SO2 Poisoning of Cu-CHA deNO x Catalyst: The Most Vulnerable Cu Species Identified by X-ray Absorption Spectroscopy

Abstract

Cu-exchanged chabazite zeolites (Cu-CHA) are effective catalysts for the NH₃-assisted selective catalytic reduction of NO (NH₃-SCR) for the abatement of NO _x 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^II ₂(NH₃)₄O₂]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^II ₂(NH₃)₄O₂]²⁺ is a crucial intermediate in the low-temperature NH₃-SCR catalytic cycle.