The mechanism of nitrite hydrogenation over a Pd/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. Nitrite hydrogenates to NO(ads), NH2(ads), and NH4+ on the palladium surface. Hydrogenation of adsorbed NO on palladium results in the formation of a reaction product that is not infrared-active (most likely nitrogen), whereas no NH4+ is formed from NOW,). NH4+ is formed solely from hydrogenation of the NH2(ads) intermediate. The present study clearly shows that formation of nitrogen and NH4+ proceeds via two separate pathways, based on which a revised reaction scheme is proposed. (C) 2008 Elsevier Inc. All rights reserved.