Propriedades estruturais e eletrônicas de partículas de 13 e 55 átomos de metais de transição

Abstract

In this thesis we performed a theoretical study of the structural, electronic, and magnetic properties of transition metal (TM) particles using two models, with 13 and 55 atoms to describe clusters, nanoparticles (NPs), nanoalloys, protected NPs, and adsorption on clusters by Density Functional Theory. Firstly, we performed a systematic study for 3d, 4d, and 5d TMs of the Periodic Table using clusters with 13 atoms. This study gives the trends of the properties as function of the d occupation. We implemented a strategy to obtain the clusters structures, which is based on high-temperature molecular dynamic calculations and simulated annealing. New lower energy configurations were identified for some 13 atom clusters and previous known structures were confirmed. The following conclusions were identified: (i) The analysis of the binding energies and average bond lengths show a parabolic-like shape as a function of the occupation of the d states and hence, most of the properties can be explained by the chemistry picture of occupation of the bonding and antibonding states. (ii) Ground state structures are seen to depend on the d band occupation, with compact icosahedral-like (ICO) forms at the beginning of each metal series, more opened structures such as hexagonal bilayer-like (HBL) and double simple-cubic (DSC) layers at the middle of each metal series, and structures with an increasing effective coordination number occur for large d states occupation. (iii) For Au13, we found that spin-orbit coupling favors 3D structures, i.e., a 3D structure is about 0.10 eV lower in energy than the previously assumed lowest energy 2D configuration. (iv) The magnetic exchange interactions play an important role for particular systems such as Fe, Cr, and Mn. Several trends are similar for clusters and bulk, however, the atomic structures for Ru13, Rh13, Os13, and Ir13 are considered unexpected, since the respective elemental crystals crystallize in compact structures. In this context, we employed different local, semilocal, and non-local exchange and correlation energy functional, to understand the performance of different exchange and correlation schemes in the prediction of the physical and chemical properties of TM clusters. The local and semilocal functionals confirm the DSC configuration as the lowest energy structure for the studied TM13 clusters. A good agreement in the relative total energies is obtained even for structures with small energy differences, i.e., the PBE (Perdew, Burke, and Ernzerhof) results are confirmed. With the study employing PBE+U and hybrid functionals we found that a partial correction of the self-interaction problem decreases the relative stability of opened structures such as the DSC, and hence, compact structures became the lowest energy ones. The sd hybridization helps to explain the dependence of the structural stabilities with the self-interaction correction. We found that, for Co13 and Rh13, the sd hybridization decreases for DSC and increases for ICO. The study of NO adsorption on TM13 clusters, such as: Rh13, Pd13, Ir13 and Pt13, and the comparison with the results obtained for the respective TM(111) surfaces, allowed the finding that the adsorption on clusters changes significantly, with a strong dependence of the chemical environment close to the adsorption sites, whereas the trend obtained for the characteristic geometric parameters are similar to those observed for NO/TM (111). For the TM55 we get that Co55 and Rh55 NPs have ICO lowest energy structures, contrarily to the respective 13 atoms clusters. For Pt55 and Au55 NPs we found a non-icosahedral structure, with lower symmetry and the reduced core size, 7 - 9 atoms, which is very important for catalysis due to the larger number of atoms at the surface. After the TM55 study, we performed the study for PtnTM55-n (TM = Co, Rh, Au) nanoalloys as a function of the composition (n). It is confirmed that PtTM NPs prefer a composition pattern where the Co and Rh (Pt) atoms are in the core region and Pt (Au) atoms are at the surface region. Furthermore, we get that PtnRh55-n and, especially, PtnCo55-n tend to form alloys, mainly between n = 28 42 and n = 20 42, where the core-shell ICO configurations (Pt42Co13 and Pt42Rh13) are stable for both systems, due to the different atomic sizes that cause a release of stress in the NPs. For PtnAu55-n nanoalloys only n = 13 is energetically favorable, forming a core-shell structure. For the other compositions of PtAu we have the same trend as for the crystalline alloys reported experimentally, i.e., non-alloy formation. The effects on the catalytic properties of mixing two-TMs can be understood through the shift of the gravity center of the d occupied states. In this analysis, we observed that it is possible to obtain PtTM nanoalloys that can be more affordable and have better catalytic properties than pure Pt NPs. In terms of magnetic properties, we found that Pt55 and Co55 have smaller and larger values of magnetic moments, respectively, so PtCo follows the tendency where the Co atoms dominate the magnetic properties. For PtRh, the magnetic moment values are higher than for pure NPs. In the case of PtAu we observed the same trend, although with a lower magnitude. The lowest energy structures for Pt55 and Au55 are non-icosahedral, with an unexpectedly small core. Thus, we study these systems adding ligands, and verifying the changes in the stability. We studied the interaction of TM NPs with ligands such as: PH3, PH2, and SH2, in order to verify the changes in stability, structural, and electronic properties. We obtained that the relative stability differences between ICO and LOW (lowest energy configuration) structures decreases with the use of ligands. The LOW structures are not the most stable (Au) or very similar in energy than ICO structures (Pt) when 18 ligands are added to NPs.