Kinetic and isotopic data on Pt clusters and activation free energy barriers from density functional theory (DFT) on Pt(1 1 1) are used to assess the elementary steps involved in NO-H-2 reactions. Pt clusters 1-10 nm in diameter gave similar turnover rates, indicating that these elementary steps are insensitive to surface-atom coordination. N-O cleavage occurs after sequential addition of two chemisorbed H-atoms (H*) to NO* which are quasi-equilibrated with H-2 and NO co-reactants. The first step is equilibrated and forms HNO*, while the second addition is irreversible and forms *HNOH*; this latter step limits NO-H-2 rates and forms OH* and NH* intermediates that undergo fast reactions to give H2O, N2O, NH3, and N-2. These conclusions are consistent with (i) measured normal H/D kinetic isotope effects; (ii) rates proportional to H-2 pressure, but reaching constant values at higher pressures; (iii) fast H-2-D-2 equilibration during catalysis; and (iv) DFT-derived activation barriers. These data and calculations, taken together, rule out N-O cleavage via N-O* reactions with another NO* (forming O* and N2O) or with vicinal vacancies (forming N* and O*), which have much higher barriers than H*-assisted routes. The cleavage of N-O bonds via *HNOH* intermediates is reminiscent of C-O cleavage in CO-H-2 reactions (via (HCOH)-H-**) and of O-O cleavage in O-2-H-2 reactions (via OOH* or *HOOH*). H*-addition weakens the multiple bonds in NO, CO, and O-2 and allows coordination of each atom to metal surfaces; as a result, dissociation occurs via such assisted routes at all surface coverages relevant in the practice of catalysis. (C) 2014 Published by Elsevier Inc.