3
574
J. Iniesta et al. / Electrochimica Acta 46 (2001) 3573–3578
[TOC] −[TOC] }/6
{
0
t
evolution has been proposed [14–16]. This mechanism
can explain the complete oxidation of organics to CO2
by electrogenerated hydroxyl radicals on ‘non-active’
electrodes and the selective oxidation on ‘active’ elec-
trodes. According to this mechanism, boron-doped dia-
mond, a ‘non-active’ electrode, is a promising anode for
the complete combustion of organics to CO2 for
wastewater treatment.
%
CO =
×100
(1)
2
[
Ph] −[Ph]
0 t
where [TOC] and [TOC] are the total organic carbon
0
t
−3
at times 0 and t (in mmol dm ), respectively, [Ph]0
and [Ph] the concentration of phenol at times 0 and t
t
−
3
(in mmol dm ). The term {[TOC] −[TOC] }/6 repre-
0
t
sents the mole number of phenol converted into CO2.
The percentage of phenol converted into aromatic
intermediates, relative to the amount of phenol con-
verted, has been defined as:
The oxidation of some organic compounds on BDD
®
compared to traditional DSA has been investigated
also in a previous paper [12]. It has been found that at
IrO2 anodes, mainly selective oxidation occurs by
chemisorbed oxygen at electrogenerated active sites,
and at diamond anodes, organics are fully oxidized at
high overpotentials via intermediation of electrogener-
[
Aromatics]
%Aromatics=
×100
(2)
[
Ph] −[Ph]
0 t
where [Aromatics] is the concentration of aromatic
intermediates (1,4 benzoquinone, hydroquinone and
catechol) in mmol dm
−
3
.
ated OH radicals.
The instantaneous current efficiency (ICE) for the
anodic oxidation of phenol has been calculated from
the values of COD using the following relation [18]:
In this work we investigate the possibility to use the
synthetic diamond film electrodes not only for phenol
complete combustion but also for electro-organic syn-
thesis of aromatic compounds such as benzoquinone,
hydroquinone and catechol. The electrochemical oxida-
tion of phenol on BDD has been investigated in
perchloric acid medium by cyclic voltammetry,
chronoamperometry and bulk electrolysis.
[
(COD) −(COD)t+Dt]
t
ICE=FV
(3)
8IDt
where (COD) and (COD)
are the chemical oxygen
t+Dt
t
−
3
demands at times t and t+Dt (in g dm ), respectively,
I is the current (A), F is the Faraday constant (96487 C
−
1
3
mol ), V is the volume of electrolyte (dm ) and 8 is
−
1
the equivalent mass of oxygen (g eq ).
2
. Experimental
Boron-doped diamond films were synthesized by the
3
. Results and discussion
hot filament chemical vapor deposition technique (HF
CVD) [11] on conducting p-Si substrate (Siltronix). The
thickness of the obtained diamond film was about 1
mm. Electrochemical measurements were carried out in
a conventional three-electrode cell using a computer
controlled EG&G potentiostat model M 273. Diamond
has been used as working electrode, Hg/Hg SO ·K SO
4
3
.1. Cyclic 6oltammetry and chronoamperometry
Fig. 1 shows typical cyclic voltammetric curves for
BDD in a solution containing 2.5 mM of phenol in 1 M
HClO at a scan rate of 100 mV s . In the first scan
−
1
4
2
4
2
(
curve a, Fig. 1) an anodic current peak corresponding
(
sat) as a reference and Pt as counter electrode.
Bulk oxidation of organics was performed in a one-
compartment electrolytic flow cell under galvanostatic
to the oxidation of phenol is observed at about 1.67 V.
As the number of cycles increase, the anodic current
peak decreases until almost zero after about five cycles
conditions. Diamond anode was a disk (80 mm diame-
(
curve b, Fig. 1). Even if diamond is well known to
have weak adsorption properties due to its inert surface
3], this decrease in electrode activity appears to be
2
ter) with a geometric area of 50 cm . More details on
the equipment used in bulk electrolyses are given else-
where [10,13].
The total organic carbon (TOC) of the solutions was
monitored during the electrolyses using a Shimadzu
050. The oxidation products formed during the anodic
oxidation of phenol were monitored by HPLC on a
Shimadzu series 6, using a Nucleisil C18 column, with
[
originated by deposition of polymeric adhesive prod-
ucts on the electrode surface. However, the electrode
deactivation seems to be less pronounced than with
platinum and other traditional electrodes [19].
Washing with organic solvents (isopropanol) the elec-
trode cannot reactivate. However, the electrode surface
can restore its initial activity by an anodic polarization
in the same electrolyte in the potential region of water
decomposition (E\2.3 V vs. SHE). In fact, this poten-
tial is in the region of water discharge and on BDD it
involves the production of active intermediates, proba-
bly hydroxyl radicals, that oxidize the polymeric film
on the surface.
5
4
0% water/60% acetonitrile as the mobile phase at a
3
−1
flow rate of 0.80 cm min . The chemical oxygen
demand (COD) of the solutions was monitored during
the electrolyses using a HACH DR200 system.
The percentage of phenol converted into CO , rela-
2
tive to the amount of oxidized phenol, has been calcu-
lated from the values of TOC using the relation [17]: