Microkinetic modeling of the fast selective catalytic reduction of nitrogen oxide with ammonia on H-ZSM5 based on first principles

The reaction mechanism of the fast selective catalytic reduction (SCR) of NOx has been investigated by means of a microkinetic model based on DFT calculations. First, potential additional reaction mechanisms are presented that include the intermediate formation of Z−[NOx]+ (x = 1, 2) from the decomposition of N2Oy (y = 3, 4) on a Brønsted acid. The formed NOx+ reacts with ammonia to nirosamine or nitramide. The DFT results are applied in a microkinetic model together with prior studies of Brüggemann et al. The simulated conversion of NOx is in agreement with experimental data over a wide range of temperature. The activity of the H-ZSM5 for the fast SCR is based on the reaction sequence via Z−[NOx]+, the decomposition of nitrosamine and nitramide, and the reaction of nitrous and nitric acid with adsorbed ammonia at low temperatures. Ammonia blocks the active sites at low temperatures while thermodynamic limitations of the surface species N2Oy restrict the conversion at high temperatures. Heat of formations and reaction rate constants were adjusted within the accuracy of the applied method for important elementary steps to cope with experimental data.

The fast selective catalytic reduction of NOx with ammonia on H-ZSM5 was investigated using microkinetic modeling based on DFT results. The NOx conversion proceeds via the formation of nitrosyl on the zeolite framework.Figure optionsDownload high-quality image (82 K)Download as PowerPoint slideHighlights
► DFT calculations and microkinetic modeling of fast SCR reveal pathway on H-ZSM5 via nitrosyl.
► Adsorption of ammonia with up to four molecules per active site.
► NOx conversion blocked by adsorbed ammonia at low temperatures.
► NOx conversion limited by thermodynamic equilibrium conditions of N2Oy (y = 3, 4) at high temperatures.
► Minor influence of ammonia oxidation on fast SCR.