Selective Catalytic Reduction of NOxwith NH3over 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 NOxwith 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, Z2CuIIand ZCuIIOH, 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 O2, two isolated ZCuIIOH sites readily transform into a double O-bridged dicopper species with a Cu–Cu distance of 3.37 Å, whereas Z2CuIIspecies cannot undergo such a transformation. In addition, ZCuIIOH displays stronger activity than Z2CuIIduring both reduction by NH3and NO oxidation to bidentate nitrates. Despite these differences, Z2CuIIand ZCuIIOH sites exhibit similar spectroscopic features under both NH3oxidation 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.