A DFT study of the NO adsorption on Pdn (n=1–4) clusters

Abstract

We report a density-functional study of some properties of the adsorption process of the NO molecule on small palladium clusters (n = 1-4). The interaction between NO and the Pd n clusters is studied on various adsorption sites. Both, NO and Pd n geometrical relaxations are taken into account. The significant conformational reconstruction of the metallic cluster upon NO adsorption induces a large decrease of the NO adsorption energy. Nevertheless, the NO binding energy is strongly weakened when the molecule is adsorbed on the small Pd n clusters due essentially to an electrostatic repulsion between both N and O atoms. The possible dissociation process of NO on Pd 4 cluster is then investigated within two processes: the NO molecule does not dissociate on Pd 4 with process (i) (dissociation of the isolated gas phase NO molecule followed by the adsorption of both nitrogen and oxygen atoms on the cluster). Process (ii) which presents three successive steps (adsorption of the NO molecule, dissociation of the NO molecule adsorbed on Pd 4 , adsorption of the O atom on the cluster) is studied in details and we propose a reaction pathway locating transition states and intermediate species. The activation energy for process (ii) is high and the dissociation of the NO molecule on the Pd 4 cluster is thus highly improbable.

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

“…The energy for atop adsorption on the distorted arrangement of Pd 4 (the only adsorption mode found for supported Pd 4 ) 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 Pd n cluster, as found in previous studies, 22,23,50 with palladium atoms having higher coordination number showing lower ability to accept the NO molecule.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 energy for atop adsorption on the distorted arrangement of Pd 4 (the only adsorption mode found for supported Pd 4 ) 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 Pd n cluster, as found in previous studies, 22,23,50 with palladium atoms having higher coordination number showing lower ability to accept the NO molecule.…”

“…22,23 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. (Figures 1f and 1g).…”

“…13 The adsorption of NO on the supported catalyst surface has been investigated by both experimental 7,[14][15][16][17][18][19][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.…”

“…In the vast literature on theoretical studies of NO adsorption on metal surfaces, the catalyst is frequently represented as a single crystal 29 or as a small atomic cluster, [21][22][23][24]27,28 although evidence shows that the support Vol. 27, No.…”