Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
365
or interstitial, forms the localised occupied states in the band gap (shift of absorption
edge up to 2 eV) [209]. The Ti-substitution in rutile does not form the localised states
in the band gap and (in opposition to anatase) does not narrow the band gap.
Nitrogen in most cases adopts the substitutional positions in TiO 2 , in rare cases
it can, however, be present in interstitials [208, 210]. The substitutional N-doping
does not lead to any multi-atom effects [209, 210]. The nitrogen (2 p) states lie a few
hundreds of meV above the valence band top.
Self-organisation
The issue of self-organisation of organic adsorbates on metallic (coinage metals)
surfaces yielding graphene nanoribbons has been extensively studied [211, 212].
However, the rutile (110) surface turned out to be also very perspective and better suited substrate for the bottom-up assembly of such structures. The process of
self-organisation on the substrate of functionalised semiconductor surfaces, e.g. TM
oxides, can be much more tunable, for example by hydration and thus decorating
by protons, than on metal surfaces. On the other hand, the TMO have interesting
optical, photo- and electrochemical properties which can be tuned, e.g. sensitised,
by the adsorbates. Kolmer et al. [213] demonstrated that the presence of surface
hydroxyl groups (protons, particularly) is crucial in the polymerisation of aryl halide
precursors on the (2 × 1) reconstructed rutile TiO 2 (011) surface ([213], Fig. 1C).
The moderate concentration of hydroxyl leads to the formation of long molecular
oligomers. Increasing the hydroxyl coverage of the surface results in the formation
of shorter oligomers and the hydroxyl-free surface suppresses the polymerisation
utterly. Kolmer et al. used 10,10
-dibromo-9,9
-bianthryl (DBBA, see, e.g., [213],
Fig. 1) as a precursor and performed both experimental and computational studies of
the reaction thermodynamics and also the STM microscopy imaging ([213], Fig. 1,
right panel). These studies were based on PW91+D functional. The reaction route on
rutile differs distinctively from the route on metallic substrates: the most important
difference stems from their very different chemical nature and properties emerging
thereof. The concerted process of early C–C bond formation together with late C–Br
bond cleavage followed by the multistep proton-assisted coupling was found to be
the easiest (the lowest activation energy, modelled computationally, below 1.95 eV)
among the several conceivable routes. The heterolytically formed protons, specific
to the metal oxides, transferred from surface hydroxyl groups to DBBA admolecules
(1.39 eV), facilitate the coupling, the subsequent migration of protons on the aromatic
framework is easy (0.65–1.13 eV), and then preferred attachment to the Br-bound
carbon atom occurs. The subsequent relevant weakening of the C–Br bond facilitates
the C–C bond formation and release of Br 2 molecules (0.82 eV).
In the articles of Zasada et al. [214], the issue of terephthalic anhydride monolayer
on rutile (110) surface was studied computationally (Ti 48 O 96 supercell; PW91+D and
PBE+D; 2 × 2 × 1 sampling of IBZ; 400 eV planewave cut-off energy). For the high
coverage limit, several packing domains were tested and similar adsorption energies
were found suggesting possible coexistence of such domains. It was also shown that
365
or interstitial, forms the localised occupied states in the band gap (shift of absorption
edge up to 2 eV) [209]. The Ti-substitution in rutile does not form the localised states
in the band gap and (in opposition to anatase) does not narrow the band gap.
Nitrogen in most cases adopts the substitutional positions in TiO 2 , in rare cases
it can, however, be present in interstitials [208, 210]. The substitutional N-doping
does not lead to any multi-atom effects [209, 210]. The nitrogen (2 p) states lie a few
hundreds of meV above the valence band top.
Self-organisation
The issue of self-organisation of organic adsorbates on metallic (coinage metals)
surfaces yielding graphene nanoribbons has been extensively studied [211, 212].
However, the rutile (110) surface turned out to be also very perspective and better suited substrate for the bottom-up assembly of such structures. The process of
self-organisation on the substrate of functionalised semiconductor surfaces, e.g. TM
oxides, can be much more tunable, for example by hydration and thus decorating
by protons, than on metal surfaces. On the other hand, the TMO have interesting
optical, photo- and electrochemical properties which can be tuned, e.g. sensitised,
by the adsorbates. Kolmer et al. [213] demonstrated that the presence of surface
hydroxyl groups (protons, particularly) is crucial in the polymerisation of aryl halide
precursors on the (2 × 1) reconstructed rutile TiO 2 (011) surface ([213], Fig. 1C).
The moderate concentration of hydroxyl leads to the formation of long molecular
oligomers. Increasing the hydroxyl coverage of the surface results in the formation
of shorter oligomers and the hydroxyl-free surface suppresses the polymerisation
utterly. Kolmer et al. used 10,10
-dibromo-9,9
-bianthryl (DBBA, see, e.g., [213],
Fig. 1) as a precursor and performed both experimental and computational studies of
the reaction thermodynamics and also the STM microscopy imaging ([213], Fig. 1,
right panel). These studies were based on PW91+D functional. The reaction route on
rutile differs distinctively from the route on metallic substrates: the most important
difference stems from their very different chemical nature and properties emerging
thereof. The concerted process of early C–C bond formation together with late C–Br
bond cleavage followed by the multistep proton-assisted coupling was found to be
the easiest (the lowest activation energy, modelled computationally, below 1.95 eV)
among the several conceivable routes. The heterolytically formed protons, specific
to the metal oxides, transferred from surface hydroxyl groups to DBBA admolecules
(1.39 eV), facilitate the coupling, the subsequent migration of protons on the aromatic
framework is easy (0.65–1.13 eV), and then preferred attachment to the Br-bound
carbon atom occurs. The subsequent relevant weakening of the C–Br bond facilitates
the C–C bond formation and release of Br 2 molecules (0.82 eV).
In the articles of Zasada et al. [214], the issue of terephthalic anhydride monolayer
on rutile (110) surface was studied computationally (Ti 48 O 96 supercell; PW91+D and
PBE+D; 2 × 2 × 1 sampling of IBZ; 400 eV planewave cut-off energy). For the high
coverage limit, several packing domains were tested and similar adsorption energies
were found suggesting possible coexistence of such domains. It was also shown that