Understanding the dynamic behavior of catalysts under operational conditions is crucial for their rational development. This highlights the need for in-situ and operando experimental techniques. In this thesis, a combined operando continuous-wave electron spin resonance (cw ESR)/microwave cavity perturbation technique (MCPT) apparatus was developed to investigate the interaction of heterogeneous catalysts with gases. These bulk-sensitive methods were applied to several catalytic systems, including: Cu/ZnO:Al (CZA), ZnO:Al (AZO), Ni/silica, and LaxFe0.7Mn0.3O3-δ (x < 1) catalysts. The dielectric loss properties of the CZA catalyst were found to correlate with the yield of CO production in the reverse water-gas shift (r-WGS) reaction. The results provide evidence for the redox mechanisms, i.e., the reduction of CO2 where an oxygen atom released by the cleavage of CO2 is transiently stored by the catalyst, and the Schottky barrier model formed at the Cu-ZnO:Al interface, which can be modified by altering the composition of the gas phase. Although the oxidizing interaction of CO2 with defects in ZnO:Al is confirmed, these interactions are less pronounced than compared to CZA, showing the essential role of Cu in the dissociation of the carbon-oxygen bond of CO2. Additionally, comparative studies of CZA and AZO under reducing conditions indicate that the presence of Cu alters the structural stability of AZO, as it prevents the formation of ferromagnetic ZnO-phases. For the Ni/silica catalyst, operando ESR measurements reveal an inverse correlation between the growth of Ni nanoparticles and both CH4 selectivity and CO2 conversion. While ESR spectroscopy proves the presence of superparamagnetic metallic Ni particles, in-situ X-ray diffraction measurements at a synchrotron facility did not confirm the presence of crystalline Ni phases, demonstrating the high sensitivity of ESR measurement. In the final project of this thesis, the effects of La deficiency in LaxFe0.7Mn0.3O3-δ perovskite oxides are explored using in-situ ESR, utilizing again the superparamagnetic properties of the system as a sensitive probe for structural changes during reduction and oxidation treatments. Magnetic phase heterogeneity during the reduction process, as well as the samples’ response to the oxidation potential of the surrounding gas phase, are found to depend on the level of La deficiency. Lastly, in-situ ESR results show that the superexchange interactions are relevant for the magnetic properties of the studied perovskite oxides.
