Abstract
Zeolites have been found to be promising sensor materials for a variety of gas molecules such as NH3, NOx, hydrocarbons, etc. The sensing effect results from the interaction of the adsorbed gas molecules with mobile cations, which are non-covalently bound to the zeolite lattice. The mobility of the cations can be accessed by electrical low-frequency (LF; mHz to MHz) and high-frequency (HF; GHz) impedance measurements. Recent developments allow in situ monitoring of catalytic reactions on proton-conducting zeolites used as catalysts. The combination of such in situ impedance measurements with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which was applied to monitor the selective catalytic reduction of nitrogen oxides (DeNOx-SCR), not only improves our understanding of the sensing properties of zeolite catalysts from integral electric signal to molecular processes, but also bridges the length scales being studied, from centimeters to nanometers. In this work, recent developments of zeolite-based, impedimetric sensors for automotive exhaust gases, in particular NH3, are summarized. The electrical response to NH3 obtained from LF impedance measurements will be compared with that from HF impedance measurements, and correlated with the infrared spectroscopic characteristics obtained from the DRIFTS studies of molecules involved in the catalytic conversion. The future perspectives, which arise from the combination of these methods, will be discussed.
Highlights
Zeolites are crystalline, microporous solids bearing tetrahedral TO4 (T denotes as Si, Al, Ti, etc.) units as primary building blocks
Sensing performance may be further improved by optimizing the preparation of zeolite films. In zeolites, when they are applied as gas-sensing materials, interaction of the gas molecules with mobile cations, which are non-covalently bound to the zeolite lattice, can be monitored at different length scales and frequency scales
The electrical properties of zeolites were changed upon interacting with guest molecules, in particular with NH3, which can be analyzed by impedance spectroscopy (IS) in a broad frequency range
Summary
Microporous solids bearing tetrahedral TO4 (T denotes as Si, Al, Ti, etc.) units as primary building blocks. In the last few years, both low-frequency (100 mHz–10 MHz; LF) and high-frequency (1 GHz–10 GHz, denoted as microwave region; HF) impedance spectroscopies have been applied to study the electric properties of zeolites [7,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27] Detailed investigations combining both experimental and theoretical methods were performed to understand the influence of framework type (e.g., FAU, MFI, CHA), Si/Al ratio (10–5000), and cation type (e.g., H+, Na+) on the proton motion in zeolites [14,16].
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