The photocatalytic reaction of nitric oxide (NO) on TiO2 and transition metal-loaded M (Cu, V, and Cr)/TiO2 catalysts was studied using in Situ FTIR spectroscopy under UV irradiation. TiO2 and M/TiO2 catalysts were prepared by the sol-gel method via controlled hydrolysis of titanium (IV) bUtoxide. Copper, vanadium, or chromium was loaded onto TiO2 during the sol-gel procedure. After treatment at 500 degrees C under air flow, a large amount of surface peroxo species and OH groups were detected on the TiO2 and M/TiO2 catalysts. Nitric oxide was adsorbed oil TiO2 and M/TiO2 in the form of bidentate nitrites and nitrates by reacting with OH groups, peroxo, or M=O species. In addition, NO can also be adsorbed oil Mn+ in the form of nitrosyls. Under UV irradiation, bidentate nitrite was oxidized to either monodentate or bidentate nitrate. Such oxidation was suggested to be induced by superoxo Species generated by oxidizing peroxo species via photogenerated holes. The existence of nitrosyls deferred the oxidation of nitrites to nitrates due to the prior oxidation of nitrosyls by superoxo. The XRD and UV-vis spectra showed that the structures and the abilities of absorbing UV light of all catalysts were not influenced by the photocatalytic NO reaction. Possible mechanisms were proposed for the photocatalytic NO oxidation on TiO2 and M/TiO2 baSed on the intermediates found from the ill Situ FTIR study. (c) 2005 Elsevier Inc. All rights reserved.