Modeling the role of CO and C3H6 in NOx reduction on a Cu-CHA SCR catalyst

Kinetic models for CO-NO2 and C3H6-NO2 reactions are developed to analyze the effect of the reductant species on NH3-SCR reactions over a Cu-CHA monolithic catalyst. A nitrate-based mechanism is used to predict the reduction of NO2 by CO, for which a kinetic model for the adsorption of NO2 is first developed and is then incorporated into another model to include the effect of CO on NO2 adsorption and reaction. The interaction of C3H6 with NO2 is explained by global reactions leading to the formation of NO, N-2 and CO. The kinetic models for CO-NO2 and C3H6-NO2 when combined with the NH3-SCR model are able to explain the experimental trends reported in the literature. Under NO2-SCR reaction conditions, the promotional effect of C3H6 on NOx conversions at low temperatures is explained by the generation of local fast SCR conditions. CO shows a more pronounced promotional effect on NOx conversion than C3H6 at low temperatures. This is explained by the synergistic role of CO and NH3 in enhancing the formation of nitrites and nitrates by NO2 adsorption. The increased formation of NO at high temperatures is used to explain the negative effect of C3H6 on the conversion of NOx during fast SCR and NO2-SCR reaction conditions. The negligible effect of CO during fast SCR reaction conditions is explained by the consumption of nitrites by NH3 instead of their reaction with NO2 to form additional NO. The insights gained in this work can also be used for the analysis of SDPF systems, wherein CO produced during the regeneration of a DPF can affect the NOx reduction performance of the SCR catalyst.