Dependence of the Al Distribution in CHA Zeolite on the Presence of Na⁺ during the Synthesis. : An EPR Investigation of Cu Species in CuCHA

Copper-exchanged zeolites, such as CuCHA are active catalysts for removing NO and NO₂ from automotive exhaust. The zeolite synthesis is decisive for the aluminium distribution in the framework and the properties of the catalyst. We have applied in-situ EPR spectroscopy on CuCHA based on CHA synthesized in four different ways. We found that different mineralizing agents led to differences in the Cu distribution in the final product. Several distinct Cu sites were present after thermal activation according to the EPR spectra and we found that the presence of Na⁺ during the synthesis boosted the forming of “paired” Al close to Cu, but at the same time limited the ion-exchange capacity and the redox activity. Two types of well-defined EPR spectra were found, Site A and Site B. Site B was redox active and catalytically active for all investigated materials whereas the properties of Site A depended on the CHA material and consisted of two subtypes distinguished by their reactivity towards NO and NH₃. Studies with isotope labeled NH₃ showed that the system was highly dynamic. In-situ EPR was proved to be an excellent spectroscopic technique to investigate the Cu distribution directly and investigate the Al distribution indirectly for CuCHA zeolites.