Photocatalytic reduction of NO with CO has been carried out using various single-site photocatalysts under UV-light irradiation. The single-site photocatalysts prepared by incorporating transition metal oxides within SiO2 (M/Sio(2): M=Mo, V, and Cr) display different photocatalytic activities. Mo/SiO2 promotes photocatalytic reduction of NO with CO efficiently to produce N-2 via the intermediate N2O. V/SiO2 was observed to promote the reaction that leads to selective production of N2O that does not undergo subsequent reduction to form N-2. When Cr/SiO2 is used as the photocatalyst, reduction of NO does not take place. The results of Fourier transform infrared investigations reveal that differences in photocatalytic activities are due to different reactivity of reactants-adsorbed transition metal oxides in M/SiO2 and, consequently, the reaction mechanisms for the photocatalytic reduction of NO with CO over M/SiO2 are proposed. (C) 2012 Elsevier Inc. All rights reserved.