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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...

The role played by the metal − support (MSI) and metal − metal (MMI) interactions on two important processes in controlling the catalyst performance -nucleation and molecular adsorptionhas been investigated using DFT (B3LYP) combined with LMOEDA and NBO calculations, with aid of a Pd 4 /γalumina (110D) model (Pd 4 /Al 13 O 23 H 7 ). Our results indicate the occurrence of an electronic effect (EMSI) at the metal − support interface which induces a most intense charge transfer in the Pd 4 →γ-alumina backdonation direction, most expressive in Pd→Al, promoting an electronic redistribution within the units and attenuating the MMI. Nevertheless, the MSI/MMI synergistic effect seems to favor slightly the nucleation of a fth palladium atom, leading to a distorted square pyramidal arrangement for Pd 5 . The LMOEDA analysis points to a mostly covalent character in the Pd − Al bonds, whereas the Pd − O bonds are mainly electrostatic in nature. The palladium atoms deposited on oxygen anions are the acid centers, where both NO molecule and an additional palladium atom anchor more strongly. In addition, the MSI/MMI effect, through the electronic and geometric contributions, drives the adsorption of the NO molecule to the mode which most favors the Pd→NO (4d z 2 →2π * ) backdonation (bridge mode).

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The role played by the metal − support (MSI) and metal − metal (MMI) interactions on two important processes in controlling the catalyst performance - nucleation and molecular adsorption – has been investigated using DFT (B3LYP) combined with LMOEDA and NBO calculations, with aid of a Pd4/γ-alumina (110D) model (Pd4/Al13O23H7). Our results indicate the occurrence of an electronic effect (EMSI) at the metal − support interface which induces a most intense charge transfer in the Pd4→γ-alumina backdonation direction, most expressive in Pd→Al, promoting an electronic redistribution within the units and attenuating the MMI. Nevertheless, the MSI/MMI synergistic effect seems to favor slightly the nucleation of a fifth palladium atom, leading to a distorted square pyramidal arrangement for Pd5. The LMOEDA analysis points to a mostly covalent character in the Pd − Al bonds, whereas the Pd − O bonds are mainly electrostatic in nature. The palladium atoms deposited on oxygen anions are the acid centers, where both NO molecule and an additional palladium atom anchor more strongly. In addition, the MSI/MMI effect, through the electronic and geometric contributions, drives the adsorption of the NO molecule to the mode which most favors the Pd→NO (4dz2→2π*) backdonation (bridge mode).