SO2 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 NH3-assisted selective catalytic reduction of NO (NH3-SCR) for the abatement of NOx emission from diesel vehicles. However, the presence of a small amount of SO2 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 SO2 using in situ X-ray absorption spectroscopy. By varying the pretreatment procedure prior to the SO2 exposure, we have selectively prepared Cu-I and Cu-II species with different ligations, which are relevant for the NH3-SCR reaction. The highest reactivity toward SO2 was observed for Cu-II species coordinated to both NH3 and extraframework oxygen, in particular for [Cu-2(II) (NH3)(4)O-2](2+) complexes. Cu species without either ammonia or extraframework oxygen ligands were much less reactive, and the associated SO2 uptake was significantly lower. These results explain why SO2 mostly affects the low-temperature activity of Cu-CHA catalysts, since the dimeric complex [Cu-2(II) (NH3)(4)O-2](2+) is a crucial intermediate in the lowtemperature NH3-SCR catalytic cycle.