Identification of Distinct Copper Species in Cu-CHA Samples Using NO as Probe Molecule. A Combined IR Spectroscopic and DFT Study

Abstract

[EN] Combining IR spectroscopy of NO adsorption on copper exchanged molecular sieves with the chabazite structure, i.e. Cu-SAPO-34 and Cu-SSZ-13, and theoretical calculations, different types of copper species have been identified. On one hand, [Cu¿OH]+ species can be accurately distinguished, characterized by a ¿NO frequency at 1788¿ 1798 cm¿1 depending on their location in the chabazite structure (6R vs. 8R) and composition (Cu-SAPO-34 vs. Cu-SSZ-13). On the other hand, dimeric copper oxo [Cu¿O¿ Cu]2+ species have been properly identified by means of DFT modelling, that proposes a ¿NO stretching frequency of 1887 cm¿1, which has been confirmed experimentally in the Cu-SAPO-34 sample. Finally the location of isolated Cu2+ ions either in the 6R units or in the 8R positions of the chabazite cavity could be accurately defined according to DFT data, and validated in the experimental IR spectra with IR bands between 1907 and 1950 cm¿1. Regarding to Cu+ species, IR spectroscopy of CO reveals different types of Cu+ species as evidenced by their ability to form mono, di and try carbonyls. The unambiguous differentiation of different types of copper species is of great interest in further identification of active sites for the NH3- SCR reaction.