Urea and NH3 Selective Catalytic Reduction

While the first reaction is easily obtained from a thermal point of view the formation of the second NH3 molecule through isocyanic add hydrolysis represents a key step in the process [45]. The latter can be accelerated by using a catalyst, with the added advantage of avoiding secondary reactions (see below) that lead to the formation of larger molecules and possibly to the deactivation of the system  

Three types of commercial catalysts have been developed for SCR-NH3 noble metal, metal oxides, and zeolites. At low temperature (398 K), platinum catalyst can reduce NO with a very good selectivity in N2. However, noble metals are no longer used, since they oxidize completely NH3 at 473 K. 

The catalysts that are typically used for reducing the NO produced by stationary sources and heavy goods vehicles are currently of the V20s-W03/Ti02 type [49-52]. These catalysts present high efficiencies for NO reduction by NHs/urea [50,52]. Their development nevertheless requires an optimum vanadium composition, since inordinately high levels lead to a loss of selectivity with increased formation of N2O. Also, these systems suffer from low activity and 

however, display a beginning of deactivation for hydrothermal treatments carried out at 773 K [63]. For processing temperatures above 973 K, more than 90% of the acidity is destroyed due to the dealumination as well as the collapse of the structure [64]. One mean of improving the stability and efficiency of these zeolitic systems is to change the zeolite structure, and research efforts have therefore focused on chabazite structure [65-67]. Recently, some papers relate the use of perovskite catalyst for SCR-NH3 due to their remarkable hydro-thermal stability [68-72]. 

2 Single Brick Solution for Lean-Burn DeNO and Soot Abatement 

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