Emission of NH3 and N2O during NO reduction over commercial aged three-way catalyst (TWC): Role of individual reductants in simulated exhausts

Abstract

The NH3 and N2O emissions over commercial aged three-way catalysts (TWC) is closely dependent on individual reductant species in a complex reaction network. Under simulated full mixture feed composition containing CO, H2 and a hydrocarbon mixture of C3H6 and C3H8 as reductants, the NH3 formation was maximized under rich conditions (0.97 ≤ λ < 1.00) at high temperature ranges from 350 to 500 °C. The N2O formation was more pronounced under a stoichiometric condition (λ = 1.00) at low temperatures from 250 to 350 °C. To determine the role of individual reductant species in NH3 and N2O emissions characterized from full feed conditions, we conducted simple feed tests containing a single reductant (H2, CO or C3H6) and NO. It was found that NH3 is mainly generated through the NOH2 reaction via -NH2 intermediates when H2 is present in the feed gas under the stoichiometric condition. On the other hand, the role of H2 on the N2O formation is mainly low temperature reactions below 200 °C. The major contribution of the N2O formation at warmed-up temperatures is the CONO reaction forming surface NCO- species, where additional surface CO and H2 species can be also produced by steam reforming of C3H6.