Effect of the Si/Al ratio on SO2 poisoning of Cu-CHA zeolites studied by in situ DR UV-Vis spectroscopy and deactivation measurements

Cu exchanged chabazites (Cu-CHA) are effective catalysts for ammonia-assisted selective catalytic reduction of NOx (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. SO2 selectively reacts at low temperature with an oxygen-bridged diamine dicopper (II) complex , a crucial intermediate in the NH3-SCR cycle. In this study, we measured in situ Diffusion Reflectance UV-Vis spectra, SO2 uptake, and deactivation of SO2 poisoned Cu-CHA zeolites with the same Cu loading (3.2 wt%) and different Si/Al ratio (6.7, 11, and 15). The catalysts were pre-treated in situ to obtain the complexes, followed by exposure to SO2 at 200 oC. Exposure to SO2 leads to a partial reduction of the CuII in, which appears in the UV-Vis spectrum as a reduction in the absorption around 12000 cm-1 (hatched areas). The observed decrease in absorption in the UV-Vis spectra correlates with the S/Cu ratio measured by the SO2 uptake, confirming that the deactivation is related to the interaction of SO2 with the complex. To relate the deactivation to the SO2 uptake, we have performed 6 consecutive cycles of SO2 exposure and activity measurements at 200 °C in a separate reactor setup. The deactivation shows a linear dependence on the S/Cu ratio for each Si/Al ratio. The Cu-CHA catalysts with a higher Si/Al ratio generally show a stronger deactivation.