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NH3 Selective Catalytic Reduction of NOx

It is widely recognized that the Diesel engine vehicles are fated to significantly increase their worldwide penetration. This is mainly due to the fact that Diesel engines are inherently more thermodynamically efficient than gasoline engines, thus offering the perspective of reducing fuel consumptions and carbon dioxide emission. However, Diesel produce high emissions of nitrogen oxides and particulates. A widely accepted DeNOx technology for heavy-duty applications, already introduced in Europe with the EURO IV legislation, is the Selective Catalytic Reduction (SCR) of NOx with NH3/urea.
The design and application of SCR systems is a complex process involving the optimization of different parameters like urea dosing strategy, SCR catalyst position, size, cell density of the monolith substrate and last but not least the optimization of the catalytic material itself. In view of shortening development cycles and reducing development costs, numerical simulation is a key factor. The development of simulation models requires however a dedicated study of the NH3-NO/NO2 SCR reactivity in order to obtain a deep knowledge of the reaction network and of how the reaction takes place on the catalyst. Only a clear comprehension of the reaction network and of the related reaction mechanisms permits to develop a kinetic model able to predict the catalyst performances in the large range of conditions, which characterize the automotive applications of SCR. In front of these requests, the work carried out at LCCP addresses the experimental and modeling study of new commercial vanadium-based and metal-exchanged zeolite catalysts for SCR mobile applications.

Even though nowadays the SCR reaction represents a well established technology, the tightening of the emission limits and the need of more efficient Diesel engines, responsible for a lower exhaust temperature, demand improved DeNOx performances in the low temperature range. Additionally, due to the limited volume available on board and to the fuel consumption constraints, minimal volume and weight are as important as the performance when designing an aftertreatment system for vehicle applications. In this context, two new DeNOx technologies are under investigation at LCCP, namely the SDPF concept and the Enhanced-SCR reaction.
The SDPF is a highly porous ceramic Diesel Particulate Filter washcoated with a SCR active phase. This set up enables to place the SCR catalyst closer to the engine in a way to improve the catalyst temperature behavior. Moreover, coupling two unit operations into a single compact device enables to satisfy the need of reduced volume. The main drawback of this concept, however, is that its performance strongly depends on the nature of the feed NOx: in particular, NO2 represents a key reagent for both the SCR and the soot combustion reactions involved in the filter regeneration. Current research activities at LCCP are focused on the fundamental investigation of the complex interactions between the two chemistries.
The Enhanced-SCR is a new technology consisting in the injection of an aqueous solution of ammonium nitrate together with NH3/urea upstream of the SCR converter. The reaction between ammonium nitrate, NO and NH3 is associated with superior NO removal efficiencies in the 200 - 350°C range, similar to those observed in the well known highly active Fast SCR reaction, but does not require partial oxidation of NO to NO2 upstream of the SCR unit. LCCP is investigating a number of fundamental aspects of the Enhanced SCR reaction (the reaction pathway, the relation with the Fast SCR reaction, the selectivity to N2O, the role of the catalyst active element, ...): elucidation of such features has a number of practical implications in view of the scale-up to the industrial level.

The research activity on mobile SCR at LCCP is performed in cooperation with Daimler, Germany (since 2001), MTU, Germany (since 2011), Johnson Matthey, UK (since 2013), and within the EU H2020 project "Heavy Duty Gas Engines integrated into Vehicles - HDGAS" (since 2015).

LCCP faculties involved in the project are: Isabella Nova, Enrico Tronconi