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 °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 °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.