“…We note that in the current literature only the first method is used. 17,[60][61][62][63] Our results suggest that estimates of anharmonicity based on a purely onedimensional approach are of limited validity. Furthermore our MD studies emphasize that anharmonic frequency shifts are necessarily temperature dependent.…”
We present a periodic density-functional investigation of the adsorption and the vibrational spectroscopy of CO in mordenite. Our results highlight a pronounced sensitivity of the strength of the hydrogen bond between the acidic hydroxyl groups and the adsorbed molecule, and hence of the induced red shift of the OH, and the blue shift of the CO stretching mode on the choice of the exchange-correlation functional. The popular Perdew-Wang (PW) gradient-corrected functional strongly overestimates the frequency shifts and the interaction energies. We demonstrate that the revised Perdew-Burke-Ernzerhof (RPBE) functional leads to an improved description of hydrogen bonding. For bridging OH groups, terminal silanol groups and for Lewis sites formed by tricoordinated Al atoms, we predict adsorption energies and frequency shifts in good agreement with experiment. The calculated difference in the binding energies of CO in purely siliceous mordenite and at Brønsted acid sites in the main channel agrees very well with microcalorimetry data. We find that Brønsted acid sites in the small channels (the side-pockets) of mordenite do not adsorb CO, which is adsorbed only in the main channel via the C atom. For these adsorption complexes we find reasonable (though not perfect) agreement of the predicted blue shift of the CO-mode and of the red shift of the OH-mode with experiment. Our prediction that the side-pockets are inaccessible to CO correlates well with the microcalorimetric studies and with experimental observation of the adsorption of O(2), N(2) and H(2) molecules but contradicts the current interpretation of experimental adsorption studies for CO.
“…We note that in the current literature only the first method is used. 17,[60][61][62][63] Our results suggest that estimates of anharmonicity based on a purely onedimensional approach are of limited validity. Furthermore our MD studies emphasize that anharmonic frequency shifts are necessarily temperature dependent.…”
We present a periodic density-functional investigation of the adsorption and the vibrational spectroscopy of CO in mordenite. Our results highlight a pronounced sensitivity of the strength of the hydrogen bond between the acidic hydroxyl groups and the adsorbed molecule, and hence of the induced red shift of the OH, and the blue shift of the CO stretching mode on the choice of the exchange-correlation functional. The popular Perdew-Wang (PW) gradient-corrected functional strongly overestimates the frequency shifts and the interaction energies. We demonstrate that the revised Perdew-Burke-Ernzerhof (RPBE) functional leads to an improved description of hydrogen bonding. For bridging OH groups, terminal silanol groups and for Lewis sites formed by tricoordinated Al atoms, we predict adsorption energies and frequency shifts in good agreement with experiment. The calculated difference in the binding energies of CO in purely siliceous mordenite and at Brønsted acid sites in the main channel agrees very well with microcalorimetry data. We find that Brønsted acid sites in the small channels (the side-pockets) of mordenite do not adsorb CO, which is adsorbed only in the main channel via the C atom. For these adsorption complexes we find reasonable (though not perfect) agreement of the predicted blue shift of the CO-mode and of the red shift of the OH-mode with experiment. Our prediction that the side-pockets are inaccessible to CO correlates well with the microcalorimetric studies and with experimental observation of the adsorption of O(2), N(2) and H(2) molecules but contradicts the current interpretation of experimental adsorption studies for CO.
“…41,50 Note that 4090 cm À1 band, associated with the disappearance of the band due to LF component, represents the lowest value observed for H 2 adsorbed on proton-exchanged zeolites, 15 confirming the very strong character of these acidic sites. At higher P H 2 , the growth of a component at 4135 cm À1 is associated to H 2 interacting with silanols, 25,51 as it is accompanied by the perturbation of the band at 3740 cm À1 (see the corresponding spectra in Fig. 2a).…”
Section: K Ir Spectroscopy Of H 2 Adsorbed On H-ssz-13 Materialsmentioning
We have recently highlighted that H-SSZ-13, a highly siliceous zeolite (Si/Al = 11.6) with a chabazitic framework, is the most efficient zeolitic material for hydrogen storage [A. Zecchina, S. Bordiga, J. G. Vitillo, G. Ricchiardi, C. Lamberti, G. Spoto, M. Bjørgen and K. P. Lillerud, J. Am. Chem. Soc., 2005, 127, 6361]. The aim of this new study is thus to clarify both the role played by the acidic strength and by the density of the polarizing centers hosted in the same framework topology in the increase of the adsorptive capabilities of the chabazitic materials towards H2. To achieve this goal, the volumetric experiments of H2 uptake (performed at 77 K) and the transmission IR experiment of H2 adsorption at 15 K have been performed on H-SSZ-13, H-SAPO-34 (the isostructural silico-aluminophosphate material with the same Brønsted site density) and H-CHA (the standard chabazite zeolite: Si/Al = 2.1) materials. We have found that a H2 uptake improvement has been obtained by increasing the acidic strength of the Brønsted sites (moving from H-SAPO-34 to H-SSZ-13). Conversely, the important increase of the Brønsted sites density (moving from H-SSZ-13 to H-CHA) has played a negative role. This unexpected behavior has been explained as follows. The additional Brønsted sites are in mutual interaction via H-bonds inside the small cages of the chabazitic framework and for most of them the energetic cost needed to displace the adjacent OH ligand is higher than the adsorption enthalpy of the OH...H2 adduct. From our work it can be concluded that proton exchanged chabazitic frameworks represent, among zeolites, the most efficient materials for hydrogen storage. We have shown that a proper balance between available space (volume accessible to hydrogen), high contact surface, and specific interaction with strong and isolated polarizing centers are the necessary characteristics requested to design better materials for molecular H2 storage.
“…9,12 The silanol molecule being markedly less acidic than the t SiOH group, a possible alternative minimal cluster is the trifluorosilanol molecule F 3 SiOH (structure 2 in Figure 1, referred to as FSL), which turns out, however, to be exceedingly acidic. 9,14 On the other hand, it is possible to consider larger clusters.…”
Section: Introductionmentioning
confidence: 99%
“…The isolated species ⋮SiOH is the best studied (see ref and references therein): computational studies dealing with geminal species as well as with freely interacting species are available. Before the advent of DF techniques, in order to adopt an adequate level of treatment in the study of interaction with molecules, a minimal-size cluster was normally chosen, i.e., the silanol molecule H 3 SiOH, represented as structure 1 in Figure and also referred to as SIL. , The silanol molecule being markedly less acidic than the ⋮SiOH group, a possible alternative minimal cluster is the trifluorosilanol molecule F 3 SiOH (structure 2 in Figure , referred to as FSL), which turns out, however, to be exceedingly acidic. , On the other hand, it is possible to consider larger clusters. 1 Optimized structures of the adopted clusters (structures 1 − 6 ).…”
Structural, energetical, and vibrational features of cagelike clusters mimicking the isolated hydroxyl
group at the surface of siliceous materials, both free and interacting with an ammonia molecule, are
studied ab initio at the B3-LYP/DZP level of calculations. Two minimal model clusters, H3SiOH and
F3SiOH, and four cagelike clusters differing in size and in the geometrical strain of the Si−O ring structures
have been studied. The computed value of 3762 cm-1 for the anharmonic ω01 (OH) of cagelike clusters with
an anharmonicity constant of 78 cm-1 is in fair agreement with the experiment, as it is the computed value
of 1454 ± 10 kJ/mol for the deprotonation energy obtained by averaging values for the four different
cagelike clusters. For the interaction with the NH3 molecule, the computed average value of 841 ± 69 cm-1
for the shift of the ω01 (OH) stretching frequency is in excellent agreement with the experimental value
measured on amorphous silica at low temperature; the computed average value of 29 ± 3 kJ/mol for the
heat of interaction of NH3 is definitely underestimated with respect to the microcalorimetric measurements,
probably because of the neglect of dispersive interactions and long-range forces not accounted for by the
cluster approach.
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