Local dynamics of copper active sites in zeolite catalysts for selective catalytic reduction of NOx with NH3

Abstract

In Cu-zeolite based selective catalytic reduction of NOx with NH3 (NH3-SCR), Cu species (in particular CuI) solvated by NH3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH3-SCR reactivity at low temperatures (<250 °C). Direct experimental observation of the Cu mobility after NH3 solvation, however, has not been achieved yet. Here we show that complex impedance-based modulus spectroscopy, performed by following the corresponding dielectric relaxation processes at high frequencies (104 to 106 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH3-SCR related reaction conditions. Simultaneous in situ impedance and infrared spectroscopy studies, assisted by periodic DFT calculations with reliable van der Waals dispersion corrections, allowed us to identify the key factors determining the local dynamics of Cu ions in two representative Cu-zeolites, i.e. Cu-ZSM-5 and Cu-SAPO-34. The co-adsorption and interaction of NO and NH3 on CuII sites led to the formation of highly mobile CuI species and NH4+ intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of CuI, which is the rate-determining step of NH3-SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH4+ intermediate and Cu site promoting the formation of CuII-NO2/NH4+. As a result, the overall local dynamics of Cu, largely determined by CuI species, is less dependent on the NH4+ intermediate in Cu-SAPO-34 than in Cu-ZSM-5.