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

H2-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,Z2Cu (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 H2-TPR and EPR to elaborate on the possible quantification of Cu-sites. Proper quantification of the Z2Cu sites required simulation of the EPR spectra and analysis of the Gaussian fit of the H2-TPR data for each family of species. To reach an agreement between EPR and H2-TPR on the quantification of Z2Cusites, the two high-temperature reduction peaks in the TPR (ca. 350and 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, Z2Cu, 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 Z2Cu sites, a non-even distribution exists between the two possible types in this particular CHA zeolite. H2-TPR combined with in situ EPR further reveals that one of the Z2Cu sites is more easily reduced than the other.