Development of Materials for Solar Thermal Production of Nitrogen as a Basis for Regenerative Fertilizer Production

One of the main challenges of our generation is the supply of nutrition to a steadily increasing world population, which is unthinkable without the use of fertilizers. Nitrogen‐based fertilizer production requires large amounts of ammonia (NH3), and the current state of the art method for ammonia synthesis is attributed to at least 1 % of the world’s primary energy consumption and a significant fraction of the worldwide greenhouse gas emissions. [3] It requires a stream of highly pure Nitrogen and hydrogen, and the production of those gases is typically very energy intensive. Thermochemical redox cycles enable the use of concentrated solar power or process heat for water splitting or air separation. The latter process is studied within this thesis, which is focused on redox materials preparation and investigation to remove oxygen from air. This is achieved by oxidation of oxygendeficient oxides, which can be regenerated in a second step at high temperatures via thermal reduction. Mainly perovskite based materials have been studied comprehensively within this work, and after theoretical considerations based on thermodynamics and kinetics, a broad materials screening was carried out. As an additional aspect, liquid redox materials are discussed in this thesis, and Initial experiments on redox active glasses were performed. The properties of the perovskite materials have been studied via thermogravimetric analysis and X‐Ray diffraction. Two promising candidate materials (SrFeOx and CaMnOx) have been identified, and their air separation performance was further optimized via doping. The prepared materials gain about 2 % in mass by absorbing oxygen from the air and do not require inert atmospheres or low oxygen partial pressures for reduction. The observed reactions are completely reversible and the temperature window of application is in the range of 400‐1200 °C. To study the properties of the most promising materials more in detail, thermogravimetric experiments at different oxygen partial pressures were carried out, and reaction enthalpies and entropies were determined. The lattice expansion and phase relations during reduction were observed in‐situ via X‐Ray diffraction, and long term tests were carried out in order to evaluate possible degradation effects. Moreover, the composition of the materials has been verified via energy‐dispersive X‐Ray spectroscopy. Their application for air separation processes appears highly promising and the feasibility of perovskite‐based air separation on the laboratory scale has been demonstrated