Reaction kinetics of NH3-SCR over Cu-CHA from first principles

Licentiate thesis, 2021

Ammonia assisted selective catalytic reduction (NH3-SCR) is a leading technology that is used for NOx reduction to N2 and H2O in oxygen excess. Thanks to its high activity, high selectivity, and durability, Cu-CHA is commercialized as an NH3-SCR catalyst. Despite the superior catalytic performance, small amounts of nitrous oxide (N2O) are formed during the NH3-SCR as an unwanted by-product. N2O has a strong greenhouse potential and should be avoided. To further enhance the performance of NH3-SCR catalysts to handle the increasingly stringent emission standards, understanding the mechanism for NH3-SCR and, in particular, N2O formation over Cu-CHA is essential.

In this thesis, density functional theory (DFT) calculations and first principles microkinetic simulations are used to investigate the reaction path and the reaction kinetics for low temperature-NH3-SCR. Based on a previously proposed catalytic cycle for NH3-SCR over Cu-CHA, an N2O formation path is put forward. It is proposed that N2O can form over linear [Cu(NH3)2]+ complexes, which are present during low temperature operation. N2O is formed from H2NNO, which is generated via NH2-NO coupling over a Cu-OOH-Cu site. The reaction proceeds with a low barrier and rationalizes the low-temperature
N2O emission peak observed experimentally at high Cu-loadings. N2O formation at high temperatures is instead proposed to occur through the decomposition of NH4NO3.

With a catalytic cycle including N2O formation, a first principles microkinetic model is developed to investigate the reaction kinetic of NH3-SCR over Cu-CHA. When developing the model, special attention is paid assessing the change in entropy for each reaction step. The results from the kinetic model show good agreement with the experimental data of apparent activation energies, reaction orders and N2O selectivity. The model links the catalytic performance with structure and forms the basis for further developments of the NH3-SCR technology.