FT-IR STUDY OF NO-2 COADSORPTION ON H-ZSM-5 - REASSIGNMENT OF THE 2133 CM(-1) BAND TO NO+ SPECIES(O)

Whereas NO adsorption at room temperature on activated H-ZSM-5 (Si/Al = 29) caused only negligible changes in its IR spectrum, addition on O -2 to NO led to the appearance of bands at 2133 and 977 cm(-1). Concom itantly, the number of acidic zeolite OH groups decreased while H2O hy drogen-bonded to zeolite OH groups developed. Introduction of small am ounts of O-18(2) did not change the 2133 cm(-1) band wavenumber, nor t he use of a partly deuteroxylated D-H-ZSM-5 sample. In such a case, HO D formation was detected. The results obtained evidence that the 2133 cm(-1) band, generally considered as characterizing NO2+ species, is, in fact, due to NO+ species occupying cationic positions in the zeolit e. The 977 cm(-1) band is attributed to the O-lattice-NO+ vibration. A scheme of the NO+ formation, involving NO2 molecules as NO oxidizing agent, is proposed.