Investigation of photocatalytic and photothermocatalytic ammonia production from molecular nitrogen and water under elevated temperature and pressure conditions

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Ammonia is one of the most important chemical commodities. However, its production is also associated with up to two percent of the worldwide CO2 emissions. The photocatalytic reduction of molecular nitrogen and simultaneous oxidation of water to dioxygen is an interesting approach to achieve a sustainable NH3 production. However, there is currently a profound lack of fundamental knowledge of the influence of the various reaction parameters on both the reaction mechanism and rate - a gap the present proposal aims to fill.As a first step we will study how temperature influences the reaction. While certainly, the energy required for the photocatalytic reaction is supplied by the high-energy photons, we have recently seen that elevated reaction temperatures positively influence the kinetics of photocatalytic reactions and can help to significantly increase the efficiency of the conversion. In this regard, our own preliminary study already hints at tremendous potential also for the reduction of N2 to NH3. Starting from there, we will further systematically investigate this temperature effect. The next unanswered question is whether the availability of nitrogen at the catalyst is a limiting factor for both the selectivity towards NH3 and the overall observed reaction rate. To study this, we will investigate the effect of the employed nitrogen partial pressure which directly increases the amount of N2 dissolved in the aqueous system.Particularly under these conditions of elevated temperature and pressure, the observed ammonia generation rate may not be exclusively from direct photocatalytic reduction of N2 but may be superimposed by photocatalytic and/or thermal “Haber-Bosch” reactions, i.e., the formation of ammonia from the elements, fueled by H2 generated as a byproduct from water reduction. We will therefore study in depth under which conditions these reactions occur and what contributions they have to the overall observed reaction rate. This knowledge will be critical in a knowledge-driven catalyst design as it determines for example if the (photo)catalysts need to suppress H2 evolution or not.As photocatalyst materials, we will systematically explore bismuth oxyhalides as they are currently the most promising material class for this reaction. Nonetheless, there is no systematic study on this material class as of yet regarding synthesis routes, materials and performance properties. By varying the compositions of the halide in the material as well as the particle morphology and size we will first find the most suitable candidate as photocatalyst through an initial screening. We will also study if these materials are sufficiently stable within the whole range of intensified reaction conditions planned herein. Then, by analyzing the reaction rate response of the catalysts towards varying experimental conditions, we will be able to calculate fundamental optoelectronic and catalytic properties based on a detailed kinetic model.