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Abstract

The need for clean, energy efficient and environmental friendly technologies to close
the resource cycle and help building a sustainable economy is higher than ever. A
promising approach to achieve this goal is the utilization of atmospheric CO₂ by its electro-catalytic reduction towards non-fossil fuels and chemical resources.
Despite numerous dedicated studies carried out in the past decades, detailed understanding of the reaction mechanism and the reaction’s dependence on specific
parameters such as the surface orientation and structure on an atomic scale, the
chemical environment under which the reaction is carried out, the oxidation state of
the employed catalysts or quantum size effects possibly becoming relevant at small
length scales, is missing.
The scientific work presented here is addressing many of these issues focusing
on examining the influence of oxides when running the reaction as well as on the
selectivity-structure correlations on an atomic scale. Utilizing various existing ultra
high vacuum based techniques together with operando, synchrotron-based, electrochemical experiments and developing new techniques, this work demonstrated for
the first time that oxide species are particularly important for the reaction path-
way towards multi-carbon chain oxygenate products. Surprisingly, unlike thought,
the selectivity towards especially ethylene is independent on the presence of those
species. Employing well defined single crystal surfaces it is revealed that structural
properties are the key parameter determining the reduction reaction selectivity towards non-oxygenate products. By controlling and characterising such surfaces on the atomic scale this work significantly contributes to the field by further pointing out that the pristine undisturbed crystal orientation of the surface is, unlike believed on the basis of the existing theoretical predictions and experimental data, not as important as the presence of defects in the atomic lattice and high index facets on the surface. Furthermore, those results are put to use and into perspective when analysing the behaviour of high surface area nano-structured catalysts that are also
considered for large scale up industrial applications. Using a combination of spectro-
scopic and electrochemical techniques the work provides detailed information about
the importance of the chemical environment in the catalytic performance and points
out new directions to further improve existing catalysts. For instance synergistic
effects in bi-metallic nano structured systems are utilized, and the crucial role of
the support in determining the reaction outcome explained. Similarly the effects
of the electrolyte are explored, showcasing that the performance of already existing
highly selective catalysts can be further improved due to an enhanced stability of
crucial reaction intermediates on the surface as a consequence of the exposure to
certain halides and alkaline cations.
These results clearly indicate that structural and morphological properties of catalysts are the key factors in determining the reaction selectivity towards specific
products while only the oxygenate selectivity is found to be significantly affected
by the chemical state and environment.