Reduction over zeolite-based catalysts of nitrogen oxides in emissions containing excess oxygen: Unraveling the reaction mechanism

Abstract

Reduction of NO x to N 2 over zeolite-based catalysts is a multi-step reaction for which transition metal (TM) ions are helpful but not indispensable. The present state of understanding the mechanism is reviewed for the catalyzed NO x reduction with ammonia or alkanes; reduction with acetaldehyde is also sketched, but will be described in detail in a separate paper. A decisive step is the interaction of two reaction intermediates containing N atoms in different oxidation states. A favorable reaction path uses an organic molecule to reduce part of the NO x to ammonia, this product then reacts with NO+NO 2 to give N 2. Isotopic labeling shows that each N 2 molecule has one N atom from the NH 3 intermediate, the other from NO x . N 2 is formed at room temperature, if an 1:1 mixture of NO and NO 2 is in contact with an Fe/MFI catalyst covered with NH 3. During NO x reduction with CH 4 over Pd/zeolite catalysts, H/D exchange of methane has been observed, indicating that methane is dissociatively adsorbed on Pd 0 clusters that are present in the steady-state of surface oxidation and reduction steps. BaNaY catalysts, containing negligible impurities of TM elements, catalyze NO x reduction with acetaldehyde at 200 °C. In the prevailing mechanism acetaldehyde is transformed via acetate ions and nitromethane to isocyanic acid, which is hydrolyzed to NH 3. Again, N 2 is ultimately produced from NH 3 and N 2O 3 via ammonium nitrite.