Wettability control of defective TiO2 with alkyl acid for highly efficient photocatalytic ammonia synthesis

Ammonia is an important chemical raw material and non-carbon-based fuel. Photocatalytic ammonia production technology as a mild alternative to the traditional Harbor–Bosch route is carried out at the air, liquid, and solid three-phase interface. Promoting the activation of N2, depressing hydrogen evolution reaction (HER), and increasing the local N2 concentration around the catalyst surface are critical factors in achieving high conversion efficiency. In this paper, we proposed that defective TiO2 is surface-modified by alkyl acids with different carbon chain lengths (C2, C5, C8, C11, and C14) to tune the catalyst surface properties. The defect sites greatly promote N2 adsorption and activation. The wettability of the catalyst can be regulated from hydrophilic to hydrophobic by the length of the alkyl chain. The hydrophobic surface enhances the N2 adsorption and increases the local N2 concentration due to its aerophile. Meanwhile, it depresses the proton adsorption and HER. Overall, the nitrogen reduction reaction (NRR) is greatly promoted. Among the series of samples, they present a systematic change and have a maximal NRR performance for n-octanoic acid-defective TiO2 (C8-Vo-TiO2; Vo = oxygen vacancy). The rate of ammonia production can be as high as 392 µmol·g−1h−1. This work provides a new strategy for efficient ammonia synthesis at the three-phase interface using photocatalyst technology.