DECOMPOSITION OF NO OVER [CO]-ZSM-5 ZEOLITE - EFFECT OF COADSORBED O-2

The decomposition of NO over four Co-containing ZSM-5 zeolites and Pr- , Ga-, and Cu-exchanged ZSM-5 zeolites was investigated using the isot ope labeled (NO)-N-15-O-18 and a temperature-programmed desorption (TP D) technique. We found that [Co]-ZSM-5 that contains Co in the framewo rk had the highest activity for NO decomposition (TOF: 2.05 x 10(-3) s (-1) at 442 degrees C and 0.2 vol% NO and 0.8 vol% O-2), almost an ord er of magnitude greater than that previously reported for a zeolite ca talyst, namely Cu-ZSM-5 (TOF: 2.27 x 10(-4) s(-1) at 335 degrees C and 0.2 vol% NO and 0.8 vol% O-2) obtained under steady-state conditions. The phenomenally high activity of [Co]-ZSM-5 is due to the unique inc orporation of Co2+ in the siliceous MFI structure. For all the catalys ts investigated, coadsorption of NO and O-2 led to a substantial incre ase in the amount of NOx adsorbed. However, the adsorbed species were not necessarily NO2 as reported by others. We believe that the interac tion between adsorbed NOx species and O-2 is responsible for enhancing the rate of NOx decomposition. It is obvious that the framework Co2behaves very differently from Co2+ in the countercation position and f rom extra-framework CoO Such as that supported on or dispersed on the surface of silicalite also having the same MFI structure. (C) 1998 Aca demic Press.