A Kinetic Model for the Selective Catalytic Reduction of NO_<i>x</i> with NH₃ over an Fe−zeolite Catalyst

Abstract

The selective catalytic reduction of NO x with ammonia over an Fe−zeolite catalyst was investigated experimentally and a transient kinetic model was developed. The model includes reactions that describe ammonia storage and oxidation, NO oxidation, selective catalytic reduction (SCR) of NO and NO 2, formation of N 2 O, ammonia inhibition and ammonium nitrate formation. The model can account for a broad range of experimental conditions in the presence of H 2 O, CO 2, and O 2 at temperatures from 150 to 650 °C. The catalyst stores ammonia at temperatures up to 400 °C and shows ammonia oxidation activity from 350 °C. The catalyst is also active for the oxidation of NO to NO 2 and the oxidation reaches equilibrium at 500 °C. The SCR of NO is already active at 150 °C and the introduction of equal amounts of NO and NO 2 greatly enhances the conversion of NO x at temperatures up to 300 °C. The formation of N 2 O is negligible if small fractions of NO 2 are fed to the reactor, but a significant amount of N 2 O is formed at high NO 2 to NO ratios. An ammonia inhibition on the SCR of NO is observed at 200 °C. This kinetic model contains 12 reactions and is able to describe the experimental results well. The model was validated using short transient experiments and experimental conditions not used in the parameter estimation and predicted these new conditions adequately.