Probing the effect of Si/Al ratio in Cu-CHA zeolite catalyst on SO۲ exposure: in situ DR UV-vis spectroscopy and deactivation measurements

exchanged chabazites (Cu-CHA) are effective catalysts for ammonia-assisted selective catalytic reduction of NOx (NH۳-SCR) in diesel exhaust systems. However, this catalyst is sensitive to small amounts of SO۲ in the exhaust gases, causing deactivation after prolonged exposure [۱]. SO۲ selectively reacts at low temperature with an oxygen-bridged diamine dicopper (II) complex (...), a crucial intermediate in the NH۳-SCR cycle [۲]. In this study, we measured in situ Diffusion Reflectance UV-Vis spectra, SO۲ uptake, and deactivation of SO۲ poisoned Cu-CHA zeolites with the same Cu loading (۳.۲ wt%) and different Si/Al ratio (۶.۷, ۱۱, and ۱۵). The catalysts were pre-treated in situ to obtain the(....) complexes, followed by exposure to SO۲ at ۲۰۰ oC. Exposure to SO۲ leads to a partial reduction of the CuII in(....) [۲], which appears in the UV-Vis spectrum as a reduction in the absorption around ۱۲۰۰۰ cm-۱ (hatched areas in Figure ۱a). The observed decrease in absorption in the UV-Vis spectra correlates with the S/Cu ratio measured by the SO۲ uptake, confirming that the deactivation is related to the interaction of SO۲ with the(....) complex. To relate the deactivation to the SO۲ uptake, we have performed ۶ consecutive cycles of SO۲ exposure and activity measurements at ۲۰۰ °C in a separate reactor setup. The deactivation shows a linear dependence on the S/Cu ratio for each Si/Al ratio (Figure ۱b). The Cu-CHA catalysts with a higher Si/Al ratio generally show a stronger deactivation.