CO PRODUCTION BY THERMOCHEMICAL SPLITTING AND REVERSE WATER GAS SHIFT: STRUCTURAL AND CHEMICAL INSIGHT OVER IRON SPINELS

The urgent need to reduce carbon dioxide emissions while enabling sustainable routes for fuel and chemical production has stimulated growing interest in high-temperature CO2 conversion processes. Among these, thermochemical CO2 splitting and the Reverse Water Gas Shift (RWGS) reaction represent attractive pathways for producing carbon monoxide, a key reactant for syngas-based technologies. The performance of these processes strongly depends on the properties of redox-active materials, which must combine sufficient activity with controlled reducibility and structural stability under severe operating conditions. In this PhD thesis, a wide range of iron-based oxide materials was systematically investigated with the aim of elucidating structure-property-performance relationships relevant to both thermochemical CO2 splitting and RWGS. The study focused primarily on spinel-type ferrites, including cation-substituted systems (Ni, Mg, Co, Cu), mechanically mixed composites with secondary oxides (CuO, NiO, ZrO2), and perovskite-type materials. A comprehensive experimental approach was adopted, combining catalytic testing with extensive structural and chemical characterization by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). Thermochemical CO2 splitting experiments demonstrated that iron-based spinels are intrinsically capable of activating CO2 through lattice oxygen participation. Among the investigated materials, nickel-containing spinels exhibited the most favourable overall performance, providing the highest effective CO production when normalized to material mass and oxygen exchange capacity. These systems displayed a more efficient utilization of lattice oxygen and a redox behaviour better suited to thermochemical operation. In contrast, copper- and cobalt-substituted spinels, despite exhibiting rapid redox kinetics and pronounced oxidation features, were strongly affected by over-reduction, metal segregation, and irreversible structural changes, which limited their effective and reproducible CO productivity. Mechanical mixing with secondary oxides allowed partial tuning of redox behaviour, with zirconia-containing composites showing enhanced structural stability at the expense of activity, and nickel oxide addition offering a compromise between performance and durability. Perovskite-type materials, although characterized by non-stoichiometric reduction mechanisms, underwent extensive structural decomposition and showed significantly lower activity compared to spinel-based systems. Under RWGS conditions, all investigated iron-containing oxides exhibited catalytic activity, achieving high CO2 conversion at elevated temperatures. At 1000 °C, carbon dioxide conversion values approaching 90 % were obtained for several materials, with complete selectivity toward CO and no detectable methane formation. Differences among materials were primarily observed at intermediate temperatures and in terms of structural evolution under reducing conditions. Nickel-substituted spinels again emerged as the most balanced systems, combining high conversion with relatively improved structural integrity, whereas copper-containing materials suffered from irreversible reduction and severe sintering, and zirconia-stabilized systems prioritised stability over activity. Overall, this thesis demonstrates that iron-based spinel oxides constitute a versatile and tunable materials platform for high-temperature CO2 conversion. While high activity and selectivity can be achieved for both thermochemical CO2 splitting and RWGS, long-term redox stability and reversibility remain the key challenges. The insights gained provide clear guidelines for future material design, including optimized spinel compositions, rational composite architectures, and integrated thermochemical strategies for syngas and hydrogen production.