The Impact of CO, C₃H₆, and SO₂ on Cu-CHA and V₂O₅-WO₃/TiO₂ Catalysts for NH₃-SCR for Close-Coupled Operation

In diesel vehicles, selective catalytic reduction by ammonia (NH₃-SCR) is used to abate the emission of NO_x. Traditionally, the SCR catalyst is placed downstream towards the outlet of the exhaust cleaning system, where heating of the SCR catalyst is slow. This leads to a significant emission of NO_x during cold start as it takes time to reach the operating temperature of 180 °C for the SCR catalyst. When the SCR catalyst is placed directly at the engine outlet (close-coupled position), it is heated faster, resulting in lower NO_x emissions. However, a close-coupled SCR catalyst is exposed to higher concentrations of unburnt fuel, CO and SO₂, compared to the traditional position further downstream. To assess the performance of close-coupled SCR catalysts, Cu-CHA and V₂O₅-WO₃/TiO₂-based monolithic SCR catalysts were exposed to NH₃-SCR feeds with controlled amounts of CO, propylene, propane, or SO₂. WHTC cold start test cycles showed 10–25% lower NO_x emissions for the close-coupled position. Exposing the catalysts to 950 ppm_v CO or 450 ppm_v of propylene results in a reversible decrease of the NO_x conversion by 1–2% points or 7–8% points, respectively. The presence of 2 ppm_v SO₂ has a minor effect on the V₂O₅-WO₃/TiO₂ catalyst, while the NO_x conversion for Cu-CHA decreases from 75 to 63% after exposure to 1.6 g SO₂/L_cat. The Cu-CHA catalyst can be regenerated by heating to 500 °C. NH₃ enhances the impact of SO₂ on the Cu-CHA catalyst, implying that the urea dosing strategy affects the response of a Cu-CHA-based system to SO₂. The results indicate that both conventional types of SCR catalysts are suitable for use in the close-coupled position.