Cu-exchanged chabazites (Cu-CHA) are the most relevant catalysts for the selective catalytic reduction (SCR) of NO$_{x}$ with NH$_{3}$ in automotive exhaust after-treatment. The mobility of NH$_{3}$-solvated Cu active sites [i.e. Cu$^{I}$/$^{II}$(NH $_{3}$)$_{n}$] in the CHA framework was recently found to be an essential element of the SCR catalytic cycle. Even though theoretical simulations agree on this crucial aspect, only the support of in situ analysis methods for the quantification of

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... ion mobility of Cu$^{I}$/$^{II}$(NH $_{3}$)$_{n}$ species can lead to a deeper understanding of the SCR mechanism in Cu-CHA catalysts. This work shows how in situ impedance spectroscopy (IS) is capable of detecting such local movement of Cu active sites in CHA. In situ IS was applied to monitor ion conduction processes under typical SCR reaction conditions by analysing the resonance peaks in the high-frequency range (HF). Initially, Cu-CHA were studied in NH$_{3}$ atmosphere in order to ensure the presence of NH$_{3}$-solvated Cu$^{2+}$ and evaluate its specific interaction with the CHA framework. Afterwards, the complexity of the study was increased by performing in situ IS measurements in gas atmospheres that initiate the SCR reaction and the Cu $^{2+}$/Cu$^{+}$ redox cycle. In a third step, the site-specific mobility of Cu$^{I}$/$^{II}$(NH $_{3}$)$_{n}$ was investigated by analysing two Cu-CHA materials in which Cu coordinates only to isolated or paired Al sites, respectively. In this way, the short-range motion of Cu$^{I}$/$^{II}$(NH $_{3}$)$_{n}$ was successfully monitored by variations of the HF resonance frequency. The observed mobility trends agree with those predicted by theoretical simulations. Moreover, it was for the first time possible to discriminate the contribution to the overall ion conductivity given by Cu$^{I}$/$^{II}$(NH $_{3}$)$_{n}$ from the one given by other NH$_{3}$-solvated species formed during SCR, like NH$_{4}$$^{+}$. The results obtained by in situ IS were subsequently used to interpret the behav [...]