Alla ricerca della comprensione della dinamica dell'ossigeno in catalizzatori Cu-perovskite per abbattimento di inquinanti atmosferici

In this work, perovskite and Cu-perovskite nanocomposite catalysts are prepared via different methods and tested as air pollution abatement materials. The general characterization, carried out via XRD, SEM, EDX, XPS, H2-TPR and N2-isotherm absorption to measure BET surface area, as well as the catalytic characterization in model reactions, were mainly directed towards the investigation of the redox properties of these materials. Chapter 1 explores the possibility of obtaining active Three Way Catalysts using only non-Critical Raw Materials in the perovskite formulation. A series of Ba1-xSrxMg0.2Mn0.8-yCuyO3 catalysts has been studied. General considerations include the fact that perovskites are only partially activated towards NO reduction, materials are quite active towards oxidation reactions but not commercially viable. Chapter 2 focused on the oxidation of carbon soot, achieving great performances on La0.9K0.1M0.9Co0.1O3, thanks to the surface-enhancement of reactivity given by K-doping and the increased mobility in the lattice generated by Co presence. In Chapter 3 the study of several CuO@perovskite materials was fundamental to assess a Metal-perovskite interaction that strongly modifies reactivity. In particular, Mn containing supports proved to increase the oxidative character of CuO, by somehow disposing a higher oxygen content to the contact interface, whereas Cr-based supports were effective in allowing Cu to reduce and activate towards NO reduction. The proposed description of the phenomena observed relies on the description of a M-O-M system in which the relative acidity of cations determines how the electron density on the O-bridging atom is shifted from one side or the other of the contact interface upon interaction with the surface chemical potential imposed by the different reaction atmospheres. The considerations drawn led to the synthesis of Mn-doped Cr-based supports, with conversely different A-site dopings. CuO@La0.8Sr0.2Cr0.9Mn0.1O3 proved to be the most active material in a CO/NO/O2 mixture. The effect of Sr seems more related to transport of oxygen in comparison with K, Mn helps in boosting the oxidation performances of the perovskite increasing O-supply to Cu. In Chapter 4 and 5 the Cu-perovskite interaction is explored and systematically investigated in function of different Sr-dopings in the A-site, changing B-site element from Mn to Fe, and testing the materials at various temperatures, in steady-state and unsteady-state conditions. It is found that the stability of the B-site M4+/M3+ generated by A-site doping determines whether this modification will increase or decrease the oxygen supply at the interface with Cu. Stable Mn4+ proves to be more effective in inducing chemisorption and storage of oxygen than unstable Fe4+, that instead is more activated towards O-transport. Two directions are possible in the Cu-O-M system, depending on which side attracts oxygen the most. Reducible substrate may donate O to Cu in response to a reducing surface potential, whereas if transport in the support is not activated, Cu could be depleted of oxygen by interaction with the perovskite. It can be inferred that there is a certain amount of active oxygen that it is possible to generate in such catalysts, and that it concentrates on different active sites depending on the reaction conditions. Chapter 6 targets direct NO decomposition, exploiting the amazing possibilities given by exsolution technique, that maximizes the Cu-perovskite interaction via creation of the socket interface, that preserves them from sintering as well. The performances obtained are strongly encouraging: catalysts prove to be active up to 90% conversion in a wide range of temperatures and also in presence of small percentages of oxygen in the reaction atmosphere. Performances are stable in time and not influenced by the exposure to oxidative atmospheres.