Bioinspired catalyst for sustainable ammonia production

The ammonia synthesis process developed by Haber and Bosch in 1909 is considered one of the most important milestones in the human history. With this process it has been possible to convert atmospheric nitrogen in NH3 which can easily be converted to fertilizers. Indeed, the ammonia process allowed to increase the agricultural productivity and to sustain the global demand of foods. Nowadays ammonia is still produced using the Haber-Bosch process and considering the high volume of its production, 170 million tons in 2021, it is responsible of the 2% of the global world energy consumption and 1.6% of the global CO2 emission[1]. It is clear how the necessity to develop a more sustainable process for ammonia synthesis is mandatory to decrease its environmental impact. This PhD work is carried out in collaboration with Casale SA and focuses on the development of a new class of bio-inspired photocatalysts for ammonia production at ambient pressure and temperature. Nitrogen is already fixed by nature under these conditions within bacteria like Azotobacter Vinelandii at nitrogenase’s enzyme active site. Scope of this research has been to mimic the enzymatic active site mechanism using different strategies such as Metal Organic Frameworks (MOF) or metalloclusters systems. Moreover, the development of a proper reaction rig has been also taken in investigation to maximize catalysts performances. The first part of the research has involved the study of Metal Organic Frameworks as a viable photocatalyst analogue of the enzymatic cluster. Matériaux de l′Institut Lavoisier-53 (MIL-53) framework has been used as a model and it has been opportunely modified with the introduction of different amount of Fe2+ centers during the solvothermal synthesis to imitate the enzymatic cluster. The prepared catalysts have been deeply investigated with several characterization techniques like FT-IR, XRD, SEM and DRS to understand the structural and optical properties modifications after the introduction of Fe2+ centers. What emerged is that the introduction of iron (II) has increased the absorption properties in the Vis region with no significant modification of the structure. However, the materials showed negligible activity toward ammonia synthesis. To increase the poor performances their promotion with Carbon Dots (CDs) introduction have been investigated. CDs would act as promoter in different ways: forming heterojunction to hinder electro-hole recombination, increase the absorption spectra of the material, and introduce a photosensitizer which can act as electron reservoir. Despite the effective increase in optical performances of these hybrid materials the ammonia production was still negligible. The second part of the research was focused on the study of Fe-S for ammonia photo catalyzed synthesis. In particular, (NH4)3[Fe6Mo2S8(SPh)9] cluster was synthetized in oxygen free atmosphere and characterized using several techniques like cyclic voltammetry, 1H-NMR, SEM, XPS, NEXAFS, and DRS. From these characterizations it was possible to confirm the structure of the cluster and asses that cluster has similar redox properties to the one of the real enzyme, furthermore it possess wide light absorption range from the UV up to the visible region. This catalyst was also tested for ammonia synthesis in liquid batch conditions and under UV irradiation, reaching up to 200 ppb of ammonia concentration in 30h of reaction and part of this research was carried out at Casale SA. This material has proven also to be active both in liquid and gas phase conditions. Catalyst performance has also been improved by supporting it on different materials to increase the number of available active sites reaching an overall 10-fold increment in ammonia productivity.