Hutton, James T; (2024) In-situ X-ray Absorption Spectroscopic studies coupled with Electrochemistry of open and dense framework materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis, the thermal reduction of electrodeposited doped CeO2 and the electrochemical reactivity of copper-based MOFs were investigated using in situ X-ray absorption spectroscopy. CeO2-based films were studied using high-energy resolution fluorescence X-ray absorption spectroscopy (HERFD-XANES). The in situ study at the Ce L3 edge, revealed that Ce(IV) in pure ceria films undergo a marginal reduction to Ce(III) to ca. 15% when heated to 450 oC but, upon cooling in hydrogen, revert to Ce(IV), a phenomenon also observed for high-surface area powder samples1. However, when Cu(I) ions were co-deposited, the Ce(IV) reduction takes place at ca. 50% conversion; Sm(III) dopants reduced this to ca. 20% conversion, and Gd(III) dopants showed the least conversion. The onset of reduction temperature of Ce(IV) appears to vary. Copper-doped systems are the only one which showed resistance to reoxidation of reduced Ce(III) to Ce(IV). Inclusion of cobalt in copper-based MOFs, specifically Cu-BTC, Cu/Co BTC (with 90/10, 50/50 and 10/90) and pure Co-BTC along with Cu/Ni and Cu/Zn (50/50) doped analogues were investigated for electrochemical CO2 conversion at various applied potentials. In situ Cu K-edge measurements at potentials of +0.3 V, -0.2 V, -0.4 V, -0.6 V and returning to +0.3V were undertaken for these compounds with CO2 gas bubbled through the electrolyte. The degree of copper reduction and reoxidation at various voltages was determined. There appears to be a considerable variation in the nature of Cu(II) reduction to Cu(I) and subsequently to Cu(0) at these potentials. Reoxidation occurs when voltage was increased to +0.3 V, however the extent of reoxidation depends on the system studied. Electrocatalytic conversion of CO2 at -1.0 V and -2.0 V were conducted and the products were analysed using 1H NMR spectroscopy. It was found that formate or ethanol is formed in certain systems. Details are discussed in the respective chapters in the thesis.

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