Ce_0.3–TiO_x nanoparticle carrier was prepared by the sol–gel method, and a series of Cd–Ce–Ti nanoparticle catalysts with variable Cd contents were prepared by the means of an improved incipient-wetness impregnation. The catalysts were characterized by means of XRD, N₂ adsorption–desorption analysis, SEM, TEM, NH₃-TPD and in situ DRIFTS. The catalytic activities for deNO_x were evaluated by NH₃-SCR reaction. All these nanoparticle catalysts contain mesopores with a pore size around 7 nm, and the average particle size is 20 nm. Among the catalysts, 2 wt% Cd/Ce_0.3TiO_x catalyst exhibits the best NH₃-SCR performance with a wide temperature window of 250–400 °C for NO conversion above 90%. Moreover, in situ DRIFTS spectra of NO_x reduction over 2 wt% Cd/Ce_0.3TiO_x catalyst were also investigated. The results show that this reaction mainly follows the Langmuir–Hinshelwood mechanism at room temperature, while Eley–Rideal mechanism plays more important role when the reaction temperature is higher than 150 °C. The adsorbed NH₃ coordinately linked to Lewis acid site is easy to react with NO_x at high temperature.