The conversion of dinitrogen into nitrous compounds like amino acids is essential for sustaining life on Earth. In addition to natural nitrogen fixation, industrial methods are indispensable to fulfil the immense global requirements, particularly for fertilizer manufacturing, mostly dependent on ammonia. Presently, ammonia synthesis relies on the energy-intensive Haber-Bosch process. In consideration of the current climate and energy challenges, investigating alternative methods for ammonia production is imperative. Our focus lies in investigating the photocatalytic reduction of nitrogen to ammonia using various bismuth oxyhalides as catalysts.

The main challenge for the photocatalytic nitrogen reduction is the low quantum yield due to the difficulty of breaking the very strong N-N triple bond. Enhancements in this process can be pursued through two approaches: identifying more suitable catalysts and refining the reaction conditions to promote the desired outcomes. Bismuth based catalysts showed the most promising results in the last years of research into photocatalytic ammonia generation. Especially bismuth oxyhalides (BiOX, X=Cl, Br, I) are an interesting material class due to their well-fitting band positions and high variability in composition, which allows to tune their reactivity.[1, 2, 3] Therefore, different bismuth oxyhalides were synthesized, which we have been tested for reactivity, as well as ammonia yields. Preliminary findings have revealed that BiOBr demonstrated the highest ammonia yields. Consequently, we have shifted our focus towards the modification and enhancement of this catalyst.

Furthermore, we investigated the influence of temperature, pressure, and light intensity on the reaction. Temperature holds particular interest due to its well-documented enhancement of reaction kinetics, while pressure augments nitrogen availability, and light intensity accelerates the generation of electron-hole pairs. For the photoreactions, we employed closed vessels under a nitrogen atmosphere containing ultrapure water and the corresponding photocatalyst. To control temperature and light intensity, we positioned the samples in an oil bath and illuminated them with a high-power 365 nm LED. To assess ammonia yield, we utilized two distinct methods: the salicylate assay which is an optical assay based on the common indophenol blue method,[4] and isocratic flow ion chromatography.

To define reaction parameters for the comparison, we will show results of using different temperatures, pressure, light intensity, and reaction time by using the example of titanium dioxide (Figure 1). Endowed with these defined parameters, we will then proceed to compare the performance of different bismuth oxyhalides alongside titanium dioxide. To ensure the reliability of the results and eliminate the possibility of false positives, strict control experiments were conducted. These included reactions under an argon atmosphere, in the absence of light, and at ambient temperature. Furthermore, internal standards were integrated into the employed analytical methods to validate the measurements. Such precautions are critical due to the very low ammonia yields and the potential risk of contamination, which could otherwise compromise data integrity.