The Effect of Gamma-Al₂O₃Support on the NO Adsorption on Pd₄Cluster

Abstract

The effect of γ-Al 2 O 3 support on the NO adsorption on Pd 4 clusters was investigated by means of density functional theory (DFT) calculations. Pd 4 adsorbed on γ-Al 2 O 3 (represented by a Al 14 O 24 H 6 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 . Charge density analysis show that the electron flux between the NO molecule and Pd 4 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 IntroductionPalladium catalysts have found wide appllication in several areas of chemistry. [1][2][3] One of the most common and established use of palladium catalyst is to control automobile exhaust gas emission. [4][5][6] 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, 7 more fundamental studies are still necessary for a full comprehension of the catalytic process, particularly at the molecular level. 8,9 Commonly, the platinum and palladium catalysts are supported on metal oxides, among them γ-Al 2 O 3 , although oxides of cerium, zirconium and titanium are also frequently employed. [10][11][12] These supported catalysts are thus employed to remove carbon monoxide (CO), nitric oxide (NO), NO x and SO x contaminants, among others, from the exhaust engine. [4][5][6] 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. 13 The adsorption of NO on the supported catalyst surface has been investigated by both experimental 7,14-20 and theoretical procedures. [21][22][23][24][25][26][27][28][29] NO adsorbs on small Pd clusters through its nitrogen atom in a tilted orientation, preferentially in the hcp threefold and bridge sites, 27 although the relative binding energies are strongly dependent on the size and geometry of the cluster employed. 27 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. 29 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. 29 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 N...

Effect of the Metal–Support Interaction on the Adsorption of NO on Pd4/γ-Al2O3: A Density Functional Theory and Natural Bond Orbital Study

“…The ground electronic spin state for the Pd 4 /Al 14 O 24 H 6 complex is the triplet state, which is the same as that found for an isolated tetrahedral Pd 4 cluster. By approximation, the singlet state of the isolated Al 14 O 24 H 6 aggregate was considered .…”

“…The N–O bond distance does not vary much with the adsorption site and is 1.151 and 1.156 Å for adsorption in the on-top 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, the NO molecule adsorbs in a tilted orientation on the palladium atoms in the Pd 4 /Al 14 O 24 H 6 model. The Pd–N–O bond angles are 135.07° and 129.18° for adsorption on Pd(1) and Pd(3), respectively.…”

“…In the present work, we continue our previous study concerning the adsorption of NO on isolated Pd 4 and on Pd 4 supported on the (110C) face of γ-Al 2 O 3 . As previously reported, the model used to represent the alumina support is restricted to three sheets of atoms containing 14 aluminum and 24 oxygen atoms.…”

“…In the present work, we continue our previous study concerning the adsorption of NO on isolated Pd 4 and on Pd 4 supported on the (110C) face of γ-Al 2 O 3 . As previously reported, 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 Al 14 O 24 H 6 stoichiometry.…”

“…Several studies have reported that the SMSI effect is promoted by different metal oxide supports, such as TiO 2 , − ,, CeO 2 , ,, SiO 2 , ZrO 2 , and Al 2 O 3 . ,, The latter support, mainly in the γ phase (γ-Al 2 O 3 ), is one of the most employed oxides in supported catalysts ,− due to its high specific area, well-defined pore size, stability over a large temperature range and acid/base properties. , The structure of γ-Al 2 O 3 includes octahedral and tetrahedral aluminum cations occupying the interstitial sites between oxygen anions . 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. ,− In a DFT study on formaldehyde adsorption on a Pd 4 /γ-Al 2 O 3 model, we showed that the highly coordinated octahedral aluminum cations can donate electron density to the supported Pd 4 cluster . In contrast, tetrahedral aluminum cations have acidic properties and therefore act as electron acceptors.…”