Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
355
The comparison of the performance of pure GGA (PBE), PBE+U(4.5), hybrid
HSE06, and van der Waals functional vdW-DF+U on the adsorption of water on
the (111) surface of ceria is found in the article by Fernández-Torre et al. [132]
They considered a few associative two dissociative geometries of adsorbed waters
and found, based on their own results and those reviewed from the literature, that
the inclusion of the van der Waals interactions increases (to the absolute value) the
binding energy of water by ca. 0.18 eV per molecule but do not change the water
overall binding behaviour. They also show that water adsorbs on top of a Ce
4+ atom
either as a molecule (forming one hydrogen bond with the surface O atom) or as
a hydroxyl pair, and these states differ very slightly in energy (in agreement with
[133], where PW91+U(5.0) is used) so both forms—associated and dissociated—can
coexist.
More involved and unbiased approach, namely the ab initio-MD (AIMD), within
the DFT+U(7.0) level of theory (PBE functional, pseudopotentials, double-ζ Gaussian basis set with auxiliary planewave basis set, Γ -point integration of irreducible
Brillouin zone), was used by Ren et al. [128] who studied three low-index surfaces of
ceria, (111), (110), and (100), in contact with vapour and liquid water. They conclude,
based on the Wulff construction, that in the vapour phase at lower T two low-index
partially hydroxylated surfaces, (111) and (100), dominate and the contribution of
(100) is lowered for the nanograins at higher temperatures, when the grain becomes
dehydrated. In the aqueous conditions, however, the hydroxylated (111) surface is
dominant.
Formation of O Vacancy
The energy of formation of O vacancy on the surface is relevantly lower than in
the bulk [102] and is significantly dependent on the exposed face, the lowest is
for the (110) face [105]. The structure of the surface vacancy, crucial in the catalytic
processes, is non-trivial. For years it was believed that the localisation of the electrons
emerging from the formation of the oxygen vacancy, i.e., the position of the reduced
Ce
3+ cations, was next to the vacancy. As was shown by, e.g., Ganduglia-Pirovano
et al. [101], there are multiple structures of almost identical stabilities (differences
within the range of a few tenths of eV) in the first to fourth coordination sphere and
in the first to second cationic layer [81]. Such mobile O vacancies can be formed by
doping with aliovalent cations [134], by exposition to the oxygen-lean conditions at
high temperatures [135–137], or by chemical reduction [138].
The electronic state of Ce in the reduced CeO 2 surface has been devoted a number
of experimental articles. The evidences for the existence of Ce
3+ in the surface or
intrafacial region of ceria have been obtained by means of XPS, (HR) EELS [139–
141], IR [111, 142], and EPR [143]. The experiments show that in the reduced ceria
the electrons reducing Ce
4+ to Ce
3+ are localised at the 4 f orbitals. The morphology
of the defects have been studied by STM, and it was shown that the oxygen defects
created thermally on the (111) surface are either isolated or form the aggregates
[121, 122, 144]. The computational studies of the surface of reduced CeO 2 clearly
355
The comparison of the performance of pure GGA (PBE), PBE+U(4.5), hybrid
HSE06, and van der Waals functional vdW-DF+U on the adsorption of water on
the (111) surface of ceria is found in the article by Fernández-Torre et al. [132]
They considered a few associative two dissociative geometries of adsorbed waters
and found, based on their own results and those reviewed from the literature, that
the inclusion of the van der Waals interactions increases (to the absolute value) the
binding energy of water by ca. 0.18 eV per molecule but do not change the water
overall binding behaviour. They also show that water adsorbs on top of a Ce
4+ atom
either as a molecule (forming one hydrogen bond with the surface O atom) or as
a hydroxyl pair, and these states differ very slightly in energy (in agreement with
[133], where PW91+U(5.0) is used) so both forms—associated and dissociated—can
coexist.
More involved and unbiased approach, namely the ab initio-MD (AIMD), within
the DFT+U(7.0) level of theory (PBE functional, pseudopotentials, double-ζ Gaussian basis set with auxiliary planewave basis set, Γ -point integration of irreducible
Brillouin zone), was used by Ren et al. [128] who studied three low-index surfaces of
ceria, (111), (110), and (100), in contact with vapour and liquid water. They conclude,
based on the Wulff construction, that in the vapour phase at lower T two low-index
partially hydroxylated surfaces, (111) and (100), dominate and the contribution of
(100) is lowered for the nanograins at higher temperatures, when the grain becomes
dehydrated. In the aqueous conditions, however, the hydroxylated (111) surface is
dominant.
Formation of O Vacancy
The energy of formation of O vacancy on the surface is relevantly lower than in
the bulk [102] and is significantly dependent on the exposed face, the lowest is
for the (110) face [105]. The structure of the surface vacancy, crucial in the catalytic
processes, is non-trivial. For years it was believed that the localisation of the electrons
emerging from the formation of the oxygen vacancy, i.e., the position of the reduced
Ce
3+ cations, was next to the vacancy. As was shown by, e.g., Ganduglia-Pirovano
et al. [101], there are multiple structures of almost identical stabilities (differences
within the range of a few tenths of eV) in the first to fourth coordination sphere and
in the first to second cationic layer [81]. Such mobile O vacancies can be formed by
doping with aliovalent cations [134], by exposition to the oxygen-lean conditions at
high temperatures [135–137], or by chemical reduction [138].
The electronic state of Ce in the reduced CeO 2 surface has been devoted a number
of experimental articles. The evidences for the existence of Ce
3+ in the surface or
intrafacial region of ceria have been obtained by means of XPS, (HR) EELS [139–
141], IR [111, 142], and EPR [143]. The experiments show that in the reduced ceria
the electrons reducing Ce
4+ to Ce
3+ are localised at the 4 f orbitals. The morphology
of the defects have been studied by STM, and it was shown that the oxygen defects
created thermally on the (111) surface are either isolated or form the aggregates
[121, 122, 144]. The computational studies of the surface of reduced CeO 2 clearly