Copper Site Motion Promotes Catalytic NO Reduction under Zeolite Confinement.

Ammonia-mediated selective catalytic reduction (NH-SCR) is currently the key approach to abate nitrogen oxides (NO) emitted from heavy-duty lean-burn vehicles. The state-of-art NH-SCR catalysts, namely, copper ion-exchanged chabazite (Cu-CHA) zeolites, perform rather poorly at low temperatures (below 200 °C) and are thus incapable of eliminating effectively NO emissions under cold-start conditions. Here, we demonstrate a significant promotion of low-temperature NO reduction by reinforcing the dynamic motion of zeolite-confined Cu sites during NH-SCR. Combining complex impedance-based spectroscopy (IS) and extended density-functional tight-binding molecular dynamics simulation, we revealed an environment- and temperature-dependent nature of the dynamic Cu motion within the zeolite lattice. Further coupling IS with infrared spectroscopy allows us to unravel the critical role of monovalent Cu in the overall Cu mobility at a molecular level. Based on these mechanistic understandings, we elicit a boost of NO reduction below 200 °C by reinforcing the dynamic Cu motion in various Cu-zeolites (Cu-CHA, Cu-ZSM-5, Cu-Beta, etc.) via facile postsynthesis treatments, either in a reductive mixture at low temperatures (below 250 °C) or in a nonoxidative atmosphere at high temperatures (above 450 °C).