Probing the effect of the Si/Al ratio in Cu-CHA zeolite catalysts on SO2 exposure: in situ DR UV-vis spectroscopy and deactivation measurements

Abstract

Cu-exchanged chabazite zeolite (Cu-HA) is one of the most effective catalysts for ammonia-assisted selective catalytic reduction (NH₃-SCR) in diesel exhaust systems. However, this catalyst is sensitive to small amounts of SO₂ in the exhaust gases, causing deactivation after prolonged exposure. To have a better understanding of the effect of the Si/Al ratio of zeolite on the SO₂ exposure of Cu-CHA catalysts, we measured in situ diffuse reflectance UV-vis NIR spectroscopy, SO₂ uptake, and deactivation of SO₂ poisoned Cu-CHA catalysts with the same Cu loading (3.2 wt%) and different Si/Al ratios (6.7, 11 and 15) at 200 °C. SO₂ selectively reacts with an oxygen-bridged diamine dicopper(II) complex [Cu^II₂(NH₃)₄O₂]²⁺, resulting in 50% deactivation in all catalysts, with an SO₂ uptake which varies from a 0.2 S/Cu ratio for the catalyst with Si/Al = 6.7, to S/Cu = 0.12 for Si/Al = 15. For the fresh catalysts, the NH₃-SCR activity decreases as the Si/Al ratio increases from 6.7 to 15, as also indicated by the amount of [Cu^II₂(NH₃)₄O₂]²⁺ complexes. After exposure of the [Cu^II₂(NH₃)₄O₂]²⁺ complex to SO₂, the change in UV-vis spectra correlates well with the SO₂ uptake and the expected Cu-species formed for all three Si/Al ratios. This suggests that, under the applied conditions, the SO₂ reaction with the [Cu^II₂(NH₃)₄O₂]²⁺ complex in Cu-CHA does not depend on the Si/Al ratio.