Estudo Computacional de Primeiros Princípios dos Efeitos de Ligantes nas Propriedades Estruturais, Energéticas e Eletrônicas de Nanoclusters e Nanoligas

Abstract

In this thesis we performed a theoretical study based on density functional theory cal culations within van der Waals D3 corrections, in order to investigate the properties of molecular adsorption and effects of ligands in nanoclusters with 13 and 55 atoms and na noalloys with 55 atoms. First, we performed the molecular adsorption of CO2 on 13-atom TM nanoclusters (TM = Ru, Rh, Pd, Ag), where our results showed that the adsorption energy is more intense for metals from the beginning of the 4d series, increasing from Ru to Ag, except in the ICO (icosahedral) configuration. Furthermore, adsorption on these sys tems changed the geometry of the molecule, taking on an angular shape. In these cases, the adsorption process on clusters happens through the interaction mechanism namely chemisorption for angular CO2 and physisorption for linear CO2 . Thus, the activation of CO2 occurs only through chemisorption, in which the cluster transfers charge to the molecule, making it anionic, being proven by our analysis of Bader effective charge and vibrational frequency. To study the effect of ligand on these nanoclusters, we performed the adsorption of 6CO on five structural models for each system. Thus, we verify that there is a change stability, where the lowest energy configurations (LOW) are ICO, except for Ru. The change stability occurs due to the increase in the average bond length of the LOW configuration compared to the more stable configuration without a molecule, where the geometry of the nanocluster plays an important role in stabilization. In these systems, the C-TM interaction is more intense than the C-O interation, which makes the LOW configuration more stable. In addition, we verified that the stability change occurs with 3 (Ru13), 4 (Rh13), 2 (Pd13) and 4 (Ag13) CO molecules, where the adsorption energy re duces the intensity with the increase in number of ligands. Thus, the greater the number of ligands, the more stable the system will be, as there is a reduction in the number of active sites and consequently, it reduces the intensity of the adsorption energy. Finally, we performed the adsorption of CO, NO and H2 on larger systems (nanocluster and na noalloys of PdAu with 55 atoms), where we consider the main energetic, structural and electronic properties in relation to the composition of Pd. We obtained the lowest energy configurations for nanoclusters and nanoalloys and, based on excess energy, we identified the configuration with maximum stability, n = 20. For molecular adsorption, we consider Pd55, Au55 and Pd20Au35, in which we verified that systems with adsorbed NO (H2 ) have more (less) intense adsorption energies (in modulus). Thus, the molecules prefer the top site, where the Au atoms are found in the surface region (atoms directly exposed to the vacuum) and Pd atoms in the core region. However, the molecule-Pd interactions are more intense (in modulus) than the molecule-Au interactions, since the center of gravity of the occupied d states of the Pd is closer to the Fermi energy than of Au. Thus, we verified that the size and composition of the nanocluster is important in the adsorption process.