The chemical nature of SO2 poisoning of Cu-CHA-based SCR catalysts for NOx removal in diesel exhausts

This study addresses the impact of SO₂ exposure on the catalytic performance of a Cu-chabazite-based SCR catalyst, as used in diesel exhausts, to reduce the emission of NOx through the NH₃-SCR reaction. The SCR activity is determined by a reaction of NO with the [Cu₂^II(NH₃)₄O₂]²⁺ intermediate. The same intermediate is also the most reactive Cu-species towards SO₂. We demonstrate here that the reaction with NO at 200 °C is limited after exposure of the [Cu₂^II(NH₃)₄O₂]²⁺ complex to SO₂ or SO₂/O₂. Heating the catalyst to 300 °C in NO restores the reaction, albeit at a significantly lower rate. The lower reactivity towards NO indicates that exposure of [Cu₂^II(NH₃)₄O₂]²⁺ to SO₂ induces changes in the chemistry of Cu in the catalyst. This implies that poisoning of Cu-chabazite catalysts by SO₂ is, at least in part, of the chemical nature, and may be not limited to the physical pore blocking.