Photodegradation of p-nitrophenol using octahedral Cu2O particles immobilized on a solid support under a tungsten halogen lamp

Article abstract of DOI:10.1016/j.apcata.2012.12.032

Octahedral Cu₂O particles were prepared on an indium-tin oxide glass via electrodeposition and were employed in the catalytic degradation of p-nitrophenol in the presence of H₂O₂. Under irradiation of a warm visible-light source, tungsten halogen lamp, Cu₂O particles not only acted as a photocatalyst, but might also act as a thermal-catalyst to induce the decomposition of H₂O₂ and produce O₂ at higher temperatures. The photogenerated electrons and holes could react with H₂O₂, O₂, and H₂O to produce abundant OH radicals, resulting in the effective oxidation of p-nitrophenol. High-performance liquid chromatography measurements of degradation intermediates showed that p-nitrophenol was first decomposed into hydroquinone and benzoquinone and then mineralized. The degradation efficiency was dependent on electrodeposition time, light intensity, H₂O₂ amount, and solution temperature. This catalyst could be easily recycled and used in the efficient degradation of other phenolic compounds.

Full text of DOI:10.1016/j.apcata.2012.12.032

Applied Catalysis A: General 454 (2013) 59–65
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Applied Catalysis A: General
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Photodegradation of p-nitrophenol using octahedral Cu2O particles
immobilized on a solid support under a tungsten halogen lamp
Wei Zhaia, Fengqiang Suna,b,c,∗, Wei Chena, Zizhao Pana, Lihe Zhanga, Shaohua Lia,
Shuilan Fenga, Yiyi Liaoa, Weishan Lia
a School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
b Engineering Research Center of Materials and Technology for Electrochemical Energy Storage, Ministry of Education, South China Normal University,
Guangzhou 510006, PR China
c Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, South China Normal University, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Octahedral Cu2O particles were prepared on an indium-tin oxide glass via electrodeposition and were
employed in the catalytic degradation of p-nitrophenol in the presence of H2O2. Under irradiation of a
warm visible-light source, tungsten halogen lamp, Cu2O particles not only acted as a photocatalyst, but
might also act as a thermal-catalyst to induce the decomposition of H2O2 and produce O2 at higher
temperatures. The photogenerated electrons and holes could react with H2O2, O2, and H2O to pro-
duce abundant •OH radicals, resulting in the effective oxidation of p-nitrophenol. High-performance
liquid chromatography measurements of degradation intermediates showed that p-nitrophenol was first
decomposed into hydroquinone and benzoquinone and then mineralized. The degradation efficiency
was dependent on electrodeposition time, light intensity, H2O2 amount, and solution temperature. This
catalyst could be easily recycled and used in the efficient degradation of other phenolic compounds.
© 2013 Elsevier B.V. All rights reserved.
Received 10 October 2012
Received in revised form
18 December 2012
Accepted 21 December 2012
Available online xxx
Keywords:
Octahedral Cu2O particles
Electrodeposition
p-Nitrophenol
Photodegradation
1. Introduction
been used in the photocatalytic degradation of PNP, but they are
activated only under UV-light irradiation. Moreover, in all reports,
p-Nitrophenol (PNP) is one of the most hazardous refractory pol-
lutants with high stability and solubility in water. It can be found in
industrial and agricultural wastewaters, posing a significant envi-
ronmental and public risk. Generally, these PNP molecules can be
effectively decomposed by several advanced oxidation processes
(AOPs), for example, microwave-assisted oxidation [1], electro-
catalytic oxidation [2], radiation-induced catalytic oxidation [3],
and photoelectrocatalytic oxidation [4]. However, the require-
ments of various devices, the complex designs of oxidation systems,
or low degradation efficiency limit the practical applications of
these methods. Photo-induced oxidation, which is also an AOP,
is economical, environmentally friendly, and promising in treat-
ing contaminated water. In addition to the homogenous catalyst
systems of UV/H2O2 photo-oxidation [5] and photo-Fenton oxida-
tion [6], several TiO2-based heterogeneous photocatalysts, such as
nanosized TiO2 or TiO2/SiO2 composite particles [7,8], organic acid-
modified TiO2 particles [9], and Fe-doped TiO2 particles [10], have
the catalyst particles have to be dispersed in solution and are
difficult to be recycled. The preparation or the construction of a
recyclable photocatalyst or photocatalytic system for PNP pho-
todegradation under the irradiation of a low-cost, harmless, and
easily operated lamp remains a challenge.
Recently, Cu2O, as a new type of photocatalyst, has attracted
much attention because it has low toxicity, is environmentally
friendly, and has a narrow direct band gap (2.17 eV) that can be
easily activated by visible light. In many studies, Cu2O can be effi-
ciently used in the photocatalytic degradation of various dyes and in
the decomposition of water into H2 and O2 [11,12]. In photodegra-
dation of PNP and other organic pollutants, only Cu2O/TiO2 [13]
heterojunction network, Cu2O@TiO2 core–shell particles [14] and
Cu2O/Ag composite nanoparticles [15] have ever been reported
exhibiting high efficiency; when Cu2O particles alone were used
directly [16], the degradation process took longer to obtain a low
efficiency. In addition, Cu2O particles cannot be immobilized and
have to be directly dispersed in solutions, which inevitably results
in the aggregation of different particles, a decrease in the activities,
and make the particles difficult to recycle.
∗
Corresponding author at: School of Chemistry and Environment, South China
In this study, we introduce for the first time the application of
octahedral Cu2O particles on an indium-tin oxide (ITO) glass for PNP
photodegradation in the presence of H2O2. The particles obtained
Normal University, Guangzhou 510006, PR China. Tel.: +86 20 39310187;
fax: +86 20 39310187.
E-mail address: fqsun@scnu.edu.cn (F. Sun).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.032

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W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
via electrodeposition directly grew on the substrate and were iso-
lated from each other. A Cu2O crystal with octahedral shape was
hypothesized to possess the highest stability and the best activi-
ties [17]. A warm visible-light source, tungsten halogen lamp, was
employed. The lamp provides the visible light required in the pho-
tochemical reactions and increases the temperature of the solution.
The increase in the temperature could excite the thermal-catalytic
activity of Cu2O to induce the catalytic decomposition of H2O2 and
produce O2 [18]. The combined effects of both oxidants, H2O2 and
O2 [19], caused the production of more •OH radicals [20], thereby
oxidizing PNP more efficiently. The method required no protection
measure, stirring apparatus, cooling system, and aeration device.
These requirements are commonly found in a general photocat-
alytic system. The catalyst could also be used in the degradation of
many other pollutants and could be easily recycled.
sample was characterized via X-ray powder diffraction [XRD,
Bruker D8 Advance X-ray diffractometer, with Cu K␣ radiation
˚
(ꢀ = 1.54178 A)]. A UV–vis spectrophotometer (UV-2500PC) was
used to measure the diffuse reflectance spectrum of the octahedral
Cu2O particles formed on the ITO substrates. During the degra-
dation reactions, the photolyzed solutions were analyzed using a
UV–vis spectrophotometer (UV-752), and the absorbance was mea-
sured at a wavelength of 320 nm, corresponding to the maximum
absorption wavelength of PNP. The total organic carbon (TOC) of the
PNP solutions was analyzed using a Shimadzu TOC-VCPH analyzer
to evaluate the mineralization degree of the organic material.
3. Results and discussion
3.1. Morphologies of the particles
2. Materials and methods
The morphologies and compositions of the Cu2O particles grow-
ing on the ITO glass through electrodeposition are shown in Fig. 1.
When the electrodeposition time was increased from 15 min to
90 min, the morphologies of all Cu2O particles became consis-
tent and exhibited octahedron shapes. However, the sizes had a
tendency to increase (from ∼3 ␮m to ∼6 ␮m). According to the
crystallographic character of Cu2O particles, the surfaces of such
particles should be only composed of the {1 1 1} crystalline faces.
The formation of the octahedron particles was closely related to
the addition of PVP in the electrolyte. Similar to other previously
reported surfactants [21], PVP would be preferably adsorbed onto
the {1 1 1} facets once the crystal nuclei were formed to slow down
the corresponding growth rate. A suitable amount of PVP (e.g.,
1 g 100 mL−1 electrolyte) caused the {1 1 1} facets to have the slow-
est growth rate, resulting in the formation of octahedral particles
with naked {1 1 1} crystalline faces. Using a lower current density
(0.18 mA cm−2), the nuclei could preferably form on certain sites
of the polycrystalline ITO glass, which had higher conductivity in
the micro-scale. Thus, most particles adhered to the substrate alone
without coming into contact with each other in a large area over
a short deposition time (e.g., 15 and 30 min; Fig. 1A and B). Every
crystalline face could be exposed except the one in contact with
the substrate. When the time was increased, the particles gradu-
ally grew and the distances between the particles decreased. At the
same time, some newly formed particles were observed among the
enlarged particles. Therefore, more particles began to come into
contact with each other (Fig. 1C–E), which reduced the exposure
of the crystalline faces. We speculate that an increasing number of
particles would come into contact with each other as the electrode-
position time is increased. Thus, a closed film with slightly exposed
crystalline faces could be formed.
2.1. Materials
Analytical-grade
reagents
including
copper
sulfate
(CuSO4·5H2O), hydrogen peroxide (H2O2), poly(vinyl pyrrol-
idone) (PVP), PNP, phenol, and p-chlorophenol were all purchased
from the Aladdin Reagent Company and were used without further
purification. ITO-glass substrates were employed as the conducting
substrates and carriers for the preparation of the Cu2O particles.
These substrates were ultrasonically and sequentially cleaned in
acetone, ethanol, and distilled water prior to use.
2.2. Electrodeposition of the Cu2O particles
Octahedral Cu2O particles on the ITO substrates were gal-
vanostatically electrodeposited at an ambient temperature in an
electrolyte solution composed of copper sulfate (0.02 M) and PVP
(8.0 × 10−4 M). ITO substrates (4.0 × 1.5 cm2) immersed in the elec-
trolyte were used as the working electrode, and a graphite plate
was used as the auxiliary electrode. The distance between the
two electrodes was maintained at 10 cm. Electrodeposition was
carried out at a current density of 0.18 mA cm−2 for a specific
period. Subsequently, the samples were obtained and rinsed with
abundant distilled water to remove impurities. The samples were
labeled as “sample-15,” “sample-30,” “sample-45,” “sample-60,”
and “sample-90” according to different electrodeposition times
(min). The electrodeposition area for each sample was maintained
at 3.0 cm × 1.5 cm.
2.3. Photodegradation experiments
The XRD patterns of sample-30 (Fig. 1B) are shown in Fig. 1F.
Four characteristic peaks appeared at 29.87◦, 36.27◦, 42.30◦, 61.37◦,
and 73.26◦, which correspond to the (1 1 0), (1 1 1), (2 0 0), (2 2 0),
and (3 1 1) crystal faces of Cu2O (JCPDS No. 782076), respectively.
The peaks of the ITO substrate were also determined. The other
samples showed the same test results.
In a typical experiment, one piece of ITO glass covered with
octahedral Cu2O particles was immersed in 50 mL of 20 mg L−1 PNP
solution in a beaker. 1 mL of 3 vol% H2O2 was added into the solution
using a pipette. After the beaker was covered with a polyethylene
film, it was directly placed beside a 150 W tungsten halogen lamp
while maintaining a 15 cm distance. Agitation in the solution or
any protection measure was not necessary. When the sample was
illuminated, approximately 3.0 mL of the reactive pollutant solu-
tion was taken from the beaker to analyze the variations in the
concentration of the degraded solution every 20 min. The analyzed
solution was poured back into the beaker to keep the total volume
of the solution constant.
3.2. Abilities of Cu2O particles in the photodegradation of PNP
The kinetics of PNP degradation with different samples in 3 h
is shown in Fig. 2. In the dark, no degradation was observed in
the presence of both Cu2O particles and H2O2, suggesting that
lamp irradiation was essential for PNP degradation by Cu2O. Even
under irradiation, PNP was still difficult to degrade in the H2O2-
only solution. This result suggests that the auto-degradation of
PNP was a rather slow process under the irradiation of the tung-
sten halogen lamp. Moreover, the bare ITO glass support lacked
active sites for H2O2 activation. All samples shown in Fig. 1 had high
2.4. Characterization and analytical methods
The morphologies of the Cu2O particles on the ITO glass
were directly investigated via field emission scanning elec-
tron microscopy (JSM-6630F). The phase identification of the

W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
61
Fig. 1. SEM images of the Cu2O particles on the ITO glass obtained using different electrodeposition times: (A) 15, (B) 30, (C) 45, (D) 60, and (E) 90 min. (F) XRD spectra of B
and ITO glass (scale bar = 20.0 ␮m).
activities in PNP degradation in the presence of H2O2. However, the
total weight of the octahedral Cu2O particles in the deposited area
(3.0 cm × 1.5 cm) for each sample was only approximately 1.0 mg.
When the time was increased, the concentration of PNP gradually
decreased because of the decomposition processes. About 180 min
later, nearly all (more than 98%) PNP was completely decomposed
in a solution containing any of the aforementioned Cu2O samples.
However, a slight difference was observed in the degradation rates
among the different samples obtained at different electrodeposi-
tion times. The sample with an electrodeposition time of 30 min
(sample-30) had the best activity, which resulted in the highest
degradation rate of PNP. Compared with the other four samples,
the complete degradation time was shortened by approximately
40 min in sample-30. Regardless of the decrease or increase in the
electrodeposition time, the obtained samples (samples-15, 45, 60,
and 90) all had reduced activities, which resulted in lower degrada-
tion rate of PNP. The variations in the activities might be dependent
on the sizes and distributions of the electrodeposited Cu2O parti-
cles. When the electrodeposition time was too short (e.g., 15 min),
the obtained particles had smaller sizes, and the total surface area
of all particles in the electrodeposition area was also smaller than
those of others. As a result, the activities decreased. By contrast, a

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W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
Fig. 2. Degradation of 50 mL 20 mg L−1 PNP solution by different Cu2O particle sam-
ples obtained using different electrodeposition times.
long electrodeposition time would cause more particles to come
into contact with each other. Thus, the exposed part of the active
crystalline faces would be suppressed, reducing the total activities
of all particles in the sample (e.g., samples-45, 60, and 90).
The Cu2O particles on the ITO glass exhibited strong abil-
ities in the catalytic degradation of PNP.
A piece of glass
covered with the particles led to the complete degradation of
more than 1000 mL of PNP solution in a reasonable time. The
kinetics of the degradations of PNP solutions with different vol-
umes by sample-30 is shown in Fig. 3A. The concentrations
of PNP and H2O2 were kept constant, and the volumes of the
solutions were changed from 25 mL to 1000 mL. All solutions could
be fully degraded; however, the consumed time increased when
the volume was increased. When 20 mg L−1 PNP was degraded,
the degradation processes of 25, 50, 100, 200, and 1000 mL of PNP
solution took 100, 140, 180, 220, and 400 min, respectively. Accord-
ing to this trend, a higher volume of the PNP solution would be
efficiently degraded. Moreover, compared with the powder cata-
lysts, the Cu2O particles growing on the ITO glass could be recycled
easier. After the photocatalytic reaction was finished, the support
covered with particles was simply taken out with a pair of twee-
zers and was then flushed with deionized water. The support could
be then directly placed into a new solution that contains pollut-
ants. The catalyst (sample-30) could be repeatedly used at least 7
times. In addition, it did not exhibit a significant loss of activity in
the degradation of 25 mL of 20 mg L−1 PNP solutions (Fig. 3B). The
Cu2O particles were stable and could not be oxidized after recycled
7 times (Fig. S1 in the supporting data).
Fig. 3. Degradation of different volumes of PNP by sample-30 (A) and catalyst
recycling in 25 mL solution during degradation (B). Concentrations of PNP and H2O2
were kept at 20 mg L−1 and 0.06 vol%, respectively.
PNP reached 79.19% after 180 min when no PNP molecules were
detected (Figs. 2 and 4A). The fast mineralization of PNP indicated
that the Cu2O particles irradiated by the halogen tungsten lamp
could effectively remove PNP and organic intermediates in the
presence of H2O2.
3.4. Influencing factors in PNP degradation
Further experiments revealed that the degradation of PNP was
strongly dependent on light intensity, H2O2 addition, and solu-
tion temperature. To describe the influencing factors easily, other
factors were kept constant when a particular influencing factor is
discussed.
The use of sample-30 in the degradation of 50 mL of PNP solution
was taken as an example to discuss further the degradation process,
the influencing factors, and the corresponding mechanism.
3.3. Decolorization and mineralization of PNP
3.4.1. Effect of light intensity
The absorption spectrum of the PNP solution in the presence of
sample-30 under irradiation of the 150 W halogen tungsten lamp
at various durations is shown in Fig. 4A. The absorption peak at
320 nm, which corresponds to the maximum absorption peak of
the PNP molecule, gradually weakened as the exposure time was
increased. This peak disappeared after 180 min. At this time, the
degradation efficiency reached 98.57% (curve of sample-30, Fig. 2).
The mineralization degree of PNP was examined by measuring the
decrease in the TOC during the degradation process (Fig. 4B). Dur-
ing the irradiation, the TOC removal ratio gradually increased as
the irradiation time was prolonged. The maximum TOC removal of
Most of the light from the halogen tungsten lamp was in the vis-
ible region, while some was in the UV region. The intensity of the
254 nm UV light ranged from 11.4 ␮W cm−2 to 112.0 ␮W cm−2 and
was related to the distance of the samples from the lamp, which
was equivalent to that of sunlight. The Cu2O particles on the ITO
glass had high absorption values in visible light and showed a direct
bandgap of 2.1 eV (Fig. 5A). These results indicate that the Cu2O
particles could be activated by the light from the lamp to induce a
series of photochemical reactions in the PNP solution. Light inten-
sity had an important function in the photodegradation reactions.
In this study, the light intensity was tuned by changing the distance

W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
63
Fig. 4. (A) UV–vis spectra of PNP at different intervals, and (B) changes in TOC during PNP degradation.
between the lamp and the beaker containing the PNP solution. At
the same time, when the light intensity was increased, the solu-
tion temperature also increased. A glass cold trap refrigerator with
a continuous flow of water was used to maintain the temperature
at 41 ◦C and avoid the effect of temperature. The distances were
changed from 20 cm to 15 and 8 cm, which gradually increased light
intensity and further increased degradation efficiency. Although all
cases showed lower degradation rates with lower temperatures,
the changes in the samples placed at different locations could still
be determined (Fig. 5B). When the distance was decreased from
20 cm to 8 cm, the final degradation ratio increased from 8.47% to
18.39% in 3 h (inset in Fig. 5B). These results indicate that light alone
could lead to PNP degradation without the assistance of tempera-
ture.
3.4.2. Effect of H2O2 amount
The kinetics of PNP degradation by the octahedral Cu2O parti-
cles on the substrates (sample-30) containing different amounts of
3 vol% H2O2 is shown in Fig. 5C. When no H2O2 was added into
the solution, only 7.58% of the PNP was degraded in 3 h. Although
the degradation rate was very low, the results proved that the
octahedral Cu2O particles on the substrates could be activated by
Fig. 5. UV–vis diffuse reflectance spectra of sample-30 (A) and different effects on PNP degradation by different factors. (B) Effect of light intensity, (C) effect of H2O2 amount,
(D) effect of solution temperature. The inset of A shows the variation of (˛hꢁ)2 as a function of photo energy (hꢁ); the inset of B shows the final degradation ratios of the PNP
solutions with different distances from the lamp.

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W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
light from the halogen tungsten lamp and that PNP degradation
was a photo-initiated reaction. The addition of H2O2 significantly
enhanced the degradation reactions. The presence of 0.1 mL H2O2
in the solution increased the degradation ratio to 80.13%. The addi-
tion of more H2O2 (e.g., 1, 5, and 10 mL) significantly increased the
degradation rates of all the samples. Further increasing the amount
of H2O2 slightly increased the degradation. In all cases, the samples
nearly had the same degradation ratio after 2 h. This result sug-
gested that the continuous increase in the amount of H2O2 alone
did not enhance the degradation, and 1 mL of H2O2 was sufficient
for the degradation of 50 mL of 20 mg L−1 PNP.
3.4.3. Effect of solution temperature
The increase in the solution temperature caused by the irradi-
ation of the halogen tungsten lamp could significantly affect PNP
degradation. The solution temperature was kept at 63 ◦C when no
cooling system was employed and the distance between the beaker
containing the PNP solution and the lamp was set at 15 cm. By
changing the immersion depth of the glass cold trap refrigerator,
the temperature could be maintained within the range of 41 ◦C
to 63 ◦C, and the degradation efficiency of PNP could be changed
accordingly. As shown in Fig. 5D, the degradation rate at 41 ◦C
was significantly lower than that at higher temperatures. The final
degradation ratio was only 13.15% in 3 h. When the temperature
was increased to 52 ◦C, the rate significantly improved, and the
degradation ratio reached 86.37%. At 63 ◦C, the corresponding data
reached the maximum (98.57%).
Fig. 6. A comparison of degradation dynamics of PNP added with and without t-
butanol.
has a high activity in the catalytic decomposition of H2O2 and that
a higher temperature could also enhance the decomposition [17].
Many bubbles were continuously produced around the Cu2O par-
ticles when the temperature was increased to 63 ◦C. However, no
such phenomenon was observed at 41 ◦C or in the solution with-
out a Cu2O sample at any temperature. This result indicates that
O2 could be produced at a higher temperature during the degra-
dation reactions. In other studies, O2 was externally introduced. In
the present study, O2 was directly produced on the surfaces of the
Cu2O particles. Thus, O2 could easily scavenge the photo-generated
electrons accumulated on the Cu2O {1 1 1} facets to produce super-
3.5. Degradation mechanism and pathway
Based on the experimental results, the degradation mechanism
of PNP in the presence of Cu2O particles and H2O2 was determined.
Once the Cu2O microcrystals were irradiated by UV and visible
light (from the lamp), the electrons of the valence band could be
excited to the conduction band, which would leave positive holes
on the {1 1 1} surfaces (Eq. (1)), regardless if the temperature in the
PNP solution was increased or not. The photo-generated valence
band holes could directly oxidize the pollutants into inorganic or
nontoxic materials (Eq. (2)). However, the electrons and holes easily
recombined to reduce the number of holes and the corresponding
activities of the Cu2O particles, which resulted in the low degra-
dation ratio (7.58%) of PNP when the Cu2O particles alone were
used (Fig. 5C). H2O2 has been used as a green additive in numer-
ous studies to enhance catalytic activities. The enhancements were
generally attributed to two reasons. First, H2O2 is a good electron
acceptor and can be converted to •OH radicals after accepting the
electrons [18] (Eq. (3)). The recombination of electron–hole pairs
is therefore hindered, promoting the applications of holes. Second,
H2O2 might be directly photolytically split to produce •OH under
UV radiation of up to ∼370 nm [22] (Eq. (4)). The light from the
halogen tungsten lamp contained a small amount of UV light, which
facilitated the occurrence of the reaction in Eq. (4). All the formed
•OH radicals could directly oxidize PNP (Eq. (5)). Thus, the degrada-
tion rate was enhanced compared with the solution without H2O2
(Fig. 5B and C). However, the enhancement was still very limited at
lower temperatures (≤41 ◦C).
•−
•−
then reacted with H2O2 to
oxide ions O2
(Eq. (7)) [12]. O2
produce hydroxyl radicals (•OH) [26] (Eq. (8)) and further accel-
erate the oxidation of PNP. When the temperature was increased,
the decomposition rate of H2O2 also increased, which resulted in
a rapid decomposition of PNP. A radical scavenger (t-butanol) was
added into the PNP solution in a control experiment to evaluate the
effect of oxidation via hydroxyl radical (Fig. 6). The removal rate
significantly decreased, and the degraded ratio was only 5.21% in
3 h. This value was much lower than that without t-butanol. There-
fore, the hydroxyl radical had the most important function in the
degradation process.
hꢁ
Cu2O−→ Cu2O(h+) + Cu2O(e−)
(1)
(2)
(3)
h
+ + PNP → degraded product
H2O2 + e− → OH− + •OH
hv
H2O2−→ 2•OH
(4)
(5)
(6)
(7)
(8)
•OH + PNP → degraded product
ꢂ,Cu
O
2
2H2O2 −→ 2H2O + O2
↑
•
−
O2 + e− → O2
•
O2 − + H2O2 → OH− + O2 + •OH
The irradiated solutions were qualitatively analyzed via high-
performance liquid chromatography (HPLC) to determine the
reaction intermediates and reveal some details of the reaction pro-
cess (Fig. S2 in the supporting data). During the irradiation, PNP
could be degraded gradually into two intermediates: hydroquinone
and 1,4-benzoquinone. After 3 h, PNP and 1,4-benzoquinone dis-
appeared, and most of the hydroquinone was decomposed. Based
on the variations in the intermediates, the degradation of PNP fol-
lowed the hydroquinone pathway [3], as shown in Scheme 1. The
phenolic hydroxyl group is an electron donor, which can increase
the electron density at the ortho- and para-positions, whereas
The temperature induced by the lamp was mainly responsi-
ble for the high increase in the degradation rate of PNP. On one
hand, increasing the temperature might accelerate the aforemen-
tioned reactions [23]. On the other hand, several new reactions
might occur. Based on the composition of the catalytic system, the
decomposition of H2O2 was thought to have an important function
in PNP degradation. H2O2 is not stable and can be decomposed to
molecular O2 (Eq. (6)) at elevated temperatures. The decomposi-
tion rate increases with increasing temperature and the addition
of a certain catalyst [24,25]. A previous study reported that Cu2O

W. Zhai et al. / Applied Catalysis A: General 454 (2013) 59–65
65
the photogenerated electrons and holes were first produced on the
surfaces of the Cu2O particles and then reacted with H2O2 and O2
to produce an abundant amount of •OH radicals. Thus, PNP was
decomposed into hydroquinone and benzoquinone, and was finally
mineralized to CO2 and H2O. A TOC removal of ca. 80% was achieved.
The degradation efficiency regularly varied with electrodeposition
time, light intensity, H2O2 amount, and solution temperature. The
particles were easily recycled and maintained their high activities.
In addition to PNP, other phenolic and dye pollutants in wastewa-
ters could be effectively degraded (Fig. S3 in the supporting data).
Acknowledgments
This work was co-supported by National Nature Science
Foundation of China (Grant No. 31071057) and the National
Nature Science Foundation of Guangdong Province (Nos.
10351063101000001 and S2011010003499).
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2012.12.032.
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Scheme 1. Degradation pathway of PNP in the presence of Cu2O particles and H2O2
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the NO2 group is an electron-withdrawing substituent and is a
very good leaving group. In such molecular structure, electrophilic
attack preferentially occurs in the para-positions with respect to
the OH group. When the PNP solution containing H2O2 and Cu2O
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generated. These radicals could attack the para-positions to form
a kind of unstable OH-adduct. Subsequently, the adduct could be
immediately decomposed to release the nitro group and produce
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could be formed as an intermediate, which subsequently dispro-
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formed. At the same time, 1,4-benzoquinone was unstable and
might be converted into hydroquinone in a weak acid environ-
ment or might be quickly decomposed by further oxidization. Thus,
hydroquinone was more easily detected than benzoquinone in the
HPLC spectra. Finally, hydroquinone and 1,4-benzoquinone could
be further oxidized by the •OH radical and oxygen, resulting in the
aromatic ring cleavage and the mineralization into CO2 and H2O.
The nitro group released from PNP was oxidized to nitric acid.
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In summary, the octahedral Cu2O particles that were isolated
from each other on the ITO glass showed high catalytic abilities
during PNP degradation in the presence of a small amount of H2O2
under a halogen tungsten lamp. During the degradation process,