Cooperative Cation Migrations upon CO Addition in Cu~I- and Alkali-Exchanged Faujasite: A DFT Study

摘要

CO adsorption in Al-rich faujasite zeolite containing copper and alkali cations has been investigated using DFT methods in order to determine how CO interacts and may modify the original position of the cations. Whether a cluster or a periodic model is used, addition of CO induces the formation of stable complexes labeled DI_(CO) in which CO interacts by both its C-end and its O-end, resulting from a cooperative rearrangement of cations. In addition to a Cu~I migration from site II to the supercage, a migration of alkali from site III' to site III may occur. DI_(CO) also induces a downshift of the v_(CO) mode in comparison with the complex containing CO interacting with a single cation, SI_(CO) These results suggest a new assignment of the IR spectra of CO adsorbed in YCu~I and YNa~+: for YCu~I, the upshifted signal at ca. 2160 cm~(-1) in comparison with v_(COgas) at 2143 cm~(-1) could be assigned to a SI_(CO) structure, whereas the downshifted signal at ca. 2140 cm~(-1) could be assigned to a DI_(CO) complex. For YNa~+, the upshifted signal at ca. 2170 cm~(-1) could be assigned to SI_(CO) Na_(SII)…CO and/or to DI_(CO) Na_(SII)…CO…Na_(SIII), whereas the downshifted signal at ca. 2122 cm~(-1) could be assigned to DI_(CO) complex Na_(SII)…CO…Na_(SIII). This study shows that the DI_(CO) interaction is a key ingredient for understanding the metal-exchanged zeolite properties.