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Abstract:
Ammonia is the starting point of all nitrogen chemistry in the chemical industry and, as a starting material for the production of fertilizers, is also of enormous relevance to our society as a whole. For over 100 years, ammonia has been produced mainly by the Haber Bosch process catalytically from its elements at high pressures and high temperatures. In contrast to this multi-million ton scale process, efforts are being made to carry out ammonia synthesis under milder conditions using alternative catalysts, electro-, photo- or plasma chemistry. In recent years, it has been shown several times that heterogeneously catalyzed conversion of gases is also possible by mechanical stressing of a catalyst. While only a handful of reports on the mechanocatalytic ammonia synthesis exist, all of which fail to show beyond doubt that it is a catalytic synthesis of ammonia from N2 and H2 and not a formation enabled by side reactions, or which need to divide up the synthesis into two independent steps, this can clearly be realized in this thesis for the first time.
In a batch screening approach, numerous systems are tested for their suitability as mechanocatalysts. With a large number of systems that were not active, this also revealed the complexity of this project through hydrolytic formation of ammonia due to presence of reducible oxidic materials. At the same time, these systems paved the way for the development of a catalytic process based on alkali-promoted transition metal catalysts, which are formed by combining elemental alkali metals (Na, K, Rb, Cs) with pure transition metals (Fe, Co, Ni, Ru).
One of the most promising catalysts, Cs promoted Fe, was selected in the further course to realize a continuous mechanocatalytic ammonia synthesis. In addition to existing equipment for mechanocatalytic gas reactions, further developments were needed at this point to allow a stable, long-term continuous mechanocatalytic process with gases under pressure. It will be shown that continuous ammonia synthesis is also possible at room temperature and down to atmospheric pressure.
Furthermore, this work deals with ex situ characterizations of the physicochemical changes of the mechanocatalyst that occur during milling, leading to the development of a further optimized process in which no loss of ammonia synthesis activity is observed over the whole tested period of almost 90 h. With this, the intrinsically already very challenging mild condition ammonia synthesis represents one of the mechanocatalytic gas reactions carried out over the longest period of time.
In addition to numerous experiments and results, which up to this point demonstrate that it is indeed a (mechano)catalytic synthesis of ammonia from its elements, the temperature dependence of the process in a wide temperature range from −10 oC to 100 oC is investigated in the last part of this work.
