Elucidating the reaction mechanism of SO₂ with Cu-CHA catalysts for NH₃-SCR by X-ray absorption spectroscopy

The application of Cu-CHA catalysts for the selective catalytic reduction of NO by ammonia (NH₃-SCR) in exhaust systems of diesel vehicles requires the use of fuel with low sulfur content, because the Cu-CHA catalysts are poisoned by higher concentrations of SO₂. Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO₂ is crucial for elucidating the SO₂ poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO₂ reacts with the [Cu₂^II(NH₃)₄O₂]²⁺ complex that is formed in the pores of Cu-CHA upon activation of O₂ in the NH₃-SCR cycle. In order to determine the products of this reaction, we use X-ray absorption spectroscopy (XAS) at the Cu K-edge and S K-edge, and X-ray emission spectroscopy (XES) for Cu-CHA catalysts with 0.8 wt% Cu and 3.2 wt% Cu loadings. We find that the mechanism for SO₂ uptake is similar for catalysts with low and high Cu content. We show that the SO₂ uptake proceeds via an oxidation of SO₂ by the [Cu₂^II(NH₃)₄O₂]²⁺ complex, resulting in the formation of different Cu^I species, which do not react with SO₂, and a sulfated Cu^II complex that is accumulated in the pores of the zeolite. The increase of the SO₂ uptake upon addition of oxygen to the SO₂-containing feed, evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO₂, is explained by the re-oxidation of the Cu^I species into the [Cu₂^II(NH₃)₄O₂]²⁺ complexes, which makes them available for reaction with SO₂.