Characterization of active sites on copper ion-exchanged mordenite for dinitrogen adsorption by using CO as a probe molecule

The peculiar feature of copper ion-exchanged mordenite in dinitrogen (N₂) adsorption has been investigated through the measurements of infrared (IR) and emission spectra (ES) and X-ray absorption fine structure (XAFS). IR spectra provided the definitive evidence for the existence of at least two kinds of adsorption sites on the 873 K-treated copper ion-exchanged mordenite; the absorption bands centered at 2160 and 2150 cm^-1 appeared when CO molecules were adsorbed. The adsorbed species assigned to the former band was desorbed at 573 K in vacuo and that to the latter one at 473 K. It was found that the site responsible for the latter band acts as effective sites for N₂ adsorption. XAFS spectra also showed new bands due to the CO adsorption on copper ion at 1.48 Å, 2.52 Å, and 8.981 keV, together with bands at 1.85 Å and 8.993 keV. The former set of bands was concomitant with the appearance of an IR band at 2150 cm^-1 due to the adsorbed CO species. The two bands (at 8.981 and 8.993 keV) observed in the XAFS spectra for the CO-adsorbed sample are explained in terms of the formation of a planar three- or four-coordinate site through the reaction of copper ions with CO molecules. CO adsorption also caused a significant shift of the emission band from 470 to 430 nm. This band reverted to the original position by evacuating the sample at 473 K, accompanying a liberation of CO. This makes possible the interpretation that the adsorption site responsible for respective bands at 2150 cm^-1 in IR, at 430 nm in ES, and at 8.981 keV in XAFS spectra through the interaction with CO molecules is effective for N₂ adsorption at ambient temperature.