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Test bench measurements were performed demonstrating low temperature non-thermal plasma enhanced selective catalytic reduction of NOx in the exhaust of a 1.9 liter direct injection Diesel engine. The plasma-catalytic hybrid system consisted of an oxidation catalyst, a modular dielectric barrier discharge reactor for plasma treatment, a semiconductor switched pulse voltage source for electrical excitation, and an urea SCR system based on a monolithic V2O5-WO3/TiO2 catalyst. In a test cycle simulating urban driving conditions the average NOx-mass flow was reduced from 27 g/h down to 11.5 g/h. Maximum NOx-reduction rates of 28 g/h were obtained at an energy cost of 10 Wh/g NOx. The ammonia storage capability of the catalyst was utilized for reduction at temperatures below 140 °C.

The influence of adsorption and desorption processes on the non-thermal plasma enhanced catalytic reduction of NOx on NaZSM5- and Al2O3-based lean-NOx catalysts (Pt-NH4ZSM5, Cu-NaZSM5, Fe-NaZSM5, Pt-Al2O3, Pd-Al2O3, CuO-Al2O3, Ag-Al2O3) was investigated by temperature programmed reaction experiments in the temperature range from 100 °C to 600 °C. Dodecane was used as a reducing agent. Strong HC adsorption- and desorption effects were observed on the zeolite catalysts, which were not influenced by plasma-pretreatment. Adsorption of NO2 and desorption of NO occurred on Al2O3-based catalysts. By plasma-pretreatment adsorption of NO2 was induced at low temperatures. NOx-reduction rates of the catalysts Cu-NaZSM5, Fe-NaZSM5, and the Ag-Al2O3 were increased substantially by plasma-pretreatment. Both plasma-induced and catalytic oxidation of HCs were limiting factors of the NOx-reduction obtained on these catalysts.

For practical application of non-thermal plasma enhanced selective catalytic reduction of NOx for Diesel cars compact low cost power supplies are necessary, which enable efficient plasma generation. The influence of pulsed electrical excitation of dielectric barrier discharges (DBD) on the conversion of NO to NO2 was investigated for various discharge gaps and exhaust gas temperatures: Using a thyratron switched HV power supply with a voltage pulse duration of about 100 ns, DBDs with discharge gaps up to 8 mm could be excited. Plasma energies of up to 50 mJ per pulse were obtained. At 220 °C the energy requirements for an NO-conversion of 50 % were determined to be 9.2 Wh/g NO. Up to 80 % plasma enhanced selective catalytic reduction of NO were obtained at an energy requirement of less than 6 Wh/g NOx. Further experimental work was done using a compact semiconductor switched power supply with a voltage pulse duration of about 500 ns.

The potential of plasma enhanced selective catalytic reduction (PE-SCR) for Diesel-exhaust treatment at temperatures between 60 °C and 180 °C has been investigated in test bench measurements with a 1.9 liter 66 kW VW Passat TDI engine. Non-thermal plasmas were generated by pulsed electrical excitation of dielectric barrier discharge (DBD) modules each having a flow cross section of 9.5 cm2 and an electrode length of 26 cm. Monolithic V2O5-WO3/TiO2-catalysts with cell densities of 150 cpsi and 200 cpsi were used for selective catalytic reduction. First experiments were performed with a single DBD module and a catalyst volume of 3.5 liters. For temperatures between 100 °C and 160 °C and exhaust gas flow rates below 1200 liters (STP)/min NOx-reduction rates up to 14 g/h were obtained with an energy cost of about 20 Wh/g NOx. At larger gas flow rates NOx-reduction rates decreased even at higher temperatures.

The mechanisms of plasma enhanced selective catalytic reduction of NOx on a V2O5-WO3/TiO2-catalyst were investigated for temperatures between 100 °C and 200 °C by applying various analytical methods. In experiments with synthetic gas mixtures containing Ar instead of N2 as a carrier gas the formation of N2 as a main product and thus catalytic reduction of NOx in reactions with NH3 has been proven using mass spectroscopy. More detailed information on the reaction kinetics of NO-formation and removal induced by non-thermal plasmas has been obtained from experiments in gas mixtures containing isotopically marked 15NO. FTIR-absorption spectroscopy was applied to measure the concentrations of 14N- and 15N-containing molecules. Particles formed by the combined plasma- and catalytic treatment of Diesel exhaust with NH3 as reducing agent were analyzed by energy dispersive X-ray scattering. The spectra indicate, that mainly sulfur containing particles were formed.

The potential of dielectric barrier discharges for the reduction of NO emitted from Diesel cars has been investigated. Without additional measures the non-thermal plasma induced oxidation of NO to NO2 is favored over reduction to N2 and O2. Therefore a combination of plasma and a catalyst for the selective catalytic reduction of NO with ammonia as reducing agent has been tested: An NO conversion of about 70 % was achieved at a temperature as low as 100 °C, which cannot be explained by simply adding the reduction rates obtained by plasma and by selective catalytic reduction.