THE ADSORPTION OF NO AND REACTION OF NO WITH O-2 ON H-ZSM-5, NAH-ZSM-5, CUH-ZSM-5, AND CU-ZSM-5 - AN IN-SITU FTIR INVESTIGATION

The adsorption of NO and the reaction of NO with O-2 On H-, NaH-, CuH- , and Cu-ZSM-5 zeolites were studied at 300 K using in situ Fourier tr ansform infrared spectroscopy (FTIR), At this temperature, NO readily adsorbs on the Cu+ sites of CuH- and Cu-ZSM-5 catalysts and decomposit ion of NO is observed for all catalysts, although the rate of decompos ition is vastly different on these materials. In comparison, this reac tion is negligible over the Hand NaH-ZSM-5 samples. The time evolution of several nitrogen-containing molecules after controlled O-2 exposur e to the NO/ZSM-5 systems has allowed the spectral correlation of thes e species, These nitrogen-containing species can interact with either the protonic sites of bridging hydroxyls forming hydrogen bonding comp lexes or the metal cations producing primarily surface nitrates and ni trites. The hydrogen bonded NxOy complexes were characterized with the ir IR absorption features: (a) NO2, 2133 cm(-1); (b) N2O3, 1875 and 15 87 cm(-1); and (c) N2O4, 2185 and 1745 cm(-1). The stretching vibratio nal frequency of the acidic OH groups of ZSM-5 redshifts due to the in teraction with nitrogen-containing molecules and forms ''ABC'' band st ructures characteristic of medium and strong hydrogen bonding complexe s. Although the adsorbed NxOy species (N2O3, N2O4) interacting with th e Bronsted protons exhibit characteristics of a strong Lewis base, ads orption enthalpies are sufficiently weak that their existence is not o bserved even after brief evacuation. Nitric oxide, oxygen coadsorption produces metal cation (Na+ and Cu-n+) bonded surface species possessi ng IR absorption bands between 1400 and 1650 cm(-1) characteristic of nitrite and nitrate species. The thermal stability of the nitrite and nitrate adsorbed species is much greater than the NxOy species nucleat ed at free Bronsted acid sites. (C) 1996 Academic Press, Inc.