First-Principles Microkinetic Model for Low-Temperature NH₃-Assisted Selective Catalytic Reduction of NO over Cu-CHA

High Resolution Image Download MS PowerPoint Slide A first-principles microkinetic model is developed to investigate the low-temperature ammonia-assisted selective catalytic reduction (NH 3 -SCR) of NO over Cu-chabazite (Cu-CHA). The reaction proceeds over NH 3 -solvated Cu sites by the formation of H 2 NNO and HONO, which decompose to N 2 and H 2 O over Brønsted acid sites. Nonselective N 2 O formation is considered by H 2 NNO decomposition over the Cu sites. The adsorption of NH 3 at oxidized Cu sites is found to inhibit the reaction at low temperatures by hindering NO adsorption. For the reactions, we find positive reaction orders with respect to NO and O 2, whereas the reaction order with respect to NH 3 is negative. The reaction orders and the obtained apparent activation energy are in good agreement with experimental data. A degree of rate control analysis shows that NH 3 -SCR over a pair of Cu(NH 3 ) 2 + is mainly controlled by NO adsorption below 200 °C, whereas the formation of HONO and H 2 NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH 3 -SCR rationalizes previous phenomenological models and links the kinetic behavior with materials properties, which results in unprecedented insights into the function of Cu-CHA catalysts for NH 3 -SCR.