Ab Initio Investigation of Self-Assembled Nanostructures for Catalytic Applications

This work is focused on the characterization of different nanostructures self-assembled on appropriate substrates and on the preliminary investigation of their potential cat- alytic performance using quantum mechanical simulations based on ab-initio Density Functional Theory. The investigation is carried on in close comparison with recent experimental data. The systems under investigation include metallic Platinum nanoclusters and hybrid ordered structures with organic molecules, assembled on regular 2D templates, which could prevent syntering and deactivation of the catalysts. The interaction of such nanostructures with carbon mono- and di-oxide molecules is addressed, studying in particular CO oxidation on Platinum nanoclusters and CO2 activation on organic molecules. Firstly, the growth of Platinum nanoclusters on different regions of the moiré pattern of graphene on Ir(111) is investigated: a stronger pinning of the graphene beneath and in the vicinity of the adsorbed clusters explains their higher stability in specific regions of the moiré, named hcp, from the registry of the center of the carbon hexagon of graphene with the underlying substrate. Remarkably, the steadiness of the metallic aggregates depends on their size: larger nanoclusters are more stable than very small ones (containing few atoms only), but this is not sufficient to guarantee their stability upon CO interaction. In fact, nanoclusters of about 20-30 atoms are affected by a deep restructuring under an increasing flux of the reactant. Metal Phtalocyanines can be considered in a biomimetic approach to efficiently model single atom catalysts. The self-assembling of such molecules, in particular Iron Phtalocyanines (FePcs) forms regular arrays of catalytically active single atoms at their centers. Efficient templates, in addition to the moiré pattern of graphene on Ir(111) previously mentioned, are oxide surfaces such as Al2O3/Ni3Al(111). The molecule-surface and molecule-molecule interactions induce a regular molecular array on this surface, with molecular vacancies forming a hexagonal lattice with the same periodicity of the substrate. For high molecular coverage, multilayers with the same structure but alternate chirality are formed, even before the completion of the first monolayer. The self-assembled FePcs pattern can be controlled by dosing metal deposition on the Al2O3/Ni3Al(111) surface. In presence of Cu clusters, that likely fill the oxygen vacancies at the surface making the alumina surface potential smoother, the FePcs fully cover the oxide template and form uniform long range ordered structure with an almost square periodicity. The size of the Cu clusters affects the structural stability of the molecular array. Scanning Tunneling Microscopy images show the presence of a few molecules with a dark protrusion at their center, not observed while depositing FePcs on the pristine oxide. The comparison between simulated and experimental images and energetic arguments suggest that they are demetalated phthalocyanines (Pcs). The catalytic activity of FePcs was exploited with respect to the interaction with CO2. Tuning the chemical environment of FePc molecules, for instance oxidizing a graphene substrate, CO2 can be activated through an electron transfer from the FePc. In summary, this thesis shows that the atomic-scale study of nanostructured cat- alysts allow to understand the mechanisms governing their self-assembling and their activity, opening the way to a full control of their stability and performance.