Copper-zinc oxide based catalysts are applied industrially in methanol synthesis for more than 45 years in the low temperature process, commercially implemented by the ICI company. The growing use of methanol as a convenient energy storage molecule will require further improvement of the catalyst and adjustment to changing feedstock and process parameters. Many parameters of the catalyst preparation influence the activity to different degrees. The complex industrial recipe was developed from a combination of empirical trial and error methods. In order to understand the relationships of preparation, structure and function of the final catalyst, careful and systematic studies are necessary which include the realistic features of the active catalyst. In this thesis different aspects were investigated: (i) the influence of the precursor structure, (ii) the role of the promoter and (iii) the influence of residual carbonates, known as high temperature cabonates (HT-CO3) in the calcined catalyst. It was shown that neither zincian malachite (ZM) nor aurichalcite (AU) – both are mixed Cu-Zn hydroxy carbonate phases commonly used as precursors for Cu-Zn based catalysts – are a priori superior to the other. In the binary case, AU seems the better precursor, as it allows higher Zn/Cu ratios leading to smaller Cu-particles and more efficient ZnO spacers hindering Cu particle sintering, which is one of several reasons for catalyst deactivation. Upon promotion with small amounts of Al the situation changes drastically. For instance, the morphology of the promoted precursors is influenced such, that the BET surface area increases significantly. Most important, the deactivation of catalysts derived from both ZM and AU is much slower and sintering is no longer the primary deactivation mechanism. This leads to the situation, that ZM, with the higher Cu content, becomes now indeed superior to AU. In addition, it was shown that the Al promoter has a significant effect on the ZnO support. Al3+ introduces additional defects into the ZnO, which lead to higher conductivity of the support. In a similar manner also Ga3+ acts as efficient promoter. Mg2+ on the other hand, leads to a strong decrease of defects, of the intrinsically n-doped ZnO, and consequently in a decreased conductivity. It turned out, that the conductivity of the support scaled with the activity of the copper impregnated support in reverse water-gas shift (rWGS) and methanol synthesis. Another part of this work dealed with the role of the HT-CO3, a fraction of the carbonate that only decomposes at temperatures > 600 K, and showed that its role is ambivalent. On the one hand, HT-CO3 prevents the formation of large, crystalline CuO and ZnO particles during calcination. On the other hand, it leads to a dilution of the active copper-zinc component and thus leads to a decrease of the activity. Furthermore, it was shown that the amount of HT-CO3 is linked to the number of oxygen vacancies in the ZnO support. In summary, the results reported in this thesis show how the catalytic performance can be influenced by a careful tuning of synthesis parameters resulting in structural and electronic control of the active phase. This better understanding will enable the knowledge-based development of the catalyst for the requirements of future sustainable processes.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/1(E), Dissertation Abstracts International|
|Subjects:||Inorganic chemistry, Computational chemistry|
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