Mechanism of low-temperature selective catalytic reduction of NO with NH 3 over carbon-supported Mn 3O 4: Role of surface NH 3 species: SCR mechanism

Abstract

In this work a complete mechanism for describing the low-temperature (125 °C) selective catalytic reduction of NO with NH 3 over carbon-supported Mn 3O 4 is discussed. This study sets out to explain for the first time certain specific interactions among NH 3, NO, O 2, and a manganese-based catalyst. A set of SCR reactions was obtained through a detailed TPD analysis of the surface NH 3 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 NO 2, and to a lesser extent NO, reacts from the gas phase with surface-active NH 3 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/NO 2. These NH 3-based species are related to the local phases that coexist in Mn 3O 4: (a) SCR by aminooxy groups (steady-state mechanism). Aminooxy groups formed on the locally octahedral environment of Mn 3O 4 (Mn 2O 3) react with gaseous NO 2. 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 2O. (b) SCR by ammonium ions (pseudo-steady-state mechanism). This mechanism occurs on the locally tetrahedral environment of Mn 3O 4 (MnO) and initially accounts for ∼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 NO 2. The overall SCR process is 4NO+4NH 3+O 2→4N 2+6H 2O.