Electrochemical oxidation of trans-3,4-dihydroxycinnamic acid at PbO2 electrodes: Direct electrolysis and ozone mediated reactions compared

Article abstract of DOI:10.1016/S0013-4686(00)00590-9

A comparative study of the oxidative degradation of trans-3,4-dihydroxycinnamic acid was carried out using three different methods, two of which have in common the use of electrogenerated ozone as a potential oxidant: (i) direct electrolysis; (ii) external cell chemical reactions with O₃; and (iii) a `cathodic oxidation' in which an O₂/O₃ mixture is fed into the cathodic compartment containing the organic substrate; H₂O₂ produced reacts with ozone to yield radicals that bring about the demolition of the organic compound. The nature products formed by the direct electrochemical oxidation depends on potential. The degradation of the dihydroxy-acid is observed only at relatively high potentials, where radical oxygen species are formed in a high amount by the oxidation of water. At relatively lower potentials the process shows stirring dependence and leads essentially to one oxidation product which is adsorbed on the electrode. A comparative examination of the results, considering also the requested amount of ozone as a parameter, shows that the external cell and cathodic oxidation are the most efficient methods. This confirms our previous data on the oxidation of phenolic compounds, and the cathodic oxidation with ozone emerges as a promising approach in the abatement of pollutants.

Full text of DOI:10.1016/S0013-4686(00)00590-9

Electrochimica Acta 46 (2000) 341–347
www.elsevier.nl/locate/electacta
Electrochemical oxidation of trans-3,4-dihydroxycinnamic
acid at PbO electrodes: direct electrolysis and ozone
2
mediated reactions compared
a,
a
b
b
R. Amadelli *, A. De Battisti , D.V. Girenko , S.V. Kovalyov ,
b
A.B. Velichenko
a
Centro di Studio su Fotoreatti6it a` e Catalisi (CNR) and Dipartimento di Chimica, Uni6ersit a` di Ferrara, 6ia L. Borsari,
4
6-44100 Ferrara, Italy
Department of Physical Chemistry, Ukrainian State Chemical Technology Uni6ersity, Gagarin A6e., 8-Dniepropetro6sk,
20005, Ukraine
b
3
Received 26 November 1999; received in revised form 10 February 2000
Abstract
A comparative study of the oxidative degradation of trans-3,4-dihydroxycinnamic acid was carried out using three
different methods, two of which have in common the use of electrogenerated ozone as a potential oxidant: (i) direct
electrolysis; (ii) external cell chemical reactions with O ; and (iii) a ‘cathodic oxidation’ in which an O /O mixture is
3
2
3
fed into the cathodic compartment containing the organic substrate; H O2 produced reacts with ozone to yield
2
radicals that bring about the demolition of the organic compound. The nature products formed by the direct
electrochemical oxidation depends on potential. The degradation of dihydroxy-acid is observed only at relatively high
potentials, where radical oxygen species are formed in a high amount by the oxidation of water. At relatively lower
potentials the process shows stirring dependence and leads essentially to one oxidation product which is adsorbed on
the electrode. A comparative examination of the results, considering also the requested amount of ozone as a
parameter, shows that the external cell and cathodic oxidation are the most efficient methods. This confirms our
previous data on the oxidation of phenolic compounds, and the cathodic oxidation with ozone emerges as a
promising approach in the abatement of pollutants. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Lead dioxide; 3,4-Dihydroxycinnnamic acid; Ozone; Electrocatalysis; Pollutants
1
. Introduction
as OH radicals, are formed during the anodic oxidation
of water which are able, in turn, to oxidise most
organic compounds. There is a vast literature on this
subject, concerning both conducting [1–9] and semicon-
ducting anodes [10–13].
The electrooxidation of a large number of organic
and inorganic compounds on different electrode materi-
als, including PbO , proceeds simultaneously with the
2
evolution of oxygen. Highly oxidising oxygen species,
At the high anodic potentials involved, the same
oxygen species may be involved in the formation of O3
in addition to O [4–20], as illustrated by the pathway
below:
2
*
Corresponding author. Tel.: +39-532-291120; fax: +39-
5
0
32-240709.
+
−
E-mail address: amr@dns.unife.it (R. Amadelli).
H O“H +(OH)ads+e
(1)
2
013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0013-4686(00)00590-9

3
42
R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
+
−
−
(
OH)ads“(O)ads+H +e
(2)
(3)
(4)
(5)
2. Experimental
+
2
2
(OH)ads“O +2H +2e
All other chemicals were Fluka reagents and were
used as received. b-PbO electrodes were prepared by
2
2
(O)ads“O2
electrodeposition at constant current from Pb(NO3)2
acid solutions onto Ti substrates previously etched in
hot oxalic acid and then platinised. The electrodeposi-
tion of Pt was carried out from a solution containing
(
O)ads+O “O3
2
A mechanistic study on the role reactive oxygen
intermediates in the formation of ozone at PbO and at
platinum has been published by Wabner and Gramow
20], using p-nitrosodimethylaniline as a radical trap.
The authors also proposed that singlet oxygen, in addi-
tion to OH radicals, can be involved in oxidation
reactions.
In our recent research work [21] we have compared
different methods for the abatement of pollutants in-
volving or not ozone formation at PbO2 anodes: (i)
−
3
−3
32.5 g/dm
K PtCl6 in 30 g/dm
KOH at 75°C,
2
2
2
using a constant current of 8 mA/cm for 2.5 min. The
approximate thickness of the resulting deposit is 0.25
mm. Lead dioxide was electrodeposited onto these sub-
[
−
3
strates from a solution containing 0.1 mol dm
−
3
HNO +0.1 mol dm
Pb(NO ) at room temperature
3
3 2
2
and a constant current of 5 mA/cm until a PbO2
deposit with an estimated thickness of 30.8 mg/cm was
obtained.
2
Electrochemical experiments were conducted using
an EG&G/PARC model 273A potentiostat with
EG&G software using a conventional three compart-
ments cell. Impedance measurements were carried out
using a EG&G model 5210 lock-in amplifier. A satu-
rated calomel electrode was used as reference.
Analyses of the reaction products were conducted by
HPLC using a TSP instrument equipped with a Spectro
Monitor detector 5000 and a 25 cm Econosphere C 18
direct electrolysis; (ii) ex-situ use of O and (iii) com-
3
bined use of anodically generated O3 and H O2 pro-
2
duced at an O2 cathode (cathodic oxidation). We
showed that the last method gives better results in the
oxidation of 4-chlorophenol.
The encouraging results stimulated our research
along this direction and in the present work we examine
the degradation of trans-3,4-dihydroxycinnamic acid
(
henceforth abbreviated as DHCA) as a representative
5U column. Ozone analysis was carried out mostly by
phenolic acid present in wastewater from olive oil in-
dustry. The large content in phenolic compounds repre-
sent a serious problem of environmental contamination
and the reduction of the pollutant content has been the
target of several recent investigations using different
methods [22,23].
iodometric titration. In some cases the results so ob-
tained were checked by the spectrophotometric method.
In the oxidation experiments by the different meth-
ods, the reactions were not carried out exhaustively,
and the kinetic analysis of the results is done in terms
of residual DHCA as a function of time. In some cases,
the chemical oxygen demand (COD) was evaluated as
described in [6]. The formation of coloured compounds
during electrolysis was followed by UV-visible spec-
troscopy using a Kontron Uvikon 940 spectrometer.
3. Results and discussion
3.1. Direct electrolysis
All experiments were conducted in a phosphate
buffer electrolyte at pH 7.2. In this medium DHCA is
easily soluble, whereas some solubility problems were
encountered in acid solutions. On the other hand, we
noticed that at pH values higher than 10 the DHCA
solutions were not stable, as indicated by changes of the
UV-vis spectrum with time.
In Fig. 1 are shown the steady-state polarisation
curves for PbO in phosphate buffer both without and
2
Fig. 1. Quasi- steady-state potential vs. log i curves for PbO2/
Pt electrodes in stirred phosphate buffer solutions (pH=7.2)
with added DHCA. In the latter case one notices a
pronounced stirring dependence in the lower potential
region, which is actually seen only during the first run
in the direction of increasing potentials. However, in
−
3
in the absence and in the presence of 0.5 mmol dm
trans-
3
,4-dihydroxycinnamic acid. Electrode geometric area: 0.42
2
cm . T=23°C.

R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
343
The results presented above indicate that the mecha-
nism of DHCA oxidation changes with changing cur-
rent or potential. To prove this convincingly, we carried
out an UV-visible spectroscopy analysis of DHCA so-
lutions following electrolysis at different constant cur-
rents corresponding to the stirring dependent and
stirring independent domains of the Tafel region. In the
low currents region the UV-visible spectra feature a
clear isosbestic point at 340 nm, indicating that the
oxidation of DHCA gives mainly one stable product, in
the solution phase, which is characterised by higher
absorption at the longer wavelengths (Fig. 3A). The
reaction most likely corresponds to the oxidation of the
OH groups of the aromatic ring to give a coloured
conjugated quinone.
Conversely, a plain decrease of the absorbance above
250 nm is observed when the electrolysis is carried out
in the higher current range (Fig. 3B), which indicates
the degradation of the organic compound. The fact that
this reaction occurs despite the steady-state electro-
chemical data provide no evidence of it in this region
Fig. 2. Variations of the electrode potentials for repeated
chronopotentiometric measurements with PbO /Pt electrodes
2
in stirred phosphate buffer solutions (pH=7.2) in the absence
−
3
(curve 1) and in the presence (curve 2) of 0.5 mmol dm
trans-3,4-dihydroxycinnamic acid. Applied constant current:
2
(
A) 0.3 mA; (B) 1 mA. Electrode geometric area: 0.42 cm .
T=23°C.
consecutive steady-state experiments, the effect of solu-
tion stirring is no longer observed and the polarisation
curves are similar to those observed with no DHCA
present. As a matter of fact, for repeated runs we
observed that in the low potential region the curves
shifted to slightly higher potentials.
This phenomenon is well illustrated by the data
reported in Fig. 2 referring to repeated experiments
carried out at constant applied current in stirred solu-
tions, both in the absence and in the presence of
DHCA. For the case of successive runs at relatively low
currents (Fig. 2A), corresponding to the stirring depen-
dent region in the polarisation curve, the potential
increases to higher values than those observed in phos-
phate buffer alone. If the electrode is then cleaned in a
solution of CH COONH4 and a new experiment is
3
performed in the DHCA solution, the observed steady-
state potential returns to a low value. This behaviour
can be ascribed to the adsorption of products deriving
from the oxidation of the dihydroxy-acid such as, for
example, polycondensation species.
This conspicuous time dependence of the potential is
not seen in analogous experiments with higher constant
applied currents (Fig. 2B). In this case, the potentials
measured in the absence and in the presence of DHCA
differ by few tens of a mV evidently indicating that, in
Fig. 3. UV-visible spectra as a function time of electrolysis of
−
3
0
.5 mmol dm
trans-3,4-dihydroxycinnamic acid at PbO /Pt
2
electrodes in stirred phosphate buffer solutions (pH=7.2).
Curves 1–5 correspond to: (A) 0 to 135 min at a constant
2
current of 1.8 mA/cm (E=1.55 V, SCE); (B) 0 to 100 min at
2
this region, O evolution dominates the anodic process.
a constant current of 50 mA/cm (E=2.03 V, SCE).
2

3
44
R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
appreciable after the concentration of DHCA has de-
creased below ꢀ20% of its initial value. Ozone forma-
tion finally reaches a plateau as the concentration of the
dihydroxy-acid drops further.
In our previous work on the oxidation of 4-
chlorophenol [21] we explained an analogous behaviour
by a mechanism that involves the reaction of active
oxygen intermediates formed upon discharge of water.
These intermediates either combine to give O and O3
2
evolution (reactions 3–5) or react with the organic
compound [for a review see [24]].
Since the steady-state approach can suffer from
severe limitations, especially at high positive potentials,
we conducted an impedance spectroscopy analysis in
order to obtain further insights into the oxidation of
DHCA occurring in parallel with O evolution. Indeed
2
the data reported in Fig. 5 show some interesting
aspects from the point of view of the reaction mecha-
nism in the higher currents range. The impedance plane
exhibits a capacitive arc in the high frequency range
and an inductive arc at low frequencies. The latter
disappears as the positive potential (or the DHCA
concentration in solution) increases.
Fig. 4. Oxidation of trans-3,4-dihydroxycinnamic acid and O3
formation at PbO /Pt electrodes in phosphate buffer solutions
2
2
(
pH=7.2), at a constant applied current of 23 mA/cm (cir-
cles) and 50 mA/cm (triangles). Initial concentration: 0.5
2
−
3
mmol dm
.
The conditions that determine the appearance of an
inductive loop have been discussed in the literature
(
Fig. 1 and Fig. 2B), suggests that the process is largely
[25–28] and are associated with an electrode two-step
controlled by the discharge of water to give highly
reactive OH radicals that are able to initiate the oxida-
tive demolition of DHCA and prevent the poisoning of
the surface by adsorbed intermediates.
reaction path involving an adsorbed intermediate gen-
erally written as [28]:
−
reactant“adsorbate+e step 1
Fig. 4 illustrates the conversion of DHCA in phos-
phate buffer as a function of electrolysis time for two
different values of the current density in the stirring
independence range, where the evolution of both O2
−
reactant+adsorbate“products+e step 2
For this reaction sequence the predicted evolution of
the impedance diagrams as a function of increasing
potential involves an attenuation of the inductive loop,
in agreement with the data of Fig. 5. A mechanism of
this type describes, for example, the evolution of chlo-
and O is possible. The same figure shows that, with
3
DHCA present, the efficiency of O formation is negli-
3
gible at the beginning of the electrolysis and becomes
Fig. 5. Nyquist impedance plots at different potentials for PbO /Pt electrodes in phosphate buffer solutions (pH=7.2) containing
2
−
3
2
0
.5 mmol dm
trans-3,4-dihydroxycinnamic acid. Electrode geometric area: 0.42 cm .

R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
345
concentration of the substrate remains essentially con-
stant. This occurs because ozone is consumed by sec-
ondary reactions with intermediates [30].
Since the ozonization reaction is generally first order
in ozone and in the organic substrate [23], one obvious
way of keeping the reaction going is, therefore, that of
increasing the yield of O formation. We could achieve
3
this by using metal doped PbO anodes [8]. Thus, for
2
example, we obtained current efficiencies of 6.8 and
8
2
.85% for Fe(III) and Co(II) modified PbO2 [31] at
0°C and 100 mA/cm . Lowering the temperature to
2
+
592°C increased the yields to 18 and 12%, respec-
2
tively, for an applied current of 50 mA/cm .
The effect of increasing the amount of O3 is illus-
trated in Fig. 7A, where curves 1, 2 refer to the con-
Fig. 6. Comparison of different methods for the oxidation of
−
3
trans-3,4-dihydroxycinnamic acid (0.5 mmol dm ): (1) direct
2
electrolysis method at 50 mA/cm ; (2) external-cell chemical
oxidation with O3 electrogenerated on PbO with a current
2
2
efficiency of 6–7% at 50 mA/cm and 22°C; (3) cathodic
oxidation experiment (see text), O3 electrogenerated on PbO
with a current efficiency of 6–7% at 50 mA/cm and 22°C.
2
2
rine, the dissolution of metals ([28] and refs. therein)
and the oxidation of carboxylic acids [29]. The last case,
i.e. the decarboxylation reaction:
−
−
R−COO “(R−COO)ads+e
(6)
(7)
−
−
(
R−COO)ads+R−COO “products+e
provides a possible explanation of the impedance re-
sults. We then propose that, to some degree, this pro-
cess occurs in parallel with the above discussed reaction
−
of DHCA with OH radicals formed by the discharge
of water.
3
.2. External cell reactions with electrogenerated ozone
This approach exploits the chemical reaction of
DHCA with O that is formed at a PbO anode. The
3
2
electrogenerated gas (O +O ) was flowed through an
3
2
external vessel containing DHCA in phosphate buffer,
following the reaction course by HPLC or UV-visible
spectroscopy.
Typical results are illustrated by curve 2 in Fig. 6, for
an ozone current efficiency of 6–7% at 50 mA/cm and
2
Fig. 7. Effect of an increase in the amount O on the chemical
3
oxidation of trans-3,4-dihydroxycinnamic acid in phosphate
buffer solutions (pH=7.2). Graph A: the current efficiency of
O3 electrogeneration was 6–7% for curve 1 and 14–16% for
curve 2; curves 1% and 2% show the formation and disappear-
ance of coloured intermediates. Graph B: comparison of the
external-cell data (1) and cathodic oxidation (2) using O3
electrogenerated with current efficiencies of 6–7% and 14–
16%, respectively.
room temperature. From this summary figure we can
now compare the results of the chemical reaction with
O with those of the direct electrolysis (curve 1), under
3
the same conditions of applied constant current. It is
interesting to note that while the rate of disappearance
of DHCA is initially higher for ex-situ method than for
the electrolytic reaction, for prolonged experiments the

3
46
R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
sumption of DHCA and curves 1%, 2% to the bleaching
of coloured intermediates as measured by the ab-
sorbance in the visible spectrum. It is clear that the
latter process strongly conditions the rate of ozoniza-
tion of DHCA. We have not tried to establish the exact
nature of the coloured intermediates; HPLC data indi-
cate a mixture of compounds probably originating from
the reaction of some unstable intermediates [32].
process (curve 1) in which about 50% less ozone is used.
Actually, this improvement is not even observed on the
whole time scale of the experiments (Fig. 7B): in the
long run, the situation is inverted, and COD measure-
ments after a reaction time of 120 min gave values that
were lower for the cathodic oxidation than for the
external-cell method by a factor of 1.5–1.7.
The behaviour described above can explained an
effective contribution of the radical route under the
operating conditions of the cathodic oxidation process.
In fact, hydrogen peroxide formed at a graphite
The homogeneous phase reaction of O with organic
3
compounds, has been previously investigated
[
23,30,32–36]. In principle, the oxidation can be ini-
tiated either by the direct attack of O on the organic
substrate or by secondary oxidants, such as OH radi-
cathode reacts with O giving rise to a highly oxidising
environment due to the occurrence of reaction:
3
3
−
cals, originating from the O decomposition [32,35]:
3
−
−
’
−
O +HO “OH +O +O
2
(11)
3
2
2
−
−
’
OH +O “O +H O
(8)
(9)
3
2
2
in addition to the above mentioned reactions [8–10]
and to the direct oxidation of DHCA by O . The latter
−
−
’
O +OH “O +OH
3
3
3
pathway probably prevails under initial conditions, as it
seems to be indicated by the fact that at relative short
times curves 2, 3 in Fig. 6 tend to converge. The
mechanism of this reaction is very likely similar to
proposed for the analogous case of p-coumaric acid,
involving the attack of the acrylic double bond by
ozone as the first step [35]. As the concentration of the
organic substrate decreases, the radical route would
constitute the prevailing oxidation mode. This can
provide an explanation of why the cathodic oxidation
approach gives overall better results.
−
’
−
O +H O“OH +OH +O
2
(10)
3
2
In accord with the above reactions, ozone stability in
aqueous solutions is reported to decrease as the pH
increases [37], and the contribution of the radical chain
route correspondingly augments.
3
.3. Cathodic oxidation with O /H O
3 2 2
The term cathodic oxidation has been used in con-
nection with the use of oxygen cathodes that produce
hydrogen peroxide as a mediator in oxidation processes
[
38]. In this case, the organic substrate is present in the
cathodic compartment and its degradation is initiated
by hydroxylation with OH radicals produced by the
decomposition of hydrogen peroxide. In principle, the
oxidation efficiency can be increased by the use of a
Fenton system based on the addition of iron salts. In
this work, however, we chose the O /H O approach
4. Concluding remarks
The oxidative degradation of trans-3,4-dihydroxycin-
namic acid was investigated using three different meth-
ods, two of which have in common the use of
electrogenerated ozone as a potential oxidant: (i) direct
electrolysis; (ii) external cell chemical reactions with O3
and (iii) a cathodic oxidation in which an O /O mix-
3
2
2
[
21] in which an O stream is used to sweep the O /O
2
2
3
gas mixture produced at the PbO2 anode into the
cathodic compartment.
The results for the cathodic oxidation of DHCA are
plotted as curve 3 in the summary Fig. 6 and as curve
2
3
ture is fed into the cathodic compartment, where the
H O produced reacts with ozone to yield radicals that
2
2
bring about the demolition of the organic compounds.
The direct electrolysis is a complex process in which
the nature of the oxidation processes depends on poten-
tial. At relatively low potentials the process shows
stirring dependence and leads essentially to one oxida-
tion product in solution and to adsorbed species on the
electrode. At some higher potential the reaction be-
comes stirring independent and poorly distinguishable
1
in Fig. 7B. It is noteworthy that UV-vis spectroscopic
measurements show that DHCA undergoes no change
at an O cathode in the absence of O . In Fig. 6 the
2
3
data of curve 3 can be compared with the results of the
direct electrolysis and with the chemical oxidation. It is
apparent that, under the same conditions of applied
constant current and O yield, the cathodic oxidation
3
provides the more efficient method.
from the O evolution. Nevertheless, the demolition of
2
This conclusion is still valid if a comparison is made
between the results of Fig. 6 (curve 3) and those of Fig.
the organic substrate is observed in this potential range,
and this suggest a mechanism in which the active
oxygen radical intermediates, formed from the dis-
charge of water, act as oxidation mediators. The reac-
tion of these intermediates with the organic substrate
competes with the process of O formation: no O was
7A (curve 2) obtained upon increasing the efficiency of
O electrogeneration. To facilitate the comparison, the
3
data are plotted again in the graph of Fig. 7B. The
salient point is that the chemical oxidation (curve 2) is
but slightly more efficient than a cathodic oxidation
3
3
detected until the concentration of the di-hydroxyacid

R. Amadelli et al. / Electrochimica Acta 46 (2000) 341–347
347
dropped below ꢀ20% of its initial value. Interestingly,
in our cited work on the oxidation of 4-chlorophenol
we observed the same behaviour.
From an analysis of impedance data in the region of
high potentials, it seems possible to conclude that, in
addition to the processes involving the reaction of
DHCA with oxygen radical species, an electron transfer
reaction takes place that can be ascribed to the decar-
boxylation of the acid.
[11] M Schiavello, Photocatalysis and Environment, Kluwer
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The homogeneous chemical decomposition of DHCA
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involves the direct reaction of O3 with the organic
[
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3
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examination of the results shows that the ex-situ and
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methods. This confirms the conclusions of our previous
investigation on the oxidative degradation of 4-
chlorophenol and indicates this method as a promising
one in the abatement of noxious chemicals.
(
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