DFT investigation of CO oxidation over Mg exchanged periodic zeolite models

Theoretical identification of carbonate geometry in zeolites from IR spectra

“…On the opposite, for LTA types, the BS values of the bi-dentate forms are close or slightly smaller than the one observed in NaA (300 cm À1 for the less stable group and 195 cm À1 for the more stable [5], 337-295 cm À1 [6], 476-201 cm À1 [7]) and NaKA (310, 265 cm À1 [5], 469, 294 cm À1 [6], 334, 308 cm À1 [7]) while a symmetric carbonate form could not be optimized in agreement with experiment [5][6][7]. The good coincidence between the BS values for the cluster models and experimental values for carbonates in AEZ was already discussed in details [9][10][11][12][13]. As measure of the CO 2À 3 distortion, we induced:…”

“…None, one or two Al atoms are present in both 8R and 6R + 4R models as in our previous studies [9][10][11][12][13]. The initial chemical compositions of the cluster models, without carbonate species, with two Al atoms are Me n Al 2 Si 6 O 8 H 16 and Me n Al 2 Si 6 O 9 H 14 for 8R and 6R + 4R, respectively, Me being the alkali (n = 2) or AE (n = 1) cation.…”

“…The unit cell (UC) parameters were fixed as optimized for the respective zeolites in the absence of carbonates. If for the AE forms we considered the carbonate formation via the CO 2 reaction with the MeO X Me species as demonstrated in [9][10][11][12][13], such mechanism is forbidden for dehydrated alkali zeolites 1 . For these last ones, the mechanism of carbonate formation is not known.…”

“…Some of us already discussed the aspects related to the formation of bi-dentate carbonate groups in alkaline earth zeolites (AEZ) [9][10][11][12][13]. From calculations at the cluster and periodic levels, it was possible to propose a direct way for a barrierless formation of carbonate groups in AEZ via the interaction between CO 2 and stable MeO X Me species (X = 1 or 3 and Me = Mg, Ca, Sr) [9][10][11][12][13].…”

“…From calculations at the cluster and periodic levels, it was possible to propose a direct way for a barrierless formation of carbonate groups in AEZ via the interaction between CO 2 and stable MeO X Me species (X = 1 or 3 and Me = Mg, Ca, Sr) [9][10][11][12][13]. Even if bi-dentate carbonates dominate in the IR spectra for some zeolites like NaA, they seem to possess a lower stability versus symmetric carbonates.…”