The study of the effect of particle size and low coordination sites in metal nanoparticles (Cu-n, Ag-n and Au-n with n = 19, 38, 55, 79 and 116) on the reduction of NO2 to NO + O, was carried out using density functional theory (DFT) calculations. All metal nanoparticles have shown to be more favorable for the NO2 adsorption than the (111) extended surfaces. The adsorption energy order of NO2 found for both configurations, O-down (most stable) and N-down, was Cu > Ag > Au. The dissociation energy values of NO2 on Cu and Ag nanoparticles decrease with the increment of the particle size, however, considering the activation barrier values, the most reactive substrates evaluated were Cu(111) surface, and the Cu-19, Cu-116. The activation barriers (E-act) for the larger nanoparticles, were obtained using a non-traditional relationship of Bronsted Evans Polanyi (BEP), from the E-act calculated on the smaller ones. The BEP relations highly depend on the reaction product configurations and also on the structure of the active site. Notwithstanding that the nanoparticles improve the NO2 adsorption and the dissociation energies are lower than those corresponding to the extended surfaces, the activation barriers are higher.