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In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of Cu -(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic reduction of NO with NH (NH -SCR). In particular, we investigated the reaction of NO and O at low temperature (200 and 50 °C) on a series of Cu-CHA zeolites with different composition (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidation part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200 °C. At lower temperature a mixture of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO , whose amount was found to be proportional to the zeolite Brønsted site concentration. Nitrates were found to mainly form with Cu ions stabilized by one negative framework charge (Z), Z-[Cu(OH] or Z-[Cu(O ] , without involvement of Z -Cu ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH . Finally, water was found to replace preformed chelating copper nitrates and deplete NO (though with different kinetics) at both temperatures, while favouring the presence of monodentate ones.

Cu-exchanged zeolites of the CHA structure are state-of-the-art catalysts for selective catalytic reduction of NOx with NH3 in diesel aftertreatment systems. However, these catalysts deactivate in the presence of SO2, which is a constituent of diesel exhaust gas. In this article, the deactivation behavior and mechanisms of a Cu-SAPO-34 catalyst were studied with reactor tests and DFT calculations. Exposure of the catalyst to two different SO2 concentrations and durations, but with the same total SO2 exposure, calculated as the product of partial pressure of SO2 and exposure time, lead to the same degree of deactivation. Exposure of the Cu-SAPO-34 catalyst to SO2 in the presence and absence of NO and NH3 at different temperatures between 200-600 °C showed different trends for the deactivation. Below 400 °C, the S/Cu ratio on the catalyst increased with temperature in absence of NO and NH3, while it decreased with increasing temperature in the presence of NO and NH3. This is explained by the ability of NO and NH3 to reduce Cu(II) to Cu(I). DFT calculations show that SO2 adsorbs more strongly on Cu(I) than on Cu (II). Above 400 °C, the S/Cu ratio decreased with temperature irrespective of the presence of NO and NH3. In all cases, the S/Cu ratio is lower than 1. This is not compatible with extensive deposition of ammonium sulfate when co-feeding SO2, H2O and NH3. A more likely explanation for the deactivation is that SO2 is mainly related to the Cu sites. This is further corroborated by DFT calculations showing that SO2 and SO3, which is possibly formed by oxidation of SO2 over Cu sites, interact similar with Cu in Cu-SAPO-34 and Cu-SSZ-13.

Highlights Deactivation of Cu-CHA catalysts for NH3-SCR by SO2 occurs fast. Deactivation by SO2 can be limited to 20 % by regeneration at 550 °C. Regeneration probably feasible for handling SO2-poisoning of Cu-CHA SCR catalysts. SO2-poisoning appears to depend on the structural properties of Cu-CHA materials.