(582cw) Evaluating the Surface Science of Photocatalytic Nitrogen Fixation

Photocatalysis is the ideal solution to sustainable ammonia synthesis, as it will enable the distributed production of fertilizers and fuels from reactants that are readily available at ambient conditions: air, water and photons. Given the massive amount of energy currently expended to produce and transport fertilizer, such a process would have an enormous impact on energy consumption and carbon emissions. Photocatalytic nitrogen reduction has been demonstrated over titania catalysts, although the production rates are too low to be practical. Despite the enormous promise of this reaction, first demonstrated 40 years ago, there is no understanding of the fundamental chemical and physical processes that enable it.

Previous investigations of photocatalytic fixation of nitrogen over titania have produced a myriad of contradictory results, including reduction to ammonia and hydrazine, oxidation to nitrate ions, and absence of nitrogen fixation activity. The reaction mechanisms have therefore been intensely debated for decades^{[3, 6, 7]}, with no consensus reached. It has been pointed out that the single-photon reduction of nitrogen and water to ammonia is thermodynamically unfeasible, yet ammonia production has been consistently observed by several independent groups, suggesting a complex multi-photon mechanism. Furthermore, it has been shown that photocatalytic ammonia production can be enhanced by the addition of transition-metal co-catalysts. Since these reports, tremendous progress has been made towards understanding the molecular-level processes involved in titania photocatalysis and thermochemical nitrogen fixation; however, a modern surface-science perspective has not been applied to photocatalytic nitrogen fixation. Here we will detail initial experiments which were conducted to evaluate surface properties of titania under dark and conditions with various reactants.