Mechanocatalytic Synthesis of Ammonia by Titanium Dioxide with Bridge-Oxygen Vacancies: Investigating Mechanism from the Experimental and First-Principle Approach.

Mechanochemical ammonia (NH 3 ) synthesis is an emerging mild approach derived from nitrogen (N 2 ) gas and hydrogen (H) source. The gas-liquid phase mechanochemical process utilizes water (H 2 O), rather than conventional hydrogen (H 2 ) gas, as H sources, thus avoiding carbon dioxide (CO 2 ) emission during H 2 production. However, ammonia yield is relatively low to meet practical demand due to huge energy barriers of N 2 activation and H 2 O dissociation. Here, six transition metal oxides (TMO) such as titanium dioxide (TiO 2 ), iron(III) oxide (Fe 2 O 3 ), copper(II) oxide (CuO), niobium(V) oxide(Nb 2 O 5 ), zinc oxide (ZnO), and copper(I) oxide (Cu 2 O) are investigated as catalysts in mechanochemical N 2 fixation. Among them, TiO 2 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 N 2 molecules prefer to side-on chemisorb on the mechano-induced bridge-oxygen vacancies in the (101) crystal plane of TiO 2 catalyst, while H 2 O molecules can dissociate on the same sites more easily to provide free H atoms, enabling an alternative-way hydrogeneration process of activated N 2 molecules to release NH 3 eventually. This work highlights the cost-effective TiO 2 mechanocatalyst for ammonia synthesis under mild conditions and proposes a defect-engineering-induced mechanocatalytic mechanism to promote N 2 activation and H 2 O dissociation.