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The role of surface nitrogen in the kinetics of the NO+CO conversion reaction on Rh(111) under steady-state catalytic conditions was explored by using collimated molecular beams and mass spectrometry detection. Two types of kinetically different nitrogen atoms were identified on the surface. The buildup of a critical nitrogen coverage was determined to be required for the start of the nitrogen recombination step to N2. This threshold coverage is quite large at low temperatures, amounting to over half a monolayer around 400 K, but decreases abruptly with increasing reaction temperature, and becomes almost insignificant above 600 K. The actual value of this coverage is quite insensitive to the ratio of NO to CO in the reaction mixture, but displays an inverse correlation with the steady-state reaction rate under most conditions. An additional small amount of nitrogen appears to be present on the surface during catalysis but to desorb rapidly after the removal of the gas-phase reactants. The NO reduction rate displays an approximately first-order dependence on the coverage of these labile N atoms. Isotope switching experiments indicated that the two types of kinetically different nitrogens are not likely to represent different adsorption sites, but rather similar adsorption states with adsorption energetics modified by their immediate surrounding environment on the surface. The data are explained here by a model in which the nitrogen atoms form surface islands and where the atoms at the perimeter of those islands react preferentially via N+N recombination to N2.

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