Estudo de nanopartículas bimetálicas de metais de transição com 55 átomos: Pt em combinação com Y - Tc via Teoria do Funcional da Densidade

Abstract

In this master’s thesis we performed a theoretical study about transition-metal (TM) bimetallic nanoparticles (NPs) with 55-atoms, using the Density Functional Theory (DFT). The study was based on Pt-based nanoalloys in combination with TMs from the beginning of the 4d series, PtnMT55−n (MT = Y, Zr, Nb, Mo, and Tc), we have tried to understand the energetic stabilization, structural patterns, and the changes in the energetic, structural, and electronic properties in relation to the Pt composition (from 0% to 100%). Firstly, we performed the study of the unary systems, in both configurations, bulk phase and TM55 NPs. From these results, we have confirmed the adopted methodology and obtained good agreement with the literature. In the 55-atoms unary systems study we focused on to prove that distorted reduced-core (DRC) structures are obtained for systems with a large occupation of the d states, as is the case of Zn55 (3d), Cd55 (4d), Pt55 and Au55 (5d). The DRC structures, with 8 − 11 atoms in the core, are obtained in contrast to compact high-symmetry structures, such as the icosahedron (ICO), with 13 atoms in the core. As a result, we found that Y55 adopts ICO structure, while Nb55, Mo55, and Tc55 adopt bulk fragment structures. Zr55 adopted a DRC structure, similar to Zn55, Cd55, Pt55, and Au55, so, we conclude that the preference for DRC structure is not related to the occupation of anti-ligand d-states, but with different effects, a combination of electronic and structural effects. Furthermore, the binding energy follows a trend of the occupation of the ligand and anti-ligand states, thus, the energetic stability increases from Y to Tc, which may also explain the averaged bond lengths tendency. For PtTM nanoalloys, we have obtained the most stable structures, as well as, the most interesting compositions from the experimental point of view, by analyzing the excess energy, i.e., n = 42 for PtY, n = 35 for PtZr and PtNb, n = 28 − 42 for PtMo, and n = 42 for PtTc. The structural patterns follow the trend where for low or no Pt composition the lowest energy structures belong to the TM55 family, for high or maximum Pt composition it occurs the formation of the DRC structural model (Pt55), and for intermediate Pt compositions, it occurs variations in the structural models, which may be DRC, ICO, or bulk fragments. The nanoalloy formation occurs through a combination of factors, specifically, the atomic size (atomic radius) and the strength of the nanoalloy bonds. The localization of the smaller atoms (small atomic radius) in the core region and larger atoms (large atomic radius) in surface region results in an internal strain relief in the NP, which represents an energetic gain for the system. However, the atomic radius differences argument is not enough to fully explain the TM localization (surface or core region), it is necessary to also consider information about the strength of the bonds between the nanoalloy constituents. For Pt42Mo13 and Pt42Tc13 core-shell structures are found, with Pt atoms in the surface and Mo/Tc atoms in the core, favouring the Pt-Pt and TM-TM bonds. About electronic properties, we found that only Y55 and PtY nanoalloys with low Pt percentage present a significant magnetic moment value. Considering the center of gravity of the d-band as an indicator of favourable systems for catalysis, we found that the change in Pt composition consists of a good way to change the center of gravity of the occupied d-states.