Zeolite H-ZSM-5: A Microporous Proton Conductor for the in situ Monitoring of DeNO x -SCR
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Zeolite H-ZSM-5: A Microporous Proton Conductor for the in situ Monitoring of DeNOx-SCR Thomas Simons1 and Ulrich Simon1 1 Institut für Anorganische Chemie and JARA – Future Information Technology, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany ABSTRACT Impedance spectroscopy was applied on zeolite H-ZSM 5 as a proton conducting NH3 sensor material and DeNOx-SCR (selective catalytic reduction of NOx with NH3) catalysts at the same time under SCR conditions. We show that in situ monitoring of the NH3 conversion with NOx becomes feasible when the zeolite is loaded with NH3 first and NOx is applied afterwards to the gas phase. Temperature dependent measurements allow discriminating NH3 desorption from catalytic conversion and time dependent measurement give first hints on the thermal activation of the conversion. INTRODUCTION Zeolites are crystalline, microporous alumosilicates composed of [TO4] tetrahedra (T = Si, Al). H-form zeolites are proton conductors due to the mobility of protons which compensate the negative charge of the [AlO4] tetrahedra by forming bridging Si-OH-Al groups, so-called Brønstedt sites. By means of impedance measurements [1,2,3] and quantum chemical calculations [4,5] of zeolite H-ZSM-5 we showed that protons can move through the zeolite framework by thermally activated hopping. The probability of proton hopping depends on the spatial distance between an occupied and the spatially nearest unoccupied lattice-site, and by this on the SiO2/Al2O3 ratio in the zeolite lattice. In N2 as well as in O2 containing atmospheres the presence of solvent molecules, like H2O or NH3 in concentrations of 100 ppmv and above, leads to an increase of the proton mobility up to a temperature of 420 °C [3]. While the proton transport in the solvent free zeolite can be described by means of classical hopping models, a Grotthus-like and a vehicle-like transport was indentified in the presence of NH3, depending on the NH3 concentration and temperature range, respectively. Hence, H-form zeolites can be applied as NH3-sensor materials [6]. At the same time H-form zeolites serve as catalysts for DeNOx-SCR (selective catalytic reduction of NOx with NH3), which is of great relevance in exhaust gas after treatment of lean burning diesel engines as well as in NOx emitting technical plants. For the conversion of NOx stoichiometric amounts of NH3 need to be dosed into the exhaust gas stream. Today technically established DeNOx-SCR-systems apply e.g. zeolite ZSM-5 as a catalyst and an aqueous solution of urea, from which NH3 is released via hydrolysis (AdBlue® technology [7]). Hence, precise measurements of the NH3 and NOx concentration in the exhaust gas prior and after the SCR catalyst are needed for the control of the reducing agent dosage in order to ensure quantitative conversion as well as to avoid NH3 slip [8]. In view of these technical requirements it is conceivable to use the SCR catalyst itself as the sensing element in order to follow the catalytic conversion in situ. A first step in this direction was
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