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

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