Composite metal oxide catalyst Cu(2)M(9)CeOx (M = Fe, Co, Ni) was prepared by citric acid complexation method for NOx reduction. Prepared catalysts activities were tested in a fixed bed reactor for NOx reduction by CO and H-2. The sequence of activity of the three catalysts for NOx reduction by CO is: Cu(2)Co(9)CeOx > Cu(2)Fe(9)CeOx > Cu(2)Ni(9)CeOx. The catalytic efficiency of Cu(2)Co(9)CeOx increased to 100% at 250 degrees C. For NO reduction by H-2, the sequence of activity follows: Cu(2)Fe(9)CeOx > Cu(2)Co(9)CeOx > Cu(2)Ni(9)CeOx. The catalytic activity of Cu(2)Fe(9)CeOx was ranged from 60% to 100% during 250-325 degrees C. Additionally, characterization was conducted to investigate the physical and chemical properties of Cu(2)M(9)CeOx catalysts. In-situ Fourier transform-infrared spectra was also used to study the mechanism of the reaction. The results indicated that Cu-square-Co synergistic oxygen vacancies in Cu(2)Co(9)CeOx and unstable CuFe2O4 (spinel) in Cu(2)Fe(9)CeOx promote NO reduction. The reasons of activities over different Cu(2)M(9)CeOx catalysts were then carefully explored. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.