Studio della conversione di metano in metanolo in Cu-zeoliti tramite spettroscopie in situ NEXAFS e X-ray Raman Scattering

This thesis focuses on the data treatment and analysis of two experiments performed at APE-HE (ELETTRA) and at ID20 (ESRF), both targeting soft-XAS-based characterization of Cu-exchanged zeolites using different techniques, namely NEXAFS and XRS. In addition, ab initio calculations of O K-edge spectra on seven different Cu-exchanged zeolites models were realized. The first part of the thesis activity concerns O K-edge FDMNES simulations of a set of DFT optimized chabazite (CHA) and mordenite (MOR) models containing Cu-oxo species that are of interest for direct conversion of methane to methanol (DMTM) applications. These theoretical spectra are of general interest, allowing us to identify general trends through the systematic screening of different models. Moreover, they will be useful as a reference in the interpretation of the experimental data from the ID20 experiment. The second part of this work consists in the data treatment of NEXAFS spectra concerning Cu chabazite (Cu-CHA) catalysts with two different compositions obtained under in situ conditions. The measurements monitored the O K-edge and Cu L-edges during three main treatment steps, namely: (1) activation of the zeolite at 350 °C under He gas flow, (2) adsorption of CO at the catalysts surface and (3) desorption of CO under He gas flow. The spectra shown the characteristic self-reduction process during the activation and the presence of different Cu-carbonyls depending on the temperature conditions of the second and third experimental phase, giving an insight to the possible Cu speciation present at the surface after the He-Activation. The third part of this thesis contemplates an X-ray Raman Scattering (XRS) in situ experiment on different Cu-zeolites frameworks, under conditions relevant to DMTM, foreseen in April 2021 but postponed to September 2021 due to COVID restrictions. In these experiments the Si L2,3-edges and O K-edge were monitored after two main treatment steps: (1) O2 activation at 500 °C and (2) CH4 loading at 200 °C. The results show small differences between the different states and the sample composition that are linked to the activated state. Unfortunately, due to the rescheduling of the experiment, only a preliminary analysis is presented.