Adsorção molecular em subnanoclusters de metais de transição: um estudo via Teoria do Funcional da Densidade

Abstract

Transition Metal Subnanoclusters are extensively explored for their potential applications, particularly in nanocatalysis. The reduced volume of the nanoparticle, contrasting with a high number of atoms with low coordination exposed to the surface, results in properties distinct from those observed at the crystalline scale. At the nanoscale, these systems tend to be more reactive, facilitating the study of adsorption in molecular environments. In this study, we employed Density Functional Theory to investigate molecular adsorption in Transition Metal Subnanoclusters, describing their key properties and the primary mechanisms of adsorption of molecular species in accordance with the Sabatier principle. We conducted studies on the adsorption of CO, NO, N2, and O2 species on Ag, Au, and Pt nanoclusters, ranging in size from 2 to 7 atoms. Using an energetic criterion (stability function), the most stable sizes for Ag and Au systems were 4 and 6 atoms, while for Pt, they were 3 and 6 atoms—all chosen for the adsorption of molecular species. Ag and Au systems exhibit electronic behaviors similar to hydrogenoids, distinguishing them from Pt systems, which predominantly show contributions from s and d orbitals in their electronic properties. Therefore, we separated these systems into two studies. In the first study, we investigated molecular adsorption in Ag and Au systems in the gas phase, describing the main contributions in terms of interaction and adsorption to classify them as ideal catalysts. The interaction energy, ∆Eint, proved to be an excellent classifier for describing our systems as ideal catalysts. The condition where 0 (physisorption) < ∆Eint < Ebclu (chemisorption) establishes the limits of interaction between the catalyst and reaction intermediates, with a notable example being the physisorption of N2 on Ag6. Additionally, the energy decomposition analysis revealed the contributions of physical nature to the interaction, where closed-shell (open-shell) molecular systems predominantly exhibited electrostatic (covalent) contributions. In the second study, we investigated the mechanisms of molecule adsorption on Pt systems in the gas phase and supported on graphene, describing the main contributions to characterizing these systems as ideal catalysts. Once again, ∆Eint proved to be an excellent classifier for our systems as ideal catalysts, establishing the limits of interaction between the catalyst and reaction intermediates. We highlighted the chemisorption of O2 on Pt6, characterized by an oxidation process. Bader charge analysis revealed the influence of sd and pd hybridizations, as well as the electronegativity of molecular species, on the trend of charge density transfer, allowing us to evaluate oxidative processes associated with molecular adsorption. Furthermore, in this study, we investigated the stability of Pt nanosystems when supported on graphene nanosheets, describing changes in their main properties and mechanisms of molecule adsorption. Pair interactions between nanoclusters and molecules, as well as between nanoclusters and graphene nanosheets, were characterized as the primary interactions compared to secondorder interactions, such as surface interactions with molecules, providing a qualitative analysis and a quantitative estimation of the interaction in the system of adsorbed molecules on supported subnanoclusters. With this study, we shed light on investigations in nanocatalysis, enabling experimental studies on the main mechanisms of molecule adsorption in subnanometric systems, such as metallic subnanoclusters.