Spectroscopic identification and catalytic relevance of NH₄⁺ intermediates in selective NO_x reduction over Cu-SSZ-13 zeolites

Reduction of harmful nitrogen oxides (NO_x) from diesel engine exhausts is one of the key challenges in environmental protection, and can be achieved by NH₃-assisted selective catalytic reduction (NH₃-SCR) using copper-exchanged chabazite zeolites (i.e. Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) as catalysts. Understanding the redox chemistry of Cu-CHA in NH₃-SCR catalysis is crucial for further improving the NO_x reduction efficiency. Here, a series of Cu-SSZ-13 catalysts with different Cu ion exchange levels were prepared, thoroughly characterized by different techniques such as X-ray diffraction, diffuse reflectance ultraviolet-visible spectroscopy and temperature-programmed desorption using NH₃ as a probe molecule, etc., and tested in NH₃-SCR reactions under steady-state conditions. In situ studies by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), supplemented with density-functional theory calculations, provided solid evidence for the formation of ammonium ion (NH₄⁺) intermediates resulting from the reduction of Cu²⁺ to Cu⁺ by co-adsorbed NH₃ and NO molecules on Cu-SSZ-13. Catalytic relevance of the NH₄⁺ intermediates, as demonstrated by an increase of NO conversion over Cu-SSZ-13 pre-treated in NH₃/NO atmosphere, can be attributed to the formation of closely coupled Cu⁺/NH₄⁺ pairs promoting the Cu⁺ re-oxidation and, consequently, the overall NH₃-SCR process. This study thus paves a new route for improving the NH₃-SCR efficiency over Cu-CHA zeolite catalyst.