Supported Pd-Cu NO3 reduction (NO3R) under H-2 atmosphere is considered as a promising way to achieve NO3 removal directly in water through converting into N-2, which is a harmless product in environment. However, desirable NO3R kinetics and N-2 selectivity remain challenges. In this research, 2.5 wt% Cu and 10 wt% Pd are loaded on various oxides supports and combine with electrolytic in-situ H-2 generation for NO3R. Pd10%Cu2.5%/TiO2 demonstrates a faster NO3R rate constant than Pd10%Cu2.5%/gamma-Al2O3 and even greater than Pd10%Cu2.5%/CeO2. Pd10%Cu2.5%/gamma-Al2O3 obtains better N2 selectivity than the other two. Crucially, the oxygen vacancies (VO) existing in TiO2 and CeO2 can reduce the nearby Cu2+ to Cu+, also Cu2+ can attract electrons from Pd-0 to form Cu' on the gamma-Al2O3 surface. Verified by experiments and density functional (DFT) calculations, these Cu' based active sites on the surface of substrates are vital for effective NO3 adsorption and high N-2 selectivity. (C) 2020 Elsevier Inc. All rights reserved.