New Catalysts for Methanol Synthesis

In the context of a fossil-fuel free and CO₂-neutral society, where we inevitably will still depend on liquid fuels, methanol is considered an important chemical. Methanol is suggested as the key chemical in the ‘methanol economy’ and so it has the potential to aid in the reduction of greenhouse gas emissions. If CO₂ hydrogenation is to be applied within a sustainable infrastructure, either new catalysts or the optimization of the currently used one is required. This thesis presents research within the development and characterization of Cu-based catalysts for the synthesis of methanol from CO₂ and H₂. Furthermore, it describes the findings from the study of a model system that can potentially aid in the understanding of the Cu/ZnO/Al₂O₃ catalyst.
For methanol synthesis from the hydrogenation of CO₂, a series of new promoters for a Cu-based catalyst were investigated in close collaboration with theory. The study focused on the Cu-Ge system and investigated methods for synthesizing supported Cu_(1x)Ge_x nanoparticles with very small amounts of Ge promoter. The investigations lead to a synthesis that produced metallic Cu_(1x)Ge_x nanoparticles with variations in the Ge- molar fraction and Cu particle sizes. Activity tests of SiO₂-supported Cu_(1x)
Ge_x nanoparticles showed that while catalysts of high Ge-loading performed worse than those of low Ge-loading, the series of catalysts within the latter category performed similarly. Characterization of the Cu-Ge catalysts showed a large variance in Cu particle sizes across the range of tested catalysts, which prompted the need to separate the effect of active surface area from that of promotion by Ge. The Cu surface-averaged mass activities indicate that the Cu-Ge catalyst with the lowest fraction of Ge perform slightly better. However, when compared to an industrial grade Cu/ZnO/Al₂O₃/MgO, none of the CuGe catalysts performed better both neither in terms of activity nor selectivity. The question of the state of Ge remains since neither analysis of PXRD, PDF analysis of Xray total scattering data, nor (HR-)TEM imaging proved useful in determining the presence and state of Ge in the most relevant Cu-Ge catalysts.
A CuZn/C model for the industrialtype methanol catalyst was studied using a combination of PXRD, X-ray total scattering, STEM-EDX, and activity measurements. By combining a weakly interacting carbon support of high surface area with the stepwise increase of Zn/Cu, the relevant fraction of Zn could be determined: The highest mass activities were obtained for carbon-supported catalysts with Zn/(Zn+Cu) ratios above 0.2. This was in line with previous findings and hypothesized to be the gradual reduction of ZnO to Zn in the surface of Cu. Characterization of the catalysts post activity-testing illustrates the important role of ZnO as a structural promoter: Smaller Cu nanoparticles are stabilized with larger Zn/Cu fractions.
Work on bringing back a combined HPCUHV chamber was done, and now a foundation has been laid for the future characterization of the chemical speciation of the relevant fraction of any promoter. Whether it be Zn in the  CuZn/C model, or Ge in the new Cu-Ge-based catalysts for CO₂ hydrogenation, the combination of quasi insitu XPS/ISS and high-pressure measurements will prove very useful in the future.