NO adsorption on ex-framework [Fe,X]MFI catalysts:: novel IR bands and evaluation of assignments

IR spectra of NO adsorbed on isomorphously substituted [Fe,AI]MFI, [Fe,Ga]M Fl and [FC]MFI after steaming at 873 K in 30 vol% H2O are presented. On ex-[Fe,Al]MFl, NO adsorption leads to bands at 2133 cm(-1) and a doublet at 1886 and 1874 cm(-1). The 2133 cm(-1) band is assigned to NO+ occupying cationic positions in the zeolite structure. Of the doublet, the 1874 cm(-1) band is much more susceptible to reaction with O-2 than the 1886 cm(-1) band, yielding adsorbed NO, with an absorption frequency of 1635 cm(-1). After evaluation of the constitution of the catalyst and (sometimes contradictory) literature assignments, the 1886 cm-1 band is assigned to NO adsorbed on Fe ions located in isolated positions, and/or (FeO), clusters inside the zeolite channels, whereas the 1874cm(-1) band is proposed to be induced by 2nm FeAl0, nano-particles. The ex-[Fe,Ga]MFI catalyst showed a similar absorption pattern (doublet), which is shifted to lower wavenumbers (1881 and 1867cm(-1)), suggesting that both frequencies are affected by the vicinity of Ga (or Al) to the Fe site involved. The absence of bands at 1765 and 1835 cm(-1) suggests that the isolated sites causing these absorptions are in the Fe-III state in ex-[Fe,AI]MFI and ex-[Fe,Ga]MFI. For the ex-[Fe]MFI sample, which did not contain any 2nm FeOx nano-particles, an NO absorption band at 1854cm(-1) is assigned to mono-nitrosyl on extra-framework oligonuclear ((FeO)-O-II) species in the zeolite channels.