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

Density functional theory (B3LYP) was employed to analyze the metal–support interaction in a Pd₄ cluster supported on a γ-alumina model <b>(</b>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