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

H-2-temperature-programmed reduction (TPR)and electronicparamagnetic resonance (EPR) spectroscopy are widely used to quantifytwo 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 investigatedwith different amounts of copper using both H-2-TPR andEPR to elaborate on the possible quantification of Cu-sites. Properquantification of the Z(2)Cu sites required simulation ofthe EPR spectra and analysis of the Gaussian fit of the H-2-TPR data for each family of species. To reach an agreement betweenEPR and H-2-TPR on the quantification of Z(2)Cusites, the two high-temperature reduction peaks in the TPR (ca. 350and 530 & DEG;C) were added together. The composite low-temperaturereduction peak in TPR (ca. 215 and 250 & DEG;C) is speculated to consistof different types of ZCuOH where some are EPR silent and some givea broad and indistinct EPR spectrum. Our analysis shows that the amountof Cu in the most stable sites, Z(2)Cu, is close to the amountpredicted by the assumption of a random distribution of Al in theframework. However, the hyperfine coupling pattern of the EPR spectrareveals that within the category of Z(2)Cu sites, a non-evendistribution exists between the two possible types in this particularCHA zeolite. H-2-TPR combined with in situ EPR further revealsthat one of the Z(2)Cu sites is more easily reduced thanthe other.