Characterization of specific N-2-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 (N-2) 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 N-2 and CO. In the IR spect ra an intense band appears at 2295 cm(-1), which is attributed to the adsor bed N-2 species. The amount of adsorbed CO, as well as the adsorbed N-2, in creases with increasing copper-ion-exchange level of the ZSM-5 sample. By u se 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 C O adsorption; these are responsible for giving the TR bands due to the adso rbed CO species at 2159, 2151, and 2135 cm(-1). The adsorption behavior of N-2 molecules on the samples, which have various copper-ion-exchange levels and preadsorbed CO species, has been investigated, and it was found that t he 2151 cm(-1) band in the IR spectra reflects an N-2 adsorption site. Emis sion spectra were also obtained at each step of N-2 adsorption; the emissio n band due to the exchanged copper-ion species decreases in intensity with increasing pressure of N-2. These results can be interpreted as follows. Th e monovalent copper ion (Cu+) formed during the evacuation of the sample at 873 K acts as an effective site for N-2 adsorption. Moreover, the site res ponsible for giving the IR band at 2151 cm(-1) and the ES band at 18500 cm( -1) plays an important role in the N-2 adsorption. By reference to the resu lts of X-ray absorption fine structure (XAFS) measurements reported previou sly, it is thought that the active site, i.e., Cu+ species, for N-2 adsorpt ion interacts with the lattice oxygen and with CO or N-2 molecules to produ ce a pseudotetrahedral four-coordination structure. The relation between th e heat values and the frequency of the IR band due to the adsorbed CO speci es gives a linear regression, which indicates that sigma-bonding is predomi nantly operative in the bonding of Cu+ with CO or N-2 molecule.