Mechanocatalytic Synthesis of Ammonia by Titanium Dioxide with Bridge-Oxygen Vacancies: Investigating Mechanism from...

Mechanochemical ammonia (NH₃) synthesis is an emerging mild approach derived from nitrogen (N₂) gas and hydrogen (H) source. The gas-liquid phase mechanochemical process utilizes water (H₂O), rather than conventional hydrogen (H₂) gas, as H sources, thus avoiding carbon dioxide (CO₂) emission during H₂ production. However, ammonia yield is relatively low to meet practical demand due to huge energy barriers of N₂ activation and H₂O dissociation. Here, six transition metal oxides (TMO) such as titanium dioxide (TiO₂), iron(III) oxide (Fe₂O₃), copper(II) oxide (CuO), niobium(V) oxide(Nb₂O₅), zinc oxide (ZnO), and copper(I) oxide (Cu₂O) are investigated as catalysts in mechanochemical N₂ fixation. Among them, TiO₂ shows the best mechanocatalytic effect and the optimum reaction rate constant is 3.6-fold higher than the TMO-free process. The theoretical calculations show that N₂ molecules prefer to side-on chemisorb on the mechano-induced bridge-oxygen vacancies in the (101) crystal plane of TiO₂ catalyst, while H₂O molecules can dissociate on the same sites more easily to provide free H atoms, enabling an alternative-way hydrogeneration process of activated N₂ molecules to release NH₃ eventually. This work highlights the cost-effective TiO₂ mechanocatalyst for ammonia synthesis under mild conditions and proposes a defect-engineering-induced mechanocatalytic mechanism to promote N₂ activation and H₂O dissociation.