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

In Cu-zeolite based selective catalytic reduction of NO x with NH 3 (NH 3 -SCR), Cu species (in particular Cu I ) solvated by NH 3 molecules are predicted theoretically to be highly mobile with their mobility being decisive for the NH 3 -SCR reactivity at low temperatures (<250 °C). Direct experimental observation of the Cu mobility after NH 3 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 (10 4 to 10 6 Hz), can be applied to monitor directly the dynamic local movement of Cu ions in zeolite catalysts under NH 3 -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 NH 3 on Cu II sites led to the formation of highly mobile Cu I species and NH 4 + intermediates, and, consequently, significantly enhanced local dynamics of Cu ions in both zeolite catalysts. The re-oxidation of Cu I , which is the rate-determining step of NH 3 -SCR reaction, was more favorable in Cu-SAPO-34 than in Cu-ZSM-5, which can be attributed to the close coupling of NH 4 + intermediate and Cu site promoting the formation of Cu II -NO 2 /NH 4 + . As a result, the overall local dynamics of Cu, largely determined by Cu I species, is less dependent on the NH 4 + intermediate in Cu-SAPO-34 than in Cu-ZSM-5. [ABSTRACT FROM AUTHOR]