Experimental modeling of nanostructured and single metal atom supported catalysts at close-to-ambient conditions.

This Thesis work deals with the growth and characterization of model nanostructured surface systems in ultra-high vacuum environment (UHV, <10−9 mbar) and with their evolution at near ambient pressure (NAP, 0.1 - 100 mbar) conditions. The investigations are performed with the aid of specific in situ techniques (IR-Vis SFG, NAP-XPS, etc.) in order to probe the structural, electronic, chemical and catalytic properties of the models. The latter span from ordered lattices of metal nanoparticles to 2D metallorganic crystals, where stabilized mono-metallic centers act as the active cores. These systems, based on single metal atom centers, represent the main topic of this manuscript and they will be referred to as Single Metal Atom Catalysts (SMAC). The discussion of the scientific findings will first focus on the evolution of graphene supported Pt nanoclusters in CO atmosphere, varying both surface temperature and CO pressure to test the stability of the nanostructures. As degradation of this nanosystem occurs at realistic reaction conditions, the attention was shifted to the design and synthesis of model SMAC systems, where the single metal atom is stabilized in a metallorganic cage, thus preventing structural degradation. A first, prototype SMAC model system consisted of a single layer of Nickel tetraphenyl porphyrins (Ni-TPPs) deposited on the Cu(100) surface. We proved that, following to NO exposure, a hyponitrite species (N2O2) readily forms at the Ni sites already at UHV conditions and is stable at room temperature. The NO conversion is observed only on the NiTPP monolayer interacting with the underlying copper surface, showing that the substrate plays a major role, governing the properties of the nanostructured system through trans-effects associated with a strong surface-to-molecule charge transfer. A single Iron Phthalocyanine (FePc) layer was instead considered for a model carbonylation reaction. The metalorganic molecules were deposited both on a single foil of graphene, grown on the Ir(111) surface (FePc/GR), and on an alumina ultra-thin film, grown on the Ni3Al(111) surface (FePc/alumina). In both cases, we exploited CO adsorption to probe the molecular active sites. On the FePc/GR layer, IR-Vis SFG evidenced unexpected CO stretching modes in 1-10 mbar CO at 300 K. We ascribe the observed vibrational features to the production of long-lived molecular excitons (induced by the visible radiation). The long lifetime of these excitons and their efficient production through singlet-fission mechanisms represent intriguing findings for innovative organic devices for solar energy conversion. We also investigated the interaction of the same system with gas-phase CO2 We found that oxidation of the underlying graphene support yields the control of the charge transfer to the active sites, thus reducing the threshold pressure for CO2 adsorption and activation at 300 K by at least two orders of magnitude. As CO2 catalytic conversion is hindered by its low reactivity, enhancing its adsorption to metal sites is crucial in the framework of the efficient conversion of this waste gas to valuable chemicals. A practical route to alter the mesoscopic properties of the single metal atom centers has been found, and in parallel we proved a novel graphene oxidation route employing molecular oxygen at near ambient pressure. Concerning instead the FePc/alumina film, we demonstrated that decoration by Cu nanoclusters tunes the surface potential energy, inducing a different symmetry in the molecular overlayer lattice, scarcely affecting the reactivity of the metallic sites, as proved by the vibrational modes of adsorbed CO molecules. Thus, we succeeded in tailoring the motif of a self-assembled metallorganic layer while preserving its active sites properties.