(697c) Carbon-Mediated Photocatalytic Nitrogen Fixation on Titania

Photocatalytic conversion of nitrogen to ammonia has been a topic of interest for decades. Despite numerous studies, the exact role of the photocatalyst in the dissociation of dinitrogen and the formation of ammonia are still debated, which greatly hinders the development of this technology. Two prevailing hypotheses suggest that nitrogen fixation occurs through a direct reduction process at the oxygen vacancies on the photocatalyst or through an oxidation process in which hydroxyl groups play a role in the reaction. However, neither of these two hypotheses can fully explain the reaction and lacks direct experimental evidence.

According to recent theoretical computational studies, hydrocarbons may have a more direct role in the reaction, beyond their traditional function of acting as hole scavengers. These studies propose a carbon-assisted mechanism, which suggests that hydrocarbons can interact strongly with dinitrogen and potentially act as an active site. Here, we use several spectroscopic and computational techniques to identify the interactions between dinitrogen, hydrocarbon (methanol), and photocatalyst (titania) that enable the formation of ammonia. With electron paramagnetic resonance spectroscopy, we observed methanol forms carbon radicals upon exposure to ultraviolet radiation. These carbon radicals then transform into diazo- and nitrogen-centered radicals during photocatalysis in the nitrogen environment. In situ infrared spectroscopy revealed the presence of C-N stretching on titania under the same conditions. Furthermore, density functional theory calculations indicate that nitrogen adsorption and the thermodynamic barrier to photocatalytic nitrogen fixation are significantly more favorable in the presence of methanol or surface carbon species. These results provide compelling evidence that carbon radicals formed by the oxidation of hydrocarbons play a crucial role in the fixation of dinitrogen on titania, and suggest that the conventional understanding of carbon-based "hole scavengers" and the inertness of nitrogen atmospheres in photocatalysis may need to be re-evaluated.