Pd catalysts are well known for their ability to hydrogenate nitrite to form molecular nitrogen as well as ammonium. Removal of nitrite and nitrate form drinking water requires extreme high selectivity to molecular nitrogen. This work shows that selectivity to ammonium can suddenly rise when complete conversion of nitrite is achieved in semi-batch operation. Surprisingly, formation of ammonium is continuing after exhaustion of nitrite. These observations were confirmed for unsupported colloidal Pd particles stabilized with PVA, colloid Pd NPs supported on Al2O3, as well as conventional Pd/alumina catalysts prepared via impregnation. The amount of ammonium formed after exhaustion of nitrite was quite similar to the number of accessible Pd surface sites, indicating that N-containing species adsorbed on the Pd surface are responsible for the effect. ATR-IR experiments prove that the responsible adsorbed species is not IR active and we propose that the surface of the catalyst is importantly covered with rather unreactive atomic nitrogen. These N atoms are almost unreactive, and neither the formation of ammonium via hydrogenation, nor the formation of N-2 via dimerization contribute significantly during steady-state operation. (C) 2016 Elsevier Inc. All rights reserved.