Dottorato in SCIENZE CHIMICHE

Titolo della tesi: Advanced experimental, theoretical and data analysis methods for the investigation of chemical processes by X-ray spectroscopy

Achieving accurate chemical knowledge of dynamic chemical processes is of considerable interest both for fundamental understanding and applications. This task however may prove to be challenging, especially when trying to rationalize the often intricate mechanisms of chemical transformations that may involve labile intermediates. The main aim of the dissertation that follows is to address the need of new methods capable of shedding light onto chemical processes whose previously proposed mechanisms are still controversial, also by accurately determining the structural and electronic properties of the key intermediates. Herein, this problem is tackled through the combination of advanced spectroscopic, theoretical and data analysis methods. Specifically, a variety of chemical systems are investigated with such experimental and theoretical approach, such as different chemical reactions occurring in solution on the hour to millisecond time scales and chemical processes occurring in the solid phase and on the surface of materials. The main spectroscopic technique employed to study all the investigated systems is X-ray Absorption Spectroscopy (XAS), which is a sensitive and element-specific probe of both the structural and electronic properties of the given photoabsorbing species. The complexity of the investigated systems was addressed in the first place by leveraging advanced experimental setups, such as those allowing energy dispersive and surface-selective XAS measurements. In addition, coupling XAS with other spectroscopic techniques such as UV-Vis and Nuclear Magnetic Resonance (NMR) provided important complementary information that could not be accessible with either one of the techniques alone. The experimental XAS data capturing the evolution of the given system of chemical interest were then thoroughly analyzed employing multivariate and theoretical tools. In particular, Principal Component Analysis (PCA) and Multivariate Curve Resolution (MCR) spectral un-mixing techniques were employed to determine the number, nature, and concentration time evolution of the key species contributing to the given chemical process. In addition, theoretical methods relying on the Multiple Scattering (MS) and Density Functional Theory (DFT) approaches were applied to interpret the experimental XAS data, focusing on the X-ray Absorption Near Edge Structure (XANES) region. In all the presented studies, the theoretical modeling allowed a quantitative interpretation of the XAS data and provided extensive structural and electronic insights into the given chemical transformation. In particular, the dissertation contains the following studies: (i) XAS-NMR investigations of transition metal exchange processes between prototypical coordination complexes, (ii) a study of the Belousov-Zhabotinskii oscillating reaction by employing Br K-edge XAS and UV-Vis spectroscopies, (iii) investigations of the reactivity of a class of nonheme iron complexes on the second to millisecond time scales; (iv) studies of transition metal-based high entropy oxides (HEOs), an emerging class of multicomponent materials, as anodes for Li–ion batteries and catalytic platforms for the low-temperature CO oxidation; (v) an analysis of the nature of the defective sites of the Cu-based metal-organic framework (MOF) HKUST-1 made possible by surface-sensitive soft XAS; (vi) an investigation of the interfacial solvation of Mg(II) ions on the magnesium oxide surface exposed to water and methanol vapour fluxes; (vii) a study of the interaction established between water and the magnesium chloride surface at elevated temperatures. Altogether, it is shown that the combination of complementary spectroscopic techniques, rooted in XAS and supported by theory, enables a detailed comprehension of the investigated processes. The implement experimental and theoretical method can provide a protocol of general interest that may be extended to investigate interfacial, liquid- and solid-phase systems of chemical relevance.