Importance of the Cu oxidation state for the SO₂-poisoning of a Cu-SAPO-34 catalyst in the NH₃-SCR reaction

Cu-exchanged zeolites of the CHA structure are state-of-the-art catalysts for selective catalytic reduction of NO_x with NH₃ in diesel aftertreatment systems. However, these catalysts deactivate in the presence of SO₂, which is a constituent of diesel exhaust gas. In this article, the deactivation behavior and mechanisms of a Cu-SAPO-34 catalyst were studied with reactor tests and DFT calculations. Exposure of the catalyst to two different SO₂ concentrations and durations, but with the same total SO₂ exposure, calculated as the product of partial pressure of SO₂ and exposure time, lead to the same degree of deactivation. Exposure of the Cu-SAPO-34 catalyst to SO₂ in the presence and absence of NO and NH₃ at different temperatures between 200–600 °C showed different trends for the deactivation. Below 400 °C, the S/Cu ratio on the catalyst increased with temperature in absence of NO and NH₃, while it decreased with increasing temperature in the presence of NO and NH₃. This is explained by the ability of NO and NH₃ to reduce Cu(II) to Cu(I). DFT calculations show that SO₂ adsorbs more strongly on Cu(I) than on Cu (II). Above 400 °C, the S/Cu ratio decreased with temperature irrespective of the presence of NO and NH₃. In all cases, the S/Cu ratio is lower than 1. This is not compatible with extensive deposition of ammonium sulfate when co-feeding SO₂, H₂O and NH₃. A more likely explanation for the deactivation is that SO₂ is mainly related to the Cu sites. This is further corroborated by DFT calculations showing that SO₂ and SO₃, which is possibly formed by oxidation of SO₂ over Cu sites, interact similar with Cu in Cu-SAPO-34 and Cu-SSZ-13.