SO 2 Poisoning of Cu-CHA deNO x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy.

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH 3 -assisted selective catalytic reduction of NO (NH 3 -SCR) for the abatement of NO x emission from diesel vehicles. However, the presence of a small amount of SO 2 in diesel exhaust gases leads to a severe reduction in the low-temperature activity of these catalysts. To shed light on the nature of such deactivation, we characterized a Cu-CHA catalyst under well-defined exposures to SO 2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO 2 exposure, we have selectively prepared Cu I and Cu II species with different ligations, which are relevant for the NH 3 -SCR reaction. The highest reactivity toward SO 2 was observed for Cu II species coordinated to both NH 3 and extraframework oxygen, in particular for [Cu II 2 (NH 3 ) 4 O 2 ] 2+ complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO 2 uptake was significantly lower. These results explain why SO 2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [Cu II 2 (NH 3 ) 4 O 2 ] 2+ is a crucial intermediate in the low-temperature NH 3 -SCR catalytic cycle.