Characterization of Specific N2-Adsorption Site Existing on CuZSM-5 Type Zeolite:  Effect of Ion-Exchange Level on Adsorption Properties

The specific adsorption property of copper-ion-exchanged ZSM-type zeolite (CuZSM-5) for dinitrogen molecules (N2) has been elucidated by methods such as infrared (IR) and emission (ES) spectroscopy and by the measurements of heat of adsorption and adsorption isotherms of N2 and CO. In the IR spectra an intense band appears at 2295 cm-1, which is attributed to the adsorbed N2 species. The amount of adsorbed CO, as well as the adsorbed N2, increases with increasing copper-ion-exchange level of the ZSM-5 sample. By use of CO as a probe molecule, it was found that on the 873 K treated CuZSM-5 sample there are at least three types of adsorption sites available for CO adsorption; these are responsible for giving the IR bands due to the adsorbed CO species at 2159, 2151, and 2135 cm-1. The adsorption behavior of N2 molecules on the samples, which have various copper-ion-exchange levels and preadsorbed CO species, has been investigated, and it was found that the 2151 cm-1 band in the IR spectra reflects an N2 adsorption site. Emission spectra were also obtained at each step of N2 adsorption; the emission band due to the exchanged copper-ion species decreases in intensity with increasing pressure of N2. These results can be interpreted as follows. The monovalent copper ion (Cu+) formed during the evacuation of the sample at 873 K acts as an effective site for N2 adsorption. Moreover, the site responsible for giving the IR band at 2151 cm-1 and the ES band at 18500 cm-1 plays an important role in the N2 adsorption. By reference to the results of X-ray absorption fine structure (XAFS) measurements reported previously, it is thought that the active site, i.e., Cu+ species, for N2 adsorption interacts with the lattice oxygen and with CO or N2 molecules to produce a pseudotetrahedral four-coordination structure. The relation between the heat values and the frequency of the IR band due to the adsorbed CO species gives a linear regression, which indicates that σ-bonding is predominantly operative in the bonding of Cu+ with CO or N2 molecule.