Improvement in Efficiency of Dinitrogen Fixation Using Conducting Polymer/Titanium Oxide Junction System

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Nitrogen fixation systems driven by white light under mild conditions was investigated. The typical system consists of titanium oxide(TiOx) and poly(3-methylthiophene)(P3MeT), and their heterojunction is previously shown to produce ammonium salt crystals (NH_4ClO_4) when exposed to white light in air at 20℃ and 40%RH. The aims of this study have been directed to the collection of experimental data which would allow us to elucidate the fixation mechanism and to the investigation of material conditions for increasing the fixation rate and the fixation yield.First, the mechanistic study was performed by the chemical and physical approaches. The former approach revealed that the fixation process consists of the formation of ammonia at the junction interface and its conversion to ammonium salt in the polymer bulk. The latter approach gave the data suggesting that the oxygen vacancy on the titanium oxide surface plays the role of a site for the adsorption, activation, and reduction of dinitrogen.Secondly, we investigated the material conditions to affect the fixation yield and the fixation rate. The results indicated that the use of polycarbazole films leads to the enhancement in the fixation yield and that the use of titanium oxide prepared electrochemically in aqueous phosphoric acid increases the fixation rate. The former improvement was explained by the high concentration of dopant anion species in the polycarbazole (i.e., high doping density), and the latter by the high density of oxygen deficiency on the titanium oxide surface synthesized in phosphoric acid.