; Li, W.-X.; Phys. Chem. Chem. Phys. 2010, 12, 2459. Adsorption on the energetically less favorable top-site results in tilted orientation with the NO bent to the metal surface.

In the vast literature on theoretical studies of NO adsorption on metal surfaces, the catalyst is frequently represented as a single crystal2929 Zeng, Z.-H.; da Silva, J. L. F.; Li, W.-X.; Phys. Chem. Chem. Phys. 2010, 12, 2459. or as a small atomic cluster,2121 Begum, P.; Gogoi, P.; Mishra, B. K.; Deka, R. C.; Int. J. Quantum Chem. 2015, 115, 837.

22 Piotrowski, M. J.; Piquini, P.; Zeng, Z.; da Silva, J. L. F.; J. Phys. Chem. C 2012, 116, 20540.

23 Lacaze-Dufaure, C.; Roques, J.; Mijoule, C.; Sicilia, E.; Russo, N.; Alexiev, V.; Mineva, T.; J. Mol. Catal. A: Chem. 2011, 341, 28.
-2424 Grybos, R.; Benco, L.; Bučko, T.; Hafner, J.; J. Comput. Chem. 2009, 30, 1910.,2727 Duarte, H. A.; Salahub, D. R.; Top. Catal. 1999, 9, 123.,2828 Bertin, V.; delAngel, G.; Mora, M. A.; Poulain, E.; Olvera, O.; López-Rendón, R.; J. Mex. Chem. Soc. 2008, 52, 93. although evidence shows that the support plays a relevant role in the reactivity of the supported catalyst.1010 Blaser, H.-U.; Indolese, A.; Schnyder, A.; Steiner, H.; Studer, M.; J. Mol. Catal. A: Chem. 2001, 173, 3.

11 Agostini, G.; Groppo, E.; Piovano, A.; Pellegrini, R.; Leofanti, G.; Lamberti, C.; Langmuir 2010, 26, 11204.
-1212 Lan, L.; Chen, S.; Cao, Y.; Gong, M.; Chen, Y.; Catal. Sci. Technol. 2015, 5, 4488.,1818 Wang, C.-B.; Yeh, T.-F.; Lin, H.-K.; J. Hazard. Mater. 2002, 92, 241.

19 Auvray, X.; Olsson, L.; Appl. Catal., B 2015, 168-169, 342.
-2020 Kaneeda, M.; Iizuka, H.; Hiratsuka, T.; Shinotsuka, N.; Arai, M.; Appl. Catal., B 2009, 90, 564.,2525 Grybos, R.; Benco, L.; Bučko, T.; Hafner, J.; J. Chem. Phys. 2009, 130, 104503.,2626 Geng, L.; Han, L.; Cen, W.; Wang, J.; Chang, L.; Kong, D.; Feng, G.; Appl. Surf. Sci. 2014, 321, 30. The oxidation of Pd nanoparticles is improved in the presence of the γ-Al2O3 support.3030 Kacprzak, K. A.; Czekaj, I.; Mantzaras, J.; Phys. Chem. Chem. Phys. 2012, 14, 10243.,3131 Valero, M. C.; Raybaud, P.; Sautet, P.; J. Catal. 2007, 247, 339.

In the supported catalyst, the size of the metal particles may vary to some degree. Dispersed clusters are usually formed by small number of atoms. Extended X-ray absorption fine structure (EXAFS) analysis of a fresh platinum catalyst supported on Al2O3 indicated an average Pt-Pt coordination number of 2.6 ± 0.4, suggesting a platinum cluster size of 3 to 4 atoms dispersed on the γ-alumina surface.3232 Chang, J. R.; Chang, S.-L.; Lin, T. B.; J. Catal. 1997, 169, 338. Therefore, simulation with a small cluster of atoms considering the presence of the support should grasp the main features of the real system.

The structure of γ-Al2O3 is characterized by the presence of tetrahedral and octahedral aluminum atoms. In a study employing density functional theory (DFT) to compute the adsorption of formaldehyde on palladium supported on γ-Al2O3 we showed that the Pd-Al2O3 interactions result in charge transfer from octahedral aluminum atoms to the deposited Pd cluster.3333 Carneiro, J. W. M.; Cruz, M. T. M.; J. Phys. Chem. A 2008, 112, 8929. The tetrahedral aluminum site has high acididity, acting as electron acceptors, while octahedral aluminum sites act as electron donors.3333 Carneiro, J. W. M.; Cruz, M. T. M.; J. Phys. Chem. A 2008, 112, 8929. Therefore, it was possible to establish the electron flux between the γ-Al2O3 support and the metal cluster.

In the present study we return to the discussion on the the adsorption properties of small palladium clusters supported on γ-Al2O3. We employed the DFT B3LYP approach to analyze the energies for adsorption of nitric oxide on a Pd4 cluster supported on a model for γ-Al2O3. The goal was to quantify geometric, electronic and energetic parameters involved in the adsorption of NO on the palladium clusters and evaluate the effect of the support γ-Al2O3 on the adsorption process.

Methodology

In the present work we report data for adsorption of NO (Figure 1a) on planar (Figure 1b) and tetrahedral (Figure 1c) Pd4 clusters and on the Pd4 cluster supported on a model for the (110) surface of γ-Al2O3 (Figure 1d).

Figure 1
Structures of the (a) nitric oxide (NO) molecule; (b) planar Pd4 cluster; (c) tetrahedral Pd4 cluster; (d) general representation of γ-Al2O3; (e) Al14O24H6 unit used to represent the γ-Al2O3 support; and (f) lateral and (g) superior views of Pd4 optimized over the Al14O24H6 unit. Distances are given in Å, Alt is for tetrahedral aluminum atoms and Alo is for octahedral aluminum atoms.

The model for the γ-Al2O3 surface was obtained from the unit cell of γ-Al2O3 by cutting a slice in the (110) direction of the unit cell. The resulting surface exposes both tetrahedral and octahedral aluminum atoms, as well as oxide anions, to external interactions, in addition to being the most stable face for γ-alumina.3434 Márquez, A. M.; Sanz, J. F.; Appl. Surf. Sci. 2004, 238, 82.,3535 Alvarez, L. J.; Sanz, J. F.; Capitán, M. J.; Centeno, M. A.; Odriozola, J. A.; J. Chem. Soc., Faraday Trans. 1993, 89, 3623. Due to computational cost, the γ-Al2O3 model was restricted to three sheets of atoms containing a total of 14 aluminum and 24 oxygen atoms. To balance charge and yield a neutral model, 6 hydrogen atoms were added to terminal oxygens, leading to a final model with Al14O24H6 stoichiometry. In the γ-Al2O3 model, each oxygen atom is coordinated to at least two aluminum (or one aluminum and a hydrogen, in the case of the terminal oxygen atoms) and each aluminum is coordinated to at least three oxygen atoms, therefore reducing any border effect. Experimental distances and angles were used to construct the γ-Al2O3 model.3636 Wyckoff, R. W. G.; Crystal Structures; John Wiley Interscience Publishers: New York, 1968.

On the (110) face of the γ-alumina we deposited the Pd4 cluster, starting from a planar geometry for the Pd4 moiety and Pd-Pd distances of 2.751 Å (Figure 1b), with the palladium atoms interacting with both tetrahedral and octahedral aluminum atoms. The Pd4 cluster was then optimized on the γ-Al2O3 surface, leading to a distorted geometry (Figures 1f and 1g). Considering that tetrahedral Pd4 cluster has a triplet ground state3333 Carneiro, J. W. M.; Cruz, M. T. M.; J. Phys. Chem. A 2008, 112, 8929. and that the γ-Al2O3 model has a singlet ground state,3737 The electronic ground state computed for AlxOy cluster is dependent on the AlxOy arrangement. However, computations of single-point MP2/6-31g(d) relative energies (not reported) confirm the singlet state as the ground state for all AlxOy forms. all calculation involving the Pd4 deposited on the γ-Al2O3 was carried out in the total triplet electronic state, using the unrestricted formalism (see Table S1). For all unrestricted calculations the stability of the final wave function was tested and the adsorption energies were computed using the most stable wave function in each case.

After having prepared the Pd4/γ-Al2O3 aggregate for adsorption, the NO molecule was optimized in several orientations over the Pd4/Al14O24H6 cluster. Initially, the calculations were performed relaxing only the NO distance and orientation, while keeping the positions of the atoms in the Pd4/Al14O24H6 cluster in their original positions. In the final step, both the NO molecule and the Pd4 unit were allowed to relax.

To have a reference for the effect of the γ-alumina surface on the NO adsorption energy, we also computed the NO adsorption on a naked Pd4 cluster. The NO molecule was optimized on the following three clusters, all containing four palladium atoms, each in different arrangements: (i ) the planar arrangement in D2h rhombohedral symmetry (Figure 1b); (ii ) the tetrahedral arrangement with Td symmetry (Figure 1c); and (iii ) the distorted arrangement obtained in the previous optimization of Pd4 on the Al14O24H6 cluster. In these cases only the NO molecule was allowed to relax during the optimization process, whereas the palladium atoms were kept fixed in their original positions. Several adsorption modes for the NO molecule on Pd4 were simulated: atop, bridge, hollow, di-σ and π, as shown in Figure 2. When possible, symmetry was imposed.

Figure 2
Different positions for adsorption of nitric oxide on the palladium (111) surface. Atop (O) and atop (N) are for adsorption of NO on a specific palladium atom, via the oxygen or the nitrogen atom, respectively. Bridge (O) and bridge (N) are for adsorption of NO on a bridge position, via the oxygen or the nitrogen atom, respectively. Hollow (O) and hollow (N) are for NO adsorption on the hollow positions, via the oxygen or the nitrogen atom, respectively. Di-σ and π are for adsorption in parallel orientations, as shown in the figure.

The NO adsorption energies were computed by summing the energies of the NO molecule and of the Pd4 or Pd4/Al14O24H6 cluster and subtracting the result from the energy of the NO/Pd4 or NO/Pd4/Al14O24H6 complex, according to equation 1.

(1)

The adsorption energies were subjected to correction due to the basis set superposition error (BSSE),3838 van Duijneveldt, F. B.; van Duijneveldt-van de Rijdt, J. G. C. M.; van Lenthe, J. H.; Chem. Rev. 1994, 94, 1873.,3939 Lopes, J. F.; Rocha, W. R.; dos Santos, H. F.; de Almeida , W. B.; J. Braz. Chem. Soc. 2010, 21, 887. using equations 2 and 3.

(2)
(3)

In these equations Ead/corr. is the BSSE corrected adsorption energy; A is the cluster (Pd4 or Pd4/Al14O24H6) and B is the NO molecule; E(AB) is the energy of the NO/Pd4 or the NO/Pd4/Al14O24H6 complexes; E(AB) and E(AB) are the absolute energies of the NO/Pd4 or the NO/Pd4/Al14O24H6 complexes, considering the nuclei of B and A, respectively, as absent; Edef is the deformation energy of NO, computed from the difference in the energy of NO in the geometry of the complex (EB(AB)) and in the fully optimized geometry (E(B)).

All calculations were carried out using the Gaussian 03 computational package,4040 Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.; Gaussian 03, Revision B.04; Gaussian, Inc., Pittsburgh, 2003. with the B3LYP hybrid functional as proposed and parameterized by Becke.4141 Becke, A. D.; J. Chem. Phys. 1992, 96, 2155. This is a mixture of Hartree-Fock and DFT exchange terms with the gradient-corrected correlation functional of Lee et al.4242 Lee, C.; Yang, W.; Parr, R. G.; Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785. The palladium atoms and the γ-alumina atoms were described by the LANL2DZ pseudopotential4343 Hay, P. J.; Wadt, W. R.; J. Chem. Phys. 1985, 82, 270. and the D95V4444 Dunning Jr., T. H.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer III, H. F., ed.; Plenum: New York, 1976. basis set for the valence electrons. The NO molecule was described by the 6-311+G(d) basis set.4545 Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. R.; J. Comput. Chem. 1983, 4, 294.

Charge densities were computed using the natural bond orbital (NBO) approach, to identify the main electron flux between the several units interacting in the NO/Pd4, Pd4/Al14O24H6 and NO/Pd4/Al14O24H6 complexes.

Results and Discussion

Adsorption of NO on Pd4 clusters

The preferential site for adsorption of NO on Pdn cluster seems to have not yet been defined precisely. Recent studies proposed adsorption either on the atop2121 Begum, P.; Gogoi, P.; Mishra, B. K.; Deka, R. C.; Int. J. Quantum Chem. 2015, 115, 837. or on the hollow and bridge sites,4646 Liu, X.; Tian, D.; Ren, S.; Meng, C.; J. Phys. Chem. C 2015, 119, 12941. following previous computational studies that also suggested threefold hollow4747 Loffreda, D.; Simon, D.; Sautet, P.; Chem. Phys. Lett. 1998, 291, 15. or threefold hollow plus atop sites as the preferential adsorption mode.4848 Hansen, K. H.; Sljivancanin, Z.; Hammer, B.; Laegsgaard, E.; Besenbacher, F.; Stensgaard, I.; Surf. Sci. 2002, 496, 1.

In the present study, we computed the adsorption of NO on three different Pd4 clusters, planar (Figure 1b), tetrahedral (Figure 1c) and the distorted cluster obtained after optimization of Pd4 over the γ-alumina surface (Figures 1f and 1g). Several electronic spin states were considered, using either the restricted (singlet) or the unrestricted (doublet to septet states) formalisms. It has been shown4949 Cruz, M. T. M.; Carneiro, J. W. M.; Aranda, D. A. G.; Bühl, M.; J. Phys. Chem. C 2007, 111, 11068. that the ground electronic state for small Pd7 and Pd10 clusters have triplet (S = 1) and septet (S = 3) spin states, respectively. For the Pd4 cluster in any of the three arrangements computed, we found the triplet as the ground state, with the tetrahedral arrangement always more stable than the corresponding planar form. The distorted arrangement is less stable than tetrahedral and planar palladium clusters.

We computed the several alternatives for interaction between the NO molecule and Pd4, as shown in Figure 3. The most stable form for adsorption of NO on planar Pd4 is on the bridge site, with adsorption energy of -41.2 kcal mol-1 (Table 1), followed by adsorption on the hollow, atop 1 and atop 2 sites, which have adsorption energies lower by 3.2, 8.5 and 11.1 kcal mol-1, respectively (Figures 3a- 3d). On the tetrahedral Pd4 cluster (Figures 3e- 3g) the atop adsorption site is preferential, with adsorption energy of -30.6 kcal mol-1, followed by adsorption on the bridge and hollow sites, with adsorption energy lower by 1.4 and 9.6 kcal mol-1, respectively (Table 1). The energy for atop adsorption on the distorted arrangement of Pd4 (the only adsorption mode found for supported Pd4) is -31.8 kcal mol-1, in the midway between atop adsorptions on the tetrahedral and on the planar arrangements. Therefore, the preferential adsorption site is dependent on the arrangement of the Pdn cluster, as found in previous studies,2222 Piotrowski, M. J.; Piquini, P.; Zeng, Z.; da Silva, J. L. F.; J. Phys. Chem. C 2012, 116, 20540.,2323 Lacaze-Dufaure, C.; Roques, J.; Mijoule, C.; Sicilia, E.; Russo, N.; Alexiev, V.; Mineva, T.; J. Mol. Catal. A: Chem. 2011, 341, 28.,5050 Oemry, F.; Escano, M. C.; Kishi, H.; Kunikata, S.; Nakanishi, H.; Kasai, H.; Maekawa, H.; Osumi, K.; Tashiro, Y.; J. Nanosci. Nanotechnol. 2011, 11, 2844. with palladium atoms having higher coordination number showing lower ability to accept the NO molecule.

Figure 3
Different arrangements for adsorption of NO on planar, tetrahedral and distorted Pd4 clusters.

Table 1
Adsorption energy (Ead., kcal mol-1, corrected for BSSE), charge density (e-) on Pd4 and selected geometrical parameters for nitric oxide adsorbed on planar and on tetrahedral Pd4 (distances in Å and bond angles in degrees)

Adsorptions on the atop sites lead to Pd-N distances that are in general lower than the corresponding Pd-N distances for adsorption on the bridge or hollow sites, reflecting the lower coordination number of the adsorbed NO molecule. In any case, the adsorption occurs in a tilted orientation, with O-N-Pd angles between 113 and 136°. The higher tilting was found for the bridge modes.

One important point is the electron flux between the two moieties. This is also a parameter which depends on the adsorption site. Previous studies reported that the NO adsorption process happens with charge back-donation from the Pd4 cluster to the NO molecule.2121 Begum, P.; Gogoi, P.; Mishra, B. K.; Deka, R. C.; Int. J. Quantum Chem. 2015, 115, 837. Our results indicate that the amount of charge transferred between the two moieties depends on the adsorption site, but not on the arrangement. In both arrangements, adsorption in the atop mode, where back-donation to the 2π* orbital of NO seems to be less efficient, results in charge transfer from the NO molecule to the Pd4 cluster, while for adsorption in the bridge and hollow modes, charge is transferred from the Pd4 cluster to the NO molecule. As a consequence, the NO distances in the NO adsorbed on the bridge or hollow sites are larger than those for the NO adsorbed on the atop sites. This should have consequences for the NO reduction process and, therefore, for the catalyst efficiency.

In summary, our computations for adsorption of NO on Pd4 show that adsorption energies and preferential adsorption mode are strongly dependent on the arrangement of the Pdn cluster, in agreement with previous studies.2222 Piotrowski, M. J.; Piquini, P.; Zeng, Z.; da Silva, J. L. F.; J. Phys. Chem. C 2012, 116, 20540.,2323 Lacaze-Dufaure, C.; Roques, J.; Mijoule, C.; Sicilia, E.; Russo, N.; Alexiev, V.; Mineva, T.; J. Mol. Catal. A: Chem. 2011, 341, 28. In spite of these facts, the general trends are that adsorption on palladium atoms with higher coordination number results in lower adsorption energies and the back-donation is less efficient for adsorption on atop sites.

The Pd4/Al14O24H6 cluster

Starting from a planar arrangement with palladium atoms interacting with both tetrahedral and octahedral aluminum atoms, Pd4 was fully optimized on the Al14O24H6 cluster, while keeping the Al14O24H6 moiety unchanged. The final optimized geometry for Pd4 resembles one intermediate between the planar and the tetrahedral arrangements (Figures 1f and 1g). In a previous study on the adsorption of Pd4 on the surface of α-Al2O3(0001),5151 Gomes, J. R. B.; Lodziana, Z.; Illas, F.; J. Phys. Chem. B 2003, 107, 6411. the arrangement of the Pd4 cluster that resulted in the highest adsorption energy is similar to the almost flat arrangement we found in the present case. Palladium atoms interact with both aluminum (tetrahedral and octahedral) and oxygen atoms. Three of the four palladium atoms adsorb in bridge positions, between either two octahedral aluminum or two oxygen atoms. The fourth one assumes an atop position over a tetrahedral aluminum atom. In general, the positions of the palladium atoms are close to the vacant sites of the alumina framework (Figures 1f and 1g).

After adsorption on the support, the palladium atoms become more dispersed than in the isolated cluster, with the peripheral atoms showing elongated Pd-Pd distances, while the central Pd-Pd distance is decreased (Figure 1f). On average, the peripheral Pd-Pd distances in the supported Pd4 are 2.975 Å, as compared to 2.751 Å in bulk palladium. One of the central Pd-Pd distances becomes shorter (2.677 Å) than in the bulk palladium.

The smallest Pd-O distances are 2.160 and 2.206 Å, while the smallest Pd-Al distances are 2.527 and 2.616 Å. Among all Pd-Al distances in the metal-alumina interface, those distances involving the octahedral aluminum atoms (Alo) are shorter than the distances involving the tetrahedral aluminum atoms (Alt). Therefore, although one of the palladium atoms sits atop on a tetrahedral aluminum atom, the Pd-Alt distance is larger than for the Pd-Alo case, where the palladium atoms are adsorbed on a site between two aluminum atoms.

The strong distortion promoted upon adsorption of Pd4 on the alumina support is a consequence of the strong interaction between the two subunities. The energy for adsorption of Pd4 on the alumina support is 197.7 kcal mol-1. If we add to this the energy necessary to distort the Pd4 cluster from the planar arrangement to the arrangement it has on the alumina surface (18.7 kcal mol-1), it results that the total interaction energy between the Pd4 and the alumina surface is 216.4 kcal mol-1.

The total NBO atomic charge on the Pd4 unit adsorbed on the alumina surface is positive (0.475 e-), concentrated on the palladium atoms adsorbed in bridge positions over oxygen atoms (Pd(47) and Pd(48), Figure 1f). When the palladium atom is adsorbed bridge on the octahedral aluminum (Pd(46)) or atop on the tetrahedral aluminum (Pd(45)) the NBO atomic charges are slightly negative. Therefore, we should expect a stronger influence of the aluminum atoms on the catalyst process than of the oxygen atoms.

Adsorption of NO on the Pd4/Al14O24H6 cluster

After optimization of the positions of the four palladium atoms on the alumina surface, the NO molecule was adsorbed on the Pd4/Al14O24H6 cluster, keeping the atomic nuclei of Pd4/Al14O24H6 fixed in their previously optimized positions. Initially, four adsorption modes were tested: atop, bridge, di-σ and hollow. After optimization, all arrangements converged to the atop adsorption mode, with the NO unit bent to the alumina surface (Figure 4). Due to the unpaired electron of the NO molecule and considering the results found for the Pd4/Al14O24H6 cluster, four electronic spin states were calculated: doublet, quartet, sextet and octet. The electronic state of lowest energy depends on the position where NO adsorbs. The lowest energy electronic state for adsorption on the palladium atoms Pd(46) and Pd(47) is the doublet, while for adsorption on Pd(45) the lowest energy electronic state is the quartet. The most stable site for adsorption is Pd(47), as shown in Table 2, with adsorption energy of -25.6 kcal mol-1, close to the experimental value of -27.2 kcal mol-1.1818 Wang, C.-B.; Yeh, T.-F.; Lin, H.-K.; J. Hazard. Mater. 2002, 92, 241. The second site for adsorption is on Pd(45), with adsorption energy of -18.4 kcal mol-1 (all values corrected for BSSE, which amounts to 2-3 kcal mol-1 (see Tables S2 and S3)). The preferential site for adsorption of NO is on the palladium atom that is closer to the oxygen atoms, with the highest positive electronic charge. Therefore, our data indicate that the main parameter that determines the intensity of the interaction of NO with the palladium atoms is the charge on the corresponding palladium. More positive palladium atoms have higher ability to accept the σ electrons from the NO molecule, making the adsorption process stronger.

Figure 4
NO adsorption in different positions on the Pd4/Al14O24H6 cluster: (a and b) two lateral views for adsorption on Pd(47); (c) lateral view for adsorption on Pd(45); and (d) lateral view for adsorption on Pd(46).

Table 2
Geometrical parameters, adsorption energy (Ead., kcal mol-1, corrected for BSSE) and charge density (e-) on the Pd4 cluster and on the nitric oxide (NO) for nitric oxide adsorbed on both Pd4 supported on Al14O24H6 and on the isolated Pd4 with distorted geometry (distances in Å and bond angles in degrees)

The lower Pd-N distance is found for adsorption on Pd(47), reflecting the stronger interaction of the NO molecule with this palladium atom. It is worth noting that the N-O distance after adsorption is high for adsorption on Pd(47), accompanying the order of the interaction energy, although close to the N-O distance for adsorption on Pd(46), probably reflecting the higher ability of this palladium atom (Pd(46)), due to its more negative charge, to donate electron back to the 2π* orbital of NO.

For all adsorption sites the NO molecule adsorbs in a bent geometry, in accordance with the literature.2424 Grybos, R.; Benco, L.; Bučko, T.; Hafner, J.; J. Comput. Chem. 2009, 30, 1910. The Pd-N-O angles varies between 131 and 136°. Adsorption in the bent arrangement favors overlap between the dz22 Liao, F.; Lo, T. W. B.; Tsang, S. C. E.; ChemCatChem 2015, 7, 1998. orbital on the palladium atom and the polarized 2π* orbital of NO.5252 Viñes, F.; Desikusumastuti, A.; Staudt, T.; Görling, A.; Libuda, J.; Neyman, K. M.; J. Phys. Chem. C 2008, 112, 16539.

Reflecting the main type of interaction between the NO molecule and the supported palladium cluster, the total NBO charge density on the NO molecule is positive for all adsorption sites. Therefore, in general, there is charge transfer from the NO molecule to the palladium cluster, with higher values for charge transfer found for the adsorption sites Pd(45) and Pd(47). The palladium Pd(46), which has the most negative electronic charge when interacting with the alumina surface, has higher ability for electron back-donation, therefore making the NO not so positive. Indeed, calculation of charge density with the alternative Merz-Kollman method indicates that NO adsorbed on Pd(46) becomes negatively charged.

Conclusions

The present computational study allowed us to derive relevant informations about the energetic, geometric and electronic parameters for adsorption of NO on palladium and on palladium supported on γ-alumina. The adsorption mode and adsorption energy for the NO molecule adsorbed on the Pd4 clusters is dependent on the metal arrangement. When adsorbing on the planar Pd4, the bridge adsorption mode is preferential, with adsorption energy of -41.2 kcal mol-1. For adsorption on the tetrahedral arrangement of Pd4 the atop adsorption form becomes the most stable, with adsorption energy of -30.6 kcal mol-1. Atop adsorption on a distorted Pd4 obtained by optimizing Pd4 on the alumina surface has adsorption energy of -31.8 kcal mol-1. The electron flux between the NO molecule and the Pd4 cluster also depends on the adsorption mode, with electron transfer from the NO to the Pd cluster in the atop adsorption mode and in the opposite direction for adsorption in the bridge arrangement.

When interacting with the alumina surface, Pd4 adsorbs with a strong deformation in the Pd4 arrangement, leading to a structure with more dispersed palladium atoms and arrangement that is intermediate between the planar and the tetrahedral forms. The adsorption of Pd4 on the alumina surface occurs with a high interaction energy (above 200 kcal mol-1) and charge transfer from the palladium atoms to the alumina, particularly due to the strong interaction with the surface oxygen atoms, although palladium atoms directly placed over octahedral aluminum atoms become negatively charged.

NO adsorbs in an atop orientation on the Pd4/Al14O24H6 cluster, with adsorption energy of -25.6 kcal mol-1. The NO molecule adsorbs preferentially on the palladium atoms with the highest positive charge, mainly due to the interaction via the σ electrons of NO. Charge transfer in this interaction mode is found from the NO molecule to the Pd4/Al14O24H6 cluster. However, when adsorbing on the more negative palladium atoms, charge back-donation to the NO molecule is also observed. Therefore, the effect of the alumina support on the palladium cluster is mainly to increase the dispersion of the palladium atoms, although with a slightly reduced energy for adsorption of NO.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

The authors would like to thank CAPES for scholarship (L. M. P.) and the financial support given by CNPq (grant 478302/2012-6) and FAPERJ (grants E-26/201.302/2014 and E-26/111.708/2013) for research fellowship received by J. W. M. C.

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Publication Dates

  • Publication in this collection
    Nov 2016

History

  • Received
    14 Jan 2016
  • Accepted
    30 Mar 2016
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