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Effect of the Metal#Support Interaction on the Adsorption of NO on Pd/#-AlO: A DFT and NBO Study 4

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Letícia Maia Prates, Glaucio Braga Ferreira, José Walkimar de Mesquita Carneiro, Wagner Batista De Almeida, and Maurício Tavares de Macedo Cruz J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 12, 2017

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The Journal of Physical Chemistry

Effect of the Metal─Support Interaction on the Adsorption of NO on Pd4/γ-Al2O3: A DFT and NBO Study

Letícia M. Prates,a Glaucio B. Ferreira,b José W. de M. Carneiro,b Wagner B. de Almeida b and Maurício T. de M. Cruza(*)

a

Departamento de Química Geral e Inorgânica, Instituto de Química, Universidade do Estado do

Rio de Janeiro, Campus Maracanã, 20550-900, Rio de Janeiro-RJ, Brasil. b

Departamento de Química Inorgânica, Instituto de Química, Universidade Federal Fluminense,

Campus do Valonguinho, 24020-141, Niterói-RJ, Brasil. *corresponding author: [email protected]

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Abstract Density functional theory (B3LYP) was employed to analyze the metal─support interaction in a Pd4 cluster supported on a γ-alumina model (Al14O24H6) and its effect on the adsorption of a single NO molecule. Our results show that the Pd4─Al14O24H6 interaction leads to a reduction in the cohesion energy among palladium atoms, promoting greater dispersion on the γ-alumina surface. NO preferentially adsorbs in a tilted orientation on the palladium atom anchored on two oxygen atoms, with an adsorption energy of –25.4 kcal mol-1, which is in good agreement with the experimental result of –27.2 ± 1.4 kcal mol-1. The palladium─alumina interaction causes a significant reduction in the NO adsorption energy, suggesting the possible existence of a strong metal–support interaction (SMSI). We observe that the larger the decrease in the adsorption energy, the higher the electronic component of the metal─support interaction. NBO (natural bond orbital) calculations show that such effect also leads to attenuation of the Pd─NO back-donation process. This effect is observed mainly in the bridge adsorption mode, where 91% of the decrease in the adsorption energy is due to metal─support electronic effects.


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Introduction The effect of the support on the activity of metal particles has been extensively discussed for more than 30 years.1–6 The term strong metal–support interaction (SMSI) was first introduced in 1978 by Tauster et al.1 and refers to an effect that hinders or even suppresses the chemisorption process in a catalyst, altering its efficiency.7,8 For example, the performance of palladium and platinum supported on alumina-pillared clays is modified by a possible SMSI effect inhibiting the hydrogen chemisorption.9 The SMSI effect can be related to morphological contributions.10,11 However, electronic factors, which can favor or not the chemisorption process, have also been reported in both experimental12–18 and theoretical19–28 studies. Rodriguez and coworkers used photoemission experiments and density functional theory (DFT) calculations to show that the SMSI effect produces electronic perturbations in a Pt particle interacting with a CeO2(111) surface, which significantly enhances the adsorption of water and the dissociation of O─H bonds.29 Periodic DFT calculations showed that ZrO2(111) and CeO2(111) supports change the preferential CO adsorption mode on Pt4 and Pd4 clusters, promoting a higher CO activation.30 In contrast, other works show electronic contributions in SMSI state that impair the chemisorption capability and effectiveness of a catalyst.15–17 The electronic perturbations on a nickel nanoparticle promoted by its interaction with CeO2(111) support decreases the catalyst activity for CO methanation.18 Additional studies suggest a relation between the metal particle size and metal─support electron transfer.6,31 Several studies have reported that the SMSI effect is promoted by different metal oxide supports, such as TiO21–3,7,31, CeO26,13,29, SiO232, ZrO25 and Al2O37,33,34. The latter support, mainly in the gamma phase (γ-Al2O3), is one of the most employed oxides in supported catalysts26,35–40 due to its high specific area, well-defined pore size, stability over a large temperature range and acid/base properties.38,41 The structure of γ-Al2O3 includes octahedral and tetrahedral aluminum cations occupying the interstitial sites between oxygen anions.42 Theoretical studies have indicated that the metal─support electronic flux depends on the coordination number of the aluminum site at which the metal atom sits.27,43–46 In a DFT study on formaldehyde adsorption on a Pd4/γ-Al2O3 model, we 3 ACS Paragon Plus Environment


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showed that the highly coordinated octahedral aluminum cations can donate electron density to the supported Pd4 cluster.45 In contrast, tetrahedral aluminum cations have acidic properties and therefore act as electron acceptors. Although a large amount of research has been dedicated to understanding the SMSI effect, the rationalization of its origin is still under debate.12 The nature and magnitude of metal-support interactions can be better evaluated by means of adsorption of molecular prototypes. The present work is an extension of a previous study44 addressing NO adsorption on a catalyst model involving palladium supported on γ-Al2O3 (Pd4/Al14O24H6). Here, we provide a detailed analysis of the influence of the different sites on the γ-Al2O3 surface on the metal─support interaction and on the NO adsorption process. DFT calculations using the B3LYP functional were performed to optimize Pd4 and Pd1 on an Al14O24H6 model and a single NO molecule on a Pd4/Al14O24H6 cluster. Natural bond orbital (NBO) analysis47–49 was used to evaluate electron transfer in the NO/Pd4/Al14O24H6 adsorption interfaces. Hence, we aim to rationalize the nature of the Pd─Al2O3 SMSI effect in the NO adsorption process.

Computational Details In the present work, we continue our previous study44 concerning the adsorption of NO on isolated Pd4 and on Pd4 supported on the (110C) face of γ-Al2O3. As previously reported,44 the model used to represent the alumina support is restricted to three sheets of atoms containing 14 aluminum and 24 oxygen atoms. To balance the total charge and yield a neutral model, 6 hydrogen atoms were added to the terminal oxygens, leading to a partially hydroxylated model with Al14O24H6 stoichiometry. On the (110C) face of γ-alumina we deposited a Pd4 cluster, starting with a planar geometry for the Pd4 moiety and Pd‒Pd distances of 2.751 Å. The (110C) γ-alumina face was chosen because it exposes a variety of distinct sites,50 allowing the palladium atoms to interact with both tetrahedral and octahedral aluminum atoms. The Pd4 cluster was then optimized on the γ-Al2O3 surface, leading 4 ACS Paragon Plus Environment

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to a distorted geometry.44 In addition, single-point calculations were performed after removing three of the four palladium atoms, keeping a single palladium atom in its original position. Finally, this palladium atom was allowed to relax on the Al14O24H6 sites to determine the effect on the Pd‒Al interactions. Herein, these models are denoted Pd1(sp)/Al14O24H6 and Pd1(opt)/Al14O24H6, respectively. Considering that the tetrahedral Pd4 cluster has a triplet ground state45 and that the γ-Al2O3 model has a singlet ground state,51 all calculations involving Pd4 deposited on γ-Al2O3 were carried out in the total triplet electronic state, using the unrestricted formalism. 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. Several orientations and adsorption modes (ontop, bridge, hollow, di-σ and π) were considered for the adsorption of a NO molecule on the Pd4/Al14O24H6 cluster, as previously reported.44 The calculations for NO adsorption were performed by relaxing only the NO distance and orientation, while keeping the atoms in the Pd4/Al14O24H6 clusters in their original optimized positions. As a reference for the effect of γ-alumina on the NO adsorption energy, we also computed NO adsorption on a naked Pd4 cluster in the same arrangement as that obtained in the optimization of Pd4 on the Al14O24H6 cluster. The ontop, bridge and hollow adsorption modes were also tested in this model. When possible, symmetry was imposed. The adsorption energies were computed by summing the energies of the NO molecule and the Pd4 or Pd4/Al14O24H6 cluster and subtracting the resulting sum from the energy of the NO/Pd4 or NO/Pd4/Al14O24H6 complex, according to Equation 1. The adsorption energies were corrected by the basis set superposition error (BSSE), calculated by the counterpoise method.52,53

Ead = E( NO/ Pd4 / Al14O24H6 ) − [E( NO) + E(Pd4 / Al14O24H6 ) ]

(1)

To quantify the distortion of the Pd4 cluster due to adsorption on the Al14O24H6 moiety, the deformation energy (Edef) was calculated in relation to the planar and tetrahedral Pd4 models, according to Equation 2. 5 ACS Paragon Plus Environment


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Edef = EPd4( dist ) − [EPd4( I ) ]

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(2)

where EPd4( dist ) is the energy of Pd4 in the distorted arrangement, obtained from Pd4 optimized on Al14O24H6, and EPd4( I ) is the energy of the planar or tetrahedral Pd4 cluster. To evaluate the nature of the electronic interactions at the adsorption interface inside the complexes, Natural bond orbital (NBO)47–49 calculations were performed. According to secondorder perturbation theory, the hyperconjugative interaction energy between a filled (i) and an unoccupied (j) NBO is calculated as described in Equation 3, E 2 = ∆Eij = qi

F (i , j ) 2 ε j − εi

(3)

where qi is the occupancy of the donor orbital, F(i,j)2 is the NBO Fock matrix between orbitals i and j and εj – εi is the difference between the energies of the j and i NBOs.54,55 The charge densities were also computed using the NBO approach to identify the main electron flux between the multiple interacting units. All calculations were carried out using the Gaussian 03 computational package56 with the B3LYP hybrid functional, as proposed and parameterized by Becke.57 This functional includes a mixture of Hartree-Fock and DFT exchange terms with the gradient-corrected correlation functional of Lee et al.58 The palladium atoms were described by the LANL2DZ pseudopotential59 and the D95V60 basis set for the valence electrons. The 6-31G(d) and 6-311+G(d) basis sets61,62 were used to model the γ-alumina support and the NO molecule, respectively.


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Results and Discussion Adsorption of Pd4 on Al14O24H6: cohesion and dissociation energies and charge distribution The ground electronic spin state for the Pd4/Al14O24H6 complex is the triplet state,44 which is the same as that found for an isolated tetrahedral Pd4 cluster. By approximation, the singlet state of the isolated Al14O24H6 aggregate was considered.51 In the triplet state, Pd4 adsorbs on Al14O24H6 in a distorted arrangement (Figure 1) with an adsorption energy of –181.5 kcal mol-1. The distorted Pd4 structure corresponds to a deformation energy (Equation 2) of 29.6 and 52.0 kcal mol-1, calculated in relation to the planar and tetrahedral arrangements, respectively.

Figure 1. (a) Superior view of the Al14O24H6 cluster, (b) superior and (c) lateral views of Pd4 optimized over the Al14O24H6 unit. Distances are given in Å.

The adsorption sites at which the four palladium atoms were anchored involve one aluminum atom with tetrahedral geometry (Altc), two aluminum atoms with octahedral geometry positioned at the center of the alumina surface (Aloc), and four oxygen atoms, with two placed at the center (Oc) and two at the border (Ob1) of the Al14O24H6 model. The Pd(1) atom adsorbs on Altc in the ontop mode, whereas Pd(2) adsorbs in the bridged mode on Aloc atoms. Pd(3) and Pd(4) adsorb in the bridge mode, involving Oc and Ob1 atoms (Figure 1). The shortest Pd─O distances in the Pd4/Al14O24H6 complex are Pd(3,4)─Oc = 2.158 Å and Pd(3,4)─Ob1 = 2.255 Å (Table S1). The Pd─Al distances involving octahedral aluminum atoms are shorter (Pd(2)─Aloc = 2.527 Å) than those involving tetrahedral aluminum atoms (Pd(1)─Altc = 2.616 Å). 7 ACS Paragon Plus Environment


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The deformation energy given above is an indication of the stability of the distorted arrangement in Pd4. However, one additional parameter associated with the stability is the cohesion energy, which is the energy necessary to maintain the bonding of all palladium atoms in the Pd4 cluster (Ecoh), either in the isolated form or in the presence of the support.63 The cohesion energy was computed using Equations 4 and 5:

Ecoh = EPd4 − 4EPd1 Ecoh(ad ) = Ead ( Pd4 ) −

(4)

∑E

ad ( Pd1 )

(5)

where Ead(Pd4) and Ead(Pd1) are the adsorption energies of Pd4 and Pd1, respectively. The cohesion energy for the isolated distorted Pd4 (Equation 4) is –58.2 kcal mol-1, which is smaller than the corresponding value for the planar and the tetrahedral arrangements (–76.0 kcal mol-1 and –99.0 kcal mol-1, respectively, Table S2, Supplementary Material). The estimated cohesion energy for Pd4 in the presence of Al14O24H6 (Equation 5) is –19.1 kcal mol-1. Therefore, the Pd4─Al14O24H6 interaction reduces the cohesion energy among the palladium atoms by 39.1 kcal mol-1, in addition to the reduction promoted by the distortion itself. This indicates that the bonding among the adsorbed palladium atoms is considerably reduced compared to the corresponding bonding in the planar and tetrahedral arrangements. These data allow the effect of the alumina surface on the Pd4 cohesion energy to be decomposed into two contributions: one due to the geometric distortion of Pd4, 40.8 kcal mol-1 (51.1%), and another that we attribute to an electronic effect due to the presence of alumina, 39.1 kcal mol-1 (48.9%). We also evaluated the dissociation energy of each individual palladium atom within the Pd4 cluster.63 Equations 6 and 7 were used to calculate this energy in the isolated and supported Pd4 models.

Ediss = E(Pd1 ) + E(Pd3 ) − E(Pd4 )

(6)

Ediss(ad ) = Ead ( Pd1 ) + Ead ( Pd3 ) − Ead ( Pd4 )

(7)


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In the presence of γ-alumina, each palladium atom becomes more weakly bound within the Pd4 cluster, with exception of Pd(3) (Table S2, Supplementary Material). The major influence of γ-alumina is on the palladium atom anchored on the two Aloc (Pd(2)), where its dissociation energy within Pd4 decreased by 26.9 kcal mol-1 after adsorption on alumina. For the palladium atom adsorbed on Altc, the corresponding change is 19.4 kcal mol-1. However, for Pd(3) (positioned on Oc and Ob1), the dissociation energy increases by 6.0 kcal mol-1. These results suggest that, while the Pd─Al interaction weakens the metal─metal bond, the Pd─O interaction strengthens the metal─metal bond. NBO charges calculated for the Al14O24H6 model show positive values for aluminum atoms and negative values for oxygen atoms (Table S3). The charges on the octahedral aluminum atoms (Alo) are less positive than those on the tetrahedral ones (Alt). The highest positive value is observed for Altc (+2.02 ē), followed by the Alob atoms (+1.75 ē). The lowest values are on the Aloc atoms (+1.54 ē). The Oc atoms present the most negative charge (–1.40 ē), whereas the Ob1 atoms are less negative (–1.08 ē). There is no significant modification in the charges of the atoms inside the Al14O24H6 agglomerate after adsorption of the Pd4 cluster, probably due to compensation effects between the atoms in the γ-alumina structure. The greatest change in the NBO charges was computed for Altc, which becomes 0.20 ē less positive after the adsorption of Pd4 (+1.82 ē), which is a possible indication of its Lewis acid behavior, also pointed by some works.45,46 The palladium atoms accumulate different charges, according to the adsorption site on the γ-alumina cluster. Pd(3), adsorbed in the bridged mode on Oc and Ob1, becomes positive (+0.36 ē). In contrast, Pd(1) and Pd(2) become negative (–0.05 ē and –0.19 ē, respectively).



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Pd1/Al14O24H6 aggregates: adsorption energies and charge analysis

Identification of the influence of the distinct sites of the support surface on the metal─support interaction is not a simple task, since the metal presents intrinsic effects that are difficult or even impossible to eliminate.12 This fact was observed in our NBO results, which indicates important Pd─Pd interactions within Pd4/Al14O24H6 that hamper the analysis of Pd─alumina electron transfer (see discussion further on). Thus, we decided to remove the selfinteraction among the palladium atoms. Single-point calculations were performed on a modified model, removing three of the palladium atoms and keeping only the Pd atom on the site of interest (Pd1(sp)/Al14O24H6). For this new system, the singlet state was obtained as the lowest energy state. The highest adsorption energy, –56.8 kcal mol-1, was computed for Pd(2), which was anchored on two Aloc (Table S4). Pd(3), coordinated to Oc and Ob1, adsorbs with the second highest adsorption energy, –43.3 kcal mol-1, whereas Pd(1), which is bound to Altc, adsorbs with an energy of –19.5 kcal mol-1. The NBO charges reveal changes in the electronic nature of the palladium atoms. In the absence of neighboring palladium atoms, Pd(1) assumes the most positive charge (+0.16 ē), in contrast to the value it has in the Pd4/Al14O24H6 complex (–0.05 ē), which indicates some acidity of the tetrahedral aluminum atoms. The charge on Pd(2) is close to zero (+0.02 ē), while in the Pd4/Al14O24H6 complex it is negative (–0.19 ē). Pd(3) has positive charge in both cases (+0.29 ē in Pd1/Al14O24H6 and +0.36 ē in Pd4/Al14O24H6). These results show that palladium atoms that adsorb closer to oxygen atoms have more positive charges, probably due to the ionic character of the Pd─O bond, which induces charge polarization. The higher positive charge of Pd(3) in Pd4/Al14O24H6 than in Pd1/Al14O24H6 indicates a possible electronic flux from the palladium atoms coordinated to oxygen to the palladium atoms coordinated to aluminum. There is no significant change in the charges of both the oxygen and aluminum atoms in the models containing one and four palladium atoms. 10 ACS Paragon Plus Environment

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Structures with a single palladium atom were also optimized on the γ-alumina cluster (Pd1(opt)/Al14O24H6 complex) (Figure S1). The most relevant change compared to the single-point calculations was computed for Pd(1), which moves towards Ob4, increasing the adsorption energy (from –19.5 kcal mol-1 to –51.4 kcal mol-1). Pd(3) slightly moves towards the Aloc and Alob atoms, with a smaller increase in the adsorption energy (–43.3 kcal mol-1 to –52.4 kcal mol-1). In contrast, the position of Pd(2) is essentially unaltered, and Pd(2) remains bonded to two Aloc. As a consequence, its adsorption energy also remained essentially the same (–56.8 kcal mol-1 to –57.7 kcal mol-1). Therefore, the strongest Pd─alumina interaction must involve two neighboring Aloc atom, when the effect caused by interactions among the palladium atoms is removed.

NO adsorption on Pd4/Al14O24H6 To evaluate the influence of γ-alumina (Al14O24H6) on the catalytic activity of palladium, a single NO molecule was adsorbed in different modes (ontop, bridge, hollow, di-σ and π) on Pd4/Al14O24H6. For all complexes, the quartet electronic spin state is the most stable. The di-σ and π adsorption modes converged to the ontop mode. Considering that NO adsorption does not occur via the oxygen atom on neither palladium nor other transition metal clusters, all computed adsorption forms are through the nitrogen atom.64 In the ontop mode, the NO molecule can adsorb on three different sites, corresponding to the three non-equivalent palladium atoms, Pd(1), Pd(2) and Pd(3) (Figure S2). The bridge mode was obtained only for adsorption on Pd(1) and Pd(3) (Figure 2). For the other possible adsorption modes, the bridge mode converts to the ontop mode, whereas the hollow mode converts to the bridge mode. The adsorption energy shows that NO preferentially adsorbs in the ontop mode on Pd(3) with an adsorption energy of –25.4 kcal mol-1 (Table 1), which is in good agreement with the experimental result (–27.2 ±1.4 kcal mol-1).65 The second preferential site is the ontop mode on Pd(1), with an adsorption energy of –17.6 kcal mol-1. The NO adsorption energy in the bridge mode is –14.5 kcal mol-1, making this mode the third most stable.



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Figure 2. Bridge mode for NO adsorbed on Pd4/Al14O24H6 cluster. The terminal atoms are not represented here only for clarity.

The geometric parameters vary according to the sites and adsorption mode. For the ontop adsorption mode, the N─Pd distances are 2.007 Å and 1.889 Å, for adsorption on Pd(1) and Pd(3), respectively. The N─O bond distance does not vary much with the adsorption site and is 1.151 Å and 1.156 Å for adsorption in the ontop mode on Pd(1) and on Pd(3), respectively, which is only slightly higher than the computed value for the isolated NO molecule (1.148 Å). As shown in a previous study,44 the NO molecule adsorbs in a tilted orientation on the palladium atoms in the Pd4/Al14O24H6 model. The Pd─N─O bond angles are 135.07° and 129.18° for adsorption on Pd(1) and Pd(3), respectively. For adsorption in the bridge mode, the Pd─N bond distances are similar to those observed for adsorption in the ontop mode (2.033 Å and 2.032 Å for Pd(1) and Pd(3), respectively). However, the N─O bond distance is much higher (1.182 Å), whereas the Pd─N─O bond angles (121.52° for Pd(1) and 119.86°for Pd(3)) are lower than those observed for adsorption in the ontop mode.


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Table 1. NO adsorption energy (Ead., kcal mol-1, corrected for BSSE); NBO charge density (|e|) on the Pd atom (bonded to NO), on the γ-alumina aggregate and on the NO molecule; and selected geometrical parameters for NO adsorbed on Pd4/Al14O24H6 (distances in angstroms (Å) and bond angles in degrees (°)).

(1)

(2)

bridge (3)

(1,2)

(1,3)

Hollow (2,3)

(1,2,3)

Convert to bridge (1,3)

Label

ontop

Convert to ontop (3)

Form

(1)

N─Pd

2.007

2.096

1.889

2.033 2.032(3)

N─O

1.151

1.156

1.156

1.182

Pd─N─O

135.07

131.29

129.18

qAlumina

–0.451

–0.381

–0.387

qPd

–0.143

–0.294

+0.154

qNO

+0.070

+0.011 +0.072

–0.106

Ead

–17.55

–5.37

–14.47

–25.36

Convert to ontop (1)

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121.52(1) 119.86(3) –0.412 +0.076(1) +0.251(3)

The NBO charges on NO/Pd4/Al14O24H6 (Table 1) show that, for adsorption in the ontop mode, Pd(1) and Pd(2) become more negative compared with the model in the absence of NO (Pd4/Al14O24H6) (from –0.05 ē to –0.14 ē and from –0.19 ē to –0.29 ē, respectively), whereas Pd(3) becomes less positive (from +0.36 ē to +0.15 ē). The NO molecule assumes a slightly positive charge when it adsorbs in the ontop mode, regardless of the site. For adsorption on Pd(1), Pd(2) and Pd(3), the charges on NO are +0.07 ē, +0.01 ē and +0.07 ē, respectively. For adsorption in the bridge mode, the charges on the palladium atoms are positive (+0.08 ē for Pd(1) and +0.25 ē for Pd(3)) and on the NO molecule the charge is negative (–0.11 ē). The adsorption of NO on the Pd4 cluster supported on γ-alumina also leads to a reorganization of the charges inside the support. The Al14O24H6 agglomerate acquires a negative charge (‒0.47 ē) after Pd4 adsorption. However, in the presence of NO, this charge decreases, depending on the site at which the NO molecule adsorbs.



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To compare the behavior of the palladium atoms in the absence of γ-alumina, the adsorption of NO on isolated Pd4, in the geometry obtained from the previous optimization on Al14O24H6, was also tested. For Pd4 the most stable electronic spin state is the triplet spin state. When NO is adsorbed, the doublet spin state becomes the most stable. The preferential NO adsorption mode on this distorted and isolated structure of Pd4 is the hollow mode (H2 form), with an adsorption energy of –52.0 kcal mol-1, followed by the bridge mode (B2 form, –42.8 kcal mol-1) and ontop modes (A5, –31.9 kcal mol-1; A3, –26.8 kcal mol-1; and A4, –25.8 kcal mol-1) (Figures 3 and S3 and Table 2). The geometric parameters for the NO/Pd4 system show that the N─O bond distance stretches as the palladium coordination with NO increases (Table 2). The shortest Pd─N bond lengths are computed for the ontop modes. The NO molecule adsorbs in a tilted orientation on the palladium atoms, with Pd─N─O bond angles varying in the range of 112.7–131.8°. The NBO charge also varies according to the number of palladium atoms coordinated to NO. For adsorption in the ontop mode, the charge on NO is negative but close to zero (–0.012 ē for Pd(1), –0.019 ē for Pd(2) and –0.032 ē for Pd(3)), while for adsorption in the bridge and hollow modes, NO presents highly negative charges of –0.236 and –0.410 ē, respectively. If we consider the relevance to the catalytic process of an isolated Pd4 cluster, it seems that adsorption in the hollow mode most highly favors NO catalysis, since it involves the highest adsorption energy, the longest N─O distance and the highest charge transfer to the NO molecule. However, the presence of the support inhibits the formation of this mode. The second mode that could better promote NO dissociation is the bridge adsorption mode, which is only the third-most stable in the supported model. Adsorption in the ontop mode, although preferential in the supported model, has the lowest influence in the NO charge and geometry.


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Figure 3. (B2) bridge and (H2) hollow arrangements for NO adsorption on distorted Pd4 clusters. Table 2. Adsorption energy (Ead., kcal mol-1, corrected for BSSE), NBO charge density (|e|) on the NO molecule and selected geometrical parameters for NO adsorbed on a distorted Pd4 cluster (distances in angstroms (Å) and bond angles in degrees (°)).

Cluster

Distorted

Mode

ontop

bridge

hollow

Form

A3

A4

A5

B2

H2

Pd─N

1.843

1.900

1.865

1.935

1.987

N─O

1.158

1.165

1.157

1.185

1.205

Pd─N─O

131.24

112.69

131.81

120.86

117.14

qNO

–0.012

–0.019

–0.032

–0.236

–0.410

Ead

–26.81

–25.78

–31.93

–42.75

–51.98

The decrease in the adsorption energy and the change in the NO adsorption mode suggest the existence of strong metal–support interaction (SMSI).1 In order to evaluate the influence of palladium─alumina interaction at different sites, the total effect in the NO adsorption (E(T), Equation 8) was split into two contributions: electronic (E(E)) and geometric (E(G)), according to Equations 9, 10 and 11.

∆E(T ) = Ead ( NO/Pd4 ) − Ead ( NO/ Pd4Al14O24H6 )

(8)

∆E(T ) = ∆E(E) +∆E(G)

(9)

∆E( E) = Ead ( NO( sp ) / Pd4 ) − Ead ( NO/ Pd4 Al14O24H6 )

(10)

∆E(G) = Ead ( NO/ Pd4 ) − Ead ( NO( sp ) / Pd4 )

(11) 15

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where ∆E(T) is the difference in the adsorption energy for NO on the isolated and supported Pd4 models and ∆E(E) and ∆E(G) are the contributions to this difference due to electronic and geometric effects, respectively. The ∆E(E) term is calculated by subtracting the NO adsorption energy on Pd4/Al14O24H6 ( Ead ( NO/Pd4Al14O24H6 ) ) from the adsorption energy on isolated Pd4 in the same coordinates of NO/Pd4/Al14O24H6 ( Ead ( NO( sp) / Pd4 ) ). The ∆E(G) term is obtained by subtracting the term

Ead ( NO( sp) / Pd4 ) from the NO adsorption energy on isolated Pd4. Applying the procedure described above, we found a decrease in the adsorption energy due to the palladium─alumina interaction, regardless of the adsorption mode or position. Table 3 shows the ∆E(T) values for the different adsorption sites. Considering the ontop mode, there is a decrease of 9.3, 20.4 and 6.6 kcal mol-1, respectively, for Pd(1), Pd(2) and Pd(3). For adsorption on either Pd(1) and Pd(2), which are both anchored on aluminum atoms, the electronic contribution is the same at approximately 57%. Nonetheless, it is important to note that even though they present a similar electronic effect, the NO adsorption energy on Pd(2) is more strongly influenced by the palladium‒alumina interaction. For adsorption on Pd(3), besides having the lowest ∆E(T), almost all of the decrease in the adsorption energy is due to the distortion of the NO molecule (geometric effect, 98%). For adsorption in the bridge mode, the effect of the metal─support interaction is the highest, decreasing the adsorption energy by 28.3 kcal mol-1, almost exclusively, due to electronic effects (91%).


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Table 3. Influence of γ-alumina on the adsorption energy of NO. ontop

bridge

Pd

(1)

(2)

(3)

(1)─(3)

∆E(T)

9.26

20.41

6.57

28.28

∆E(E)

5.26

11.63

0.11

25.77

%(E)

56.80

56.98

1.67

91.12

∆E(G)

4.00

8.78

6.46

2.51

%(G)

43.20

43.02

98.33

8.88

These results suggest that the intensity and nature of the influence of the metal─support interaction on the adsorption energy vary according to the site and mode in which NO adsorbs. Considering the ontop mode, when the involved palladium atom is on aluminum cations (Pd(1) and Pd(2)), the change in the NO adsorption energy occurs mainly due to an electronic effect (57%), consequence of metal‒support interaction. However, Pd(2), which has the highest interaction energy with the alumina surface (–56.8 kcal mol-1), shows the largest decrease in the NO adsorption energy (20.4 kcal mol-1). This value is approximately twice as high as that observed for Pd(1) (9.3 kcal mol-1). Intriguingly, the palladium atom which anchors with the second higher interaction energy (Pd(3), –43.3 kcal mol-1) promotes the lowest influence in the NO adsorption (6.6 kcal mol-1). Additionally, in this case, the electronic effect promoted by Pd(3)‒O interaction is small (2%). For the bridge mode, the greatest decrease in the NO adsorption energy is observed (28.3 kcal mol-1). In this case, the influence of the metal‒support interaction in the NO anchorage has the highest electronic character (91%). Adsorption in the other bridge and hollow modes is not observed. Thus, the electronic nature of the effect promoted by the metal─oxide interaction seems to be the determinant component for the magnitude of the NO adsorption energy and can even inhibit some adsorption modes. In summary, our results suggest that the higher the electronic nature of the 17 ACS Paragon Plus Environment


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influence promoted by the Pd─support interaction, the larger the decrease in the NO adsorption energy, at least for the γ-alumina model employed in the present study.

Natural bond orbital analysis NBO calculations were performed to investigate the electron transfer processes at the Pd4/Al14O24H6 and NO/Pd4 interfaces. The NBO analysis shows that the interaction between Pd4 and Al14O24H6 is governed by a donation process, where the major contribution is from Aloc→Pd(2), involving the 3sp0.51 and 5s orbitals of the respective atoms (33.2 kcal mol-1, see Tables S5 and S6 and Figure S4). This energy is five times higher than that observed for Pd(2)→Aloc back-donation (6.4 kcal mol-1), which is in agreement with the negative charge (–0.19 ē) on Pd(2) and the occupation of the 5s orbital (0.25 ē) in the Pd4/Al14O24H6 complex. However, NBO analysis indicates an important electron transfer from Pd(1) to Pd(2) (87.2 kcal mol-1). Therefore, the negative charge on Pd(2) cannot be attributed entirely to the γ-alumina model. Consequently, Pd(1) maintains only a slightly negative charge in Pd4/Al14O24H6 (–0.05 ē). For the palladium atoms close to the oxygen anions (Pd(3) and its counterpart, Pd(4)), the NBO calculation does not show any significant Pd─O electron transfer; instead, the calculation indicates the high ionic character of this bond (>95% polarization on the oxygen anions). The observed donation effect promoted by the aluminum cations (Aloc and Alob) does not seem to be high enough to attenuate the positive charge on the palladium atoms (Table S5, Supplementary material). In addition, the energy for Pd→Alo back-donation is small (approximately 4.0 kcal mol-1). Thus, the positive charge on Pd(3) may be a consequence of the ionic character of Pd─O. The NBO calculations for the systems containing the NO molecule show that the intensity of the NO─Pd electron transfer is influenced by the NO adsorption mode. In the ontop mode, the charge of NO is close to zero, regardless of the presence or absence of the alumina surface, resulting in a small elongation of the N─O bond for NO adsorbed in this mode. 18 ACS Paragon Plus Environment

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For NO adsorbed in the bridge mode, Pd→NO back-donation is the main effect. In the isolated Pd4 clusters, this effect involves energies ranging from 37.5 to 43.1 kcal mol-1 and a negative charge on NO, from –0.19 ē to –0.24 ē. In the presence of Al14O24H6, the back-donation decreases to 28.9 kcal mol-1 (Figures 4 and S5 and Table 4). Consequently, the charge on the adsorbed NO also decreases from the isolated to the supported Pd4 model (in the same arrangement), changing from –0.24 ē to –0.11 ē, respectively. In addition, an increase in the occupation of the 4d orbital on palladium (Pd(1), occ. = 0.81 ē; Pd(3), occ. = 0.92 ē) and a decrease in the occupation of the 2π* molecular orbital on NO (occ. = 0.60 ē) are observed (Table 4). These NBO results agree with the findings of the electronic analysis given above. In the sites where the influence of the metal─support interaction in the NO adsorption is mainly electronic in nature, the most intense palladium─alumina electron transfers are observed. In contrast, the Pd─O interaction is governed by a predominantly ionic character, which can explain the small electronic contribution in the decrease of adsorption energy for NO on this site. In addition, the decrease in the Pd→NO back-donation in the presence of γ-alumina can also be attributed to the highest electronic effect observed for NO adsorbed in bridge mode. Therefore, the strong electronic palladium─alumina interaction not only contributes to the decrease of the NO adsorption energy but also affects the ability of palladium to transfer electrons to the NO molecule. The strong electronic metal─support interaction, promoted by charge transfer between the metal particle and the support, is described in the literature by the term EMSI.14 Some works suggest that the EMSI effect enhances the adsorption ability of the catalyst,6,12,29 although interpretation in the opposite direction has also been given.15–18 Our results suggest that this process depends not only on the nature of the metal and oxide used as support, but also on the site and adsorption mode in which the molecule adsorbs.



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Figure 4. Orbitals involved in Pd→NO back-donation in the distorted (a and a’) and supported (b and b’) Pd4 clusters for adsorption in the bridge mode. The terminal atoms in Al14O24H6 cluster are not represented here only for clarity.

Table 4. NBO parameters for orbitals involved in Pd→NO back-donation for NO adsorption on the isolated and supported Pd4 clusters (hybridization (s, p and d, %), occupancy (occ., |e|), participation coefficient (2π*, %) and interaction energy (Eback, kcal mol-1)). Palladium Agglomerate

NO

Label

s

p

d

occ.

(%)N

(%)O

occ.

Eback

(1) (3)

1.33 2.55

0.25 0.41

98.42 97.05

0.776 0.753

66.44

33.56

0.682

43.08

(1,3)

1.43

0.30

98.27

0.752

66.55

33.45

0.642

37.50

Distorted

(1) (3)

7.34 1.66

0.76 0.24

91.91 98.10

0.742 0.789

67.53

32.47

0.748

41.81

Supported

(1) (3)

0.70 0.31

0.28 0.58

99.02 99.10

0.810 0.924

66.14

33.86

0.604

28.93

Planar

Tetrahedral


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Conclusion In this work, a detailed analysis of the interactions found in the Pd4/Al14O24H6, NO/Pd4/Al14O24H6 and NO/Pd4 adsorption interfaces was performed. The NO adsorption energy calculated on the proposed catalyst model is in good agreement with the experimental results.65 Some important observations can be highlighted concerning the Pd4‒alumina interaction and its effect in the NO adsorption process: i) The presence of the γ-alumina support decreases the cohesion energy between the palladium atoms, where 48.9% of this energy decrease results from electronic effects promoted by the Pd4‒Al14O24H6 interaction. However, Pd‒Pd interactions are still observed, causing troubles to the NBO analysis. When Pd–Pd interactions are removed (Pd1/Al14O24H6), Pd(2)–Aloc presents the greatest contribution to the SMSI. ii) The classical SMSI effect decreases the adsorption ability of the catalyst. The proposed Pd4/Al14O24H6 model indicates the same behavior. Thus, when the Pd‒alumina interaction is the strongest (Aloc), the NO adsorption energy is the weakest. iii) The energy of the 4d→2π* electronic transfer from palladium to NO increases with increasing NO coordination to the palladium atoms. The Pd4 arrangement obtained on Al14O24H6 indicates that the NO molecule prefers the hollow adsorption mode. iv) The stronger the electronic nature of the effect promoted by the palladium‒alumina interaction (EMSI), the larger the depletion in the NO adsorption energy. Thus, the NO molecule adsorbs more strongly on the palladium atom anchored on the Oc and Ob1 sites, where the effect provided by palladium─alumina interaction is only 2% electronic in nature. v) The EMSI effect observed in Pd4/Al14O24H6 decreases the capacity of palladium atoms to back-donate electrons to the 2π* orbital of NO, including preventing the formation of the hollow adsorption mode. In sum, this work suggests that the SMSI effect for a palladium catalyst supported on γalumina provides an important electronic contribution in the adsorption of a NO molecule. 21 ACS Paragon Plus Environment


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However, this contribution also depends on the γ-alumina site at which the palladium atom interacts as well as the adsorption mode of the adsorbate molecule.

Associated content

Supporting Information: The Supporting Information is available free of charge on the http://pubs.acs.org website. Calculated optimized structures for Pd1/Al14O24H6, NO/Pd4/Al14O24H6 and NO/Pd4 models; NBOs involved in electron transfers on Pd4/Al14O24H6 and NO/Pd4 models; bond distances on Pd/Al14O24H6 complexes; Pd4 dissociation and cohesion energies; NBO charges on Al14O24H6 and Pd/Al14O24H6 models; adsorption energies for Pd1/Al14O24H6 complexes; details of NBO analysis for interacting orbitals on Pd/Al14O24H6 complexes.

Acknowledgement The authors are grateful to CAPES for their scholarship (L. M. P.) and for the financial support given by CNPq (grant 478302/2012-6) and FAPERJ (grant E-26/201.302/2014 and E-26/111.708/2013). CNPq research grants for J. W. de M. Carneiro and W.B. de Almeida are also acknowledged.

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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 27 ACS Paragon Plus Environment


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TOC Graphic



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Figure 1. (a) Superior view of the Al14O24H6 cluster, (b) superior and (c) lateral views of Pd4 optimized over the Al14O24H6 unit. Distances are given in Å. 177x58mm (300 x 300 DPI)


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Figure 2. Bridge mode for NO adsorbed on Pd4/Al14O24H6 cluster. The terminal atoms are not represented here only for clarity. 82x58mm (300 x 300 DPI)



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Figure 3. (B2) bridge and (H2) hollow arrangements for NO adsorption on distorted Pd4 clusters. 82x25mm (300 x 300 DPI)


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Figure 4. Orbitals involved in Pd→NO back-donation in the distorted (a and a’) and supported (b and b’) Pd4 clusters for adsorption in the bridge mode. The terminal atoms in Al14O24H6 cluster are not represented here only for clarity. 134x92mm (300 x 300 DPI)



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82x40mm (300 x 300 DPI)


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Effect of the Metal–Support Interaction on the ... - ACS Publications

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