Infrared Spectroscopic Study of the NH₃-SCR on Fe-Zeolites under Transient Conditions

The standard NH₃-SCR (selective catalytic reduction) of NO/O₂-mixtures on Fe-zeolites shows significant maxima in the NO-conversion as transient response, when NH₃ is turned on and off. Introducing in situ DRIFT-spectroscopy of the catalyst two remarkable effects were observed. On the one hand a kink in the kinetics of the bands assigned to the 2H- and 3H-structure of adsorbed NH₃ was detected. Furthermore the kinetics of several bands show a maximum, which gives hint to the existence of an intermediate, in the following called SCR-SI (SCR-Surface Intermediate). Due to the observed correlation between both, the maximum in the NO-conversion and the maxima in the band kinetics on the catalyst, the SCR-SI plays an important role in the mechanism and seems to be responsible for the increased NO-conversion. In order to gain more information about the structure and role of SCR-SI the influence of temperature, iron content, O₂ concentration and NO/NH₃–ratio was studied.