Effects of Extra-Framework Metal Centers on Zeolite-Based NH3-Sensors for Exhaust Gas Abatement
Authors: Rizzotto Valentina , Simon Ulrich 
Published: May 2020

Introduction

The abatement of nitrogen oxides (NOx) from lean-burn engine exhausts is nowadays mainly achieved by selective catalytic reduction using NH3 as reducing agent (NH3-SCR). In these systems, a solution of urea (AdBlue®) is injected in the exhaust, upstream the SCR unit, and converted to NH3 under operative conditions. Precisely dosing the NH3 inlet is necessary in order to achieve the highest NOx conversion and, simultaneously, to avoid NH3 slip. Since the currently employed NOx sensors are cross sensitive to NH3, precise NH3 sensors are also necessary for a correct determination of the outlet NOx concentration [1]. In this context, the emerging concept of utilizing “catalysts as sensors” can play an important role for the on-board diagnosis, specially by application of zeolite materials [2].
Zeolites are proton conductors and their properties as sensing materials for NH3 detection are well documented: in situ impedance spectroscopy (IS) has proved that the adsorption of NH3 on zeolites increases the ion conductivity by forming highly mobile NH4
+ species [3]. Moreover, their Cu-exchanged derivatives (e.g. Cu-SSZ-13, Cu-ZSM-5) are widely applied as NH3-SCR catalysts, due their high and stable activity in NOx conversion [1]. Nevertheless, after NH3-solvation, Cu2+ and Cu+ ions are mobile under SCR condition, potentially affecting the sensing response to NH3, as well. Our research group has recently investigated this phenomenon selectively in small-pore zeolites (i.e. Cu-SSZ-13 and Cu-SAPO-34) [5]. Here we extend our study to zeolite materials with different crystal structure and Cu-loading in order to gain deeper understanding of structural and compositional effects.

Materials and Methods

We synthesized a series of zeolite catalysts with different framework types, namely H-SSZ-13 (CHA), H-ZSM-5 (MFI), H-beta (BEA). On each proton-form zeolite, wet Cu ion-exchange was performed, to obtain the Cu-exchanged counterparts (i.e. Cu-SSZ-13, Cu-ZSM-5, Cu-beta) with selected levels of Cu-loading, analytically controlled via ion-coupled plasma optic emission spectroscopy (ICP-OES) in terms of Cu/Al ratio. The samples have been further characterized by powder X-ray diffraction, diffuse reflection UV/Vis spectroscopy and scanning electron microscopy. Temperature-programmed desorption using NH3 as the probe molecule (NH3-TPD) was performed over the synthesized Cu zeolites and the corresponding proton-form zeolites in order to specify the respective NH3-adsorption sites.
For IS measurements, thick films of zeolite materials were deposited on gold inter-digital electrodes chips (IDE) with alumina substrate. The IDE chips are also equipped with a heating circuit on the back side to adjust the temperature. In the measuring chamber, the gas dosing was operated by means of mass flow controllers. An alternating voltage of 0.1 V was applied to the zeolite sensor in at a measuring frequency of 10 kHz. The here-presented NH3-sensing experiments were performed alternating N2 and NH3 (100 ppm in N2) flow at typical SCR operative temperature (200, 350 and 400 °C), with a total flow of 100 sccm. Prior to each sensing measurement, the investigated sample was pretreated in O2 at 450 °C for 1 h.

Results and Conclusions

The material characterization, before and after deposition on the IDE chips, shows that the composition and microstructure of the samples was preserved. The Cu-exchanged samples have been named after their Cu/Al ratio [e.g. Cu(x)-SSZ-13, where x = Cu/Al].
Based on NH3-supported proton transport, in situ IS measurements were performed to study the NH3-zeolite interactions. The absolute impedance |Z| was employed to evaluate the sensing performances of the zeolite materials after normalization, expressing the sensing signal as IIS
signal [IIS
= (|Z0|-|Z|)/|Z0|, where Z0
is the impedance measured in pure N2).
The results obtained at 200 °C over Cu-SSZ-13 zeolites are shown in Figure 1. As can be seen that the IIS
signal of Cu-exchange SSZ-13 zeolites became significantly higher than H-SSZ-13 after exposure in NH3. Moreover, the conductivity appears to increase with increasing Cu-loading. This trend was also observed for ZSM-5 and beta samples, even though not as pronounced. This effect may be surprising, if NH4
+ and H+ were considered the only mobile cationic species, as usual for proton-transport-based sensors: indeed, the IIS
would be expected to decrease, since non-mobile Cu2+ substitutes H+ in the zeolite framework. Nevertheless, at the measuring frequency, short-range motion of NH3-solvated Cu complexes [i.e. Cu2+(NH3)n] are activated [4]. The contribution of these mobile cations lead to the increase of the overall ion conductivity, even compensating the loss of conventional charge carriers.
These in situ IS results give important information for the implementation of Cu-zeolite as NH3-sensors in SCR systems, following the concept of “catalysts as sensors”. Further comparative measurements over BEA and MFI zeolites are ongoing, aiming at a comprehensive understanding of the interaction between NH3 and Cu-zeolite with different framework types.

References

[1] M.F. Hsieh, et al., (eds.), Urea-SCR Technology for DeNOx After Treatment of Diesel Exhausts, Springer, New York, NY (2014); doi:10.1007/978 1 4899 8071-7_14
[2] R. Moos, Catalysts as Sensors—A Promising Novel Approach in Automotive Exhaust Gas Aftertreatment, Sensors 10, 6773–6787 (2010); doi:10.3390/s100706773
[3] P. Chen, et al., Correlating the Integral Sensing Properties of Zeolites with Molecular Processes by Combining Broadband Impedance and DRIFT Spectroscopy—A New Approach for Bridging the Scales, Sensors 15, 28915–28941 (2015); doi:10.3390/s151128915
[4] C. Paolucci, et al., Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites, J. Am. Chem. Soc. 138, 6028−6048 (2016); doi: 10.1021/jacs.6b02651
[5] P. Chen, et al., Mechanistic Understanding of Cu-CHA Catalyst as Sensor for Direct NH3-SCR Monitoring: The Role of Cu Mobility , ACS Appl. Mater. Interfaces 11, 8097–8105 (2019); doi:0.1021/acsami.8b22104

Figure 1