On the mechanism of the selective catalytic reduction of NO to N-2 by H-2 over Ru/MgO and Ru/Al2O3 catalysts

Steady-state and transient kinetic experiments were performed in a versatil e microreactor flow set-up with magnesia- and alumina-supported ruthenium c atalysts in order to elucidate the mechanism of the selective catalytic red uction (SCR) of nitric oxide with hydrogen. Both Ru/MgO and Ru/gamma-Al2O3 were found to be highly active catalysts converting NO and H-2 into N-2 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 N-2 and H-2. Frontal chromatography experiments with NO at room temperature revealed th at NO and its dissociation products displace adsorbed atomic hydrogen (H-*) almost completely from hydrogen-precovered Ru surfaces. Obviously, NO and H-2 compete for the same adsorption sites, H-* being the weaker bound adsor bate. Temperature-programmed surface reaction (TPSR) experiments in H-2 sub sequent to NO exposure demonstrated that higher heating rates and lower par tial pressures of H-2 shift the selectivity from NH3 to N-2. Therefore, the coverage of H-* is concluded to govern the branching ratio between the rat e of associative desorption of N-2 (2N-*--> N-2 + 2*) and the rate of hydro genation of N-* (N-* +3H-* --> NH3 + 4*). Finally, the steady-state coverag es of N- and O-containing adsorbates were derived by interrupting the SCR r eaction and hydrogenating the adsorbates off as NH3 and H2O. By solving the site balance, the Ru surfaces were found to be essentially saturated by a N+O coadsorbate layer. Thus, the observed high steady-state SCR selectivity to N-2 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 i ts hydrogenation to NH3.