Ammonia storage studies on H-ZSM-5 zeolites by microwave cavity perturbation: correlation of dielectric properties with ammonia storage

Abstract

Abstract.To meet today's emission standards, the ammonia-based selective catalytic reduction (SCR) has become the major NO x control strategy for light and heavy diesel engines. Before NO x reduction can proceed, adsorption of ammonia on the acidic sites of the catalyst is necessary. For improvements in efficiency and control of the exhaust gas aftertreatment, a better understanding of the ammonia storage on the acidic sites of zeolite-based SCR catalysts is needed. Thereby, the correlation of dielectric properties of the catalyst material itself with the ammonia storage is a promising approach. Recently, a laboratory setup using microwave cavity perturbation to measure the dielectric properties of catalyst material has been described. This study shows the first experimental data on zeolite-based SCR materials in their H-form. The SCR powder samples are monitored by microwave cavity perturbation while storing and depleting ammonia, both with and without admixed NO x at different temperatures. Its complex dielectric permittivity is found to correlate closely with the stored mass of ammonia. The influence of the temperature and the Si / Al ratio of the zeolite to the ammonia storage behavior are also examined. These measurements disclose different temperature dependencies and differing sensitivities to ammonia storage for both real and imaginary parts of the complex permittivity. The apparent constant sensitivity of the real part can be related to the polarity of the adsorbed ammonia molecules, whereas the imaginary part depends on the Si / Al ratio and is related to the conductivity mechanisms of the zeolite material by proton hopping. It provides information about the zeolite structure and the number of (and the distance between) acidic storage sites, in addition to the stored ammonia mass.

Novel radio-frequency-based gas sensor with integrated heater

“…(1), this is caused by an increase in the permittivity of the sensitive layer. Thus, these measurements are consistent with the results of previous research (Dietrich et al, 2017;Rauch et al, 2017). The increase in sensor temperature results in a significantly lower resonant frequency shift.…”

“…It has already been successfully demonstrated that the stored ammonia in zeolite-based selective catalytic reduction (SCR) catalysts can be determined using a radio-frequency-based technology. The excitation and measurement of the resonances was carried out in a contactless way by coaxial probe antennas in a cavity resonator, which is formed by the metallic catalyst canning (Dietrich et al, 2017). This was possible since the ammonia storage in the zeolite structure causes a change in the complex permittivity ε = ε −j ε (Rauch et al, 2017).…”

“…The evaluation of changes in electrical material properties due to the presence of the analyte to be detected is widely used. Most sensors currently determine these changes by measuring the resistance or the conductance of the sensitive material (Comini et al, 2009). For this purpose, a thin layer of the sensitive material can be applied on (interdigital) electrodes, as is usual, for example, with metal oxide sensors (Barsan et al, 2007).…”

Simultaneous neutron powder diffraction and microwave dielectric studies of ammonia absorption in metal–organic framework systems

“…It has been shown that the microwave cavity perturbation technique can be used to effectively characterise solid-state ammine materials, including the determination of ammonia stoichiometry when appropriately calibrated. 9,[41][42][43] This technique provides a non-invasive, nondestructive method of measuring the intrinsic dielectric properties of solid-state materials as they absorb ammonia gas, and is effective as ammonia is a highly polar molecule with an electric dipole moment of 1.47 D. 44 The absorption of ammonia within the solid matrix causes a change in the polarisation (quantified by e 1 ) of the sample, which can be seen as a shift in the resonant frequency (Df 0 ) of the microwave cavity resonator (MCR) from when it is empty ( f 0 ), with chemisorbed (strongly bound to metal centres) ammonia causing the greatest perturbation in polarisation. 9 Changes in dielectric loss (quantified by e 2 ), are most closely associated with physisorbed (weakly bound to surfaces) ammonia 9 and cause a change in the resonant bandwidth of the system (Df B ).…”

Monitoring NH3 storage and conversion in Cu-ZSM-5 and Cu-SAPO-34 catalysts for NH3-SCR by simultaneous impedance and DRIFT spectroscopy

“…Therefore, it is of great academic and technological interest to gain in depth information on the proton transport properties of these Cu-promoted zeolites, as well as on the impact of proton transport in the direct electrical determination of stored NH3 and in the electrical in operando monitoring of NH3-SCR catalysis [28,29].…”