Fluorinated phosphonium ionic liquid boosts the N2-adsorbing ability of TiO2 for efficient photocatalytic NH3 synthesis

Abstract

Photocatalytic N₂ reduction (pNRR) is considered to be an important pathway for green NH₃ synthesis. However, in the traditional photocatalytic system, the adsorption capacity of N₂ by an aqueous phase reaction solution and a semiconductor photocatalyst is limited, which seriously restricts the performance improvement of photocatalytic N₂ reduction synthesis of NH₃. In this work, a TiO₂ photocatalyst with a positively charged surface (TiO_2−x) was synthesized, and low-cost fluorinated phosphonium ionic liquid (IL) tetrabutylphosphate–hexafluorophosphate ([P_4,4,4,4][PF₆]) was innovatively added to the water phase reaction system. Notably, the anion [PF₆]⁻ of [P_4,4,4,4][PF₆] can be electrostatically driven to enrich on TiO_2−x (IL-TiO_2−x), creating an outermost layer that transforms the surface properties from hydrophilic to aerophilic, thus effectively promoting N₂ adsorption. Under full solar irradiation, the NH₄⁺ generation rate of IL-TiO_2−x substantially increased by 2.6 times (22.7 μmol g_cata⁻¹ h⁻¹) compared to the system without IL was rendered. This intriguing work offers a feasible strategy for developing an efficient N₂ capture–fixation reaction system for pNRR.