Photoreduction of Nitrogen on TiO2 and TiO2-Containing Minerals

Abstract

A report appearing in 1975 demonstrated that incompletely out-gassed, powdered titanium dioxide photo-reduces acetylene to ethylene and prompted attempts to achieve the photoreduction of molecular nitrogen under similar conditions. Whereas pure, powdered anatase was only slightly active, a substantial enhancement of its photocatalytic activity was achieved by impregnating it with a small percentage of iron, causing its partial conversion to rutile by a heat-treatment at 1000°C. After humidification, this TiO2 substratum, on exposure to light in an argon atmosphere, exhibited activity in the water-splitting reaction and yielded NH3 in the presence of molecular nitrogen. Subsequent studies demonstrated that this N2 photoreduction occurs in a stepwise fashion via diazene and hydrazine as the intermediates. These reactions were shown to take place under simulated terrestrial conditions on the surface of rutile-containing minerals on exposure to sunlight. Evidence for the secondary photoxidation of NH3 on rutile-containing minerals was also obtained. In view of the wide distribution of titanium minerals on the Earth's crust, these abiological, light-driven reactions contribute to the nitrogen ecology in semiarid regions of the Earth and have been suggested to occur on the surface of Mars.