Urea Hydrolysis for the Selective Catalytic Reduction of NO_x: Comparison of Liquid‐ and Gas Phase Decomposition

Abstract

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.

Investigations on the Liquid‐Phase Decomposition of AdBlue Urea for the Selective Catalytic Reduction Process

“…Also, TiO 2 and ZrO 2 , which were shown to be active in the decomposition of urea in the gas phase [21], did not show remarkable catalytic activity in the liquid-phase decomposition. This observation goes along with findings of Peitz et al [21,22] who could find only very low catalytic effects for these materials in liquid phase. The reason for the high activity of the metal oxides being dissolved in the reaction mixture might be explained by the hydrophilicity of urea.…”

“…In the experiments with all the other compounds, the ammonia concentration was not remarkably increased after 30 min at 165°C. Also, TiO 2 and ZrO 2 , which were shown to be active in the decomposition of urea in the gas phase [21], did not show remarkable catalytic activity in the liquid-phase decomposition. This observation goes along with findings of Peitz et al [21,22] who could find only very low catalytic effects for these materials in liquid phase.…”

“…Since by-product formation during urea hydrolysis is strongly promoted by local lack of water but, as can be derived from Eq. (1), can as well be counteracted by excess of water to an even larger extent [21,22], the studies on decomposition of urea were carried out in liquid phase using elevated pressure.…”

“…Gangadharan et al [24] did further studies on the decomposition of urea in the presence of Al 2 O 3 in a batch reactor for application in fly ash removal. Peitz et al [21,22] compared the decomposition of AdBlue in the gas phase with the decomposition in the liquid phase under elevated pressure in the presence of TiO 2 , ZrO 2 , and Al 2 O 3 . Besides the influence of different inorganic materials on the decomposition efficiency of AdBlue urea, in the present study, also the influence of acidic and alkaline conditions was studied.…”

Potential Technical Approaches for Improving Low‐Temperature NOx Conversion of Exhaust Aftertreatment Systems

“…By catalyzing the hydrolysis of isocyanic acid, TiO 2 hinders the formation of byproducts [76]. Kröcher and Bernhard investigated the catalytic activity of different materials in decomposing urea [72,77] (see Fig. 6).…”