The reactivity in the selective catalytic reduction of the individual and binary components of NbO x −MnO x −CeO2 catalysts has been studied with coated cordierite monoliths in the temperature range of 150−450 °C. FTIRS, DRIFTS, TA, XRD, BET, and XAS have been used to elucidate the structural and catalytic properties. The results confirmed the contribution of the manganese oxides, particularly to the low-temperature NO-to-NO2 oxidation reaction. The significant increase of the surface acidity as a result of niobium oxide addition has been established. The data obtained revealed also the strong interaction between the manganese and niobium catalytic active sites. This phenomenon leads to a very good distribution of the oxidizing and acidic sites in the catalyst structure and also diminishes the unselective NH3 oxidation at higher temperatures. However, in order to keep the low-temperature catalytic activity, an excess of manganese relative to the niobium content is needed.

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The selective catalytic reduction (SCR) of nitrogen oxides (NO x ) in the presence of NH 3 was investigated on Printex U and three diesel soot samples in the temperature range between 200 and 350°C. In the presence of NO + NO 2 , a stoichiometry similar to that of the fast SCR reaction was observed, whereas in the absence of NO, the stoichiometry shifted to values typical of the NO 2 SCR reaction. In the presence of only NO, no NO x reduction was observed. Carbon oxidation and SCR reaction minimally influenced each other. On the basis of these experimental results, a mechanism for NO x reduction over diesel soot was developed. The first step in the reaction is the disproportionation of NO 2 , which is followed by the formation of ammonium nitrates and nitrites. The nitrites decompose directly into N 2 and H 2 O, whereas the nitrates have to be reduced to nitrites either by NO or in its absence by NH 3 , which also leads to the formation of N 2 and H 2 O. The observed byproducts indicated that the acidity and the amount of surface functional groups for NH 3 are important preconditions for the occurrence of the SCR reaction over soot.

The catalytic activity of soot samples for the selective catalytic reduction (SCR) of NO x with NH 3 was investigated in dependence of the NO 2 , NO and NH 3 concentration in the temperature range between 200 and 350°C. The highest NO x reduction of up to 25 % was measured in the presence of both NO 2 and NO at a GHSV of 35,000 h-1. Decreasing space velocities resulted in an increase of the SCR activity. In the absence of NO 2 , NO x reduction was not observed. Carbon oxidation and SCR reaction occurred in parallel due to the presence of NO 2 and O 2 , but hardly influenced each other, which suggested that in the NO x reduction on soot most probably physisorbed species were involved. The observed stoichiometries indicated the action of the fast SCR reaction in the presence of NO and the NO 2 SCR reaction in the absence of NO, while the observed gas phase and surface species pointed at reaction steps similar to those on classical SCR catalysts.

Zur Reduzierung der Stickoxid-Konzentration in Autoabgasen wird meist die selektive katalytische Reduktion mit Ammoniak verwendet. Um jedoch das Mitführen von Ammoniak in Kraftfahrzeugen zu vermeiden, wird auf Harnstoff als NH 3 -Vorläufer zurückgegriffen. Zur kontrollierten Harnstoffzersetzung bereits vor dem Eindüsen in den Abgasstrang bietet sich der Einsatz eines separaten Reaktors an. Die Harnstoffzersetzung zu Ammoniak in der Flüssigphase unter Druck in einem beheizten Reaktor wurde mit der Zersetzung in der Gasphase verglichen. Hierbei wurden in der Flüssigphase weit höhere Umsatzraten pro Reaktorvolumen erreicht. Katalysatoren, die in der Gasphase eine hohe Aktivität zeigten, hatten nur einen kleinen Einfluss auf die Flüssigphasenzersetzung.In order to reduce the NO x concentration in car exhausts usually the selective catalytic reduction with ammonia is used.However, to avoid the transport of ammonia in vehicles urea is applied as NH 3 precursor. Controlled urea decomposition before the injection into the exhaust gas system itself may be accomplished by the use of a separate reactor. Urea decomposition to ammonia in the liquid phase under pressure in a heated reactor was compared to its decomposition in the gas phase. In the liquid phase, higher conversion rates relative to the reactor volume were realized than in the gas phase. Catalysts which showed high activity in the gas phase influenced the decomposition in the liquid phase only slightly.