Site selective adsorption and relocation of SOx in deactivation of Cu–CHA catalysts for NH3-SCR

Abstract

The presence of SO₂ in diesel exhaust gases causes severe deactivation of the Cu–CHA catalysts for the reduction of NO_x by selective catalytic reduction with ammonia (NH₃-SCR). The deactivation of Cu–CHA catalysts after exposure to SO₂ at 550 °C for 0.5, 4, 8, 16 and 32 h, and subsequent regeneration in SO₂-free gas at 550 °C was related to the site-dependent interactions of SO₂ with Cu ions associated with one or two framework Al centers (Z-CuOH or Z₂-Cu), as determined by electron paramagnetic resonance (EPR). SO₂ primarily interacts with the EPR-silent Z-CuOH sites, but a new, EPR-active Z₂-Cu phase develops with SO₂ exposure time as well. A part of the original Z₂-Cu species remain unaffected by SO₂, which is associated with a maximum deactivation level of about 90%. Regeneration at 550 °C leads to the release of most of the SO₂ from the Z-CuOH sites and some relocation of sulfur to Z₂-Cu sites occurs. The activation energy for NH₃-SCR on the SO₂ exposed catalysts decreases with S content from about 65 kJ mol⁻¹ to 30 kJ mol⁻¹. For the regenerated catalysts, the activation energy is restored to about 65 kJ mol⁻¹, showing the importance of the Z-CuOH sites for the NH₃-SCR reaction.