Deactivation of Cu‐Exchanged Automotive‐Emission NH₃‐SCR Catalysts Elucidated with Nanoscale Resolution Using Scanning Transmission X‐ray Microscopy

Abstract

To gain insight into the underlying mechanisms of catalyst durability for the selective catalytic reduction (SCR) of NOx with an ammonia reductant, we employed scanning transmission X‐ray microscopy (STXM) to study Cu‐exchanged zeolites with the CHA and MFI framework structures before and after simulated 135 000‐mile aging. X‐ray absorption near‐edge structure (XANES) measurements were performed at the Al K‐ and Cu L‐edges. The local environment of framework Al, the oxidation state of Cu, and geometric changes were analyzed, showing a multi‐factor‐induced catalytic deactivation. In Cu‐exchanged MFI, a transformation of CuII to CuI and CuxOy was observed. We also found a spatial correlation between extra‐framework Al and deactivated Cu species near the surface of the zeolite as well as a weak positive correlation between the amount of CuI and tri‐coordinated Al. By inspecting both Al and Cu in fresh and aged Cu‐exchanged zeolites, we conclude that the importance of the preservation of isolated CuII sites trumps that of Brønsted acid sites for NH3‐SCR activity.

New insights into the NH3-selective catalytic reduction of NO over Cu-ZSM-5 as revealed by operando spectroscopy

“…[11][12][13][14] In our previous study of steamed Cu-ZSM-5 zeolites we have observed an unusual NO conversion curve with a 'dip' shape at around 300 °C. 15 A similar drop of NO conversion was reported at ~270 °C with the hydrothermally treated zeolite Cu-SSZ-13 and was simply explained by the accelerated unselective NH3 oxidation promoted by CuxOy clusters/nanoparticles. 16,17 However, the detailed structural reasons for the low NH3-SCR activity have not yet been well understood due to the interference of multiple Cu sites and side reactions.…”

“…In the 850 °C steamed Cu-ZSM-5, nanoparticles with particle sizes of 2-5 nm were prone to migrate and aggregate on the surface of zeolite particle, which is in agreement with a Scanning Transmission X-ray Microscopy (STXM) study that revealed Cu zoning on the edge of individual catalyst particles in steamed zeolites. 15 The agglomerated Cu species have a less reducible nature than isolated Cu 2+ , as indicated by the higher reduction temperature in the steamed Cu-ZSM-5 observed in H2-Temperature Programmed Reduction (H2-TPR) (Fig. S6).…”

Confining isolated atoms and clusters in crystalline porous materials for catalysis

“…The advantage of Cu-zeolite materials over Cu supported on conventional solid carriers can be related to the synergistic effect of acid sites in the zeolite framework and subnanometric Cu species confined in the cavities. Control of the spatial distribution of Cu and Al species is also a key issue on preparation of highly stable Cu-zeolite materials for NH3-SCR reaction 185 .…”

Rare‐earth Yttrium Exchanged Cu‐SSZ‐39 Zeolite with Superior Hydrothermal Stability and SO2‐Tolerance in NH3‐SCR of NOx

“…It has been proposed that the hydrothermal stability of the Alcontaining zeolite framework is one of the critical factors for the NH 3 -SCR. [24] In this regard, we previously reported that the addition of rare-earth ions, especially Y, can stabilize the framework Al of a CHA matrix. [5a] In the present study, the effects of different rare-earth ions on activity and hydrothermal ChemCatChem stability of Cu-AEI were also investigated.…”