Mechanism on reduction and nitridation of micrometer-sized titania with ammonia gas

Ammonia gas can be simultaneously used as a reductant and nitrogen source to prepare TiN from titania. In this work, the mechanisms on reduction and nitridation of micrometer-sized anatase with ammonia gas have been investigated, using both thermodynamic and experimental studies. The thermodynamic analysis indicated that reduction and nitridation of TiO2 by NH3 was feasible. Anatase will undergo different paths to form TiN, depending on the reaction temperature. Upon heating, NH3 was seen to partially decompose into N-2 and H-2, although the actual NH3 decomposition ratio was less than the theoretical value. The experimental results indicated that the obtained titanium nitride was non-stoichiometric (TiNxO1-x, x <= 1), as it contained a certain amount of oxygen. Based on the phase transformation and X-ray photoelectron spectroscopy analysis, the reduction and nitridation routes were deduced: TiO2 reacted with NH3 to form TiNxO1-x directly, at lower temperatures, and followed the path TiO2 -> TinO2n-1 -> TiNxO1-x, at higher temperatures. TinO2n-1 was determined to be Ti4O7 and Ti3O5 at 1100 degrees C and 1200 degrees C, respectively. Reaction temperature and time significantly affected the oxygen and nitrogen contents in TiNxO1-x, with the lattice parameter of roasted products gradually increasing-approaching those of pure TiN-with an increase in reaction temperature and holding time. At the same time, the content of oxygen in TiNxO1-x decreased, and its nitrogen content correspondingly increased.