Mechanistical investigations of CO2 hydrogenation to methanol over Cu/CeO2 catalysts

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Rising atmospheric levels of carbon dioxide (CO2) led on the one hand to global warming caused by greenhouse effect and on the other hand to an acidification of the ocean. A possibility to reduce CO2 emissions is the conversion of CO2 and hydrogen (H2) over a catalyst to a usable chemical such as methanol. Methanol can be used as a solvent, alternative fuel and intermediate chemical for producing petrochemicals. In Industry, methanol is synthesized over a Cu/ZnO/Al2O3 catalyst at 50-100 bar and 175-325°C. This process requires a high pressure to gain a sufficient methanol yield. However, since CO2 is a stable molecule, its activation is associated with high temperatures and therefore on the one hand with high costs and on the other hand with a lower methanol yield caused by the shift in thermodynamic equilibrium. Therefore, development of a catalyst that works at lower temperatures is highly desirable. While working at lower temperatures, copper-loaded ceria (Cu/CeO2) shows comparable activities to Cu/ZnO/Al2O3 catalysts. Ceria has the ability to create defects due to its Ce(III)/Ce(IV) redox system. These defects in turn assist the activation of CO2 for methanol synthesis. The aim of this project is to elucidate the mechanism of CO2 hydrogenation to methanol over copper-loaded ceria (Cu/CeO2). Since little is known about CO2 hydrogenation over ceria, primarily pure ceria and then copper-loaded ceria will be investigated. A special attention will be focused on the establishment of structure-activity-relationships by operando spectroscopy and proof of active intermediates during methanol synthesis, while active intermediates shell be clarified by the use of isotopes. If the active intermediates of the methanol synthesis are known, a catalyst with adsorption sites for these active species can be designed and consequently the process can be optimized. For optimization of the catalyst, (i) the influence of adsorption sites by synthesis of ceria with different facets [(100), (110), (111)] and (ii) the influence of various copper-loadings shell be investigated. Experimental results will then be compared with DFT calculations. Furthermore, the influence of pressure and temperature in methanol synthesis will also be analysed. Therefore, a new operando measurement setup consisting of a combination of Raman and DRIFT spectroscopy coupled with mass spectrometry will be developed, which allows investigations for pressures up to 30 bar. With the aid of this new operando setup changes on the surface (DRIFT spectroscopy) and in the subsurface region (Raman spectroscopy) of the catalyst will be correlated with the respective gas phase composition (mass spectrometry).