Elucidating the reaction mechanism of SO 2 with Cu-CHA catalysts for NH 3 -SCR by X-ray absorption spectroscopy.

The application of Cu-CHA catalysts for the selective catalytic reduction of NO x by ammonia (NH 3 -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 2 . Understanding the mechanism of the interaction between the Cu-CHA catalyst and SO 2 is crucial for elucidating the SO 2 poisoning and development of efficient catalysts for SCR reactions. Earlier we have shown that SO 2 reacts with the [Cu 2 II (NH 3 ) 4 O 2 ] 2+ complex that is formed in the pores of Cu-CHA upon activation of O 2 in the NH 3 -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 2 uptake is similar for catalysts with low and high Cu content. We show that the SO 2 uptake proceeds via an oxidation of SO 2 by the [Cu 2 II (NH 3 ) 4 O 2 ] 2+ complex, resulting in the formation of different Cu I species, which do not react with SO 2 , and a sulfated Cu II complex that is accumulated in the pores of the zeolite. The increase of the SO 2 uptake upon addition of oxygen to the SO 2 -containing feed, evidenced by X-ray adsorbate quantification (XAQ) and temperature-programmed desorption of SO 2 , is explained by the re-oxidation of the Cu I species into the [Cu 2 II (NH 3 ) 4 O 2 ] 2+ complexes, which makes them available for reaction with SO 2 .