New light on the state of active sites in CuZSM-5 for the NO decomposition reaction and N2 adsorption

Abstract

Ion-exchange of ZSM-5-type zeolites with copper ion was carried out by three different methods to obtain information on the states and roles of the effective sites for NO decomposition and N₂ adsorption activity of copper-ion-exchanged ZSM-5-type zeolites (CuZSM-5). The first method of preparation is chemical vapour deposition (CVD) using bis(1,1,1,5,5,5-hexafluoroacetylacetonato)copper(II), [Cu(hfac)₂], as a volatile complex, and the second one is to utilize CuCl as a vaporizing source. These were compared with the third method, that is, the ordinary ion exchange method using an aqueous solution of CuCl₂. It is apparent that in the first method the Cu²⁺ species deposited on ZSM-5 as [Cu(hfac)₂] (i.e., via the hydrogen bonding between a Brønsted acid site and a pseudo-aromatic ring of ligands) is reduced to the monovalent species (Cu⁺) by evacuation at 573 K, and also that the reducibility of Cu²⁺ is superior to that in the sample prepared by the conventional ion-exchange in an aqueous solution. As for the sample prepared by using CuCl, Cu⁺ deposited as CuCl was exchanged with H⁺ on a Brønsted acid site in the HZSM-5 sample through treatment at temperatures above 573 K with a release of HCl. The CuZSM-5 sample prepared by the CVD method gave a single IR band at 2159 cm⁻¹ due to the adsorbed CO species, while the sample prepared by evaporation of CuCl at 573 K and also the sample ion-exchanged in an aqueous solution gave a broad band at around 2155 cm⁻¹ (composed of two bands at 2159 and 2151 cm⁻¹) for the adsorbed CO species, indicating the existence of at least two dominant types of exchangeable sites in these CuZSM-5 samples. Each sample reveals different features for NO decomposition reactivity and N₂ adsorption, and such behaviours are explained by the difference in ratio of the respective sites occupied by copper ions. As a result, it was clearly demonstrated that the simultaneous existence of two types of sites lying close together has important implications for the catalytic activity for NO decomposition by CuZSM-5, and that the site giving the 2151 cm⁻¹ band is the effective site for N₂ adsorption.