Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface
The paper presents the results of quantum chemical calculations using the extended Huckel method for adsorption of O₂ and CO molecules on BaTiO₃ surface. It is shown that on (001) surface, the physical adsorption is dominant, whereas on (002) surface the chemical adsorption prevails, the Ti ions...
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| Zitieren: | Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface / I.D. Sejkovskij, V.N. Zhiharev, Yu.V. Popik // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 281-292. — Бібліогр.: 11 назв. — англ. |
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irk-123456789-1207172017-06-13T03:03:13Z Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface Sejkovskij, I.D. Zhiharev, V.N. Popik, Yu.V. The paper presents the results of quantum chemical calculations using the extended Huckel method for adsorption of O₂ and CO molecules on BaTiO₃ surface. It is shown that on (001) surface, the physical adsorption is dominant, whereas on (002) surface the chemical adsorption prevails, the Ti ions being the most active adsorption centres. The possible dissociation mechanisms of the adsorbed oxygen molecule and a heterogeneous oxidation reaction of CO in CO₂ are considered. У роботі наведено результати квантово-хімічних розрахунків розширеним методом Хюккеля адсорбції молекул O₂, CO₂, CO та атомів O і C на поверхні BaTiO₃. Показано, що на поверхні (001) переважає фізична адсорбція, а на поверхні (002) – хімічна, причому найбільш активними центрами адсорбції є іони Ti. Розглянуті можливі механізми дисоціації адсорбованих молекул кисню і гетерогенної реакції окислення CO в CO₂. 2003 Article Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface / I.D. Sejkovskij, V.N. Zhiharev, Yu.V. Popik // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 281-292. — Бібліогр.: 11 назв. — англ. 1607-324X PACS: 77.84.-s, 77.84.Dy DOI:10.5488/CMP.6.2.281 http://dspace.nbuv.gov.ua/handle/123456789/120717 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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The paper presents the results of quantum chemical calculations using
the extended Huckel method for adsorption of O₂ and CO molecules on
BaTiO₃ surface. It is shown that on (001) surface, the physical adsorption
is dominant, whereas on (002) surface the chemical adsorption prevails,
the Ti ions being the most active adsorption centres. The possible dissociation
mechanisms of the adsorbed oxygen molecule and a heterogeneous
oxidation reaction of CO in CO₂ are considered. |
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Article |
| author |
Sejkovskij, I.D. Zhiharev, V.N. Popik, Yu.V. |
| spellingShingle |
Sejkovskij, I.D. Zhiharev, V.N. Popik, Yu.V. Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface Condensed Matter Physics |
| author_facet |
Sejkovskij, I.D. Zhiharev, V.N. Popik, Yu.V. |
| author_sort |
Sejkovskij, I.D. |
| title |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface |
| title_short |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface |
| title_full |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface |
| title_fullStr |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface |
| title_full_unstemmed |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface |
| title_sort |
calculation of adsorption and interaction mechanisms of o₂ and co₂ molecules on batio₃ surface |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2003 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120717 |
| citation_txt |
Calculation of adsorption and interaction mechanisms of O₂ and CO₂ molecules on BaTiO₃ surface / I.D. Sejkovskij, V.N. Zhiharev, Yu.V. Popik // Condensed Matter Physics. — 2003. — Т. 6, № 2(34). — С. 281-292. — Бібліогр.: 11 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT sejkovskijid calculationofadsorptionandinteractionmechanismsofo2andco2moleculesonbatio3surface AT zhiharevvn calculationofadsorptionandinteractionmechanismsofo2andco2moleculesonbatio3surface AT popikyuv calculationofadsorptionandinteractionmechanismsofo2andco2moleculesonbatio3surface |
| first_indexed |
2025-07-08T18:27:34Z |
| last_indexed |
2025-07-08T18:27:34Z |
| _version_ |
1837104372326072320 |
| fulltext |
Condensed Matter Physics, 2003, Vol. 6, No. 2(34), pp. 281–292
Calculation of adsorption and
interaction mechanisms of O2 and CO2
molecules on BaTiO3 surface
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
Uzhgorod National University, 54 Voloshin Str., 88000 Uzhgorod, Ukraine
Received October 16, 2002, in final form January 11, 2003
The paper presents the results of quantum chemical calculations using
the extended Huckel method for adsorption of O2 and CO molecules on
BaTiO3 surface. It is shown that on (001) surface, the physical adsorption
is dominant, whereas on (002) surface the chemical adsorption prevails,
the Ti ions being the most active adsorption centres. The possible dissoci-
ation mechanisms of the adsorbed oxygen molecule and a heterogeneous
oxidation reaction of CO in CO2 are considered.
Key words: surface, electron density, semiempirical quantum chemical
methods and calculations, adatoms, clusters, barium titanate BaTiO3
PACS: 77.84.-s, 77.84.Dy
1. Introduction
We have carried out the calculations for interaction of the adsorbate molecules
with every surface point of clusters simulating the surface of BaTiO3 crystal; and
have built the energy surfaces corresponding to potential energy minimum of a
complex “adsorbed molecule+cluster” (Eads = Emin − E
∞
).
The calculations were carried out using the Extended Huckel method (EHM)
which takes into account only the electronic interaction energy. Here, a base pro-
gram “Paramol-1” updated by us was used [1]. Earlier we had used this method to
calculate the adsorption of oxygen O2 and hydrogen H2 on small clusters of BaTiO3.
We have also accounted for the interaction of thermal sputtering vapour components
with the substrate surface in deposition and formation process of compound films
[2–4]. The modified EHM was used in [5] to calculate the geometry of polyatomic
molecules. The use of large clusters resulted in a considerable decrease of the “edge”
effects on the character of interaction of an adsorbed molecule with the adsorption
centre. When choosing the clusters of BaTiO3 single crystal surface, the shifts of
ferroelectric and surface atoms were taken into account [6,7]. While calculating the
adsorption, the orientation of the adsorbate molecules relative to the adsorbent sur-
c© I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik 281
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
face is to be taken into account. In case of adsorption of a two-atomic molecule O2
and CO we have considered two basic orientations: a) a molecule axis is parallel to
the adsorbent surface plane (s-type orientation adsorption); b) a molecule axis is
normal to the adsorbent surface (c-type orientation adsorption). There are two pos-
sible orientations for CO molecule: by oxygen or carbon atoms towards the surface
(the adsorption centre). The similar calculations for MgO and SnO2 surfaces, ac-
complished ab initio like in the Hartree-Fock approximation, are presented in papers
[8,9].
The “TiO2” surface layer of BaTiO3 single crystal with TiO2 adsorbing plane
(a crystallographic plane (00 1
2
) was modelled by a cluster of 32 atoms: 16 atoms of
TiO2 plane, 13 atoms of an underlying BaO plane, two O atoms and Ba atom of
still more underlying planes for symmetry and electroneutrality of the cluster (106
valence electronic orbitals). The second cluster with BaO adsorbing plane consisted
of 48 atoms: 18 Ba atoms, 26 O atoms and 4 Ti atoms (146 valence electronic
orbitals). The coordinate axes X and Y are in the plane of the cluster surface and
Z axis is normal to it coinciding with C axis of the crystal.
The calculations have shown the “BaO” cluster surface to be poorly active for
adsorption; here the physical adsorption of O2 and CO molecules dominates, which
weakly influences the electric and physical properties of BaTiO3 single crystal and
the strongest electronic density redistribution takes place at adsorption on “TiO2”
surface. Therefore, we present the results of the calculations of the interaction of the
adsorbed molecules with various points of “TiO2” plane of BaTiO3 adsorber as well
as the possibility of O2 dissociation and a heterogeneous oxidation reaction of CO
in CO2.
2. Theoretical analysis
2.1. Adsorption of O2 and CO on the atomically pure surface
Figure 1 shows the minimum interaction energy surface of O2 adsorbed molecule
(c-type) with the “TiO2” cluster surface and the surface of the corresponding mini-
mum distances Rmin. It is obvious that Emin and Rmin sharply decrease in the vicinity
of Ti atoms, changing slightly above the interstitial sites and oxygen atoms of the
cluster. The potential wells in the vicinity of Ti are practically of a conic form. The
potential energy Emin sharply increases moving away for over 0.05 nm from Ti atom
along X and Y axes that testifies to a distinct direction of binding O2 absorbed
molecule and Ti atom of the cluster.
We have also carried out similar calculations for O2 s-type adsorption and CO
adsorption on the atomically pure plane of the “TiO2” cluster. The values of binding
energies of the adsorbed molecules with the cluster (Ebinds = |Eads|) and their
corresponding equilibrium distances from the surface adsorption centre are given in
table 1.
The comparison of the data in figure 1 and in table 1 shows that the most
energetically favourable adsorption centres for O2 and CO molecules on the “TiO2”
282
Adsorption and interaction of O2 and CO2 on BaTiO3
Figure 1. The minimum potential energy surfaces and the corresponding mini-
mum distances (Rmin) at the interaction of O2 molecule (c-type orientation) with
the atomic pure surface of the “TiO2” cluster of BaTiO3 single crystal. (In figure
the coordinates of Ti atoms are X,Y = ±0.2 nm).
cluster surface are Ti atoms with c-type adsorption geometry and for CO molecules –
by the oxygen downward (closer to Ti atom). The most pronounced redistribution of
the electron density both inside adsorbate molecules and between the adsorbed ones
takes place and the surface also corresponds to this localization geometry (table 2)
[10]. Below, a possibility of dissociation of O2 molecule bound with Ti atom is
considered.
The dissociation of O2 molecule along Z-coordinate after a c-type adsorption on
Ti (Rads = 0.1 nm) has been considered: one of the molecule atoms (O2) moves away
from the other (O1, localized above Ti) in the direction normal to the surface; O1 in
its turn changes the position above the surface reaching the potential energy mini-
Table 1. The energy (in eV) and length (in nm) of the adsorbed molecules bond
on the most energetically favourable localization centres of “TiO2” surface for
BaTiO3 single crystal.
Locali- O2 molecule adsorption CO molecule adsorption
zation s-type c-type s-type c-type c-type
centre O downward C-downward
Eads Rads Eads Rads Eads Rads Eads Rads Eads Rads
Ti –0.25 0.21 –0.77 0.10 –0.29 0.22 –0.65 0.14 –0.28 0.16
O –0.09 0.26 –0.03 0.28 –0.12 0.25 –0.04 0.29 –0.02 0.32
M –0.06 0.24 –0.04 0.24 –0.06 0.25 –0.05 0.23 –0.04 0.29
283
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
Table 2. Charges Q (in electron charge units, e) of localization centres and the
atoms of molecules adsorbed on a “pure” “TiO2” surface of BaTiO3 crystal.
O2 adsorption CO adsorption, c-type CO adsorption, s-type
Atom c-type s-type Atom O-down- C-down- C-down- Charge Atom Charge
ward ward ward
Ti +3.61 +3.880 Ti +3.75 +3.74 intersti- +3.86 Olat –1.93
tial site
O1 +0.27 –0.014 C +0.43 +0.56 +0.448 +0.45 C +0.46
O2 –0.15 –0.014 O –0.30 –0.49 –0.441 –0.45 O –0.46
O2 +0.12 –0.028 CO +0.13 +0.07 +0.007 –0.04 CO 0.006
Charges before the adsorption: Q(Olat) = −1.943, Q(CO) = 0(±0.441)
mum of the adsorption complex. The surface of minimum energies in Z-coordinates
of O1 and O2 atoms was calculated. It was obtained that if O2 atom moves away
from the surface, O1 atom approaches the surface, the depth of a dissociation poten-
tial well decreases approximately from 2.4 eV (note that the dissociation energy of
O2 isolated molecule is equal to 4 eV) to ≈ 1.6 eV, and Rbond(O1) = 0.05–0.06 nm
(above Ti). If we take into account the energy equal to about 1 eV that can be
liberated during adsorption and given to the molecule, then the additional dissocia-
tion energy becomes equal to 0.6–0.7 eV. This is much less compared to the isolated
molecule dissociation, but total energy of the adsorption complex after desorption
of O2 becomes larger (+0.3 eV) than before the interaction of O2 with the surface.
According to EHM evaluations, the O1 atom becomes almost neutral (its charge
is +0.2e and just −0.2e goes to Ti), and the desorbed O2 becomes doubly charged
(it is given −0.5e from each of the four nearest O2− ions of the TiO2 surface) due to
the closeness to Ti and due to strong overlap of the electron orbitals. The formation
and desorption of such an ion are hardly possible. Therefore, we have also studied
some other possible ways of O2 dissociation.
At the O2 absorption of s-type above Ti atom or above Olat oxygen atom of the
cluster by O1 and O2 orientation in the direction of Ti atoms, the O2 dissociation also
becomes possible without the desorption of oxygen atom but with the localization
of O1 and O2 atoms in a form of O− ions in the vicinity of Olat (the free migration
radius of Oad above the atoms along the surface equals about 0.1 nm, Rads ≈ 0.16 nm,
Ebond = 0.4 eV, Q = −1e – figure 2) or, in the second case, approximately above
the centre of the O–Ti bonds along the O–Ti–O line (see Y -coordinate in figure 2)
at the height Rads ≈ 0.1 nm (Ebond = 1.1 eV, Q = −0.85e). In both cases the energy
barrier height equals ≈1 eV and with the account of the adsorption energy (see
table 1) for the O2 dissociation, an energy of Edis ≈ 0.7–0.8 eV is needed. The Oad
atom is charged by Olat electrons. In the first case Oad–Olat bond is like O2 with
−2e charge, and in the second case a complex O1–Olat–O2 (or O1–Ti–O2) is formed
with a geometry similar to the water molecule H2O (H–O–H). After dissociation, the
energy Emin of “O–O-cluster” adsorption complex decreases to −0.4 eV (after O1
284
Adsorption and interaction of O2 and CO2 on BaTiO3
and O2 bond formation with Olat) or before dissociation −1.1 eV (after localization
of O1 and O2 between Ti and Olat).
Figure 2. Potential curves of the interaction of O1 and O2 atoms of O2 oxygen
molecule with the cluster of “TiO2” surface: 1) at O1–O2 molecule dissociation
along the surface on Y coordinate after its s-type localization above Ti (the
numbers near points show the height (z) of O1 and O2 atoms above the surface
plane; 2) at O2 atom desorption (the upper axis of Z-coordinates, the numbers
near • points show the height z above O of atom O1 that remained; 3) at O2
desorption after the two atoms localization (O1 and O2) in Oad position on both
sides of Ti (curves series, Z-coordinates axis). 4,5) the behavior of oxygen atoms
charge of O2 molecule (4) and the oxygen atom of the Olat lattice (5) which is
approached by the molecule O2 atom in the process of its dissociation (see “1”).
After O2 dissociation of s-type the activation desorption of one of the two oxygen
ions can take place. Consequently, a free O2− ion is formed and bound with the
nearly neutral surface oxygen atom Oad. If O1 and O2 ions were situated in the
vicinity of their neighbouring Olat, then one of them, getting about 0.65 eV, would
be desorbed and the other one as a result of a charge loss, would be bound to the
surface atom O2− (Rads = 0.11 nm, total energy drop of “molecule-cluster” system
Emin = −0.7 eV). If O1 and O2 ions are definitely localized on Ti–O bond, then for
desorption of O2, for example, 0.4–0.5 eV are needed with the simultaneous shift
of O1 closer to Ti–O bond centre and decrease in height to Rads ≈ 0.01 nm, and
the system energy decreases to Emin = −1.5 eV. Thus, we see that if in the first
285
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
Figure 3. The chart of isoenergies migration levels of a dissociated oxygen atom
(O1) along the “TiO2” cluster of BaTiO3 surface. The second atom (O2) of O2
molecule after its dissociation is localized above the Olat atom of the surface. The
region of negative energies are shadowed. M – interstitial site.
case the desorption barrier exceeds “zero” energy E
∞
by ≈ +0.3 eV, then in the
second case it does not. So, the desorption here is energetically more favourable.
Both after the dissociation and the desorption of one of O2 molecules for the atoms
which remained Oad, it becomes energetically favourable to localize above Ti–Olat
surface bond (figure 3, dark regions between Ti and O).
2.2. Adsorption of CO molecules at the presence of a chemosorbed oxygen
on the surface
In practice, CO adsorption takes place on the surface that is already partially
filled with molecules of other gases, in particular, oxygen molecules which have a
large binding energy of c-type on Ti atoms (see figure 1 and table 1). Thus, we have
calculated the potential interaction energy curves of CO molecules with adsorption
centres O, Ti and M (interstitial site) of the “TiO2” surface cluster of BaTiO3 single
crystal in the presence of O2 c-type localized molecule on the neighbouring titanium
atom at a distance of Rmin = 0.11 nm, showing the possibility of a direct localization
of CO on the adsorbed molecule O2.
For CO adsorption by s-type orientation on the partially oxidated TiO2 plane,
the potential energy minimum for the system “adsorbed molecule-cluster” is not
reached. This energy minimum is attained only in case of c-type orientation – at CO
286
Adsorption and interaction of O2 and CO2 on BaTiO3
Figure 4. c-type adsorption of CO molecule by C atom to the surface on the
adsorption centres (M, Ti, On, Od) of “TiO2” cluster of BaTiO3 surface and on
the oxygen (O2) adsorbed on the surface, which is also c-type localized above
the titanium atom (by O1 atom to Ti, Rmin(O2 − Ti) = 0.11 nm); on the right
ordinate the change in the charges of CO molecule atoms (Q(C−) and Q(−O))
and O2 (Q(O1) and Q(O2) at CO approaching O2 is shown; curve 5 – CO ad-
sorption on Ti by O atom to titanium; curve 6 – CO2 desorption (i.e. O2–CO)
from the surface after the interaction of CO with O2 (O1 oxygen atom remains
bound with Ti).
localization on Ti the minimum is observed for both orientations of the adsorbed
molecule, while for the rest of adsorption centres – only at CO orientation with
carbon nearer to the surface. Thus, in the presence of an adsorbed oxygen on the
surface, the energetically favourable is c-type adsorption of CO, which, however,
has the opposite orientation in comparison with the adsorption on “TiO2” “pure”
surface. The results of these evaluations are given in figure 4. The curve of Od is
calculated for the case of CO localization on the surface oxygen atom distant from Ti
atom with which O2 molecule is c-type bound; the curve of On is calculated for the
interaction of CO with the oxygen atom of lattice nearest to Ti atom. It is seen that
the binding energy of CO with such an “unclean” surface increases almost 10 times.
The strongest decrease in the potential energy of the “adsorbed molecule-cluster”
system is observed at the interaction of CO with O2 c-type molecule localized on
Ti. Possible mechanisms of the oxidation of CO into CO2 are presented below.
287
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
2.3. Low degree of the surface covering with the adsorbate molecules (O2
and CO); a non-dissociative form of oxygen adsorption
Let CO molecule on Ti2 atom be adsorbed, then on the nearest neighbouring
Ti1 atom O2 molecule of c-type is localized. For such a succession, the energetically
favourable is c-type of CO orientation with O atom to Ti (Eads = −0.65 eV, see
table 1). In the opposite case more probable is CO localization with carbon atom
to titanium (see figure 4, Eads = −1.2 eV), although the binding energy of opposite
orientation is also large, approximately 0.7 eV (curve 5, figure 4). The oxygen atom
of CO molecule is 0.14 nm away from Ti2 atom, R(C–O) = 0.11 nm, and O1
atom of O2 adsorbed molecule is at a distance of 0.10 nm from Ti1. An equilibrium
distance between O1–O2 of O2 adsorbed molecule is equal to 0.16 nm (whereas
far from the surface it is 0.12 nm). Thus, the O2 atom is 0.26 nm away from Ti1.
The “extension” of O2 molecule favours its dissociation. Along the coordinate Z,
this leads to desorption of O2 atom with the activation energy of 0.7 eV. Along
the coordinate Y (or X), the O2 atom migrates along the surface (figure 5) to CO
molecule adsorbed on titanium with the formation along the axis Z (normal to
the adsorbing plane) of Ti–O–C–O complex, i.e. CO2 adsorbed molecule. It is seen
that the system energy remains practically unchanged with quite large (±0.11 nm)
distances of O2 atom from a geometric centre above Ti1 atom, and permits it to
approach CO “without difficulty”. To form CO2 adsorbed molecule (Ti2–O–C–O2)
O2 atom needs to overcome a potential barrier about 0.6 eV high which is followed by
an energy well, 8 eV deep (i.e. the reaction of oxidation of CO into CO2). However,
the bond of CO2 with Ti is comparatively weak, Ebond ≈ 0.4 eV.
2.4. Adsorption of CO with dissociative adsorption of oxygen on the “TiO2”
cluster surface
As stated above, O2 adsorbed molecule after having received additional energy
of 0.6–0.8 eV in the process of adsorption, can dissociate with localization of Oad on
Ti or approximately above the binding line centre of the surface atoms Ti and O at
a distance of 0.09 nm from the surface. The second case of dissociation process is
energetically more probable. For this geometry of Oad, the calculations of CO inter-
action with the surface show that the potential energy minimum is observed neither
at CO localization in the interstitial sites nor at s-type orientation on titanium or
oxygen atoms. The energy minimum is attained only for c-type CO localization on
O and Ti atoms of the surface, CO adsorption being of an activation character: at
the adsorption on the lattice oxygen, the barrier height is the largest (≈ 1.8 eV),
and on Ti atom it is the smallest (≈ 1 eV). In the latter case, an interesting effect
was found: after c-type CO localization on Ti at a distance Rbond = 0.17 nm, cor-
responding to Emin ≈ −1.8 eV, the movement of the adsorbed molecule becomes
energetically favourable. It is almost parallel to the Ti–Olat surface in the direction
towards the atomically adsorbed ion of Oad oxygen at the height Rads = 0.09 nm
above the “TiO2” surface and thus forms CO2. The total energy drop of the system
is 3.7 eV. After having overcome a somewhat higher barrier (≈ 1.5 eV) CO molecule
288
Adsorption and interaction of O2 and CO2 on BaTiO3
Figure 5. The minimum energy surface at dissociative movement above the
“TiO2” surface of BaTiO3 crystal of O2 atom of O2 oxygen molecule c-type
localized above the Ti atom to CO molecule which is also c-type bound with the
neighboring Ti atom (with O atom to Ti).
can also be localized directly above Oad with Rbond ≈ 0.08 nm (Emin ≈ −3.7 eV). In
both cases, the desorption of CO2 molecule being formed is energetically favourable.
2.5. CO adsorption at a high coverage of the “TiO2” surface with oxygen
molecules
In such a case, a molecular form of c-type oxygen absorption on Ti atoms prevails
(Ti–O1–O2). Energetically more favourable localization centre of CO is the O2 atom
of c-type adsorbed oxygen (i.e. O2 adsorbed molecule) with CO2 molecule formation
(O2–C–O, curve “O2” in figure 4). As it is seen from the curve 6 of the same figure,
the simulation of a detachment of O–C–O2 molecule (i.e. CO2) from Ti–O1 bond
is accompanied by a further energy decrease of the system, and CO2 molecule can
either stay on the surface being localized above Ti–O1 bond with Rads = 0.14 nm and
the energy decreases in the system additionally by 1 eV (total energy drop is 9.1 eV),
or desorbs having overcome the potential barrier of 2 eV, which is considerably less
than total energy drop due to CO molecule adsorption and its subsequent oxidation
to CO2.
3. Conclusion
To explain the experimental data concerning the effect of the adsorption on elec-
trical and physical properties of the solid states, it is important to know the charge
289
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
Table 3. The charges (in e units) of atoms O2 and CO molecules in the process of
CO2 desorption (figure 4, curve 6) from the surface of “TiO2” cluster of BaTiO3
crystal.
R, nm Ti O1 Olat O2 C O CO2
0.110 3.457 –0.592 –1.950 0.834 1.228 –1.064 0.998
0.130 3.507 –0.662 –1.951 0.691 1.306 –0.979 1.018
0.150 3.526 –0.669 –1.951 0.531 1.360 –0.884 1.007
0.170 3.534 –0.661 –1.949 0.346 1.400 –0.761 0.985
0.190 3.539 –0.686 –1.941 0.149 1.429 –0.603 0.975
0.210 3.545 –0.742 –1.910 –0.060 1.446 –0.446 0.941
0.230 3.552 –0.826 –1.841 –0.176 1.448 –0.399 0.873
0.250 3.606 –0.854 –1.670 –0.650 1.402 –0.666 0.086
0.270 3.611 –0.847 –1.686 –0.697 1.397 –0.698 0.002
0.310 3.611 –0.846 –1.685 –0.698 1.398 –0.699 0.001
distribution in the system “adsorbed molecule-adsorbent”. For the atomically pure
“TiO2” surface of BaTiO3 crystal, c-type geometry adsorption on Ti atoms is the
most energetically favourable for O2 and CO molecules. A more obvious redistribu-
tion of the charges also corresponds to this geometry of adsorption (table 2).
1. It was obtained for a c-type absorption of O2 in the interstitial site, that
approaching Emin (Rmin = 0.24 nm), the charge of a molecule is practically
unchanged, and only a slight charge redistribution between the atoms of the
molecule itself takes place. If an oxygen is c-type localized on Ti (Rmin =
0.1 nm), then the charges of all the nearest neighbours in the cluster change
evidently as well as does the electronic charge density on the atoms of the
adsorbed molecules. However, the lack of free electrons in a cluster predeter-
mines a weakly pronounced donor mechanism of O2 adsorption on a “pure”
“TiO2” surface: the charge of −0.12 eV goes from O2 on Ti, where the redis-
tribution of electron density from the neighbouring oxygen atoms also takes
place (−0.14e, i.e. approximately −0.035e from each of the four nearest sur-
face atoms), decreasing titanium charge by 0.26e. The similar situation also
takes place at CO absorption. Probably this explains a weak effect of O2 and
CO adsorption on the electrical conductivity value of BaTiO3 monocrystalline
samples and their substantial effect on the spontaneous polarization value of
the ferroelectric due to the appearance of the induced dipole moments of the
adsorbed molecules [11].
2. Quite a different picture is obtained at CO adsorption on a “real” surface,
when there is already an oxygen on it bound according to c-type on Ti atoms
(see figure 4 and table 3, table 4). From the analysis of the calculation data
it follows that: a) the charge of adsorbed molecules and surface atoms sub-
stantially changes in the process of adsorption and in many cases it does not
290
Adsorption and interaction of O2 and CO2 on BaTiO3
change monotonously (see, e.g., curves Q in figure 4); b) CO molecule changes
its charge state depending on the localization centre; c) if on a pure ideal sur-
face the molecular adsorption of CO and O2 follows a weak donor mechanism
separately, then at CO, appearing on the surface, a charge of ≈ −1.5 eV is
localized on O2 adsorbed molecule, i.e. an acceptor mechanism of molecular
adsorption of O2 appears.
3. It is possible to predict the change in the charge state of a “weakly oxidated”
surface on the whole depending on its occupation with CO molecules. Initially,
i.e. at a small surface occupation, the most probable is localization of CO on
Olat (figure 4). This causes a relocalization of the electron density from these
atoms to O2, forming a surface negatively charged layer, and Olat and CO
create, on the whole, a practically neutral adsorption pair O–CO, an external
atom O of the adsorbed molecule CO having also the charge −1.1e (table 4).
The occupation of the surface with the adsorption atoms will lead to the
interaction of CO with O2 and to the formation of CO2. This causes the
disappearance of a negative charge on O2 and the appearance of a positive
charge +1e on CO2 (the total value with the centre O1 charge equals +0.34e).
In case of its desorption, there remains an oxygen ion localized on Ti with a
charge equal to −0.85e. As it is seen from the calculations, this charge passes
to it from Olat atoms surrounding the Ti. This should substantially effect the
electrical conductivity of a crystal.
4. The calculations were carried out for a dielectric. Free electrons will be eas-
ily caught by the adsorbed molecules, – this should substantially effect the
electrical conductivity of a crystal.
Table 4. The distribution of charges Q (in e units) between the atoms of adsorbed
molecules O2, CO, CO2 and the TiO2 layer atoms of the surface cluster of BaTiO3
crystal after adsorption of CO on a surface “contaminated” with O2 molecules.
CO ads. Rads, Ti
centre nm C– –O CO Tilat Olat below O1 O2 O2
charge O2
Ti: 3.94 0.07 +1.54 –0.22 +1.32 +3.87 –1.81 +3.33 –0.52 –0.95 –1.47
O: –0.23 0.07 +1.28 –1.10 +0.18 +3.92 –1.94 +3.33 –0.52 –0.95 –1.47
M 0.08 +1.56 –0.18 +1.38 +3.88 –1.80 +3.35 –0.52 –0.95 –1.47
O2 0.08 +1.23 –1.06 +0.17 +3.87 –1.94 +3.46 –0.59 +0.83 +0.24
– ∞ +0.44 –0.44 0.00 +3.87 –1.94 +3.56 0.30 –0.06 +0.24
CO2 on CO2
O1: 0.14 +1.36 –0.88 +1.01 +3.87 –1.94 +3.53 –0.67 +0.53 –
CO2 on ∞ +1.40 –0.70 0.00 +3.87 –1.94 +3.61 –0.85 –0.70
291
I.D.Sejkovskij, V.N.Zhiharev, Yu.V.Popik
References
1. Zhogolev D.A., Volkov V.B. Methods, Algorithms and Programs for Quantum-
Mechanical Calculations of Molecules. Kiev, Naukova dumka, 1976.
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ium Titanate Family. Moscov, Khimiya, 1985.
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Розрахунок механізму адсорбції і взаємодії
молекул O2 і CO2 на поверхні BaTiO3
І.Д.Сейковський, В.М.Жихарєв, Ю.В.Попик
Ужгородський національний університет,
88000 Ужгород, вул. Волошина, 54
Отримано 16 жовтня 2002 р., в остаточному вигляді – 11 січня
2003 р.
У роботі наведено результати квантово-хімічних розрахунків розши-
реним методом Хюккеля адсорбції молекул O2, CO2, CO та атомів
O і C на поверхні BaTiO3. Показано, що на поверхні (001) переважає
фізична адсорбція, а на поверхні (002) – хімічна, причому найбільш
активними центрами адсорбції є іони Ti. Розглянуті можливі меха-
нізми дисоціації адсорбованих молекул кисню і гетерогенної реакції
окислення CO в CO2.
Ключові слова: поверхня, електронна густина, напівемпіричні
квантово-хімічні методи та розрахунки, кластери, титанат барію
BaTiO3
PACS: 77.84.-s, 77.84.Dy
292
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