Catalytic Properties of Selected Transition Metal Oxides—Computational Studies
357
O Chemisorption
The process of chemisorption of O 2 was studied both computationally and experimentally by the Raman spectroscopy [153], which can distinguish between reactive
oxygen species of different O-O bond length and thus different stretching frequency,
i.e. superoxo (O
•−
2 ) and peroxo (O
2−
2 ). The former can be also identified by the EPR
spectroscopy [153]. The mostly dispersive nature of the species adsorption on the
pristine surface can be deduced from the work of Huang et al. where the adsorption energy obtained for the PBE+U method was negligible [154], in line with the
fact that the dispersion effects were not included in the computational method. The
adsorption on the defected (111) surface, however, is exoenergetic by 1.72 eV and the
peroxy group is formed: [2Ce
3+
, V O ] + O 2 → [2Ce
4+
, O
2−
2 ]—this is the process
of healing of oxygen defects upon reoxidation. The detachment of the O atom from
the peroxy group costs ca. 2.6 eV, twice less than the removal of oxygen from pristine
(111) surface (not counting the O 2 association energy) [81]. On the other hand, Hu et
al. [155] reported the formation of superoxo group (confirmed by EPR [143]), thus
only one Ce
3+ ion is reoxidised: [2Ce
3+
, V O ] + O 2 → [Ce
4+
, Ce
3+
, O
•−
2 ].
Catalysis
Among the processes catalysed by ceria, the most important industrially are the watergas shift (WGS, CO + H 2 O → CO 2 + H 2 ), the selective NO x reduction (SCR) by
ammonia, methanation of CO 2 (Sabatier reaction, CO 2 + 4 H 2 → CH 4 + 2 H 2 O,
which is the combination of the reversed water-gas shift reaction, H 2 + CO 2 → CO +
H 2 O, and a CO methanation, 3 H 2 + CO → CH 4 + H 2 O [156]).
The WGS process is industrially used for the production of hydrogen and for the
CO removal from the feed stream for fuel cells [157] or ammonia synthesis [158]. In
the computational study of Bruix [159] on the Pt nanoparticles on ceria, the following
parameters were set: PW91+U(4.0) functional, the energy cut-off of 415 eV. The slab
of three CeO 2 (nine atomic) layers and a 3 × 4 surface cell was used as a model. The
separating vacuum layer was 15 Å thick. The atoms in the bottom O–Ce–O layer were
fixed at the bulk positions during the geometry optimisation. The conclusion of the
computational part of Bruix et al. article [159] is that ceria influences the electronic
structure of the small noble metal (Pt) particles in such a way that the O-H bond
dissociation is facilitated to such extent that when moving from Pt(111) surface
to the Pt 8 clusters on CeO 2 (111), the dissociation of water becomes exoenergetic
(ΔE reac = −0.24 to −0.32 eV), what is beneficial comparing to other WGS catalytic
systems (e.g. for Pt(111) ΔE reac = +0.65 eV, for Pt 79 cluster ΔE reac = −0.03 eV).
This property was attributed to the exposition of low-coordinative Pt atoms and to
the high flexibility of the nanoparticle.
The similar conclusion about non-sufficient activity of Pt was taken by Flaherty
et al. [160], who modelled computationally the WGS on Pt(111) surface concluding
that CO dissociates via two pathways, the Boudouard reaction (disproportionation
357
O Chemisorption
The process of chemisorption of O 2 was studied both computationally and experimentally by the Raman spectroscopy [153], which can distinguish between reactive
oxygen species of different O-O bond length and thus different stretching frequency,
i.e. superoxo (O
•−
2 ) and peroxo (O
2−
2 ). The former can be also identified by the EPR
spectroscopy [153]. The mostly dispersive nature of the species adsorption on the
pristine surface can be deduced from the work of Huang et al. where the adsorption energy obtained for the PBE+U method was negligible [154], in line with the
fact that the dispersion effects were not included in the computational method. The
adsorption on the defected (111) surface, however, is exoenergetic by 1.72 eV and the
peroxy group is formed: [2Ce
3+
, V O ] + O 2 → [2Ce
4+
, O
2−
2 ]—this is the process
of healing of oxygen defects upon reoxidation. The detachment of the O atom from
the peroxy group costs ca. 2.6 eV, twice less than the removal of oxygen from pristine
(111) surface (not counting the O 2 association energy) [81]. On the other hand, Hu et
al. [155] reported the formation of superoxo group (confirmed by EPR [143]), thus
only one Ce
3+ ion is reoxidised: [2Ce
3+
, V O ] + O 2 → [Ce
4+
, Ce
3+
, O
•−
2 ].
Catalysis
Among the processes catalysed by ceria, the most important industrially are the watergas shift (WGS, CO + H 2 O → CO 2 + H 2 ), the selective NO x reduction (SCR) by
ammonia, methanation of CO 2 (Sabatier reaction, CO 2 + 4 H 2 → CH 4 + 2 H 2 O,
which is the combination of the reversed water-gas shift reaction, H 2 + CO 2 → CO +
H 2 O, and a CO methanation, 3 H 2 + CO → CH 4 + H 2 O [156]).
The WGS process is industrially used for the production of hydrogen and for the
CO removal from the feed stream for fuel cells [157] or ammonia synthesis [158]. In
the computational study of Bruix [159] on the Pt nanoparticles on ceria, the following
parameters were set: PW91+U(4.0) functional, the energy cut-off of 415 eV. The slab
of three CeO 2 (nine atomic) layers and a 3 × 4 surface cell was used as a model. The
separating vacuum layer was 15 Å thick. The atoms in the bottom O–Ce–O layer were
fixed at the bulk positions during the geometry optimisation. The conclusion of the
computational part of Bruix et al. article [159] is that ceria influences the electronic
structure of the small noble metal (Pt) particles in such a way that the O-H bond
dissociation is facilitated to such extent that when moving from Pt(111) surface
to the Pt 8 clusters on CeO 2 (111), the dissociation of water becomes exoenergetic
(ΔE reac = −0.24 to −0.32 eV), what is beneficial comparing to other WGS catalytic
systems (e.g. for Pt(111) ΔE reac = +0.65 eV, for Pt 79 cluster ΔE reac = −0.03 eV).
This property was attributed to the exposition of low-coordinative Pt atoms and to
the high flexibility of the nanoparticle.
The similar conclusion about non-sufficient activity of Pt was taken by Flaherty
et al. [160], who modelled computationally the WGS on Pt(111) surface concluding
that CO dissociates via two pathways, the Boudouard reaction (disproportionation