Evaluation of titanium dioxide and cerium oxide as anodes for the electrooxidation of toluene: A theoretical approach of the electrode process

Article abstract of DOI:10.1016/j.electacta.2004.04.014

Cerium oxide and titanium dioxide were prepared by thermal decomposition of the precursor salts and thermal treatment of titanium plates. In aqueous medium, the metal oxides show a well-defined electrochemical reaction; a solid state redox process takes place in the cathodic range of potentials and only water discharge reaction occurs in the anodic region. At the experimental conditions, the prepared materials were not totally active for the electrooxidation of toluene. The theoretical modeling suggests that the lack of activity is due to the weak interaction between toluene and the metal oxide surface.

Full text of DOI:10.1016/j.electacta.2004.04.014

Electrochimica Acta 49 (2004) 4197–4203
Evaluation of titanium dioxide and cerium oxide as anodes
for the electrooxidation of toluene
A theoretical approach of the electrode process
a,∗
b
a
Luis F. D’Elia , L. Rincón , R. Ort ´ı z
a
Universidad de los Andes, Facultad de Ciencias, Departamento de Qu ´ı mica, Laboratorio de Electroqu ´ı mica, Mérida 5101, Venezuela
Universidad de los Andes, Facultad de Ciencias, Departamento de Qu ´ı mica, Grupo de Qu ´ı mica Teórica, Mérida 5101, Venezuela
b
Received 20 February 2004; received in revised form 6 April 2004; accepted 10 April 2004
Available online 24 May 2004
Abstract
Cerium oxide and titanium dioxide were prepared by thermal decomposition of the precursor salts and thermal treatment of titanium plates.
In aqueous medium, the metal oxides show a well-defined electrochemical reaction; a solid state redox process takes place in the cathodic
range of potentials and only water discharge reaction occurs in the anodic region. At the experimental conditions, the prepared materials were
not totally active for the electrooxidation of toluene. The theoretical modeling suggests that the lack of activity is due to the weak interaction
between toluene and the metal oxide surface.
©
2004 Elsevier Ltd. All rights reserved.
Keywords: Titanium dioxide; Cerium oxide; Toluene; Electrooxidation; Theoretical modeling; YAeHMOP
1
. Introduction
reduction of oxygen [11] and the photocatalytic and photo-
electrochemical oxidation of some organic compounds [12].
Cerium oxide (CeO2) seems to be a useful material for the
electrooxidation of organic compounds [13] and an interest-
ing component of SOFCs anodes [14]. It is known that these
During the last ten years there has been a growing inter-
est in electrocatalyst materials composed by metal oxides.
They have been used for the treatment of industrial wastew-
ater streams [1] and testing the electrochemical oxidation
of phenol and chloroorganic compounds in aqueous media
◦
energy devices work at high temperatures (T = 1000 C)
[15]; which constitutes one of the main problems, if they are
compared to other low temperature operation systems; such
as: Direct Methanol Fuel Cell (DMFC), Polymer Electrolyte
Fuel Cell (PEFC), Phosphoric Acid Fuel Cell (PAFC) and
others.
[2,3]. The activity of this sort of electrodes has been also
evaluated for the electrooxidation of methanol and formic
acid [4,5]. Metal oxides and its composites are promising
materials to be used as anodes in Solid Oxide Fuel Cells
(
SOFCs), in which liquid and gaseous hydrocarbons are uti-
The electrochemical oxidation of toluene has been per-
formed on platinum and glassy carbon surfaces, by using
different electrolytes [16–20]. Treimer and coworkers re-
port that toluene is electrooxidized to benzoic acid at
Fe(III)-␤-PbO2, Bi(V)-␤-PbO2 and ␤-PbO2 film electrodes
in acid media [21]. The electrochemical reaction is kineti-
cally favored at Fe(III)-␤-PbO2 electrode, since adsorption
interaction of aromatic molecules at the Fe(III) sites is
stronger than those expected for Bi(V) and Pb(IV).
Exploring the electrochemical properties in aqueous and
organic electrolytes of TiO2 anatase and CeO2, we found
lized as fuels [6,7].
Due to the high surface area of titanium dioxide (TiO2),
it has been employed as a support of active materials for
chemical and electrochemical reactions [8,9]. TiO2, anatase
[
10], has shown good activity for the electrochemical
∗
Corresponding author. Tel.: +58-274-2401393;
fax: +58-274-2401286.
E-mail address: luisf@ula.ve (L.F. D’Elia).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.04.014

4
198
L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
that these materials were not totally active for the electrox-
idation of toluene at ambient temperature. The theoretical
modeling reinforced this fact, since adsorption interactions
between molecular orbitals of the aromatic compound and
the valence and conduction bands of the metal oxides are
not significant. The calculated energy values suggest that
the interactions magnitude strongly depend on the electronic
characteristics of the exposed atoms at the surfaces.
(Ag/AgCl, KCl saturated) was separated from the work-
ing electrode compartment by a Luggin capillary. A plat-
inum gauze (Aldrich, 99.99%) was employed as counter
electrode.
Electrochemical measurements were carried out employ-
ing an Autolab 20 potentiostat, controlled with a General
Purpose Electrochemical System software (GPES) version
4.8.
2.4. Other instrumentation
2
. Experimental
Film morphology and chemical analyses of the materials
2
.1. Preparation of the electrode materials
were done using a scanning electron microscope (Philips,
Model XL30) coupled with an energy dispersive X-ray de-
tector (EDAX, Model IDX4).
Before any use, the titanium plates (Aldrich, 99.99%)
were treated as follows [22]: (i) abraded with silicon carbide
paper, grades P600A and P1200A (E.A & C.L.); (ii) washed
X ray photoelectron spectroscopy (XPS) analyses were
carried out employing a spectrometer (Leybold Her-
aeus) with a non monochromatic aluminum (Al) ra-
diation (Model LH-1, 1486.6 eV) operating at 370 W.
The analyses chamber pressure was maintained at about
5 × 10 mbar during all measurements. Binding en-
ergy values were corrected using the signal located at
284.6 eV, which is assigned to carbon (C) contamina-
tion.
−
1
with deionized water (18 Mꢀ cm ) and (iii) etched in 8 M
◦
HCl (Riedel de Haën, 37%) at 30 C for 10 min. Finally the
plates were rinsed with ultrapure water and ultrasonically
cleaned in water for 10 min.
−
8
2
.1.1. Preparation of TiO2 anatase
The TiO2/Ti electrode was prepared by thermal oxidation
2
of a titanium plate (geometric area, 1.0 cm ) in a muffle
furnace (Lindberg Blue) under oxygen atmosphere following
the temperature program: (i) the temperature was scanned
2.5. Theoretical modeling
◦
◦
−1
from 25 to 450 C at a rate of 2 C. min ; (ii) maintained
The band structure calculation and the density of states
(DOS) of the metal oxide surfaces were obtained using the
computational program YAeHMOP; Bind 3.0 [23].The crys-
tal structure data for the program was obtained from Re-
trive, Powder Difraction Files and Diamond 2002 packages
[24–26].
◦
◦
at 450 C for 6 h and (iii) scanned from 450 to 25 C at a
rate of −2 C min
◦
−1
.
2
.1.2. Preparation of CeO2
CeO2 was obtained by thermal decomposition of
Ce(NO3)3·6H2O (Aldrich, 99%) on TiO2/Ti plates as fol-
The parameters used in the calculations are standard lit-
erature values and are collected in Table 1 [23].
lows: (i) 0.0414 g of Ce(NO3)3·6H2O were dissolved in
5
␮l of ultrapure water; (ii) the prepared solution was spread
out drop wise on the TiO2/Ti surface until all of it was
wet; (iii) the wet plate was introduced in the muffle furnace
under oxygen atmosphere, and the same temperature pro-
gram used for TiO2/Ti electrode preparation was applied.
Steps (ii) and (iii) were repeated several times until total
consumption of the solution.
Table 1
Hückel atomic parameter used in the calculations
Hiia (eV)
ζ1
b
C1c
ζ2
C2
Atoms
Orbital
1s
H
C
−13.600
−21.400
−11.400
1.300
–
–
–
2s
1.625
1.625
–
–
–
–
–
–
2
.2. Chemicals
2p
O
2s
−32.300
2.275
2.275
–
–
–
–
–
–
The electrochemical behaviour of the electrodes in
2p
−14.800
−2
1
0
M K3Fe(CN) + 1 M NaOH was evaluated. Finally,
6
Ti
4s
−8.970
−5.440
1.075
1.075
4.550
–
–
–
–
–
–
the electrooxidation of toluene (Riedel de Haën, HPLC
grade) was performed in 0.1 M H2SO4 (Riedel de Haën,
4
4
p
d
−10.810
0.421
1.400
0.784
9
(
5–97%) and 0.1 M But4NPF (Aldrich, 99.9%) + CH3CN
6
Ce
6s
−4.970
−4.970
−6.430
−11.280
1.799
1.799
2.747
3.907
–
–
–
–
–
–
–
–
–
–
–
–
Aldrich, HPLC grade) solutions.
6
5
p
d
2
.3. Cells and instrumentation
4f
a
Orbital ionization potential.
Orbital exponent.
Orbital coefficient.
◦
The electrochemical studies were done at 25.0 ± 0.1 C
in a two compartment cell. The reference electrode
b
c

L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
4199
Fig. 1. Scanning electron microphotographs of the prepared metal oxide surfaces: (a) TiO2/Ti, (b) CeO2/TiO2/Ti.
3
. Results and discussion
the CeO2/TiO2/Ti electrode. It was certainly found that these
electrodes were very robust and could be used for many days
without any change in their electrochemical behaviour.
3
.1. Morphological and chemical nature of TiO2 and
CeO2 surfaces
3.2. Electrochemical behaviour of TiO2/Ti and
Scanning electron micrographs of TiO2 and CeO2 sur-
faces (Fig. 1) show complete and compact layers with a
high surface area; resulting both from cracking and the pres-
ence of rather large pores. Energy dispersive X-ray analyses
CeO2/TiO2/Ti electrodes in aqueous solutions
Cyclic voltammograms were recorded scanning the po-
tential from 600 to −800 mV in 0.1 M H2SO4. A typical
voltammogram of a TiO2/Ti electrode is shown in Fig. 3a. It
may be noted that (i) on the forward scan, there is a reduction
process (A) at around −640 mV prior to hydrogen evolution;
(ii) on the backward scan, there is an oxidation process (B) at
about −590 mV; (iii) the charge ratio of the peaks is approx-
imately one. The form of the recorded voltammogram and
the fact that the peak current varies linearly with the sweep
rate imply the presence of reversible redox reactions, in
which surface processes are involved. The TiO2/Ti electrode
showed a similar electrochemical response to that reported
for TiO2 anatase prepared using different methods [22]. Au-
thors stated that the electrochemical reaction involves a solid
stated surface transformation of Ti(IV) to Ti(IIII), and its
Nernst potential is shifted 59 mV by a unity of pH [22].
(EDAX) show only the presence of the metal (titanium or
cerium) and oxygen.
Unequivocally, XPS spectra (Fig. 2) determined that only
TiO2 and CeO2 are the components of each deposit; the dif-
ference energy values in Fig. 2 (TiO2, 5.7 eV; CeO2, 18.1 eV)
exactly correspond with those reported for these oxides [27].
Fig. 2b shows two important aspects: (i) a total of six peaks
can be easily appreciated; two of them are produce by the
electronic transition of 3d electrons and the other four peaks
(labeled as S in the figure) are satellites signals produced by
the oxygen atoms in CeO2 [27]; (ii) the binding energy val-
ues are shifted in about 0.5 eV compared to those reported
for CeO2 [27]; which is produced by the TiO2/Ti support.
These facts reaffirm that only CeO2 compose the surface of
Fig. 2. XPS spectra of the prepared metal oxide electrodes: (a) TiO2/Ti, (b) CeO2/TiO2/Ti.

4
200
L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
Fig. 3. Cyclic voltammograms of the electrodes in N2-saturated 0.1 M H2SO4: (a) TiO2/Ti, (b) CeO2/TiO2/Ti. Sweep rate, 50 mV s 1. Geometric area
of the electrodes: 1 cm .
−
2
A very similar behaviour was found for the CeO2/TiO2/Ti
electrode at the same conditions (Fig. 3b). For this case,
all the electrode processes are cathodically shifted in about
The electrochemical response of the electrodes in Fe(III)
containing solutions was also evaluated (Fig. 4). Báez and
coworkers have well-characterized the system, TiO2 anatase
in this medium [22]. They showed that the cathodic reaction
is initiated by the surface process:
8
0 mV, and the voltammetric peaks correspond to the sur-
face redox titanium transformation. Although only CeO2
is exposed at the surface of CeO2/TiO2/Ti electrode, the
electrolyte solution (0.1 M H2SO4) diffuses into the CeO2
porous films and the TiO2/Ti support generates the electro-
chemical response. The CeO2 films produce the cathodic
peak potential displacement, since the oxide film acts as a
barrier that limits the charge transfer reaction.
When the electrochemical response of the electrodes was
evaluated in the anodic region (between 0 and 2000 mV)
the cyclic voltammograms did not show any evidence of
electrode transformation. For both material, TiO2/Ti and
CeO2/TiO2/Ti, water discharge reaction starts at around
Ti(IV) + e− → Ti(III)
(1)
which is followed by the chemical reaction:
Ti(III) + Fe(III) → Ti(IV) + Fe(II)
(2)
Eq. (2) explains the absence of the peak associated to the
Ti(III)–Ti(IV) transformation in the cycle voltammogram
(Fig. 4a).
Fig. 4b shows the cyclic voltammogram of a CeO2/TiO2/Ti
electrode in a Fe(III) containing solution. Compared to
TiO2, the cathodic peak current is similar and the peak
potential is cathodically shifted in 60 mV (TiO2, 780 mV;
6
00 mV but it is kinetically favored at the TiO2 surface.
Fig. 4. Cyclic voltammograms of the electrodes in N2-saturated 10 M K3Fe(CN)6 + 1 M NaOH: (a) TiO2/Ti, (b) CeO2/TiO2/Ti. Sweep rate, 50 mV.s-1
−
2
.
2
Geometric area of the electrodes: 1 cm .

L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
4201
CeO2, 840 mV). In sum, all the oxidation and reduction
processes evaluated in this work were kinetically favored at
TiO2 surfaces.
condition. In general, the presence of toluene in the elec-
trolyte leads to a current decreasing compared to that ob-
served in free toluene electrolyte solutions; toluene is not
electrooxidized at this surface.
For both electrodes, their electrochemical responses in
toluene organic solution are very similar to those found in
aqueous medium. TiO2 shows some activity for the elec-
trooxidation of toluene; however, CeO2 is totally inactive for
this reaction. In the organic medium, toluene could be trans-
formed at TiO2 surface following the reaction route labeled
as (i); since routes (ii) and (iii) only take place in aqueous
media.
3
.3. Electrochemical response of TiO2/Ti and
CeO2/TiO2/Ti electrodes in toluene solutions
Toluene could be transformed following three different
routes: (i) direct electron transfer reaction at the electrode
surface; (ii) reaction with the species produced during wa-
ter discharge reaction (wdr); (iii) chemical reaction with
the electrode surface follows by the electrochemical re-
generation of the active sites. The last route of reaction
is illustrated by Eqs. (3) and (4), where M represents the
metallic atom of the oxide; Ti or Ce.
3.4. Theoretical consideration of the electrode process at
the metal oxides surfaces
Chemical reaction:
Any electrochemical process involves three fundamental
steps [28]: (a) diffusion of the electroactive specie from the
bulk solution to the electrode surface; (b) charge transfer
reaction; (c) diffusion of products from the surface electrode
to the bulk solution. The nature of the electrode reaction
depends on the intrinsic characteristics of the electroactive
species and the surface electrode.
Definitely, interactions of the electroactive species with
the electrode surface limit the charge transfer reaction. Ab-
sence of any sort of interaction that modifies the energy
levels of the system does not lead to any electrochemical
reaction.
Based on the crystallographic data of the materials,
YAeHMOP allowed the estimation of DOS of the extended
surfaces [29,30]. It was also possible to calculate the theo-
retical energy interactions between the metal oxide surface
and the molecular orbitals of a molecule of toluene placed at
2 Å from the surface in different geometrical configurations
(Fig. 6). For the surface modeling, four slabs metal oxide
surface (Fig. 6) is composed by four primitive unit cells.
M(IV) + Toluene → Products + M(III)
Electrochemical generation of the active sites:
(3)
M(III) → M(IV) + e−
(4)
Voltammetric evidences indicate that addition of toluene
to the electrolyte (0.1 M H2SO4) produces small anodic cur-
rent increments (Fig. 5a). This fact suggests that the organic
compound reacts following the routes (i) and (ii).
As it was done for the reduction of Fe(III) on TiO2 sur-
faces, the effect of toluene on the voltammetric peaks of
titanium redox processes was evaluated to confirm if the
third reaction route takes place. From the cycle voltammo-
gram (Fig. 5b), a small suppression of the redox reaction
Ti(IV)–Ti(III) is observed while the process Ti(III)–Ti(IV)
seems to be slightly favored. These two features confirm that
toluene could be also transformed following the chemical
activation reaction.
The electrochemical response of CeO2/TiO2/Ti electrode
in toluene aqueous solutions was also studied at the same
Fig. 5. Cyclic voltammograms of TiO2/Ti in N2-saturated solution: (—) 0.1 M H2SO4, (- - -) toluene-saturated 0.1 M H2SO4. Sweep rate, 50 mV s 1.
Geometric area of the electrode: 1 cm . (a) Anodic region, (b) cathodic region.
−
2

4
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L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
Fig. 6. Different geometrical configuration of a molecule of toluene interacting with the four slabs oxide surface: (a) only metallic atoms are exposed,
b) only oxygen atoms are exposed, c) both, metallic and oxygen atoms, are exposed.
(
Fig. 7. Theoretical values of toluene-metallic atoms (Me) and toluene–oxygen atoms (O) interactions.
As is well known the Extended Hückel method is not re-
the Fermi level (EF) of the materials and the frontier orbitals
(HOMO or LUMO) of the electroactive species [25]. Com-
paring the frontier orbital energy values of toluene (HOMO,
−12.5048 eV; LUMO −8.6927 eV) and EF of the metal ox-
ides (Table 2), we conclude that toluene should be oxi-
dized at the oxide surface. However, we have experimentally
shown that this is not the fact at all, so other factors must
be considered.
liable for geometry optimization [30,31]. Thus, a significant
orbital interaction criteria, based on overlap population, was
used to select 2 Å as the appropriate interaction distance be-
tween toluene and metal oxide surface. However, due to lim-
itations of Extended Hückel methods, our conclusion is not
qualitatively change under small variation of the interaction
distance.
The calculated band gap values suggest that the free ox-
ides are semiconductors [32] (Table 2). In general, an elec-
trode process involves interactions between bands close to
Fig. 7 shows the energy values (ꢁE) of toluene-metallic
atoms (Me) and toluene–oxygen atoms (O) interactions. In
the figure, toluene-nMe means that a molecule of toluene
is interacting with n metallic atoms (n could be 1, 2 or 3)
while toluene-nO means that a molecule of toluene is inter-
acting with n oxygen atoms (n could be 1or 2); the number
of neighbor atoms with respect to the center of a toluene
molecule were changed moving the toluene molecule over
the oxide surface. Negatives values of ꢁE mean stabiliza-
tion while positive values reflect repulsive interaction or
Table 2
Calculated energy values for the extended metal oxide surfaces
Metal oxide
Band gap (eV)
Fermi energy level (EF) (eV)
TiO2 anatase
CeO2
4.22
3.21
−14.5470
−14.4946

L.F. D’Elia et al. / Electrochimica Acta 49 (2004) 4197–4203
4203
destabilization; this is compared to the system toluene-surface
Acknowledgements
oxide without interaction. It is necessary to consider that the
calculated energy values show the real qualitative tendency;
however, these are slightly overestimated.
It may be noted that attracting interactions increase with
the numbers of the metallic atoms involved. However, in-
teractions via atoms of oxygen are generally repulsive;
these are much higher for CeO2 than TiO2. Attractive in-
teractions lead to light changes in the Fermi level of TiO2
Luis F. D’Elia would like to thank to PDVSA Intevep
(Filial de Petróleos de Venezuela, PDVSA) for the finan-
cial support and technical assistance. Special mention to Dr.
Victor Báez (PDVSA-Intevep) for his interest and excellent
advice for the development of this work.
(14.0325 eV); nevertheless, for CeO2 these interactions do
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same electrochemical behaviour in supporting electrolytes
with very different chemical and physical properties (0.1 M
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