Selective Catalytic Reduction of NOx with NH3 over Cu/SSZ-13: Elucidating Dynamics of Cu Active Sites with In Situ UV-Vis Spectroscopy and DFT Calculations

Cu/SSZ-13 is the current state-of-the-art catalyst for selective catalytic reduction of NQ with NH3 (NH3-SCR) in diesel after-treatment systems. Recent investigations under in situ or operando conditions yielded rich molecular level understanding about dynamic transformations of Cu sites during reactions. However, in situ investigations aiming at the distinction of the two SCR-active Cu species in this catalyst, that is, Z(2)Cu(II) and ZCu(II)OH, are still scarce. Herein, we apply in situ UV-vis spectroscopic studies combined with theoretical calculations to investigate the two Cu sites under SCR-relevant conditions at low temperatures. We demonstrate that in the presence of O-2, two isolated ZCu(II)OH sites readily transform into a double O-bridged dicopper species with a Cu-Cu distance of 3.37 angstrom, whereas Z(2)Cu(II) species cannot undergo such a transformation. In addition, ZCu(II)OH displays stronger activity than Z(2)Cu(II) during both reduction by NH3 and NO oxidation to bidentate nitrates. Despite these differences, Z(2)Cu(II) and ZCu(II)OH sites exhibit similar spectroscopic features under both NH3 oxidation and NH3-SCR conditions. These findings demonstrate that UV-vis spectroscopy is a powerful tool to be used in situ to provide rich information on the NH3-SCR mechanism and on the rational design of Cu/SSZ-13 catalysts.

Automated summary

In this study, we use in situ UV-vis spectroscopic studies combined with theoretical calculations to investigate the transformation behavior of Z(2)Cu(II) and ZCu(II)OH active sites in Cu/SSZ-13 catalyst under SCR-relevant conditions. We find that ZCu(II)OH readily transforms into a double O-bridged dicopper species, while Z(2)Cu(II) does not undergo such a transformation. Despite these differences, both species exhibit similar spectroscopic features under NH3 oxidation and NH3-SCR conditions, providing important information for understanding the NH3-SCR mechanism and catalyst design.