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

Abstract

The necessary condition of the active sites for the NO decomposition reaction on copper ion-exchanged ZSM-5-type zeolite (CuZSM5) has been investigated by using an adsorbed N₂ species that is one of the products of the decomposition reaction of NO. Two dominant types of exchangeable sites in the CuZSM5 sample were identified by means of IR spectra using CO as a probe molecule; these sites are responsible for giving a 2159 and 2151 cm^-1 band due to the chemisorbed CO species. From exploration of the decomposition reaction of NO on the samples having different amounts of preadsorbed CO molecules, it was found that the NO decomposition reaction occurs only under the condition that both types of sites coexist. The quantitative relationship between the number of these sites and the ion-exchange capacity of the sample was also evaluated from the IR spectra for CO adsorption. Combination with a similar relationship between the NO decomposition activity and the copper ion-exchange capacity found in the reference convinces us that the presence of both types of sites located closely to each other is a necessary condition for the NO decomposition reaction. The structure of the copper ion in CuZSM5 under different exchange levels was also studied by Cu-K-edge X-ray absorption spectroscopy, from which, evidence supporting an existence of dimer species of copper ions was obtained for samples that have excessive ion-exchanged copper ion exceeding the stoichiometric amount. In addition, the oxidation–reduction process of copper ion species was also examined during NO adsorption and subsequent heat treatment invacuo. It is concluded that zeolite having an appropriate Si/Al ratio, in which it is possible for the copper ion to exist as dimer species, may provide the key to the redox cycle of copper ion as well as catalysis in NO decomposition.