On the mechanism of nitrogen photofixation at nanostructured iron titanate films

Abstract

The photofixation of dinitrogen to ammonia at a nanostructured iron titanate thin film, prepared from iron(III) chloride and titanium tetraisopropylate, was established by isotopic labeling employing ^15,15N₂. It is found that traces of iron chloride in the film are required to observe significant amounts of ammonia. It is therefore proposed that the photogenerated hole oxidizes chloride to an adsorbed chlorine atom and the latter subsequently oxidizes ethanol, the reducing agent necessary for ammonia formation. However, thin films obtained from a chloride-free precursor like iron tris-acetylacetonate are also active. Upon prolonged irradiation ammonia is oxidized to nitrate by traces of oxygen. It is found that this final reaction step does not require photoexcitation of the iron titanate thin film but occurs thermally. Titania films exhibit about the same catalytic activity in ammonia oxidation whereas iron oxide films are much less active. Contrary to this thermal reaction step, the reduction of intermediate hydrazine by ethanol occurs only photochemically.