Unveiling the overlooked role of oxygen in nitrogen oxidation reaction for enhanced photocatalytic ammonium production over TiO2: Mechanism and pilot-scale system studies

Abstract

Synthesis of ammonium (NH4+) via solar-driven N2 oxidation reaction (NOR) and nitric oxide (.NO) reduction is a potential alternative to the energy-intensive Haber-Bosch process. However, the efficient production of NO for NH4+ via photocatalysis remains a critical knowledge gap. In this study, we investigated the effect of the O2 ratio on the photocatalytic NH4+ production over TiO2. Consequently, the addition of 20 % O2 (80 %N2/20 %O2) achieved the highest NH4+ production (242.39 mg L−1) and it was 12.81 fold higher than that of a non-oxygen environment. Changing the N2/O2 gas ratio for the formation of an oxygen-covered hydrophobic surface can alter the behavior of photo-excited electrons/hole transport and reactive oxygen species production, resulting in the promotion of NOx formation. Furthermore, in-situ SERS analysis provide insights into the promotion of lattice charge interactions between the TiO2 surface and O2. A 6-L outdoor pilot scale photocatalytic experiment was conducted, which achieved up to 107.71 mg L m−2 of NH4+ production performance at 38-h operation. In addition, the photocatalytic NH4+, namely photocatalysis activated water (PcAW) produced from NOR can promote the growth of tomato and bak choi plants, compared to a standard irrigation system. These findings offer a unique perspective on approaches to induce oxic conditions to enhance photocatalytic NH4+ production under ambient conditions and their potential application in agriculture. © 2025 Elsevier B.V.