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 Pd-4, cluster supported on a gamma-alumina model (Al14O24H6)) and its effect on the adsorption of a single NO molecule. Our results show that the Pd-4-Al14O24H6 interaction leads to a reduction in the cohesion energy among palladium atoms, promoting greater dispersion on the gamma-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 effect 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 support electronic effects.