Adsorption properties of copper ion-exchanged mordenite (CuM) for dinitrogen molecules (N2) were examined at 298 K. The intensive IR absorption band observed at 2299 cm-1 was attributed to the N2 species strongly adsorbed on CuM. The interaction of N2 with CuM is explored using adsorption calorimetry, X-ray absorption fine structure (XAFS), electron spin resonance (ESR), and photoemission spectroscopy. The differential heat and entropy of adsorption for N2 on CuM were 60 kJ mol-1 and 60 J K-1 mol-1 at the initial stage of adsorption, respectively, and those for N2 on NaM (sodium-type mordenite) gave the values of 32 kJ mol-1 and 130 J K-1 mol-1, revealing that the N2 molecules are in the localized state resulting from the strong interaction with CuM. The monolayer capacity is estimated to be 4.12 cm3 g-1 for N2 on CuM-150, which gives a value of 0.22 for the N2/Cu ratio. XAFS and emission data for CuM degassed at 873 K exhibit pair bands at 8.983 and 8.994 keV and 18 700 and 20 800 cm-1, respectively. The former pair band is assigned to the 1s-4p transition, and the latter pair band is assigned to the 3d94s1-3d10 transition. It is also found that the ESR band intensity for Cu(II) decreases with increasing pretreatment temperature. These spectral data are reasonably explained by assuming the presence of Cu(I) species in mordenite. It is proved from the emission data that the adsorption site including Cu(I) species easily formed by heat treatment at 873 K in vacuo is effective for N2 adsorption. Such easy conversion of Cu(II) to Cu(I) may be due to the spatial distribution of ion-exchanged sites on mordenite. The appearance of a strong IR band at 2299 cm-1 is due to the adsorption of N2 on the Cu(I) species and to the induction of a transition moment by the strong field of this site. Although a rather high value of heat of adsorption might suggest chemisorption, it is made plausible that this type of N2 adsorption is physisorption.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry