Spectroscopic identification and catalytic relevance of NH intermediates in selective NO reduction over Cu-SSZ-13 zeolites.

Reduction of harmful nitrogen oxides (NO) from diesel engine exhausts is one of the key challenges in environmental protection, and can be achieved by NH-assisted selective catalytic reduction (NH-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 NH-SCR catalysis is crucial for further improving the NO 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 NH as a probe molecule, etc., and tested in NH-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 (NH) intermediates resulting from the reduction of Cu to Cu by co-adsorbed NH and NO molecules on Cu-SSZ-13. Catalytic relevance of the NH intermediates, as demonstrated by an increase of NO conversion over Cu-SSZ-13 pre-treated in NH/NO atmosphere, can be attributed to the formation of closely coupled Cu/NH pairs promoting the Cu re-oxidation and, consequently, the overall NH-SCR process. This study thus paves a new route for improving the NH-SCR efficiency over Cu-CHA zeolite catalyst.