Photoassisted Selective Catalytic Reduction of NO with Ammonia in the Presence of Oxygen over TiO₂

Selective catalytic reduction (SCR) of NO with NH 3 in the presence of O 2 takes place over titanium oxide under photoirradiation at low temperature. The active site of TiO 2 is suggested to be the Ti 4+ Lewis acid site. TiO 2 exhibits the weak Lewis acid property. NH 3 is adsorbed easily on the Lewis acid site as compared with NO. It is conceivable that NO in the gas phase attacks NH 3 adsorbed on the Lewis acid site. N 2 and H 2 O are generated via a nitrosamide intermediate. Consequently, the reduced Ti 3+ species is formed and is reoxidized to the Ti 4+ species by oxygen. NO can be also adsorbed directly on the Lewis acid site which is unoccupied by NH 3 and transformed to inactive the nitrate species. In the absence of oxygen, NO reoxidizes the Ti 3+ species and N 2 O was produced.