Selective Catalytic Reduction of NO<sub><i>x</i></sub> with NH<sub>3</sub> over Cu/SSZ-13: Elucidating Dynamics of Cu Active Sites with In Situ UV–Vis Spectroscopy and DFT Calculations
Yani Zhang, Jun Zhang, Houlin Wang, Weinan Yang, Chizhong Wang, Yue Peng, Jianjun Chen, Junhua Li, Feng Gao
Abstract
Cu/SSZ-13 is the current state-of-the-art catalyst for selective catalytic reduction of NOx 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, Z2CuII and 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 Z2CuII species cannot undergo such a transformation. In addition, ZCuIIOH displays stronger activity than Z2CuII during both reduction by NH3 and NO oxidation to bidentate nitrates. Despite these differences, Z2CuII and ZCuIIOH 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.