Catalytic Reduction of Nitrate to Nitrite and beyond

The nitrate anion NO3 is, together with the phosphate anion PO4, one of the two main ingredients of artificial fertilizers. Without them, feeding by now over 8 billion people on this planet would simply be impossible. Nitrate itself is ubiquitous in nature, occurring in form of minerals and salts, but for a long time, profitable mining was only possible in a few places in the world. Soon it became clear that mining could not satisfy the growing global demand of nitrates, especially as nitrates are also the starting material for manufacture of explosives. Therefore, research into alternative methods for nitrate production were initiated, which came to a breakthrough due to groundbreaking work of Fritz Haber in 1908 and later on by Carl Bosch from BASF Ludwigshafen. From now on, ammonia NH3 could be produced via metal catalysis directly from N2 and H2, which could then be oxidized to nitrates. Nowadays though, the overuse of fertilizers by modern agriculture leads to increasing levels of nitrate in ground water, which can lead to health related issues for seniors and small children. Therefore, regulations for limits of nitrate in tap water are in place. Technologically, nitrate removal processes fully rely on physical separation techniques like ion exchange chromatography or osmosis. The common problem with these technologies is that the nitrate is only being filtered out, but is still there afterwards, often as a high-nitrate containing sludge that needs special disposal. The goal of the herein funded project by the FWF is the development of a chemical disposal of nitrate, where the nitrate is not simply removed by filtration, but is chemically reduced to harmless N2. The inspiration comes from nature itself, where a series of metalloenzymes (mainly Mo, Fe and Cu) catalyze the very same reaction, if an excess of nitrate is present. In our project, we will turn to the exceptional catalytic properties of the non-toxic, non-noble metal Rhenium (Re), to enable this chemically very challenging reaction. Initial results are very promising.