Evidence for the electrochemical production of persulfate at TiO2 nanotubes decorated with PbO2

Article abstract of DOI:10.1039/c7nj02604h

It is well known that PbO₂-based electrodes are considered to be non-active anodes, producing higher concentrations of hydroxyl radicals in aqueous solutions, and consequently, favouring the electrochemical degradation of organic pollutants. However, no evidence has been reported on the production of persulfates using this kind of electrode in sulphate aqueous solutions. For this reason, the aim of this work is to prepare (by an electrochemical procedure (anodization and electrodeposition)) and characterize (by X-ray diffraction, scanning electron microscopy, and potentiodynamic measurements) Ti/TiO₂-nanotubes/PbO₂ disk electrodes (with a geometrical area of 65 cm²) in order to evaluate the electrochemical production of persulfate using Na₂SO₄ solution as the support electrolyte and applying current densities of 7.5 and 60 mA cm^-2, as well as the influence of the electrosynthesis of hydroxyl radicals, in concomitance. The results clearly showed that significant production of hydroxyl radicals and persulfate is achieved at the Ti/TiO₂-nanotubes/PbO₂ surface, but this depends on the current density. The production of OH at the Ti/TiO₂-nanotubes/PbO₂ surface in Na₂SO₄ solution was confirmed by a RNO spin trapping reaction. The results were compared with those of a Ti/Pt electrode in order to understand the effect when a lower amount of OH is produced at the active anode surface. Based on the results, the Ti/TiO₂-nanotubes/PbO₂ anode could exhibit good electrocatalytic properties for environmental applications involving persulfate oxidants.

Full text of DOI:10.1039/c7nj02604h

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Volume 40 Number
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January 2016 Pages 1–846
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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1
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3
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Evidence of the electrochemical production of persulfate at TiO
2
-
nanotubes decorated with PbO
2
1
2
2
José Eudes L. Santos , Marco Antonio Quiroz , Monica CerroꢀLopez , Dayanne
1
1,3,*
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
Chianca de Moura , Carlos A. MartínezꢀHuitle
1
Federal University of Rio Grande do Norte, Institute of Chemistry, Lagoa Nova ꢀ CEP
59.072ꢀ970, RN, Brazil. Tel/Fax.: +55 (84) 3211ꢀ9224
2
Universidad de las Américas Puebla. Grupo de Investigación en Energía y Ambiente.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
ExHda. Sta. Catarina Martir s/n, Cholula 72820, Puebla, México.
3
Unesp, National Institute for Alternative Technologies of Detection, Toxicological
Evaluation and Removal of Micropollutants and Radioactives (INCTꢀDATREM),
Institute of Chemistry, P.O. Box 355, 14800ꢀ900, SP, Brazil.
Corresponding author: carlosmh@quimica.ufrn.br
2
ABSTRACT: It is wellꢀknown that PbO ꢀbased electrodes are considered as nonꢀactive
anodes, producing higher concentrations of hydroxyl radicals in aqueous solutions, and
consequently, favouring the electrochemical degradation of organic pollutants.
However, no evidences have been reported about the production of persulfates by
using this kind of electrodes in sulphate aqueous solutions. For this reason, the aim of
this work is to prepare by electrochemical procedure (anodization and
electrodeposition) and characterize (by Xꢀray diffraction, scanning electron microscopy
2
and potentiodynamic measurements) Ti/TiO ꢀnanotubes/PbO disk electrodes (65 cm
2
2
of geometrical area) to evaluated the electrochemical production of persulfate in
−2
Na
2 4
SO solution as support electrolyte by applying 7.5 and 60 mA cm as well as the
influence of electrosynthesis of hydroxyl radicals, in concomitance. Results clearly

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29
30
31
32
33
34
35
showed that, a significant production of hydroxyl radicals and persulfate is achieved at
●
Ti/TiO
2
ꢀnanotubes/PbO
2
, but it depends on the current density. The production of OH
at Ti/TiO
2
ꢀnanotubes/PbO
2
surface in Na
2
SO
4
solution was confirmed by RNO spin
trapping reaction. The results were compared with Ti/Pt electrode to understand the
●
effect when lower amount of OH is produced at active anode surface. Based on the
results, Ti/TiO
for environmental applications involving persulfate oxidant.
2
Keywords: TiO nanotubes; Persulfate; Hydroxyl Radical; PbO electrodeposition
2 2
ꢀnanotubes/PbO anode could exhibit good electrocatalytic properties
2
36
1. INTRODUCTION
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
Advanced oxidation processes (AOPs) emerge as an alternative due to their
efficiency in oxidizing a wide variety of organic contaminants by the in-situ generation
1
ꢀ4
•
1
2−
of highly oxidative species, such as hydroxyl radicals ( OH) , persulfate (S
2
O
8
) and
•−
•
2−
•−
sulfate radical (SO4 ). Recently, OH and S O /SO4 ꢀbased advanced oxidation
2
8
processes (AOPs) have attracted extensive attention for degradation of persistent
1
,2
organic contaminants in waters and wastewaters. Metal ions and photochemical
•
•−
radiation are the common methods to produce OH and SO
4
by activation of their
precursors. For this reason, the development of efficient activation approach is an
5
ꢀ7
important target. In this frame, electrochemical technology has drawn increasing
4
,8
attention in recent years as a promising green tool with electrons acting as the green
reactant to produce/activate this class of oxidants.
•
Anodic oxidation (AO) is the most used approach to produce OH via
electrolysis of water to O at an anode (M) with high O ꢀoverpotential to generate,
2
2
•
physically or chemically, adsorbed hydroxyl radical M( OH) as intermediate (reaction
8
ꢀ11
(1))
.

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•
+
ꢀ
52
M + H
2
O → M( OH) + H + e
(1)
2
−
•−
53
54
55
Meanwhile, S O8 and SO4 oxidants can be also electrochemically produced
2
•
via indirect AO of sulphate (equation 2) or indirect oxidation via OH (equation 3 and
8
,9,12, 13
4).
2
−
2−
56
57
58
2SO
4
→ S
2
O
8
(2)
2
−
●
−●
SO
4
4
+ OH → SO
4
+ H
2
O
(3)
−
●
−●
2−
SO
+ SO
4
→ S
2
O
8
(4)
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
However, the production or activation of these strong oxidants is not feasible at
1, ,8,14
4
all electrodes and as it is wellꢀknown
this mainly depends on the nature of
electrocatalytic material. Among the anodes tested, boronꢀdoped diamond (BDD)
•
2−
•− 15ꢀ26
electrode is preferred to produce electrochemically OH, S
2
O
8
and SO
4
.
In the
•
•
case of OH, the weak BDDꢀ OH interaction and greater O overpotential endorses the
2
•
generation of higher amounts of reactive physisorbed BDD( OH) radicals that
8
,9,11,13
mineralise more effectively organic pollutants than other anodes.
Meanwhile, the
2
8
−
•−.
in situ electrogeneration of S
2
O
and SO
4
using BDD anode was wellꢀestablished in
4
,12
recent years.
Nonꢀradical oxidation mechanism was generally proposed for
•
electrochemical production (equation 2). But sulphate can also react with OH at BDD
•
− 4
anode surface to generate SO
4
.
Pt anodes have been typically used as anodes for persulfate production by
5
,14
4,9,11,13
sulphateꢀelectrolysis.
However, based on the existing literature,
BDD anodes
•
2−
•−
have been preferentially investigated to produce OH, S O
and SO4 oxidants.
2
8
Nevertheless, Pt is good electrocatalyst for oxygen evolution reaction, limiting an
efficient production of hydroxyl radicals or persulfate on its surface, for this reason, a
5
,14
depressor for oxygen evolution is frequently added.
Meanwhile, the material

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94
95
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97
98
99
acquisition cost of BDD anodes and their higher energy consumption incentivize the
8
,11,19
use of alternative cheaper materials
. For this reason, the research about the
2
performance of other nonꢀactive electrodes for producing persulfate, such as SnO and
PbO , could be very interesting.
2
2
Among the nonꢀactive anodes, PbO ꢀbased electrodes clearly emerges as an
attractive electrocatalytic material due to its high oxygen evolution potential, low price,
relatively stable under the high positive potentials required, stability at high
27
temperatures and ease of preparation. Nevertheless, a common disadvantage when
PbO electrodes are used for electrolytic approaches is that corrosion phenomenon
can occur on anode surface dissolving Pb ions and pollute the solution, limiting its
2
2
+
2
7
applicability in some cases . Therefore, the preparation of more stable PbO
is the aim of recent investigations in order to increase its economic value and
accelerate the practical applications of PbO electrodes. In this way, some research
groups have proposed TiO nanostructured materials, such as nanotubes, nanorods
and nanowires, as support to deposit PbO2
ratio of TiO nanostructured materials include large surface area, high cation
2
anodes
2
2
28ꢀ35
. The unique properties of high aspect
2
exchangeability, high catalytic activity, easier separation and recyclability. Then, the
use of nanotubes TiO is an option that can improve the characteristics of PbO2
2
3
6
electrodes, such as long term stability and high corrosion resistance.
Therefore, the aim of this work is to study the viability of the use of TiO
2
nanotubes arrays decorated with PbO to produce electrochemically persulfate. Also,
2
the effect of hydroxyl radicals and hydrogen peroxide formation during persulfate
production was investigated, at different applied current densities. Finally, the
performances of Ti/TiO ꢀnanotubes/PbO were compared with active anode (Ti/Pt),
2
2
1
00
01
without oxygen evolution depressor, to understand the differences during persulfate
electroꢀsynthesis.
1

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02
03
2. EXPERIMENTAL
1
2.1. Chemicals and Materials
104
105
106
107
Methanol (vetec), ethanol (sigmaꢀaldrich), acetone (anidrol), glycerol (vetec),
sodium sulfate, and sodium fluoride (NaF) were purchased from Anidrol, Brazil. All
solutions were prepared with high purity water obtained from a Millipore MilliꢀQ system
with resistivity >18 Mꢁ cm at 25°C.
108
2.2. TiO nanotubes Array and Ti/TiO -nanotubes/PbO Preparation
2
2
2
109
110
111
112
113
114
115
116
117
118
119
120
2
The TiO nanotube array on the surface of titanium metal disk (1.8 mm thick
2
31
and nominal surface area of 63.5 cm ) was grown by anodization method . Prior to
anodization, the titanium metal plate was polished with abrasive paper and degreased
sonication in acetone, ethanol, and methanol, respectively. The titanium metal disk was
dried, and it was used as an anode while a stainless steel disk was employed as
cathode. The electrolyte was composed of a mixture of glycerol and ultrapure water in
2 4
a 1.3:1 ratio (v/v) containing 0.5 wt% NaF and 0.2 M Na SO as the support electrolyte
by applying 30 V for 2 h. After anodization, the surface was washed with deionized
water, dried, and calcined at 500ºC for 1.5 h under air atmosphere. The
2 2
electrodeposition of PbO onto a Ti/TiO ꢀnanotubes disk array was achieved using a
galvanostatic method by using a solution of 0.25 M Pb(NO ) + 0.1 M HNO by
3
2
3
ꢀ2
applying 30 mA cm during 10 min at 25°C.

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21
22
2.3. Surface characterization of Surface TiO2 nanotubes and Ti/TiO2-
nanotubes/PbO
1
2
123
124
125
126
127
2 2 2
Surface morphology of Ti/TiO ꢀnanotubes and Ti/TiO ꢀnanotubes/PbO
deposits were characterized using a scanning electron microscope (SEM, TESCAN
VEGA model 3SBH), and Xꢀray diffractometer (DRX 6100 SHIMADZU) using Cu Kα
(λ=1.54 Å) radiation. Electrochemical measurements were carried out using an
AutoLab 302N (Metrohm workstation).
128
2.4. Electrochemical Measurements
129
130
131
132
133
134
135
136
137
138
For the study of electrochemical characteristics of Ti/TiO
polarization measurements were performed in 0.05 M Na SO between 0.3 and 4 V vs
and Ti/Pt electrodes was also
2 2
ꢀnanotubes/PbO ,
2
4
−
1
Ag/AgCl (3M) at 50 mV s . A comparison with Pb/PbO
2
2 2
performed. The threeꢀelectrode cell was constituted by Pb/PbO , Ti/Pt or Ti/TiO ꢀ
2
nanotubes/PbO
2
(1 cm of exposed area), Ag/AgCl (KCl 3 M) and Pt wire, as working,
reference and counter electrodes, respectively. Bach experiments were also performed
for salicylic acidꢀtrapping hydroxyl radicals by using a 200 mLꢀcell with Tacussel model
PJT24ꢀ1 potentiostatꢀgalvanostat at 25°C with 0.025M of Na
electrolyte. Ti/Pt or Ti/TiO ꢀnanotubes/PbO
2
SO
4
as supporting
2
2
2
was used as anode (1 cm ), and a
stainless steel plate was used as cathode.
139
2.5. Electro-synthesis of oxidants
140
141
142
143
Electrochemical experiments were performed using an electrolytic flow cell with
a single compartment with parallel plate electrodes with nominal surface area of 63.5
2
cm (Ti/TiO
2
ꢀnanotubes/PbO
2
or Ti/Pt as anode and stainless steel as cathode) for 1 L
ꢀ2
of solution (Fig. 1). Different current densities (ranging from 7.5 to 60 mA cm ) were

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145
146
147
148
used in 0.05 M of Na
2 4
SO as supporting electrolyte for assessing the production of
hydroxyl radicals and persulfate. In the case of persulfate concentration, the effect of
concentration of sulphate in the solution was studied when Ti/TiO
was used.
2 2
ꢀnanotubes/PbO
1
49
50
1
151
152
153
154
155
Figure 1. a: 1) anodic part; 2) electric support to anode; 3) anode (Ti/TiO ꢀnanotubes/PbO or
2 2
Ti/Pt); 4) reaction compartment, 5) cathode (stainless steel disk); 6) metallic support to electrical
contact with cathode and 7) cathodic part. B) electrochemical system: 1) reservoir, 2)
thermometer, 3) electrochemical cell and 4) peristaltic pump. C: image of electrochemical cell
with each one of its compartments assembled.
156
2.6. Analytical Methods
157
158
159
160
161
162
163
Hydroxyl radicals were detected by the bleaching of N,Nꢀdimethylꢀ4ꢀ
nitrosoaniline (RNO). The originally yellow solution was decolorized through reaction
●
37,38
ꢀ5
between RNO and OH forming an adduct
. The bleaching of the 2×10 M RNO
solution in 0.05 M Na SO was followed by spectrophotometry using an Analytik Jena
2
4
Specord 210 Plus UV/Vis spectrophotometer in 440 nm. For persulfate quantification
was performed by the spectrophotometric ISCO methodology proposed by Liang et al.
3
9
(2008) . Samples were analyzed for determining the concentration changes of

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64
65
salicylic acid and byꢀproducts by HPLC. More details about the salicylic acidꢀtrapping
40
1
hydroxyl radicals method can be found elsewhere.
1
66
67
3. RESULTS AND DISCUSSION
1
2 2
3.1. Surface analysis of Ti/TiO -nanotubes/PbO
168
169
170
171
172
173
174
175
Ti/TiO ꢀnanotubes were successfully synthesized in solution of glycerol and
2
2 4
ultrapure water containing with NaF and Na SO by galvanostatic electrolysis with a
potential of 30 V for 2 h. The surface morphology of deposits was observed by SEM,
showing TiO nanotubes with diameters of approximately 100ꢀ200 nm as observed in
2
2
the images (Fig 2). The TiO nanotubes grow vertically on the substrate and form a
dense array (Fig. 2a). XDR measurements were also carried out at different spots (Fig.
3) showed that the composition corresponds to the anatase phase of TiO , which
2
3
6
demonstrates that the nanotubes were successfully synthetized.
a
b
1
76
77
1
2 2
Figure 2. (a) SEM images of anodized Ti/TiO ꢀnanotubes, (b) magnification of grown of Ti/TiO ꢀ
nanotubes and their dimensions.
178
79
1

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Titânio (44ꢀ1294)
3000
2000
1000
0
Anatase (86ꢀ1157)
Ti/TiO ꢀnanotubos
2
20
40
60
80
2
θ/degree
1
80
81
1
Figure 3. XRD spectrum from TiO
crystalline structure.
2
nanotubes showing that anatase phase is the predominant
182
83
1
184
185
186
187
188
189
190
191
192
193
On the other hand, PbO
2 2
crystals were successfully deposited onto Ti/TiO ꢀ
nanotubes arrays, according to SEM images (Fig. 4), by galvanostatic method at 30
ꢀ2
mA cm during 10 min at 25°C. The PbO
tetragonal PbO crystals. However, these tetragonal crystals can organize in tree form
when the electrodeposition time is increased, as already reported by CerroꢀLopez et al.
2
deposit was composed of orderly arranged
2
3
6
. For this reason, the electrolysis time of 10 min is very important to obtain
2
7, 36
homogenous PbO
2
deposit
and it was chosen as experimental condition of our
3
6
preparation work, based on the existing literature
.

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1
95
96
2 2
Figure 4. SEM image of complete PbO deposit into Ti/TiO ꢀnanotubes.
1
197
198
199
200
201
The deposit of PbO
2 2
into the nanotubular structure of TiO was confirmed by means
of XRD and SEM analyses. Fig. 5 shows the XRD pattern for a PbO deposit during 10
2
min on the TiO
2
nanotubes, which exhibits the characteristic peaks of the Plattnerite
2
7
phase (βꢀPbO ) in a tetragonal crystalline structure, confirming the success of deposit
2
36
formation.
ß-PbO
2
Ti/TiO
2
-NTs/PbO
2
2
0
40
60
80
2θ/degree
202
2
03
04
Figure 5. XRD diffractogram for the PbO
3 2 3
electrolysis time in a 0.25 M Pb(NO ) + 0.1 M HNO solution.
2
deposit on TiO
2
ꢀnanotubes array after 10 min of
2

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3.2. Electrochemical measurements
206
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208
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210
211
212
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223
224
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226
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228
229
It is well established that oxygen evolution reaction (OER) results from
reactions in series and hydroxyl radicals are one of the most important intermediates
4
, 9, 11
produced during it
. The first common step is the discharge of water molecules to
hydroxyl radicals (Eq. 1). The following steps depend on the strength of the interaction
between hydroxyl radicals and the electrode surface which, in turn, depends on the
nature of the electrode material.
Two classes of materials can be distinguished: "active" or "nonꢀactive"
4
, 8, 9, 11
electrodes
. In the former case, a strong interaction with hydroxyl radicals exists
and the oxygen evolution reaction occurs via the formation of higher oxides, while, in
the latter, the substrate does not participate in the process and the oxidation is assisted
by hydroxyl radicals that are weakly adsorbed at the electrode surface. Then, prior to
electroꢀsynthesis experiments and on the basis of the electrocatalytic nature of each
one of the electrodes used, electrochemical measurements were performed to increase
the understanding of the production of oxidants. Figure 6 shows linear polarization
ꢀ1
curves in 0.05 M Na
2
SO
4
solution at 25°C registered at a scan rate of 10 mV s for
Ti/Pt, Pb/PbO and Ti/TiO
2
2
ꢀnanotubes/PbO electrodes (Pb/PbO was used as
2
2
comparative material). The curves are very different from one another and show that
oxygen evolution potential depends on the electrocatalytic material; 1.24 V, 1.9 V and
2.2 V versus Ag/AgCl (3.0 M) for Ti/Pt, Pb/PbO
2 2 2
and Ti/TiO ꢀnanotubes/PbO ,
respectively. Ti/Pt exhibited a low oxygen evolution overpotential starting near of 1.10
41,42
V increased slightly to 1.24 V where oxygen evolution becomes more apparent
.
This low potential makes that Ti/Pt be a good electrocatalysts for the oxygen evolution
reaction (Eq. 5); while Pb/PbO2 and Ti/TiO ꢀnanotubes/PbO2 have high oxygen
2
evolution overpotential, and consequently are a poor electrocatalysts for the OER (it

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230
231
232
2
starts at 1.65 V, increasing more strongly over 1.85 V for Pb/PbO and 2.1 V for
41,42
Ti/TiO ꢀnanotubes/PbO (Fig. 6)).
2
2
●
+
−
OH→ ½ O + H + e
(5)
2
233
234
235
236
237
238
239
240
241
242
243
244
245
246
The value of Tafel slope was estimated (data not shown), and these were
ꢀ1
almost the same for all electrodes (1.46, 1.28 and 1.22 V decade for Ti/TiO
2
ꢀ
nanotubes/PbO2, Pb/PbO and Ti/Pt, respectively). Considering the data obtained in
2
potentiodynamic measurements, no similar behaviours were obtained at all anodes,
●
even if the main reactions seem to be the same (O
2
and OH). Then, this indicates that,
the first step is water discharge at all anodes with the formation of hydroxyl radicals
(Eq. 1). After that, the electrogenerated hydroxyl radicals can be involved in the
oxidation of supporting electrolyte (Eqs. 2 to 4) in the proximity of anode surface, and,
2
subsequently, O evolution. However, in the case of Ptꢀsupported on Ti electrode, Tafel
curve can be divided in two segments, which may indicate that, two main reactions are
●
ꢀ2
favoured at this anode ( OH and O
2
) at current densities below 30 mA cm .
Conversely, only reaction is favoured at Pb/PbO
2
and Ti/TiO
2
ꢀnanotubes/PbO
2
anodes,
●
and according the experimental information discussed above, the production of OH is
the main reaction on the surface, before the oxygen evolution.
247

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248
249
250
2 4
Figure 6. Polarization curves at different electrodes obtained in 0.05 M Na SO with a scan rate
of 10 mv s . Electrode area: 1 cm .
ꢀ
1
2
251
252
253
254
255
2 2
This behaviour indicates that Ti/TiO ꢀnanotubes/PbO could exhibit good
electrocatalytic properties for production of oxidants as well as for degradation of
organic pollutants, due to a low adsorption of oxidizing species generated on the
1
9, 41, 42
surface
. For this reason, its comparison was carried out with Pt anode in order
3
,27,41
to avoid the drawbacks achieved when PbO is used.
2
256
3.3. Electro-generation of hydroxyl radicals
2
57
58
2
Hydroxyl radicals formed by water discharge at Ti/Pt and Ti/TiO ꢀ
2
nanotubes/PbO
2
electrodes (Eq. 6) can be detected using UVꢀvisible measurements.
●
+
−
259
H
2
O → OH + H + e
(6)
260
261
262
263
264
265
The indirect technique for the detection of low concentrations of hydroxyl
radicals is carried out by trapping these radicals via an addition reaction (spin trapping)
in order to produce a more stable radical (spin adduct). In this work, pꢀnitrosoꢀ
dimethylaniline (RNO) was used as spin trap (Eq. 7). It is important to remark that this
3
7
technique is reliable to determine qualitatively the formation of hydroxyl radicals, even
38
when other trapping species can be used.
266
(7)
2
67
68
The use of this particular spin trap is particularly advantageous because under
2
these conditions of concentration and pH, it is electrochemically inactive. Moreover, the

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1
0
−1 ꢀ1 37, 43
269
270
271
272
273
274
275
276
277
278
279
280
addition reaction occurs at a very high rate constant (1.3×10 M s )
and hence is
very selective. Figure 7 shows the adsorption spectrum of aqueous RNO solution
ꢀ5
(2×10 M) during galvanostatic electrolysis at Ti/Pt and Ti/TiO
2
ꢀnanotubes/PbO
2
ꢀ2
electrodes by applying 7.5 and 60 mA cm . These current densities were chosen to
41ꢀ45
understand the behaviour at lower and higher electrical conditions.
As can be observed from Fig. 7, a different decrease in optical density at 440
nm during electrolysis is attained at both electrocatalytic materials under different
ꢀ2
current densities. For example, at 7.5 mA cm , absorbance quickly decreased at Ti/Pt
(Fig. 7c), as a function of time; while a slower disappear of band is observed at Ti/TiO
nanotubes/PbO (Fig. 7a) under similar experimental conditions. This result seems to
2
ꢀ
2
indicate that, Ti/Pt anode produces higher concentrations of hydroxyl radicals on its
surface, and these react with RNO to form the adduct.
2
81
ꢀ
5
282
283
284
285
Figure 7. Adsorption spectra of aqueous RNO solution (2×10 M) obtained at different
electrolysisꢀtime intervals under galvanostatic conditions at Ti/TiO ꢀnanotubes/PbO (a, b) and
2
2
ꢀ
2
ꢀ2
Ti/Pt (c, d) electrodes by applying 7.5 mA cm (a, c) and 60 mA cm (b, d) at 25°C.
2
86
87
However, the UVꢀvis spectrum showed the existence of intermediates that absorb
irradiation approximately at 245 and 323 nm, which were not observed at Ti/TiO
2
2
ꢀ

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ꢀ
2
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
2
nanotubes/PbO electrode during electrolysis by applying 7.5 mA cm . Similar
ꢀ2
●
behaviour was attained at 60 mA cm . These results clearly demonstrated that, OH
radicals could react with RNO to form adduct species, but, RNO and adduct are rapidly
33
oxidized on Ti/Pt surface, producing intermediates.
2
Meanwhile, for Ti/TiO ꢀ
●
nanotubes/PbO anode, there is accumulation of OH radicals on its surface, promoting
2
3
1
only the formation of adduct. In fact, no absorption bands were achieved at 245 and
323 nm. Then, generally speaking, these figures confirm that the nature of the
electrocatalytic material plays an important role in the electrochemical production of
●
significant concentrations of OH radicals, but their concentration also depends on the
applied current density used. This effect is more evident when the evolution of
normalized absorbance values is plotted as a function of time (Fig. 8), indicating that
●
Ti/TiO
2
ꢀnanotubes/PbO
2
anode produces efficiently OH radicals. These results are in
3
7
agreement with the proposed model by Comninellis in which the PbO
2
electrocatalytic
●
material is considered a nonꢀactive anode due to the significant production of OH
radicals, physically adsorbed.
3
03
04
3
Figure 8. Normalized absorbance decay of RNO solution as consequence of hydroxyl
305
306
307
radicals electrogeneration at Ti/TiO
2
ꢀnanotubes/PbO
2
(a) and Ti/Pt (b) electrodes by
SO at 25°C.
ꢀ2
applying 7.5 and 60 mA cm during electrolysis in 0.05 M Na
2
4
4
0
ꢀ1
308
309
310
By using salicylic acidꢀtrapping method, electrochemical oxidation of 100 mg L of
ꢀ2
salicylic acid, at Ti/TiO
2 2
ꢀnanotubes/PbO or Ti/Pt, by applying 7.5 and 60 mA cm in
sulphateꢀsupporting electrolyte showed that salicylic acid is converted to 2,3ꢀdihydroxy

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312
313
314
315
316
317
318
319
320
321
322
323
benzoic acid (2,3ꢀDHBA); 2,5ꢀdihydroxy benzoic acid (2,5ꢀDHBA); and catechol (CT)).
In the case of Ti/TiO ꢀnanotubes/PbO anode, higher concentrations of 2,3ꢀDHBA and
2
2
ꢀ1
2,5ꢀDHBA, preferentially, were produced; 36 and 15 mg L , respectively, after 60 min.
Meanwhile, at Ti/Pt, insignificant amounts of CT, 2,3ꢀDBHA and 2,5ꢀDBHA were
ꢀ1
detected (minor than 2.5 mg L ). The rate of salicylic acid is faster due to the reactivity
●
of OH formed at Ti/TiO
2
ꢀnanotubes/PbO
2
surface. The formation of 2,3ꢀDBHA and 2,5ꢀ
●
DBHA occurs with the organic molecules in the proximity of the surface layer of OH
●
and via homogeneous reaction between the organic molecules and the OH confined
within a reaction cage nearby the electrode surface. But, at Ti/Pt, no enough
●
concentration of OH is present in its surface, avoiding the evolution of higher
concentration of aromatics compounds and increasing the rate of direct oxidation
mechanism. These results clearly confirm the production of hydroxyl radicals at both
electrocatalytic materials in different amounts.
324
325
326
327
328
329
330
However, the main goal of our investigation is to show that, the
electrogeneration of persulfate is feasible at Ti/TiO ꢀnanotubes/PbO anode, and the
2
2
understanding the effect of hydroxyl radicals in its formation. In this context, the results
reported until now have confirmed the efficacy of Ti/TiO ꢀnanotubes/PbO anode to
produce hydroxyl radicals, but the formation of other oxidants, such as H O (equations
2
2
2
2
2 4
8 and 9) and persulfates (equations 2, 3 and 4) must be verified by using Na SO as
4
4
supporting electrolyte.
331
332
333
In the case of H O , it is a powerful oxidant that can be formed through water oxidation
2 2
by either a direct or an indirect mechanism. For this reason, its determination is useful
to better clarify the role of generated oxidizing agents.
+
−
3
34
35
2H O → H O + 2H + 2e
(8)
(9)
2
2
2
●
3
2 OH → H
2
O
2

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336
337
338
339
340
341
342
343
However, no significant concentrations were measured by applying different
current densities at both anode materials. This behaviour can be related to the
competition between the formations of free hydroxyl radicals/persulfates with hydrogen
●
2ꢀ
peroxide at both anodes because higher concentration OH radicals and S O may be
2
8
44
produced and consequently, it does not allow the production of H
O
2 2
.
Another
important feature is that, it is also expected that after formation, hydrogen peroxide can
be decomposed through oxidation at the electrode surface (equation 10), limiting its
presence in the solution.
+
−
344
H
2
O
2
→ O
2
+ 2H + 2e
(10)
4
,10ꢀ11,16,18,19,22,41,42
345
346
347
348
349
350
351
352
353
354
355
356
In the case of persulfate, based on the existing literature
its
formation occurs in sulphate medium due to the oxidation of the sulfuric acid or
sulphate in solution. To evaluate the amount of persulfate formed during water
2 4 2 2
oxidation in Na SO , bulk electrolyzes at Ti/TiO ꢀnanotubes/PbO and Ti/Pt electrodes
ꢀ2
by applying different current densities (7.5, 15, 30 and 60 mA cm ) were performed.
The concentration of persulfate was determined in the anolyte by ISCO method.
2−
8
Figure 9 shows the influence of current density on the S
2
O
concentration.
As can be observed, significant differences were attained at both electrocatalytic
2
ꢀ
materials. At Ti/TiO ꢀnanotubes/PbO anode, an increase on the S O concentration is
2
2
2
8
achieved when an increase on the applied current density is achieved. Conversely,
when higher current density is applied at Ti/Pt, lower persulfate concentration is
produced.

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357
2
−
358
359
360
Figure 9. Electrosynthesis of S
2
O
8
at Ti/TiO
2
ꢀnanotubes/PbO
2
(a) and Ti/Pt (b) electrodes by
applying different applied current densities in 0.05 M Na
2
SO at 25°C.
4
361
362
363
364
365
366
In the former case, a gradual increase on the amount of hydroxyl radicals produced is
●
attained at different applied current densities, favouring the reaction between OH and
2
−
SO
4
2 2
in the Nernst layer at Ti/TiO ꢀnanotubes/PbO anode, and consequently,
producing efficiently persulfate. In fact, when faradaic efficiencies (fE) were estimated,
these values increase when the current density increases, however, no higher fE were
ꢀ
obtained (0.14%; 0,065%; 0,054% and 0,045% by applying 7.5, 15, 30 and 60 mA cm

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2
367
368
369
370
371
372
373
374
375
376
377
378
379
380
, respectively at Ti/TiO ꢀnanotubes/PbO ) which indicate that other parasite reactions
2 2
are being favoured. Meanwhile, at Ptꢀsupported on Ti electrode, the higher is the
current density, the higher is the oxygen evolution (equation 5). In effect, fE values are
minor than those obtained at Ti/TiO ꢀnanotubes/PbO2 anode, confirming that this
2
material spent the electrical energy promoting the oxygen evolution reaction (0,14%;
ꢀ2
0,062%; 0,0087% and 0,0012% by applying 7.5, 15, 30 and 60 mA cm , respectively at
Ti/Pt)). These results in agreement with the production of hydroxyl radicals reported in
Fig. 7. In fact, these reactions explain the rapid decrease of RNO absorbance spectra
2 4
at Ti/Pt when Na SO is used as supporting electrolyte because this electrode favours
the electrogeneration of persulfate, consuming RNO by parallel reactions (direct to
anode surface or persulfateꢀmediated oxidation) instead of favouring the rapid
formation of adduct. Conversely, the heterogeneous production of hydroxyl radicals at
Ti/TiO
2 2
ꢀnanotubes/PbO surface is the main reaction attained, and after that, the
formation of persulfate is involving homogenous catalytic reaction:
●
2−
−●
ꢀ
3
81
82
TiO ꢀnanotubes/PbO ( OH) + SO → TiO ꢀnanotubes/PbO (SO ) + OH
(11)
(12)
2
2
4
2
2
4
−
●
2−
4
2−
−
3
TiO
2
ꢀnanotubes/PbO
2
(SO
4
) + SO
→ S
2
O
8
+ e
383
384
385
386
387
388
389
390
391
It is interesting to note that, even when 14 ꢂM of persulfate is obtained at Ti/Pt
ꢀ2
anode at 7.5 mA cm
(more than the concentration obtained at Ti/TiO2ꢀ
nanotubes/PbO electrode at more higher current density), it is desirable that a
2
significant range of applied currents during the process in order to improve the
operating conditions (e.g.: temperature increase and concentration of sulphate) or no
use of oxygen reaction depressor.
A set of new experiments was performed to give more evidences about the formation
of persulfate by homogeneous catalytic reaction, as previously proposed. Figure 10
2−
shows the trends of S
2
O
8
2 2
synthesized at Ti/TiO ꢀnanotubes/PbO anode by applying

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ꢀ
2
392
393
394
395
396
397
398
399
400
60 mA cm at different concentrations of sodium sulphate at 25 ºC. As can been see,
the higher is the sodium sulphate concentration, higher persulfate concentration is
produced, in according to the reactions 11 and 12. This behaviour confirms that, a
2 2
rapid production of persulfate is achieved at Ti/TiO ꢀnanotubes/PbO anode due to its
efficient production of hydroxyl radicals on its surface as well as the direct oxidation of
sulphate ion. Similar experiments were performed at Ti/Pt electrode, however, a slight
increase on the concentration of persulfate this behaviour is probably due to the
increase on the conductivity, which favoured the oxygen evolution reaction than the
18
hydroxyl radicals and persulfate production.
401
402
403
404
405
Figure 10. Electrochemical production of persulfate, as a function of time, with Ti/TiO ꢀ
2
ꢀ2
nanotubes/PbO
sulfate.
2
anode obtained by applying 60 mA cm at different concentrations of sodium
406
407
408
409
410
411
Finally, the whole process is highly dependent on the mediated oxidation
−
●
surface generation of intermediate TiO
feasible than the direct electrochemical formation of S
is important to consider that the decomposition of S
2
ꢀnanotubes/PbO
2
(SO
(equation 2). Additionally, it
to H is feasible, reducing
4
), which is more
2−
2
O
8
2−
2
O
8
2 2
O
its oxidation power efficiency in the bulk solution, as mainly observed at Ti/Pt electrode
(see Fig. 9b).

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4
4
4
4
4
4
4
4
4
4
4
4
12
13
14
15
2 2
In order to propose the use of Ti/TiO ꢀnanotubes/PbO as anode for
producing electrochemically persulfate, its electrochemical stability was
evaluated by fixing a current density for prolonged electrolysis times. Fig. 11
16 shows that the variation of Eappl as a function of time, revealing that no increase
17
18
19
20
21
22
23
in the values of Eappl was achieved up to 120 h. This is due to the tetragonal
organizedꢀarray of the PbO crystals which give high electrochemical stability
for longꢀterm applications. Another interesting feature is that, no pollution of
2
2+
Pb
nanotubes/PbO
electrode is used. The release of Pb was monitored by electroanalytical
was detected after longer times of electrolysis with Ti/TiO2ꢀ
2
anode, in contrast with the results achieved when Pb/PbO
2
2+
46
measurements.
424
425
426
427
Figure 11. Variation of Eappl at Ti/TiO
fixed current densities values for prolonged electrolysis times.
2 2
ꢀnanotubes/PbO anode, as a function of time, during

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428
CONCLUSIONS
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
In summary, the large PbO
2
electrodes composed of orderly arranged TiO
2
nanotubes were successfully synthesized via a simple, rapid, and efficient
electrochemical approach. Also, the study about the effect on the production of
2 2
hydroxyl radicals and persulfate was studied, for first time, at Ti/TiO ꢀnanotubes/PbO
●
anodes. Based on the results obtained, a significant amount of OH is produced at
Ti/TiO ꢀnanotubes/PbO surface is achieved when Na SO is used as supporting
2
2
2
4
electrolyte (confirmed by RNO spin trapping reaction), in comparison with Ti/Pt. These
radicals play an important role in secondary reactions to the production of strong
oxidants (persulfate). In fact, an evaluation of the concentration of oxidants indicates
2 2
that Ti/TiO ꢀnanotubes/PbO promotes the formation of hydroxyl radicals and persulfate
with lower amounts of hydrogen peroxide. At the same time, the concentration of
persulfate is improved by an increase on the applied current density as well as on the
sulphate concentration in solution. In light of these results, this study open new
2
−
considerations about the real mechanism followed to form S O8 species as well as we
2
2 2
can propose the use of Ti/TiO ꢀnanotubes/PbO anode for environmental applications,
such as anodic oxidation of organic pollutants.
445
ACKNOWLEDGMENTS
446
447
448
449
450
451
Financial supports from National Council for Scientific and Technological Development
(CNPq – 465571/2014ꢀ0; CNPq ꢀ 446846/2014ꢀ7 and CNPq ꢀ 401519/2014ꢀ7) and
FAPESP (2014/50945ꢀ4) are gratefully acknowledged as well as the Microscopy
Laboratory of Mechanical Engineering Academic Unit of Federal University of Campina
Grande supervised by Prof. Renato Santana.

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New Journal of Chemistry
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DOI: 10.1039/C7NJ02604H
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