322
J. Handzlik
with the PW91 and TPSS functionals to account for the amorphous nature of silica.
On the basis of test calculations, the PW91 method was found to be most accurate, among many density functionals, in predicting relative energies of chromium
oxo compounds. It was concluded that the relative stability of the monooxo Cr(VI)
species, compared to the dioxo one, is much lower than the relative stability of the
monooxo Mo(VI) species in the molybdena–silica system, which was earlier computationally studied using analogous models [55].
High-resolution
1 H MAS NMR and
29 Si CP/MAS NMR spectroscopy was
combined with DFT (B3LYP) computations employing polyhedral oligomeric
silsesquioxane (POSS) models to investigate the role of various silanol groups in
formation of surface Cr(VI) species [56]. It was suggested that monomeric dioxo
Cr(VI) species are most favourable located on pairs of a single and an adjacent
geminal silanols accompanied with a geminal silanol left. Replacing a pair of geminal silanols by the chromium species is not preferred energetically. The calculated
enthalpies of the grafting reactions indicate also higher preference for monomeric
Cr(VI) species, compared to Cr(VI) dimers.
Although cluster models of various complexity, representing CrO x /SiO 2 catalyst,
were successfully applied in many computational works, especially those combined
with spectroscopic studies, they are usually more or less arbitrarily constructed. To
develop more advanced models, being able to reproduce heterogeneity of the surface
chromium oxide species, realistic and well-validated models of amorphous silica
[12–16] are necessary. Such a model, developed and validated by Tielens et al. [13],
representing hydroxylated surface (5.8 OH nm
−2 ) in the unit cell including 120
atoms (Si 27 O 54 ·13 H 2 O), was employed in systematic computational studies of the
Cr(VI)/SiO 2 system [7]. Mono-, di-, tri- and tetragrafted monomeric chromium(VI)
species at different degrees of hydration were modelled (Fig. 5). Their formation
can be regarded as grafting H 2 CrO 4 unit with dehydroxylation of surface silanols.
Different types of silanols were involved in grafting reactions: isolated (Si–OH), vicinal (HO–Si–O–Si–OH), geminal (HO–Si–OH) and nonvicinal (two Si–OH groups
not directly connected). The relative stabilities of the surface chromium species in
a wide range of temperatures were determined from the atomistic thermodynamic
approach, based on the calculated energies (PBE functional) for grafting reactions.
The monografted hydroxy dioxo Cr species was predicted to be most stable at lower
temperatures. An increase of temperature favours digrafted dioxo Cr species (Fig. 2a)
and then tetragrafted monooxo Cr species (Fig. 2b). The comparison of the calculated
Cr=O stretching frequencies for the surface chromium species with experimental
Raman spectra indicated the digrafted dioxo Cr species as the most representative
for the Cr(VI)/SiO 2 system.
A large number of periodic and cluster models of the SiO 2 surface were applied
in extensive DFT (PW91) investigations of chromium(VI) oxide species on partly
dehydroxylated silica [4]. The periodic model of Tielens et al. [7, 13] was properly
modified to achieve the initial surface with 3.1 OH nm
−2 , and, 2.4–1.3 OH nm
−2
after grafting the Cr species, which roughly corresponds to the conditions of the
catalyst thermal treatment before the reaction [17–19, 30, 57, 58]. Similar to the
original model, the chromium coverage was about 0.4 atoms nm
−2 , in agreement
J. Handzlik
with the PW91 and TPSS functionals to account for the amorphous nature of silica.
On the basis of test calculations, the PW91 method was found to be most accurate, among many density functionals, in predicting relative energies of chromium
oxo compounds. It was concluded that the relative stability of the monooxo Cr(VI)
species, compared to the dioxo one, is much lower than the relative stability of the
monooxo Mo(VI) species in the molybdena–silica system, which was earlier computationally studied using analogous models [55].
High-resolution
1 H MAS NMR and
29 Si CP/MAS NMR spectroscopy was
combined with DFT (B3LYP) computations employing polyhedral oligomeric
silsesquioxane (POSS) models to investigate the role of various silanol groups in
formation of surface Cr(VI) species [56]. It was suggested that monomeric dioxo
Cr(VI) species are most favourable located on pairs of a single and an adjacent
geminal silanols accompanied with a geminal silanol left. Replacing a pair of geminal silanols by the chromium species is not preferred energetically. The calculated
enthalpies of the grafting reactions indicate also higher preference for monomeric
Cr(VI) species, compared to Cr(VI) dimers.
Although cluster models of various complexity, representing CrO x /SiO 2 catalyst,
were successfully applied in many computational works, especially those combined
with spectroscopic studies, they are usually more or less arbitrarily constructed. To
develop more advanced models, being able to reproduce heterogeneity of the surface
chromium oxide species, realistic and well-validated models of amorphous silica
[12–16] are necessary. Such a model, developed and validated by Tielens et al. [13],
representing hydroxylated surface (5.8 OH nm
−2 ) in the unit cell including 120
atoms (Si 27 O 54 ·13 H 2 O), was employed in systematic computational studies of the
Cr(VI)/SiO 2 system [7]. Mono-, di-, tri- and tetragrafted monomeric chromium(VI)
species at different degrees of hydration were modelled (Fig. 5). Their formation
can be regarded as grafting H 2 CrO 4 unit with dehydroxylation of surface silanols.
Different types of silanols were involved in grafting reactions: isolated (Si–OH), vicinal (HO–Si–O–Si–OH), geminal (HO–Si–OH) and nonvicinal (two Si–OH groups
not directly connected). The relative stabilities of the surface chromium species in
a wide range of temperatures were determined from the atomistic thermodynamic
approach, based on the calculated energies (PBE functional) for grafting reactions.
The monografted hydroxy dioxo Cr species was predicted to be most stable at lower
temperatures. An increase of temperature favours digrafted dioxo Cr species (Fig. 2a)
and then tetragrafted monooxo Cr species (Fig. 2b). The comparison of the calculated
Cr=O stretching frequencies for the surface chromium species with experimental
Raman spectra indicated the digrafted dioxo Cr species as the most representative
for the Cr(VI)/SiO 2 system.
A large number of periodic and cluster models of the SiO 2 surface were applied
in extensive DFT (PW91) investigations of chromium(VI) oxide species on partly
dehydroxylated silica [4]. The periodic model of Tielens et al. [7, 13] was properly
modified to achieve the initial surface with 3.1 OH nm
−2 , and, 2.4–1.3 OH nm
−2
after grafting the Cr species, which roughly corresponds to the conditions of the
catalyst thermal treatment before the reaction [17–19, 30, 57, 58]. Similar to the
original model, the chromium coverage was about 0.4 atoms nm
−2 , in agreement