Strikingly distinctive NH3-SCR behavior over Cu-SSZ-13 in the presence of NO2

Cu-SSZ-13 zeolites are highly active for standard NH3-SCR, but their activity is unexpectedly inhibited in the presence of NO2. This work demonstrates that strong oxidation by NO2 forces Cu ions to exist mainly as Cu-II species with low mobility, which is responsible for this distinctive behavior. Commercial Cu-exchanged small-pore SSZ-13 (Cu-SSZ-13) zeolite catalysts are highly active for the standard selective catalytic reduction (SCR) of NO with NH3. However, their activity is unexpectedly inhibited in the presence of NO2 at low temperatures. This is strikingly distinct from the NO2-accelerated NOx conversion over other typical SCR catalyst systems. Here, we combine kinetic experiments, in situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations to obtain direct evidence that under reaction conditions, strong oxidation by NO2 forces Cu ions to exist mainly as Cu-II species (fw-Cu2+ and NH3-solvated Cu-II with high CNs), which impedes the mobility of Cu species. The SCR reaction occurring at these Cu-II sites with weak mobility shows a higher energy barrier than that of the standard SCR reaction on dynamic binuclear sites. Moreover, the NO2-involved SCR reaction tends to occur at the Bronsted acid sites (BASs) rather than the Cu-II sites. This work clearly explains the strikingly distinctive selective catalytic behavior in this zeolite system.

Automated summary

This study demonstrates that the inhibition of Cu-SSZ-13 zeolite catalysts in the presence of NO2 is due to the strong oxidation leading to the formation of Cu-II species with low mobility. This distinctive behavior is explained by the higher energy barrier of the SCR reaction occurring at the Cu-II sites compared to the standard SCR reaction on dynamic binuclear sites.