Preparation and characterization of Cu-rare earth/Al2O3 catalysts and their application in the electrochemical removal of p-nitrophenol

Article abstract of DOI:10.1039/c6ra21164j

Cu and rare earth composite catalysts (Cu-rare earth/Al₂O₃) were prepared on Al₂O₃ particles using an impregnation method for the electro-catalytic oxidation of p-nitrophenol in this study. The surface topography and crystal structure of the products were characterized using scanning electron microscopy and X-ray diffraction. Results show that the Cu-rare earth/Al₂O₃ particles have a beneficial effect on p-nitrophenol removal by improving the ^?OH radical generation rate, especially the Cu-Bi/Al₂O₃ catalyst. The effects of the heat treatment conditions for the Cu-Bi/Al₂O₃ catalyst on its electro-catalytic activity were investigated. For a temperature of 550 °C and a time of 5 h, the obtained Cu-Bi/Al₂O₃ catalyst exhibited the highest p-nitrophenol removal, nearly 100% after 60 min electrolysis, and this was decreased by only 4.5% after 10 cycles. In addition, the intermediates formed during degradation were identified using high-performance liquid chromatography, and the major intermediates formed during degradation were benzoquinone, hydroquinone, catechol, p-aminophenol, phenol, maleic acid, succinic acid, and oxalic acid. On the basis of the reaction products identified, a possible degradation pathway for p-nitrophenol was proposed.

Full text of DOI:10.1039/c6ra21164j

View Article Online
View Journal
RSC Advances
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: X. Duan, F. Ren
and L. Chang, RSC Adv., 2016, DOI: 10.1039/C6RA21164J.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
rsc.li/rsc-advances

Page 1 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
Preparation and characterization of Cu-rare earth/Al2O3 catalysts and
their application for electrochemical removal of p-nitrophenol
c
Xiaoyue Duana,b, Fang Renc, Limin Chang
*
a School of Environmental Science and Engineering, Jilin Normal University, Siping 136000, China
b Key Laboratory of Environmental Materials and Pollution Control, the Education Department of Jilin Province,
Siping 136000, China
c Key Laboratory of Preparation and Application of Environmental Friendly Materials, Ministry of Education, Jilin
Normal University, Siping 136000, China
*Corresponding author:
Tel:+ 86ꢀ434ꢀ3292094, fax:+ 86ꢀ434ꢀ3292233
Eꢀmail: changlimin2139@163.com (L. Chang)
Address of Corresponding Author : Haifeng Street 1301, Tiexi District, Siping City, Jilin Province, P. R. China.
Abstract: The Cu and rare earth composite catalysts (Cuꢀrare earth/Al2O3) were prepared on Al2O3 particles by
impregnation method for electroꢀcatalytic oxidation of pꢀnitrophenol in this study. The surface topography and crystal
structure of the products were characterized by scanning electron microscopy and Xꢀray diffraction. Results show that
the Cuꢀrare earth/Al2O3 particles have a beneficial effect on pꢀnitrophenol removal by improving the •OH radicals
generation rate, especially for CuꢀBi/Al2O3 catalyst. The effect of heat treating conditions of CuꢀBi/Al2O3 catalyst on its
electroꢀcatalytic activity was investigated. Under the temperature of 550 ºC and the time of 5 h, the obtained
CuꢀBi/Al2O3 catalyst exhibited the highest pꢀnitrophenol removal of nearly 100% after 60 min electrolysis, and it was
decreased by only 4.5% after 10 cycles. In addition, the intermediates formed during the degradation were identified by
highꢀperformance liquid chromatography and the major intermediates formed during the degradation were
benzoquinone, hydroquinone, catechol, pꢀaminophenol, phenol, maleic acid, succinic acid, and oxalic acid. On the basis
of the reaction products identified, a possible degradation pathway of pꢀnitrophenol was proposed.
Keywords: Composite catalyst; Electroꢀcatalytic oxidation; pꢀNitrophenol; Rare earth
1
Introduction
Nitrophenols (NPs) are important raw materials for productions of explosives, herbicides, fungicides and various
synthetic compounds1. So NPs are among the most common organic pollutants in industrial and agricultural
wastewaters. However, the NPs are typical biorefractory organic compounds and listed in the 114 priority toxic
pollutants by the US Environmental protection Agency2. It is therefore important to develop effective methods to
remediate NPsꢀcontaining effluents.
A large number of methods can be considered to removal NPs in water, such as adsorption3, biological process4,
chemical oxidation5,6, photoꢀcatalysis7 and electrochemical oxidation8. The latter method has received
considerable attention in the removal of organic pollutants in water because of its great advantages, such as high
removal rates, easiness of implementation, and no byproducts formation at the end of the treatment9. The anodes
for traditional electrochemical oxidation process are usually twoꢀdimensional (2D) plate electrodes, which can
effectively degrade organic pollutants. However, the life time of •OH radicals generated on the anodes is short,
1

RSC Advances
Page 2 of 13
View Article Online
DOI: 10.1039/C6RA21164J
and they are easily converted to second oxidizers (O3, H2O2 and O2)10, whose oxidation capacities are much
weaker than that of •OH radicals. Therefore, the 2D electrodes have some weaknesses such as low current
efficiency and high energy consumption11. Thus, threeꢀdimension (3D) electrode emerged in the late 1960s12. 3D
electrode, also named particle electrode or bed electrode, is basically granular materials which are filled between
two main counter electrodes. When an appropriate voltage is applied on two counter electrodes, these particles
will be polarized and a large number of microelectrodes were formed13. Therefore, the specific surface area of 3D
electrode is generally dozens or hundreds of times that of 2D electrode14,15. The larger specific surface area of 3D
electrode can provide more activity sites and accelerate the electroꢀgeneration of •OH radicals16,17. Also, the
migration distance of reactants is short and the mass transfer rate is increased because of the significantly reduced
distance of anode and cathode of microelectrodes18ꢀ21. Besides, the 3D electrodes can effectively adsorb and
electroꢀadsorb the organic pollutants due to their wellꢀdeveloped porosity and high surface area, which will
promote the aggregation of organics on the surface of electrodes. As a result, the current efficiency of
electrochemical oxidation of organic pollutants can be enhanced significantly.
Since Cu ions were extracted from solution by graphite packed bed in 197422, 3D electrodes were applied in the
removal of organic pollutants by researchers. The granular activated carbons (GAC) have been widely used as
particle electrodes20,23ꢀ26. In addition, the feasibility of carbon aerogel (CA) used as particle electrodes for
decoloration of reactive brilliant red Xꢀ3B dye solution in a threeꢀdimensional electrode was also studied27.
However, both GAC and CA only have high specific surface area and excellent adsorptive performance, they can’t
act as catalysts. Thus, some 3D particle catalysts were developed by loading solid catalysts on GAC, γꢀAl2O3,
molecular sieve, SiO2, kaolin and CA supporters19,28ꢀ31 to accelerate the degradation of pollutants by reducing
reaction barriers.
The aim of the present work is to prepare the Cu and rare earth (RE) composite catalysts on Al2O3 particles
(marked as CuꢀRE/Al2O3) by impregnation method for electrochemical oxidation. Al2O3 was chosen as the carrier
for CuꢀRE catalysts due to its advantages of high surface area, excellent thermo stability and chemical inertness in
electrolyte. The surface topography and crystal structure of products were characterized by scanning electron
microscopy and Xꢀray diffraction. The •OH radical generation capacity and pꢀnitrophenol (pꢀNP) degradation
efficiency in 3D reactor (also called heterogeneous electrochemical oxidation) with different CuꢀRE/Al2O3
particle catalysts were compared. In addition, effects of heat treating temperature and time of CuꢀBi/Al2O3 catalyst
and initial pH of electrolyte on pꢀNP removal were described, and a possible electrochemical degradation pathway
of pꢀNP was also discussed.
2
Experimental section
2.1 Materials
The Al2O3 particles, La(NO3)3·6H2O, Bi(NO3)3·5H2O, Ce(NO3)3·6H2O, and pꢀNP were purchased from
Sinopharm Chemical Reagent Co., Ltd, China. All chemicals were of analytical grade and were used without
further purification. All solutions were prepared using deionized water.
2.2 Preparation procedure of catalyst particles
Firstly, the commercial Al2O3 particles were pretreated in boiling deionized water for 1 h and washed with
deionized water thoroughly, and then dried at 105 ºC for 24 h. Then, 10 g pretreated Al2O3 particles were
respectively put into 7% Cu(NO3)2, 7% Cu(NO3)2 + 2% La(NO3)3, 7% Cu(NO3)2 + 2% Bi(NO3)3, 7% Cu(NO3)2 +
2% Ce(NO3)3 solutions, and stirred mechanically for 24 h at room temperature. After filtering and drying, the
Al2O3 particles loaded nitrates were charged into a temperatureꢀprogrammed vacuum furnace and heat treated at
400 ºC with a heating rate of 5 °C min–1 to pyrolyze nitrates. After reaching the target temperature, the samples
were maintained at this temperature for 5 h and then cooled to room temperature to afford the samples of
2

Page 3 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
Cu/Al2O3, CuꢀLa/Al2O3, CuꢀBi/Al2O3, and CuꢀCe/Al2O3. In order to investigate the effect of temperature and time
of calcination on the electroꢀcatalytic activity of particles, the samples loaded Cu(NO3)2 + Bi(NO3)3 were heat
treated at temperatures of 150 ºC, 250 ºC, 400 ºC, 550 ºC or 700 ºC for 5 h and at temperatures of 400 ºC for 1 to
10 h, respectively.
2.3 Characterization of catalyst particles
Xꢀray diffraction (XRD) patterns of catalyst particles were recorded on a Dꢀmax/3C Xꢀray diffractometer (Rigaku,
Japan) with Cu Kα radiation source (45 kV, 30 mA) at a scan rate of 16º min–1 from 10º to 90º. The XRD samples
were prepared by grinding the catalyst particles to powders and pressing the powders onto a glass substrate. The
surface morphology of the catalyst particles was characterized by scanning electron microscopy (SEM; Hitachi
Sꢀ570, Japan) with an accelerating voltage of 20 kV. The SEM samples were directly stuck onto the sample stage
and subjected to the goldꢀspraying.
2.4 Hydroxyl radical generation
The quantity of •OH radical generation during electrolysis processes was estimated by means of a method using
terephthalic acid (TA) as a fluorescence probe, in which 2ꢀhydroxyterephthalic acid (HA) was formed by the
reaction between TA and electrochemicalꢀgenerated •OH radicals32. An aqueous solution of a volume of 200 mL
containing 0.5 mmol L–1 terephthalic acid, 0.5 g L–1 NaOH and 0.25 mol L–1 Na2SO4 was used as electrolytic
solution. •OH radical generation was performed at a current density of 30 mA cm–2 at 30 °C. During the
experiments, samples were drawn from the reactor every 5 min and diluted 20 times with deionized water, then
analyzed by a fluorescence spectrophotometer (Perkin Elmer LSꢀ50, American). Fluorescence spectra were
recorded in the range of 370ꢀ520 nm, using an excitation wavelength at 315 nm.
2.5 Procedure for electrochemical oxidation experiments
The heterogeneous electrochemical oxidation experiments were carried out by batch processes. The experimental
setup is shown in Fig. 1. The electrolysis was performed in a rectangular reactor (plexiglass). A PbO2 electrode
with a size of 3 cm × 5 cm was used as the anode, which was verified to be efficient for the degradation of pꢀNP in
previous study33, and a stainless steel sheet with a size of 3 cm × 5 cm was used as the cathode. They were
positioned vertically and parallel to each other with a distance of 3 cm. Approximately 50 g developed
CuꢀRE/Al2O3 catalyst particles were packed into space between the anode and cathode. Some polyethylene (PE)
net separated the catalysts and main electrodes. The degradation of pꢀNP was performed at a constant current
density of 30 mA cm–2. The volume of pꢀNP solution was 200 mL with an initial concentration of 50 mg L–1, and
0.05 mol L–1 Na2SO4 was added to the solution as supporting electrolyte. The reaction temperature was kept at 30
ºC during all experimental runs. The determination of pꢀNP concentration during the oxidation degradation was
determined on highꢀperformance liquid chromatography (HPLC, Shimadzu, Japan) with a UV detector set at 265
nm. The mobile phase consisted of 55% methanol and 45% water (volume fraction). The separation was
performed using a Cꢀ18 column at a column temperature of 30 ºC and at a flow rate of 1.0 mL min–1. For
comparison, pꢀNP degradation was performed in a 2D PbO2 anode system (no catalyst particles were added
between the anode and cathode) under the same conditions.
3

RSC Advances
Page 4 of 13
View Article Online
DOI: 10.1039/C6RA21164J
Fig. 1. (a) Diagram of the electrochemical setup, (b) photogragh of the electrochemical setup and (c) photogragh
of the electrode design.
3
Results and discussion
3.1 Surface morphology and crystal structure of particle electrodes
The pahse constitution of the particle electrodes were investigated by Xꢀray diffractomery and shown in Fig. 2.
For the Cu/Al2O3 particles, the main diffraction peaks are attributed to the Al2O3. The weak peaks of CuO are also
displayed in the XRD pattern, indicating that the CuO has been loaded on the surface of Al2O3 particles. However,
the peak intensity of CuO was much weaker than that of Al2O3 because of its low loading amount. The diffraction
patterns for the CuꢀLa/Al2O3, CuꢀCe/Al2O3, and CuꢀBi/Al2O3 also display La2O3, CeO2 and Bi2O3 diffraction
peaks except the broad peaks from the Al2O3 and CuO. Another notable observation in Fig. 1 is that the peak
intensity of sharp peaks of Bi2O3 are significantly stronger than those of La2O3 and CeO2. Thus, it can be deduced
that the amount of loaded Bi2O3 was more than those of CuO, La2O3 and CeO2.
♠
♦
La2O3
♥
CeO2
♠ Bi2O3
♣ γ-Al2O3
•
CuO
♠
♠
♠
♠
Cu-Bi/Al2O3
Cu-Ce/Al2O3
♥
♥
♥
♦
Cu-La/Al2O3
•
•
•
•
•
•
•
Cu/Al2O3
Al2O3
80
♣
♣
♣
♣
♣
♣
10
20
30
40
50
/deg.
60
70
90
2θ
Fig. 2. XRD spectra of different particle catalysts.
4

Page 5 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
The morphologies of Cu/Al2O3, CuꢀLa/Al2O3, CuꢀBi/Al2O3, and CuꢀCe/Al2O3 particle catalysts are displayed by
the SEM images in the Fig. 3. As can be seen, compared with pure Al2O3 (Fig. 3a), some new particulates were
formed on the surface of Al2O3 for Cu/Al2O3, CuꢀLa/Al2O3, CuꢀCe/Al2O3, and CuꢀBi/Al2O3 particles. According
to the results of the Xꢀray diffraction patterns in Fig. 2, it can be deduced that these formed new particulates on the
Cu/Al2O3, CuꢀLa/Al2O3, CuꢀCe/Al2O3, and CuꢀBi/Al2O3 are CuO, CuO + La2O3, CuO + CeO2, CuO + Bi2O3,
respectively. It can also be observed in Fig. 3 that the shapes of CuO, La2O3 and CeO2 were irregular, while the
shape of Bi2O3 was sixꢀsided cylinder. Moreover, the amount of Bi2O3 was significantly more than those of CuO,
La2O3 and CeO2, which is consistent with the result of XRD.
Fig. 3. SEM images of different particle catalysts. (a) Al2O3, (b) Cu/Al2O3, (c) Cu-La /Al2O3, (d) Cu-Bi/Al2O3, (e)
Cu-Ce/Al2O3.
3.2 Hydroxyl radical generation of different systems
•OH radicals are extremely reactive and often considered to be the major species responsible for the
electrochemical oxidation reaction34, so the •OH radicals production rates of 2D electochemical oxidation system
with a plate PbO2 used as anode (2DEO) and 3D electrochemical oxidation systems with a plate PbO2 used as
5

RSC Advances
Page 6 of 13
View Article Online
DOI: 10.1039/C6RA21164J
main anode and Cu/Al2O3, CuꢀLa/Al2O3, CuꢀBi/Al2O3 and CuꢀCe/Al2O3 used as particle catalysts
(3DEOꢀCu/Al2O3, 3DEOꢀCuꢀLa/Al2O3, 3DEOꢀCuꢀBi/Al2O3 and 3DEOꢀCuꢀCe/Al2O3) were measured and
compared by fluorescent spectrometry. The fluorescence spectra of HA formed by the reaction between TA and
electrochemicalꢀgenerated •OH radicals in different systems are shown in Fig. 4aꢀe. It can be observed that the
continuous electrolysis of the solution led to the gradual increase in the fluorescence at about 425 nm for HA,
indicating that the •OH radicals were continuously formed during the electrochemical oxidation reactions in all
systems. Fig. 4f shows the plots of fluorescence intensity at 425 nm against electrolysis time. It can be observed
that the fluorescence intensity for all systems increased linearly against electrolysis time. So the •OH radicals
produced during electrolysis were proportional to the electrolysis time obeying zeroꢀorder reaction kinetics under
the alkaline experiment conditions. From the slopes of the fitting graphs, one can also see that the •OH radicals
production rates increased in the following order under the same conditions: 2DEO (kform,HA=3.7683) <
3DEOꢀCu/Al2O3 (kform,HA=7.2971)
<
3DEOꢀCuꢀLa/Al2O3 (kform,HA=17.844)
<
3DEOꢀCuꢀCe/Al2O3
(kform,HA=20.0652) < 3DEOꢀCuꢀBi/Al2O3 (kform,HA=28.078). So the 3DEOꢀCuꢀBi/Al2O3 system had the highest
kform,HA, which was 7.45 times that of 2DEO, and it should oxidize pollutants more effecitvely than the 2DEO
system. This may be due to that H2 and O2 produced on the surface of cathode and anode could react to generate
H2O2 in the presence of CuꢀRE/Al2O3 catalysts (Equation 1), and H2O2 was further catalytically decomposed to
•OH radicals (Equation 2)35.
H2 + O2 → H2O2
H2O2 → 2 •OH
(1)
(2)
140
250
200
150
100
50
30 min
30 min
a
b
120
100
80
60
40
20
0
20 min
10 min
20 min
10 min
0 min
0 min
0
380
400
420
440
460
480
500
520
380 400 420 440 460 480 500 520
Walvelength/nm
Walvelength/nm
700
600
500
400
300
200
100
0
600
500
400
300
200
100
0
30 min
30 min
d
c
20 min
20 min
10 min
0 min
10 min
0 min
380
400
420
440
460
480
500
520
380 400 420 440 460 480 500 520
Walvelength/nm
Walvelength/nm
6

Page 7 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
1000
800
600
400
200
0
1000
800
600
400
200
0
2DEO
3DEO-Cu/Al2O3
30 min
20 min
e
f
3DEO-Cu-La/Al2O3
3DEO-Cu-Ce/Al2O3
3DEO-Cu-Bi/Al2O3
10 min
0 min
380 400 420 440 460 480 500 520
Walvelength/nm
0
5
10
15
20
25
30
Time/min
Fig. 4. Fluorescence spectra of HA for 2DEO (a), 3DEO-Cu/Al2O3 (b), 3DEO-Cu-La/Al2O3 (c),
3DEO-Cu-Ce/Al2O3 (d) and 3DEO-Cu-Bi/Al2O3 (e) systems, and plots of fluorescence intensity against time (f).
3.3 Electrochemical degradation of p-NP wastewater assisted by 3D particle electrodes
To investigate the influence of the CuꢀRE/Al2O3 catalysts on the electroꢀcatalytic degradation of organic
pollutants, degradation experiments of pꢀNP were carried in 2DEO, 3DEOꢀCu/Al2O3, 3DEOꢀCuꢀLa/Al2O3,
3DEOꢀCuꢀBi/Al2O3 and 3DEOꢀCuꢀCe/Al2O3 systems. The removal percentage change of pꢀNP with electrolysis
time is shown in Fig. 5a. It can be clearly seen that the pꢀNP removal percentage was nearly up to 100% with 60
min for the 3DEOꢀBiꢀCu/Al2O3 system and only 68.62% for the 2DEO. In addition, the degradation processes
were fitted with a pseudoꢀfirstꢀorder model for all systems by using the following expression (3).
[ p ꢀ NP]0
ln
= kappt
(3)
[ p ꢀ NP]
[pꢀNP]0 and [pꢀNP] are the pꢀNP concentrations at time 0 and t, respectively, and kapp is the kinetic rate constant.
The fitting results are shown in Fig. 5b. According to the good linear correlation between the logarithm values of
the normalized pꢀNP concentration and electrolysis time, the degradation of pꢀNP in these systems was fitted to
the pseudoꢀfirstꢀorder kinetic expression. The values of kapp of 2DEO, 3DEOꢀCu/Al2O3, 3DEOꢀCuꢀLa/Al2O3,
3DEOꢀCuꢀBi/Al2O3 and 3DEOꢀCuꢀCe/Al2O3 were 0.0205, 0.0351, 0.0426, 0.0564 and 0.0467, respectively. As
can be seen, the values of kapp of 3DEO was much higher than that of 2DEO, and the kapp,3DEOꢀCuꢀBi/Al2O3 was higher
than kapp,3DEOꢀCuꢀLa/Al2O3 and kapp,DEOꢀCuꢀCe/Al2O3. This may be attributed to higher •OH radicals generetion rate of
3DEO systems. In addition, the CuꢀRE/Al2O3 particles can provide more reactive sites than twoꢀdimensional
electrode for pꢀNP pollutant adsorption and catalytic reaction. However, comparing the values of kform,HA and kapp
for all systems, it can be noticed that the value of kapp,3DEOꢀCuꢀBi/Al2O3 was only 2.75 times the value of kapp,2DEO
,
while the kform,HA of 3DEOꢀCuꢀBi/Al2O3 was 7.45 times greater than that of 2DEO. This suggested that only a
small part of •OH radicals was used for the degradtion of pꢀNP, and most of them were inevitably converted
oxygen through water oxidation36:
[•OH] + H2O → O2 +3H+ + 3e–
(4)
7

RSC Advances
Page 8 of 13
View Article Online
DOI: 10.1039/C6RA21164J
7
6
5
4
100
80
60
40
20
0
a
b
2DEO
3DEO-Cu/Al2O3
3DEO-Cu-La/Al2O3
3DEO-Cu-Ce/Al2O3
3DEO-Cu-Bi/Al2O3
2DEO
3DEO-Cu/Al2O3
3
2
1
0
3DEO-Cu-La/Al2O3
3DEO-Cu-Ce/Al2O3
3DEO-Cu-Bi/Al2O3
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time/min
Time/min
Fig. 5. p-NP removal efficiency as a function of electrolysis time for different systems: (a) p-NP remval percentage
change. (b) kinetic analysis of the curves. Operating conditions: [p-NP]0 = 50 mg L–1, Na2SO4 concentration =
0.05 mol L–1, current density =30 mA cm–2, temperature = 30 ºC.
The influence of temperature and time of heat treating of CuꢀBi/Al2O3 particles on their electroꢀcatalytic oxidation
activity was investigated at a fixed heat treating time of 5 h and a fixed heat treating temperature of 400 °C,
respectively. Fig. 6 shows the comparison of activity of CuꢀBi/Al2O3 particles treated under different temperature
(Fig. 6a) and different time (Fig. 6b) in electrochemical oxidation processes of pꢀNP. It can be seen in Fig. 6 that
the removal percentage of pꢀNP increased firstly, and then trend to decrease with increasing temperature and
prolonging time. The highest pꢀNP removal efficiency was obtained by particles treated at the temperature of 550
ºC and the time of 5 h. Thus, the CuꢀBi/Al2O3 particles treated for 5 h at 550 ºC had the highest electroꢀcatalytic
oxidation activity. This result may be due to the fact that, if the temperature is not high enough and the time is not
enough long, the nitrates can not be converted to metal oxides; if the heat treating temperature was too high or the
time was too long, an interaction between CuO and Al2O3 may take place and CuAl2O4 was formed37.
100
80
60
40
20
0
100
80
60
40
20
0
a
b
1 h
3 h
5 h
7 h
10 h
150
250
400
550
700
°
°
°
°
°
C
C
C
C
C
0
20
40
60
80
100
120
0
20
40
60
80
100
120
Time/min
Time/min
Fig. 6 Influence of temperature (a) and time (b) of heat treating of Bi-Cu/Al2O3 on p-NP removal efficiency
during the electrolysis process. Operating conditions: [p-NP]0 = 50 mg L–1, Na2SO4 concentration = 0.05 mol L–1,
current density =30 mA cm–2, temperature = 30 ºC.
The longꢀterm cycle stability of CuꢀBi/Al2O3 particles was also evaluated by repeating the electrochemical
oxidation of pꢀNP tests. Fig. 7 shows pꢀNP removal percentage versus cycle number for CuꢀBi/Al2O3 particles.
The pꢀNP removal percentage after 10 cycles was found to be 93.41%, lower by only 4.5% than the initial pꢀNP
removal percentage. The results of this cyclic performance tests indicate the excellent stability of CuꢀBi/Al2O3
8

Page 9 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
particles for electrochemical oxidation.
100
90
80
70
60
50
1
2
3
4
5
6
7
8
9
10
Cycle number
Fig. 7 The p-NP removal percentage after 60 min electrolysis versus cycle number for Bi-Cu/Al2O3 particles.
Operating conditions: [p-NP]0 = 50 mg L–1, Na2SO4 concentration = 0.05 mol L–1, current density =30 mA cm–2,
temperature = 30 ºC.
All the experimental tests so far presented have been performed without a pH correction, leaving the pH to natural
value of 7.2. Therefore, to evaluate the influence of initial pH on pꢀNP degradation, the initial pH was adjusted to
3.1 and 10.4 by the addition of H2SO4 and NaOH. Fig. 8 shows the curve of pꢀNP removal percentage with
electrolysis time for different initial pH. It was can be observed slight improvement of pꢀNP removal from 10 to
40 min at initial pH of 3.1 and 10.4. This may be attributed that, at a lower pH of 3.1, the decreasing pH increased
the oxygen overpotential, leading to the improvement of pꢀNP efficiency38, and at a higher pH of 10.4, more •OH
radicals were formed in alkaline condition through the reaction39,40
OH– → •OH + e–
(5)
:
which was also verified by fluorescent spectrometry (shown in Fig. S1). However, it also can be seen from Fig. 8
that there was no substantial improvement of pꢀNP removal at initial pH 3.1 and 10.4 after 1 h. This phenomenon
could be due to three factors. i) Cyclic voltammograms of pꢀNP obtained in 1 mol L–1 H2SO4, Na2SO4, and NaOH
solutions on PbO2 electrode showed that the pꢀNP can not be directly oxidized on PbO2 electrode in acidic
medium (in Fig. S2), and thus the degradation efficiency of pꢀNP on anode is restrained under acidic condition. ii)
Equation 2 also suggests that at a higher pH, the the reaction will be shifted to the right, more production of the
oxygen, and more •OH radicals will be wasted and does not react with NP in alkaline medium. iii) According to
the the oxygen evolution reaction (OER) at the anode:
Acidic and neutral: 4H2O → 4H+ + O2 + 4e–
Alkaline: 4OH– → 2H2O + O2 + 4e–
(6)
(7)
the OER is greater at the anode surface in alkaline medium due to the attraction of the OH– towards the electrode,
and serves as a competing reaction for the generation of the •OH radicals. Furthermore, the attraction of the OH–
towards the electrode also competes with the surface adsorption of the NP ions, therefore limits their degradation.
Therefore, the initial pH has little effect on the pꢀNP degradation in our study.
9

RSC Advances
Page 10 of 13
View Article Online
DOI: 10.1039/C6RA21164J
100
80
60
40
20
0
pH=3.1
pH=7.2
pH=10.4
0
20
40
60
80
100
120
Time/min
Fig. 8. Influence of initial pH on p-NP removal efficiency. Operating conditions: [p-NP]0 = 50 mg L–1, Na2SO4
concentration = 0.05 mol L–1, current density =30 mA cm–2, temperature = 30 ºC.
The intermediates of pꢀNP degradation were identified by HPLC. Fig. 9 shows the changes of the HPLC
chromatograms of pꢀNP degradation with electrolysis time in 3DEOꢀCuꢀBi/Al2O3 system. The stable
intermediates were characterized to be benzoquinone, hydroquinone, catechol, pꢀaminophenol, maleic acid,
succinic acid, oxalic acid and phenol by comparing the retention time of the standard compounds. Basing on the
identification, the possible degradation pathway of pꢀNP was proposed and is shown in Fig. 10. In the first
instance, –NO2, as an electronꢀwithꢀdrawing group, was attacked by •OH radicals, and the aromatic products of
hydroquinone and phenol were formed due to that the –NO2 group was removed from the aromatic ring and the
–OH group was added in this position. The phenol was further transfered to hydroquinone and catechol. In additon,
the pꢀaminophenol was also presented as one of principal intemediates during the initial 30 min, indicating that
some pꢀNP was reduced to pꢀaminophenol on the cathode, which was also further converted to hydroquinone.
Subsequently, the products of hydroquinone and catechol were oxidized into benzoquinone, which were opened
and decomposed to succinic acid, oxalic acid, glutamic acid, and formic acid. However, in this test, the glutamic
acid and formic acid were not to be detected, which may be due to their too low amounts. Finally, these acids was
decoposed to CO2 and H2O. Comparing this pathway with the degradation pathway of pꢀNP at CNTꢀCeꢀPbO2
anode33 and Ti/BiꢀPbO2 anode41, it was found that the major intermediates formed in 3DEO and 2DEO systems
are the same, only the pꢀNP and intermediates were degraded faster in 3DEO system. So the CuꢀBi/Al2O3 particles
might only accelerate the generation of •OH radicals and the degradation of pꢀNP, and couldn’t change the
degradation pathway of pꢀNP.
10

Page 11 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
12
9
p-NP
6
0 min
3
0
4.5
3.0
1.5
benzoquinone
catechol
15 min
succinic acid
phenol
0.0
2.4
hydroquinone
maleic acid
1.6
0.8
p-aminophenol
30 min
0.0
0.9
maleic acid
oxalic acid
0.6
0.3
0.0
60 min
6
0
1
2
3
4
5
7
8
Retention time/min
Fig. 9. HPLC chromatograms of p-NP degradation at different electrolysis time.
OH
OH
O
O
O
C
C
O
OH
O
CH
OH
OH
+
OH
CH
O
Catechol
O
Orthoꢀbenzoquinone
COOH
Phenol
OH
Muconic acid
Oxalic acid
O
C
O
NO2
OH
pꢀNitrophenol
C
C
O
OH
OH
+
C
O
OH
COOH
OH
O
Hydroquinone
Bezoquinone
Maleic acid
Succinic acid
Oxalic acid
OH
NH2
pꢀAminophenol
Fig. 10. Proposed pathway of p-NP in the 3DEO-Cu-Bi/Al2O3 system.
4
Conclusions
The results of this study showed that the CuꢀRE/Al2O3 composite catalysts were effective for assisting
electrochemical oxidation of pꢀNP. Compared with the CuꢀLa/Al2O3 and CuꢀCe/Al2O3, the CuꢀBi/Al2O3 was more
suitable as catalyst due to its stronger capacity of •OH radicals generation and faster degradation rate for pꢀNP
pollutant, which were 7.45 and 2.75 times those of 2DEO, respectively. The effect of heat treating temperature and
time of CuꢀBi/Al2O3 particles on the electrochemical degradation of pꢀNP showed that the optimal heat treating
temperature and time were 550 ºC and 5 h, respectively. The electrochemical stability of CuꢀBi/Al2O3 assessed
11

RSC Advances
Page 12 of 13
View Article Online
DOI: 10.1039/C6RA21164J
from pꢀNP degradation revealed its excellent stability, in that there was a decrease of only 4.5 % in the pꢀNP
removal percentage after 10 cycles. Finally, the possible mechanism of electrochemical degradation of pꢀNP was
also proposed.
Acknowledgments
This project was supported financially by the Young People Fund of Jilin Science and Technology Department, China
(No. 20150520079JH). This work was also supported by the Science and technology Project of Jilin Education Bureau,
China (No. 2016225 and No. 2016227).
References
1 R. Vargas, S. Díaz, L. Viele, O. Núñez, C. Borrás, J. Mostany, B.R. Scharifker, Appl. Catal. B: Environ., 2014, 144,
107ꢀ111.
1 M. Zhou, L. Lei, Chemosphere, 2006, 63: 1032ꢀ1040.
2 J. Xu, T. Kou, Z. Zhang, Cry. Eng. Comm., 2013, 15: 7856ꢀ7862.
3 T.A. Saleh, V.K. Gupta, Adv. Colloid Interface, 2014, 211: 93ꢀ101.
4 P.P. Liu, J.J. Zhang, N.Y. Zhou, Int. Biodeter. Biodegr., 2010, 64: 293ꢀ299.
5 V.K. Gupta, I. Ali, T.A. Saleh, A. Nayak, S. Agarwal, RSC Adv., 2012, 2: 6380ꢀ6388.
6 A.G. Trovó, A.K. Hassan, M. Sillanpää, W.Z. Tang, Int. J. Environ. Sci. Technol., 2016, 13: 147ꢀ158.
7 T.A. Saleh, V.K. Gupta, J. Colloid Interf. Sci., 2012, 371: 101ꢀ106.
8 L. Wei, H. Zhu, X. Mao, F. Gan, Sep. Purif. Technol., 2011, 77: 18ꢀ25.
9 J.M. Aquino, R.C. RochaꢀFilho, N. S.R. Bocchi, Biaggio, J. Environ. Chem. Eng., 2013, 1: 954ꢀ961.
10 N.N. Rao, R. Misra, N. Gedam, P.N. Parameswaran, J.K. Astik. Chemosphere, 2009, 76: 1206ꢀ1212.
11 X. Zhu, J. Ni, X. Xing, H. Li, Y. Jiang, Electrochim. Acta, 2011, 56: 1270ꢀ1274.
12 J.R. Backhurst, J.M. Coulson, F. Goodridge, R.E. Plimley, M. Fleischmann, J. Electrochem. Soc., 1969, 116:
1600ꢀ1607.
13 Y. Chen, W. Shi, H. Xue, W. Han, X. Sun, J. Li, L. Wang, Electrochim. Acta, 2011, 58: 383ꢀ388.
14 H.Z. Zhao, Y. Sun, L.N. Xu, J.R. Ni, Chemosphere, 2010, 78: 46ꢀ51.
15 M. Mascia, A. Vacca, S. Palmas, Chem. Eng. J., 2012, 211-212: 479ꢀ487.
16 G. Lv, D. Wu, R. Fu, J. Hazard. Mater., 2009, 165: 961ꢀ966.
17 C. Wang, Y.K. Huang, Q. Zhao, M. Ji., Chem. Eng. J., 2014, 243: 1ꢀ6.
18 J. Yang, J. Wang, J. Jia, Environ. Sci. Technol., 2009, 43: 3796ꢀ3802.
19 L. Liu, H. He, C. Zhang, Q. Wang, M. Zhou, Curr. Org. Chem. 2012, 16: 2091ꢀ2096.
20 N.N. Rao, R. Misra, Chem. Eng. J., 2012, 184: 23ꢀ32.
21 I.A. Khattab, M.F. Shaffei, N.A. Shaaban, H.S. Hussein, S.S. Abd ElꢀRehim, Egypt. J. Petrol., 2014, 23: 87ꢀ91.
22 A.K.P. Chu, M. Fleischmann, G.J. Hills, J. Appl. Electrochem., 1974, 4: 323ꢀ330.
23 J. Putzien, Z. Wasser Abwasser Forsch., 1983, 16: 224ꢀ230.
24 Y. Xiong, C. He, H.T. Karlsson, X. Zhu, 2003, Chemosphere, 50: 131ꢀ136.
12

Page 13 of 13
RSC Advances
View Article Online
DOI: 10.1039/C6RA21164J
25 M. Zhou, Z. Wu, X. Ma, Y. Cong, Q. Ye, D. Wang, Sep. Purif. Technol., 2004, 34: 81ꢀ88.
26 L. Wang, J. Fu, Q. Qiao, Y. Zhao, J. Hazard. Mater., 2007, 144: 118ꢀ125.
27 X. Wu, X. Yang, D. Wu, R. Fu, Chem. Eng. J., 2008, 138: 47ꢀ54.
28 E. Fockedey, A. Van Lierde, Water Res., 2002, 36: 4169ꢀ4175.
29 W. Kong, B. Wang, H. Ma, L. Gu, J. Hazard. Mater. B, 2006, 137: 1532ꢀ1537.
30 Y. Hardjono, H. Sun, H. Tian, C.E. Buckley, S. Wang, Chem. Eng. J., 2011, 174: 376ꢀ382.
31 X. Zhao, A. Li, R. Mao, H. Liu, J. Qu, Water Res., 2014, 51: 134ꢀ143.
32 K. Ishibashi, A. Fujishima, T. Watanabe, K. Hashimoto, Electrochem. Commun., 2000, 2: 207ꢀ210.
33 X. Duan, Y. Zhao, W. Liu, L. Chang, X. Li, J. Taiwan Inst. Chem. E., 2014, 45: 2975ꢀ2985.
34 T.A. Enache, A.M. ChiorceaꢀPaquim, O. FatibelloꢀFilho, A.M. OliveiraꢀBrett, Electrochem. Commun., 2009, 11:
1342ꢀ1345.
35 S. Yuan, X. Mao, A.N. Alshawabkeh, Environ. Sci. Technol., 2012, 46: 3398–3405.
36 M. Panizza, G. Cerisola, Chem. Rev., 2009, 109: 6541ꢀ6569.
37 X.Y. Jiang, R.X. Zhou, P. Pan, B. Zhu, X.X. Yuan, X.M. Zheng, Appl. Catal. A: Gen., 1997, 150: 131ꢀ141.
38 Y. Wang, Z. Shen, X. Chen, J. Hazard. Mater., 2010, 178: 867ꢀ874.
39 S. Song, J. Fan, Z. He, L. Zhan, Z. Liu, J. Chen, X. Xu, Electrochim. Acta, 2010, 55: 3606ꢀ3613.
40 J. Wu, H. Xu, W. Yan, RSC Adv., 2015, 5:19284ꢀ19293.
41 Y. Liu, H. Liu, Y. Li, Appl. Catal. B: Environ., 2008, 84: 297ꢀ302.
13