Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
385
O 2 chemical potentials, dimeric VO x species and then large clusters (V 6 O 15 ) are
expected. The characteristic feature of all these stable species is the presence of
surface vanadyl groups. Importantly, all vibrational frequencies observed for the
vanadia/SiO 2 /Mo(112) model system were reproduced theoretically for the V 4 O 10
cluster anchored to the silica surface by two V–O(2)–Si interface bonds.
Vanadium oxide catalysts are frequently used in selective oxidation reactions, and
it is well established that their catalytic properties depend strongly on their ability
to provide surface oxygen as a reactant following the MvK mechanism. Since V 2 O 5
possesses two kinds of V–O–V bonds and a vanadyl V=O bond, several authors have
tried to relate experimentally the activity and/or selectivity to specific bonds of the
oxide [448–450]. However, despite extended structural and spectroscopic studies,
the role of bond types was not elucidated by experimental approach. In this context,
the theoretical investigation of V 2 O 5 lattice oxygen stability was reported.
Hermann et al. [420] reported the cluster studies of the different oxygen vacancies
at the V 2 O 5 (010) surface, showing that removal of oxygen is energetically demanding while the pre-adsorption of hydrogen can facilitate this process substantially. The
oxygen extraction leads to the local relaxation affecting interlayer binding which may
lead to major reconstruction of the surface as proposed from experiments [451]. The
electronic structure analysis reveals that the VO formation results in chemical reduction of neighbouring vanadium centres, which is expressed by increased vanadium
3d states occupation. The effect of the support for vanadia-based catalysts in terms
of the V=O bond dissociation energetic was also tested [452], and it was shown that
both polyhedral oligomeric vanadia species on silica and isolated vanadia species on
α-Al 2 O 3 have similar V=O bond dissociation energies (250 to 300 kJ mol
–1 ). These
findings explain why the vanadium oxide particles supported on Al 2 O 3 films are
easily covered with vanadyl groups even at UHV conditions [453]. The computational comparison of the reduced states of different phases of V 2 O 5 , γ -V 2 O 5 (001)
and α-V 2 O 5 (001), was performed for wide range of defect concentrations, (1/6 ML
to full ML of defects), with the main conclusion that in reducing environment the
α-V 2 O 5 (001) surface would be fully reduced, whereas the γ -V 2 O 5 -(001) surface is
only partially reduced [454] and, still exposing the surface vanadyl oxygen groups,
may be more reactive in such conditions.
The formation of oxygen vacancies in H 2 -rich environment was also studied [455],
and it was revealed that O 2 does not adsorb on the fully oxidised surface of the catalyst
but reoxidises oxygen vacancies. It was also shown that nucleophilic oxygen O
2−
(s)
is present above 550 K on the catalyst surface and is probably responsible for the
propene formation in the propane oxidative dehydrogenation (ODH) reaction. The
formation of oxygen vacancies on less abundant low-index surfaces of V 2 O 5 was
studied by Goclon et al. [456], and it was concluded that although in equilibrium
only about 15% of the surface area of a crystallite consists of (100) and (010) facets,
these surfaces will make considerable contribution to the activity of V 2 O 5 as an
oxidation catalyst.
385
O 2 chemical potentials, dimeric VO x species and then large clusters (V 6 O 15 ) are
expected. The characteristic feature of all these stable species is the presence of
surface vanadyl groups. Importantly, all vibrational frequencies observed for the
vanadia/SiO 2 /Mo(112) model system were reproduced theoretically for the V 4 O 10
cluster anchored to the silica surface by two V–O(2)–Si interface bonds.
Vanadium oxide catalysts are frequently used in selective oxidation reactions, and
it is well established that their catalytic properties depend strongly on their ability
to provide surface oxygen as a reactant following the MvK mechanism. Since V 2 O 5
possesses two kinds of V–O–V bonds and a vanadyl V=O bond, several authors have
tried to relate experimentally the activity and/or selectivity to specific bonds of the
oxide [448–450]. However, despite extended structural and spectroscopic studies,
the role of bond types was not elucidated by experimental approach. In this context,
the theoretical investigation of V 2 O 5 lattice oxygen stability was reported.
Hermann et al. [420] reported the cluster studies of the different oxygen vacancies
at the V 2 O 5 (010) surface, showing that removal of oxygen is energetically demanding while the pre-adsorption of hydrogen can facilitate this process substantially. The
oxygen extraction leads to the local relaxation affecting interlayer binding which may
lead to major reconstruction of the surface as proposed from experiments [451]. The
electronic structure analysis reveals that the VO formation results in chemical reduction of neighbouring vanadium centres, which is expressed by increased vanadium
3d states occupation. The effect of the support for vanadia-based catalysts in terms
of the V=O bond dissociation energetic was also tested [452], and it was shown that
both polyhedral oligomeric vanadia species on silica and isolated vanadia species on
α-Al 2 O 3 have similar V=O bond dissociation energies (250 to 300 kJ mol
–1 ). These
findings explain why the vanadium oxide particles supported on Al 2 O 3 films are
easily covered with vanadyl groups even at UHV conditions [453]. The computational comparison of the reduced states of different phases of V 2 O 5 , γ -V 2 O 5 (001)
and α-V 2 O 5 (001), was performed for wide range of defect concentrations, (1/6 ML
to full ML of defects), with the main conclusion that in reducing environment the
α-V 2 O 5 (001) surface would be fully reduced, whereas the γ -V 2 O 5 -(001) surface is
only partially reduced [454] and, still exposing the surface vanadyl oxygen groups,
may be more reactive in such conditions.
The formation of oxygen vacancies in H 2 -rich environment was also studied [455],
and it was revealed that O 2 does not adsorb on the fully oxidised surface of the catalyst
but reoxidises oxygen vacancies. It was also shown that nucleophilic oxygen O
2−
(s)
is present above 550 K on the catalyst surface and is probably responsible for the
propene formation in the propane oxidative dehydrogenation (ODH) reaction. The
formation of oxygen vacancies on less abundant low-index surfaces of V 2 O 5 was
studied by Goclon et al. [456], and it was concluded that although in equilibrium
only about 15% of the surface area of a crystallite consists of (100) and (010) facets,
these surfaces will make considerable contribution to the activity of V 2 O 5 as an
oxidation catalyst.