Electrochemical preparation of peroxodisulfuric acid using boron doped diamond thin film electrodes

Article abstract of DOI:10.1016/S0013-4686(02)00688-6

We have investigated the electrochemical oxidation of sulfuric acid on boron-doped synthetic diamond electrodes (BDD) obtained by HF CVD on p-Si. The results have shown that high current efficiency for sulfuric acid oxidation to peroxodisulfuric acid can be achieved in concentrated H₂SO₄ (≥ 2 M) at moderate temperatures (8-10 °C). The main side reaction is oxygen evolution. Small amounts of peroxomonosulfuric acid (Caro's acid) have also been detected. A reaction mechanism involving hydroxyl radicals, HSO₄^- and undissociated H₂SO₄ has been proposed. According to this mechanism electrogenerated hydroxyl radicals at the BDD anode react with HSO₄^- and H₂SO₄ giving peroxodisulfate.

Full text of DOI:10.1016/S0013-4686(02)00688-6

Electrochimica Acta 48 (2002) 431ꢁ436
/
www.elsevier.com/locate/electacta
Electrochemical preparation of peroxodisulfuric acid using boron
doped diamond thin film electrodes
a,
b
b,1
a,1
K. Serrano ꢀ, P.A. Michaud , C. Comninellis , A. Savall
Laboratoire de G e´ nie Chimique UMR 5503, Universit e´ Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex 04, France
a
b
Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland
Received 23 July 2002; received in revised form 14 October 2002
Abstract
We have investigated the electrochemical oxidation of sulfuric acid on boron-doped synthetic diamond electrodes (BDD)
obtained by HF CVD on p-Si. The results have shown that high current efficiency for sulfuric acid oxidation to peroxodisulfuric
acid can be achieved in concentrated H
evolution. Small amounts of peroxomonosulfuric acid (Caro’s acid) have also been detected. A reaction mechanism involving
2
SO
4
(ꢀ
/
2 M) at moderate temperatures (8ꢁ
/
10 8C). The main side reaction is oxygen
ꢂ
4
hydroxyl radicals, HSO
radicals at the BDD anode react with HSO
2002 Elsevier Science Ltd. All rights reserved.
and undissociated H
2
SO
4
has been proposed. According to this mechanism electrogenerated hydroxyl
SO giving peroxodisulfate.
ꢂ
4
and H
2
4
#
Keywords: Diamond electrode; Electrosynthesis; Sulfuric acid; Peroxodisulfuric acid
1
. Introduction
The electrochemical production of peroxodisulfates
strongly depends on the electrode material selected for
the oxidation of sulfuric acid. The main criterion is the
high overpotential value for oxygen evolution reaction
provided by the anode material.
The electrolysis of sulfuric acid solution could allow,
in certain conditions, the electrosynthesis of persulfuric
acid H S O . Persulfuric acid is widely used as oxidant
2
2
8
in many applications. In wastewater treatment it is used
for cyanides removal of effluents. Other applications
concern dyes oxidation, fibres whitening, promotion for
radical polymerization, total organic compound
measurements. . . [1]. Persulfuric acid is an important
intermediate product in the electrochemical production
This study concerns the peroxodisulfate formation
using as anode the synthetic diamond film electrodes.
Diamond films electrodes show high overpotentials for
both hydrogen and oxygen evolution in aqueous solu-
tions. The electrochemical window at diamond electro-
ꢂ1
des is about 3 V large for a 1 mol l
sulfuric acid
of hydrogen peroxide [1ꢁ
/
3].
solution, and exhibits a low background current in the
solvent stability domain [4ꢁ6]. It has been shown that
2
ꢂ
The S O
ꢂ
ions are formed from the oxidation of
ꢂ
2
8
/
2
4
SO
and HSO4 species at very high potentials:
only reactions involving simple electron transfer are
active on diamond electrodes in the potential region of
water stability. For reactions with more complex
mechanisms, oxidation can take place on diamond
electrodes only in the potential region of supporting
electrolyte or/and water discharge [7].
The diamond electrodes have been also successfully
used for the anodic oxidation of cyanide and the
cathodic recovery of heavy metals [8].
2
2
HSO ꢂ4 0 S O8 ꢃ2H ꢃ2e E ꢄ2:123 V
2ꢂ
ꢃ
ꢀ
(1)
(2)
2
2ꢂ
2ꢂ
ꢀ
SO 0 S O8 ꢃ2e E ꢄ2:01 V
4
2
The discharge of water could be a secondary reaction:
ꢃ
ꢀ
H O 0 1=2O ꢃ2H ꢃ2e E ꢄ1:023 V
(3)
2
2
ꢀ Corresponding author. Tel.: ꢃ
56139
/
33-5-61-558194; fax: ꢃ
/
33-5-61-
Actual electrochemical processes for persulfates
synthesis use noble metal coated anodes (platinized
titanium, tantalum or niobium). The anodic formation
5
E-mail address: serrano@chimie.ups-tlse.fr (K. Serrano).
1
ISE member.
0013-4686/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 1 3 - 4 6 8 6 ( 0 2 ) 0 0 6 8 8 - 6

432
K. Serrano et al. / Electrochimica Acta 48 (2002) 431ꢁ436
/
of persulfates has been studied [9ꢁ
/
16] but the reaction
controlled EG&G potentiostat model M 273. Diamond
was used as working electrode, Hg/Hg SO , K SO (sat)
as a reference and Pt as counter electrode.
mechanism is still unsettled. Two contradictions of
reaction paths on platinum electrode were reported in
literature. Some authors reported that formation of
2
4
2
4
The oxidation of sulfuric acid is performed in a two
compartments electrolytic flow cell under galvanostatic
conditions (Fig. 1). Diamond is used as anode and
zirconium as cathode. Both electrodes were disks (90
2
8
ꢂ
ꢂ
ꢂ
4
2ꢂ
S2O ions result from direct oxidation of HSO [9] or
ꢂ
2
4
SO
[10] or simultaneous by HSO4 and SO4 [11]
according to equations (1) and (2). Alternatively to this
mechanism, others suggest a primary formation of
radicals which is stemming from water discharge
2
mm diameter) with a geometric area of 63 cm each. The
inter-electrode gap was 10 mm.
[
12,13] followed by a reaction with sulfate.
In most of these studies, one or some adsorption
The set-up is illustrated in Fig. 2. The electrolytes
(catholyte and anolyte) were stored in two 250 ml
thermoregulated glass reservoirs (B1 and B2) and
circulated through the electrolytic cell using two cen-
trifugal pumps (P1 and P2). The flow rate of the
phenomena were shown to determine the kinetics of
ꢂ
2
8
S2O
formation on platinum electrode. Elbs and
Sch o¨ nherr [14] observed a considerable rise of the
current efficiency from 45 to 85% of persulfate forma-
tion after addition in the sulfuric acid solution of
ammonium cations. They ascribed the effect of various
cations on the current efficiency to an unspecified
catalytic effect. According to Smit and Hoogland [10]
the coverage of the surface of platinum by oxygen is
ꢂ1
electrochemical cell was 200 l h . The flow rate of
electrolyte supply to the cell stored in the tanks B3 and
ꢂ1
B4 was 0.04 l h . The value of the current density used
ꢂ2
was 23 mA cm . The electrolysis duration is 20 h. The
peroxomonosulfuric acid and the hydrogen peroxide are
simultaneously titrated by back titration with As(III)
and Ce(IV).
2
4
ꢂ
increasingly replaced by patchs of adsorbed SO
2
which S O
2
on
formation proceeds instead of oxygen
ꢂ
The sum of peroxomonosulfuric acid, hydrogen
peroxide and peroxodisulfuric acid is back titrated using
Fe(II) and KMnO . The peroxodisulfuric acid is de-
8
evolution. Another explanation is given by Kasatkin
and Rakov [15]: these additives block up the active
centers and favor recombination of atomic oxygen
which reacts with sulfate species. According to Frumkin
4
duced from the two preceding results.
[16], oxygen is chemisorbed resulting in a discontinuous
rise of oxygen overvoltage.
3. Results and discussion
The aim of this study, following a previous paper [17],
2
ꢂ
is to propose the reaction path of the S O
2
formation
on synthetic diamond film electrodes in sulfuric acidic
medium.
8
The cyclic voltamogramm, Fig. 3, shows the cathodic
part before (A) and after (B) the scan in the anodic part
in a sulfuric acid solution (1 M). During the second cycle
The first part is the electrochemical study carried out
by cyclic voltammetry. The second part concerns the
preparation of persulfate by galvanostatic electrolysis.
The influences of the temperature and the concentration
of sulfuric acid are pointed out.
(
B) a specie, formed during the anodic scan, is reduced
at ꢂ1 V versus ref. This phenomenon is not observed in
/
perchloric acid. This specie should be peroxodisulfate
formed by oxidation of the sulfate ions during the first
scan. In order to confirm the formation of peroxodi-
sulfate in the anodic scan, an anodic polarization is
holded at 2.4 V versus ref. (corresponding to a charge of
2
. Experimental
Boron-doped diamond films, Diachem , were synthe-
1.2 C) and immediately a cathodic scan is performed.
†
sized by the hot filament chemical vapor deposition
technique (HF CVD) on conducting p-Si substrate (0.1
V cm, Siltronix). The filament temperature ranged from
2
440 to 2560 8C and the substrate one was kept at
30 8C. The reactive gas used was methane in an excess
8
of dihydrogen (1% CH in H ). The doping gas was
4
2
trimethylboron with a concentration of 3 ppm. The gas
mixture was supplied to the reaction chamber, providing
ꢂ1
a 0.24 mm h
growth rate for the diamond layer. The
diamond films were about 1 mm thick. This HF CVD
process, produces columnar, randomly textured, poly-
crystalline films.
Electrochemical measurements were carried out in a
conventional three-electrode cell using a computer
Fig. 1. Electrochemical cell used for electrochemical oxidation of
sulfuric acid. Aƒ, anode top; A?, anode support; A, anode; B,
separating plate; C, cathode; C?, cathode support; Cƒ, cathode top.

K. Serrano et al. / Electrochimica Acta 48 (2002) 431ꢁ
/436
433
Fig. 4. I vs. E curve after an anodic polarization at ꢃ
chargeꢄ
acid and (b) in sulfuric acid (1 mol dm ) (c) after addition of K
/
2.4 V vs ref.
Fig. 2. Set up used for electrochemical oxidation of sulfuric acid on Si/
diamond anodes. C1, electrochemical cell; B1 and B2, anodic and
cathodic (respectively) thermoregulated tank (250 ml); B3 and B4 feed
tank anodic and cathodic (10 l); B5 and B6, purge tank; P1 and P2,
centrifugal pump feeding the anodic and cathodic compartment.
(
/
1.2 C) of boron doped diamond electrode (a) in perchloric
ꢂ3
2 2 8
S O
ꢂ2
ꢂ1
(
4ꢅ/10
M) in sulfuric acid. Scan rateꢄ
/
50 mV s , Tꢄ20 8C,
/
reference electrode, Hg/Hg
num.
2
SO
/K
4 2
SO
4
(sat); counter electrode, plati-
ꢂ3
dissociated sulfuric acid ([H SO ]) in mol dm
2
are
4
given by:
ꢂ
4
2
[
HSO ]ꢄ0:07663ꢃ0:7716Cꢂ0:03052C ꢃ2:303
ꢅ10ꢂ3C3
(5)
(6)
ꢂ3
2
[
H SO ]ꢄ5:549ꢅ10 ꢂ0:03389Cꢃ0:02862C
2
4
ꢃ4:984ꢅ10ꢂ4C3
ꢂ3
ꢂ
where C (mol dm ) is defined as: Cꢄ
/
[HSO ]ꢃ
/
4
2
ꢂ
[
undissociated sulfuric acid. These equations are for
SO ]ꢃ[H SO ], in which [H SO ] represents the
/
4
2 4 2 4
ꢂ3
0
the concentrations of different species as a function of
B
/
C B
/
8 mol dm
and Tꢄ25 8C. Fig. 5 illustrates
/
the concentration C.
According to refs [28ꢁ
30], only HSO 4ꢂ and molecular
Fig. 3. Cyclic voltammogram of boron doped diamond electrode in
ꢂ3
sulfuric acid (1 mol dm ) [A] before and [B] after the scan in the
50 mV sꢂ1, Tꢄ
Hg/Hg SO /K SO (sat); counter electrode, platinum.
/
anodic part. Scan rateꢄ
/
/20 8C; reference electrode,
+
H SO react with OH radicals to form sulfate radicals,
2
4
2
4
2
4
following reactions (7) and (8):
k1
Fig. 4 represents the plots obtained in perchloric and
sulfuric acid, respectively. The lack of the cathodic wave
after the anodic polarization in perchloric acid confirms
that the cathodic wave is formed after oxidation of the
sulfate ions. Besides, the cathodic wave observed at
ꢂ
+
ꢂ+
HSO ꢃOH 0 SO ꢃH O
(7)
(8)
(9)
4
4
2
k2
+
ꢂ+
ꢃ
H SO ꢃOH 0 SO ꢃH O
2
4
4
3
k3
0 S
ꢂ+
ꢂ+
2ꢂ
8
SO
ꢃSO
O
4
4
2
ꢂ1 V versus ref. increases after an addition of K S O
in the bulk, curve (c).
/
2 2 8
At the same time, a competitive reaction between the
hydroxyl radicals may take place:
Thus the formation of the peroxodisulfuric acid
coincides with the oxygen formation. The mechanism
of anodic evolution of O has been studied extensively at
2
different electrodes in aqueous media [21,22]. The first
step in the mechanism is the anodic discharge of H O to
2
+
produce hydroxyl radicals (OH ) as indicated by eq (4):
+
ꢃ
H O 0 OH ꢃH ꢃe
(4)
2
In sulfuric acid solutions, sulfate anion, hydrogeno-
sulfate anion and undissociated sulfuric acid coexist
2
4
ꢂ, HSOꢂ
as a function of C according to Librovich and Maiorov [23].
[
concentrations of hydrogenosulfate ([HSO ]) and un-
23ꢁ
/
26]. According to Librovich and Maiorov [23] the
ꢂ
Fig. 5. Concentration of SO
4
ions and undissociated H
2
SO
4
4

434
K. Serrano et al. / Electrochimica Acta 48 (2002) 431ꢁ
/436
k4
+
+
ꢃ
OH ꢃOH 0 O ꢃ2H ꢃ2e
(10)
2
The values of the rate constants k1-4 are reported in
Table 1.
If we consider that the hydroxyl radicals are free and
the concentration is constant in the reaction layer and
according to steps 7 to 10, the rate of disappearance of
the hydroxyl radicals is given by the following expres-
sion:
+
d[OH ]
vꢄ
dt
ꢂ
4
+
+
+ 2
ꢄk [HSO ][OH ]ꢃk [H SO ][OH ]ꢃk [OH ]
1
2
2
4
4
Fig. 6. Plot of hexp vs. C on boron doped diamond electrode. Current
23 mA cmꢂ2, Tꢄ
(11)
densityꢄ
/
/
9 8C.
The current efficiency, h, is defined as the ratio
between the number of electrons used for the sulfate
radical formation and the total number of the electrons
used.
The influence of the concentration C on the experi-
mental current efficiency at Tꢄ9 8C is reported in Fig.
/
6
. It appears two zones.
ꢂ
+
+
.
For C B
concentration up to 90%.
For C ꢀ2 M, the current efficiency is constant to a
/
2 M, the current efficiency increases with the
k [HSO ][OH ] ꢃ k [H SO ][OH ]
1
4
2
2
4
hꢄ2
k [OH ] ꢃ k [HSO ][OH ] ꢃ k [H SO ][OH ]
+
2
ꢂ
4
+
+
4
1
2
2
4
.
/
(
12:a)
maximum value of 95%.
The relation (Eq. (12.a)) could be linearized:
It appears that in these experimental conditions of
ꢂ2
temperature (9 8C) and current density (23 mA cm ),
h
1
ꢅ
ꢂ
the maximum current efficiency is obtained with C ꢀ2
/
1
ꢂ h [HSO ]
4
M. It is interesting to note that the maximum value of
the current efficiency in this study is higher than those
obtained in industrial electrosynthesis on platinum
k1
k2
[H SO ]
2
4
ꢄ2
ꢃ
ꢅ
+
(12:b)
ꢂ
4
+
k [OH ] 2k [OH ] [HSO ]
4
4
electrode (about 70ꢁ75%) [1,27].
/
Galvanostatic electrolyses have been performed with
regular samples which are taken at the input and output
The plot Fig. 7 checks the relation (Eq. (12.b)). The
linear representation is in accordance with the reaction
path of the peroxodisulfuric proposed in the set of
reactions (7), (8), (10). The slope allows to calculate the
average concentration of hydroxyl radicals in the
reaction layer. Considering the rate constants given in
of the set-up. The variation of the concentration of
2
ꢂ
ꢂ3
peroxodisulfuric acid [S O ] (mol m ), allows to
2
8
calculate the experimental current efficiency hexp ex-
pressed as:
+
ꢂ5
ꢂ1
Table 1, one obtains [OH ]:
/10
mol l
.
zFF
2
2ꢂ
hexp
ꢄ
([S O8ꢂ]
ꢂ[S O8 ]inlet
)
(13)
The plot reported in Fig. 8 evidences the influence of
the temperature on the current efficiency of the perox-
odisulfate production (curve A) in a sulfuric acid
2
outlet
2
I
where z is the number of exchanged electrons (two
2
ꢂ
solution (1 M) by galvanostatic electrolysis (iꢄ23 mA
/
electrons for 1 mol of S O ), F the Faradic constant (C
8
2
molꢂ1), F the flow rate of the electrochemical cell (m
3
ꢂ1
s
), and I is the current (A).
Table 1
Values of the constant rate k
1
, k
, k
2 3
and k
4
of the sulfate radical at pH
1
3
(dm mol
ꢂ1
sꢂ1
5
k
1
)
4.7ꢅ10 ꢀꢀ [28];
3
5
.5ꢅ10 ꢀ [29];
5
6
.9ꢅ10 ꢀ [30]
3
(dm mol
ꢂ1 -1
7
k
k
k
2
3
4
s )
ꢂ1 ꢂ1
1.4ꢅ10 ꢀꢀ [28]
3
(dm mol
8
s
)
(7.691)10 ꢀꢀ [28]
3
(dm mol
ꢂ1
ꢂ1)
10
s
1.1ꢅ10 ꢀꢀ [28]
ꢀ
, Tꢄ25 8C and ꢀꢀ, Tꢄ20 8C [28ꢁ
/
30].
Fig. 7. Graphical representation of the model given by the Eq. (12.b).

K. Serrano et al. / Electrochimica Acta 48 (2002) 431ꢁ
/436
435
ꢃ
H SO ꢃH O 0 O ꢃH SO ꢃ2H ꢃ2e
(19)
2
5
2
2
2
4
According to Balej et al. [31], the oxidation rate of
ꢂ
2
Caro’s acid under common conditions of S O8 elec-
2
trosynthesis is higher than the rate of the eventual
formation of H SO . This is the reason why the
2
5
concentration of Caro’s acid does not increase with
the temperature (Fig. 8).
Acknowledgements
Authors thank the Swiss Center for Electronics and
Microtechnology (CSEM) for preparing the boron-
doped diamond electrodes. K. Serrano wishes to express
her thanks to the Institute of Chemical Engineering,
Swiss Federal Institute of Technology, Lausanne for
financial support.
Fig. 8. Current efficiency of peroxodisulfate (A) and Caro’s acid (B)
production vs. temperature (8C) in sulfuric acid (1 M), current
ꢂ2
densityꢄ
/
23 mA cm
.
cmꢂ2). We can note that the current efficiency decreases
with the temperature. At 65 8C, it falls down to 27%.
The persulfuric acid is unstable in aqueous solution,
decomposing in dilute sulfuric acid solutions with
liberation of oxygen according to reaction (14):
References
2
8
ꢂ
ꢂ
4
1
2
S2O ꢃH O 0 2HSO ꢃ O
(14)
2
2
[1] N. Ibl, H. Vogt, in: J.O.’M. Bockris, B.E. Conway, E. Yeager,
R.E. White (Eds.), Comprehensive Treatise of Electrochemistry,
vol. 2, Plenum Press, New York, 1981, p. 224.
In more concentrated sulfuric acid solutions hydrogen
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and (16)).
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H S O ꢃH O 0 H SO ꢃH SO
(15)
(16)
2
2
8
2
2
5
2
4
[5] W. Haenni, H. Baumann, C. Comninellis, D. Gandini, P.
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2
5
2
2
2
2
4
(
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5
[
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peroxide.
The decomposition rate of potassium persulfate in
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1
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[
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[
[
[
[
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2
ꢂ
2ꢂ
ꢃ
2ꢂ
8
ꢂd[S O8 ]=dtꢄk? [S O ]ꢃk?[H ][S O
]
(17)
(18)
2
1
2
8
2
2
¨ ¨
[13] F. Foerster, in: J.A. Barth (Ed.), Elektrochemie wassiger Losun-
gen, Leipzig, 1923.
14] K. Elbs, O. Sch o¨ nherr, Z. Elektrochem. 2 (1895) 245.
2ꢂ
2ꢂ
ꢃ
ꢂd[S O8 ]=dtꢄ [S O8 ](k? ꢃk?[H ])
2
2
1
2
[
[
[
[
ꢃ
with k ꢄ
/k
ꢃk [H ].
? ?
2
/
15] E.V. Kasatkin, A.A. Rakov, Electrochim. Acta 10 (1965) 131.
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Electrochem. Solid-State Lett. 3 (2) (2000) 77.
0
1
The decrease of the current efficiency with the
temperature can be explained by the increase of the
constant rate of the decomposition of the persulfate [30],
[
18] J.P. Hoare, The Electrochemistry of Oxygen, Wiley, New York,
1968.
k .
0
[
[
[
[
[
19] J.O’.M. Bockris, J. Chem. Phys. 24 (1956) 817.
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Khim. 3 (1977) 684.
Caro’s acid is present in the bulk but the quantities
are low (see Fig. 8, curve B), since Caro’s acid (which is
2
ꢂ
the product of the hydrolysis of S O
2
(eq. 15)) is
oxidized during the electrolysis following the reaction
below:
8

436
K. Serrano et al. / Electrochimica Acta 48 (2002) 431ꢁ436
/
[
[
[
24] P.Y. Jiang, Y. Katsumura, R. Nagaishi, M. Domae, K. Ishikawa,
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