In situ impedance spectroscopy for monitoring the mobility of cu active sites in zeolite catalysts for DeNO$_{x}$ NH$_{3}$-SCR

  • In-situ-Impedanzspektroskopie zur Überwachung der Mobilität von Cu-Aktivzentren in Zeolith-Katalysatoren für DeNO$_{x}$ NH$_{3}$-SCR

Rizzotto, Valentina; Simon, Ulrich (Thesis advisor); Palkovits, Regina (Thesis advisor); Chen, Peirong (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

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 the 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}$$^{+}$. The results obtained by in situ IS were subsequently used to interpret the behaviour of Cu-CHA in the NH$_{3}$-sensing and to optimise their SCR catalytic performances. It was revealed that the Cu mobility alters NH$_{3}$-sensing response of Cu-CHA in SCR conditions, and a pretreatment under reducing conditions was found beneficial to enhance the catalytic performances of a commercial Cu-CHA. Finally, a novel microwave-assisted method for the synthesis of the chabazite SSZ-13 was developed, which allowed to reduce the conventional reaction time and to obtain materials with tailored composition and structure for future in situ investigations.

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