Catalytic activation of dioxygen to hydroxyl radical and efficient oxidation of o-aminophenol by cobalt(II) ions in bicarbonate aqueous solution

Article abstract of DOI:10.1016/j.apcata.2014.01.048

Increasing attention has been paid to the activation of the ideal oxidant O₂ to reactive oxygen species for environmental pollutants transformation and complete degradation. In this work, the oxidation of o-aminophenol (OAP), a poor biodegradable intermediate, by simple Co ²⁺ ions in HCO₃^- aqueous solution with O ₂ under ambient conditions was investigated. The results reveal that OAP is efficiently transformed to a less harmful compound 2-aminophenoxazine-3- one (APZ) by the Co²⁺-HCO₃^- system. HCO ₃^- is necessary for the reaction by promoting the protonation of the two-electron reduction species of O₂ to produce H₂O₂. The metal-bound hydroxyl radical ( ^?OH) generated from the intermediacy of H₂O ₂ contributes to APZ formation. Based on the analysis of electron spin resonance spin-trapping technologies, radical scavenging measurements and kinetics of APZ formation under different conditions, a possible pathway for ^?OH radical formation and APZ production is proposed. This study can provide new insight on the mechanism of the molecular oxygen activation by simple metal complexes and their application on pollutant removal under mil reaction conditions.

Full text of DOI:10.1016/j.apcata.2014.01.048

Applied Catalysis A: General 475 (2014) 297–304
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic activation of dioxygen to hydroxyl radical and efficient
oxidation of o-aminophenol by cobalt(II) ions in bicarbonate aqueous
solution
a
b
b
b
b
b
Xiaoxia Li , Wei Shi , Qiang Cheng , Lianghua Huang , Mingyu Wei , Long Cheng ,
b
b,∗
Qingfu Zeng , Aihua Xu
School of Chemistry and Chemical Engineering, Wuhan Textile University, Wuhan 430073, China
School of Environmental Engineering, Wuhan Textile University, Wuhan 430073, China
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Increasing attention has been paid to the activation of the ideal oxidant O2 to reactive oxygen species
for environmental pollutants transformation and complete degradation. In this work, the oxidation of
o-aminophenol (OAP), a poor biodegradable intermediate, by simple Co ions in HCO3 aqueous solution
Received 22 October 2013
Received in revised form 21 January 2014
Accepted 22 January 2014
Available online 28 January 2014
2+
−
with O2 under ambient conditions was investigated. The results reveal that OAP is efficiently transformed
2
+
−
−
to a less harmful compound 2-aminophenoxazine-3-one (APZ) by the Co –HCO3 system. HCO3 is
necessary for the reaction by promoting the protonation of the two-electron reduction species of O2
Keywords:
o-Aminophenol
Dioxygen
Bicarbonate
Cobalt ion
•
to produce H2O2. The metal-bound hydroxyl radical ( OH) generated from the intermediacy of H2O2
contributes to APZ formation. Based on the analysis of electron spin resonance spin-trapping technologies,
radical scavenging measurements and kinetics of APZ formation under different conditions, a possible
•
pathway for OH radical formation and APZ production is proposed. This study can provide new insight
Hydroxyl radical
on the mechanism of the molecular oxygen activation by simple metal complexes and their application
on pollutant removal under mil reaction conditions.
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
oxidize 4-chlorophenol [10]. When combination with ethylene-
diaminetetraacetic acid, the NZV could even nonselectively and
deeply oxidize various compounds including 4-chlorophenol, pen-
tachlorophenol, phenol, and malathion to low molecular weight
acids and CO2 at pH 5.5 [11]. Upon irradiation with ultraviolet or
solar light, the destruction of pollutants with nano semiconduc-
tor is also facilitated mainly by a series of hydroxylation reactions
Efficient removal of organic contaminants is one of the major
targeted objectives of water treatment and wastewater reclama-
tion unit operators [1]. As dioxygen from air is the cleanest and the
most abundant oxidative agent, the catalytic activation of dioxy-
gen to reactive oxygen species (ROS) for pollutants removal from
aquatic environments has become a major public concern in almost
all parts of the world [2]. Among the dioxygen-activation tech-
nologies such as supercritical water oxidation [3], catalytic wet
air oxidation [4], zero-valent metal [5,6] and photocatalysis [7–9],
the nanoscale zero-valent iron (NZV) and semiconductor-mediated
photocatalysis have recently received greater attention, due to their
high efficiency, mild reaction conditions and the utilization of inex-
pensive reagents. For instance, Ai’s group synthesized core–shell
Fe@Fe O nanowires with different iron oxide shell thickness, and
•
initiated by OH [12]. However, these techniques have many limita-
tions in application. For example, the difficulty in catalysts recover,
the nano-toxicology issues, the remarkable leaching of metal ions
from NZV, and the rapid recombination rate of photogenerated
electron–hole pairs within semiconductor. In the past decades, var-
ious strategies have been developed in an attempt to modify these
processes and improve the catalytic performance [13–16].
The catalytic activation of dioxygen by transition metal com-
plexes under mild conditions also attracts interest in wastewater
treatment and in synthetic and biological chemistry. Aminophe-
nols (APs) are widely used as reducing agents, as intermediates
in chemical synthesis, bleaching, and hair dyes, and as mate-
rials for photography [2]. The removal of APs from aquatic
environments has become a major public concern. Recently,
copper salts [17], forroxime(II) [18], water soluble metallopor-
phyrins [19] and manganese(II) isoindoline based complexes
2
3
found that the surface bound ferrous ions on the iron oxide shell
could initiate the single-electron reduction of oxygen to activate
molecular oxygen for the generation of superoxide radical (O
−
•
),
2
•
hydrogen peroxide (H O ) and hydroxyl radical ( OH) to further
2
2
∗ Corresponding author. Tel.: +86 27 59367334; fax: +86 27 59367334.
E-mail address: xahspinel@sina.com (A. Xu).
[
20] have been demonstrated as efficient catalysts for oxidative
0
926-860X/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2014.01.048

2
98
X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
4
−1
coupling of o-aminophenol (OAP) to a less harmful compound
-aminophenoxazine-3-one (APZ) with dioxygen as the oxidant.
spectrum (ꢀmax = 361 nm, ε = 2.5 × 10 (M cm) ) with the same
2
diluted solution but without NaI as the blank sample.
However, most of these studies were achieved in organic solvents
such as acetonitrile, dimethylformamide and methanol [21]. Fur-
thermore, different from the NZV and photocatalysis, nearly no
Electron spin resonance (ESR) spectra were recorded at room
temperature using a Bruker ESR A-300 spectrometer with the
following parameters: center field 3516 G, sweep width 100 G,
microwave frequency 9.86 G, modulation frequency 100 kHz,
microwave power 1 mW. The measurement with the accumulation
of 5 successive spectra was carried out after a reaction of 30 min
by adding of 20 ␮L DMPO solution into 200 ␮L mixture solution
•
formation of the high reactive OH by these systems was found,
•
although in cellular systems, OH was suggested to be the major
contributor to overall oxidative DNA damage, leading to strand
cleavage, base and sugar modifications, as well as DNA/protein
cross-linking [22,23].
containing Co2+ ions, HCO3 , and OAP.
−
In this work, the oxidation of OAP by simple Co2+ ion at
For FT-IR analysis of the product from OAP oxidation, the col-
lected solution after a reaction of 240 min was adjusted to a pH
−
micromole concentration in HCO3 aqueous solution under mild
•
−
conditions was investigated. It is found that OH radical is gener-
of 2.5 with HCl to completely decompose HCO3 , and then was
◦
ated by the system and contributes to the pollutant oxidation. The
dried at 30 C. The resulting solid sample was finally recorded as
−
•
role of HCO3 and the mechanism of OH radical formation in this
process were further studied in detail. The results can provide new
insight on the molecular oxygen activation mechanism of transi-
tion metal complex, and will be beneficial to the development of
simple and efficient systems aimed to pollutants degradation with
dioxygen.
KBr pellets on an FT-IR 170S spectrometer.
3. Results and discussion
3.1. Catalytic activity for OAP oxidation
The oxidation of OAP by dissolved O was performed in an aque-
2
2
. Experimental
ous solution of bicarbonate in the presence of catalytic amount
of Co2+ ions at a temperature of 30 C, and was followed by
UV–Vis spectroscopy and HPLC. The products during the reaction
were identified by ESI–MS and FT-IR spectroscopy. Fig. 1(A) shows
the changes in the UV–Vis absorption spectra between 250 and
◦
2.1. Chemicals and reagents
Cobalt acetate, sodium bicarbonate, o-aminophenol (OAP),
sodium iodide, methanol and other chemicals were of analytical
grade if not noticed otherwise. Catalase (CAT) and 5,5-dimethyl-1-
pyrroline N-oxide (DMPO) were obtained from Aldrich. Hydrogen
peroxide (30% w/w) was obtained from Sinopharm Chemical
reagent Co., Ltd. The sample solutions were prepared using deion-
ized water throughout the experiments.
6
50 nm within 90 min. In good agreement with literature reports,
the characteristic absorbance at 430 nm increases with time, indi-
cating the oxidation of OAP to 2-aminophenoxazine-3-one (APZ)
[
20]. The change of the Abs at the wavelength was then plot-
ted against reaction time and is shown in Fig. 1(B). As can be
seen, the value increases linearly with time after an induction
period of about 15 min. The behavior can be explained by a rad-
ical pathway and zero-order dependence on the concentration of
the substrate.
2.2. Degradation experiments
Oxidation reactions were performed in a laboratory scale bath
Fig. 2(A) shows the result of HPLC analysis during the reaction.
The retention time of OAP is 3.52 min, and after a reaction of 90 min
its intensity decreases from 150.6 to 53.2. At the same time, APZ
appears at 8.89 min as the main reaction product, since other inter-
mediates are accumulated in insignificant amounts towards OAP
and APZ. From the spectra, it can be found that the conversion of
reactor equipped with a magnetic stirrer. The desired amounts of
NaHCO3 and OAP in 25 mL of the aqueous solution saturated with
O2 were added in a 50 mL flask and kept at 30 C; then the reaction
was initialized by adding 0.25 mL of a Co(CH COO) solution. In a
typical oxidation experiment, the concentrations of the reagents
◦
3
2
2+
−
in the reaction mixture were: Co 5 ␮M, HCO3 10 mM and OAP
2+
−
OAP with the Co –HCO3 system is 64.7% after 90 min. Whereas
after 240 min, about 92% of OAP is transformed (shown in Fig. S1),
with a calculated turn over number of 67.7 and turn over frequency
0.367 mM.
−
1
2.3. Analysis
of 0.0047 s
.
ESI–MS spectrometry at positive ionization mode was used to
confirm the molecular weight of OAP and its oxidation product.
A molecular ion with m/z at 110 corresponding to [OAP + H]+ is
anticipated, and significant change is observed for the oxidation
product. As shown in Fig. 2(B), two intensity ions appear at m/z 213
To monitor the reaction process, solution samples taken at
different time intervals were measured at 430 nm on UV–Vis spec-
trophotometer (Beijing Rayleigh Analytical Instrument Co. Ltd,
China). The HPLC–MS measurement of OAP and its oxidation prod-
ucts was carried out on an Agilent 1100 LC/MSD SL system. The
HPLC experiments were conducted using an Xbridge C-18 column
+
and 235, consistent with the molecular formula [C12H O N ] and
9
2
2
+
(C12H O N Na) , respectively, the protonated and sodium ionized
8
2
2
(
5 ␮m particle size, 150 4.6 mm) with methanol–water (2:8, v/v)
form of APZ. The product was further analyzed with FT-IR. From Fig.
S2 one can see that it is very similar to the reported FT-IR spectra of
as the mobile phase. The flow rate was 0.5 mL/min and detection
wavelength was 210 nm. The mass experiments were performed on
an Esquire LC–ion trap mass spectrometer (Bruker Daltonics, Bre-
men, Germany) equipped with an orthogonal geometry ESI source.
Nitrogen was used as the drying (3 L/min) and nebulizing (6 psi)
−1
APZ [25], with two main absorption bands at 1600 cm (stretch-
−
1
ing vibration of C=N group) and 1054 cm (stretching vibration of
C–O–C group).
−
Control experiments with H O , or with HCO in the absence
2
2
3
◦
of Co2+ ions, indicated nearly no oxidation of OAP. When instead
of NaHCO3 with other basic solutions such as NaOH at the same
pH, the reaction is still very slow. In Fig. 3(A), the zero-order
rate constant kobs was calculated from the plots of Abs at 430 nm
after the induction period, and the results indicate that the rate
of APZ formation with NaOH is 10 times slower than that with
gas at 300 C. The spray shield was set to 4.0 kV and the capillary
cap was set to 4.5 kV. Scanning was performed from m/z 70 to 800
in the standard resolution mode at a scan rate of 13 kDa/s. Before
analysis, each sample was diluted five times.
The amount of H O was determined by titration with iodide
2
2
ion [24]. The diluted solution was treated with an excess of NaI
and the amount of I3 formed was then quantified using its visible
−
−
HCO3 . Furthermore, the catalytic activity of the system decreases

X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
299
2
2
1
1
0
0
.5
.0
.5
.0
.5
.0
min
2.5
2.0
1.5
1.0
0.5
0.0
(A)
(B)
90
80
70
60
50
40
30
20
15
10
5
0
2
50 300 350 400 450 500 550 600 650
Wavelength (nm)
0
15
30
45
60
75
90
Time (min)
Fig. 1. (A) UV–Vis spectral changes for OAP oxidation with Co2+–HCO3− system; (B) plot of the Abs at 430 nm against reaction time. Conditions: Co2+ 5 ␮M, HCO3 10 mM,
−
◦
OAP 0.367 mM, 30 C.
160
140
120
100
(A)
OAP
(B)
235
0 min
80
60
40
20
0
213
APZ
90 min
2
4
6
8
10 100
150
200
M / Z
250
300
Time (min)
Fig. 2. (A) HPLC spectra of OAP oxidation with Co2+–HCO3 system; (B) ESI (+) mass spectra of APZ. Conditions: Co2+ 5 ␮M, HCO3 10 mM, OAP 0.367 mM, 30 C.
−
−
◦
2
+
dramatically when Co is replaced by other transition metal ions.
genotoxicity in a concentration-dependent manner, and can only
2
+
2+
2+
In the case of Fe , Mn and Ni , nearly no reaction is observed.
inhibit cell viability at concentrations higher than 50 ␮M [26].
2
+
2+
Simple Cu ion has been reported as an efficient catalyst for
synthesis of APZ from oxidation of OAP in aqueous solution [21];
while in the presence of the metal ions, the formation of APZ slowly
increases with a zero-order kinetics constant of 0.0068 mM/min,
which is four times slower than that with Co ions.
It should be noted that although Co ions are toxic and not
recommended to be added to water, it induces cytotoxicity and
While in our system the dosage of Co ion can be very low. As
shown in Fig. S3, under a concentration of 0.5 ␮M, the Abs at 430 nm
increases to 0.82 within 60 min. Moreover, according to the emis-
sion standard of pollutants for copper, nickel, cobalt industry in
2+
2+
china (GB 25467-2010), the concentration of Co ions is allowed
2+
2+
to be lower than 16.9 ␮M and thus the solutions with 5 ␮M of Co
ions can be discharged.
0
0
0
0
0
0
0
0
.035 (A)
0.035 (B)
2
+
-
2+
.030 Co +HCO3
0.030
0.025
0.020
0.015
0.010
0.005
0.000
Co
.025
.020
.015
.010
2
+
Cu
.005
.000
2+
-
2+
Co (pH 8.2) HCO3
Fe
2
+
2+
Ni
Blank
Mn
−
Fig. 3. (A) Effect of various reaction systems on the rate of APZ production; (B) Effect of different metal ions on the rate of APZ production. Conditions: metal ion 5 ␮M, HCO3
◦
1
0 mM, OAP 0.367 mM, 30 C.

3
00
X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
0
0
0
0
0
0
0
0
.035
.030
.025
.020
.015
.010
.005
.000
(
A)
.040 (B)
.035
0
0
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0
(
20 40 60 80 100 120 140 160
0
2
4
6
8
10
2+
-
Co concentration ( M)
HCO3 concentration (mM)
C)
120 (D)
0
0
0
0
0
0
0
.040
.035
.030
.025
.020
.015
.010
1
00
80
60
4
0
0
2
0
.15
0.30
0.45
0.60
0.75
0.90
2
4
6
8
10
12
14
-
1
OAP concentration (mM)
1/OAP (mM )
−
2+
Fig. 4. The rate of APZ formation under different reaction conditions: (A) Effect of HCO3 concentration; (B) Effect of Co concentration; (C) Effect of OAP concentration;
2+
◦
−
◦
2+
−
(
D) Lineweaver–Burk plot of the data in Fig. 4 (C). Conditions: (A) Co 5 ␮M, OAP 0.367 mM, 30 C; (B) HCO3 10 mM, OAP 0.367 mM, 30 C; (C) and (D) Co 5 ␮M, HCO3
◦
1
0 mM, 30 C.
3
.2. Effect of reaction conditions
The dependence of kobs on the concentration of OAP is shown
in Fig. 4(C). The reaction rate constants kobs increases non-
linearly with the increasing of OAP concentration. A double
reciprocal Lineweaver–Burk plot (Fig. 4(D)) shows that the rate
fittes Michealis–Menten kinetic model [19]. This indicates that
an intermediate metal complex–substrate adduct may form in a
pre-equilibrium process, most probably through the interactions
between the functional groups (–NH2 and –OH) in the substrate
and Co(II) ion, followed by the production of reactive species in the
rate determining step of the oxidation process.
To further elucidate the efficiency and mechanism of OAP oxi-
2
+
−
dation to APZ by the Co –HCO3 system, the effect of different
operating parameters, including the concentrations of HCO3 , Co
and OAP, and reaction temperature was investigated.
−
2+
−
As mentioned in the above section, HCO is absolutely neces-
3
2
+
−
sary in the Co –HCO3 system. One of the reasons may be that
the buffer can react with the released H ion during OAP oxidation
and thus enhances the rate of APZ production. Another possible
reason is that HCO3 can form complex with Co ions, making it
+
−
2+
2+
−
The advantages of the Co –HCO3 system for OAP oxidation
−
2+
much more reactive. To clarify the later role of HCO3 in the reac-
include not only the use of simple Co ions at micromole con-
tion, the effect of its concentration in the range of 0–160 mM on APZ
production was studied. As shown in Fig. 4(A), the zero-order kinet-
centration and the cheap reagent NaHCO , but also the very mild
3
reaction conditions. As shown in Fig. 5(A), the rate of APZ for-
mation decreases with the temperature decreasing. But even at
ics constant k
does not change monotonously with the increase
obs
−
◦
of HCO3 concentration, but first rises, reaching the maximum
at 20 mM HCO3 , and then decreases. This trend is similar with
that of organic dye decolorization by Co –HCO3 /H O system
a low temperature of 2.5 C, the absorbance at 430 nm increases
−
to 1.26 after 360 min. The activation parameters for the catalytic
oxidation reaction were further determined from the temperature
dependence of the kinetic constant kobs. As indicated in Fig. 5(B), the
Arrhenius plot of log kobs versus 1/T is linear with a correlation coef-
ficient of 98.3%. From the slope of this line the apparent activation
energy Ea was calculated to be 49.0 kJ/mol, which is much higher
than that of OAP oxidation by cobalt(II) phthalocyaninetetrasodi-
umsulfonate (12.07 kJ/mol) [31], copper salt (25 kJ/mol) [18] and
NiO (22.39 kJ/mol) [2], implicating a different mechanism occurred
2+
−
2
2
−
[
27,28], implying that the formed complex between HCO
and
3
2+
Co plays an important role in OAP oxidation. Thus, the increase of
−
catalytic efficiency in the presence of HCO can be attributed to the
3
formation of more active Co2+–HCO complex. But at higher con-
−
3
−
2+
centrations, the number of HCO3 complexed to Co ions increases
and there are no equatorial positions available to bind OAP and O2,
leading to the efficiency decreasing [27].
Different from organic dyes decolorization by Co2+–HCO3− sys-
tem, in which the initial rate varies linearly on the concentration
of Co2+ [29], a kinetic order less than one during OAP oxidation
is present in Fig. 4(B). If the cobalt multimer forms and accumu-
in the current Co –HCO3 system.
2+
−
3.3. Detection of ROS
2
+
lates at high Co concentrations, this nonlinear behavior can be
explained. Indeed, the molecule aggregation and deactivation of
homogeneous cobalt phthalocyanine have been observed during
organic pollutants degradation [30].
The mechanism of the six-electron oxidative condensation of
two molecules of OAP to form APZ has been widely studied dur-
ing the past decades, and it is now accepted that o-benzoquinone
monoamine (BQMI) is the key intermediate, which can be formed

X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
301
-
-
-
-
2
3
4
5
(
A)
(B)
2.5
2.0
1.5
1.0
0.5
0.0
o
2
1
.5 C
o
0 C
o
20 C
o
3
5
0 C
o
0 C
0
60
120 180 240 300 360
Time (min)
0.0031 0.0032 0.0033 0.0034 0.0035 0.0036
1 / T
Fig. 5. (A) Effect of reaction temperature on the APZ formation; (B)The corresponding kinetic parameters during the reaction. Conditions: Co2+ 5 ␮M, HCO3 10 mM, OAP
−
0
.367 mM.
-
3
mixture of Co(CH COO)2 and OAP shows obvious signals for the
HCO +OAP
3
aminophenoxyl radical between 3000 G and 3800 G (marked with
asterisks) [17]. The results indicate that the radical can be pro-
duced by the reaction between Co2+ and OAP; but its contribution
to OAP transformation is minor, for the oxidation rate is slow in
2
+
-
3
Co +HCO
−
2+
this case. When HCO3 is added into the mixture of Co and OAP,
the intensity of aminophenoxyl radical becomes very weak; so the
production of other reactive species is suggested. Then we paid
our attention to the formation of ROS during OAP oxidation in the
2+
Co +OAP
2+
−
Co –HCO3 system, as Co(II) ions can easily absorb dioxygen and
be oxidized to Co(III) with the production of ROS after complexed
with amino and polyamino ligands [32]. Here the high reactive
hydroxyl radical is found to be produced and contributes to OAP
oxidation.
2
+
-
Co +HCO +OAP
3
2
000
2500
3000
Magnetic field (G)
3500
4000
4500
5000
DMPO spin-trapping ESR technique was employed to charac-
terize the ROS production. As shown in Fig. 7(A), a strong four-line
ESR spectrum with relative intensities of 1:2:2:1 is observed after
Fig. 6. ESR spectra of wet-grinding mixture of Co(CH3COO)2, NaHCO3 and OAP.
•
Conditions: Co2 0.5 mM, HCO3 2.5 mM, OAP 1 mM, 25 C.
+
−
◦
a reaction of 90 min, which is the characteristic for DMPO/ OH spin
adduct [29]. The average intensity of the two high lines in ESR
spectra was than calculated, and plotted against the reaction time.
It can be observed from Fig. 7(B) that the intensity fast increases
linearly with time after an induction period of about 10 min, sim-
ilar to the rate of APZ formation, and then keeps unchanged.
These results confirm that hydroxyl radicals can be produced from
from disproportionation of o-aminophenoxyl radical, or the reac-
tion between OAP and metal oxo species [17,20]. Different from
the most of catalysts performed in organic solvents (acetonitrile,
dimethylformamide, methanol) [21], OAP oxidation catalyzed by
2+
−
2+
−
Co ions in this work was carried out in aqueous HCO3 solu-
tion, and the formed 2-aminophenoxyl radical is not so important.
From the ESR spectra of water-grinding mixture of Co(CH COO) ,
the aqueous solution of Co , OAP and HCO3 . However, when
using other similar substrates such as o-phenylenediamine and
•
catechol in place of OAP, the DMPO/ OH spin adduct does not
3
2
NaHCO3 and OAP (shown in Fig. 6), it can be seen that only the
appear.
(A)
(B)
1
0000
10000
8
6
4
2
000
000
000
000
0
5
000
0
-
5000
-
10000
3
475
3500
3525
3550
0
15
30
45
60
75
90
Magnetic field (G)
Time (min)
Fig. 7. (A) DMPO spin-trapping ESR spectra of radicals formed in the Co +–HCO3−–OAP system after 90 min; (B) Plot of the ESR intensity against reaction time. Conditions:
2
2+
−
◦
Co 20 ␮M, HCO3 10 mM, OAP 0.367 mM, DMPO 25 mM, 25 C.

3
02
X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
A
B
2.5
Control
N2
H O
2 2
2.0
1.5
1.0
CAT
Methanol
C
D
E
0
0
.5
.0
F
0
15
30
45
60
75
90
G
Time (min)
Fig. 9. Influence of different additives on the APZ formation. Conditions: Co2+ 5 ␮M,
HCO3 10 mM, OAP 0.367 mM, H2O2 0.1 mM. CAT 0.4 g/L, Methanol 0.5 M, 30 C.
H
−
◦
inhibition effect of CAT and the acceleration effect of H O2 further
2
3
480 3490 3500 3510 3520 3530 3540 3550
Magnetic field (G)
suggest that H O2 is formed as an intermediate for the production
2
of hydroxyl radical in the system. The concentration of H O2
2
2
+
formed in this process was then determined to confirm this
Fig. 8. DMPO spin-trapping ESR spectra under different conditions ((A) Co + OAP,
2+
−
−
2+
−
2+
−
supposition. As shown in Fig. 10, the H O2 concentration reaches
(
+
+
B) Co + HCO3 , (C) HCO3 + OAP, (D) Co + HCO3 + OAP, (E) Co + HCO3 + OAP
2
N2, (F) Co2 + HCO3 + OAP + methanol, (G) Co + HCO3 + OAP + CAT, (H) Co
+
−
2+
−
2+
a peak within 60 min, and then slowly decays within 120 min. This
is mainly attributed to the rapid regeneration of Co(II) by electrons
transfer from OAP to Co(III) at the beginning of the oxidation;
with the progression of the reaction, OAP is oxidized to APZ and
the rate of electron transfer becomes slow, as discussed in the
following Section 3.5. In addition, in the presence of 0.1 mM H O
−
2+
−
HCO3 + OAP + H2O2). Conditions: Co 20 ␮M, HCO3 10 mM, OAP 0.367 mM,
◦
DMPO 25 mM, Methanol 0. 5 M, CAT 0.4 g/L, H2O2 0.5 mM, 25 C.
3.4. Influence of radical scavengers
2
2
under the same conditions but without Co2+ ions, a very slow
ESR spectra under different reaction conditions are shown in
Fig. 8. No spin adduct caused by hydroxyl radical is observed for
the systems Co + OAP (Fig. 8(A)), Co + HCO3 (Fig. 8(B)) and
HCO3 + OAP (Fig. 8(C)), indicating that the three reagents Co
HCO3 and OAP are necessary for the radicals formation. O2 is
also essential for the reaction. When a solution of DMPO, OAP
and HCO3 is first bubbled with N2 gas before Co is added, the
observed DMPO/ OH ESR signal significantly decreases (Fig. 8(E)).
The addition of 0.5 M methanol, a commonly recognized hydroxyl
radical scavenger [27], completely reduces the DMPO/ OH ESR sig-
nal intensity with the formation of new ESR spectrum characteristic
for spin adduct of DMPO/O2 (Fig. 8(F)). The increase of the sig-
nal intensity with respect to the reaction time further evidences
the generation of superoxide radical (shown in Fig. S4) [32]. The
oxidation of OAP to APZ is observed. Hence, H O2 can not oxidize
2
OAP to APZ directly. In literature, the production of H O2 has been
2
2
+
2+
−
observed for other metal complexes [24,33,34]. For example, Mase
et al. reported that a cobalt chlorin complex can efficiently and
selectively catalyze two-electron reduction of O2 by one-electron
reductant (ferrocene derivatives) to produce H O in the presence
−
−
2+
,
2
2
−
2+
◦
of perchloric acid in benzonitrile at 25 C [34]. Several researchers
have also proposed the possible reaction mechanisms to explain
•
the generation of H O2 from dioxygen by zero-valent iron [10],
2
•
magnetite nanoparticles [35] and electrocatalytic two-electron
reduction with cobalt complexes [34]. Current evidence indicates
−•
that it arises primarily by transfer of electrons from a donor to O
2
•
−
2−
to produce O2 and O2 , which subsequently undergoes dismu-
tation and protonation, either catalyzed or uncatalyzed, to form
•
•− −
2+
absence of spin adduct of DMPO/ OCH3 adduct suggests that the
H O [10,35]. Due to the detection of O
in the Co –HCO3 –OAP
2
2
2
release of free hydroxyl radical in solution by the system appar-
ently does not occur and, hence, seems to remain tightly with
the cobalt complex. The hydroxyl radical induced by the system
is almost totally inhibited by CAT, which can catalyze the decom-
position of H O to H O and O quickly [10]. This implies that H O
18
1
1
6
4
2
2
2
2
2
2
is an intermediate for the production of the radicals via the Fenton-
2
+
−
like reaction between H O and Co –HCO complex. In addition,
12
2
2
3
+
2
−
when 0.5 mM H O is added into the Co –HCO3 –OAP system, a
2
2
1
0
8
6
4
2
0
•
much higher intensity of DMPO/ OH ESR signal is observed within
min.
To investigate whether OH participated in the oxidation of
5
•
OAP to APZ, experiments were conducted in the presence of
methanol. As illustrated in Fig. 9, after addition of 0.5 M methanol,
the absorbance of APZ at 430 nm slowly increases to 0.42 within
9
0 min. The likely explanation for this phenomenon is that a large
•
portion of generated OH is scavenged, resulted in the reduction
of the catalytic efficiency. Although superoxide radical can be
produced in the presence of methanol, its activity is much lower
0
20
40
60
Time (min)
80
100
120
than that of hydroxyl radical. When N is bubbled into the reaction
system, nearly no APZ formation is found. Moreover, the observed
Fig. 10. The concentration of H2O2 formed during OAP oxidation with Co2+–HCO3−
system. Conditions: Co 20 ␮M, HCO3 10 mM, OAP 0.367 mM, 30 C.
2
2
+
−
◦

X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
303
Scheme 1. Proposed mechanism for hydroxyl radical production and APZ formation.
system, it is then hypothesized that the mechanism of generation
of H O by the system is similar to that of cobalt chlorin complex
by OAP may not occur, otherwise, the induction period will disap-
pear and the rate of APZ formation will be very fast in the whole
reaction.
2
2
2
+
•
34]. Once formed, H O2 reacts with Co ions to produce OH
2
[
2+
−
through Fenton reaction, for the Co –HCO3 system is very effi-
cient for H O activation in the presence of substrates with high
Our previous investigations have demonstrated that
Co2+–HCO3 is an efficient Fenton-like system for organic dyes
decolorization [27–29,38,39]. The system can produce hydroxyl
radicals, but the pollutants play an important role in the rate of
−
2
2
coordination ability, just as described in our previous work [29].
•
3.5. Possible catalytic mechanism
OH formation [29]. During degradation of organic dyes such as
acid orange II with high coordination ability to Co(II), the produced
•
Based on the above results and analysis, a possible mechanism
OH radical is not free, but tightly associates with the cobalt
for hydroxyl radical production and APZ formation is proposed and
is shown in Scheme 1. According to the mechanism, OAP first binds
complex [39]. The similar mechanism can be used to explain the
current reaction with high oxidation rate. Site-specific mecha-
nisms have also been proposed for Co(II)-induced DNA damage
and deoxyribose degradation in the presence of H O [32].
II
−
to the cobalt ion in a preequilibrium. The formed Co (HCO3 )(OAP)
complex then reacts with dioxygen in a further equilibrium step,
2
2
II
•−
during which Co is oxidized to Co(III) and O is reduced to O2
(
2
2−
one-electron reduction species) and O2
(two-electron reduc-
4. Conclusion
tion species), as amino and polyamino are important ligands for
2+
Co ions in solution, and thus alter its redox potential and facile
the electron transfer [32]. This supposition can be proved by ESR
measurement of the cobalt-bicarbonate–OAP mixture, which has
been previous dissolved in methanol and dried at room tem-
The oxidation of OAP by Co2+ ions in HCO3 aqueous solution
−
was investigated with dioxygen as the oxidant under mild condi-
tions, with a particular emphasis on mechanistic details of reactive
•
oxygen species formation. The results suggest that the bound OH
2+
perature. As shown in Fig. S5, for the mixture of Co –OAP and
is produced as the main reactive species and the contribution of
2
+
−
Co –HCO3 –OAP, the signal of Co(III)-superoxide appears [36],
similar with the report by Hu, for a polymer immobilized cobalt
•
o-aminophenoxyl radical is minor. The formation of OH by the
system can be divided into two steps: (i) the O22 mediated produc-
−
2
−
accepts H+ released from the
complex [37]. Afterwards, the O2
protonation by HCO3 to form H O . When H O is present in aque-
ous solution, the complex Co (HCO )(OAP) may react directly
with it to form the stable Co (HCO )(OAP) (H O ) complex and
2+
−
tion of H O , and (ii) the reaction of formed H O with Co –HCO3
2
2
2
2
−
2
II
2
2
2
•
complex to produce OH through Fenton reaction. The results will
be helpful for the development of simple and efficient dioxygen-
based system aimed to pollutants degradation.
−
3
II
−
3
2
2
•
then dissociate it to a transient caged OH radical through an elec-
2
+
•
tron transfer from Co . Once formed, the high active OH radical
attracts rapidly the hydrogen atom from the bonded substrate to
yield BQMI. Then the product reacts uncatalytically with OAP with
the release of electrons and the production of APZ, accompanied by
Acknowledgment
This work was supported by the National Nature Science Foun-
dation of China (21207105).
II
−
III
−
regeneration of Co (HCO3 ). The direct reduction of Co (HCO3 )

3
04
X. Li et al. / Applied Catalysis A: General 475 (2014) 297–304
Appendix A. Supplementary data
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