In the Light and in the Dark: Photocatalytic Fixation of Nitrogen into Ammonia and Nitrate at Iron Titanate Semiconductor Thin Films

Abstract

This Minireview first summarizes the principle of semiconductor photocatalysis focusing on the comparability of photocatalytic activities, a central problem of the field, often leading to unjustified mechanistic conclusions. It then describes the visible light photofixation of pure and aerial nitrogen at a nanostructured Fe2Ti2O7 thin film on glass. In the presence of ethanol or humic acid, ammonia is formed via hydrazine in the photochemical, first reaction part. Since only reducing agents exhibiting a current amplification effect enable ammonia formation, it is proposed that the primary reduction of N2 is a one-photon – two-electron proton-coupled electron transfer. A consecutive aerial oxidation of ammonia to nitrate constitutes the thermally catalyzed final reaction part, indicating the dual catalysis function of the film. Since the over-all reaction proceeds also with air, and weathering of corresponding iron titanium minerals could form Fe2Ti2O7 and similar semiconducting surfaces, a solar non-enzymatic nitrogen fixation by natural reducing agents may occur in nature. This aspect of environmental chemistry is largely unexplored.