Setup and application of modulation excitation infrared spectroscopy for mechanistic elucidation of heterogeneously catalyzed reactions

This work describes the design and optimization of a setup for the application of ME-DRIFTS to a variety of catalytic reactions, such as CO oxidation, water-gas shift reaction, reverse water-gas shift reaction, as well as CO₂ activation for MeOH production at elevated pressures using gold- and copper-loaded ceria catalysts. In particular, it was possible to enrich the mechanistic understanding of these catalysts at work and provide direct evidence for the pathway taken during the catalytic reaction. For example, for the WGSR over Cu/CeO₂ catalysts, MES-PSD enabled an assessment of the type of mechanism dominating, that is, an adsorbate-driven (associative) or redox mechanism. In this case, a redox mechanism was found to be at work. In the case of CO oxidation over Au/CeO₂, carbonate and hydroxyl species were found not to be involved in product formation. All potentially active species could be attributed to the carbonyl region, where (dynamically formed) single atomic gold sites were identified as active species. RWGSR over Au/CeO₂ revealed an associative reaction mechanism mediated by adsorbates as the predominant route. Bidentate carbonates and specific hydroxyl groups were identified as active species, but formate, which was also discussed as an intermediate in the literature, could be excluded due to its formation only after the reaction. The analysis of the high pressure CO₂ activation with operando-ME-DRIFTS has revealed, that the formation of CO, methane and MeOH occur in parallel, which has hindered the unambiguous interpretation of the mechanistic information extracted via PSD. Nevertheless, it was possible to gather evidence for a possible pathway to MeOH via adsorbed methoxy species. The application of a newly developed low void volume in situ cell has greatly improved the spectra quality, so that a mechanism of MeOH formation over Au/CeO₂ at high pressures could be postulated. The newly developed cell has also advanced the mechanistic understanding of the CO oxidation over Au/CeO₂ and Cu/CeO₂. Because of a rapid total gas phase exchange within seconds, the period lengths in the experiments could be reduced, which is advantageous when targeting fast-reacting reaction intermediates. For example for the CO oxidation over Au/CeO₂, the rapid exchange drastically reduced the abundance of stable but non-reactive species. In the case of CO oxidation over Cu/CeO₂, the focus was primarily on the active oxidation states of copper during the reaction. In particular, intermediate species such as CO adsorbed on Cu²⁺ were identified. In this work it could thus be demonstrated that MES-PSD in combination with DRIFT spectroscopy represents a powerful method to gain rapid access to mechanistic information of heterogeneously catalyzed reactions. The various possible applications of the method presented in this work make it possible, on the one hand, to quickly obtain a general overview of the reaction under investigation and the active species involved, and on the other hand, to answer very specific questions, e. g., about the dynamics of individual reaction intermediates. A future extension of the repertoire to include other spectroscopic techniques such as Raman and UV/Vis spectroscopy promises additional synergies