Novel Catalysts for CO2 and CH4 Valorization/H2 Production Prepared via Mechanochemical Synthetic Approach

The scope of this PhD work was to investigate the use of a novel mild-energy ball milling technique, which gave promising results in previous studies carried out by our research group, for the synthesis of cheap novel catalysts for environmental applications. Two series of Ni/CeO2 catalysts were synthesized starting from two Ni precursors (NiCl2 . 6H2O and Ni(NO3)2 . 6H2O) by varying the milling time and tested in Methane Dry Reforming and characterized by means of XRD, BET, TPR, HRTEM and XPS. For the two series of catalysts we found that the best performing catalysts in terms of activity and stability were the ones milled at low-medium milling times, thus establishing that a mild mechanical action is useful to obtain very strong Ce-Ni interactions responsible for increased catalytic activity. In particular, for the nitrate sample milled for 10 minutes, the HRTEM analysis revealed a rugosity diffused on the surface of the ceria crystallites in which the presence of some sub-nanometric Ni entities was detected. More detailed XPS studies performed on the same materials revealed the presence of a localized interaction between cerium and nickel. The nature of this interaction is related to the formation of localized Ni-O-Ce surface arrangements. Regarding the samples milled for longer times, we found that prolongated milling action is responsible for their incorporation in the bulk of the crystallites, thus explaining the observed minor catalytic activity for these catalysts. We also prepared a series of CeO2-CuO milled composites by varying Cerium molar ratio. The purpose of this study was to promote the redox exchange between the surface Ce3+ sites of a high surface area ceria and the Cu2+ sites of the CuO with the gentle mechanical energy provided by the ball milling technique. The synthesized materials together with a reference analogue material synthesized via impregnation method, were characterized by means of XRD, BET, TPR, in-situ DRIFT and operando NEXAFS. We studied the materials with a multi analytical approach which helped us in the understanding of the mechanism of methane activation at low temperature. A careful in-situ DRIFT analysis demonstrated that the synthesized composites were able to activate methane at 250°C in contrast to the reference materials. Through operando-NEXAFS experiments, we showed that this activity was related to the formation of a stable and reversible Ce4+/Cu+ couple generated from the mechanical action provided by the synthesis. The analysis of products which was carried out during the operando measurements in a low ceria content sample, identified traces of formaldehyde and methanol. This candidates these materials to be promising catalysts for the partial oxidation of methane to oxygenates. In order to promote the surface interaction between ceria and the transition metal we have employed for the above studies samples of commercial high surface area (HSA) ceria, utilizing the presence of Ce3+ sites to promote the redox exchange with Cu/Ni particles. For this reason we developed a synthetic procedure for HSA ceria which gave good results since specific surface area of more than 300 m2/g and 40m2/g were achieved after calcination at 350°C and 900°C for 6h respectively. In conclusion, this PhD thesis we found that the mild shear stresses provided by the mechano-synthesis were functional to the creation of metastable arrangements responsible of improved catalytic activity compared to reference materials synthesized via the well-established impregnation method. The formation of these arrangements demonstrated to be favored by low milling times and confined on the surface of the ceria crystallites. This synthetic technique also demonstrated to promote redox exchanges between metal ions of different transition metal oxides, creating active catalytic sites.