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Abstract

Steady-state and transient kinetic experiments were performed in a versatile microreactor flow set-up with magnesia- and alumina-supported ruthenium catalysts in order to elucidate the mechanism of the selective catalytic reduction (SCR) of nitric oxide with hydrogen. Both Ru/MgO and Ru/ϒAl₂O₃ were found to be highly active catalysts converting NO and H₂ into N₂ and H₂O with selectivities close to 100% at full conversion, although Ru-based catalysts are known to be active in the synthesis of NH₃ from N₂ and H₂. Frontal chromatography experiments with NO at room temperature revealed that NO and its dissociation products displace adsorbed atomic hydrogen (H−_*) almost completely from hydrogen-precovered Ru surfaces. Obviously, NO and H₂ compete for the same adsorption sites, H−_* being the weaker bound adsorbate. Temperature-programmed surface reaction (TPSR) experiments in H₂ subsequent to NO exposure demonstrated that higher heating rates and lower partial pressures of H₂ shift the selectivity from NH₃ to N₂. Therefore, the coverage of H−_* is concluded to govern the branching ratio between the rate of associative desorption of N₂ (2N−_*→N₂+2_*) and the rate of hydrogenation of N−_* (N−_*+3H–_* →NH3 4_*). Finally, the steady-state coverages of N- and O-containing adsorbates were derived by interrupting the SCR reaction and hydrogenating the adsorbates off as NH₃ and H₂O. By solving the site balance, the Ru surfaces were found to be essentially N₂ is attributed to the very low coverage of H−_* due to site blocking by a N + O coadsorbate layer, favouring the recombination of N−_* instead of its hydrogenation to NH₃.