RIDUZIONE Dl CO2 SU CATALIZZATORE AD OSSIDO MISTO STUDIATA IN CONDIZIONI Dl REAZIONE CON ASSORBIMENTO E DIFFRAZIONE Dl RAGGI X

The study focuses on characterizing a Gallium/Zirconium mixed oxide using advanced spectroscopic and diffraction methods to understand Gallium role as a substitutional atom within the ZrO2 lattice, particularly its interaction with oxygen vacancies and changes in coordination geometry. Additionally, the research explores the potential of MES-PSD, an advanced technique, to enhance spectroscopic sensitivity to active sites in heterogeneous catalysis. The thesis included X-ray diffraction (XRD) and X-ray absorption (XAS) measurements at the European Synchrotron Radiation Facility (ESRF) at the Swiss-Norwegian beamline BM31. Operando measurements were carried out utilizing a 1mm capillary connected to a gas feed system capable of switching between two gas compositions automatically. XRD data, collected with a 2D detector, allowed for atomic Pair Distribution Function (PDF) analysis at high q values. XAS data focused on the Gallium K edge, with a fast 30-second scan of the XANES region due to Hafnium contamination preventing EXAFS data collection. High molar absorption and capillary thickness imposed fluorescence detection, but transmission data proved useful thanks to a novel technique, X-ray absorption quantification (XAQ). To address the surface-level catalytic activity of the powder sample, modulated emission spectroscopy with phase-sensitive detection (MES-PSD) was used. A gas composition switch and Fourier analysis along the time axis filtered out bulk contributions, enhancing surface sensitivity for both XAS and XRD. Processing the extensive raw data relied on Python, MATLAB, and specialized software such as XrayLarch, Athena, and the FullProf Suite. Emphasis was placed on creating a stable and adaptable codebase to streamline future experiments using similar techniques. This project achieved two primary objectives: firstly, it confirmed the catalytic mechanism involving the change in coordination geometry of surface Gallium atoms in the presence of CO2. This confirmation was obtained through Multivariate Curve Resolution (MCR) analysis, X-ray Absorption Near Edge Structure (XANES) fitting, and Rietveld refinement of diffraction data, all of which consistently showed the same coordination change trend when CO2 was introduced. Secondly, the study explored the MES-PSD technique to increase the sensitivity of analysis but faced limitations in time resolution. However, despite not fully meeting its objectives, this approach remains promising and could yield more insights with refined experimental parameters. The convergence of results from different techniques strongly supports the hypothesis that the observed coordination change is fundamental to the material catalytic activity. This research not only provides a detailed description of the catalytic mechanism but also serves as a proof-of-concept for future work at large facilities like the BM31 beamline at ESRF, showcasing the potential of MES-PSD.