The effect of Na on highly dispersed FeOx impregnated onto CeO2 via the unique precursor Na/Fe-ethylenediaminetetraacetate (NaFeEDTA) was investigated by comparison to a series of well-defined catalysts synthesized by the traditional precursor Fe(NO3)(3) both with and without Na addition. Catalysts were evaluated for steady-state NO reduction by CO and activities varied based on synthesis method and Fe:Na ratio. Na contributed a promoter effect when added at a stoichiometric ratio (Fe:Na = 1), providing an explanation for the higher activity of NaFeEDTA/CeO2 for NO reduction by CO. Activity decreased when excess Na was present in Fe(NO3)(3) catalysts, but the stoichiometric promoter effect persisted up to similar to 4.0 Fe/nm(2). In situ Fourier transform infrared (FTIR) spectroscopy during NO adsorption revealed the presence of unique NO adsorption species (1460 cm(-1)) on the NaFeEDTA/CeO2, suggesting enhanced NO adsorption due to Na. At reaction temperature, FTIR bands of bulk nitrates on CeO2 were quantitatively shown to more rapidly undergo NO reduction catalytic transformations over NaFeEDTA/CeO2. These results increase understanding of mechanistic effects of Na on NO reduction over FeOx/CeO2 catalysts and serve to guide future design of oxide-based emission control catalysts that are free of Pt-group metals. (C) 2016 Elsevier B.V. All rights reserved.