New light on the state of active sites in CuZSM-5 for the NO decomposition reaction and N2 adsorptionElectronic supplementary information (ESI) available: XANES and EXAFS spectra of the reference samples (1) Cu metal; (2) Cu2O; (3) CuCl; (4) [Cu(NH3)2]+; (5) CuO; (6) Cu(OH)2; (7) [Cu(hfac)2]. See http://www.rsc.org/suppdata/cp/b3/b304538m/

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.