Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
383
unit cell parameters are underestimated by ∼1.25% only. Sauer and Vyboishchikov
reported the BP86/D(T)ZVP results for bulk V 2 O 5 which reasonably reproduce both
cell parameters (within 2%) and atomic coordinates (within 2 pm) within the V 2 O 5
layer [435]. The interlayer V–O distance, however, was overestimated by 10 pm,
which may stem from its intermolecular character, and a proper description may
therefore be a challenge for DFT calculations. The strength of the interaction between
the individual layers in the crystal is very low, a few kJ/mol per V 2 O 5 unit [433, 435].
For V 2 O 5 , the natural cleavage plane is (001) surface characterised by the lowest
surface energy thus highest stability comparing to other facets of V 2 O 5 . It was studied
by means of periodic methods and the structural and electronic properties of the (001)
surface, calculated as single-layer slab, are close to those of the bulk [433, 434] in
good agreement with recent experimental results [436, 437]. It was also reported that
the vanadia surface is mostly ionic with the vanadyl bond V–O1c (for labelling, see
Fig. 12b) being mostly covalent and the atomic charges on oxygen atoms decrease
with their coordination: O3c > O2c > O1c for both bulk and (001) surface. This
may suggest a large reactivity of the threefold coordinated oxygen. On the other
hand, the pDOS shows that vanadyl oxygen levels are much closer to the Fermi level
than those of O2c or O3c. Therefore, a large reactivity can be expected for O1 site
[433]. A metal-to-insulator transition of the V 2 O 5 (001) surface was studied by DFT
and Monte Carlo simulations corroborated with in situ STM and band gap mapping,
derived from scanning tunnelling spectroscopy data. The transition restricted to the
surface layers was observed at 350–400 K and occurred anisotropically across the
surface with the formation of vanadyl oxygen vacancies preferentially growing along
the vanadyl rows. Further heating to 800 K leads to irreversible surface reduction
which proceeds sequentially V 2 O 5 (001) → V 6 O 13 (001) → V 2 O 3 (0001), as observed
by high-resolution STM [399]. Witko et al. reported theoretical results for (010)
and (100) which are characterised by stronger relaxation effect than (001) facet.
Calculated surface energies were used to obtain a Wulff shape which highly resembles
the equilibrium shape of real V 2 O 5 observed by SEM. It was concluded that even
both (100) and (010) surfaces constitute only 15.5% of the total surface area, they
cannot be excluded from the catalytic processes’ studies.
Interface Modelling
The issue of vanadia dispersion on support was addressed by gas-phase cluster calculations by Sauer et al. [438], where the neutral (V 2 O 5 ) n clusters (n = 1, . . . , 12)
were studied by DFT-GGA calculations and it was shown that gas-phase clusters are
very different from the layer structure of the solid bulk since vanadium is fourfold
coordinated and threefold coordinated oxygen is avoided. For small clusters, the
doubly-bridged OV–O 2 –VO 2 structure is observed, whereas for larger models the
polyhedral oligomeric species with vanadyl groups at the corners and V–O–V bonds
at the edges are expected. Similar geometrical parameters for the vanadium clusters
were also reported by Calatayud et al. [439] (B3LYP) together with topological analysis of the Electron Localisation Function. The latter method was used to understand
383
unit cell parameters are underestimated by ∼1.25% only. Sauer and Vyboishchikov
reported the BP86/D(T)ZVP results for bulk V 2 O 5 which reasonably reproduce both
cell parameters (within 2%) and atomic coordinates (within 2 pm) within the V 2 O 5
layer [435]. The interlayer V–O distance, however, was overestimated by 10 pm,
which may stem from its intermolecular character, and a proper description may
therefore be a challenge for DFT calculations. The strength of the interaction between
the individual layers in the crystal is very low, a few kJ/mol per V 2 O 5 unit [433, 435].
For V 2 O 5 , the natural cleavage plane is (001) surface characterised by the lowest
surface energy thus highest stability comparing to other facets of V 2 O 5 . It was studied
by means of periodic methods and the structural and electronic properties of the (001)
surface, calculated as single-layer slab, are close to those of the bulk [433, 434] in
good agreement with recent experimental results [436, 437]. It was also reported that
the vanadia surface is mostly ionic with the vanadyl bond V–O1c (for labelling, see
Fig. 12b) being mostly covalent and the atomic charges on oxygen atoms decrease
with their coordination: O3c > O2c > O1c for both bulk and (001) surface. This
may suggest a large reactivity of the threefold coordinated oxygen. On the other
hand, the pDOS shows that vanadyl oxygen levels are much closer to the Fermi level
than those of O2c or O3c. Therefore, a large reactivity can be expected for O1 site
[433]. A metal-to-insulator transition of the V 2 O 5 (001) surface was studied by DFT
and Monte Carlo simulations corroborated with in situ STM and band gap mapping,
derived from scanning tunnelling spectroscopy data. The transition restricted to the
surface layers was observed at 350–400 K and occurred anisotropically across the
surface with the formation of vanadyl oxygen vacancies preferentially growing along
the vanadyl rows. Further heating to 800 K leads to irreversible surface reduction
which proceeds sequentially V 2 O 5 (001) → V 6 O 13 (001) → V 2 O 3 (0001), as observed
by high-resolution STM [399]. Witko et al. reported theoretical results for (010)
and (100) which are characterised by stronger relaxation effect than (001) facet.
Calculated surface energies were used to obtain a Wulff shape which highly resembles
the equilibrium shape of real V 2 O 5 observed by SEM. It was concluded that even
both (100) and (010) surfaces constitute only 15.5% of the total surface area, they
cannot be excluded from the catalytic processes’ studies.
Interface Modelling
The issue of vanadia dispersion on support was addressed by gas-phase cluster calculations by Sauer et al. [438], where the neutral (V 2 O 5 ) n clusters (n = 1, . . . , 12)
were studied by DFT-GGA calculations and it was shown that gas-phase clusters are
very different from the layer structure of the solid bulk since vanadium is fourfold
coordinated and threefold coordinated oxygen is avoided. For small clusters, the
doubly-bridged OV–O 2 –VO 2 structure is observed, whereas for larger models the
polyhedral oligomeric species with vanadyl groups at the corners and V–O–V bonds
at the edges are expected. Similar geometrical parameters for the vanadium clusters
were also reported by Calatayud et al. [439] (B3LYP) together with topological analysis of the Electron Localisation Function. The latter method was used to understand