Substoichiometric formation, ammonia

The rate of the DeNO, reaction is first order in respect to NO concentration and essentially independent of NH3 concentration when ammonia is in excess. However, in SCR industrial applications a substoichiometric feed ratio (a = NH3/NO < 1) is employed in order to minimize the slip of unconverted ammonia and the formation of ammonium sulfates. A kinetic dependence on ammonia is apparent when NH3 becomes the limiting reactant. Several authors have proposed kinetic expressions for the SCR reaction that account for the observed dependences [31-37]. In particular, the simplest expressions are based on Eley-Rideal kinetics in line with the mechanistic studies, they assume that the reaction occurs between strongly adsorbed ammonia and gas-phase NO. Beckman and Hegedus [36] have proposed and fitted to experimental data obtained over commercial SCR catalysts the following kinetic expression ... 

Orsenigo et al. [47] have proposed an alternative reactor design suitable in principle to exploit NH3 inhibition for minimizing SO3 formation in the SCR process. This is based on the idea of splitting the NOx-containing feed stream in substreams fed separately to the SCR reactor in this way, a portion of the catalyst volume can operate with an excess of ammonia, while the overall NH3/NO feed ratio is still substoichiometric. 

Ammonia and numerous primary and secondary amines have been subjected to HAM. Since amines display different basicities depending on the number and nature of the AT-substituents, the transformation of the starting amine to the product amine alters the pH value during the reaction [68]. Especially in aqueous solutions this feature may become important, and control of the pH is necessary [68]. The addition of substoichiometric or stoichiometric amounts of acids or the submission of corresponding ammonium salts (e.g., sulfates) can enhance the formation of the product amines as well as the desired regioselectivity [29, 69, 70]. Thus, the addition of a substoichiometric amount of HBF increased significantly the yield of the amine [48]. At high catalyst concentrations, also the addition of acetic acid could be of value, but this additive was less efficient than HBF [51]. Formic acid or sulfuric acid decelerated the transformation. 

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