Preparation of visible-light-responsive TiO_2-xN_x photocatalyst by a sol-gel method: Analysis of the active center on TiO ₂ that reacts with NH₃

Active reaction centers for ammonia on titanium oxyhydroxide were explored to direct the search for an efficient sol-gel method for the synthesis of a titanium oxynitride (TiO_2-xN_x) sample with an efficient responsiveness to the visible light constituting a main part of the solar spectrum. The results lead to the conclusion that the site giving IR bands at around 2195 cm^-1 for the adsorbed CO molecules at 300 K is a reactive site and behaves as Lewis acid site in the coordination environment of distorted five-coordinate Ti⁴⁺ ions. Ammonia molecules are adsorbed on such a site to form -NH₂ and -OH species during the heat treatments at a temperature above 373 K, and they are ultimately incorporated into the TiO₂ lattice as nitride through the dehydration at higher temperatures of up to 723 K, resulting in the formation of an anatase type of TiO_2-xN_x.