Breaking the NO bond on Rh, Pd, and Pd3Mn alloy (100) surfaces: A quantum chemical comparison of reaction paths

Total energy calculations have been performed within the periodic density-f unctional theory framework to study the dissociation of molecularly adsorbe d nitrogen monoxide NO over three different catalytic surfaces: palladium, rhodium, and palladium-manganese (100). The potential energy surfaces for N O dissociation on these metallic surfaces have been calculated in order to determine the minimal energy paths. The accurate optimizations of the trans ition states and their characterization with a complete vibrational analysi s, including the degrees of freedom of the surface, have been presented. Th e order of increasing activation energy barrier is Rh, Pd3Mn, and Pd. Two t ypes of reaction paths have been found: one involving a horizontal molecula r precursor state and a low activation energy barrier (Rh and Pd3Mn) and th e 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 si mulation of the reaction rate constants is fully compatible with the observ ed catalytic behavior. The differences in catalytic activity have been anal yzed with a bond breaking-bond forming energetic decomposition and a Mullik en population analysis. (C) 2001 American Institute of Physics.