384
W. Piskorz and F. Zasada
the nature of bonding which showed that larger clusters are energetically favourable
rather than the smaller V x O y species and that the V–O bond can be considered as
mostly ionic.
The thermodynamic stability of dimeric V 2 O 5 species on anatase (100) and (001)
perfect surfaces at low coverage was studied by Calatayud et al. [440] with conclusion
that relaxation of the outermost surface layers leads to stabilisation of the covered
slabs, especially for the (001) surface, and seems necessary for the correct description
of the adsorption. The comparison of the electronic properties of the model active
centres for silica- and titania-supported vanadium catalysts was reported by Avdeev
et al. [441], and the results reveal that the weak interface bond is realised on SiO 2
whereas for TiO 2 the strong bonding is expected. The latter results from the structural correspondence of TiO 2 and VO x which promotes the formation of polymeric
oxovanadium groups on the carrier surface by the isomorphic substitution of Ti for V.
Additionally, the TD-DFT method was used to calculate the energies of the excited
states and optical spectra of the adsorbates which enabled for the identification of the
frequencies typical for monomeric and dimeric oxovanadium-supported species. It
was revealed that such amorphous catalyst exhibits increased acidic power compared
to the pure V 2 O 5 and that the V ions embedded in the lower layer are of great importance in this context. The possible weak and strong bonding between TiO 2 support
and V 2 O 5 monolayer were also investigated by ab initio methods [442]. In the former
case, the vanadia monolayer maintains its crystallographic structure; however, the
weak interaction is strong enough to induce a charge redistribution responsible for
vanadium acidity enhancement. In the second model, the presence of the anatase
support alters significantly the structure of the V 2 O 5 monolayer, resulting in a much
higher epitaxy. It was also observed that reactivity of the oxygen sites in H adsorption for the different catalyst models correlates well with the frontier orbital levels
of the metal oxide. The stability of reduced and reoxidated VO x species (following
the MvK cycle) on the reconstructed TiO 2 (001) surface has also been theoretically
investigated [443]. It was revealed that the low binding energy of vanadyl oxygen
species is related to the structure relaxation of the surface VO x species within a monolayer and that the stable peroxide species V(O–O) can be formed on the fully oxidised
VO x /TiO 2 surfaces. Taking into account that functional groups V=O and V–O–V and
their reduced forms V–OH and V–OH–V are the main components of the active sites
in VO x /TiO 2 catalysts [444, 445], Avdeev and Tapilin studied computationally the
effect of water on the electronic structure of active sites of supported vanadium oxide
catalyst [446]. They show that H 2 O dissociates readily on the reduced vanadium sites
forming the stable surface OH groups which are then reoxidated by gas-phase O 2
yielding hydroxylated monolayer. The calculated hydroxylation–hydration reaction
pathway reveals that the lattice oxygen of the surface vanadia species is exchanged
with water oxygen rather than with O 2 .
More involved V 2 O 5 support system of ultrathin SiO 2 /Mo(112) film was described
theoretically in the paper of Todorova et al. [447] where low-coverage differently
anchored vanadia species (monomers and dimers) and clusters have been investigated by means of first-principles thermodynamics. It was revealed that at low
V chemical potentials, monomeric species are stable and with increasing V or
W. Piskorz and F. Zasada
the nature of bonding which showed that larger clusters are energetically favourable
rather than the smaller V x O y species and that the V–O bond can be considered as
mostly ionic.
The thermodynamic stability of dimeric V 2 O 5 species on anatase (100) and (001)
perfect surfaces at low coverage was studied by Calatayud et al. [440] with conclusion
that relaxation of the outermost surface layers leads to stabilisation of the covered
slabs, especially for the (001) surface, and seems necessary for the correct description
of the adsorption. The comparison of the electronic properties of the model active
centres for silica- and titania-supported vanadium catalysts was reported by Avdeev
et al. [441], and the results reveal that the weak interface bond is realised on SiO 2
whereas for TiO 2 the strong bonding is expected. The latter results from the structural correspondence of TiO 2 and VO x which promotes the formation of polymeric
oxovanadium groups on the carrier surface by the isomorphic substitution of Ti for V.
Additionally, the TD-DFT method was used to calculate the energies of the excited
states and optical spectra of the adsorbates which enabled for the identification of the
frequencies typical for monomeric and dimeric oxovanadium-supported species. It
was revealed that such amorphous catalyst exhibits increased acidic power compared
to the pure V 2 O 5 and that the V ions embedded in the lower layer are of great importance in this context. The possible weak and strong bonding between TiO 2 support
and V 2 O 5 monolayer were also investigated by ab initio methods [442]. In the former
case, the vanadia monolayer maintains its crystallographic structure; however, the
weak interaction is strong enough to induce a charge redistribution responsible for
vanadium acidity enhancement. In the second model, the presence of the anatase
support alters significantly the structure of the V 2 O 5 monolayer, resulting in a much
higher epitaxy. It was also observed that reactivity of the oxygen sites in H adsorption for the different catalyst models correlates well with the frontier orbital levels
of the metal oxide. The stability of reduced and reoxidated VO x species (following
the MvK cycle) on the reconstructed TiO 2 (001) surface has also been theoretically
investigated [443]. It was revealed that the low binding energy of vanadyl oxygen
species is related to the structure relaxation of the surface VO x species within a monolayer and that the stable peroxide species V(O–O) can be formed on the fully oxidised
VO x /TiO 2 surfaces. Taking into account that functional groups V=O and V–O–V and
their reduced forms V–OH and V–OH–V are the main components of the active sites
in VO x /TiO 2 catalysts [444, 445], Avdeev and Tapilin studied computationally the
effect of water on the electronic structure of active sites of supported vanadium oxide
catalyst [446]. They show that H 2 O dissociates readily on the reduced vanadium sites
forming the stable surface OH groups which are then reoxidated by gas-phase O 2
yielding hydroxylated monolayer. The calculated hydroxylation–hydration reaction
pathway reveals that the lattice oxygen of the surface vanadia species is exchanged
with water oxygen rather than with O 2 .
More involved V 2 O 5 support system of ultrathin SiO 2 /Mo(112) film was described
theoretically in the paper of Todorova et al. [447] where low-coverage differently
anchored vanadia species (monomers and dimers) and clusters have been investigated by means of first-principles thermodynamics. It was revealed that at low
V chemical potentials, monomeric species are stable and with increasing V or