Selective catalytic reduction of NO over commercial DeNO x-catalysts: experimental determination of kinetic and thermodynamic parameters

Abstract

The kinetic and thermodynamic parameters have been measured for three commercial catalysts based on TiO 2-WO 3-V 2O 5 at temperatures from 200 to 450°C in the presence of water (5%) and oxygen (10%). At low temperatures, the first-order rate constants strongly depend on NO concentration; this effect decreases with increasing temperature. Therefore, a first-order rate law with respect to NO can only approximately describe the kinetic behaviour of SCR catalysts. A much better description is possible by taking into consideration the adsorption of both NH 3 and NO. However, the relevant parameters are experimentally not accessible with a satisfactory degree of accuracy. At temperatures above 350°C the thermodynamics of adsorption of SCR-active NH 3 could be studied under realistic SCR-conditions. In a similar way, detailed measurements below 300°C allowed determination of the thermodynamics of NO adsorption. NH 3, reactive in SCR, is strongly adsorbed ( H ad, NH 3 ≈ −100 to −130 kJ/mol) and NO is weakly adsorbed ( H ad, NO ≈ −20 kJ/mol). Due to this, the temperature dependence of the respective adsorption constant is strong for NH 3 but weak for NO. The observed adsorption of NO calls either for a reaction mechanism of the Langmuir-Hinshelwood or of the nitrosamidic intermediate type.