Mechanism of Fast SCR

To reduce the emissions during cold start of the engine and extended operation at low loads in vehicles, it has been proposed to use the so-called fast SCR reaction in which NH3 reacts with NO and NO2 according to 

This reaction has been known since the 1980s to be faster by one order of magnitude than the standard SCR (Equation 13.1) at low temperatures [64]. 

13 Catalytic RemamI of NO, Under Lean Conditions from Stationary and Mobile Sources 

Indeed, it is possible to install on vehicles a preoxidizing catalyst upstream ofthe SCR catalyst that converts part ofthe NO to NO2 the DOC also oxidizes CO and UHCs to CO2. 

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

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The mechanism of fast SCR over a zeolite-based catalyst has also been addressed by Sachtler and co-workers using an IR technique [69, 70]. They concluded that nitrogen is produced through fast decomposition of ammonium nitrite (the hydrated form of nitrosamide), vhich is formed from equimolar NO/NO2 feeds via N2O3 and its reaction vith water and ammonia ... 

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