The Adsorption of NO and Reaction of NO with O2on H-, NaH-, CuH-, and Cu-ZSM-5: Anin SituFTIR Investigation

Abstract

The adsorption of NO and the reaction of NO with O2on H-, NaH-, CuH-, and Cu-ZSM-5 zeolites were studied at 300 K usingin situFourier transform infrared spectroscopy (FTIR). At this temperature, NO readily adsorbs on the Cu+sites of CuH- and Cu-ZSM-5 catalysts and decomposition of NO is observed for all catalysts, although the rate of decomposition is vastly different on these materials. In comparison, this reaction is negligible over the H- and NaH-ZSM-5 samples. The time evolution of several nitrogen-containing molecules after controlled O2exposure to the NO/ZSM-5 systems has allowed the spectral correlation of these species. These nitrogen-containing species can interact with either the protonic sites of bridging hydroxyls forming hydrogen bonding complexes or the metal cations producing primarily surface nitrates and nitrites. The hydrogen bonded NxOycomplexes were characterized with their IR absorption features: (a) NO2, 2133 cm−1; (b) N2O3, 1875 and 1587 cm−1; and (c) N2O4, 2185 and 1745 cm−1. The stretching vibrational frequency of the acidic OH groups of ZSM-5 red-shifts due to the interaction with nitrogen-containing molecules and forms “ABC” band structures characteristic of medium and strong hydrogen bonding complexes. Although the adsorbed NxOyspecies (N2O3, N2O4) interacting with the Brønsted protons exhibit characteristics of a strong Lewis base, adsorption enthalpies are sufficiently weak that their existence is not observed even after brief evacuation. Nitric oxide, oxygen coadsorption produces metal cation (Na+and Cun+) bonded surface species possessing IR absorption bands between 1400 and 1650 cm−1characteristic of nitrite and nitrate species. The thermal stability of the nitrite and nitrate adsorbed species is much greater than the NxOyspecies nucleated at free Brønsted acid sites.