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

The application of Cu-CHA catalysts for the selective catalytic reduction of NO by ammonia (NH-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 SO. Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO is crucial for elucidating the SO poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO reacts with the [Cu(NH)O] complex that is formed in the pores of Cu-CHA upon activation of O in the NH-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 SO uptake is similar for catalysts with low and high Cu content. We show that the SO uptake proceeds via an oxidation of SO by the [Cu(NH)O] complex, resulting in the formation of different Cu species, which do not react with SO, and a sulfated Cu complex that is accumulated in the pores of the zeolite. The increase of the SO uptake upon addition of oxygen to the SO-containing feed, evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO, is explained by the re-oxidation of the Cu species into the [Cu(NH)O] complexes, which makes them available for reaction with SO