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

Abstract

A first-principles microkinetic model is developed to investigate the low-temperature ammonia-assisted selective catalytic reduction (NH3-SCR) of NO over Cu-chabazite (Cu-CHA). The reaction proceeds over NH3-solvated Cu sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Brønsted acid sites. Nonselective N2O formation is considered by H2NNO decomposition over the Cu sites. The adsorption of NH3 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 O2, whereas the reaction order with respect to NH3 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 NH3-SCR over a pair of Cu(NH3)2+ is mainly controlled by NO adsorption below 200 °C, whereas the formation of HONO and H2NNO becomes controlling at higher temperatures. The successful formulation of a first-principles microkinetic model for NH3-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 NH3-SCR.