Nanoscale tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts

Abstract

Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NOX with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000 mile simulation, Cu-SSZ-13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl2O4 spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale heterogeneities that lead to catalytic activity and material deactivation.

Probing the Location and Speciation of Elements in Zeolites with Correlated Atom Probe Tomography and Scanning Transmission X‐Ray Microscopy

“…However, to the best of our knowledge, there are no reports that correlate results of the two characterization techniques. Herein, we have applied APT and STXM in an integrated fashion to study the same single crystal of a laboratory aged (135,000 mile simulation) sample of copper‐exchanged zeolite SSZ‐13 (Cu‐SSZ‐13, CHA framework topology), which is the same sample as we previously studied with APT alone . We chose this sample as the ∼4 μm crystal size makes spatially resolved characterization studies meaningful, and as the sample was previously studied any potential beam damage can be assessed, a known problem when probing zeolites with electron beams .…”

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…Instead, formation of distorted amorphous phases with stoichiometry perhaps close to that of CuAl 2 O 4 spinel may better describe the Cu-Al phases formed in aged Cu/ZSM-5. In contrast, APT analysis of aged Cu/SSZ-13 shows some Cu and Al clustering but to a limited degree (relative to Cu/ZSM-5) with undetermined stoichiometries [22]. It can be concluded, therefore, that CuO x formed in aged Cu/SSZ 13 can indeed interact with Al from substrate dealumination but stoichiometries and structures of the thus formed species are still undetermined.…”

“…Notable also is that a lack of surface Al and Cu segregation evident from XPS analyses indicates that these moieties may still be located in pores and channels of SSZ-13. Weckhuysen and coworkers [22] recently utilized atom probe tomography (APT) to compare aged Cu/ZSM-5 and Cu/SSZ-13. In Cu/ZSM-5, severe Cu and Al aggregation was demonstrated and mapped in 3D and a Cu/Al atomic ratio of~2 for the aggregates indicates formation of copper aluminate.…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”

Selective Catalytic Reduction of NO with NH3 on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

“…Owing to the development of time‐resolved operando methods such as X‐ray absorption spectroscopy (XAS), the study of catalysts under the dynamic conditions at which they operate is now possible . The selective catalytic reduction (SCR) of NO x using small pore size and low metal‐loaded Cu−SSZ‐13 is such a reaction that takes place under dynamic conditions and has gained significant attention recently due to the high low‐temperature conversion rate of nitrogen oxides in the presence of NH 3 . The SCR reaction proceeds according to Equation . …”