Mechanistic insights into the promotion of low-temperature NH3-SCR catalysis by copper auto-reduction in Cu-zeolites

Abstract

The performance of Cu-chabazite (Cu-CHA) zeolite, the state-of-art catalyst for the ammonia-assisted selective reduction (NH3-SCR) of toxic NOx pollutants from heavy-duty diesel vehicles, is insufficient at low reaction temperatures and needs to be improved for meeting stringent emission regulations. Here, we demonstrate that the auto-reduction of isolated Cu sites (i.e. Cu2+[OH]→Cu+) in Cu-CHA could promote the low-temperature NOx abatement efficiency in NH3-SCR. Combining in situ spectroscopy, steady-state/transient kinetic measurements and computational simulations, we unveiled that Cu auto-reduction, which was driven by non-oxidative thermal activation, weakened the Cu tethering to CHA framework and thus increased the Cu mobility, particularly, in the presence of NH3. More importantly, auto-reduction-induced charge redistribution favored NO activation to form intimately interacted Cu+···NO+ pairs, which enabled alternative and highly efficient pathways leading to drastically promoted NO abatement at temperatures below 250 °C. These mechanistic findings shed new light on tuning the speciation of Cu single-sites to promote NOx reduction catalysis over Cu-zeolites.