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.