The reduction of NO with H₂ over Ru/MgO

Abstract

Ruthenium supported on magnesia was found to be a highly active and selective catalyst for the reduction of NO to N₂ with H₂. The adsorption of NO on Ru/MgO was studied at room temperature by applying frontal chromatography with a mixture of 2610 ppm NO in He. Subsequently, temperature‐programmed desorption (TPD) and temperature‐programmed surface reaction (TPSR) experiments in H₂ were performed. The adsorption of NO was observed to occur partly dissociatively as indicated by the formation of molecular nitrogen. The TPD spectrum exhibited a minor NO peak at 340 K indicating additional molecular adsorption of NO during the exposure to NO at room temperature, and two N₂ peaks at 480 K and 625 K, respectively. The latter data are in good agreement with previous results with Ru(0001) single‐crystal samples, where the interaction with NH₃ was found to lead to two N₂ thermal desorption states with a maximum coverage of atomic nitrogen of about 0.38. Heating up the catalyst after saturation with NO at room temperature in a H₂ atmosphere revealed the self‐accelerated formation of NH₃ after partial desorption of N₂, whereby sites for reaction with H₂ become available. As a consequence, the observed high selectivity towards N₂ under steady‐state reduction conditions is ascribed to the presence of a saturated N+O coadsorbate layer resulting in an enhanced rate of N₂ desorption from this layer and a very low steady‐state coverage of atomic hydrogen. The formation of H₂O by reduction of adsorbed atomic oxygen is the slow step of the overall reaction which determines the minimum temperature required for full conversion of NO.