Mechanism of low-temperature selective catalytic reduction of NO with NH3 over carbon-supported Mn3O4 -: Role of surface NH3 species:: SCR mechanism

In this work a complete mechanism for describing the low-temperature (125 degreesC) selective catalytic reduction of NO with NH3 over carbon-supported Mn3O4 is discussed. This study sets out to explain for the first time certain specific interactions among NH3, NO, O-2, and a manganese-based catalyst. A set of SCR reactions was obtained through a detailed TPD analysis of the surface NH3 species and by taking into account the conclusions of a previous study on the role of NO species [Phys. Chem. Chem. Phys. 6 (2004) 453]. The SCR reactions proceed via an Eley-Rideal mechanism, in which NO2, and to a lesser extent NO, reacts from the gas phase with surface-active NH3 species. The overall reaction path involves the simultaneous occurrence of two different SCR mechanisms in which either aminooxy groups or ammonium ions react with NO/NO2. These NH3-based species are related to the local phases that coexist in Mn3O4: (a) SCR by aminooxy groups (steady-state mechanism). Aminooxy groups formed on the locally octahedral environment of Mn3O4 (Mn2O3) react with gaseous NO2. O-2 cannot dissociate on this phase in order to reoxidize the reduced catalyst and therefore the overall SCR process is 6NO + 4NH(3) --> 5N(2) + 6H(2)O. (b) SCR by ammonium ions (pseudo-steady-state mechanism). This mechanism occurs on the locally tetrahedral environment of Mn3O4 (MnO) and initially accounts for similar to 60% of the total NO reduction. However, it is gradually deactivated by the nitrates formed on those same hydroxyl groups that are available for ammonium formation. The ammonium ions formed on the hydroxyl groups of this tetrahedral environment react with gas-phase NO2. The overall SCR process is 4NO + 4NH(3) + O-2 --> 4N(2) + 6H(2)O. (C) 2004 Elsevier Inc. All rights reserved.