Characterization of active sites on copper ion-exchanged ZSM-5-type zeolite for NO decomposition reaction

The necessary condition of the active sites for the NO decomposition reacti on on copper ion-exchanged ZSM-5-type zeolite (CuZSM5) has been investigate d by using an adsorbed N-2 species that is one of the products of the decom position reaction of NO. Two dominant types of exchangeable sites in the Cu ZSM5 sample were identified by means of IR spectra using CO as a probe mole cule; these sites are responsible for giving a 2159 and 2151 cm(-1) band du e to the chemisorbed CO species. From exploration of the decomposition reac tion of NO on the samples having different amounts of preadsorbed CO molecu les, it was found that the NO decomposition reaction occurs only under the condition that both types of sites coexist. The quantitative relationship b etween the number of these sites and the ion-exchange capacity of the sampl e was also evaluated from the IR spectra for CO adsorption. Combination wit h a similar relationship between the NO decomposition activity and the copp er ion-exchange capacity found in the reference convinces us that the prese nce of both types of sites located closely to each other is a necessary con dition for the NO decomposition reaction. The structure of the copper ion i n CuZSM5 under different exchange levels was also studied by Cu-K-edge X-ra y absorption spectroscopy, from which, evidence supporting an existence of dimer species of copper ions was obtained for samples that have excessive i on-exchanged copper ion exceeding the stoichiometric amount. In addition, t he oxidation-reduction process of copper ion species was also examined duri ng NO adsorption and subsequent heat treatment in vacuo. It is concluded th at zeolite having an appropriate Si/Al ratio, in which it is possible for t he copper ion to exist as dimer species, may provide the key to the redox c ycle of copper ion as well as catalysis in NO decomposition.