VIBRATION FREQUENCIES OF NCO SPECIES AS AN INTERMEDIATE FOR SELECTIVEREDUCTION OF NITROGEN MONOXIDE OVER COPPER ION-EXCHANGED ZSM-5

Vibration frequencies of the NCO stretching mode were calculated for m odel molecules of the copper ion-exchanged ZSM-5 catalytic system, Cu( OH2)(3)X(n+) (X = NCO, NCO- and HNCO, n = 1 or 2) by use of ab initio molecular orbital calculations. The complexes, Al(OH)(3)(NCO), Al(OH)( 3)(NCO-) and Al(OH)(3)(OCNH) were adopted as the models of the zeolite framework. According to the present calculations, the absorption band s observed at 2189 cm(-1) and 2251 cm(-1) are assigned to the stretchi ng vibration of the NCO- coordinating to the copper ion and to the alu minum in the zeolite (ZSM-5) framework, respectively. This result is c onsistent with the experimental assignment.