Selective catalytic reduction with ammonia as the reducing agent (NH3-SCR) is an efficient technology for NOx reduction in oxygen excess. Zeolite catalysts with Cu-functionalized chabazite (Cu-CHA) have emerged as preferred catalysts for NH3-SCR thanks to low-temperature activity and good hydrothermal stability. Significant advances have been made in recent years concerning the atomistic understanding of the active sites and the reaction mechanism of low-temperature NH3-SCR reaction over Cu-CHA. The advances are based on in situ X-ray absorption spectroscopy experiments in combination with density functional theory calculations. In particular, it is established that the ion-exchanged Cu cations do not occupy fixed positions in the zeolite during low-temperature operation but forms instead mobile NH3-solvated complexes. This dynamic character of the active site adds complexity when describing the reaction kinetics from first principles. The presentation will describe our recent work to develop a first principles model to describe the reaction kinetics of NH3-SCR over Cu-CHA. Density functional theory calculations are used to obtain the potential energy surfaces and the mean-field approximation is used to describe the reaction kinetics. We find that the potential energy surfaces of the reactions are flat, and the reaction rate depends sensitively on changes in the entropy. Thus, the evaluation of entropy changes along the reaction paths is given special attention. The reaction is found to proceeds over NH3-solvated Cu-sites by the formation of H2NNO and HONO, which decompose to N2 and H2O over Bronsted acid sites. Non-selective N2O formation is considered by H2NNO decomposition over the Cu-sites. The results from the kinetic model are in good agreement with previous kinetic experiments of apparent activation energies, reaction orders and N2O selectivity. The successful formulation of a first-principles microkinetic model for NH3-SCR rationalizes previous phenomenological models and links the kinetic behavior with fundamental materials properties. Moreover, the study exemplifies the fruitful interplay between experimental and computational approaches to elucidate complex catalytic reactions.

First-principles reaction kinetics of ammonia assisted selective catalytic reduction of NO over Cu-CHA

ACS Spring 2022

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