Electrooxidation of phenol on a Ti/RuO2 anode: Effect of some electrolysis parameters

Article abstract of DOI:10.1590/S0103-50532011000500009

The influences of electrolysis time, anodic area, current density and supporting electrolyte on phenol and its byproducts degradation on a Ti/RuO ₂ anode were investigated. It was observed that phenol and its byproducts were rapidly broken down in the presence of chloride ions. Gas chromatography/mass spectrometry (GC/MS) data have shown that the presence of chloride ions lead to chlorophenols formation, due to reactions with Cl ₂. and/or OCl generated during electrolysis. However, these intermediate products were also degraded later by the oxidizing agents. The standards established by the CONAMA (Brazilian National Council for the Environment) for phenols and chlorophenols in effluents were achieved after 360 min of electrolysis with a current density of 10 mA cm^-2. Cyclic voltammograms obtained with the anodes before and after 436 h of electrolysis under severe salinity conditions (2 mol L^-1) and current density (800 mA cm^-2) showed that Ti/RuO₂ did not lose its electrocatalytic properties. This fact indicates that Ti/RuO₂ can be used for the treatment of effluents containing phenols in a chloride environment.

Full text of DOI:10.1590/S0103-50532011000500009

J. Braz. Chem. Soc., Vol. 22, No. 5, 875-883, 2011.
Printed in Brazil - ©2011 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Article
A
Electrooxidation of Phenol on a Ti/RuO2 Anode: Effect of Some Electrolysis
Parameters
Iranildes D. dos Santos,a Júlio C. Afonso*,b and Achilles J. B. Dutraa
aPrograma de Engenharia Metalúrgica e de Materiais/Coordenação dos Programas de
Pós-Graduação em Engenharia (COPPE), Universidade Federal do Rio de Janeiro,
21941-909 Rio de Janeiro-RJ, Brazil
bDepartamento de Química Analítica, Instituto de Química, Universidade Federal do Rio de Janeiro,
21941-909 Rio de Janeiro-RJ, Brazil
Foram investigadas as influências do tempo de eletrólise, da área anódica, da densidade de
corrente e do tipo de eletrólito na degradação de fenol e de seus subprodutos de oxidação em
anodo de Ti/RuO2. Foi observado que na presença de cloreto ocorreu rápida degradação de fenol
e de seus subprodutos. Resultados de cromatografia gasosa/espectroscopia de massa (GC/MS)
mostraram que a presença de cloreto levou à formação inicial de clorofenóis através do Cl2 e/ou
OCl¯ gerados durante a eletrólise. Entretanto, eles posteriormente atuaram na degradação dos
clorofenóis.As concentrações limites estabelecidas pelo ÓrgãoAmbiental Brasileiro (CONAMA)
para descartes de fenol e clorofenóis foram obtidas após 360 min de eletrólise a uma densidade
de corrente fixa de 10 mA cm-2.Voltamogramas cíclicos obtidos antes e após 436 h de eletrólise
em condições severas de salinidade (2 mol L-1) e densidade de corrente (800 mA cm-2) mostraram
que o eletrodo de Ti/RuO2 não perdeu suas propriedades eletrocatalíticas.
The influences of electrolysis time, anodic area, current density and supporting electrolyte
on phenol and its byproducts degradation on a Ti/RuO2 anode were investigated. It was observed
that phenol and its byproducts were rapidly broken down in the presence of chloride ions. Gas
chromatography/mass spectrometry (GC/MS) data have shown that the presence of chloride ions
lead to chlorophenols formation, due to reactions with Cl2 and/or OCl¯ generated during electrolysis.
However, these intermediate products were also degraded later by the oxidizing agents.The standards
established by the CONAMA (Brazilian National Council for the Environment) for phenols and
chlorophenols in effluents were achieved after 360 min of electrolysis with a current density of
10 mA cm-2. Cyclic voltammograms obtained with the anodes before and after 436 h of electrolysis
under severe salinity conditions (2 mol L-1) and current density (800 mA cm-2) showed that
Ti/RuO2 did not lose its electrocatalytic properties. This fact indicates that Ti/RuO2 can be used
for the treatment of effluents containing phenols in a chloride environment.
Keywords: electrooxidation, phenols, chlorophenols, supporting electrolyte
Introduction
There are several available processes to treat effluents
containing phenols, such as biological treatment, advanced
oxidation processes, oxidation in supercritical water and
electrochemical oxidation.6-8 Furthermore, some of those
processes, for instance photocatalytic oxidation process,
presents high operational costs and operational issues due
to catalyst deactivation.9 The electrolytic process has shown
to be an efficient low cost, an alternative to the treatment of
several aqueous effluents containing organic and inorganic
pollutants.3,4,6,10-15 In this process, different electrode
materials, such as Ti/RuO2, Ti/IrO2, Pt, Ti/PbO2, boron-
doped diamond (BDD) and Ti/SnO2-Sb have been tested.
Phenols are persistent organic compounds found in
aqueous effluents from domestic activities such as cooking,
washing and bathing, as well as petroleum refineries, steel
plants, dyeing manufacturing, pharmaceutical and plastic
industries.1-4 They are refractory to conventional treatment
process and, in the presence of chlorine, they may produce
chlorophenols, which are carcinogenic and even more
refractory to degradation than the phenols themselves.4-6
*e-mail: julio@iq.ufrj.br

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Electrooxidation of Phenol on a Ti/RuO2 Anode: Effect of Some Electrolysis Parameters
J. Braz. Chem. Soc.
Some authors16-18 reported that the “active” electrodes
indirect oxidation is due to the presence of hypochlorite/
chlorine species in solution formed by the oxidation of
chloride ions on the anode surface.22,23 Hypochlorite ion
predominates in alkaline media. While chlorine is more
stable in acidic environment since the ionization constant
of hypochlorous acid is 2.95 × 10-8 (pKa = 7.53). However,
the indirect electrochemical oxidation presents some
drawbacks, for instance the possible formation of persistent
organochloride compounds2,24,25 and the low resistance of
electrode materials in this medium.
The objective of this paper was to investigate the
electrochemical conditions for complete removal of phenol
and its byproducts using aTi/RuO2 anode, aiming at a future
application for effluents treatment. The choice of Ti/RuO2
anode was due to its wide use in industrial processes, such
as chlor-alkali industry and effluent treatment. This anode
low overpotential for oxygen and chlorine evolution, good
conductivity, thermal stability in a wide temperature range
and is fairly resistant to corrosion.5,26
such as Ti/RuO2 and Ti/IrO2 are not very effective for
phenols degradation. In these electrodes, there is a strong
interaction between the hydroxyl radical (OH•) and the
surface of the electrode (M). This interaction may lead
to the possible transition of the oxygen from OH• at the
electrode surface forming an oxide with higher oxidation
state (reaction 1). This condition is found in electrodes
with low oxygen overpotential 1.23 V vs. SHE (standard
hydrogen electrode).18 The redox pair MO/M can promote
a selective oxidation of organic compounds (reaction 2).
In such electrodes, there is a competition with the oxygen
generation during the oxidation of organic compounds
due to chemical decomposition of the oxide with higher
oxidation state (reaction 3).
M(OH•) → MO + H+ + e–
MO + R → M + RO
MO → M + ½ O2
(reaction 1)
(reaction 2)
(reaction 3)
Experimental
For “non-active” electrodes Ti/PbO2, BDD and
Ti/SnO2-Sb, there is a weak interaction between OH• and
the electrode surface. The oxidation of organic compounds
occurs via OH• radical (reaction 4) leading to their
complete removal as CO2 and H2O. Reaction 3 occurs
simultaneously to the oxygen generation (reaction 5). In
these circumstances, the electrode surface does not play
a role in the oxidation of organic compounds.18-20 This
oxidation mechanism takes place on electrodes with high
oxygen overpotential.
Voltammetric tests were carried out with an EG&G
PAR model 273A potentiostat/galvanostat connected
to a microcomputer with the M270 software. The cell
was a 250 mL glass beaker with an acrylic cover. The
working electrode was a RuO2 layered titanium plate,
provided by De Nora/Brazil; titanium plates were used as
counter electrodes and a silver-silver chloride electrode
in a 1 mol L-1 KCl solution, the reference (Eo = 0.222 V).
Unless otherwise stated, all the tests were conducted at
25 oC without stirring.
The batch electrolysis tests were conducted in the same
cell used in the voltammetry, but under constant moderate
stirring. The anodic area ranged from 14 to 27 cm2. For
the long duration tests the anodic potential was monitored
throughout the experiment.
Some tests were conducted in a 160 mL plug flow
reactor (flow-by cell), which is presented, along with
its accessories, in Figure 1. For these tests, 400 mL of a
0.34 mol L-1 NaCl solution containing 100 mg L-1 of phenol
M(OH•) + R → M + mCO2 + nH2O + H+ + e– (reaction 4)
M(OH•) → M + ½ O2 + H+ + e–
(reaction 5)
Li et al.16 used Ti/SnO2-Sb, Ti/RuO2 and Pt anodes for
the electrochemical degradation of phenol in sulfate media.
Phenol was degraded on the three anodes; however there
is a substantial difference on their degradation efficiency.
The complete phenol oxidation took 36, 18 and 5 h with
the Ti/RuO2, Pt and Ti/SnO2-Sb, respectively. Although
the Ti/RuO2 anode presented the lower phenol oxidation
efficiency in sulfate media, it is recognized as a very stable
material for chlorine evolution.21 In sulfate medium, the
phenol oxidation takes place only on the anode surface
(direct mechanism), because there is no relevant generation
of oxidizing species during electrolysis. On the other
hand, phenol degradation in the presence of chloride can
occur through both mechanisms: direct and indirect. The
Figure 1. Scheme of electrolytic plug flow reactor.

Vol. 22, No. 5, 2011
Santos et al.
877
were used. The anode area was 107 cm2, the solution
flow rate 0.23 mL s-1 and the current density was fixed at
10 mA cm2.
Results and Discussion
Voltammetry
The solution pH was adjusted with 0.1 mol L-1 HCl
or NaOH solutions whenever necessary and measured
with a pH meter. Bi-distilled water was used for solution
preparation and all the reagents were analytically grade,
without further purification. For all the experiments the
initial NaCl and phenol concentrations ranged respectively
from 0.01 to 0.34 mol L-1 and 30 to 100 mg L-1, respectively.
When used as supporting electrolyte, Na2SO4 concentration
was 0.34 mol L-1.
The concentrations of phenol and its intermediate
degradation products were monitored with an UV-Vis
spectrophotometer, according to procedure described by
Amlathe et al.27 Gas chromatography coupled to mass
spectrometry (GC/MS) was used to identify and quantify
the electrolysis products concentration at the end of the
experiments. The organic compounds present in solution
were extracted with dichloromethane at pH 2 (adjusted
with H2SO4) and analyzed with the GC/MS according
to USEPA 8270D method. The COD (chemical oxygen
demand) determination was carried out using oxidation
with potassium dichromate in sulfuric acid and heating
for 2 h at 150 °C according to Hanna method using a
spectrophotometer model HI 83099.
The voltammetric behavior of the Ti/RuO2 electrode in
solutions with different NaCl concentrations in the absence
and presence of 30 mg L-1 of phenol is presented in Figure 2.
It can be observed that the voltammogram shapes depend
on the NaCl concentration. In Figure 2A, an increase on the
current density in the potential range of 1250 and 2200 mV
(0.01 mol L-1) was observed for low NaCl concentrations
indicating phenol oxidation. Conversely, a decrease on
current density in the same potential range was observed
for high NaCl concentrations (0.34 mol L-1). This behavior
can be attributed to chlorophenols formed via reaction
of organic intermediate products with the hypochlorite
ions generated on the anode. The chlorophenols tend to
adsorb on the anode surface blocking it and partially cause
a decrease on the current density. For the lower NaCl
concentration, the low chlorophenols formation rate did
not favor the buildup of adsorbed products, which block
the anode surface.6,30-32
The current efficiencies for phenol and chlorophenol
oxidations were based on the COD measurements,
according to equation 1:1,2,11,28
(1)
where, (COD)t and (COD)t+Dt are the COD (mg O2 L-1) at
time t and t + Dt (s), respectively. I is the current (A), F the
Faraday constant (96487 C mol-1), V the solution volume
(L) and 8 is the gram equivalent of oxygen.
The energy consumption (EC) for the removal of 103 g
of COD was determined through equation 2:14,29
Figure 2. Cyclic voltammograms on a Ti/RuO2 anode in different
NaCl concentrations in the absence and presence of 30 mg L-1 phenol:
(A) 0.01 mol L-1 NaCl and (B) 0.34 mol L-1 NaCl. Anodic area: 4.5 cm2,
temperature: 25 ºC and n = 100 mV s-1.
Several researchers5,12,24,25,33-36 observed that the
decrease of the electrode efficiency due to organic matter
adsorption depends on the electrolysis conditions, such
as phenol concentration, supporting electrolyte type,
electrode material, electrode potential and time.According
to Zareie et al.,30 although high NaCl concentrations favor
the formation of adsorbed intermediate products on the
anode surface, the hypochlorite and chlorine ions act on
the destruction of this blocking layer under longer times.
This avoids the buildup of these compounds on the anode
surface with the consequent electrode deactivation and
decrease of the oxidation efficiency. Evidences of the
(2)
where, t is the electrolysis time (h), U is the cell applied
potential (V), I is the current (A), V the solution volume
(L) and DCOD the COD difference (mg L-1).
The microstructure of the electrode surface was
analyzed by scanning electron microscopy (SEM), the
equipment is a 6460 model JEOL Electron Microscope
coupled to an energy dispersive X-ray spectrometer
(EDS).

878
Electrooxidation of Phenol on a Ti/RuO2 Anode: Effect of Some Electrolysis Parameters
J. Braz. Chem. Soc.
destruction of the adsorbed blocking layer are presented
on the voltammograms in Figure 3, which indicate a
current density decrease due to the formation of a blocking
layer after five cycles. After a reactivating treatment
at 3.0 V during 60 s, the anode surface recovered its
initial properties. This statement was achieved since the
voltammogram obtained after reactivation is very similar
to the one in the presence of NaCl only. The recovery of the
catalytic properties of the electrode is due to the oxidation
of the organic matter (blocking layer on the anode surface)
by the oxidizing agents generated during NaCl solution
electrolysis (Cl2 and/or OCl–). The anodic generation of
gases, as oxygen, can help the mechanical removal of the
layer from the electrode surface.1,34,35
Figure 4. The influences of the (A) anodic area and (B) electrolysis time
using anodic areas of 27 and 107 cm2 on the phenol removal. Solution:
100 mg L-1 phenol, 0.34 mol L-1 NaCl, i: 10 mA cm-2, temperature: 25 oC.
formation of organochloride compounds that block the
electrode surface. For the larger anodic area, this effect
becomes less important. After 15 min of electrolysis, 85%
of the phenol was degraded for a 107 cm2 anodic area, while
only 5% was degraded with an anodic area of 27 cm2. For
periods of time longer than 30 min, the effect of area was
less relevant. This behavior indicates in the second half of
the electrolysis phenol is mostly oxidized by the indirect
mechanism, i.e. through the action of hypochlorite, since
the final phenol concentrations are low and basically the
same for both anodic areas.
The influences of current density and electrolysis time
on the phenol removal are presented in Figure 5. It can be
observed in Figure 5A that in the first 10 min of electrolysis
only a low phenol removal was attained, probably due
to the formation of organic intermediate compounds
adsorbed on the electrode surface. From 10 to 20 min,
as the hypochlorite concentration increased due to the
electrolysis, the phenol removal rate increased substantially.
Then, with most of the phenol already oxidized, its removal
rate decreased because phenol concentration was lower and
Figure 3. Cyclic voltammograms on a Ti/RuO2 anode of a 0.34 mol L-1
NaCl in the absence and presence of 30 mg L-1 phenol after five cycles and
anodic reactivation at 3.0 V for 60 s. Anodic area: 4.5 cm2, temperature:
25 oC and n = 100 mV s-1.
Batch cell electrolysis
The influences of anodic area and electrolysis time for
batch tests in a 190 mL cell on phenol removal are presented
in Figure 4. In Figure 4A, phenol removal continuously
increased with the anodic area increase until 27 cm2. Higher
anodic areas did not significantly increase phenol removal.
Larger anodic areas are more difficult to be completely
covered by intermediate blocking layers and also tend to
increase the hypochlorite generation rate (for a fixed current
density) acting on the indirect phenol oxidation.2,37 In this
case, the direct oxidation mechanism for phenol seems to
be less important for this electrode type.16 The effect of
the electrolysis time on phenol removal for two different
anodic areas is presented in Figure 4B. The larger area
is responsible for the fastest phenol degradation. In the
beginning of the test with the anodic area of 27 cm2, the
phenol removal rate was very slow, probably due to the
Figure 5. The influences of (A) current density and (B) electrolysis time
on the phenol removal. Solution: 100 mg L-1 phenol, 0.34 mol L-1 NaCl,
temperature: 25 oC.

Vol. 22, No. 5, 2011
Santos et al.
879
the effect of the hypochlorite excess, used in the indirect
oxidation mechanism, less noticeable. The phenol removals
after 15 min of electrolysis with current densities of 20 and
40 mA cm-2 were 70 and 80%, respectively. For a current
density of 10 mA cm-2 with an anodic area of 107 cm2
(Figure 5B), a 99.99% phenol removal was achieved after
15 min. This indicates that for a given current density the
anodic area seems to be an important parameter for the
phenol oxidation, since it provides a higher rate for the
generation of oxidizing agents (mainly hypochlorite) in
solution favoring the direct oxidation, and being more
difficult to be blocked due to its larger area.
phenol degradation in the presence of NaCl as supporting
electrolyte. This behavior can be attributed to the presence
of chloride ions in solution which are oxidized to OCl– and
Cl2 depending on the pH and act in the indirect oxidation
of phenol as oxidizing agents.22,37,39
The influence of electrolysis time on the COD removal
for different NaCl concentrations is presented in Figure 7.
It can be observed that in both cases COD dropped with
time. However, an almost complete COD removal for the
higher NaCl concentration (0.34 mol L-1) was attained after
30 min. While the drop for the lower concentration was
considerably smoother, reaching 90% only after 60 min of
electrolysis. The faster COD removal obtained for the higher
NaCl concentration can be attributed to the larger oxidizing
agent (OCl¯) generation rate during electrolysis.11,22,40,41 This
feature can be an advantage for chlorophenols degradation,
despite the inevitable chlorophenols formation during the
electrolysis in the presence of chloride ions since they are
also destroyed under longer electrolysis times.22,24,28,30,42,43
This behavior is evidenced in Figure 8, which presents
the chlorophenols concentration drop with both NaCl
concentration (Figure 8A) and time (Figure 8B). Starting
with an initial concentration of 100 mg L-1 phenol in
0.34 mol L-1 NaCl solution, almost all phenol (98.6%)
was converted to chlorophenols after 5 min (Figure 8B).
However, after 30 min the chlorophenols concentration
was close to zero corresponding to a 99.99% removal.
The residual chlorophenols concentrations were higher for
lower NaCl concentrations and the same electrolysis time
(30 min), as expected. The rapid formation of chlorophenols
is an indication that the current density drop observed
previously in Figure 2B can be attributed to the adsorption
of chlorophenols on the electrode surface.
Flow-by cell electrolysis
The influences of supporting electrolytes (NaCl or
Na2SO4) on the phenol removal as a function of electrolysis
time with the flow-by electrolytic cell are presented in
Figure 6. Phenol concentration was much lower in the
presence of NaCl than in the presence of Na2SO4. For a
Na2SO4 supporting electrolyte, around 30% of the phenol
was oxidized to intermediate products after 5 min of
electrolysis. It has been reported16,17 that the intermediate
products of phenol oxidation in sulfate media are
benzoquinone and maleic acid, which tend to degrade to
CO2 and H2O. By the other side, under similar experimental
conditions (except for a sodium chloride environment)
more than 99% of the phenol was oxidized after 5 min of
electrolysis. The intermediate products of phenol oxidation
in chloride media are basically chlorophenols.22,37-39 It
can be also observed in Figure 6 that the solution still
presented a high phenol concentration after 360 min of
electrolysis in the presence of sulfate, i.e. around 6 mg L-1.
While it was necessary only 30 min to achieve almost total
Figure 7. Influence of electrolysis time in the COD removal in the
presence of different NaCl concentrations. Solution: 100 mg L-1 phenol,
anodic area: 107 cm2, temperature: 25 oC, i = 10 mA cm-2, V = 400 mL,
flow rate: 0.23 mL s-1.
Figure 6. Influences of chloride and sulfate on the phenol removal.
Solution: 100 mg L-1 phenol, anode: Ti/RuO2, anodic area: 107 cm2,
temperature: 25 oC, i = 10 mA cm-2, V = 400 mL, flow rate: 0.23 mL s-1.

880
Electrooxidation of Phenol on a Ti/RuO2 Anode: Effect of Some Electrolysis Parameters
J. Braz. Chem. Soc.
compounds) removal in a 0.34 mol L-1 NaCl solution
with 100 mg L-1 phenol are shown in Figures 9A and 9B,
respectively. In Figure 9A, the energy consumption presents
an increasing trend, which is interrupted between 10 and
40 min, probably due the predominance of the indirect
oxidation mechanism, i.e. through the action of OCl–, in this
time interval. After this period, the cell current produced
more OCl– that is not used in the oxidation anymore, due
to the low organic concentration in solution leading to an
increase in the energy consumption. An analogous, but
opposite behavior is observed for current efficiency, as
presented in Figure 9B. An elevated current efficiency in
the beginning of the electrolysis was achieved due to the
oxidation of phenol and its by-products.11,17 After 20 min
the efficiency dropped continuously due to the decrease
of the total organic compounds concentration in solution,
since they were almost completely removed after 30 min,
as shown previously in Figure 8B. Another possible cause
for the low current efficiency and high energy consumption
throughout the electrolysis is the oxygen evolution reaction
which occurs along with OCl– generation. The generated
oxygen does not play a significant role on the phenols
oxidation, as shown previously in Figure 6.
Figure 8. The influences of (A) chloride concentration and (B) electrolysis
time on the chlorophenols degradation. Solution: 100 mg L-1 phenol,
anodic area: 107 cm2, temperature: 25 oC, i = 10 mA cm-2, V = 400 mL,
flow rate: 0.23 mL s-1.
The organic compounds concentration detected after
5 and 360 min of electrolysis, along with their respective
limits established by the National Brazilian Environmental
Council (CONAMA)44 for Class 1 saltwater are presented
in Table 1. It can be observed that most of the phenol was
converted to different chlorophenols in the first 5 min,
being almost totally destroyed at the end of the electrolysis
(360 min). The absence of other oxidation by-products
except for the chlorophenols indicates the conversion of
phenols to CO2 and water. This happened besides eventually
low molecular weight intermediate aliphatic compounds,
such as carboxylic acids, which were not quantified due to
their low hazardousness potential.
The influences of electrolysis time on the specific energy
consumption and current efficiency for COD (which can
be associated to the presence of phenol and other organic
Table 1. Organic compounds identified by GC/MS after 5 and 360 min
of electrolysis and the respective limits established by CONAMAa for
class 1 saltwater. Solution: 100 mg L-1 phenol and 0.34 mol L-1 NaCl;
anodic area: 107 cm2; temperature: 25 °C; i = 10 mA cm-2; V = 400 mL;
flow rate: 0.23 mL s-1
Figure 9. The influences of (A) electrolysis time in the energy
consumption and (B) current efficiency for COD removal. Solution:
100 mg L-1 phenol, 0.34 mol L-1 NaCl, anodic area: 107 cm2, temperature:
25 oC, i = 10 mA cm-2, V = 400 mL, flow rate: 0.23 mL s-1.
Compound
5 min
(mg L-1)
360 min
(mg L-1)
Limits
(mg L-1)b
phenol
0.0563
0.0006
6.1905
16.5314
26.8775
43.3248
5.6510
nd
0.0007
0.0600
c
The durability of the anodes is an important parameter
for the application of an electrolytic process, especially
in corrosive environments like concentrated chloride
solutions. The results of accelerated life tests for the
Ti/RuO2 anode as function of time under high salinity
(2 mol L-1 NaCl) and current density (800 mA cm-2) in the
absence and presence of 30 mg L-1 phenol are presented
in Figure 10. It can be observed only a small change in the
behavior of the electrodes in the media studied. The anode
potential in the beginning of the test was around 3.0 V in
both cases; however, a dramatic increase on the electrode
2,3,4,6-tetrachlorophenol
2,4,5-trichlorophenol
2,4,6-trichlophenol
2,4-dichlorophenol
2,6-dichlorophenol
2-chlorophenol
*
0.0108
c
*
*
*
*
*
0.0024
0.2900
c
0.1500
0.0030
pentachlorophenol
*Belowdetectionlimit;nd:notdetected; aNationalBrazilianEnvironmental
Council (CONAMA);43 bLimit for class 1 saltwater; c: Limit not
established by CONAMA for class 1 saltwater.

Vol. 22, No. 5, 2011
Santos et al.
881
potential (6 and 7 V after ca. 430 h of electrolysis in the
presence and absence of phenol, respectively) in the end of
the test was observed indicating the starting of a corrosion
process, especially in the absence of phenol.
SEM micrographs and EDS spectra of the Ti/RuO2
anodes before and after accelerated degradation testing
under the conditions mentioned in Figure 10 are presented
in Figure 11. The morphology of the electrode before the
test is shown in Figure 11A. In Figure 11B, a moderate
exfoliation was observed in the absence of phenol. However
the detail in Figure 11B suggests that other RuO2 layers
on the titanium electrode practically remain intact. The
exfoliation was probably due to the higher gas (chlorine
and oxygen) generation on the anode surface,26,45 according
to the voltammogram presented in Figure 1. Another
possibility should be the corrosion of RuO2 in the presence
of hypochlorite in alkaline environment.45 Furthermore, it
Figure 10. Accelerated life tests of Ti/RuO2 anode in the absence (anodic
area: 3.9 cm2) and presence (anodic area: 3.3 cm2) of phenol. Conditions:
NaCl: 2 mol L-1, phenol: 30 mg L-1, i = 800 mA cm-2, temperature: 25 oC.
Figure 11. SEM micrographs and EDS spectra of Ti/RuO2 anodes surface used in the accelerated life tests: (A) before test and (B) after 436 h in the
presence of 2 mol L-1 NaCl with anodic area 3.9 cm2; (B1) Magnification: ×1000; (C) After 436 h in the presence of 2 mol L-1 NaCl and 30 mg L-1 phenol
with anodic area de 3.3 cm2, temperature: 25 oC.

882
Electrooxidation of Phenol on a Ti/RuO2 Anode: Effect of Some Electrolysis Parameters
J. Braz. Chem. Soc.
should be considered that some phenolic compounds act
as corrosion inhibitors extending the anode life according
to Figure 11C.46
Voltammetric tests indicated that the anode did not
lose its electrocatalytic properties, despite its previous
use during 436 h under severe salinity and current density
conditions.
The voltammetric performance of the Ti/RuO2 anode
(5.6 cm2) for an electrolysis of a 0.34 mol L-1 NaCl solution
with 30 mg L-1 of phenol, before and after the accelerated
life test, in presence of phenol, is shown in Figure 12.
A similar performance for the phenol degradation was
observed, indicating that the anode did not lose its
electrocatalytic properties, despite its previous use during
436 h under severe salinity and current density conditions.
This suggests that this material presents good performance
and resistance to oxidation of organic matter in chloride
medium.
Acknowledgements
The authors are grateful to Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq) and
Fundação de Amparo à Pesquisa do Estado do Rio de
Janeiro (FAPERJ) for the financial support.
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