A quantitative analysis of the impact of SO2 on the activity of Cu-CHA catalysts for NH3-SCR

Copper-exchanged chabazite (Cu-CHA) catalysts for selective catalytic reduction of NOx by ammonia (NH3-SCR) in diesel exhausts deactivate in the presence of SO2 at temperatures below 300 degrees C. In this article, we develop a descriptive model to evaluate a catalyst deactivation with respect to SO2 tolerance, in terms of a disappearance of active catalyst. This leads to the SO2 sensitivity, which can be interpreted as the loss of catalyst per mol SO2 taken up by the catalyst, as a measure for the deactivation for that catalyst material. We have determined the SO2 sensitivity for three Cu-CHA catalysts, namely 1.6 and 3.2 wt% Cu with a Si/Al ratio of 6.7 and 3.2 wt% Cu with a Si/Al ratio of 15. The 3.2 wt% Cu (Si/Al = 6.7) catalyst shows a lower SO2 sensitivity at 200 degrees C, as compared to the other two catalysts. For all three catalysts, the SO2 sensitivity is highest at low SO2 uptake, and decreases linearly with the further uptake of SO2. This means that small amounts of SO2 cause a relatively strong deactivation, indicating that the deactivation is a consequence of a reaction of SO2 with the active Cu.