Prediction of Cu Zeolite NH₃-SCR Activity from Variable Temperature ¹H NMR Spectroscopy

Selective catalytic reduction (SCR) of NO_x by ammonia is one of the dominant pollution abatement technologies for near-zero NO_x emission diesel engines. A crucial step in the reduction of NO_x to N₂ with Cu zeolite NH₃-SCR catalysts is the generation of a multi-electron donating active site, implying the permanent or transient dimerization of Cu ions. Cu atom mobility has been implicated by computational chemistry as a key factor in this process. This report demonstrates how variable temperature ¹H NMR reveals the Cu induced generation of sharp ¹H resonances associated with a low concentration of sites on the zeolite. The onset temperature of the appearance of these signals was found to strongly correlate with the NH₃-SCR activity and was observed for a range of catalysts covering multiple frameworks (CHA, AEI, AFX, ERI, ERI-CHA, ERI-OFF, *BEA), with different Si/Al ratios and different Cu contents. The results point towards universal applicability of variable temperature NMR to predict the activity of a Cu-zeolite SCR catalyst. The unique relationship of a spectroscopic feature with catalytic behavior for zeolites with different structures and chemical compositions is exceptional in heterogeneous catalysis.