Production of Carbon-Neutral Fuels – Fundamental Investigations into the Heterogeneous Catalysis of CO2 Reduction to CH4 and CH3OH by Spectro-scopic and Imaging Methods

Abstract

This thesis focuses on different aspects of the synthesis of renewable methane and methanol from carbon dioxide. By modeling, a general overview about limitations of the two reactions is given. Different reactor concepts were established to overcome these limitations. Modeling showed that sorption-enhancement helps to increase the conversion of CO2 to CH3OH, a principle that has already been proven to be successful in CO2 methanation.
For understanding the fundamental reaction mechanism of the CO2 methanation, a commercial nickel catalyst was investigated by inelastic neutron scattering and diffuse reflectance infrared Fourier transform spectroscopy. We found that the byproduct water is a persistent adsorbate on the catalyst thereby impeding the catalyzed CO2 methanation. The kinetics of water removal on a microscopic scale in sorption-enhanced methanation catalysts were investigated by neutron imaging. Measurements highlight the importance of nanostructured catalysts rather than macroscopic mixtures of sorbent and catalytic moiety.
The promising outcome of the modeling of sorption-enhancement in CO2 reduction to CH3OH triggered us to apply the recipes of catalysts developed for the CO2 methanation on sorption-enhanced methanol production. Despite various similarities, there are specific challenges associated with this reaction. With the CO2 reduction to methanol being less exothermic than that of CO2 methanation, the formation of methane has to be avoided which asks for other catalysts than nickel. As an educated guess, we started with copper exchanged zeolites in order to proof the positive influence of sorption-enhanced CO2 reduction to CH3OH. Already the first attempt demonstrated the general feasibility of the concept, though the catalytic performance lied much below commercial catalysts. Careful analyses including the investigation of different catalysts and sorbent materials paved the way to sorption catalysts with enhanced activities by increasing the active copper surface and stabilizing the zeolite used. However, despite the positive influence of zinc doping, the role of this element is still under debate.
For unravelling the promotional effect of zinc, we studied a commercial Cu/ZnO/Al2O3 catalyst by operando neutron imaging. Measurements revealed that ZnO interacts with hydrogen and is therefore not a pure spectator but actively participates in the reaction.