Total energy calculations have been performed within the periodic density-functional theory framework to study the dissociation of molecularly adsorbed nitrogen monoxide NO over three different catalytic surfaces: palladium, rhodium, and palladium-manganese (100). The potential energy surfaces for NO dissociation on these metallic surfaces have been calculated in order to determine the minimal energy paths. The accurate optimizations of the transition states and their characterization with a complete vibrational analysis, including the degrees of freedom of the surface, have been presented. The order of increasing activation energy barrier is Rh, Pd3Mn, and Pd. Two types of reaction paths have been found: one involving a horizontal molecular precursor state and a low activation energy barrier (Rh and Pd3Mn) and the other involving a vertical molecular state and a high activation energy (Pd). Hence the improvement of the catalytic activity for dissociating NO by alloying manganese to palladium has been explained and interpreted. The simulation of the reaction rate constants is fully compatible with the observed catalytic behavior. The differences in catalytic activity have been analyzed with a bond breaking-bond forming energetic decomposition and a Mulliken population analysis.