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

Doktorsavhandling, 2024

Ammonia-assisted selective catalytic reduction (NH3-SCR) is an advanced technology to reduce nitrogen oxide (NOx) emissions from lean-burn engines. NH3 is added to the exhaust gas and reacts selectively with NOx, forming N2 and H2O. Small-pore Cu-exchanged chabazite (Cu-CHA) is a widely used zeolite-based catalyst for NH3-SCR thanks to its high activity, high selectivity, and durability. However, one issue is the formation of small amounts of nitrous oxide (N2O), which is a strong greenhouse gas. Atomic level understanding is valuable to improve the performance of NH3-SCR catalyst and meet the increasingly stringent emission standards.

The state of Cu-CHA during NH3-SCR reaction depends sensitively on the reaction temperature. At low temperatures, Cu-ions are solvated by NH3, forming Cu[(NH3)2]+ complexes, while framework-bound Cu dominates at high temperatures. Experimentally, a non-monotonic behavior in NO conversion is observed with increasing temperature, which reflects a change in the active site.

In this thesis, NH3-SCR over Cu-CHA is investigated using density functional theory (DFT) calculations and kinetic simulations. In the low-temperature range (<250℃), a pair of Cu[(NH3)2]+ is required for O2 adsorption to form a Cu-peroxo complex, which is a key Cu-intermediate. The reaction intermediates HONO and H2NNO can be converted to N2 and H2O over Brønsted acid sites. H2NNO is found to be the origin of N2O formation when decomposing over Cu-peroxo species. At high-temperature (>350℃), the proposed reaction mechanism suggests that the reaction proceeds over single framework Cu sites and starts from the co-adsorption of O2 and NO. The main source of N2O formation at high temperatures is suggested to be ammonium nitrate decomposition. By combining the reaction mechanisms for high and low temperatures, the non-monotonic temperature-dependent reaction activity is successfully reproduced.

The present work deepens the understanding of the reaction mechanism of NH3-SCR on Cu-CHA in a wide temperature range and provides theoretical support for further improvement of the catalyst performance.