Oxygen and Titanium Vacancies in a BiOBr/MXene-Ti₃C₂ Composite for Boosting Photocatalytic N₂ Fixation

Solar energy can be used as "green" energy by photocatalysis for the nitrogen fixation under the atmospheric conditions compared with the traditional energy-intensive industrial production of ammonia. However, the complex kinetics and high reaction barriers greatly hinder the development of the photocatalytic N₂ reduction reaction. Herein, a BiOBr/MXene-Ti₃C₂ composite catalyst is prepared by the simple electrostatic adsorption and self-assembly method. The as-prepared 10 wt % BiOBr/Ti₃C₂ exhibits the best performance for N₂ fixation to NH₃ by photocatalysis. The evolution rate of NH₃ is up to 234.6 μmol·g^-1·h^-1, which is approximately 48.8 times and 52.4 times higher than those of pure BiOBr and Ti₃C₂, respectively. It is found that the designed double vacancies of oxygen and titanium for BiOBr/Ti₃C₂ composites, with the availability of localized electrons, have the ability to adsorb and activate N₂, which can be efficiently reduced to NH₃ by the interfacial electrons transferred from the excited BiOBr/Ti₃C₂ composite. In addition, the results of in situ Fourier transform infrared show the generation of NH species by the continuous protonation processes. Moreover, titanium vacancy (V_Ti) induces a strong absorption energy for nitrogen atoms on the surface of BiOBr/Ti₃C₂ according to the density functional theory calculations. In particular, the P-electron feedback caused by V_Ti could effectively promote the weakening of the N≡N triple bond and elongate the N₂ bond length by ∼31.6%. This work might provide new insights into the synergistic effect of double defects and inspiration for the rational design of catalysts by defect engineering in the field of catalytic synthesis of ammonia.