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

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 degrees C. SO2 selectively reacts with an oxygen-bridged diamine dicopper(II) complex [Cu-2(II)(NH3)(4)O-2](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-2(II)(NH3)(4)O-2](2+) complexes. After exposure of the [Cu-2(II)(NH3)(4)O-2](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-2(II)(NH3)(4)O-2](2+) complex in Cu-CHA does not depend on the Si/Al ratio.