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 an 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.
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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