NO adsorption on Cu-ZSM-5: assignment of IR band at 2133 cm−1

The adsorption of nitrogen oxides on Cu-ZSM-5 was studied by infrared spectroscopy to elucidate the species associated with the band at 2133 cm^–1. The band was found for both NO and NO₂ adsorption. Labeling experiments with¹⁵NO revealed that the associated surface species contained nitrogen and, most likely, an N-O bond. Co-adsorption experiments of NO and oxygen produced adsorbed nitronium, NO₂⁺, as the principal, associated species. Adsorption of nitrogen oxides on dispersed CuO and the HZSM-5 support demonstrated that the 2133 cm^–1 band was not necessarily associated with copper ions. A relatively strong correlation between the bands at 2133 and 3615 cm^–1 indicates that the primary adsorption sites of NO₂⁺ are the strongly protic, bridging Si(OH)Al framework hydroxyls. Once these were filled, other, weaker acid sites began to adsorb NO₂^O.