Copper powders were subjected to high-energy ball milling under air or argon atmospheres. The Cu crystallite size, determined from the X-ray diffraction data, decreased from 57 to 33 nm, and 27 nm, after 6 It of milling under argon and air, respectively. Milling under air led to copper particles having a wide particle size distribution (70-700 mu m) with an irregular shape and to a slight decrease of surface oxides (CuO, Cu(OH)(2), Cu2O) concentration, whereas under argon, particles appear rather spherical with a larger diameter (700-1000 mu m) and a significant decrease of surface oxidation is observed. Cyclic voltammetry carried out in the presence of nitrate ions shows that the intrinsic electrocatalytic performance of copper for the reduction of nitrate is largely improved by the ball milling treatment. Moreover, the Cu powder milled under air exhibits a nitrate reduction rate 10 times higher than the one measured on Cu milled under argon, likely because of the occurrence of more surface defects that may favor the nitrate adsorption onto the electrode. The higher electroactivity of the Cu powder milled under air for nitrate reduction has been confirmed during prolonged electrolyses, where the formation of NH3 is largely favored. (c) 2006 Elsevier B.V. All rights reserved.