An operando spectroscopic study of the NO/H-2 reaction has been achieved to relate spectral features to kinetic data in order to identify key intermediates involved in the production of N-2, N2O and NH3 as the main reaction products. Transient experiments show that different nitrosyl species, essentially neutral, positively and negatively NO adsorbed over metallic Rh sites exhibit different reactivity below the light-off temperature at low conversion and temperature. In particular, stable positively charged nitrosyl species inhibit the re-adsorption and further dissociation of N2O predominantly formed during the cold start engine. This can be related to a major drawback in the use of Rh for further developments of compressed natural gas engine catalysts running typically in three-way conditions. Hydrogen was found to enhance the conversion of NOx and the transformation to nitrogen mainly by destabilizing those species further enhancing the subsequent reduction of N2O. The participation of the fast H-2/O-2 reaction at high temperature that restores the metallic character of rhodium could be an indirect effect in the selectivity enhancement changing the nature and the relative rate of steps related to nitrogen production. (c) 2012 Elsevier B.V. All rights reserved.