Cu-Speciation in Dehydrated CHA Zeolites Studied by H₂-TPR and In Situ EPR

Abstract

H 2 -temperature-programmed reduction (TPR) and electronic paramagnetic resonance (EPR) spectroscopy are widely used to quantify two families of Cu-species in dehydrated Cu-CHA zeolites, namely, Z 2 Cu (Cu associated with two Al in the 6-ring) and ZCuOH (Cu associated with one Al). Since quantification is not straightforward, discrepancies arise between investigations based on only one method. Five samples with the same parent zeolite material were investigated with different amounts of copper using both H 2 -TPR and EPR to elaborate on the possible quantification of Cu-sites. Proper quantification of the Z 2 Cu sites required simulation of the EPR spectra and analysis of the Gaussian fit of the H 2 -TPR data for each family of species. To reach an agreement between EPR and H 2 -TPR on the quantification of Z 2 Cu sites, the two high-temperature reduction peaks in the TPR (ca. 350 and 530 °C) were added together. The composite low-temperature reduction peak in TPR (ca. 215 and 250 °C) is speculated to consist of different types of ZCuOH where some are EPR silent and some give a broad and indistinct EPR spectrum. Our analysis shows that the amount of Cu in the most stable sites, Z 2 Cu, is close to the amount predicted by the assumption of a random distribution of Al in the framework. However, the hyperfine coupling pattern of the EPR spectra reveals that within the category of Z 2 Cu sites, a non-even distribution exists between the two possible types in this particular CHA zeolite. H 2 -TPR combined with in situ EPR further reveals that one of the Z 2 Cu sites is more easily reduced than the other.

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

“…occupied after the more stable Z 2 Cu sites are filled. It was previously observed by EPR that the occupation develops somewhat gradual when increasing the Cu loading [35] and the results presented here confirm this.…”

“…Specifically relevant for Cu in the vicinity of only one charge compensating Al site, three-coordinate Cu 2 + with two zeolite contacts and one OH-group is expected to be EPR inactive due to the pseudo-Jahn-Teller effect which leads to a fast relaxation of the excited spin state. [34,35,35,36] In this study, we focus on the EPR spectra of thermally activated CuCHA materials. The observed EPR spectra can be described using an axial spin Hamiltonian with S = 1/2 corresponding to one unpaired electron and I = 3/2 due to the interaction of the unpaired electron with the nuclear spin of both Cu isotopes, see Eq.…”

“…A flat line with no EPR signal still has non-zero intensity after the double integration performed in the analysis because of random noise on the measured spectrum. [27,35] The values can be corrected using calibration curves based upon prior knowledge to obtain a more precise value of the copper content in the individual samples, see Figure S3. The ICP results presented here were obtained with a mild digestion of the sample without the addition of HF since the HF-digestion step is highly problematic from a safety point of view and is best avoided if possible.…”

“…Thus, the loss of signal is more likely to be caused by the formation of either [CuOCu] 2 + dimers with antiferromagnetic coupling between the two Cu, larger "Cu x O y " clusters, or 3-coordinate Cu in ZCuOH sites. [34,35] For samples with low Cu-loading, the decrease in signal is small or nonexistent. For Cu-loading above 1 wt.%, a significant decrease in signal was observed by thermal activation, see Figure 3.…”

“…These extra peaks are marked with orange in Figure 5 and were recently assigned as a 4-coordinate ZCuOH species. [35,40] The intensities of these extra signals increase with copper loading; they do, however, remain a minor contribution to the EPR signal and are not considered in the quantification described below.…”