(Invited) Sustainable Nitrate Production by Photocatalytic Oxidation of Air

Abstract

Nitrates are essential compounds for the chemical industry and in particular fertilizer production and are therefore critical for our global food production. They are nowadays almost exclusively produced in centralized plants by burning ammonia, which presents a major energetic detour in which the majority of ammonia’s high energy content is lost as heat. It would be much more efficient to synthesize nitrates directly from the elements (i.e., air) instead.It has been shown already that air can be oxidized using heterogenous photocatalysis in aqueous suspensions to yield nitrate. However, the low nitrogen solubility and the fact that nitrate will also be available as electron scavenger (back-reaction) limit the overall efficiency and performance. We have recently demonstrated that this reaction is also possible in the gas phase.[1] Air can be directly oxidized over illuminated TiO2 surfaces to nitrogen oxides, predominantly NO2, which can then subsequently be adsorbed into water to form nitric acid.[1] This approach circumvents the limitations of the aqueous phase as nitrogen and water content can be more freely controlled and nitrates are only formed on the subsequent step, suppressing the back-reaction.Overall, this presents and interesting new synthesis approach to nitrates in a decentralized manner, which could then be the basis for sustainable (solar) fertilizers which are produced directly in the areas in which they are needed.[2] This circumvents the energetic retour via ammonia and also saves on storage and transportation costs and emissions.In this talk we will present results of our first studies on this reaction, in particular which reaction parameters (such as humidity and light intensity) are critical for the reaction and what influence they have. This reveals first insights into possible reaction mechanisms which will then be discussed. Based on our screening experiments, we will also show which catalyst materials and potential co-catalysts are suited and discuss why that may be the case, further elaborating about the reaction mechanism and potential active sites. This information is then used to predict ideal catalyst materials in a knowledge-based catalyst design approach.