Characterization of Cu-exchanged SSZ-13: a comparative FTIR, UV-Vis, and EPR study with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios

Abstract

Cu-SSZ-13 has been characterized by different spectroscopic techniques and compared with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios and prepared by the same ion exchange procedure. On vacuum activated samples, low temperature FTIR spectroscopy allowed us to appreciate a high concentration of reduced copper centres, i.e. isolated Cu⁺ ions located in different environments, able to form Cu⁺(N₂), Cu⁺(CO)_n (n = 1, 2, 3), and Cu⁺(NO)_n (n = 1, 2) upon interaction with N₂, CO and NO probe molecules, respectively. Low temperature FTIR, DRUV-Vis and EPR analysis on O₂ activated samples revealed the presence of different Cu²⁺ species. New data and discussion are devoted to (i) [Cu–OH]⁺ species likely balanced by one framework Al atom; (ii) mono(μ-oxo)dicopper [Cu₂(μ-O)]²⁺ dimers observed in Cu-ZSM-5 and Cu-β, but not in Cu-SSZ-13. UV-Vis-NIR spectra of O₂ activated samples reveal an intense and finely structured d–d quadruplet, unique to Cu-SSZ-13, which is persistent under SCR conditions. This differs from the 22 700 cm⁻¹ band of the mono(μ-oxo)dicopper species of the O₂ activated Cu-ZSM-5, which disappears under SCR conditions. The EPR signal intensity sets Cu-β apart from the others.