Adsorption of NO on the Rh₁₃, Pd₁₃, Ir₁₃, and Pt₁₃Clusters: A Density Functional Theory Investigation

Abstract

The adsorption of NO on transition-metal (TM) surfaces has been widely studied by experimental and theoretical techniques; however, our atomistic understanding of the interaction of nitrogen monoxide (NO) with small TM clusters is far from satisfactory, which compromises a deep understanding of real catalyst devices. In this study, we report a density functional theory study of the adsorption properties of NO on the TM 13 (TM = Rh, Pd, Ir, Pt) clusters employing the projected augmented wave method. We found that the interaction of NO with TM 13 is much more complex than that for NO/TM(111). In particular, for low symmetry TM 13 clusters, there is a strong rearrangement of the electronic charge density upon NO adsorption and, as a consequence, the adsorption energy shows a very complex dependence even for adsorption sites with the same local effective coordination. We found a strong enhancement of the binding energy of NO to the TM 13 clusters compared with the TM(111) surfaces, as the antibonding NO states are not occupied for NO/TM 13 , and the general relationship based on the d-band model between adsorption energy and the center of gravity of the occupied d-states does not hold for the studied TM 13 clusters, in particular, for clusters with low symmetry. In contrast with the adsorption energy trends, the geometric NO/TM 13 parameters and the vibrational N−O frequencies for different coordination sites follow the same trend as for the respective TM(111) surfaces, while the changes in the frequencies between different surfaces and TM 13 clusters reflect the strong NO−TM 13 interaction.

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.…”

σ-Holes on Transition Metal Nanoclusters and Their Influence on the Local Lewis Acidity

“…The DFT methods are, in addition, known to exhibit difficulties in describing the multi-configurational character of certain TM elements, the exceptions being e.g., Au, Ag and Cu, whereas, as discussed further in the supplementary material (Sections S1 and S3), large spin contaminations are found for Ir 13 and, especially, Rh 13 , indicating possible multi-configurational states. It can, moreover, be noted that the d-band model has also been reported to fail to reproduce adsorption trends for e.g., the Rh 13 , Ir 13 and Pt 13 nanoclusters [16].…”

“…The initial geometries for the Au 13 , Cu 13 , Pt 13 , Pt 7 Cu 6 , Pd 13 , Co 13 , Ir 13 , Rh 13 , and Ru 13 TM nanoclusters were taken from documented low-energy structures ( Figure 2) [14][15][16][17][18][19][20][21][22][23]. These were reoptimized under symmetry constraints in the Turbomole 6.4 software package [13] using the Def2-TVZP basis set; the Def2 basis set family of Ahlrichs and co-workers [12] employs effective core potentials (ECPs) for the 4d and 5d, but not for the 3d metals.…”

“…In the present work, we shall investigate the presence and absence of the various kinds of σ-holes on a series of neutral, low-energy TM 13 nanoclusters [14][15][16][17][18][19][20][21][22][23]. The nanoclusters comprise the mixed elemental Pt 7 Cu 6 nanocluster as well as the Ir 13 , Pt 13 , Au 13 , Pd 13 , Rh 13 , Ru 13 , Cu 13 and Co 13 clusters.…”